This article was downloaded by: [Universidad Autonoma de Barcelona]On: 16 October 2014, At: 02:12Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK

Synthetic Communications: AnInternational Journal for RapidCommunication of SyntheticOrganic ChemistryPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lsyc20

Oxidation of Steroidal 5-En-3β-ol with PyridiniumChlorochromate: Isolation ofKey Intermediate, Steroidal 6β-Hydroxy-4-en-3-oneS. S. Korde a , R. A. Udasi a & G. K. Trivedi aa Department of Chemistry , Indian Institute ofTechnology , Mumbai, 400076, IndiaPublished online: 22 Aug 2006.

To cite this article: S. S. Korde , R. A. Udasi & G. K. Trivedi (1997) Oxidation ofSteroidal 5-En-3β-ol with Pyridinium Chlorochromate: Isolation of Key Intermediate,Steroidal 6β-Hydroxy-4-en-3-one, Synthetic Communications: An International Journalfor Rapid Communication of Synthetic Organic Chemistry, 27:19, 3419-3430, DOI:10.1080/00397919708005643

To link to this article: http://dx.doi.org/10.1080/00397919708005643

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SYNTHETIC COMMUNICATIONS, 27( 19), 3419-3430 (1997)

OXIDATION OF STEROIDAL 5-EN-3p-OL WITH PYRIDINIUM CHLOROCHROMATE : ISOLATION OF KEY INTERMEDIATE, STEROIDAL 6P-HYDROXY-4-EN-3-ONE

S. S. Korde. R. A. Udasi and G. K. Trivedi'

Department of Chemistry, Indian Institute of Technology, Mumbai-400076. India

Abstract : The PCC oxidation of 5-en-30-01 steroids proceeds to 4-en-3,6-dione through the intermediate 5-en-3-one. The isolation of another key intermediate, steroidal 6P-hydroxy-4-en-3-0ne guides an understanding of the mechanism involved.

Steroidal 5-en-3P-01 compounds commonly occur in nature. Pregnenolone,

cholesterol and diosgenin are some examples of compounds belonging to this

class. Pregnenolone is a precursor to a number of adrenocortico and gonadal

hormones in our body' while cholesterol is an architectural component of the

biological membranes.2 Of the steroid drug precursors, diosgenin is most

important and versatile, and is used for the synthesis of nearly 50% of the total

steroid drugs in the world.3 Conversion of these homoallylic alcohols to steroidal

4-en-3-one compounds is a crucial step in the synthesis of most steroid hormones.

Oppenauer oxidation was a traditional method used during the early years of

synthesis: but reaction setups were tedious and workups were laborious. Hence

To whom correspondence should be addressed.

3419

Copyright 0 1997 by Marcel Dekker. Inc.

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3420 KORDE, UDASI, AND TRIVEDI

simple alternatives to Oppenauer oxidations were sought and found in chromium

oxidizing reagents due to facile reaction conditions and elementary procedures.5

Homoallylic steroidal alcohols have been oxidized by several chromium

reagents like Na2Cr207 in AcOH,6 pyridinium dichromate,’ CrO3 in pyridines

and Jones reagenL9 Pyridinium chlorochromate (PCC) has also proved to be a

good oxidizing agent for this purpose.10-11 Recently prolonged oxidations of

steroidal 4-ene-3p-01 compounds with PCC in dichloromethane to steroidal 4-en-

3,6-diones via the intermediate 5-en-3-one have been reported.’*

I n this paper, we wish to report short time reaction of PCC on steroidal 5-

en-3fl-01 in dichloromethane and isolation of the speculated intermediate 6p-

hydroxy-5-en-3-one during the formation of steroidal 4-ene-3,6-dione. The

stereochemical and mechanistic possibilities leading to the intermediates and

products formed are also discussed herein.

