Ionic hydrogenation-directed stereoselective construction of C-20(H) stereogenic center in steroid side chains: Scope and limitations

Article abstract of DOI:10.1016/j.tet.2017.03.029

Stereoselective synthesis of steroidal C-20 tertiary alcohols with n-butyl, vinyl, furyl, thienyl, thiazolyl, aryl and pyridyl side chains via Grignard reaction or organolithium reagents have been realized starting from readily available 16-dehydropregnen

Full text of DOI:10.1016/j.tet.2017.03.029

Accepted Manuscript
Ionic hydrogenation-directed stereoselective construction of C-20(H) stereogenic
center in steroid side chains: Scope and limitations
Bapurao B. Shingate, Braja G. Hazra
PII:
S0040-4020(17)30267-3
10.1016/j.tet.2017.03.029
TET 28535
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 30 November 2016
Revised Date: 7 March 2017
Accepted Date: 10 March 2017
Please cite this article as: Shingate BB, Hazra BG, Ionic hydrogenation-directed stereoselective
construction of C-20(H) stereogenic center in steroid side chains: Scope and limitations, Tetrahedron
(2017), doi: 10.1016/j.tet.2017.03.029.
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ACCEPTED MANUSCRIPT
Ionic hydrogenation-directed stereoselective construction of C-20(H)
stereogenic center in steroid side chains: Scope and limitations
Bapurao B. Shingate a,b* and Braja G. Hazraa
aDivision of Organic Chemistry, CSIR-National Chemical Laboratory, Pune 411 008 (MS), India
bDepartment of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431
004 (MS), India
Telephone: (91)-240-2403312; Fax: (91)-240-2403113
E-mail:* bapushingate@gmail.com (B. B. Shingate)
ABSTRACT
Stereoselective synthesis of steroidal C-20 tertiary alcohols with n-butyl, vinyl, furyl, thienyl,
thiazolyl, aryl and pyridyl side chains via Grignard reaction or organolithium reagents have been
realized starting from readily available 16-dehydropregnenolone acetate. The ionic
hydrogenation of steroidal C-20 tertiary alcohols having furyl, methylfuryl, thienyl, phenyl and
4-methoxyphenyl side chains, resulted into the deoxygenated product with C-20 natural
configuration in excellent yields. However, the alkyl, thiazolyl and pyridyl incorporated steroidal
C-20 tertiary alcohols were failed under the same reaction condition. The scope of ionic
hydrogenation is further highlighted through the stereoselective reduction of steroidal C-20,21-
ene compounds with furyl, thienyl and 4-methoxyphenyl side chains gave the saturated
compounds with C-20 natural configuration.
Keywords: Steroid side chain, Grignard reaction, Ionic hydrogenation, Steroidal tertiary
alcohols, C-C bond formation
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Graphical Abstract
Ionic hydrogenation-directed stereoselective construction of C-20(H) stereogenic center in
steroid side chains: Scope and limitations
Bapurao B. Shingate and Braja G. Hazra
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Ionic hydrogenation-directed stereoselective construction of C-20(H) stereogenic center in
steroid side chains: Scope and limitations
1. Introduction
The last few decades have witnessed intensive research on steroid side chain synthesis, yielded
many imaginative syntheses of general interest, and contributed to a great extent to the
development of stereospecific chiral carbon formation. The biological significance of both the C-
20 epimers of naturally occurring and/or synthetic sterol molecules provided not only steroid
chemists but also natural product chemists an opportunity to pursue new syntheses of sterols or
compounds with similar chain structures.1 The recent discovery of many new sterols with novel
side chain structures from marine and animal sources has focused attention on developing
stereocontrolled methods to introduce these side chains onto tetracyclic steroidal starting
materials.1,2
The isolation and development of stereochemically controlled syntheses of steroid side
chains of many biologically important new natural products containing modified cholesterol side
chains,1 such as ecdysones,2 metabolite of vitamin D3,3 brassinosteroids,4 squalamine,5 saponin
OSW-1,6 contignasterol,7
agosterol,8 xestbergsterol,9 halosterols,10 aragusterol,11
12
certonardosterol D2, physanolide,13 amaranzole,14 withaferin,15 clathsterol,16 marine sterols17
and other compounds18 with sterol like side chains have natural C-20 configuration. Steroids
with unnatural configuration at C-20 are attracting attention because of interesting biological
activities. The 20-epi-1α,25-dihydroxy vitamin D3 (20-epi calcitriol), is more potent in regulating
cell growth and differentiation19 and also exhibits immunosuppressive properties.20 It was
reported that, the fluorinated vitamin D3 analogue Ro 26-9228 (1α-fluoro-25-hydroxy-16,23E-
diene-26,27-bishomo-20-epi-cholecalciferol), increased osteoblast surface and bone mass and
inhibited bone resorption in an OVX rat model at doses that did not increase serum and urine
calcium.21
Very recently, we have reviewed various methods22 for the construction of C-20(H)
stereogenic center in the acyclic domain of steroids using named/unnamed
reactions/rearrangements, organometallic reagents and others protocols. Most of the literature
reports on the synthesis of steroidal C-20(H) chiral center in the side chain via C-20 tertiary
alcohols are multi-step reactions. Dehydration of steroidal C-20 tertiary alcohol, formation of
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either C-20(21) or C-20(22) olefin followed by hydrogenation with various reducing catalysts
have been employed (eq. 1).22 Again, we have reported23-25 the stereoselective construction of C-
20 unnatural configuration via ionic hydrogenation of steroidal C-20,22-ketene dithioacetal and
C-20 tertiary alcohols possessing dithiane moiety with excellent selectivity (eq. 2). Ionic
hydrogenation is an effective method for the removal of steroidal tertiary alcohols26 to the
corresponding hydrocarbons.
On the basis of these observation, the significance of stereoselective construction of C-
20(H) stereogenic center in steroid side chain and continuation of our work,23-25 herein, we
would like to report, stereoselective synthesis and ionic hydrogenation of steroidal C-20 tertiary
alcohols with various aliphatic, furyl, thienyl, thiazolyl, aryl and pyridyl side chains, their scope
and limitations.
2. Results and discussion
2.1. Stereoselective synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols
with aliphatic side chain
The introduction of alkyl group enhances the physiological activity and narrows the spectrum of
biological activity of steroids. Furthermore, alkyl steroids are of interest for the stereochemistry
of the reactions, which take place with participation of condensed alicyclic systems.27 The
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reaction of Grignard and other organometallic reagents with steroidal 20-ketones has been
utilized by a number of investigators to construct the side chain in one- or in multistep
sequences. In these reactions, chiral center at C-20 is created with mixture of epimers, the ratio
of epimers depending greatly upon the structure of the steroids, particularly the nature of
substituents near to the C-20 and the bulkiness of the reagent.1.22
We have already described,23-25 the synthesis of 1a-b from readily available starting
material 16-dehydropregnenolone acetate in three steps via hydrogenation-hydrolysis-silyl ether
formation. The reaction of n-butyl lithium with 3β-tert-butyldimethylsilyloxy-pregn-5-ene-20-
one 1a and 3β-tert-butyldimethylsilyloxy-5α,17α-pregn-20-one 1b in THF at -45 °C yielded
C(20S) compounds 2a and 2b in 91% and 89% yield, respectively (Scheme 1). The TBDMS
group of compound 2a and 2b was deprotected with n-Bu4NF in THF, followed by selective
acetylation of the 3β-secondary alcohol of 3,20-diol 3a and 3b with acetic anhydride and
catalytic amount of DMAP to afford 3β-acetate 4a and 4b in 89% and 90% yield, respectively
over two steps.
Scheme 1. Attempted ionic hydrogenation of steroidal C-20 tertiary alcohols with n-butyl
side chain
Reagents and conditions: (i) n-BuLi, THF, -45 ºC, 1 h, 2a (91%) and 2b (89%); (ii) n-Bu4NF,
THF, 30 ºC, 18 h, 91-93%; (iii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 95-96%; (iv) (a) Et3SiH,
.
F3CCOOH, CH2Cl2, 0 ºC, 10 min, 83%; (b) Et3SiH, BF3 OEt2, CH2Cl2, 0 ºC, 10 min, 87-92%.
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A representative transformation of alcohols is deoxygenation to alkane, since a hydroxyl
group is a poor leaving group, it should be generally activated before treating with a reducing
reagent. Tosylation or thiocarboxylation are generally employed for activation. There are very
few reports on the deoxygenation of alkyl tertiary alcohols by ionic hydrogenation to saturated
hydrocarbons.28 On the basis of these findings; we envisaged the ionic hydrogenation of steroidal
C-20 tertiary alcohols. Ionic hydrogenation of compound 2a with triethylsilane in trifluoroacetic
acid and also with triethylsilane in borontrifluoride etherate resulted in the formation of the C-
17(20) tetrasubstituted olefinic product 5a in 83% and 90% yield, respectively (Scheme 1).
.
Under similar reaction condition (Et3SiH, BF3 OEt2), compound 4a afforded compound 6a in
.
