Borinic Acid-Catalyzed Sulfation and Boronic Acid-Promoted ......work on utilizing boronic acids as protective groups for the preparation of sugar fatty acid ester surfactants is also

Borinic Acid-Catalyzed Sulfation and Boronic Acid-Promoted Esterification of Carbohydrates

by

Yu Chen Lin

A thesis submitted in conformity with the requirements for the degree of Master of Science

Department of Chemistry University of Toronto

© Copyright by Yu Chen Lin 2017

ii

Borinic Acid-Catalyzed Sulfation and Boronic Acid-Promoted

Esterification of Carbohydrates

Yu Chen Lin

Master of Science

Department of Chemistry University of Toronto

2017

Abstract

Carbohydrates and their O-sulfates play important roles in biological functions, including cellular

recognition and adhesion, neural processes, fibrosis, growth factor regulation, cancer metastasis,

and cellular entry of viruses. However, preparation of sulfated carbohydrates remains a synthetic

challenge with conventional methods requiring lengthy protection and deprotection steps.

Described herein is our work toward the development of a method for the regioselective sulfation

of fully unprotected carbohydrates using a borinic acid catalyst. Via an activated 1,2-cis-borinate

intermediate, our method was shown to be robust in the sulfation of a range of substrates, including

the synthesis of a sulfated galactosylceramide found in mammalian nervous systems. In addition,

work on utilizing boronic acids as protective groups for the preparation of sugar fatty acid ester

surfactants is also discussed.

iii

Acknowledgments

I would like to thank my supervisor, Professor Mark Taylor, for the opportunity to join his lab,

and his continued support in my studies, research, and pursuits.

I am also very grateful to all the Taylor lab members whom I’ve had the pleasure of meeting and

working together with. You have all made me feel so at home here, and although it has only been

a year, the great memories I’ve had here will stay with me long after. Furthermore, I’d like to give

a shout out to my friends in and around the Department for every laughter and drink we’ve shared.

Lastly, I would like to thank my family for their unconditional love and support throughout the

years and during my degree.

Table of Contents Acknowledgments .................................................................................................................... iii

Table of Contents ..................................................................................................................... iv

List of Tables ........................................................................................................................... vi

List of Figures ......................................................................................................................... vii

List of Abbreviations ............................................................................................................... ix

Chapter 1 Introduction ........................................................................................................1

1.1 Sulfated carbohydrates ...................................................................................................1

1.2 Direct, regioselective synthesis of sulfated carbohydrates ............................................4

1.3 Synthesis of sulfated carbohydrates via masked sulfates ............................................11

1.4 Organoboron compounds in carbohydrate chemistry ..................................................18

1.5 Scope of Thesis ............................................................................................................20

Chapter 2 Regioselective, catalytic sulfation of unprotected carbohydrates ....................21

2.1 Sulfation of carbohydrates with 2,2,2-trichloroethyl chlorosulfate .............................21

2.2 Sulfation of carbohydrates with alkyl and aryl 1,2-dimethylimidazolium salt ............22

2.3 Sulfation of carbohydrates with sulfur trioxide amine complexes ..............................29

2.4 Summary and future work ...........................................................................................32

2.5 Experimental ................................................................................................................33

2.5.1. General Information .........................................................................................33

2.5.2. General Procedure A ........................................................................................34

2.5.3. Preparation of catalyst and carbohydrate substrates ........................................34

2.5.4. Synthesis and characterization of compounds .................................................36

Chapter 3 Boronic acid-promoted Fischer esterification ..................................................47

3.1 Introduction ..................................................................................................................47

3.2 Chemical synthesis of sugar fatty acid esters ..............................................................49

v

3.3 Summary ......................................................................................................................51

3.4 Experimental ................................................................................................................51

3.4.1 General Information .........................................................................................51

3.4.2 General Procedure B ........................................................................................52

3.4.3 Synthesis and characterization of compounds .................................................52

Appendices ...............................................................................................................................58

A1.NMR spectra of reported compounds ..........................................................................58

vi

List of Tables Table 01. Optimization of methyl 6-O-TBS-α-D-mannopyranoside sulfation with 2,2,2-TCE chlorosulfate ..................................................................................................................................22

Table 02. Optimization of methyl α-L-rhamnopyranoside sulfation with TCE-sulfuryl 1,2- dimethylimidazolium triflate .........................................................................................................24

Table 03. Solvent screen of methyl α-L-rhamnopyranoside sulfation with TCE-sulfuryl 1,2- dimethylimidazolium triflate .........................................................................................................25

Table 04. Boronic acid/Lewis base co-catalyst for sulfation of methyl α-L-rhamnopyranoside ....26

Table 05. Optimization of methyl α-L-rhamnopyranoside sulfation with arylsulfuryl 1,2-dimethylimidazolium triflate .........................................................................................................28

Table 06. Optimization of methyl α-D-mannopyranoside sulfation with sulfur trioxide amine complex .........................................................................................................................................30

Table 07. Optimization of n-octyl α-D-galactopyranoside sulfation with sulfur trioxide amine

complex .........................................................................................................................................31

Table 08. Substrate scope of carbohydrate sulfation with SO3-Me3N ...........................................32

Table 09. Substrate scope for fatty acid and sugar alcohol esterification .......................................50

vii

List of Figures Figure 01. Structure of select naturally occurring sulfated carbohydrates .......................................2

Figure 02. Structure of the major tri-sulfated disaccharide repeat unit in heparin ...........................3

Figure 03. Structure of heparin derivatives .....................................................................................4

Figure 04. Conventional routes to access various patterns of carbohydrate sulfation ......................5

Figure 05. Transient boronate ester protection in the regioselective sulfation of steroids ...............6

Figure 06. Temperature-dependent regioselective sulfation of galactoside ....................................7

Figure 07. Effect of SO3-amine complexes in the sulfation of trimethylsilyl cellulose ...................8

Figure 08. Regioselective sulfation with dibutyltin oxide of 1,2-cis-diol and 1,2-cis-dioxy ...........9

Figure 09. Regioselective sulfation with dibutyltin oxide of 1,3-cis-diol ......................................10

Figure 10. Regioselective sulfation with dibutyltin oxide in the absence of 1,2-cis-diol ...............10

Figure 11. Routes of nucleophilic attack on a carbohydrate sulfate diester ...................................11

Figure 12. Preparation and unmasking of phenyl sulfate diesters ..................................................12

Figure 13. Preparation and unmasking of alkyl sulfate diesters, a) sulfation with iBu/nP

chlorosulfate, b) unmasking of iBu sulfate diesters, c) unmasking of nP sulfate diesters ...............13

Figure 14. Preparation and unmasking of trifluoroethyl sulfate diesters .......................................14

Figure 15. Stability of TFE-masked sulfate diester to further functionalizations ..........................15

Figure 16. Deprotection of TFE-masked sulfate diesters ..............................................................16

Figure 17. Preparation and unmasking of trichloroethyl sulfate diesters .......................................17

Figure 18. Regioselective TCE-masked sulfation of unprotected carbohydrates ..........................18

Figure 19. Tetracoordinate organoboron activation of diols for regioselective functionalization .19

viii

Figure 20. Synthesis of trichloroethyl chlorosulfate .....................................................................21

Figure 21. Synthesis of 2,2,2-trichloroethoxysulfuryl 1,2-dimethylimidazolium salts .................23

Figure 22. Sulfation with pre-formed cyclic arylboronate of methyl α-L-rhamnopyranoside .......26

Figure 23. Preparation of alkyl- and arylsulfuryl 1,2-dimethylimidazolium triflate .....................27

Figure 24. Decomposition study of 2.08, 2.18, 2.19 in CD3CN; A: 2.08 in 1 equiv. 1,2-

dimethylimidazole; B: 2.08 in 1 equiv. DIPEA; C: 2.18 in 1 equiv. DIPEA; D: 2.18 in 1 equiv. 3

MS-dried DIPEA; E: 2.19 in 1 equiv. DIPEA; F: 2.19 in 1 equiv. 3 MS-dried DIPEA .........29

Figure 25. Structure of select fatty acids and sugar alcohols .........................................................47

Figure 26. Preparation of sugar fatty acid esters with lipases ........................................................49

List of Abbreviations

Ac acetyl

app apparent Bn benzyl

br broad

Bu butyl

Bz benzoyl calcd. calculated

Cp cyclopentadienyl

CSA (1S)-(+)-10-camphorsulphonic acid d doublet

DAST (diethylamino)sulfur trifluoride

DBU 1,8-diazabicyclo[5.4.0]undec-7-ene

DCM dichloromethane

dec. decomposed

DIPEA N-N-diisopropylethylamine DMA N-N-dimethylacetamide

DMAP 4-(dimethylamino)pyridine

DMF N-N-dimethylformamide DMPU 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone equiv equivalents

ESI electrospray ionization

Et ethyl GML glycerol monolaurate h hour

hept heptet

HIV human immunodeficiency virus

HMDS bis(trimethylsilyl)amide

x

HMPA hexamethylphosphoramide

HRMS high-resolution mass spectra iBu isobutyl

Im imidazole

iPr isopropyl

IR infrared m multiplet (NMR), medium (IR)

m meta

Me methyl

min minute

MP 4-methoxyphenyl

Ms methanesulfonyl

MS molecular sieve NBS N-bromosuccinimide NMP N-methylpyrrolidine

NMR nuclear magnetic resonance

nP neopentyl o ortho p pentet

p para

Ph phenyl

Piv pivaloyl

PMB para-methoxybenzyl

PMP 1,2,2,6,6-pentamethylpiperidine

ppm parts per million q quartet rt room temperature s singlet (NMR), strong (IR)

SFAE sugar fatty acid ester

SIV simian immunodeficiency virus

xi

t triplet

TBAF tetrabutylammonium fluoride

TBS tert-butyldimethylsilyl tBu tert-butyl

TCE 2,2,2-trichloroethyl

TDS thexyl-dimethylsilyl

Tf trifluoromethanesulfonyl

TFA trifluoroacetic acid

TFE 2,2,2-trifluoroethyl

THF tetrahydrofuran

TMS trimethylsilyl

TREAT-HF triethylamine trihydrofluoride UV ultraviolet w weak

1

Chapter 1 Introduction

Carbohydrates present a wealth of diverse structures derived from joining together

monosaccharides, primarily in their pyranose or furanose forms. These structures can undergo

further functionalization such as alkylation, macrocyclization, phosphorylation, and sulfation.

Their structural diversity is mirrored by their broad range of biological functions and properties,

from energy storage as starch and structural support as cellulose, to cellular recognition and other

intercellular functions as glycolipids and glycoproteins.

The monosaccharides that make up complex carbohydrate structures contain several hydroxyl

groups that differ in their stereochemical arrangements. Site selectivity among similar hydroxyl

groups presents a difficult challenge for chemical modification. However, certain distinguishing

features among monosaccharides have been exploited. Reactivity differences among amines,

primary and secondary hydroxyl groups allow for site-selective modifications. cis-Diols present

in mannose and galactose can be used to distinguish between two secondary hydroxyl groups.

Differences between axial and equatorial hydroxyl groups can be used to influence stereochemical

outcomes.