16a-Methylpregnenolone (1A) was subjected to excess of PCC in dry

dichloromethane at room temperature and the reaction was monitored by thin

layer chromatography (TLC). After half an hour, TLC of the reaction mixture,

carried out in 20% ethyl acetate in petroleum ether as the solvent, showed three

spots, among which one with rf 0.4 belonged to the starting material. A dominant

spot was seen at rf 0.8 and a faint one at rf 0.5. TLC was again recorded after an

hour. Two additional spots appeared, one being close to that of the starting

material with rf 0.3 and the other just below it with rf 0.25. The reaction was

stopped and the usual workup was followed by separation of the obtained mixture

of compounds by careful column chromatography using silica gel and ethyl

acetate-petroleum ether as eluant. This resulted in the purification of the two

compounds that were less polar, but the more polar compounds were obtained as a

mixture. These were finally separated by changing the eluant to dichloromethane.

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STEROIDAL 6P-HYDROXY-4-EN-3-ONE 342 1

The mass, IR, and IH NMR spectra of the compounds were recorded. The

least polar compound was identified as 16a-methylpregn-5-en-3,20-dione (1 B),

compound with rf 0.5 as 16a-methylpregn-4-en-3,20-dione (IC), compound with

rf 0.3 as 16a-methylpregn-4-en-3,6,20-trione (ID) and the most polar compound

as 6P-hydroxy- 16a-methyIpregn-4-en-3,2O-dione (I E). The spectral data of these

compounds corroborated well with the literature reports.I3

The ‘€1 NMR spectrum of the mixture as such revealed that the ratio of

products B. C, D and E was 2 : 0.6 : I : I . 16a-Ethylpregnenolone (ZA), 16a-

prop-2’-enylpregnenolone (3A) , cholesterol (4A) and diosgenin (SA) were also

subjected to PCC reaction under identical conditions (Fig.1 ). The results were

similar to that obtained for the reaction with 16a-methylpregnenolone.

The oxidation of homoallylic steroidal alcohols with PCC led to four

products viz. B, C, D and E. Thus, it can be established that, in agreement with

the earlier report, the reaction follows a path where pyridinium chlorochromate

first oxidizes the homoallylic alcohol to a homoallylic ketone B. Due to a mild

acidic nature of PCC, some of the compound B is isomerised to the conjugated

ketone C. Excess of PCC then oxidizes the 5-en-3-one B to 6P-hydroxy 4-en-3-

one E and finally to 4-en-3,6-dione D.

Blaszezyk and Paryzek have suggested an ionic mechanism for PCC

oxidation’* while Solaja et a1 contemplate a free radical mechanism for Jones’

oxidation in ether.9 Although there is no evidence whether Cr(V1) reagents operate

through the ionic or free radical mode, a comparison of the two mechanisms can

be made i n light of the stereochemistry of isolated intermediate E.

Both the mechanisms for the formation of compound D suggest the

abstraction of the 4-H from compound B. The radical mechanism is assumed to

proceed with a conformational flip prior to the 4-H abstraction which facilitates

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3422 KORDE, UDASI, AND TRIVEDI

A

&;@ t o@R

0

0 OH

C D E

1 2 3 4 5

e \.oo R Me Et M H

+ R' COCH3 COCHj COCH3

Figure 1 : PCC oxidation of homoallylic steroidal alcohols

the removal of 4a-H. Resonance stabilised C-6 radical 6 subsequently undergoes

hydroxylation of the C-6 radical to give 6P-hydroxy-4-en-3-one E. It should be

noted that the 6P-hydroxy compound E was formed exclusively. Alternatively, the

ionic abstraction of the 4-H proton leads to an enolate 7. The attack of chromate

ion at C-6 of the enolate 7 gives 6P-hydroxy-4-en-3-one.

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STEROIDAL 6P-HYDROXY-4-EN-3-ONE 3423

H r *°Cr02CI

6

7

Figure 2 : Conformations of radical 6 and enolate 7 as seen in the model

Comparing the models of radical 6 and enolate 7 (Fig.2) clearly shows that

an exclusive P attack is more feasible on the enolate rather than on the radical.