92% yield. C-5(6)-Saturated compounds 2b, 3b and 4b on reaction with Et3SiH and BF3 OEt2 led
to the formation of C-17(20) tetrasubstituted olefinic products 5b (2b, 3b→ 5b) and 6b (4b →
6b) in an excellent yields (87-91%). In the present study, the dehydration of tertiary alcohols
takes place via formation of carbocation at C-20 and deprotonation from C-17 giving rise to the
formation of tetrasubstituted alkenes 5a,b and 6a,b. This is due to the lack of the formation of
stabilized carbocation at C-20 by the n-butyl side chain.
There are several reports,26,29 on the ionic hydrogenation of allylic alcohols and steroidal
allylic alcohols with triethylsilane and boron trifluoride etherate to the formation of
hydrocarbons. It has also been documented,30 that ionic hydrogenation of allylic alcohols does
not produce deoxygenated product, but gave eliminated product. On the basis of these findings,
several allylic C-20 tertiary alcohols were synthesized and examined the effect of ionic
hydrogenation conditions on these systems.
Compound 7 on treatment with TBDMSCl in the presence of imidazole afforded 8 in
excellent yield (Scheme 2). The TBDMS-protected keto compound 8 on treatment with n-BuLi
in THF at -78 °C resulted into C-20 tertiary alcohol 9 with epimeric mixture at C-20. The C-20
epimers of compound 9 were not separated and it was subjected for deprotection of TBDMS
group, followed by selective 3-OH acetylation with Ac2O and DMAP, to afford 11 in good yield.
.
The ionic hydrogenation of 9 or 10 with Et3SiH and BF3 OEt2 resulted into rearranged product
5a. Similarly, compound 11 on ionic hydrogenation gave compound 6a in excellent yield. In this
case, the ionic hydrogenation takes place, in which, first the formation of carbocation at C-20,
then stabilization of this carbocation by the migration of double bond from C-16(17) to C-17(20)
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and formation of new carbocation at C-16, followed by hydride transfer from Et3SiH resulted
compounds 5a/6a in good yields.
Scheme 2. Synthesis and ionic hydrogenation of allylic steroidal C-20 tertiary alcohols with
n-butyl side chain
Reagents and conditions: (i) TBDMSCl, imidazole, DMF, rt, 2 h, 91%; (ii) n-BuLi, THF, -78
ºC, 1 h, 87%; (iii) n-Bu4NF, THF, 30 ºC, 18 h, 89%; (iv) Ac2O, Pyridine, DMAP, 25 ºC, 2 h,
.
93%; (v) Et3SiH, BF3 OEt2, CH2Cl2, 0 ºC, 10 min, 81-83%.
Compound of types 15 with C-20 natural configuration are useful intermediates for the
synthesis of variety of natural products and their analogues. It is well known1a-b,22 that, addition
of vinylic organometallic reagents to 20-ketones resulted stereoselectively only one isomer.
Grignard reaction with vinyl magnesium bromide on steroidal ketones 1a and 1b in THF yielded
12a and 12b in 98% and 97% yield, respectively (Scheme 3). Deprotection of TBDMS group in
compounds 12a and 12b with n-Bu4NF in THF, resulted the compounds 13a and 13b
respectively, in excellent yields. Selective acetylation of 3-hydroxy group of 13a and 13b with
Ac2O and DMAP in pyridine yielded compounds 14a and 14b with 94% and 93% yield,
.
respectively. Ionic hydrogenation of 14a and 14b with Et3SiH and BF3 OEt2 was not clean and
afforded mixture of inseparable compounds, which were not characterized. Thus, synthesis of
compounds 15a and 15b by ionic hydrogenation of 14a and 14b were not realized by this
procedure.
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Scheme 3. Attempted ionic hydrogenation of steroidal C-20 tertiary alcohols with n-vinyl
side chain
Reagents and conditions: (i) CH2=CHMgBr, THF, 0 ºC, 1 h, 12a (98%) and 12b (97%); (ii) n-
Bu4NF, THF, 30 ºC, 18 h, 94-96%; (iii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 93-95%; (iv) Et3SiH,
.
BF3 OEt2, CH2Cl2, 0 ºC, 10 min.
2.2. Stereoselective synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols
with furyl side chain
The furan based molecules are useful intermediates for further several types of reactions.31 The
furan ring can also be oxidized by a variety of reagents including singlet oxygen, dioxiranes,
peracids, metal oxides and depending on both the reagent and substitution pattern of the ring, to
give derivatives of high synthetic utility.32 The literature survey reveals that, there are several
bioactive steroids having furan ring in their skeleton.33 Construction of the steroidal side chain
with natural and unnatural configuration at C-20 position having furyl or thienyl moiety were
realized by t-BuLi mediated Wittig rearrangement. Kametani et al35 described synthesis of (22R)-
22,25-dihydroxycholesterol and its (22S)-isomer from pregnenolone acetate via catalytic
hydrogenation of C(20-21)-olefinic steroidal furyl derivative as a key reaction.
Reaction of 1a and 1b with 2-lithio furan (prepared in situ from furan and n-butyllithium
at -10 °C) gave the steroidal furyl derivatives 16a and 16b respectively, with C(20R) tertiary
alcohols and confirmed by the literature report (Scheme 4). The formation of these tertiary
alcohols 16a and 16b was established by the absence of the carbonyl peak and presence of peak
1
at 3450 cm-1 due to C-20 hydroxy group in IR spectroscopy. This was also confirmed by H
NMR data of crude compound 16b, in which, 7.31 (d, J = 2 Hz, 1H, furan-H), 6.29 (dd, J = 1.9
and 3.25 Hz, 1H, furan-H), 6.13 (dd, J = 0.7 and 2.1 Hz, 1H, furan-H) and 1.58 (s, 3H, 21-H3)
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signals. These compounds 16a and 16b are not stable at room temperature (30 °C) and on
standing for 18 h, dehydration of tertiary alcohol takesplace and led to C-20(21) olefinic
products 19a and 19b. Even at 0 °C, slow dehydration (72 h) of the compounds 16a and 16b to
19a and 19b, respectively.
Scheme 4. Synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols with furyl
side chain
.
Reagents and conditions: (i) Furan, n-BuLi, THF, -10 ºC, 30 min; (ii) Et3SiH, BF3 OEt2, DCM,
0 ºC, 10 min, 17a (94%) and 17b (95%); (iii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 18a (93%) and
18b (96%).
The crude tertiary alocohols 16a and 16b on ionic hydrogenation with Et3SiH and
.
BF3 OEt2 led to the formation of C-20 deoxygenated products, and deprotection of TBDMS
group also took place. This was purified by column chromatography on silica gel to obtain 17a
and 17b in 94% and 95% yields, respectively. Compounds 17a and 17b are stable in silica gel
and also at room temperature. The formation of compounds 17a and 17b were confirmed by
spectroscopic data, in which the C-20 methyl group in 17a was observed at 1.28 ppm and in 17b
at 1.26 ppm in the 1H NMR spectrum. The 3β-hydroxy functionality in compounds 17a and 17b
were subjected to acetylation furnished the products 18a and 18b respectively, in excellent yields
(93% and 96%). Compounds 16a and 16b on silica gel column chromatography purification
undergo dehydration to furnish 19a and 19b in 93% and 91% yields, respectively (Scheme 5).
The formation of C-20,21-ene compounds 19a and 19b has been confirmed by spectroscopic
1
data and elemental analysis. The H NMR spectrum for 19a shows two different signals at 5.69
and 5.06 ppm for 21-methylene protons and for compound 19b at 5.67 and 5.04 ppm. 13C NMR
spectrum for 19a shows signals at 137.1 (for C-20), 110.3 (for C-21) and for compound 19b
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shows at 137.2 (for C-20), 110.3 (for C-21), confirms the formation of C-20(21) olefinic bond.
The compounds 19a and 19b on TBDMS deprotection with n-Bu4NF in THF, yielded 20a and
20b in 91% and 93% yield, respectively. Acetylation of 20a and 20b with Ac2O and catalytic
amount DMAP, resulted into the 21a and 21b in excellent yields (94% and 96%, respectively).
Ionic hydrogenation of the compounds 21a and 21b with Et3SiH and CF3COOH in CH2Cl2
furnished 18a and 18b, respectively in almost quantitative yield (97% and 99%, respectively).
Scheme 5. Synthesis and ionic hydrogenation of steroidal C-20,21-enes with furyl side chain
Reagents and conditions: (i) SiO2, CH2Cl2, 19a (93%) and 19b (91%); (ii) n-Bu4NF, THF, 30
ºC, 18 h, 20a (91%) and 20b (93%); (iii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 21a (94%) and 21b
(96%); (iv) Et3SiH, CF3COOH, CH2Cl2, 30 ºC, 1 h, 18a (97%) and 18b (99%).
The formation of the same configuration in product 18a and 18b by ionic hydrogenation
of both C-20 tertiary alcohols 16a and 16b and steroidal C-20,21-ene containing furan side chain
21a and 21b are worth mentioning here. Ionic hydrogenation of compound 21a, is
chemoselective as the 5,6-double bond is unaffected. This is the first report of construction of C-
20 natural configuration by ionic hydrogenation of steroidal C-20,21-ene containing furan side
chain.
Taking advantage of the ionic hydrogenation methodology, compounds 18a and 18b
obtained from 16a and 16b and also from compounds 21a and 21b in excellent yields with
natural C-20 stereochemistry. These intermediates 18a and 18b are useful for the synthesis of
natural products and related analogues. Stereoselective synthesis of C-20 natural stereoisomer
with a 5-methylfuran side chain has been achieved and outlined in scheme 6. The compounds
22a and 22b, on treatment with 2-lithio-5-methylfuran (prepared in situ from 2-methylfuran and
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n-butyllithium at -10 °C) in THF to afford compounds 23a and 23b, respectively. As discussed