1.1 Sulfated carbohydrates Sulfated carbohydrates are common in nature and play key roles in biological functions (Figure

01). Dermatan sulfate 1.01, found in skin and blood vessels, plays a role in coagulation and

fibrosis.1 Heparan sulfate 1.02, found on all cell surfaces, is a proteoglycan that functions in cell

adhesion, blood coagulation, and growth factor regulation.1 Sulfated sialyl-Lewisx 1.03, found on

cell surfaces, binds preferentially to lymphocyte cell-adhesion molecule L-selectin2 and has been

1 Capila, I.; Linhardt, R. J. Angew. Chem. Int. Ed. 2002, 41, 390-412.

2 Julien, S.; Ivetic, A.; Grigoriadis, A.; QiZe, D.; Burford, B.; Sproviero, D.; Picco, G.; Gillett, C.; Papp, S. L.;

Schaffer, L. Cancer Res. 2011, 71, 7683-7693.

2

shown to play a role in bladder urothelial carcinoma metastasis.3 Sulfated galactosylceramide 1.04,

found in the myelin sheath of nerve cells, play various functions in the nervous system. Abnormal

expression of these sulfoglycolipids is associated with neurological disorders such as Alzheimer’s

and Parkinson’s diseases.4 These sulfoglycolipids are also involved in the progression of other

illnesses such as diabetes mellitus and the cellular entry of HIV-1.5

Figure 01. Structure of select naturally occurring sulfated carbohydrates.

Perhaps the most well-known sulfated carbohydrates belong to the heparin polysaccharide family

(Figure 02). Discovered in 1916, naturally occurring heparin contains on average 25 units of the

disaccharide 1.05, giving a molecular weight of 5–40 kDa.1 It is the biological macromolecule

with the highest density of negative charge due to its sulfate and carboxylate groups. Heparin

3 Taga, M.; Hoshino, H.; Low, S.; Imamura, Y.; Ito, H.; Yokoyama, O.; Kobayashi, M. Urol. Oncol.: Semin. Orig.

Invest. 2015, 33, 496.e1-496.e9. 4 Eckhardt, M. Mol. Neurobiol. 2008, 37, 93-103.

5 Compostella, F.; Panza, L.; Ronchetti, F. C. R. Chim. 2012, 15, 37-45.

OO O

OH-O3SO

NHAc

O OHO

OSO3-HOOC

Dermatan sulfate 1.01

O-O3SO

O

OSO3-

O

AcHNO

OHO

OSO3-HOOC

OHO

OH

AcHN

OO

OH

HOOCOO

Heparan sulfate 1.02

OAcHN

COOH

OH

OH

HOHO

OO O

OSO3-OH

OH

OO OH

OH

NHAc

OOH

OHOH

6-Sulfo sialyl-Lewisx 1.03

OO

-O3SO

OHOH

OHC13H27

OH

HN fatty acid

O

Sulfated galactosylceramide 1.04

3

initiates the blood coagulation cascade by binding to enzyme antithrombin III, which then

inactivates thrombin and other proteases.1

Figure 02. Structure of the major tri-sulfated disaccharide repeat unit in heparin.

First approved in 1939, heparin sodium salt is an anticoagulant drug administered through

intravenous catheter or as an injection. It remains one of the oldest pharmaceutical drugs that is

still in use. Pharmaceutically relevant effects aside from anticoagulation include anti-inflammatory

properties for the treatment of ulcerative colitis and relief of obstructive pulmonary diseases like

asthma.6 Most pharmaceutical-grade heparin on the market is isolated from animal tissues,

particularly from porcine intestine or bovine lungs. However, the sulfation pattern of livestock-

grown heparin is variable and difficult to control, and the extracted heparin is prone to viral and

bacterial contamination. Recently, mammalian cell production of heparin using the Chinese

hamster ovary cell lines have been shown to be a safer and more robust alternative.6,7

Due to the success of heparin as a drug, a number of derivatives and low-molecular weight

analogues have been studied. Fondaparinux 1.06, a pentasaccharide marketed by

GlaxoSmithKline, was approved in 2001 as an anticoagulant (Figure 03).8 Fondaparinux has

advantages over heparin in that the former has a longer half-life, thus requiring a lower dosage.

6 Oduah, E. I.; Linhardt, R. J.; Sharfstein, S. T. Pharmaceuticals 2016, 9, 38.

7 Baik, J. Y.; Gasimli, L.; Yang, B.; Datta, P.; Zhang, F.; Glass, C. A.; Esko, J. D.; Linhardt, R. J.; Sharfstein, S. T.

Metab. Eng. 2012, 14, 81-90. 8 Bauer, K. A.; Hawkins, D. W.; Peters, P. C.; Petitou, M.; Herbert, J. M.; Boeckel, C. A. A.; Meuleman, D. G.

Cardiovasc. Ther. 2002, 20, 37-52.

O

O

OSO3-

O

-O3SHN

O OHO

OSO3--OOCHO

Heparin 1.05

4

Pentosan polysulfate 1.07, a plant-derived oligosaccharide, exhibits anti-HIV activity.9 Heparin

tetrasaccharide 1.08, with increased oral bioavailability, has anti-allergic activity and is being

studied for the treatment of asthma.10

Figure 03. Structure of heparin derivatives.

1.2 Direct, regioselective synthesis of sulfated carbohydrates Synthesis of sulfated carbohydrates remains a challenge despite their long history of biologically

relevant properties. Sulfation is commonly carried out with sulfur trioxide-amine complexes,

together forming a Lewis acid-base adduct. These complexes are easier to handle than liquid sulfur

trioxide, which requires distillation prior to use. The relative reactivities of the SO3 complexes

9 Baba, M.; Nakajima, M.; Schols, D.; Pauwels, R.; Balzarini, J.; De Clercq, E. Antiviral Res. 1988, 9, 335-343. 10

Ahmed, T.; Smith, G.; Abraham, W. M. Pulm. Pharmacol. Ther. 2013, 26, 180-188.

O

OSO3-

O

-O3SHNHO

O

OSO3-OH

-OOCOH

O

O

OSO3-O

-OOCOH

OOH

OHOSO3-

Heparin tetrasaccharide 1.08

O

OSO3-O

OSO3-

n

Pentosan polysulfate 1.07

O

O

OSO3-

HO

-O3SHNHO

O O-OOC

OH

HOO

O

OSO3-

-O3SHN-O3SO

O

-OOC OSO3-HO

O

OMe

OSO3-

O

-O3SHNHO

Fondaparinux 1.06

5

generally vary inversely with the strength of the Lewis base component. Common complexes used

are listed in order of decreasing basicity: Me3N ≈ Et3N > pyridine > DMF.11

Conventional routes to sulfated carbohydrates consist of numerous protection and deprotection

steps. As exemplified in Figure 04, the synthesis of galactopyranoside monosulfate requires

multiple orthogonal protecting groups, functional group manipulation, and selective deprotection

to reveal the free hydroxyl at the position of interest for sulfation.12

Figure 04. Conventional routes to access various patterns of carbohydrate sulfation.12

To overcome lengthy protection/deprotection steps, methods for direct and regioselective sulfation

have been studied.13 Boronate ester protection was utilized by McLeod to transiently mask the cis-

diols of steroid 1.09 for selective functionalization at the remaining hydroxyl group.14 The

11

Gilbert, E. E. Chem. Rev. 1962, 62, 549-589. 12 Marinier, A.; Martel, A.; Banville, J.; Bachand, C.; Remillard, R.; Lapointe, P.; Turmel, B.; Menard, M.; Harte, W. E.; Wright, J. J. K. J. Med. Chem. 1997, 40, 3234-3247. 13 Al-Horani, R. A.; Desai, U. R. Tetrahedron 2010, 66, 2907-2918. 14

Hungerford, N. L.; McKinney, A. R.; Stenhouse, A. M.; McLeod, M. D. Org. Biomol. Chem. 2006, 4, 3951-3959.

OHO OR

OHHO

OH

OO OR

OHO

OH

OO OR

OSO3-O

OH

OO OR

OPivO

OH

OO OR

OPivO

OSO3-

OHO OR

OPivAcO

OPiv

SO3-amine

O-O3SO OR

OPivAcO

OPiv

OBzO OR

OBz

OO

Ph

OBzO OR

OPivOH

OBz

SO3-amine

OBzO OR

OPiv-O3SO

OBz

SO3-amineSO3-amine

6-O-monosulfate 2-O-monosulfate 3-O-monosulfate 4-O-monosulfate

6

boronate ester of 1.10 can then be cleaved to reveal the cis-diol in 1.11 for selective sulfation to

give 1.12 (Figure 05). Although this route still requires protecting groups, it reduces the number

of separate purification steps.

Figure 05. Transient boronate ester protection in the regioselective sulfation of steroids.14

A temperature-dependent regioselective sulfation of galactoside 1.14 was described by Kondo

(Figure 06).15 Sulfation performed at room temperature with SO3-pyridine yielded the 3,4-O-bis-

sulfate 1.15 while at 0 oC, the reaction afforded only the 4-O-sulfate 1.16 without observable 3-O-

or bis-sulfate. The 3-O-sulfate 1.18, however, could only be prepared after first protecting the C4-

hydroxyl group. This method was not shown to be generalizable to other sugar moieties or to be

applicable in the presence of exposed primary or C2-hydroxyl groups.

15 Tsukida, T.; Yoshida, M.; Kurokawa, K.; Nakai, Y.; Achiha, T.; Kiyoi, T.; Kondo, H. J. Org. Chem. 1997, 62, 6876-6881.

HO

OH

OH

H

1) PhB(OH)2, DMF/CH2Cl22) TBS-Cl, imidazole

TBSOH

H2O2, aq. NaOH

THF73% over 3 steps

OBO

Ph

TBSO

OH

OH

H

SO3-pyridineDMF, pyridine

61%

TBSO

OH

OSO3- +Na

H

80% AcOH/H2O

69%HO

OH

OSO3- +Na

H

1.09 1.10

1.11

1.121.13

7

Figure 06. Temperature-dependent regioselective sulfation of galactoside.15

Following up on previous reports of sulfate insertion into O-Si bonds,16,17 Richter showed that the

Lewis base used in the sulfate complex can be tuned to influence the regiochemical outcome

(Figure 07).18 Trimethylsilyl cellulose 1.19 with SO3-DMF preferentially yields the 6-O-sulfate

1.20 while SO3-Et3N preferentially yields the 2-O-sulfate 1.21. The authors rationalize this effect

by stating that the electron-donation of Et3N polarizes the O-S bond of its SO3 complex, promoting

insertion at the more polarized O-Si bond of the O-2 position. This electron-donating effect is

absent in the DMF complex, which favors the more sterically accessible position at O-6.

16 Stein, A.; Wagenknecht, W.; Philipp, B.; Klemm, D.; Schnabelrauch, M., German Patent DD 299313, 1989. 17 Wagenknecht, W.; Nehls, I.; Stein, A.; Klemm, D.; Philipp, B. Acta Polym. 1992, 43, 266-269. 18 Richter, A.; Klemm, D. Cellulose 2003, 10, 133-138.

