Chem 115 Sharpless Asymmetric Epoxidation Reaction Myers Reivews: Katsuki, T.; Martin, V. S. Org. React. 1996, 48, 1–300. Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis, Ojima, I., Ed.; VCH: New York, 1993, pp. 103–158. Johnson, R. A.; Sharpless, K. B. In Comprehensive Organic Synthesis, Trost, B. M.; Fleming, I., Eds., Pergamon Press: New York, 1991, Vol. 7, pp. 389–436. Pfenninger, A. Synthesis 1986, 89–116. Substitution patterns: Asymmetric Epoxidation of Allylic Alcohols: R 2 R 1 R 3 OH Ti(Oi-Pr) 4 , (+)-DET t-BuOOH, 3Å-MS CH 2 Cl 2 , –20 °C R 2 R 1 R 3 OH O • 5–10 mol% catalyst in the presence of 3- or 4Å-MS. • 10–20 mol% excess tartrate vs. Ti(OiPr) 4 required. • (+)- and (–)-DET are readily available and inexpensive. • (+)- and (–)-DIPT, diisopropyl tartrate, are also available and sometimes lead to higher selectivity. Gao, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune, H.; Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 5765–5780. Mnemonic for selectivity: L-(+)-DET "O" D-(–)-DET "O" Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974–5976. Application of Mnemonic: OH CH 3 OH CH 3 O OH CH 3 O AE-(–)-DET 97%, 86% ee AE-(+)-DET 97%, 86% ee OH OH OH OH OH OH OH • Z-disubstituted olefins are least reactive and selective. OH Examples of Sharpless Epoxidation: product Ti(%) tartarate(%) °C h yield (%) ee (%) OH O 5 (+)-DIPT (6.0) 0 2 65 90 OH O 5 (+)-DIPT (7.0) –20 3 89 >98 Ph OH O 4.7 (+)-DET (5.9) –12 11 88 95 Pr OH O 10 (+)-DET (14) –10 29 74 86 C 7 H 15 OH O 5 (+)-DIPT (7.5) –35 2 79 >98 Ph CH 3 OH O 100 (+)-DET (142) –20 14 80 80 CH 3 BnO OH O 5 (+)-DET (7.4) –20 0.75 95 91 CH 3 H 3 C CH 3 OH O 120 (–)-DET (150) –20 5 90 94 Ph CH 3 Ph From: Gao, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune, H.; Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 5765-5780 and Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis, Ojima, I., Ed.; VCH: New York, 1993, pp. 103–158. R 1 R 2 R 3 HO EtO 2 C OH OH CO 2 Et (+)-DET = M. Movassaghi 1
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Reivews:Katsuki, T.; Martin, V. S. Org. React. 1996, 48, 1–300.
Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis, Ojima, I., Ed.; VCH: New York, 1993, pp. 103–158.
Johnson, R. A.; Sharpless, K. B. In Comprehensive Organic Synthesis, Trost, B. M.; Fleming,I., Eds., Pergamon Press: New York, 1991, Vol. 7, pp. 389–436.
Pfenninger, A. Synthesis 1986, 89–116.
Substitution patterns:
Asymmetric Epoxidation of Allylic Alcohols:
R2
R1
R3OH
Ti(Oi-Pr)4, (+)-DET
t-BuOOH, 3Å-MSCH2Cl2, –20 °C
R2
R1
R3OH
O
• 5–10 mol% catalyst in the presence of 3- or 4Å-MS.• 10–20 mol% excess tartrate vs. Ti(OiPr)4 required.• (+)- and (–)-DET are readily available and inexpensive.• (+)- and (–)-DIPT, diisopropyl tartrate, are also available and sometimes lead to higher selectivity.
Gao, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune, H.; Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 5765–5780.
Mnemonic for selectivity:
L-(+)-DET "O"
D-(–)-DET "O"
Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974–5976.Application of Mnemonic:
OH
CH3
OH
CH3O OH
CH3O
AE-(–)-DET
97%, 86% ee
AE-(+)-DET
97%, 86% ee
OH OH
OH
OHOH OH OH
• Z-disubstituted olefins are least reactive and selective.OH
Examples of Sharpless Epoxidation:
product Ti(%) tartarate(%) °C h yield (%) ee (%)
OHO 5 (+)-DIPT (6.0) 0 2 65 90
OHO 5 (+)-DIPT (7.0) –20 3 89 >98Ph
OHO 4.7 (+)-DET (5.9) –12 11 88 95
Pr
OHO
10 (+)-DET (14) –10 29 74 86
C7H15
OHO 5 (+)-DIPT (7.5) –35 2 79 >98Ph
CH3
OHO 100 (+)-DET (142) –20 14 80 80
CH3
BnO
OHO
5 (+)-DET (7.4) –20 0.75 95 91CH3
H3C
CH3
OHO
120 (–)-DET (150) –20 5 90 94PhCH3
Ph
From: Gao, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune, H.; Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 5765-5780 and Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis, Ojima, I., Ed.; VCH: New York, 1993, pp. 103–158.