earlier, the stability of 20-hydroxy-furyl derivatives 23a and 23b are not stable and the crude
material was utilized for the next step. Compounds 23a and 23b on ionic hydrogenation with
.
Et3SiH and BF3 Et2O in CH2Cl2 at 0 °C afforded 24a and 24b respectively, in excellent yields
(96% and 95%). The acetylation of compounds 24a and 24b with Ac2O and catalytic amount of
DMAP in pyridine yielded 25a and 25b respectively, in 98% and 99% yields. The intermediate
25a was utilized for the synthesis of (22R)-22,25-dihydroxycholesterol and its 22S-isomer.[35]
Scheme 6. Ionic hydrogenation of steroidal C-20 tertiary alcohols with 5-methyl furyl side
chain
H
OH
O
O
O
H
H
H
(i)
(ii)
H
H
H
H
H
H
H
H
H
RO
(iii)
AcO
HO
5
a
=
=
24a, 24b,
25a, 25b,
R = H
22a, 22b
23a, 23b
b
5 -H
R = Ac
OH
O
H
H
OH
OH
HO
H
H
H
H
25a
H
H
H
H
AcO
(22R)-22,25-dihydroxycholesterol
Reagents and conditions: (i) 2-lithio-5-methylfuran, THF, -10 ºC, 30 min; (ii) Et3SiH,
.
BF3 OEt2, CH2Cl2, 0 ºC, 10 min, 24a (96%) and 24b (95%); (iii) Ac2O, Pyridine, DMAP, 25 ºC,
2 h, 25a (98%) and 25b (99%).
Bisnorcholenic/cholanic acids are the useful intermediates,36 for the synthesis of natural
products and biologically active compounds. The oxidative fragmentation of aromatic
compounds (furan, substituted phenyl) to carboxylic acids in the presence of catalytic amount of
RuO4 has found several applications in organic synthesis. The use of aromatic rings as
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carboxylic acid surrogates offers the chance to perform complex synthetic manipulations.
Heteroaromatic compounds can be oxidized under very mild conditions. Furyl substituents have
been frequently used as carboxylic acid surrogates in a variety of synthetic applications.37 On the
basis of these findings, we have carried out oxidation of the synthesized steroidal furyl
compound 18b with RuO4 (prepared in situ from RuCl3 and NaIO4) to obtain product 26 in very
good yield (Scheme 7). The transformation was carried out with excess NaIO4 (15 equiv) and
with 1% of RuCl3 in CCl4-MeCN-H2O (ratio 1:1:1.5). To the best of our knowledge, this is the
first example of the introduction of carboxylic acid functionality by RuO4 mediated oxidation of
furyl moiety in steroid chemistry.
Scheme 7. Synthesis of 3β-acetoxy-20(S)-5α-bisnorcholanic acid
Reagents and conditions: (i) RuCl3, NaIO4, CCl4-MeCN-H2O, rt, 10 min. 91%.
2.3. Stereoselective synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols
with thienyl side chain
Thiophene-steroid conjugates are biologically important molecules and intermediates leading to
natural products.38 In an earlier discussion, applications of ionic hydrogenation on steroidal C-20
tertiary alcohols with furyl side chain, herein we would like to compare these results with C-20
tertiary alcohols containing thienyl side chain. Grignard reaction of thiophene-2-magnesium
bromide on steroidal ketones 1a and 1b at 0 ºC lead to the steroidal thienyl derivatives 27a and
27b (Scheme 8). Although these tertiary alcohols 27a and 27b are stable at room temperature,
flash chromatographic purification of compounds 27a and 27b could not be realized, as these
compounds are not stable in silica gel column chromatography. The crude tertiary alcohols 27a
.
and 27b on ionic hydrogenation with Et3SiH and BF3 OEt2 led to the formation of deoxygenated
products, in which the deprotection of TBDMS group also takesplace. This was purified by
column chromatography on silica gel to obtain 28a and 28b in 68% and 65% yields,
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respectively. Compounds 28a and 28b were characterised by spectroscopic data, in which the C-
20 methyl group in 28a was observed at 1.34 ppm and in 28b at 1.33 ppm in 1H NMR spectrum.
The 3β-hydroxy functionality in 28a and 28b were subjected for acetylation with Ac2O and
DMAP to furnish the acetylated products 29a and 29b in excellent yields (98% and 95%).
Scheme 8. Synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols with
thienyl side chain
.
Reagents and conditions: (i) 2-bromothiophene, Mg, THF, 0 ºC, 1 h; (ii) Et3SiH, BF3 OEt2,
CH2Cl2, 0 ºC, 10 min, 28a (68%) and 28b (65%); (iii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 29a
(98%) and 29b (95%).
Deprotection of TBDMS group of compounds 27a and 27b by n-Bu4NF, followed by
silica gel mediated dehydration of C-20 tertiary alcohols, lead to the formation of C-20,21-ene
products 30a and 30b respectively, in good yields (Scheme 9). The formation of C-20,21-ene in
compounds 30a and 30b has been confirmed by spectroscopic data and by elemental analysis.
1
The H NMR spectrum for 30a shows two different signals at 5.48 and 5.05 ppm for 21-
13
methylene protons and compound 30b shows signals at 5.47 and 5.03 ppm. C NMR spectrum
for 30a shows signals at 141.7 (for C-20), 112.4 (for C-21) and compound 30b shows signal at
141.8 (for C-20), 112.3 (for C-21). The formation of C-20(21)-olefin affects on the 18-methyl
protons. 1H NMR spectrum for 30a shows signal at 0.62 ppm for 18-methyl protons and for 30b
at 0.59 ppm. This confirms the formation of C-20(21) olefinic bonds in compounds 30a and
30b. Acetylation of 30a and 30b with Ac2O and DMAP resulted in the formation of 31a and 31b
in excellent yields (96% and 95%).
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Scheme 9. Synthesis and ionic hydrogenation of steroidal C-20,21-enes with thienyl side
chain
Reagents and conditions: (i) (a) n-Bu4NF, THF, 30 ºC, 18 h; (b) SiO2, CH2Cl2, rt, 1 h, 30a
(67%) and 30b (70%); (ii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 31a (96%) and 31b (95%); (ii)
Et3SiH, CF3COOH, CH2Cl2, 30 ºC, 1 h, 29a (98%) and 29b (97%).
Ionic hydrogenation of 31a and 31b with Et3SiH and CF3COOH in CH2Cl2 furnished
compounds 29a and 29b respectively, in almost quantitative yield (98% and 97%). The
formation of the same compound by ionic hydrogenation of both C-20 tertiary alcohols and C-
20,21-ene containing thienyl side chain are worth mentioning here. Ionic hydrogenation of
compound 31a, is chemoselective as the 5,6-double bond is unaffected. Podlech et al reported,39
the degradation of substituted thiophene to the corresponding carboxylic acid. Compounds 29a
and 29b are may be useful intermediates for the elaboration of the side chain of natural products
and its analogues.
2.4. Ionic hydrogenation of steroidal C-20 tertiary alcohols with thiazolyl side chain
The thiazoles, furans, thiophenes and pyridines appended to the steroid nucleus were positioned
on the α-face and the β-face of the steroid, and the conjugated with a 16,17-olefin, to test their
ability to coordinate the heme iron of the P450 enzyme complex. The position of the heterocycle
with respect to the steroid skeleton was determined to be important for optimum affinity, and in
general, compounds with the heterocycle attached to a trigonal center at C-17, had the best
affinity for C17(20) lyase.40a Abiraterone acetate is a prodrug of abiraterone used for the treatment
of metastatic castration-resistant prostate cancer and an orally administrated CYP17 inhibitor.40b,c
The applications of ionic hydrogenation on steroidal C-20 tertiary alcohols with furyl and thienyl
as a side chains have been described above. We would like to compare these results with C-20
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tertiary alcohols containing thiazolyl side chain. It is well known that, thiazoles are the
precursors for the aldehyde functionality.41 With this in view, we have attempted to synthesize
steroidal C(20S) aldehydes 36a and 36b from C-20 oxo-compounds 1a and 1b. We have recently
reported42 the synthesis and antimicrobial evaluation of steroidal C-20 tertiary alcohols 32a-b,
33a-b and 34a-b from 1a and 1b. Grignard reaction of steroidal ketones 1a and 1b with thiazole-
2-magnesium bromide lead to C(20R) tertiary alcohols 32a and 32b in 98% and 96% yields,
respectively (Scheme 10). It can be mentioned here that the products 32a and 32b are stable and
purified by column chromatography over silica gel in excellent yield. Deprotection of TBDMS
group of 32a and 32b with n-Bu4NF, followed by selective acetylation of 3-hydroxy group of
33a and 33b, resulted in the formation of acetylated products 34a and 34b with overall yield of
90% and 91% in two steps, respectively. The ionic hydrogenation of compounds 34a and 34b
.
with triethylsilane and BF3 Et2O at varying temperature from 0 to 30 ºC, did not gave expected
deoxygenated products 35a and 35b and the starting material remain intact. Attempted
dehydration of the C-20 tertiary alcohols 35a and 35b with SOCl2-pyridine, POCl3-pyridine, p-
TSA in CH2Cl2 resulted in the recovery of starting material. This may be due to the electron poor
character of the thiazole ring, formation of Lewis base-Lewis acid pair with BF3, due to this lack
of formation of carbocation at C-20.
Scheme 10. Attempted ionic hydrogenation of steroidal C-20 tertiary alcohols with thiazolyl
side chain
Reagents and conditions: (i) thiazole-2-magnesium bromide, THF, 0 ºC, 1 h, 32a (98%) and
32b (96%); (ii) n-Bu4NF, THF, 30 ºC, 18 h, 33a (94%) and 33b (95%); (iii) Ac2O, Pyridine,
.
DMAP, 25 ºC, 2 h, 34a (95%) and 34b (94%); (iv) Et3SiH, BF3 OEt2, CH2Cl2, 0 ºC to 30 ºC.
15