OHO OR

OBzOH

OBz

SO3-pyridineDMF, rt

73%

SO3-pyridineDMF, 0 oC

73%

OHO OR

OBzAcO

OBz

SO3-pyridineDMF, 0 oC

90%

O-O3SO OR

OBzAcO

OBz

OHO OR

OBz-O3SO

OBz

O-O3SO OR

OBz-O3SO

OBz

1.14

1.17

1.15 1.16 1.18

8

Figure 07. Effect of SO3-amine complexes in the sulfation of trimethylsilyl cellulose.18

Dibutyltin oxide was employed by Flitsch in 1994 to facilitate the regioselective sulfation of 1.22

(Figure 08).19,20 The dibutylstannylene acetal 1.23 was first formed at the cis-diol group with

super-stoichiometric amount of dibutyltin oxide, followed by a solvent-switch and addition of SO3-

Me3N complex to afford the product of sulfation at the more sterically accessible position of the

19

Guilbert, B.; Davis, N. J.; Flitsch, S. L. Tetrahedron Lett. 1994, 35, 6563-6566. 20

Guilbert, B.; Davis, N. J.; Pearce, M.; Aplin, R. T.; Flitsch, S. L. Tetrahedron: Asymmetry 1994, 5, 2163-2178.

OTMSO O

OTMS

OTMS

SO3-DMFTHF

SO3-Et3NTHF

OTMSO O

O

OTMS

SO

OO

SiO

TMSO O

OTMS

OSO

OO Si

NaOHMeOH

NaOHMeOH

OHO O

OSO3-

OH

OHO O

OH

OSO3-

major product, 1.20 major product, 1.21

1.19

9

1,2-cis-diol. The galactosylceramide glycolipid 1.24, found in mammalian nervous systems, was

synthesized in excellent yield. This strategy was also shown for disaccharides, including a

lactoside 1.25 to give the 3′-O-sulfate 1.26 as the major product with 10% of the 3′,6′-O-bis-sulfate

byproduct 1.27. In the absence of Bu2SnO, this reaction gave no observable 1.26. Maltosides 1.28

protected at the primary hydroxyl groups that do not possess a cis-diol were selectively 2′-O-

sulfated to give 1.29 in decent yield. The authors rationalize that the regioselectivity is due to either

the higher reactivity of the 2′-hydroxyl group or the C1′-C2′ cis-dioxy configuration. However,

they did not perform the control reaction to show that the maltoside sulfation is only regioselective

in the presence of Bu2SnO.

Figure 08. Regioselective sulfation with dibutyltin oxide of 1,2-cis-diol and 1,2-cis-dioxy.20

In 2004, Gelb adapted the method for the synthesis of 1.32, a fluorescent probe used in the assay

for screening newborns against Hunter syndrome, a disease caused by the deficiency of iduronate-

2-sulfatase (Figure 09).21 The dibutyltin oxide, in this case, binds to the 1,3-cis-diol of iduronic

ester 1.30, delivering the sulfate to the more nucleophilic O-2 affording 1.31.13

21

Blanchard, S.; Turecek, F.; Gelb, M. H. Carbohydr. Res. 2009, 344, 1032-1033.

OHO OR

OHOH

OH

Bu2SnO (1.5 equiv)

MeOHreflux, 2 h

ORO

O

OHO

OH

SnBu

BuSO3-Me3N (2 equiv)

THFrt, 4 h

1.2497%

O-O3SO OR

OHOH

OH

R =HN

OH

C13H27

O

C8H17

1.22 1.23

OHO

O

OHHO

OH

OOH

OH

OHSPh

1) Bu2SnO (1 equiv) MeOH, reflux, 2 h

2) SO3-Me3N (2 equiv) dioxane, rt, 30 h

ORO

O

OR′HO

OH

OOH

OH

OHSPh

1.25 1.26 R = SO3-, R′ = H1.27 R = SO3-, R′ = SO3-

76%10%

OOH

OTBS

OHO-allylO

OOH

OH

OO

Ph1) Bu2SnO (1 equiv) MeOH, reflux, 2 h

2) SO3-Me3N (2 equiv) dioxane, rt, 93 h

OOH

OTBS

OHO-allylO

OOH

-O3SO

OO

Ph

1.28 1.2956%

10

Figure 09. Regioselective sulfation with dibutyltin oxide of 1,3-cis-diol.21

In 2009, Kosma demonstrated the regioselective sulfation of xylopyranosides and xylotriosides.22

Low-molecular weight xylans and their sulfates have been shown to possess antithrombin23 and

antiasthmatic24 activities. With the dibutyltin oxide strategy, 1.33 and 1.35 gave the terminal 4-O-

sulfated products 1.34 and 1.36, respectively (Figure 10).22 However, this selectivity for substrates

without the presence of a cis-diol was not explained, and the remaining mass balance of the

sulfation of 1.35 was unaccounted for.

Figure 10. Regioselective sulfation with dibutyltin oxide in the absence of 1,2-cis-diol.22

Although activation with dibutyltin oxide has shown to be highly selective for sulfating the

equatorial hydroxyl of a cis-diol, it requires—at minimum—a stoichiometric amount of tin, long

reaction times (93 h to afford 1.29), and a two-step activation/sulfation process. That being said,

22

Abad-Romero, B.; Mereiter, K.; Sixta, H.; Hofinger, A.; Kosma, P. Carbohydr. Res. 2009, 344, 21-28. 23 Yamagaki, T.; Tsuji, Y.; Maeda, M.; Nakanishi, H. Biosci. Biotechnol. Biochem. 1997, 61, 1281-1285. 24 Kuszmann, J.; Medgyes, G.; Boros, S. Carbohydr. Res. 2005, 340, 1739-1749.

O

OHOH

O

MeOOCOH O O

O

NH

HNt-BuO

O 1) Bu2SnO (1.5 equiv) MeOH, reflux, 40 min

2) SO3-Me3N (1.5 equiv) DMF, 55 oC, 24 h O

OSO3-OH

O

MeOOCOH O O

O

NH

HNt-BuO

O

O

OSO3-OH

O

-OOCOH O O

O

NH

HNt-BuO

ONaOH

H2O

1.3261% over 2 steps

1.30 1.31

HO OHO

OHOMe

1) Bu2SnO (1.08 equiv) toluene, reflux, 15 h

2) SO3-Me3N (1.1 equiv) THF, rt, 48 h

-O3SOO

HOOH

OMe

O OHO

OHHO O

HOOH

O OHO

OHOMe

1) Bu2SnO (4 equiv) toluene, reflux, 15 h

2) SO3-Me3N (3.2 equiv) THF, rt, 72 h

O OHO

OH

-O3SOO

HOOH

O OHO

OHOMe

1.33 1.3474%

1.35 1.3627%

11

direct sulfation of carbohydrates presents a concise and efficient alternative to traditional

protecting group-based methods. However, this strategy is often only applicable as the final step

in a complex synthesis. The highly polar sulfated products are insoluble in organic solvents,

making them difficult to purify and manipulate for subsequent elaborations. To this end, masked

sulfates have been devised.

1.3 Synthesis of sulfated carbohydrates via masked sulfates Masked sulfates, or protected sulfate esters, provide a way to address the issues of sulfated

products, allowing for chemical transformations of the molecule elsewhere. Following appropriate

transformations, the sulfate can be unmasked to reveal the desired product. These masked sulfates

must be stable to conventional purification techniques and survive a wide range of reaction

conditions including, in particular, the deprotection conditions of other functional groups on the

molecule.

Sulfate diesters have generally been used, providing the added benefit of a spectroscopic handle

that is compatible with common characterization techniques. Sulfate diesters are prone to

nucleophilic attack and substitution at one of three possible sites (Figure 11).25 Substitution via

Route A leading to desulfation is generally slow, particularly on a carbohydrate’s secondary

hydroxyl group. Design of sulfate diesters would ideally disfavor attack on the sulfur center (Route

B), or premature deprotection of the ester (Route C).

Figure 11. Routes of nucleophilic attack on a carbohydrate sulfate diester.25

25

Proud, A. D.; Prodger, J. C.; Flitsch, S. L. Tetrahedron Lett. 1997, 38, 7243-7246.

SO O

O OR carbohydrate

NuA

BC

12

Perlin first introduced phenyl chlorosulfate as a masked sulfating group for carbohydrates.26 The

stable phenyl chlorosulfate is synthesized from phenol, sodium hydroxide, and sulfuryl chloride

in high yields. After reaction with 1.37 (Figure 12), unmasking of the sulfate diester 1.38 was

achieved by catalytic hydrogenolysis of the phenyl group to a cyclohexyl group, followed by alkyl

fission to give 1.39. Although yields for unmasking the phenyl group were not reported, the authors

stated that about 10% desulfation had occurred. The phenyl-masked sulfate is stable to the

TFA/CHCl3 conditions used to deprotect the 5,6-O-isopropylidene group, and also stable to 1:1

Ac2O:H2SO4, which was used to deprotect both isopropylidene groups. However, this method is

not compatible with functional groups sensitive to base and hydrogenolysis.

Figure 12. Preparation and unmasking of phenyl sulfate diesters.26

Alkyl-masked sulfating groups have been described by Widlanski with particular emphasis on

isobutyl (iBu) and neopentyl (nP) chlorosulfates.27 The carbohydrate 1.37 was first stirred in

NaHMDS, followed by the addition of the masked chlorosulfate to give sulfated products 1.40 and

1.41 (Figure 13a). Sulfation with iBu chlorosulfate was slower and required large excess of the

chlorosulfate, while sulfation with nP chlorosulfate required DMPU as a co-solvent. The stabilities

of the sulfates were studied using phenyl iBu or nP sulfate diesters as model substrates. The iBu

sulfate diester degraded completely in 6% piperidine after 24 h, but was relatively stable in 50%

TFA (7.1% degradation after 24 h). No degradation of the nP sulfate diester was observed in 20%

piperidine, versus 6.9% degradation in 50% TFA after 24 h. Both sulfate diesters were stable under

26

Penney, C. L.; Perlin, A. S. Carbohydr. Res. 1981, 93, 241-246. 27

Simpson, L. S.; Widlanski, T. S. J. Am. Chem. Soc. 2006, 128, 1605-1610.