R1
R2 R3
HO
EtO2C
OH
OH
CO2Et(+)-DET =
M. Movassaghi
1
Chiral Substrate: Kinetic Resolution:
• Products are diastereomeric.• Sense of induction is dominated by the catalyst.• The C4 center reinforces and erodes this in "MATCHED" and "MISMATCHED" cases, respectively, as shown.
Homoallylic, bishomoallylic and trishomoallylic:
Rossiter, B. E.; Sharpless, K. B. J. Org. Chem. 1984, 49, 3707–3711.
Ko, S. Y.; Lee, A. W. M.; Masamune, S.; Reed, L. A., III; Sharpless, K. B.; Walker, F. J. Tetrahedron 1990, 46, 245–264.
• Rates of epoxidation are usually slower.• Enantiofacial selectivity of the catalyst is reversed for all three.• Enantiofacial selectivity is generally lower.
R1
R2 R3
HO
(+)-DET "O"
H3C OHTi(Oi-Pr)4 (1.0 equiv)(+)-DET (1.2 equiv)
TBHP, –20 °C1–4 d
50%, 41% ee
H3C OHO
CH3
OH
Ti(Oi-Pr)4 (1.0 equiv)(+)-DET (1.2 equiv)
TBHP, 0 °C48 h
22%, 29% ee
O
OH
HCH3
H
Hosokawa, T.; Kono, T.; Shinohara, T.; Murahashi, S.-I. J. Organometal. Chem. 1989, 370,C13–C16.For other examples see: Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis,Ojima, I., Ed.; VCH: New York, 1993, pp. 103-158.and Katsuki, T.; Martin, V. S. Org. React. 1996, 48, 1–300.
• Products are diastereomeric.• Using the Sharpless mnemonic, contact between the C1 substituent (R) and the catalyst predicts the slow-reacting isomer.
RH
R1
R2 R3
HO
(+)-DET "O"
HR
slow fastkrel = kfast/kslow
• With the exception of Z-disubstituted allylic alcohols, krel > 25.• When krel = 25, the ee of unreacted alcohol is essentially 100% at 60% conversion.• Allylic tertiary alcohols are not successfully expoxidized under Sharpless conditions.• Factors may combine for high selectivity:
H3C
OH (–)-DIPT
40% conversion H3C
OHO
70% yield>95% ee
• Disubstituted olefin is more reactive than monosubstituted olefin (krel ~ 100).• kfast/kslow for chiral E-propenylcarbinols is ~100.
Excercice: Apply the Sharpless mnemonic to predict the stereochemistry of this product.
Sharpless, K. B.; Behrens, C. H.; Katsuki, T.; Lee, A. W. M.; Martin, V. S.; Takatani, M.; Viti, S.M.; Walker, F. J.; Woodard, S. S. Pure Appl. Chem. 1983, 55, 589–604.
• Allylic 1,2-diols do not follow the Sharpless mnemonic:
OH
OH
(+)-DIPTOH
OH
OOH
OH
O+
71%90% ee
10%90% ee
Excercice: What isomer would you have predicted using the Sharpless mnemonic?
Takano, S.; Iwabuchi, Y.; Ogasawara, K. J. Am. Chem. Soc. 1991, 113, 2786–2787. M. Movassaghi
• Any minor diastereomer that is produced is rapidly removed by bis-epoxidation.Exercise: Why?
Schreiber, S. L.; Schreiber, T. S.; Smith, D. B. J. Am. Chem. Soc. 1987, 109, 1525–1529.Schreiber, S. L.; Goulet, M. T.; Schulte, G. J. Am. Chem. Soc. 1987, 109, 4718–4720.
• HWE-olefination, reduction, and AE provides an iterative route to the synthesis of polyols.