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2.5. Stereoselective synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols
with phenyl and p-methoxy phenyl side chain
Hochberg reported43 the incubation of 20(R)-20-phenyl-pregn-5-en-3β,20-diol, an aromatic
analogue of 20(S)-20-hydroxycholesterol, with an adrenal mitochondrial preparation leads to the
formation of four compounds; pregnenolone, phenol, a C8 ketone (acetophenone), and a non
polar C19 compound. The substitution of an aromatic group on C-20 facilitates side chain
cleavage between the carbon atom and the nucleus whereas neither of the naturally occurring
precursors, cholesterol or its 20-hydroxylated counterpart, are metabolized C8 fragment.
Hochberg synthesized44 20-p-tolyl analogues of 20α-hydroxycholesterol and cholesterol and
tested as a precursors of pregnenolone. (20R)-20-(p-Tolyl)-pregn-5-en-3β,20-diol, when
incubated with an acetone powder preparation of bovine adrenal mitochondria forms
pregnenolone and progesterone in good yield. (20S)-20-(p-Tolyl)-pregn-5-en-3β-ol, prepared in
80% yield by Raney nickel hydrogenation of (20R)-20-(p-tolyl)-pregn-5-en-3β,20-diol, when
incubated in a similar fashion yields (20R)-20-(p-tolyl)-pregn-5-en-3β,20-diol in large quantity,
in addition to small amounts of pregnenolone and progesterone. Again, Hochberg described45 the
synthesis of various para- and meta-substituted phenyl groups attached at C-20 position via
either by Grignard reaction or organolithium reagents.
We have examined the effect of ionic hydrogenation on steroidal C-20 tertiary alcohols
with various side chain and C-20, 21-olefinic steroids. It would be interesting to compare ionic
hydrogenation results with that of C-20 tertiary alcohols containing phenyl and p-methoxyphenyl
side chain, and the obtained results are described here. Grignard reaction of phenyl magnesium
bromide on steroidal ketone 1a in THF at 0 °C afforded C-20 tertiary alcohol 37a (Scheme 11).
The isolation of this C-20 tertiary alcohol by flash chromatography was not successful, as the Rf
value of product and starting material was the same. The TBDMS group in compound 37a was
deprotected with n-Bu4NF in THF and the crude 3β-hydroxy compounds were purified by
column chromatography on silica gel to afford C-20 tertiary alcohol with epimeric mixture at C-
20. The separation of the two epimers has been achieved by fractional crystallization in
methanol. It was reported that, Grignard reaction of C-20 oxo-functionality with phenyl
magnesium bromide or phenyl lithium gave only one epimeric tertiary alcohol.43-45 In our hand,
Grignard reaction of 1a with phenyl magnesium bromide, furnished the C-20 epimeric tertiary
alcohols, 38aa and 38ab (65%) in 3:2 ratio (Scheme 11).
16