OOH

OO

OO

1) NaH, THF, 30 min

2)SO

OClPhO , 20 h

OO

OO

OO

SOPh

O O

1.3875%

K2CO3, PtO2, H2

EtOH, H2O, 20 h

OOSO3-

OO

OO

1.37 1.39

13

conditions used to deprotect benzyl and isopropylidene groups (hydrogenolysis with Pd/C and H2,

and acidification with aq. H2SO4/THF)

Figure 13. Preparation and unmasking of alkyl sulfate diesters, a) sulfation with iBu/nP

chlorosulfate, b) unmasking of iBu sulfate diesters, c) unmasking of nP sulfate diesters.27

Unmasking of the diester was examined through Route C (Figure 11) via the attack of a small

nucleophile on the alkyl group to reveal the sulfated carbohydrate. iBu sulfate diester 1.42 was

unmasked cleanly in the presence of NaI at elevated temperatures to give 1.43 (Figure 13b). nP

sulfate diester 1.44, however, was more difficult to unmask due to its increased bulk. Under the

same deprotection conditions for the iBu sulfate diester, the nP sulfate diester 1.44 yielded no

OOH

OO

OO

1) NaHMDS, THF

2) ClSO2OiBu (5–10 equiv), −15 oCor

ClSO2OnP (1 equiv), DMPU, −75oC

OO

OO

OO

SOR

O O

1.40 R = iBu 95%1.41 R = nP 95%

OiBu-O3SO

OH

HO

OH OH

NaI

acetone, 55 oCO

-O3SO

OH

HO

OH OH1.4397%

a)

b)

c)

NaI

acetone, 55 oC

OOSO3-nP

OO

OO

no reaction

OnP-O3SO

OH

HO

OH OH

O

OH

HO

OH OHN3

OOSO3-nP

OO

OO

OOSO3-

OO

OO

NaN3

DMF, 70 oC

NaN3

DMF, 70 oC

1.3998%

1.37

1.42

1.44

1.44

1.45 1.46

14

reaction (Figure 13c). NaN3 was found to be able to unmask the nP sulfate diester of the

glucofuranose sulfate 1.44 to give 1.39, but led to the azide substitution product of the

glucopyranose sulfate 1.45 to give 1.46. Although the method shown by Widlanski were not

generally applicable and showed limited scope, it does offer a masked-sulfate approach with the

potential for alkyl group manipulation to tune the reactivity of the masked sulfate.

Trihaloethyl-masked sulfates have also long been considered as an alternative to aryl-masked

sulfates. Trifluoroethyl (TFE) and trichloroethyl (TCE) are thought to hinder Routes B and C

(Figure 11) compared to alkyl and aryl masking groups due to both steric and electronic reasons.

TCE esters have been used in the protection of phosphate and carboxyl groups, and removed

selectively by Zn/AcOH. Flitsch sought to extend trihaloethyl masking to the sulfation of

carbohydrates.25 Focusing initially on TCE-masked sulfates, the authors were unable to sulfate

carbohydrates in sufficiently high yields for further studies, citing steric hindrance of the trichloro

group. Switching direction to TFE-masked sulfates, the authors were unable to introduce the TFE-

sulfate in one step from the TFE-chlorosulfate. Instead, the carbohydrate 1.47 had to be first

sulfated with an SO3-amine complex, followed by TFE protection with 2,2,2-trifluorodiazoethane

in moderate yields (Figure 14). The TFE-sulfate diester 1.48 was stable to conditions including

hydrogenation, 20% TFA/EtOH, and NaOMe/MeOH used for isopropylidene deprotection. Harsh

conditions (reflux in t-BuOK/t-BuOH) were required for attack on the sulfur center and elimination

of TFE to give the unmasked sulfate 1.49. However, deprotection of TFE-sulfate diester on O-2

or O-3 positions resulted in sulfate migration to free, adjacent hydroxyl groups. Apart from

difficult deprotection, this masked sulfate strategy was not widely adopted in carbohydrate

synthesis due to the need for a two-step sulfation process and the potentially explosive nature of

trifluorodiazoethane.

Figure 14. Preparation and unmasking of trifluoroethyl sulfate diesters.25

1) SO3-pyridine, MeCN, 80 oC

2) F3C N2citric acid, MeCN, rt

1.4851%

OO

O

OHO

O

OO

O

OSO3TFEO

O OHO

OSO3-HO

OH OH

1) 4:1 TFA:EtOH rt, 2 h, 97%

2) t-BuOK, t-BuOH, reflux, 96%

1.47 1.49

15

Linhardt then showed the versatility and stability of TFE-masked sulfate diesters in subsequent

functionalizations. Sulfate diesters 1.50 and 1.52 were stable to either TBAF in AcOH or TREAT-

HF (Figure 15), thus demonstrating orthogonality to silyl protective groups.28 The masked sulfate

diesters were also stable to fluorination and glycosylation conditions, affording TFE-masked

sulfate disaccharides 1.54 and 1.56 in decent yields.

Figure 15. Stability of TFE-masked sulfate diester to further functionalizations.28

28

Karst, N. A.; Islam, T. F.; Linhardt, R. J. Org. Lett. 2003, 5, 4839-4842.

OBzO OTDS

OSO3TFE

O

N3

OBzO

OSO3TFE

O

N3

1) TBAF, AcOH, THF, −25 oC

2) DAST, DCM, , −30 oC

OBzO OTDS

OSO3TFEBnO

N3

F

1) TREAT-HF

2) DAST, DCM, −30 oC

1.5166%

1.5363%

OBzO

OSO3TFEBnO

N3 F

OBzO

OSO3TFEBnO

N3F

+O

O

O

OHO

OAgClO4, Cp2ZrCl2

4 Å MS, DCMO

O

O

OO

O

OBzO

OSO3TFEBnO

N3

1.5464%

OO

O

OHO

O+

BF3 Et2O

4 Å MS, DCM

OBzO

OBnTFEO3SO

N3O

NH

Cl3CO

O

O

OO

O

OBzO

OBnTFEO3SO

N3

1.5649%

1.50

1.52

1.53 1.47

1.55 1.47

OCl

OCl

16

Deprotection of TFE-masked sulfate diesters proved to be quite challenging.29 Previously reported

t-BuOK deprotection conditions were effective on monosaccharide 1.57 to give 1.58, but led to

the decomposition of disaccharide 1.59 (Figure 16). However, NaOMe/MeOH was found to be

able to unmask the diester of disaccharide 1.59 in good yields to give 1.60 with minimal

decomposition. Deprotection of TFE-masked bis-sulfate disaccharide 1.61 required a two step

process. Unmasking with NaOMe/MeOH gave 1.62 in excellent yield, but the harsher t-BuOK

reagent used in the second step gave 1.63 in only moderate yield, along with disaccharide

decomposition, desilylation, and desulfonation.

Figure 16. Deprotection of TFE-masked sulfate diesters.29

In 2004, Taylor re-visited Flitsch’s attempts to develop a method for TCE-masked sulfate diesters

and succeeded in sulfating aryl alcohols with TCE chlorosulfate.30 In follow-up reports of

extending this methodology to the sulfation of carbohydrate 1.47, the authors managed to

synthesize the sulfate diester 1.64 in approximately 50% yield, with the majority of the remaining

mass balance being the chlorosugar byproduct 1.65 (Figure 17). This result, in contrary to Flitsch’s

29

Karst, N. A.; Islam, T. F.; Avci, F. Y.; Linhardt, R. J. Tetrahedron Lett. 2004, 45, 6433-6437. 30

Liu, Y.; Lien, I. F. F.; Ruttgaizer, S.; Dove, P.; Taylor, S. D. Org. Lett. 2004, 6, 209-212.

OBnO OMP

OSO3TFE

OOPh t-BuOK, t-BuOH

OO

O

OO

O

OBzO

OBnTFEO3SO

N3 NaOMe, MeOH

OBnO OMP

OSO3-O

OPh

OBzO

OBnTFEO3SO

N3

OBnO OMP

OTBS

O

OSO3TFE

NaOMe, MeOHO

HO

OBn-O3SO

N3

OBnO OMP

OTBS

O

OSO3TFE

t-BuOK, t-BuOHO

HO

OBn-O3SO

N3

OBnO OH

OTBS

O

OSO3-

OO

O

OO

O

OHO

OBn-O3SO

N3

1.5882%

1.6070%

1.6290%

1.6350%

1.57

1.59

1.61

17

report in 1997,25 showed that TCE-sulfated carbohydrates could indeed be prepared in good yields.

Encouraged by this finding, Taylor designed a sulfating reagent that did not release an effective

nucleophilic species. With the optimized 1,2-dimethylimidazolium triflate salt 1.67 in hand, the

authors achieved TFE-masked sulfation of carbohydrate 1.66 to give 1.68 in high yields, as well

as deprotection of 1.68 under mild conditions with Zn or Pd/C and ammonium formate to give

1.58 (Figure 17).31 The sulfating reagent 1.67 was stable to prolonged storage at room temperature

and the TCE-masked sulfate diester 1.68 was stable to a range of reaction conditions:

NaOMe/MeOH and ZnCl2/AcOH/Ac2O for debenzylation and deacetylation, NBS in

acetone/water, DBU, and acidic conditions for benzylidene opening.

Figure 17. Preparation and unmasking of trichloroethyl sulfate diesters.31,32

Masked sulfation provides products that can be manipulated for further synthetic transformations.

However, despite these recent advances in the field of masked sulfation of carbohydrates, there

remains a need for a reliable method toward the regioselective installation of a masked sulfate

group, particularly on fully unprotected carbohydrates. In 2016, Kaji and Makino33 bridged that

gap by using the transient boronate ester protection for the regioselective sulfation of steroids

31

Ingram, L. J.; Desoky, A.; Ali, A. M.; Taylor, S. D. J. Org. Chem. 2009, 74, 6479-6485. 32

Ingram, L. J.; Taylor, S. D. Angew. Chem. Int. Ed. 2006, 45, 3503-3506. 33

Fukuhara, K.; Shimada, N.; Nishino, T.; Kaji, E.; Makino, K. Eur. J. Org. Chem. 2016, 2016, 902-905.

OO

O

OHO

O

SO

OClTCEO

AgCN, Et3N, DMAP, THF

OO

O

OSO3TCEO

O

OO

O

ClO

O

1.64~ 50%

+

OBnO OMP

OH

OOPh

SO

OTCEO N N

OTf

1,2-dimethylimidazoleDCM, 24 h

OBnO OMP

OSO3TCE

OOPh

1.6896%

Zn or Pd/CHCO2NH4

MeOH

OBnO OMP

OSO3-O

OPh

1.58with Zn: 94%

1.47 1.65

1.66

1.67

18

described previously14 and the TCE-masked sulfate developed by Taylor.31 In one pot, unprotected

carbohydrates 1.69 with 1,2-cis- or 4,6-diols are protected by phenylboronic acid, followed by

TCE-masked sulfation at the remaining hydroxyl group of 1.70, and transesterification of the

boronate ester with pinacol to reveal the TCE-masked sulfate carbohydrate 1.71 in good yields

(Figure 18).

Figure 18. Regioselective TCE-masked sulfation of unprotected carbohydrates.33

1.4 Organoboron compounds in carbohydrate chemistry As discussed previously, organoboron compounds have been used in carbohydrate chemistry for

functional group protection14,33 among other purposes.34 In addition, organoboron compounds in

their tetracoordinate state have been used for the activation of diols. As first described by Aoyama,

the phenylboronate ester of the 1,2-cis-diol of methyl α-L-fucopyranoside 1.72 can be activated

by an amine base to give the tetracoordinate complex 1.73 (Figure 19).35 It can then undergo

regioselective alkylation at the more accessible position to give 1.74. Other regioselective

transformations following this strategy have also been explored, including the glycosidation of

34

McClary, C. A.; Taylor, M. S. Carbohydr. Res. 2013, 381, 112-122. 35

Oshima, K.; Kitazono, E.-I.; Aoyama, Y. Tetrahedron Lett. 1997, 38, 5001-5004.