The three axial protons viz. la-H, 7a-H and 9a-H completely hinder an a attack

on the enolate. On the P side, the 19-methyl is directed away from C-6 and only

one axial proton viz. 8P-H causes some hindrance. In contrast, due to the quasi

axial disposition of the 7a-H, the a face of the radical 6 is comparatively less

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3424 KORDE, UDASI, AND TRlVF rrl

hindered. The 0 side of the radical is more hindered as compared to that of the

enolate since the 19-methyl is directed more towards C-6. Some hindrance is

caused by 8p-14 as well. Another elusion to an exclusive p attack on the radical 6

is the radical inversion process. Thus, if a radical mechanism is assumed, a

question strongly arises as to why is the 6a-hydroxy-4-en-3-one not formed.

Solaja et a19 claim that there is no enolisatioii prior to reaction and that the

proton abstracted is exclusively 4a. They obtained 4-deuterio-cholest-4-en-3,6-

dione on oxidation of 4P-deuterio cholesterol. But this circumstance may well be

conceived in an ionic process, wherein, the 4a-H is abstracted by the chromate

species via a cyclic mechanism as suggested by Blaszezyk and Paryzek,I2

followed by a rapid attack of the chromate ion at C-6.

In conclusion, although the radical mechanism cannot be ruled out, the

exclusive formation of steroidal 6P-hydroxy-5-en-3-one leads us to the belief that

an ionic mechanism is operational during PCC oxidation of the steroidal

homoallylic alcohols to the 4-en-3,6-diones.

Experimental

16a-Methyl-, 16a-ethyl- and 16a-prop-2'-enylpegn-S-en-3P-o1 were

synthesized from 3 0-hydroxypregn-5, I6-dien-20-one according to the available

procedures in the literature. I4 Diosgenin and cholesterol were obtained from

Glaxo Laboratories, India and recrystallised before use.

IR spectra were recorded on a Perkin Elmer 681 spectrophotometer in

CHCI,. A Hewlett Packard MS Engine 5989-A spectrometer was used to record

the mass spectra. IH NMR spectra were recorded on a Varian VXR 300s. About 5

mg of the sample was dissolved in 0.6 mL of the solvent.

Oxidation of 3P-hydroxy-16a-methylpregn-5-en-20-0ne (1A)

3P-hydroxy-l6a-methylpregn-5-en-20-one (1A) (4 g, 12.12 mmol) was

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STEROIDAL 6P-HYDROXY-4-EN-3-ONE 3425

dissolved in dry dichloromethane (100 mL). Pyridinium chlorochromate (7.78 g,

36.36 mmol) was added and the solution stirred for one hour. The solvent was

then evaporated to half and the slurry was poured over a silica gel column and

eluted with 50 % ethyl acetate in petroleum ether. Evaporation of the solvent gave

a mixture of compounds B, C, D and E, free of the chromium reagent, which was

then subjected to silica gel column chromatography with the eluant as 20 % ethyl

acetate-petroleum ether to obtain pure compounds B and C . To obpin pure

compounds D and E, the mixture of D and E was loaded on a fresh silica gel

column and eluted with dichloromethane. The compounds lB, lC, ID and 1E

were obtaind in yields of 1.43 g, 0.40 g, 0.79 g and 0.63 g respectively.

16a-methylpregn-!%en-3,20-dione (1B)

MS: m/z = 328 (M+, 4%).

IR(CHCI3): t 3030 (OH), 2924,2875, 1712, 1464 cm-1.

'H NMR (CDCI,): 6 5.34(bs, IH, 6-H), 3.29(m, IH, 4P-H), 2.83(dd, J=16.4,2.4

Hz, IH, 4a-H), 2.69(m, I H , 16P-H), 2.49(ddd, J=15.0, 13.8, 5.7 Hz, IH, 2P-H),

2.3l(m, IH, 2a-H), 2.13(s, 3H, 21-H3), 1.19(s, 3H, 19-H3), 0.95(d, J=6.9Hz, 3H,

1 '-H3), 0.69(~, 3H, 18-HJ).