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Scheme 11. Synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols with
phenyl side chain
OH
H
OH
H
OH
H
(i)
(ii)
H
H
1a, 1b
+
H
H
H
H
H
H
TBDMSO
RO
(iii)
RO
37a, 37b
38ab, 38bb, R = H
39ab, 39bb, R = Ac
38aa, 38ba, R = H
39aa, 39ba, R = Ac
(iii)
H
H
(iv)
HO
H
37a/38aa/38ab/38aa+38ab, 37b/38ba/38bb/38ba+38bb
H
H
40a, 40b
H
H
(iv)
H
39aa/39ab, 39ba/39bb
H
H
AcO
41a, 41b
Reagents and conditions: (i) PhMgBr, THF, 0 ºC, 1 h; (ii) n-Bu4NF, THF, 30 ºC, 18 h, 65-68%
in two steps; (iii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 39aa (95%), 30ab (97%), 39ba (96%) and
.
39bb (95%); (iv) Et3SiH, BF3 OEt2, CH2Cl2, 0 ºC, 10 min, 40a (94%), 41a (95%), 40b (91%)
and 41b (91%).
In a similar way, Grignard reaction of phenyl magnesium bromide with steroidal ketone
1b in THF, followed by TBDMS deprotection of compound 37b led to the formation of C-20
epimeric alcohols. The two C-20 epimers 38ba and 38bb (63%) in 3:2 ratios were separated by
crystallization from methanol. The formation of two C-20 epimers by Grignard reaction has been
confirmed by spectroscopic data. The spectroscopic data of compound 38aa is similar to that of
1
reported.43-45 In the H NMR spectrum (in CDCl3) of compound 38aa, the signal appeared at δ
1.67 was assigned to 21-H3, whereas for compound 38ab the signal at δ 1.49 ppm. In 13C NMR
17

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spectrum (in CDCl3) the signal appeared for compound 38aa, at δ 76.5 was assigned to C-20,
whereas for compound 38ab the signal for C-20 at δ 76.1 ppm.
1
Similar type of difference was observed in compounds 38ba and 38bb. In the H NMR
spectrum (in CDCl3) of compound 38ba, the signal at δ 1.66 was assigned to 21-H3, whereas for
13
compound 38bb the signal appeared at δ 1.48 ppm. In C NMR spectrum (in CDCl3+CD3OD)
of compound 38ba, the signal at δ 76.2 was assigned to C-20, whereas for compound 38bb the
signal appeared at δ 75.7 ppm for C-20. It also affects on 18- and 19-methyl protons in 1H NMR
spectrum. This is the first report of the isolation and characterization of both the isomers
obtained from Grignard reaction of phenyl magnesium bromide on 20-oxo steroids.
Selective acetylation of 3-OH group of C-20 tertiary alcohol with phenyl side chain 38aa,
38ab, 38ba and 38bb with Ac2O and catalytic amount of DMAP, afforded compounds 39aa,
39ab, 39ba and 39bb respectively, in very good yields (95-97%) (Scheme 11). There are several
.
reports46 on the ionic hydrogenation of benzylic alcohols with Et3SiH and BF3 Et2O to the
corresponding hydrocarbons. All the steroidal C-20 tertiary alcohols 38aa, 38ab, 38ba, 38bb,
39aa, 39ab, 39ba and 39bb are at benzylic position. Ionic hydrogenation of C(20R)-hydroxy
.
compound with phenyl side chain 38aa using Et3SiH and BF3 Et2O afforded compound 40a in
excellent yield. Again, the ionic hydrogenation of C(20S)-hydroxy compound 38ab gave the
same compound 40a in good yield. Similarly, we have carried out the ionic hydrogenation of
TBDMS protected compound 37a and the mixture of 38aa and 38ab led to the formation of
same compound 40a in excellent yield. Here, deprotection of 3β-TBDMS group in compound
37a and deoxygenation of C-20 tertiary alcohol took place. There are reports26 on the
stereoselective ionic hydrogenation of C-7 epimeric alcohols into single isomer. It can be noted
here that, the ionic hydrogenation of C-20 tertiary alcohols with phenyl side chain (either
separate isomer or mixture of the isomers at C-20) lead to the formation of single product in
excellent yield (91-95%) for all the cases. Exclusive formation of the natural C(20S)-methyl
compound 40a by ionic hydrogenation of the C-20 tertiary alcohol 37a, 38aa and 38ab is
confirmed by 1H NMR, in which a single C-21 methyl at δ 1.27 ppm (d, J = 6 Hz) and a single
methyl signal at δ 22.1 ppm in the 13C NMR spectrum was observed. The 3β-acetylated
.
compounds 39aa and/or 39ab on ionic hydrogenation with Et3SiH and BF3 Et2O afforded a
single compound 41a in excellent yield. Similarly, ionic hydrogenation of steroidal tertiary
alcohols 37b, 38ba, 38bb or the mixture of compounds 38ba and 38bb with Et3SiH and
18