O

OR(HO)n

PhB(OH)2

DCMrt, 24–48 h

O

OR(HO)n

OOB

Ph

1)

SO

OTCEO N N

OTf

1,2-dimethylimidazole4 Å MS, THF or DCM20–24 h, 0 oC to rt

2) pinacol, DCM

O

OR(HO)n

TCEO3SO

Product:

OTCEO3SO O

OHHO

OH

OHO OMe

OH

HO

TCEO3SO

O

OH

TCEO3SO

OMe

OH

76% 74% 66% 72%

1.691.70 1.71

OOSO3TCE

OH

OMe

OH

19

unprotected carbohydrate 1.72, via the tetracoordinate complex 1.75, to give the disaccharide 1.76

(Figure 19).36

Figure 19. Tetracoordinate organoboron activation of diols for regioselective functionalization.34

Expanding on the reactivity of tetracoordinate boronates, our group has developed catalytic

versions employing borinic acid derivatives, avoiding the need for complexation with a Lewis

36

Oshima, K.; Aoyama, Y. J. Am. Chem. Soc. 1999, 121, 2315-2316.

OOH

OH

OMe

OH

1) PhB(OH)2

2) Ag2O, Et3N benzene, reflux

OOH

O

OMe

O

B PhEt3N

n-BuI OOH

OH

OMe

On-Bu

1.7450%

OOH

OH

OMe

OH Ag2CO3Et4N+ I-, 4 Å MS

OOH

O

OMe

O

B

OOH

OH

OMe

O

1.7693%

O BOH

O

OAcO

Br

OAc

AcO

AcO

OAcO

OAc

AcO

AcO

OHO

OMe

OTBSHO

HO

BO

H2NPh

Ph(10 mol%)

BzCl, iPr2NEt

MeCNO

O

OMe

OTBSO

HO

BPhPh

BzO NHBz

OBzO

OMe

OTBSHO

HO

1.7995%

1.721.73

1.72

1.75

1.77 1.78

20

base. Catalytic, regioselective acylation,37 sulfonylation,38 alkylation,39 and glycosylation40 of

carbohydrates have since been accomplished. An example with 1.77 is shown in Figure 19,

forming the tetracoordinate complex 1.78, to give the regioselectively benzoylated product 1.79.

1.5 Scope of Thesis Carbohydrates and their O-sulfated derivatives have long been known to play important roles in

biology. With a growing number of carbohydrate drugs being approved in recent years, it follows

that concise and elegant approaches to the synthesis of carbohydrate derivatives, including O-

sulfates, are also increasingly needed. Furthermore, organoboron compounds have shown to be

powerful in imparting regiocontrol in carbohydrate functionalization, both as protecting and

activating groups.

The remainder of this thesis discusses my research on expanding the scope of borinic acid catalysis

and boronic ester protection in carbohydrate synthesis. Chapter 2 outlines the development of a

method for the regioselective sulfation of unprotected sugars. In contrast to previously described

sulfation methods, this synthesis employs catalytic borinic acid to effect direct sulfation with high

selectivity. Chapter 3 describes the use of boronic acids in Fischer esterification between sugar

alcohols and fatty acids for the preparation of surfactants. The esterification project outlined in this

last chapter was conducted in conjunction with a fellow laboratory group member, Sanjay Manhas,

and individual contributions to the joint work are delineated therein.

37 Lee, D.; Taylor, M. S. J. Am. Chem. Soc. 2011, 133, 3724-3727. 38

Lee, D.; Williamson, C. L.; Chan, L.; Taylor, M. S. J. Am. Chem. Soc. 2012, 134, 8260-8267. 39

Chan, L.; Taylor, M. S. Org. Lett. 2011, 13, 3090-3093. 40

Gouliaras, C.; Lee, D.; Chan, L.; Taylor, M. S. J. Am. Chem. Soc. 2011, 133, 13926-13929.

21

Chapter 2 Regioselective, catalytic sulfation of unprotected carbohydrates

As discussed in Chapter 1, although regioselective methods using stoichiometric amounts of

additives have been developed for direct sulfation, they are only suitable as the final step in a

synthesis due to the highly polar nature of the products. Masked sulfates, with their various

protective ester groups, address some of the issues of sulfated products but no regioselective

strategies have been developed for sulfation. My research began by employing the catalytic

method developed in our lab using borinic acid-activation of cis-diols to prepare TCE-masked

sulfates of carbohydrates.

2.1 Sulfation of carbohydrates with 2,2,2-trichloroethyl chlorosulfate

TCE chlorosulfate 2.02 was prepared according to literature41 in 84% yield from sulfuryl chloride

2.01 and 2,2,2-trichloroethanol (Figure 20).

Figure 20. Synthesis of trichloroethyl chlorosulfate.

Initial attempts for TCE-masked sulfation of 6-O-TBS-α-D-mannopyranoside 2.03 with 2.02,

catalyzed by the borinic ester pre-catalyst 2.04 yielded the cyclic sulfate 2.05 in low yields (Table

01). After the initial sulfation reaction, the sulfate diester presumably undergoes a rapid cyclization

with the adjacent free hydroxyl group on the carbohydrate. The 2,2,2-trichloroethoxide generated

in situ as a result of the cyclization may explain the intermolecular TBS migration product 2.06

observed under 1.5 equivalents of DIPEA or Et3N (Entries 1 and 5). However, the sugar without a

TBS-group was not able to be isolated cleanly for characterization. Furthermore, the strongly basic

41

Pitts, A. K.; O'Hara, F.; Snell, R. H.; Gaunt, M. J. Angew. Chem. Int. Ed. 2015, 54, 5451-5455.

SO

Cl ClO

pyridine (1 equiv)Et2O (0.5 M)

1.5 h at −20 oC, then 0.5 h at rt

SO

O ClOCl3C

(1 equiv)2.01

2,2,2-trichloroethanol (1 equiv)

2.0284%

22

ethoxide could cause decomposition of the sulfating reagent 2.02, leading to low yields of sulfated

products. Decreasing the reaction time (Entry 2), using DCM as solvent (Entry 3), or using 1.1

equivalents of DIPEA (Entry 4) also gave cyclic sulfates with none of the desired, uncyclized

product observed. A preliminary base screen with Et3N and pyridine (Entries 5, 6) did not yield

any productive results. 1,2,2,6,6-Pentamethylpiperidine (PMP) was used as a bulky base (Entry 7)

in hopes of minimizing cyclic sulfate formation, but the TCE-masked sulfate diester was not

observed.

Table 01. Optimization of methyl 6-O-TBS-α-D-mannopyranoside sulfation with 2,2,2-TCE

chlorosulfate.

2.2 Sulfation of carbohydrates with alkyl and aryl 1,2-dimethylimidazolium salt

Following Taylor’s success using the sulfating reagent in the form of an imidazolium salt,31 we

synthesized the 2,2,2-trichloroethoxysulfuryl 1,2-dimethylimidazolium salts from the reaction of

2.02 with 2-methylimidazole to give 2.07, followed by methylation to give the

dimethylimidazolium salts (Figure 21). MeOTf gave high yields of 2.08, but methylation with MeI

or Me3OBF4 gave low conversion to 2.09 and 2.10, respectively, and were not pursued further.

OHO

HO

OH

OMe

TBSO

Base (1.5 equiv)2.04 (10 mol %)MeCN (0.2 M)

24 h, rt2.03

OHOO

O

OMe

OTBSSOO

OTBSOO

O

OMe

OTBSSOO

+

BaseEntry

2.05 2.06

2.03 Yield (%)a 2.04 Yield (%)a

DIPEADIPEA DIPEA DIPEAEt3N

pyridine 1,2,2,6,6-pentamethylpiperidine

4010636

2919

5---1--

12b3c4d567

aCrude 1H NMR yields were determined based on tetramethylsilane as an internal standard. b6 hour reaction time. cDCM used as solvent. d1.1 equivalents of 1.58 and DIPEA used.

2.042-aminoethyl phenylborinate

OB

NH2

Ph

Ph(1.5 equiv)

SO

O ClOCl3C

23

Figure 21. Synthesis of 2,2,2-trichloroethoxysulfuryl 1,2-dimethylimidazolium salts.

We then investigated TCE-masked sulfation with the imidazolium triflate salt using methyl α-L-

rhamnopyranoside as a model substrate for easier characterization compared to the TBS-protected

sugars and to avoid the possibility of silyl migration (Table 02). Conditions A with a tertiary amine

base for substrate/catalyst binding in MeCN were based on previously optimized conditions for

catalyst activity in our group.37 Conditions B with 1,2-dimethylimidazole as base in DCM at 0 oC

to room temperature were based on Taylor’s optimized conditions for TCE-sulfation.31 TCE-

masked sulfated rhamnopyranosides were observed, and a clear pattern of regioselectivity became

evident. With Conditions A, there is a slight preference for the 3-O-sulfated product 2.12 which

presumably goes through a tetracoordinate borinate complex, delivering the TCE-sulfate to the

more accessible, equatorial position at O-3. However, with Conditions B, background reaction in

the absence of catalyst was very prominent, with a slight preference for the 2-O-sulfated product

2.13. The catalyst did not appear to have a significant effect for Conditions B, affording neither an

increase in yield nor influence on regioselectivity. This may be due to the inability of 1,2-

dimethylimidazole to promote effective substrate binding to the borinic acid catalyst.

SO

OClTCEO

NHN

SO

OTCEO N N

MeOTf (1 equiv)SO

OTCEO N N

OTf

2.0776%

2.0882%

2.02

(3.6 equiv)

THF (0.025 M)1 h at 0 oC,

then 1 h at rt

Et2O (0.2 M)0 oC, 3 h

MeI (1 equiv)

THF (0.2 M)0 oC to rt, 4 d

Me3OBF4 (1 equiv)

THF (0.2 M)0 oC, 24 h

SO

OTCEO N N

I

2.09low conversion

SO

OTCEO N N

BF4

2.10low conversion

24

Table 02. Optimization of methyl α-L-rhamnopyranoside sulfation with TCE-sulfuryl 1,2-

dimethylimidazolium triflate.

Proceeding with the result that gave the most promising regioselectivity (Table 02, Entry 4,

Condition A), a solvent screen quickly revealed that amide solvents were essential (Table 03).

DMF and DMA gave the 3-O-sulfate 2.12 in improved regioselectivity (Entries 4, 5). A control

reaction in DMA without catalyst gave trace yields (Entry 6), showing that the borinic acid is

crucial. However, the switch to NMP did not prove to be productive (Entry 7). The use of solvent

mixtures, DMA/MeCN and DMA/H2O, also did not prove to be beneficial (Entries 8, 10). In an

attempt to improve yield, we increased the reaction temperature to 50 oC but were met with

decreased yield, although complete regiocontrol (Entry 9). This result was puzzling at the time,

and will be re-visited later on in the chapter.