16a-methylpregn-4-en-3,20-dione (1 C)

MS: m/z = 328 (M+, lo%), 313, 124,79.

IR(CHC13):T 3036,2930,2868, 1712,1681,1464.1389 cm-1.

'H NMR (CDCI,): 6 5.74(bs, lH, 4-H), 2.69(m, IH, 16P-H), 2.44(ddd, J=15.0,

13.8, 5.7 Hz, IH, 2P-H), 2.28(ddd, J=14.7, 4.5, 2.4 Hz, lH, 6a-H), 2.15(d, J=9

Hz, IH, 17a-H), 2.13(s, 3H, 21-H3), 1.18(s, 3H, 19-H3), 0.95(d, J=6.9 Hz, 3H, 1'-

HJ), 0.69(~, 3H, 18-H3).

16a-methylpregn-4-en-3,6,20-trione (1 D)

MS: m/z = 342 (M+, 24%). 327,299,284.

IR(CHCl3): 7 3032,2920,2875, 1736, 1700, 1464, 1380 em-1.

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3426 KORDE, UDASI, AND TFUVEDI

*H NMR (CDC13): 6 6.20(s, IH, 6-H), 2.70(m, IH, 16P-H), 2.67(dd, J=15.9, 3.9

Hz, IH, 7P-H), 2.56(ddd, J=15.0, 13.8, 5.7 Hz, IH, 2P-H), 2.19(d, J=9 Hz, IH, 17

a-H), 2.15(s, 3H, 21-H3), 1.17(s, 3H, 19-H3), 0.97(d, J=6.9 Hz, 3H, I'-H3),

0.71(~, 3H, 18-H3).

6P-hydroxy-16a-methyIpregn-4-en-3,20-dione (1E)

MS: m/z = 344 (M', 25%), 329,326,283.

IR(CHCI3):T 3400,3032,2920,2875, 1736,1700,1470,1380 cm-l.

'H NMR (CDCI,): 6 5.86(s, IH, 6-H), 4.4(t, J= 2.4 Hz, lH, 6a-H), 2.69(m, IH,

16P-H), 2.54(ddd, J=16.8, 14.7, 5.1 Hz, IH, 2P-H), 2.40(m, IH, 2a-H), 2.16(d,

J=9 Hz, IH, 17a-H), 2.14(~, 3H, 2I-H3), 1.38(~, 3H, 19-H3), 0.96(d, 5 ~ 6 . 9 Hz,

3H, l'-H3), 0.72(s, 3H, 18-H3).

Oxidation of 3P-hydroxy-16a-ethylpregn-5-en-20-one (2A)

3P-hydroxy-16a-ethyIpregn-5-en-2O-one (0.5 g, I .46 mmol) was reacted

with PCC (1.94 g, 4.39 mmol) in a similar manner as that of compound 1A. The

crude was subjected to silica gel column chromatography and the steroidal

mixture sans chromium impurities was dissolved in methanol (20 mL) to which

oxalic acid (2 g) was added. The mixture was warmed for 30 minutes and

evaporated to dryness. The organic residue was redissolved in ether, washed with

2 % NaOH solution. water and brine and dried over anhydrous NaZS04. The

compounds C, D and E were obtaind in 50 %, 15 % and 18 % yields respectively.

16a-ethylpregn-4-en-3,20-dione (2C)

MS: mlz=.342 (M', loo%), 327,313, 124.

IR(CHC13):O 3036,2926,2875, 1712,1688, 1464 cm-1.

IH NMR (CDCI,): 6 5.74(bs, IH, 4-H), 2.51(m, lH, 16P-H), 2.44(ddd, J=17.3.