Ko, S. Y.; Lee, A. W. M.; Masamune, S; Reed, L. A., III; Sharpless, K. B.; Walker, F. J. Tetrahedron 1990, 46, 245–264. Corey, E. J.; Ha, D.-C. Tetrahedron Lett. 1988, 29, 3171–3174.
Henegar, K. E.; Cebula, M. Org. Proc. Res. Dev. 2007, 11, 354–358.
OH
N3 (–)-DIPT, Ti(Oi-Pr)4TBHP, 4Å-MS
OH
N3O
90% ee 95%, 98% ee
N N
OO
OH
CO2CH3H3CO2C
HIV-1 protease inhibitor
Jadhav, P. K.; Man, H. W. Tetrahedron Lett. 1996, 37, 1153–1156.
NHCbz
OH
NHCbz
OH
O N O
O
NHAcH3C
O
An antibacterial agent
Gleave, D. M.; Brickner, S. J. J. Org. Chem. 1996, 61, 6470–6474.
F
F
OHF
FOH
O
Saksena, A. K.; Girijavallabhan, V. M.; Lovey, R. G.; Pike, R. E.; Desai, J. A.; Ganguly, A. K.; Hare, R. S.; Loebenberg, D.; Cacciapuoti, A.; Parmegiani, R. M. Bioorg. Med. Chem. Lett. 1994, 4, 2023–2028.
CH2Cl2, –15 ºC
An antifungal agent
(–)-DIPT, Ti(Oi-Pr)4TBHP, 4Å-MS
CH2Cl2, 84%, >95% ee
F
FOHO
O N N NNN
O
CH3
H3C
CH3
OH
Geraniol
CH3
H3C
CH3
OHO
(–)-DET, Ti(Oi-Pr)4TBHP, 4Å-MS
CH2Cl2, –10 ! 20 ºC99%, 91% ee
NBoc
CO2HH3CHO
Noe, M. C.; Hawkins, J. M.; Snow, S. L.; Wolf-Gouveia, L. J. Org. Chem. 2008, 73, 3295–3298.
Jacobsen, E. N. In Catalytic Asymmetric Synthesis, Ojima, I., Ed.; VCH: New York, 1993;
pp. 159-202.
Linker, T. Angew. Chem., Int. Ed. Engl. 1997, 36, 2060-2062.
(S,S)-1
(S,S)-1 (4 mol %)
NaOCl(aq)
CH2Cl2, 4 °C
olefin yield, % ee, % equiv (S,S)-1
96
63
97
94
0.03
0.15
79 84 0.006a
• Selectivity is determined through nonbonded interactions.
• Terminal olefins are poor substrates.
• Addition of substoichiometric amounts of 4-phenylpyridine N-oxide improves both catalyst
aReaction carried out in the presence of 4-phenylpyridine N-oxide.
• The observed selectivities are explained
• cis-Disubstituted conjugated olefins are epoxidized with high levels of enantioselectivity.
• trans-Disubstituted olefins react more slowly and with diminished selectivity.
epoxide
• In general, R is aryl, alkenyl or alkynyl and R' is a bulky group.
From: Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J. Am. Chem. Soc. 1991,
113, 7063-7064.
by a side-on approach of olefin:
selectivity and turnover numbers.
(S,S)-1 "O"
(R,R)-1 "O"
Mnemonics for the observed selectivities:
cis-olefin trisubstituted olefin
(S,S)-1 "O"
(R,R)-1 "O"
• cis-Olefins: Place Aryl, alkenyl or alkynyl substituent in upper-left quadrant (R) and the
corresponding trans hydrogen atom in the lower-right quadrant.
• trans-Olefins are poor substrates.
• Trisubstituted olefins: Place the hydrogen atom in the lower-right quadrant.
Trisubstituted olefins:
• Trisubstituted alkenes are excellent substrates for the Jacobsen asymmetric epoxidation.
69 93
91 95
97 92
From: Brandes, B. D.; Jacobsen, E. N. J. Org. Chem. 1994, 59, 4378-4380.