OHOHO

OH

OMe

Conditions A or Conditions B

(1.5 equiv)SO

O NOCl3C N

OTf

OHOTCEO3SO OH

OMe

OHOHO

OSO3TCE

OMe

+

Equiv. of BaseEntry Yield (%)a with

Conditions AYield (%)a with Conditions B

123456

2.11 2.12 2.13

15243937354

2.12:2.13 Ratiowith Conditions A

2.12:2.13 Ratiowith Conditions BCatalyst (XX mol%)

no catalyst2.04 (10 mol%)2.14 (5 mol%)

2.15 (10 mol%)no catalyst

2.04 (10 mol%)

1:11:1

1.5:11.8:1

1:13:1

605450--

41

1:21:21:2.3

1:1.5

2.04

OB

NH2

Ph

Ph

2.14

O BBPh

Ph

Ph

Ph

2.15

B

O

OH

1.51.51.51.50.20.2

--

aCrude 1H NMR yields were determined based on tetramethylsilane as an internal standard. Product ratio determined by 1H NMR. Conditions A: catalyst (XX mol%), DIPEA (YY equiv), MeCN (0.2 M), 24 h, rt. Conditions B: catalyst (XX mol%), 1,2-dimethylimidazole (YY equiv), DCM (0.2 M), 24 h, 0 oC to rt.

25

Table 03. Solvent screen of methyl α-L-rhamnopyranoside sulfation with TCE-sulfuryl 1,2-

dimethylimidazolium triflate.

After further optimization of the reaction conditions (base, catalyst, various methods of reagent

addition) in DMA, we were unable to raise the yield of the desired 3-O-TCE-sulfated product 2.12

above 50% and achieved only modest regioselectivity. Turning to alternate modes of activation,

we looked at complexation induced activation with boronic acid derivatives. The pre-formed cyclic

arylboronate of methyl α-L-rhamnopyranoside 2.14 was hypothesized to be activated by an

external Lewis base (Et3N) through a tetracoordinate intermediate for sulfation (Figure 22).

However, the cyclic boronate acted as a protective group instead to give the 4-O-sulfate, followed

by hydrolysis of the boronate group during work-up to give 2.15. This showed that the competing

background sulfation reaction occurs very rapidly. In the presence of the relatively unreactive 4-

hydroxyl group, the direct sulfation reaction is more favorable than sulfation through the activated

boronate.

OHOHO

OH

OMe

2.15 (10 mol%)DIPEA (1.5 equiv)

Solvent (0.2 M)24 h, rt

(1.5 equiv)SO

O NOCl3C N

OTf

OHOTCEO3SO OH

OMe

OHOHO

OSO3TCE

OMe

+

2.11 2.12 2.13

Entry Yield (%)a

123456b789c10

3745541541

tracetrace3520-

2.12:2.13 RatioSolvent

MeCNDCMTHFDMFDMADMANMP

DMA/MeCN (1:1)DMA/MeCN (1:1)DMA/H2O (20:1)

1.8:11.2:11.2:14.3:14.3:1

1:11.6:1

>20:1

aCrude 1H NMR yields were determined based on tetramethylsilane as an internal standard. Product ratio determined by 1H NMR. bNo catalyst 2.15 used. cReaction performed at 50 oC.

-

-

26

Figure 22. Sulfation with pre-formed cyclic arylboronate of methyl α-L-rhamnopyranoside.

Revisiting previous work in our group of using electron-deficient boronic acids and Lewis base

co-catalyst system for the silylation of pyranosides,42 we sought to adapt those conditions for

regioselective sulfation. However, a brief screen of phosphine oxide and phosphoramide Lewis

bases with 3,5-bis(trifluoromethyl)phenylboronic acid yielded no regioselectivity, though the 4-

O-sulfate was not isolated in this case (Table 04).

Table 04. Boronic acid/Lewis base co-catalyst for sulfation of methyl α-L-rhamnopyranoside.

42

Lee, D.; Taylor, M. S. Org. Biomol. Chem. 2013, 11, 5409-5412.

OHOO

O

OMe

B

F3C

OOHO

OH

OMe

SO

OOTCEEt3N (6 equiv)MeCN (0.2 M)

24 h, rt;aqueous work-up

2.1528%

2.14

(1.5 equiv)SO

O NOCl3C N

OTf

OHOHO

OH

OMe2.08 (1.5 equiv)

Lewis Base (20 mol%)DIPEA (1.5 equiv)

MeCN (0.2 M)24 h, rt

B(OH)2

F3C

F3C (20 mol%)

1234

1 : 1.21.1 : 1

1 : 1.11.2 : 1

35304133

n-Bu3P=OPh2MeP=O

Ph3P=OHMPA

OHOTCEO3SO OH

OMe

OHOHO

OSO3TCE

OMe

+

2.12 2.132.11

Entry Yield (%)a 2.12:2.13 RatioLewis Base

aCrude 1H NMR yields were determined based on tetramethylsilane as an internal standard. Product ratio determined by 1H NMR.

27

To evaluate the effects of the sulfate ester substituent on reactivity and selectivity, we attempted

to synthesize a series of alkyl- and arylsulfuryl 1,2-dimethylimidazolium triflate (Figure 23). The

reaction of sulfuryl chloride with phenol and para-methoxyphenol gave 2.14 and 2.15,

respectively, in good yields. However, the same reaction with benzyl alcohol, para-methoxybenzyl

alcohol, or sec-butyl alcohol gave products in trace yields or decomposed upon work-up or storage

under air at room temperature overnight. Previously attempts to prepare benzyl chlorosulfates were

reported to lead to the formation of polybenzyl,43 and thus benzyl and alkyl chlorosulfates were

not pursued further. Reaction of 2.14 and 2.15 with 2-methylimidazole gave 2.16 and 2.17,

respectively, and methylation gave the triflate salts 2.18 and 2.19, respectively, in good overall

yields.

Figure 23. Preparation of alkyl- and arylsulfuryl 1,2-dimethylimidazolium triflate.

Sulfation with 2.18 and 2.19 gave markedly better regioselectivity than with TCE-sulfuryl 1,2-

dimethylimidazolium triflate (Table 05). Amide solvents such as DMF and DMA were, again,

particularly effective (Entries 4-7 and 10) and the presence of catalyst was crucial (Entries 1, 3, 8).

The optimal conditions achieved gave 52% yield with 13:1 selectivity of 3-O-sulfate:2-O-sulfate

(Entry 6). However, this yield could not be improved further.

43

Gibbons, R. A.; Gibbons, M. N.; Wolfrom, M. L. J. Am. Chem. Soc. 1955, 77, 6374-6374.

SO

Cl ClO

pyridine, Et2OSO

O ClO(1 equiv)

2.14 R = Ph2.15 R = p-OMePh R = Bn R = PMB R = sec-Bu

R-OHR

54%94%

decompositiondecomposition

trace

NHN

SO

OO N N S

O

OO N N

OTf(3.6 equiv)

R R

2.16 R = Ph2.17 R = p-OMePh

2.18 R = Ph2.19 R = p-OMePh

82%31%

73%82%

MeOTf(1 equiv)

THF Et2O

28

Table 05. Optimization of methyl α-L-rhamnopyranoside sulfation with arylsulfuryl 1,2-

dimethylimidazolium triflate.

A closer examination of the sulfating reagents revealed that 2.08, 2.18, and 2.19 underwent

appreciable base-mediated hydrolysis in deuterated acetonitrile, as monitored over time by 1H

NMR spectroscopy (Figure 24). The triflate salts, in the absence of base, were stable in solution

(50%

decomposition after 24 h. 2.18 with 1 equiv. of DIPEA (Figure 24, C) showed >60%

decomposition, while only 50% decomposition with 1 equiv. of DIPEA dried overnight with 3

molecular sieves (Figure 24, D). The greater disparity after 24 h was seen with 2.19 in the presence

of DIPEA (Figure 24, E, 52% decomposition) and 3 MS-dried DIPEA (Figure 24, F, 28%

decomposition). This base-mediated hydrolysis of the sulfuryl imidazolium triflates explains our

(1.5 equiv)

Catalyst (10 mol%)DIPEA (1.5 equiv)

Solvent (0.2 M)24 h, rt

OHOHO

OH

OMeSO

O NO N

OTf

R = H, 2.18123456b7R = p-OMe, 2.198910

no catalyst2.15

no catalyst2.152.152.152.15

no catalyst2.15 2.15

MeCNMeCNDMFDMFDMA

DMA/MeCN (1:1)DMA/MeCN (1:1)

MeCNMeCNDMA

2:18:1

>20:1>20:1

13:1>20:1

2:18:1

>20:1

29490

30365248

344820

R

aCrude 1H NMR yields were determined based on tetramethylsilane as an internal standard. Product ratio determined by 1H NMR. b3 equiv of 2.18 and DIPEA used instead.

Entry Yield (%)a 2.12:2.13 RatioCatalyst Solvent

OHOTCEO3SO OH

OMe

OHOHO

OSO3TCE

OMe

+

2.12 2.132.11

-

29

inability to increase sulfation yields by heating the reaction or increasing the concentration—both

changes presumably accelerated the decomposition of sulfating reagents to a greater extent than

they increased the rate of sulfation. Although rigorously drying the reaction mixture would, in

theory, prevent hydrolysis and decomposition of the sulfating reagent, we decided to place this

portion of the project on hold and focus instead on direct, regioselective sulfation without masking

groups.

Figure 24. Decomposition study of 2.08, 2.18, 2.19 in CD3CN; A: 2.08 in 1 equiv. 1,2-

dimethylimidazole; B: 2.08 in 1 equiv. DIPEA; C: 2.18 in 1 equiv. DIPEA; D: 2.18 in 1 equiv. 3

MS-dried DIPEA; E: 2.19 in 1 equiv. DIPEA; F: 2.19 in 1 equiv. 3 MS-dried DIPEA.

2.3 Sulfation of carbohydrates with sulfur trioxide amine complexes We began studying direct sulfation using methyl α-D-mannopyranoside 2.20 as a model substrate

(Table 06). In the absence of catalyst (Entry 1), the reaction mixture included unreacted starting

material and a distribution of O-sulfate and bis-sulfates. A brief catalyst screen (Entries 2-4)

revealed that the reaction is highly influenced by catalyst, with the product being predominantly

the 3-O-sulfate 2.21. This is particularly promising given the presence of a more reactive primary

hydroxyl group in the substrate. Replacing SO3-pyridine with SO3-Me3N (Entry 5) diminished

conversion but afforded a cleaner reaction. Increasing the temperature to 60 oC (Entry 6) and the

30

equivalence of sulfating reagent (Entry 7) gave increased yield while maintaining the

regioselectivity of sulfation. Further increasing the amount of sulfating reagent (Entry 8) was not

beneficial and afforded more over-sulfated products than the desired mono-3-O-sulfate (3-

OSO3:over-sulfation 3:1).

Table 06. Optimization of methyl α-D-mannopyranoside sulfation with sulfur trioxide amine

complex.

Further optimization was performed on n-octyl α-D-galactopyranoside to assess the efficiency of

sulfation on a more soluble substrate with the standard conditions set out previously (Table 06,

Entry 7). As discussed by Gilbert in 1962,11 the reactivities of sulfur trioxide amine complexes

vary inversely with the strength of the Lewis base component. A survey of conventional sulfur

trioxide amine complexes in our reaction, however, afforded the opposite trend (Table 07). SO3-

Et3N (Entry 2) gave higher conversion but the regioselectivity of sulfation suffered drastically (3-

OSO3:over-sulfation 0.6:1). Weaker Lewis base complexes (Entry 3, 4) gave low or no conversion.