14.1, 5.1 Hz, IH, 2P-II), 2.28(ddd, J=14.7, 4.5, 2.4 Hz, IH, 6a-H), 2.22(d, J=9

Hz, Ill, 17a41), 2.14(s, 3€1, 21-H3), 1.18(s, 3€1, l9-113), 0.81(t, J=7.2 Hz, 3H, 2'-

H3), 0.69(~, 3H, 18-H3).

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STEROIDAL 6P-HYDROXY-4-EN-3-ONE 3427

16a-ethylpregn-4-en-3,6,20-trione (2D)

MS: m/z= 356 (M+, loo%), 327,313.

IR(CHCI3): 7 3026,2960,2875, 1720,1696,1466,1387 cm-l.

IH NMR (CDC13): 6 6.19(~, IH, 6-H), 2.69(dd, Jc15.9, 3.9 Hz, IH, 7P-H).

2.26(d, J=9Hz, IH, 17a-H), 2.16(~, 3H, 21-H3), 1.17(~, 3H, 19-H3), 0.82(t, J=7.2

Hz, 3H, 2'-H3), 0.71(s, 3H, l8-H3).

6P-hydroxy-l6a-ethylpregn-4-en-3,20-dione (2E)

MS: m/z = 358 (M', SO%), 343,329,315.

IR(CHCI3): 3 3400,2924,2870, 1730,1694,1470 cm-1.

IH NMR (CDC13): 6 5.91(~, IH, 6-H), 4.45(t, J= 2.4 Hz, IH, 6a-H), 2.21(d, J=9

Hz, IH, 17a-H), 2.14(s, 3H, 21-H3), 1.38(s, 3H, 19-H3), 1.33(t, J=7.2 Hz, 3H, 2'-

H3), 0.69(~, 3H, 18-H3).

Oxidation of 3P-hydroxy-l6a-prop-2'-enylpregn-5-en-20-one (3A)

3P-hydroxy- 16a-prop-2'-enyl pregn-5-en-20-one (2 g, 5.26 mmol) was

reacted with PCC (3.61 g, 16.86 mmol) in the same manner as that of compound

1A. The compounds B, C, D and E were obtaind in yields of 0.60 g, 0.40 g, 0.4 1 g

and 0.37 g respectively.

16a-prop-2'-enylpregn-S-en-3,20-dione (3B)

MS: m/z = 354 (M+, 8%).

IR(CtlC13): 7 3500,3030,2927,1725,1709,1660,1459,1387 cm-1.

IH NMR (CDCI,): 6 5.66(m, lH, 2'-€I), 5.62(bs, IH, 6-H), 4.91-5.01(m, 211, 3'-

Hz), 3.29(m7 IH, 4a-H), 2.84(dd, J=15.5, 2.1. 1H. 4PH) 2.73(m, IH, 16a-H),

2.49(ddd, J=15.0, 13.8, 5.7 tlz, I H , 2P-H), 2.25(d, J=9 Hz, I€{, 17a-H), 2.11(~,

3t1, 2 1-1 13), 1.18(~, 311, 19-€l3), 0.69(~, 3H, 18-H3).

16a-prop-2'-enylpregn-4-en-3,20-dione (3C)

MS: m/z = 354 (Mt, 4%), 3 13 (80%).

IR(CHC13): T 3032,2940, 1709, I68 I , 1660, 1459, 1387 cm-1.

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3428 KORDE, UDASI, AND TRIVEDI

IH NMR (CDCI3): 6 5.74(s, I H , 4-H), 5.65(m, IH, 2'-H), 4.91-5.02(m, 2H, 3'-

M2), 2.73(m, IH, I6P-€I), 2.44(ddd, J=17.4, 13.8, 4.8 Hz, IH, 2P-H). 2.47(d, J=9

€ 1 ~ 111, 17a-H), 2.10(s, 3H, 21-113), 1.18(s, 3H, 19-I13),0.69(s, 3H, 18-H3).

16a-prop-2'-enylpregn-4-en-3,6,2O-trione (3D)

MS: mlz = 368 (M+, 15%), 353,327, 124.