(R,R)-1 (3 mol %)
NaOCl(aq)
CH2Cl2 or TBME
4-Phenylpyridine N-oxide
0 °C
87%, 88% ee
olefin yield, % ee, %epoxidea
M. Movassaghi
CO2CH3RO
ROCO2CH3
O 3
CO2CH3
O
CH3
3
3
CH3PhPh
CH3O
Ph CH3
ON N
OO
Mn
Cl
HH
R
t-Bu
N
Ph
N
Ph
t-Bu
ROO
Mn
Cl
CO2EtPh Ph
CO2EtO
Ph CO2Et
O
O
TMS
TMSTMS
O
O
Br
CH3
CH3
CH3
CH3O
H3C
CH3Br
CH3
CH3
O
O
CH3Br
CH3
CH3
OCH3CH2
O
Br
CH2CH3
CH3
CH3
CH3
H3C
CH3
OCH3
CH3
CH3
CH3
H3C
CH3
O
Ph
Ph
Ph
CH3 CH3
Ph
Ph
PhO
Tetrasubstituted Olefins:
R = CH3, (S,S)-2R = OCH3, (S,S)-3
(S,S)-4
• High enantioselectivities are not yet general but may be attained in certain cases with catalysts
olefin epoxide yield, % ee, %catalyst
(S,S)-2
(S,S)-2
(S,S)-3
(R,R)-3
(S,S)-4
84 96
81 97
45 65
37 35
12 46
From: Brandes, B. D.; Jacobsen, E. N. Tetrahedron Lett. 1995, 36, 5123-5126.
From: Jacobsen, E. N. In Catalytic Asymmetric Synthesis, Ojima, I., Ed.; VCH: New York, 1993; pp. 159-202 and Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J. Am. Chem. Soc. 1991, 113, 7063-7064.
0.08
0.04
0.04
• cis-!-Substituted styrene derivatives afford cis-epoxides as major products while cis-enynes and
olefinequiv (S,S)-1
• Rotation of a radical intermediate is proposed to account for the cis " trans isomerization.
R = COCH2OPh(R,R)-1 (4 mol %)
4-Phenylpyridine N-oxideNaOClpH 11.3
Leukotriene A4 methyl ester
Chang, S.; Lee, N. H.; Jacobsen, E. N. J. Org. Chem. 1993, 58, 6939-6941.
62% (trans:cis, 8:1), 82% ee (trans)
cis-dienes produce trans-epoxides.
77, 92
52, 97
10, 64
6.7, 83
15, 78
55, 98
cis-epoxide trans-epoxideyield (%), ee (%) yield (%), ee (%) • Chromene derivatives undergo epoxidation with higher enantioselectivity as compared to indene
shown.
derivatives.
epoxide
M. Movassaghi
R1 R3O
OOO
OO
CH3
CH3
H3C
H3C
OOO
OO
CH3H3C
CH3H3C
OO
R1 R2
R3 OOO
OO
CH3H3C
CH3H3C
OO
R3 R2
R1
R1 R3O
H3C PhCH3
H3C PhCH3
H3C
H3C CH3
SO42–
O
OO
OO
CH3
CH3
H3C
H3CO
O
H3C C10H21CH3
Ph CH3CH3
H3C
O
OO
OO
CH3
CH3
H3C
H3C
O–
O OSO3–
O
OOO
OO
CH3
CH3
H3C
H3C
H3C
H3C
CH3
CH3H3C
H3C
HSO5–
Ph
O
OO
OO
CH3
CH3
H3C
H3C
OHO O
SO3–
Soojin Kwon
Chem 115Shi Asymmetric Epoxidation Reaction
Reviews:
Wong, O. A.; Shi, Y. Chem. Rev. 2008, 108, 3958–3987. Shi, Y. Acc. Chem. Res. 2004, 37, 488–496. Frohn, M.; Shi, Y. Synthesis 2000, 14, 1979–2000.
It is proposed that the Shi epoxidation proceeds through a dioxirane intermediate and a spiro transition state and that a so-called planar transition state is a main competing pathway. The spiro transition state is believed to be electronically favored as a result of a stabilizing interaction between an oxygen lone pair of the dioxirane with the !* orbital of the olefin.