Sulfating Reagent (XX equiv)

Catalyst (10 mol%)DIPEA

MeCN (0.2 M)3 h, Temperature

OHOHO

OH

OMe

HO

OHO-O3SO

OH

OMe

HO

aCrude 1H NMR yields were determined based on 1,3,5-trimethoxybenzene as an internal standard.

Entry Yield (%)aCatalystSulfating Reagent (XX equiv) Temperature (oC)

12345678

SO3-pyridine (1.5 equiv)SO3-pyridine (1.5 equiv)SO3-pyridine (1.5 equiv)SO3-pyridine (1.5 equiv)SO3-Me3N (1.5 equiv)SO3-Me3N (1.5 equiv)SO3-Me3N (3 equiv)

SO3-Me3N (4.5 equiv)

no catalyst2.142.152.162.152.152.152.15

4040404040606060

2831616627498067

2.14

O BBPh

Ph

Ph

Ph

2.15

B

O

OH2.22

B

S

OH

2.20 2.21

Conversion (%)a

7948827927499089

Equiv. of DIPEA

1.51.51.51.51.51.53

4.5

31

To test the hypothesis that a highly Lewis basic additive could accelerate the sulfation,

quinuclidine was added to the reaction with either SO3-Me3N (Entry 5) or SO3-DMF (Entry 6);

however, both changes were not fruitful.

Table 07. Optimization of n-octyl α-D-galactopyranoside sulfation with sulfur trioxide amine

complex.

With the optimized conditions in hand, we began to explore the substrate scope (Table 08). The

sulfated products as a trialkylamine salt were exchanged with DOWEX Na+-resin to give the final

product as a sodium salt. Substrates without primary hydroxyl groups gave sulfated products in

high yields (2.25, 2.26). Sulfation could also be performed in good yields in the presence of

primary hydroxyl groups for a wide range of manno- and galacto-derived pyranosides. No

significant difference in reactivity between α- and β-anomers of methyl-D-galactopyranoside was

observed (2.29 and 2.30). A known sulfated glycolipid 2.34 found in the mammalian nervous

system was also synthesized in good yield.

SO3-Me3N (3 equiv)

2.15 (10 mol%)DIPEA (3 equiv)MeCN (0.2 M)

3 h, 60 oC

OHO

O-Octyl

OHOH

OH

O-O3SO

O-Octyl

OHOH

OH

Entry Yield (%)aChanges to Above Conditions

123456

-SO3-Et3N (3 equiv) instead of SO3-Me3N

SO3-pyridine (3 equiv) instead of SO3-Me3NSO3-DMF (3 equiv) instead of SO3-Me3N

quinuclidine (3 equiv) as additiveSO3-DMF (3 equiv) instead of SO3-Me3N, quinuclidine (3 equiv) as additive

603617010

aCrude 1H NMR yields were determined based on 1,3,5-trimethoxybenzene as an internal standard.

2.23 2.24

Conversion (%)a

79>9551010

32

Table 08. Substrate scope of carbohydrate sulfation with SO3-Me3N.

2.4 Summary and future work In summary, a catalytic, regioselective sulfation method was outlined that tolerated a range of fully

unprotected carbohydrates. Our approach is similar to the one described by Flitsch in 199419,20 in

that both strategies utilize cis-diol activation to selectively functionalize at the equatorial hydroxyl

group. However, our method uses benign, organoboron compound in catalytic quantities as

opposed to toxic, organotin reagent in stoichiometric amounts. Furthermore, our one-step

O

OR(HO)n

SO3-Me3N (3 equiv)

2.15 (10 mol%)DIPEA (3 equiv)MeCN (0.2 M)

3 h, 60 oC

O

OR(HO)n

Na+ -O3SO

OOH

OH

OMe

OSO3- +Na

O

OH

HO

OMe

Na+ -O3SO

OHONa+ -O3SO

OH

OMe

HO

OHONa+ -O3SO

OH

SPh

HO

ONa+ -O3SO

OMe

OHOH

OH

ONa+ -O3SO OMe

OHOH

OH

O

Na+ -O3SO SiPr

OHOH

OH

ONa+ -O3SO

O

OHOH

OH

O

Na+ -O3SO

OHOH

AcHNOMe

OO

Na+ -O3SO

OHOH

OHC13H27

OH

HN C15H31

O

>99%2.25

72%2.26

65%2.27

54%2.28

53%2.29

57%2.30

57%2.31

54%2.32

46%2.33

42%a2.34

Reactions were performed at 0.1 mmol scale of substrate. Isolated yields are reported. aReaction was performed at 0.005 mmol scale of substrate with 6 equiv. SO3-Me3N, 40 mol% 2.15, 6 equiv. DIPEA, and 0.5 M MeCN.

33

sulfation/activation is achieved in drastically shorter reaction time compared to those reported

using the two-step process with dibutyltin oxide. There is still work to be done to understand the

mechanism of sulfation and the reactivity of the sulfur trioxide amine complexes, particularly why

the trend observed with various Lewis bases of the complex (Table 07) is opposite to that

previously reported.11

2.5 Experimental

2.5.1. General Information

All reactions were performed using a Teflon-coated magnetic stir bar under argon. All solvents

used were dried using the Pure Solv-MD solvent purification system (Innovative Technology) or

previously dried overnight with 3 molecular sieves. All reagents and carbohydrates, unless

otherwise stated, were purchased from Sigma-Aldrich or Carbosynth Ltd. Flash column

chromatography was performed using silica gel (60 , 230-400 mesh) (SiliCycle). Analytical

thin-plate chromatography was performed on aluminum-backed silica gel 60 F254 plates (EMD

Milipore) and visualized under UV or with aqueous basic permanganate stain.

1H, 13C and 2D nuclear magnetic resonance (NMR) spectra were acquired on the Agilent DD2 600

MHz or Agilent DD2 500 MHz, both equipped with a OneNMR probe. Chemical shifts (δ) are

reported in parts per million (ppm), calibrated to the residual protium in the deuterated solvent.

Spectral features are tabulated as follows: chemical shift (δ, ppm); multiplicity (app = apparent, s

= singlet, d = doublet, t = triplet, q = quartet, p = pentet, hept = heptet, m = multiplet, where the

range of chemical shift is given); number of protons, coupling constants (J, Hz); assignment.

Infrared (IR) spectra were acquired on the Fourier-transform Spectrum 100 spectrometer

(PerkinElmer) equipped with a single-bounce diamond/ZnSe ATR accessory. Spectral features are

tabulated as follows: wavenumber (cm-1); intensity (s = strong, m = medium, w = weak, br =

broad). High-resolution mass spectra (HRMS) were acquired on the Agilent 6538 UHD Q-TOF

for electrospray ionization, negative mode (ESI−). Optical rotations were acquired on the

AUTOPOL IV (Rudolph Research Analytical) in a 0.6 dm polarimeter sample cell, at 589 nm

wavelength, at 20 oC, and the sample concentrations are reported in g per 100 mL in methanol.

Melting points were acquired on the Mel-Temp II (Laboratory Devices Inc.), and reported as a

34

range of melting or decomposing (denoted by dec.).

Presence of a single sulfate group is confirmed by HRMS. Assignment of sulfation position is

based on change in NMR chemical shift between starting carbohydrate and sulfated product (1H:

approx. +0.8 ppm, 13C: approx. +7 ppm).

2.5.2. General Procedure A

To a 2-dram vial equipped with a magnetic stir bar was added the carbohydrate (0.1 mmol, 1

equiv), sulfur trioxide trimethylamine complex (42 mg, 0.3 mmol, 3 equiv), and 2.15 (2 mg, 0.01

mmol, 0.1 equiv). The reaction vial was capped with a septum and purged with argon. Acetonitrile

(0.5 mL, 0.2 M) was added to the vial, followed by N,N-diisopropylethylamine (0.06 mL, 0.3

mmol, 3 equiv). The septum was quickly replaced with a screw cap, sealed with Teflon tape, and

the reaction was stirred at 60 oC for 3 hours. The mixture was then quenched with MeOH and the

solvent was removed by rotary evaporation. The crude mixture was purified by flash

chromatography on silica gel (2% to 15% MeOH in DCM). Fractions containing the product were

combined and stirred with Dowex 50WX2 Na+-form (50–100 mesh) for 30 min. The resulting

mixture was dried, filtered through Celite with DCM, then MeOH. The MeOH fraction was

collected and dried to give the product as a solid.

2.5.3. Preparation of catalyst and carbohydrate substrates

10H-Dibenzo[b,e][1,4]oxaborinin-10-ol (2.15):

B

O

OH

O 1) n-BuLi, diphenyl ether, THF

2) tributyl borate3) 4N HCl

35

10H-Dibenzo[b,e][1,4]oxaborinin-10-ol was prepared according to literature procedure44 from

diphenyl ether and tributyl borate (Sigma-Aldrich). Spectral features are in agreement with those

previously reported.

Methyl-2-acetamido-2-deoxy-α-D-galactopyranoside:

Methyl-2-acetamido-2-deoxy-α-D-galactopyranoside was prepared according to an adapted

literature procedure45 from N-acetyl-D-galactosamine (Sigma-Aldrich). The ⍺-anomer was separated cleanly by flash chromatography on silica gel to give the desired product. Spectral

features are in agreement with those previously reported.46

β-D-galactosyl N-palmitoyl-D-erythro-sphingosine:

44

Dimitrijević, E.; Taylor, M. S. Chem. Sci. 2013, 4, 3298-3303. 45

Liang, H.; Grindley, T. B. J. Carbohydr. Chem. 2004, 23, 71-82. 46

Grönberg, G.; Nilsson, U.; Bock, K.; Magnusson, G. Carbohydr. Res. 1994, 257, 35-54.

OHO

OHOH

AcHNOMe

OHO

OHOH

AcHN OH

Amberlyst H+ resin

MeOH, reflux

OPMBO

OPMBPMBO

PMBO OH

1) Ms2O, PMP, CH2Cl2

2)

HOHN

OH

C13H27C15H31

O

(Ph2B)2O, CH2Cl2

OO

PMBO

OPMBPMBO

PMBOC13H27

OH

HN C15H31

O

3) CF3COOH, anisole, CH2Cl2

36

β-D-galactosyl N-palmitoyl-D-erythro-sphingosine was prepared according to a literature

procedure47 from 2,3,4,6-tetra-O-4-methoxybenzyl-D-galactopyranose and N-palmitoyl-D-

sphingosine (Sigma-Aldrich). Spectral features are in agreement with those previously reported.

2.5.4. Synthesis and characterization of compounds

2,2,2-Trichloroethoxysulfuryl chloride (2.02):

2.02 was prepared according to a literature procedure.31

Spectral features are in agreement with those previously

reported.