IR(CHCI3): 'v 3026,2920,2874, 1720, 1690, 1460 cm-1.

'H NMR (CDC13): 6 6.20(s. I H , 4-H), 2.77(m, IH, 16P-H), 2.68(dd. J=15.9, 3.9

Hz, 111, 7P-H), 2.55(ddd, 3~17 .4 , 14.1, 5.1 Hz, lH, 2P-H), 2.29(d, J=9 Hz, IH, 17

a-H),2.12(s, 3H, 21-H3), 1.17(s, 3H, 19-H3), 0.70(s, 3H, 18-H3).

6P-hydroxy-l6a-prop-2'-enyl pregn-4-en-3,20-dione (3E)

MS: m/z = 371 (M+I), 355,352, 124.

IR(CHCl3): 3 3500,3030,2927, 1725,1709,1660,1459,1387 cm-I.

IH NMR (CDC13): 6 5.90(s, IH, 4-H), 5.65(m, IH, 2'-H), 4.96-5.01(m, 2H, 3'-

Hz), 4.44(dd, 3=3.3, 2.4 Hz, I H , 6a-H), 2.73(m, lH, 16a-H), 2.55(ddd, J=17.4,

14.7, 5.1 Hz, IH, 2P-H), 2.24(d, J=9 Hz, IH, I~cx-H), 2.10(~, 3H, 21-H3), 1.31(~,

3H, 19-H3), 0.65(s, 3H, 18-H3).

Oxidation of iholesterol(4A)

Cholesterol (0.5 g, 1.29 mmol) was reacted with PCC (0.83 g, 3.87 mmol)

in rhe same manner as that of compound 1A. The reaction mixture was passed

through silica gel column and eluted with 50 % ethyl acetate in petroleum ether.

The mixture was concentrated and repurified to isolate the hydroxy derivative 4E

in a yield of 20 % (0.1 g).

6P-Hydroxyehoiest-4-en-3-one (4E)

MS: m/z = 401 (M+I), 341,327,313.

IH NMR (CDCI,): 6 5.89(~, lH, 4-H), 4.44(dd, J=3.9, 2.4 Hz, IH, 6a-H),

2.55(ddd, J=16.8, 14.7, 5.1 Hz, IH, 2P-H), 0.91(d, J=6.3, 3H, 21-H3), 1.33(s, 3H,

19-113). 0.87(d, J=6.6, 3H, 26-€13), 0.82(d, J=6.6, 3H, 27-H3), 0.71(~, 3H, 18-H3).

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STEROIDAL 6P-HYDROXY-4-EN-3-ONE 3429

Oxidation of diosgenin (5A)

Diosgenin (0.5, 1.21 mmol) was reacted with PCC (0.78 g, 3.62 mmol) in

the same manner as that of compound 4A. Compound 5E was obtained in a yield

of 16 Yo (0.08g).

6P-Hydroxyspirostan-22a0-4-en-3-one (5E)

MS: m/z =428 ( M', lo%), 139 ([CgH50]+, 100%)

IH NMR (CDCl,): 6 5.82(bs, lH, 4-H), 4.41(m, lH, 16a-H), 4.36(t, J=3 Hz, lH,

6a-H), 3.43- 3.52(m, lH, 26a-H), 3.36(t, J=10.8 Hz, lH, 26P-H), .1.40(s, 3H, 19-

H3). 0.98(d, 5 ~ 7 . 2 Hz, 3H, 21-H3), 0.85(~ , 3H, 18-H3), 0.79(d, J=6.3 Hz, 3H, 27-

H3),

Acknowledgements

One of us (SSK) thanks C.S.I.R. for the financial assistance. The spectral facilities

provided by R.S.I.C., Mumbai are gratefully acknowledged.

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1.

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3.

4.

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6 .

7.

8.

Murray, R.K., Gaanner, D.K., Mayers, P.A. and Rodwell, V.W. "Harper's

Biochemistry", ed. 23, Appleton & Lange, 1993.

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