•
General Transformation:
oxone, pH 10.5, base
H2O, CH3CN
Myers
Ketone 1 can be readily prepared from D-fructose ($15/kg) by ketalization (acetone, HClO4, 0 °C, 53%) and oxidation (PCC, 23 °C, 93%). L-Fructose can be prepared in 3 steps from readily available L-sorbose. Ketone 1 can be used catalytically (20–30 mol %). Oxone (a commercial mixture of 2:1:1 KHSO5:KHSO4:K2SO4) is used as the stoichiometric oxidant but H2O2/CH3CN can also be used (peroxyimidic acid is the proposed oxidant).Generally, the optimum pH for dioxirane epoxidation is 7–8. At higher pH, Oxone tends to decompose. However, at pH 7–8 the Shi catalyst decomposes due to competing Baeyer-Villiger reaction. By increasing the pH to 10.5 (by addition of K2CO3), the amount of ketone used can be reduced to a catalytic amount (30 mol %) and the amount of Oxone can be reduced to a stoichiometric amount (1.5 equiv), suggesting that at this pH the ketone is sufficiently reactive to compete with Oxone decomposition.Dimethoxymethane (DMM) and CH3CN (2:1 v/v) solvent mixtures generally provide higher ee's. Reaction temperatures range from –10 to 20 °C.
•
••
•
•
•
Spiro Planar
1
Higher ee's are observed with smaller R1 and larger R3 substituents.
Proposed Catalytic Cycle:
Useful for epoxidation of trans-disubstituted olefins (ketone 1), trisubstituted olefins (ketone 1), conjugated cis-disubstituted olefins (ketone 2, see p. 3), and styrenes (ketone 2, see p. 3).
•
Catalyst Conditions:
Examples:
1. Effect of smaller R1 (also known as "T-branch"; phenyl groups can be considered smaller than methyl).
26% ee 79% ee 81% ee 98% ee
2. Effect of larger R3 (also: "L-branch").
3. Comparing the size of R1 and R3.
76% ee 86% ee 91% ee
76% ee 97% ee
Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am. Chem. Soc. 1997, 19, 11224–11235.
R2 R2
R1 R3R2
R1 R3OR2
1
H3C OTBSCH3
H3C OCH3
H3C CH3
H3C OCH3
H3C CH3O
H3C OTBSCH3
O
TMSCH3
TMSCH3
O
Ph TMS
RPh TMSO
R
Ph
TMS
CH3
Ph TMSO
R
Ph
TMSO
CH3
CH3
O
Soojin Kwon
Monoepoxidation of conjugated dienes favors the more electron-rich or less sterically hindered olefin. The amount of catalyst used must be properly controlled (0.2–0.3 equiv) to prevent bis-epoxidation. Vinyl silanes and allylic silyl ethers are deactivated towards epoxidation (attributed to sterics and inductive deactivation, respectively).
Frohn, M.; Dalkiewicz, M.; Tu, Y.; Wang, Z.-X.; Shi, Y. J. Org. Chem. 1998, 63, 2948–2953.
Epoxidation of enynes occurs selectively at the C–C double bond.
Cao, G.-A.; Wang, Z.-X.; Tu, Y.; Shi, Y. Tetrahedron Lett. 1998, 39, 4425–4428.Wang, Z.-X.; Cao, G.-A.; Shi, Y. J. Org. Chem. 1999, 64, 7646–7650.
1,1-Disubstituted epoxides can be synthesized enantioselectively by Shi epoxidation of trisubstituted vinyl silanes followed by TBAF-mediated desilyation.
TBAF
82%
94% ee
Warren, J.D.; Shi, Y. J. Org. Chem. 1999, 64, 7675–7677.
R = HR = CH3
Ratio1:114:1
Yield31%77%
ee95%92%
Regioselectivity increases when either olefin of a 1,3-diene is trisubstituted. It is proposed that the trisubstituted olefin prevents full conjugation of the diene due to A1,2 strain, causing each olefin to present an individual steric or electronic environment, as if each were isolated.
25 mol % 1, Oxone, K2CO3
CH3CN, DMM
81%, 96% ee
1, Oxone, K2CO3,
CH3CN, DMM
1, Oxone, K2CO3,
CH3CN, DMM
64%, 94% ee
1, Oxone, K2CO3
CH3CN, DMM
74%, 94% ee
+
20 mol % 1, Oxone, K2CO3
CH3CN, DMM
65%, 89% ee
Ph Ph
Ph Cl
Ph O
O
n-C10H21 CH3
CH3
Ph PhO
Ph ClO
Ph OO O
n-C10H21 CH3
CH3
O
Examples of Shi Epoxidations:
Substrate Product Yield ee (%)
95%
93%
93%
89%
73%
61%
41%
94%
Tu, Y.; Wang, Z.-X.; Shi, Y. J. Am. Chem. Soc. 1996, 118, 9806–9807and Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am. Chem. Soc. 1997, 119, 11224–11235.