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.
BF3 Et2O afforded a single compound 40b in excellent yield (Scheme 11). Under similar reaction
conditions, ionic hydrogenation of acetate derivatives 39ba and/or 39bb resulted into the
formation of compound 41b in very good yield. It can be mentioned here, both epimeric alcohols
.
on treatment with BF3 Et2O gave carbocation at C-20, followed by hydride transfer from Et3SiH
from the less hindered side of steroid backbone, which leads to the formation of C-20 natural
configuration with 100% stereoselectivity.
Same methodology has been applied for the synthesis of steroidal C-20 tertiary alcohols
with p-methoxy phenyl side chain. Grignard reaction of p-methoxyphenyl magnesium bromide
(generated in situ from p-bromoanisole and magnesium in THF at 0 °C) on steroidal ketone 1a in
THF at 0 °C afforded C-20 tertiary alcohol 42a (Scheme 12). The TBDMS group in compound
42a was deprotected with n-Bu4NF and the crude 3β-hydroxy compounds were purified by
column chromatography on silica gel to afford C-20 tertiary alcohol with epimeric mixture at C-
20. The separation of the two epimers has been achieved by fractional crystallization in
methanol. In our hand, Grignard reaction of 1a with p-methoxyphenyl magnesium bromide,
furnished the C-20 epimeric tertiary alcohols, 43aa and 43ab (65%) in 3:2 ratio. The formation
of two C-20 epimers by Grignard reaction has been confirmed by spectroscopic data. The
spectroscopic data of compound 43aa is similar to that reported in literature.43-45 In the 1H NMR
spectrum (in CDCl3) of compound 43aa, the signal appeared at δ 1.64 was assigned to 21-H3,
whereas for compound 43ab the signal observed at δ 1.46 ppm. In 13C NMR spectrum (in
CDCl3+CD3OD) of compound 43aa, the signal appeared at δ 76.0 was assigned to C-20,
whereas for compound 43ab the signal was observed at δ 75.8 ppm. It also affects on 18- and 19-
1
methyl protons in H NMR spectrum. Selective acetylation of 3-OH group of C-20 tertiary
alcohol with p-methoxyphenyl side chain 43aa and 43ab with Ac2O and afforded compounds
44aa and 44ab respectively in excellent yields.
Ionic hydrogenation of C(20R)-hydroxy compound with p-methoxyphenyl side chain
.
43aa using Et3SiH and BF3 OEt2 afforded compound 45a in excellent yield with natural
configuration at C-20. Again, the ionic hydrogenation of C(20S)-hydroxy compound 43ab gave
the same compound 45a in good yield. Similarly, we have carried out the ionic hydrogenation of
TBDMS protected compound 42a and the mixture of 43aa and 43ab, led to the formation of
same compound 45a in excellent yield. Under similar reaction conditions, ionic hydrogenation of
19

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acetate derivatives 44aa and 44ab resulted into the formation of compound 46a in very good
yield.
Scheme 12. Synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols with p-
methoxy phenyl side chain
Reagents and conditions: (i) p-methoxyphenyl magnesium bromide, THF, 0 ºC, 1 h; (ii) n-
Bu4NF, THF, 30 ºC, 18 h, 65% in two steps; (iii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 44aa
.
(97%) and 44ab (95%); (iv) Et3SiH, BF3 OEt2, CH2Cl2, 0 ºC, 10 min, 45a (92%) and 46a (95%).
Again, Grignard reaction of p-methoxyphenyl magnesium bromide on steroidal ketone
1b in THF at room temperature for longer period (10 h) and the reaction mixture was quenched
by saturated ammonium chloride, kept for 2 h, followed by TBDMS group deprotection resulted
into the formation of compounds 47 and 48 in 42% and 23%, respectively over two steps
(Scheme 13). The formation of C-20,21-ene compound 47 has been confirmed by spectroscopic
data and by elemental analysis. The 1H NMR spectrum for 47 shows two different signals at 5.12
13
and 5.01 ppm for 21-methylene protons. C NMR spectrum for 47 shows signals at 137.6 (for
20

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C-20), 112.2 (for C-21) confirms the formation of C-20(21) olefinic bond. The formation of
1
C(20S) tertiary alcohol 48 has been confirmed by H NMR spectrum (in CDCl3), the signal
13
appeared at δ 1.45 ppm was assigned to 21-H3. In C NMR spectrum (in CDCl3) of compound
48, the signal at δ 75.8 was assigned to C-20. Acetylation of 47 and 48 with Ac2O and catalytic
amount of DMAP resulted 49 and 50 in excellent yields (97% and 94% respectively).
Scheme 13. Synthesis and ionic hydrogenation of steroidal C-20 tertiary alcohols and C-
20,21-ene with p-methoxy phenyl side chain
Reagents and conditions: (i) (a) p-methoxyphenyl magnesium bromide, THF, rt, 10 h; (b) n-
Bu4NF, THF, 30 ºC, 18 h, 65% in two steps; (ii) Ac2O, Pyridine, DMAP, 25 ºC, 2 h, 49 (97%)
.
and 50 (95%); (iii) Et3SiH, BF3 OEt2, CH2Cl2, 0 ºC, 10 min. 51 (92%) and 52 (95%); (iv) Et3SiH,
CF3COOH, CH2Cl2, rt, 1 h, 99%.
.
Ionic hydrogenation of 48 and 50 with Et3SiH and BF3 OEt2 resulted into the formation of
compounds 51 and 52 respectively in very good yields (92% of 51 and 95% of 52) (Scheme 13).
The ionic hydrogenation of compound 49 with triethylsilane and trifluoroacetic acid in
dichloromethane furnished compound 52 in almost quantitative yield (99%). The formation of
the same C(20S) natural configuration product 52 by ionic hydrogenation of C-20 tertiary
alcohol 50 and C-20,21-ene containing p-methoxyphenyl side chain 49 is worth mentioning
here.
21

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2.6. Ionic hydrogenation of steroidal C-20 tertiary alcohols with pyridyl side chain
The applications of ionic hydrogenation on steroidal C-20 tertiary alcohols with thiophene, furan,
thiazole, phenyl and p-methoxyphenyl as a side chains have been described. We would like to
compare these results with C-20 tertiary alcohols containing pyridine side chain. We have
recently reported42 the synthesis and antimicrobial evaluation of steroidal C-20 tertiary alcohols
53a-b, 54a-b and 55a-b from 1a and 1b. Grignard reaction of steroidal ketones 1a and 1b with
pyridyl-2-magnesium bromide lead to the formation of C(20R) tertiary alcohols 53a and 53b in
61% and 59% yields respectively (Scheme 14). Deprotection of TBDMS group of 53a and 53b,
selective acetylation of 3-hydroxy group of 54a and 54b resulted in the formation of acetylated
products 55a and 55b respectively. The ionic hydrogenation of compounds 55a and 55b with
.
triethylsilane and BF3 Et2O at varying temperature from 0 to 30 ºC, did not give the expected
deoxygenated products 56a and 56b and the starting material remain intact. This may be due to
the electron poor character of the pyridine ring, formation of Lewis base-Lewis acid pair with
BF3, due to this lack of formation of carbocation at C-20.
Scheme 14. Attempted Ionic hydrogenation of steroidal C-20 tertiary alcohols with
pyridine side chain
H
OH
H
N
N
H
(iv)
H
(i)
H
X
1a, 1b
H
H
H
H
AcO
RO
53a, 53b,
R = TBDMS
56a, 56b
(ii)
(iii)
54a, 54b, R = H
55a,55b,
R = Ac
Reagents and conditions: (i) 2-Pyridyl magnesium bromide, THF, 0 ºC, 1 h, 53a (61%) and 53b
(59%); (ii) n-Bu4NF, THF, 30 ºC, 18 h, 54a (93%) and 54b (91%); (iii) Ac2O, Pyridine, DMAP,
.
25 ºC, 2 h, 55a (95%) and 55b (93%); (iv) Et3SiH, BF3 OEt2, CH2Cl2, 0 to 30 ºC, 30 min.
22