Methyl 6-O-tert-butyldimethylsilyl-α-D-mannopyranoside 2,3-cyclic sulfate (2.05):

1H NMR (399 MHz, CDCl3) δ 5.00 (app s, 1H, H-1), 4.94

(dd, J = 5.3, 0.9 Hz, 1H, H-2), 4.89 (dd, J = 7.8, 5.3 Hz, 1H,

H-3), 4.25 (dd, J = 9.8, 7.8 Hz, 1H, H-4), 3.96 (dd, J = 10.6,

4.9 Hz, 1H, H-6a), 3.88 (dd, J = 10.6, 5.7 Hz, 1H, H-6b), 3.65

(dddd, J = 9.8, 5.6, 4.9, 0.6 Hz, 1H, H-5), 3.42 (s, 3H, 1-

OCH3), 0.91 (s, 9H, TBS), 0.12 (d, J = 3.2 Hz, 6H, TBS); 13C

NMR (100 MHz, CDCl3) δ 95.4 (C-1), 85.3 (C-3), 78.9 (C-

2), 69.1 (C-4), 67.8 (C-5), 64.0 (C-6), 55.3 (1-OCH3), 25.7 (TBS), 18.2 (TBS), -5.5 (TBS), -5.6

(TBS).

47

D’Angelo, K. A.; Taylor, M. S. Chem. Commun. 2017, 53, 5978-5980.

OHOO

O

OMe

OTBSSOO

Chemical Formula: C13H26O8SSiMolecular Weight: 370.4880

SO

O ClOCl3C

Chemical Formula: C2H2Cl4O3SMolecular Weight: 247.8950

37

Methyl 4,6-bis-O-(tert-butyldimethylsilyl)-α-D-mannopyranoside 2,3-cyclic sulfate (2.06):

1H NMR (400 MHz, CDCl3) δ 4.99 (d, J = 0.8 Hz, 1H, H-1),

4.94 (dd, J = 5.2, 0.9 Hz, 1H, H-2), 4.80 (dd, J = 7.7, 5.3 Hz,

1H, H-3), 4.19 (dd, J = 9.9, 7.8 Hz, 1H, H-4), 3.88 (dd, J =

11.5, 2.0 Hz, 1H, H-6a), 3.82 (dd, J = 11.5, 4.8 Hz, 1H, H-

6b), 3.55 (dddd, J = 9.9, 4.8, 2.1, 0.6 Hz, 1H, H-5), 3.40 (s,

3H), 0.90 (s, 9H), 0.89 (s, 9H), 0.15 (d, J = 11.1 Hz, 6H), 0.08

(d, J = 3.3 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ 95.3 (C-

1), 87.0 (C-3), 79.7 (C-2), 70.6 (C-5), 67.2 (C-4), 61.7 (C-6), 54.9 (1-OCH3), 25.8 (TBS), 25.7

(TBS), 18.3 (TBS), 18.0 (TBS), -4.7 (TBS), -5.2 (TBS), -5.3 (TBS), -5.4 (TBS).

1-(2,2,2-Trichloroethoxysulfuryl)-2-methyl imidazole (2.07):

2.07 was prepared according to a literature procedure.31

Spectral features are in agreement with those previously

reported.

1-(2,2,2-Trichloroethoxysulfuryl)-2,3-dimethylimidazolium triflate (2.08):

2.08 was prepared according to a literature procedure.31

Spectral features are in agreement with those

previously reported.

SO

OO N N

OTfCl3C

Chemical Formula: C8F3H10Cl3N2O6S2Molecular Weight: 308.5775

SO

OO N N

Cl3C

Chemical Formula: C6H7Cl3N2O3SMolecular Weight: 293.5430

OTBSOO

O

OMe

OTBSSOO

Chemical Formula: C19H40O8SSi2Molecular Weight: 484.7510

38

Methyl 3-trichloroethylsulfo-α-L-rhamnopyranoside (2.12):

1H NMR (500 MHz, CDCl3) δ 4.85 (q, J = 10.8 Hz, 2H,

CH2CCl3), 4.85 (dd, J = 9.2, 3.1 Hz, 1H, H-3), 4.71 (d, J =

2.0 Hz, 1H, H-1), 4.32 (dd, J = 3.2, 2.0 Hz, 1H, H-2), 3.78

(app t, J = 9.3 Hz, 1H, H-4), 3.75–3.68 (m, 1H, H-5), 3.39 (s,

3H, 1-OCH3), 1.38 (d, J = 6.1 Hz, 3H, 5-CH3); 13C NMR (100

MHz, CDCl3) δ 100.6 (C-1), 86.8 (C-3), 80.0 (CH2), 70.7 (C-

4), 69.3 (C-2), 68.3 (C-5), 55.3 (1-OCH3), 17.7 (5-CH3).

Methyl 2-trichloroethylsulfo-α-L-rhamnopyranoside (2.13):

1H NMR (399 MHz, CDCl3) δ 4.97 (d, J = 10.7 Hz, 1H,

CH2CCl3), 4.93 (d, J = 1.7 Hz, 1H, H-1), 4.89–4.82 (m, 1H,

H-2), 4.78 (s, 1H, CH2CCl3), 4.01 (dd, J = 9.5, 3.3 Hz, 1H,

H-3), 3.71–3.61 (m, 1H, H-5), 3.48 (app t, J = 9.5 Hz, 1H,

H-4), 3.40 (s, 3H, 1-OCH3), 1.34 (d, J = 6.2 Hz, 3H, 5-CH3); 13C NMR (101 MHz, CDCl3) δ 97.7, 79.8, 77.9, 72.9, 69.4,

68.0, 55.3, 17.4.

Benzenesulfonyl chloride (2.14):

2.14 was prepared according to a literature procedure.48

Spectral features are in agreement with those previously

reported.

48 DeBergh, J. R.; Niljianskul, N.; Buchwald, S. L. J. Am. Chem. Soc. 2013, 135, 10638-10641.

OHOO

OH

OMe

SO

OO

Cl3C

Chemical Formula: C9H15Cl3O8SMolecular Weight: 389.6210

OHOHO

O

OMe

SO

O

OCl3C

Chemical Formula: C9H15Cl3O8SMolecular Weight: 389.6210

SO

OO

Cl

Chemical Formula: C6H5ClO3SMolecular Weight: 192.6130

39

4-Methoxybenzenesulfonyl chloride (2.15):

2.15 was prepared according to a literature procedure.48

Spectral features are in agreement with those previously

reported.

1H NMR (400 MHz, CDCl3) δ 7.25 (d, J = 9.2 Hz, 2H, Ph),

6.89 (d, J = 9.2 Hz, 2H, Ph), 3.77 (s, 3H, CH3); 13C NMR (101

MHz, CDCl3) δ 159.5, 143.5, 122.8, 115.1, 55.8.

2-Methyl-1-(phenylsulfonyl) imidazole (2.16):

2.16 was prepared according to a literature procedure.49

Spectral features are in agreement with those previously

reported.

1-[(p-Methoxyphenyl)sulfonyl]-2-methyl imidazole (2.17):

2.17 was prepared according to an adapted literature

procedure.49 Spectral features are in agreement with those

previously reported.50

49 Desoky, A. Y.; Hendel, J.; Ingram, L.; Taylor, S. D. Tetrahedron 2011, 67, 1281-1287. 50

Reuillon, T.; Bertoli, A.; Griffin, R. J.; Miller, D. C.; Golding, B. T. Org. Biomol. Chem. 2012, 10, 7610-7617.

SO

OO

Cl

MeO

Chemical Formula: C7H7ClO4SMolecular Weight: 222.6390

SO

O NO N

Chemical Formula: C10H10N2O3SMolecular Weight: 238.2610

SO

O NO N

MeO

Chemical Formula: C11H12N2O4SMolecular Weight: 268.2870

40

1,2-Dimethyl-3-(phenylsulfonyl)-imidazolium triflate (2.18):

2.18 was prepared according to a literature procedure.49

Spectral features are in agreement with those previously

reported.

1-[(p-Methoxyphenyl)sulfonyl]-2,3-dimethylimidazolium triflate (2.19):

2.19 was prepared according to an adapted literature

procedure.49

1H NMR (400 MHz, CDCl3) δ 7.51 (d, J = 2.4 Hz, 1H,

Im), 7.33 (d, J = 2.4 Hz, 1H, Im), 7.21 (d, J = 9.2 Hz, 2H,

Ph), 6.96 (d, J = 9.2 Hz, 2H, Ph), 4.09 (s, 3H, 3-CH3),

3.80 (s, 3H, OCH3), 2.93 (s, 3H, 2-CH3); 13C NMR (101

MHz, CDCl3) δ 160.0, 147.9, 142.5, 123.9, 122.5, 120.6, 115.9, 55.8 (OCH3), 37.3 (3-CH3), 11.9

(2-CH3).

Methyl α-L-fucopyranoside 3-(sodium sulfate) (2.25):

2.25 was prepared according to General Procedure A from

methyl α-L-fucopyranoside (18 mg, 0.1 mmol, 1 equiv) and

purified by flash chromatography on silica gel (5% to 15%

MeOH in DCM) to give a beige solid in >95% yield (28 mg).

SO

O NO N

OTf

MeO

Chemical Formula: C13F3H15N2O7S2Molecular Weight: 283.3215

SO

O NO N

OTf

Chemical Formula: C12F3H13N2O6S2Molecular Weight: 253.2955

OOH

OH

OMe

OSO3- +Na

Chemical Formula: C7H13NaO8SMolecular Weight: 280.2228

41

[α]#$% = −20.2° (c 0.1, MeOH) (lit. 51: [α]&'( = −157.4° (c 1.3, H2O)); m.p.: 212–214 (dec.) (lit.51, monohydrate: 161–162 oC); 1H NMR (600 MHz, MeOD-d4) δ 4.68 (d, J = 3.9 Hz, 1H, H-

1), 4.47 (dd, J = 10.2, 3.2 Hz, 1H, H-3), 4.07 (dd, J = 3.2, 1.2 Hz, 1H, H-4), 3.97–3.92 (m, 2H, H-

2, H-5), 3.39 (s, 3H, 1-OCH3), 1.23 (d, J = 6.5 Hz, 3H, 5-CH3); 13C NMR (151 MHz, MeOD-d4)

δ 100.0 (C-1), 78.0 (C-3), 70.3 (C-4), 66.6 (C-2), 65.8 (C-5), 54.1 (1-OCH3), 15.1 (5-CH3); IR

(thin film, cm-1): 3438 (br, m), 2946 (w), 1650 (br, m), 1216 (s), 1046 (s), 987 (s), 839 (s), 748

(m); HRMS (ESI−): calcd for C7H13O8S [M − Na]− 257.0337 m/z; found 257.0341 m/z.

Methyl α-L-rhamnopyranoside 3-(sodium sulfate) (2.26):

2.26 was prepared according to General Procedure A from

methyl α-L-rhamnopyranoside (18 mg, 0.1 mmol, 1 equiv)

and purified by flash chromatography on silica gel (5% to

15% MeOH in DCM) to give a white solid in 79% yield (22

mg).

[α]#$% = −94.3° (c 0.27, MeOH); m.p.: 201–202 (dec.); 1H NMR (600 MHz, MeOD-d4) δ 4.58 (d, J = 1.8 Hz, 1H, H-1), 4.42 (dd, J = 9.5, 3.3 Hz, 1H, H-3), 4.16 (dd, J = 3.3, 1.8 Hz, 1H, H-2),

3.65–3.60 (m, 1H, H-5), 3.55 (app t, J = 9.5 Hz, 1H, H-4), 3.36 (s, 3H,1-OCH3