Indian Journal of Chemistry Vol. 43B, June 2004, pp. 1275-1281 Ceric ammonium nitrate (CAN) catalyzed Baeyer-Villiger oxidation of carbonyl compounds, specially 20-oxosteroids Papori Goswami, Saroj Hazarika, Archana M Das & Pritish Chowdhury* Natural Products Chemistry Division, Regional Research Laboratory, Jorhat 785006, India e-mail: [email protected]Received 4 February 2003; accepted (revised) 10 December 2003 The role of ceric ammonium nitrate (CAN) as an effective catalyst in the peracid induced Baeyer-Villiger oxidation of carbonyl compounds with special reference to steroids has been demonstrated. IPC: Int.C1. 7 C 07 K 1/00 Ceric ammonium nitrate (CAN) finds application in synthetic organic chemistry for various chemica l transformations, viz., nitration 1, nitroacetamidation 2 , complex formation with various alcoho ls3 etc. Its role as single electron oxidant has been reported in a num- ber of publications including some recent reviews 4 - 8 . CAN-induced oxidative radical trans formations of steroids have also been reported 9 . We too have re- ported lO the catalytic action of CAN in the esterifica- tion of carboxylic acids in very high yield. This in conjugation with our interest on steroid transforma- tions ll - 14 persuaded us to study its role in 8aeyer- Villiger oxidation 1S - 16 of 20-oxopregnanes to 17- acetoxysteroids of potent sex hormones l7 and also of the formation of the steroidal D-ring lactones many of which are biologically active l8 . Thus several steroidal carbonyl compound s 1-11 (Table I) which are available in our laboratory un- derwent Baeyer- Villiger oxidation to furnish their respective oxidation products 1a-lla (Table I) in high yield, when treated with m-chloroperbenzoic acid (m-CPBA) in the pre sence of cata lytic amount of CAN in dichloromethane keeping just for 4-6 hr at room temperature. The method has been found to be effective for some non-steroidal carbonyl compound s 12-15 also (Table II) which give respective ester s 12a-15a Cfable II) . It is pertinent to note that although Andre et have earlier reported the Baeyer-Villiger oxidation of some 20-oxopregnanes lIsing peracid alone, the reac- tion mixture had to be kept for 3 weeks in dark for completion. In the present case we did not find Baeyer- Villiger oxidation of 20-oxopregnanes 1-4 (Table I) when treated with either CAN or peracid alone at room temperature even when kept for more than 48 hr. Fur- ther, GLC experiment confirmed 40-55% conversion of benzophenone 12 to phenyl benzoate 12a in 5 hr when CAN was used as a catalyst along with peracid whereas only 8% conversion was observed in the ab- sence of CAN when kept for more than 24 hr. During our investigation, we also found that for all the ca ses (substrates 5-8, 10, 11, 13-15) the oxidation furnished only one isomer viz. 5a-8a, lOa , lla, 13a-15a as con- firmed by TLC as well as analytical and spectral data. The stereochemistry at C-17 position (17 Ha) in case of C-17 acetox y steroids fonned was confirmed by comparing the specific rotational value of products viz. 1a and 2a with those of the authentic compounds 2112 when those were found to be completely identical. Earlier Mehta et al. 2o reported the Baeyer-Villiger oxidation of some carboxylic s ystem by using slurr y of CAN in acetonitrile. However , no Baeyer-Villiger oxidation occurred in our hands when the carb onyl compounds as listed in Table I were subjected to similar reaction conditions . Therefore , the pre se nt CAN catalyzed Baeyer-Villiger oxidation provides a useful way specially in the conve rsion of 20- oxopregnanes to 17-acetoxy steroids (C-19 steroids) of sex hormone series and synthesis of steroidal ring- A and ring-D lactones. Regarding thi s interesting ob servatio n about CAN which can act both as a Lewis acid or as a SET ox i- dant , it seems to be reasonable that CAN co-ordinates with the ketone carbony l to facilitate nucleophilic attack of the peracid to form a complex which breaks- down to the products after the rearra ngement steps. Recently , a nice modification of the 8aeyer-Villi ger oxidation of some non-reactive substrates through
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Indian Journal of Chemistry Vol. 43B, June 2004, pp . 1275-1281
Papori Goswami, Saroj Hazarika, Archana M Das & Pritish Chowdhury*
Natural Products Chemistry Division, Regional Research Laboratory, Jorhat 785006, India e-mail: [email protected]
Received 4 February 2003; accepted (revised) 10 December 2003
The role of ceric ammonium nitrate (CAN) as an effective catalyst in the peracid induced Baeyer-Villiger oxidation of carbonyl compounds with special reference to steroids has been demonstrated .