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3. Conclusions
In summary, we have achieved the stereoselective synthesis of steroidal C-20 tertiary alcohols
with n-butyl, vinyl, furan, methylfuran, thiophene, thiazole, phenyl, p-methoxyphenyl and
pyridine side chain by Grignard reaction or organolithium reagents with C-20 keto compounds.
The C-20 epimers (obtained by Grignard reaction of phenyl and p-methoxyphenyl magnesium
bromide) were separated by fractional crystallization. Ionic hydrogenation of these C-20 tertiary
alcoholic compounds with furyl, methylfuryl, thienyl, phenyl and p-methoxyphenyl side chains
resulted into deoxygenated product with C-20(S) natural configuration in 100% stereoselectivity.
Ionic hydrogenation of steroidal C-20,21-ene compounds with furan, thiophene and p-
methoxyphenyl side chain with triethylsilane and trifluoroacetic acid gave the saturated
compounds with natural stereochemistry at C-20. However, the alkyl, thiazolyl and pyridyl
incorporated steroidal C-20 tertiary alcohols were failed under the same reaction condition.
These findings have the potential to greatly simplify access to structural analogues of important
steroid scaffold for applications in synthetic, biological, pharmaceutical and medical sciences.
4. Experimental section
4.1. General Information
All melting points were determined on Yanco Micro melting point apparatus and are
uncorrected. Optical rotations were obtained on Bellingham and Stanly ADP-220 Polarimeter.
Yields refer to chromatographically and spectroscopically (1H and 13C NMR) homogeneous
materials, unless otherwise stated. Reactions were monitored by thin-layer chromatography
(TLC) using TLC aluminium sheets, silica gel 60-F254 precoated, Merck, Germany and locating
the spots using UV light as the visualizing agent or spraying with ethanolic phosphomolybdic
acid (PMA) solution followed by heating. Flash column chromatography was carried out with
silica gel (300-400 mesh). Preparative thin-layer chromatography separations were carried out on
1
13
0.25 mm E Merck silica gel plates (60-F254). H and C NMR spectra were recorded on Bruker
AC-200 (200 MHz) at 200.13 and 50.32 and on a Bruker MSL-300 (300 MHz) at 300.13 and
75.47 or on a Bruker DRX-500 (500 MHz) spectrophotometer at 500.13 and 125.78,
respectively. Chemical shifts are given in δ values relative to TMS (tetramethylsilane) as internal
standard. IR spectra were recorded on Shimadzu 8400 series FTIR instrument and values are
reported in cm-1 unit. Specific rotations ([α]D) are reported in deg/dm and the concentration (c)
23

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is given in g/100 ml in the specific solvent. Mass spectra were recorded by either LC-MS or MS-
TOF API QSTAR PULSAR spectrophotometer, samples introduced by infusion method using
Electrospray Ionization Technique. Elemental analyses were performed by CHNS-O EA 1108-
Elemental analyzer, Carloerba Instrument (Italy) or Elementor Vario EL (Germany) and were
within ± 0.4% of calculated values.
4.2. General Procedure for synthesis of compounds 16a, 16b, 23a and 23b:
In a two-necked round bottom flask equipped with a magnetic needle and a septum, furan or 2-
methylfuran (5 mmol) was placed in dry tetrahydrofuran (5 mL). The solution was cooled to -10
ºC and n-BuLi in hexane (1.7 M, 3 mL, 5 mmol) was added to it dropwise under argon
atmosphere. It was stirred at that temperature for a period of half hour. To it steroidal ketones 1a
and 1b (2 mmol) in anhydrous tetrahydrofuran (15 mL) was added dropwise at -10 ºC under
argon atmosphere. After the completion of the reaction, it was quenched with crushed ice and
THF was removed under reduced pressure. The residue was extracted with ethyl acetate (2 x 100
mL). The organic layer was washed with water (2 x 25 mL), brine (2 x 25 mL), dried over
Na2SO4. Solvent was removed under reduced pressure afforded crude material.
4.3. 3β-tert-Butyldimethylsilyloxy-(20R)-20-hydroxy-20-(2-furyl)-5α-pregnane (16b):
IR (Nujol, cm-1): 3450 (-OH). 1H NMR (200 MHz, CDCl3) δ = 7.31 (d, J = 2 Hz, 1H, furan-H),
6.29 (dd, J = 1.9 and 3.25 Hz, 1H, furan-H), 6.13 (dd, J = 0.7 and 2.1 Hz, 1H, furan-H), 3.54 (m,
1H, 3-H), 1.58 (s, 3H, 21-H3), 0.88 (s, 9H, SiCMe3), 0.77 (s, 3H, 19-H3), 0.70 (s, 3H, 18-H3),
0.04 (s, 6H, SiMe2).
4.4. 3β-Hydroxy-20(S)-20-(2-furyl)-pregn-5-ene (17a):
The solution of steroidal crude tertiary alcohol 16a in dichloromethane (15 mL) was cooled to 0
°C. To it triethylsilane (1.92 mL, 12 mmol) was added, stirred for 10 minutes and then
borontrifluoride-diethyl etherate (2.52 mL, 20 mmol) in dichloromethane (10 mL) was added
dropwise. The mixture was stirred for 10 minutes, 10% sodium bicarbonate (20 mL) was added.
The reaction mixture was extracted with dichloromethane (2 × 100 mL). The combined organic
extracts were washed with brine (2 × 25 mL), dried over Na2SO4 and concentrated under reduced
pressure to afford crude compound (0.718 g). Flash chromatographic purification over silica gel
24