Chem 115Shi Asymmetric Epoxidation ReactionMyers
•
•
•
•
2
OTMSPh
OTMSPh
OTMS
OPh
Ph
OTBS
Ph
OTBS
Ph
OTBS
O
Soojin Kwon
Kinetic resolution of racemic 1,3- and 1,6-disubstituted cyclohexenes can provide optically enriched allylic silyl ethers.
35 mol % 1
49% conversion96% ee trans:cis >20:1
95% ee trans
Frohn, M.; Zhou, X.; Zhang, J.-R.; Tang, Y.; Shi, Y. J. Am. Chem. Soc. 1999, 121, 7718–7719.
35 mol % 1
70% conversion99% ee trans:cis 4:1
81% ee trans
Ph CH3
O
Ph CH3
O
OAc
Ph CH3
O
OH
Ph CH3
O
OH
Enol esters can be used as substrates for the preparation of !-hydroxyketones in either enantiomeric form.
91% ee
K2CO3, MeOH
90%
94% ee
195 °C, 0.5 h
92%
K2CO3, MeOH
88% ee
Zhu, Y.; Tu, Y.; Yu, H.; Shi, Y. Tetrahedron Lett. 1998, 39, 7819–7822.
66%, 91% eeO
CH3
Ph CH3
O O
CH3
O
Ph CH3
O
OOO
NBocO
CH3H3C
O
ONBocO
OO
OO
O
CH3CH3
R"R1
O
Ph CH3
OH
H
A modified catalyst is useful for epoxidation of cis-disubstituted olefins and styrenes.
Oxone, K2CO3, DME, DMM
82%, 91% ee
In both cases, it is proposed that the "-substituent of the substrate prefers to be proximal to the spiro oxazolidinone.
Tian, H.; She, X.; Shu, L.; Yu, H.; Shi, Y. J. Am. Chem. Soc. 2000, 122, 11551–11552.
Oxone, K2CO3, DME, DMM
100%, 81% ee
Tian, H.; She, X.; Xu, J.; Shi, Y. Org. Lett. 2001, 3, 1929–1931.Tian, H.; She, X.; Yu, H.; Shu, L.; Shi, Y. J. Org. Chem. 2002, 67, 2435–2446.
2
The enantiomeric excess is generally high for cyclic olefins and for acyclic olefins conjugated with an alkynyl or aromatic group.
The original Shi catalyst decomposes (via the Baeyer-Villiger pathway) faster than it reacts with electron-deficient !,"-unsaturated esters. A second-generation catalyst, incorporating electron- withdrawing acetate groups, slows the Baeyer-Villiger decomposition.
Wu, X.-Y.; She, X.; Shi, Y. J. Am. Chem. Soc. 2002, 124, 8792–8793.
CSA, toluene, 0 °C, 1 h31% (2 steps)
originally proposed structure of Glabrescol
Xiong, Z.; Corey, E. J. J. Am. Chem. Soc. 2000, 122, 4831–4832.
H3C CH3
CH3CH3CH3
CH3CH3
OH
CH3HO
Applications in Synthesis:
88% ee
Glabrescol:
squaleneasymmetric
dihydroxylation
73%
H3C O NHFmoc
H3C O OHO
H3C CH3
O
HN
OO
CCl3
CH3
OH Cl
OCH3
O
HN
ONH
CH3
Cl
OCH3H3C O
H3CO O
O
H3C CH3
O
HN
OO
CCl3
CH3
O Cl
OCH3H3C O
H3CO
O
CH3
CH3
NHFmoc
O
HN
OO
CCl3
CH3
OH R
O
The Shi epoxidation system provided the desired epoxide in a 6:1 diastereomeric ratio, while other epoxidation methods never exceeded a 2:1 ratio.
6.5:11.5:1
Conditions
Ketone 1m-CPBA
",! ratio
DMAP, DCC, CH2Cl271% (2 steps)
Conditions
Hoard, D. W.; Moher, E. D.; Martinelli, M. J.; Norman, B. H. Org. Lett. 2002, 4, 1813–1815.
McDonald, F. E.; Wei, X. Org. Lett. 2002, 4, 593–595.
(proposed)
(proposed)
Post epoxidation, only one bromohydrin diastereomer cyclized to the bromotetrahydropyran. The unreactive diastereomer was separated from the cyclization product and isolated in 30% yield.