IPC: Int.C1.7 C 07 K 1/00
Ceric ammonium nitrate (CAN) finds application in synthetic organic chemistry for various chemical transformations, viz., nitration 1, nitroacetamidation2
,
complex formation with various alcohols3 etc. Its role as single electron oxidant has been reported in a number of publications including some recent reviews4
-8
.
CAN-induced oxidative radical transformations of steroids have also been reported9
. We too have reported lO the catalytic action of CAN in the esterification of carboxylic acids in very high yie ld . This in conjugation with our interest on steroid transformations ll
-14 persuaded us to study its role in 8aeyer
Villiger oxidation 1S-16 of 20-oxopregnanes to 17-
acetoxysteroids of potent sex hormones l7 and a lso of the formation of the steroidal D-ring lactones many of which are biologically active l8
.
Thus several steroidal carbonyl compounds 1-11 (Table I) which are available in our laboratory underwent Baeyer-Villiger oxidation to furnish their respective oxidation products 1a-lla (Table I) in high yield, when treated with m-chloroperbenzoic acid (m-CPBA) in the presence of catalytic amount of CAN in dichloromethane keeping just for 4-6 hr at room temperature. The method has been found to be effective for some non-steroidal carbonyl compounds 12-15 also (Table II) which give respective esters 12a-15a Cfable II) .
It is pertinent to note that although Andre et all ~ . have earlier reported the Baeyer-Villiger oxidation of some 20-oxopregnanes lIsing peracid alone, the reaction mixture had to be kept for 3 weeks in dark for completion . In the present case we did not find BaeyerVilliger oxidation of 20-oxopregnanes 1-4 (Table I) when treated with either CAN or peracid alone at room
temperature even when kept for more than 48 hr. Further, GLC experiment confirmed 40-55 % conversion of benzophenone 12 to phenyl benzoate 12a in 5 hr when CAN was used as a catalyst along with peracid whereas only 8% conversion was observed in the absence of CAN when kept for more than 24 hr. During our investigation, we also found that for all the cases (substrates 5-8, 10, 11, 13-15) the oxidation furni shed only one isomer viz. 5a-8a, lOa, lla, 13a-15a as confirmed by TLC as well as analytical and spectral data.
The stereochemistry at C-17 position (17 Ha) in case of C-17 acetox y steroids fonned was confirmed by comparing the specific rotational value of products viz. 1a and 2a with those of the authentic compounds2112
when those were found to be completely identical. Earlier Mehta et al.2o reported the Baeyer-Villiger
oxidation of some carboxylic system by using slurry of CAN in acetonitrile. However, no Baeyer-Villiger oxidation occurred in our hands when the carbonyl compounds as listed in Table I were subjected to similar reaction conditions . T herefore, the present CAN catalyzed Baeyer-Villiger oxidation provides a useful way specially in the conversion of 20-oxopregnanes to 17-acetoxy steroids (C- 19 steroids) of sex hormone series and synthesis of steroidal ringA and ring-D lactones.
Regarding thi s interesting observation about CAN which can act both as a Lewi s acid or as a SET ox idant , it seems to be reasonable that CAN co-ordinates with the ketone carbony l to facilitate nucleophilic attack of the peracid to form a complex which breaksdown to the products after the rearrangement steps. Recently , a nice modification of the 8aeyer-Villiger oxidation of some non-reactive substrates throu gh
1276
S.No.
1.
AcO
2 .
3 .
CI
4.
HO
5.
6 .
AcO
INDIAN 1. CHEM., SEC B, JUN E 2004
Table I - CAN catalyzed BV oxidation of carbonyl compounds in the presence of peracids.
Substrate Product
1a
2a
o
CI
4a
Sa
6a
Yield a (% )
80
75
91
80
77
78
376
318
352
354(M++2)
334
290
348
--Col1ld
GOSWAMI ('I a/.: CATALYZED BAEYER-VILLlG ER OXIDATION OF 20-0XOSTEROIDS
Table 1- CA N catalyzed BV oxidation of carbonyl co mpound s in the presence of peracids-Collld
S.No. Substrate Product Yielda mlz(M+)
(%)
0
83 324
326(M++2)
7. CI H
C H 7a b
0 79 306
8.
0 0
0 8a
78 .8 390
9.
Ac
9ac SH 17
82 402
10.
10a sH 17.;., "
78 388
11 .
11 a
(a) Yie lds refer to the isolated products which were fully characterised by spectral analys is. (b) The compounds shown has two molecular ion peak clue to J5C1 and ,17C1 isotopes. (c) The stereochemi stry of the epoxide was tentatively confirmed as Sa, 6a, o n the basis of the comparison of tr. ':! physi cal data of with that o f authentic 5u, 6u-epoxy cho lesterol: I nl D -9 .20 (c2, EtOH) [lit 20 I a) D -10.40 (c2, EtO H)J, mp 132-36°C ll it 21l mp 136°C).
1277
their hemi ketals or ketals was reported~ l, wherein il was the acti on of a Lewis acid that promoted the generation of the reactive oxycarbonium ion to which the peracid added smooth ly. It is reported that CAN oxi-
dizes ketones as SET oxidant leading to radi cal cations, which usually undergo fragmentation~~. In situ generation of nitric acid from CAN may also be responsi ble for the observed BY oxidation reactions.
1278 INDIAN 1. CHEM ., SEC B, JUNE 2004
Table II-CAN c<ltalysed BY oxid<lt ion of non-steroid carbonyl compounds in the presence of peracids
S .No . Substrate Product Yield a m/z(M+)
(%)
12 . o°-U 56 198
12a d
13. ~o ~0'10 66 212
13a d
0 0
72 170 14 .
14ad
15 . ~CH3 :)-Jl [: CH 3 66
16. ~ t(d
16ae
(<I) Yields refer to the isolated products which were fully ch<lracterised by spectral analysis. (b) Yield c<llculated on the basis of GLC. (c) No re<lction W<lS observed and substrate was recovered gU<lntitatively.
The following points are to be noted: (i) only catalytic amount of CAN is necessary in
the reaction (CAN:Substrate:: 0.10: 1.5 mmole) . Oi) oxidation is complete within 4-6 hr at room
temperature. (iii) the yield of product is high, specially in the ster
oids. (iv) the method is also applicable to the regioselec
tive transformation of acyclic ketones to esters.
Experimental Section
Melting points were determined with an electrothermal melting point apparatus and are uncorrected. All the chemicals used were of reagent grade of AI-
drich Chemical Co. and were used without further purification. m-CPBA used was purchased from Merck-Schuchardt, Germany and its purity was 55 %. Freshly distilled dichloromethane was used. The progress' of the reactions were monitored by TLC using silica gel (E Merck) and the plates were activated at 100°C before use. IR spectra (in cm-!) were recorded on a Perkin-Elmer model 2000 series FT IR spectrometer in CHCI3; 'H NMR spectra on a Bruker DPX (300 MHz) spectrometer with TMS as internal standard (chemical shifts in 0, ppm); and mass spectrometric analysis was performed by positive mode electro spray ionization with Bruker Esquire 3000 LC-MS instrument. Specific rotations (a\) were recorded on a Perkin-Elmer Polarimeter 343
GOSW AM! et al.: CATALYZED BAEYER-V!LLIGER OXIDA nON OF 20-0XOSTEROTDS 1279
corded on a Perkin-Elmer Polarimeter 343 instrument. Elemental analysis was carried out in Varian CHN Analyzer.
Cerie ammonium nitrate (CAN) induced selective Baeyer-Villiger oxidation of carbonyl compounds with m-CPBA: General method. To the solution of a substrate (1.5 mmoles) in 10 mL of di chloromethane was added CAN (0.10 mmole) and m
CPBA (Merck-Schuchardt, Germany, 55% pure) (2.0 mmoles). The reaction mixture was kept at room temperature for 4-6 hr. The reaction mixture was worked up by pouring into cold water (150 mL) and was extracted with petroleum ether (60-80°C). The organic extract was first treated with aqueous solution of potassium iodide and the liberated iodine was neutralized with sodium thiosulfate and finally washed with sodium bicarbonate solution . The organic extract was evaporated under reduced pressure after drying over anhydrous sodium sulfate to get the desired oxidation product, which was further purified by preparative TLC (EtOAc-pet. ether). Since the reaction with all the substrates were carried out on a small scale with minimum amount of CAN, the catalyst could not be recycled and went into the aqueous phase during work-up.
The authors thank Director of the Insti tute for providing necessary facilities for thi s work. One of the authors (PG) thanks CSlR, New Delhi for the award
of a Senior Research Fell owship. The Quality Control Center of our laboratory is thankfull y acknowledged for lR , NMR, mass and GLC experimen ts.
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