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using ethyl acetate-petroleum ether as an eluent afforded 3β-hydroxy-20(S)-20-(2-furyl)-pregn-
1
5-ene 17a (0.692 g, 94%) as a colourless solid. Mp: 91-93 °C (Methanol). H NMR (200 MHz,
CDCl3) δ = 7.26 (d, J = 2 Hz, 1H, furan-H), 6.25 (dd, J = 1.9 and 3 Hz, 1H, furan-H), 5.91 (d, J
= 2.4 Hz, 1H, furan-H), 5.35 (d, J = 4.8 Hz, 1H, 6-H), 3.53 (m, 1H, 3-H), 2.74 (m, 1H, 20-H),
13
1.28 (d, J = 6.0 Hz, 3H, 21-H3), 1.03 (s, 3H, 19-H3), 0.75 (s, 3H, 18-H3). C NMR (100 MHz,
acetone-d6) δ = 161.9 (C), 142.3 (C), 141.1 (CH), 121.5 (CH), 110.6 (CH), 104.2 (CH), 71.6
(CH), 57.5 (CH), 56.0 (CH), 51.2 (CH), 43.3 (CH2), 43.0 (C), 40.4 (CH2), 38.2 (CH), 37.3 (C),
36.9 (CH), 32.8 (CH), 32.5 (CH2), 32.4 (CH2), 28.8 (CH2), 24.7 (CH2), 21.8 (CH2), 20.3 (CH3),
19.8 (CH3), 12.1 (CH3). MS (LCMS) m/z: 368.2 (M+), 391.3 (M+Na). Anal. Calcd for C25H36O2:
C, 81.47; H, 9.85; Found: C, 81.33; H, 9.57.
4.5. 3β-Aceoxy-20(S)-20-(2-furyl)-pregn-5-ene (18a):
To the solution of 3β-hydroxy-20(S)-20-(2-furyl)-pregn-5-ene 17a (0.37 g, 1 mmol) in dry
pyridine (2mL), was added acetic anhydride (0.2 mL, 2 mmol) and catalytic amount of DMAP
(0.02 g, 0.2 mmol). The reaction mixture was stirred at 25 °C for 2 h, quenched with crushed ice
and extracted with ethyl acetate (2 × 100 mL). The combined organic extracts were washed with
brine (25 mL), dried over Na2SO4. The solvent was removed under reduced pressure to give
crude compound 18a (0.4 g). Column chromatographic purification over silica gel using ethyl
acetate-petroleum ether (6:94, Rf 0.5, 10% EA/PE) as a eluent afforded pure product 18a (0.38 g,
93%) as a colourless solid. Mp: 141-143 °C. 1H NMR (200 MHz, CDCl3) δ = 7.26 (d, J = 2 Hz,
1H, furan-H), 6.25 (dd, J = 1.9 and 3 Hz, 1H, furan-H), 5.91 (d, J = 2.4 Hz, 1H, furan-H), 5.37
(d, J = 4.8 Hz, 1H, 6-H), 4.63 (m, 1H, 3-H), 2.74 (m, 1H, 20-H), 2.04 (s, 3H, OCOCH3), 1.27 (d,
J = 6.0 Hz, 3H, 21-H3), 1.03 (s, 3H, 19-H3), 0.75 (s, 3H, 18-H3). 13C NMR (100 MHz, CDCl3) δ
= 170.5 (C), 161.4 (C), 140.0 (C), 139.6 (C), 122.6 (CH), 109.7 (CH), 103.2 (CH), 73.9 (CH),
55.6 (CH), 55.1 (CH), 50.0 (CH), 42.3 (C), 39.5 (CH2), 38.1 (CH2), 37.0 (CH2), 36.6 (C), 36.2
(CH), 31.9 (CH), 31.8 (CH2), 29.7 (CH2), 28.2 (CH2), 27.8 (CH2), 24.1 (CH2), 21.4 (CH3), 21.0
(CH2), 19.9 (CH3), 19.3 (CH3), 11.8 (CH3). MS (LCMS) m/z: 433.3 (M+Na). Anal. Calcd for
C27H38O3: C, 78.98; H, 9.33; Found: C, 78.74; H, 9.38.
25

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4.6. 3β-Hydroxy-20(S)-20-(2-furyl)-5α-pregnane (17b):
Compound 17b was prepared from 16b using the same procedure for synthesis of 17a from 16a.
Yield: 95%. Mp: 151-153 °C (Methanol). 1H NMR (200 MHz, CDCl3) δ = 7.26 (d, J = 2 Hz, 1H,
furan-H), 6.25 (dd, J = 1.9 and 3 Hz, 1H, furan-H), 5.91 (d, J = 2.4 Hz, 1H, furan-H), 3.60 (m,
1H, 3-H), 2.74 (m, 1H, 20-H), 1.26 (d, J = 6.0 Hz, 3H, 21-H3), 0.82 (s, 3H, 19-H3), 0.72 (s, 3H,
13
18-H3). C NMR (50 MHz, CDCl3) δ = 161.5 (C), 140.0 (C), 109.7 (CH), 103.2 (CH), 71.3
(CH), 56.3 (CH), 54.3 (CH), 44.8 (CH), 42.5 (C), 39.8 (CH2), 38.1 (CH2), 36.9 (CH2), 36.2
(CH), 35.5 (CH), 35.4 (C), 32.0 (CH2), 31.4 (CH2), 28.7 (CH2), 28.2 (CH2), 24.0 (CH2), 21.2
(CH2), 19.8 (CH3), 12.3 (CH3), 12.0 (CH3). MS (LCMS) m/z: 370.2 (M+), 393.3 (M+Na). Anal.
Calcd for C25H38O2: C, 81.03; H, 10.34; Found: C, 80.87; H, 10.18.
4.7. 3β-Acetoxy-20(S)-20-(2-furyl)-5α-pregnane (18b):
Compound 18b was prepared from 17b using the same procedure for synthesis of 18a from 17a.
1
Yield: 96%. Mp: 145-146 °C. H NMR (200 MHz, CDCl3) δ = 7.26 (d, J = 2 Hz, 1H, furan-H),
6.24 (dd, J = 1.9 and 3 Hz, 1H, furan-H), 5.89 (d, J = 2.4 Hz, 1H, furan-H), 4.69 (m, 1H, 3-H),
2.74 (m, 1H, 20-H), 2.02 (s, 3H, OCOCH3), 1.25 (d, J = 6.0 Hz, 3H, 21-H3), 0.83 (s, 3H, 19-H3),
13
0.71 (s, 3H, 18-H3). C NMR (50 MHz, CDCl3) δ = 170.7 (C), 161.5 (C), 140.0 (C), 109.7
(CH), 103.2 (CH), 73.7 (CH), 56.3 (CH), 55.2 (CH), 54.2 (CH), 44.6 (CH), 42.5 (C), 39.7 (CH2),
36.7 (CH2), 36.2 (CH), 35.5 (CH), 35.5 (C), 34.0 (CH2), 31.9 (CH2), 28.5 (CH2), 28.2 (CH2),
27.4 (CH2), 24.0 (CH2), 21.4 (CH3), 21.2 (CH2), 19.8 (CH3), 12.2 (CH3), 12.0 (CH3). MS (LC-
MS) m/z: 435.3 (M+Na). Anal. Calcd for C27H40O3: C, 78.6; H, 9.77; Found: C, 78.47; H, 9.68.
4.8. 3β-tert-Butyldimethylsilyloxy-20-(2-furyl)-pregn-5,20(21)-diene (19a):
The solution of steroidal crude tertiary alcohol 16a in dichloromethane (10 mL) was added silica
gel (2 g) and stirred at room temperature for 1 h. The reaction mixture was filtered and the
solvent was removed under reduced pressure to afford the crude compound 19a. Flash
chromatographic purification over silica gel using ethyl acetate-petroleum ether as an eluent
afforded 3β-tert-Butyldimethylsilyloxy-20-(5-methyl-2-furyl)-pregn-5,20(21)-diene 19a (93%)
1
as a colourless solid. Mp: 121-123 °C. H NMR (200 MHz, CDCl3) δ = 7.34 (d, J = 2 Hz, 1H,
furan-H), 6.27-6.36 (m, 2H, furan-H), 5.69 (s, 1H, 21-Ha), 5.34 (d, J = 6 Hz, 6-H), 5.06 (s, 1H,
21-Hb), 3.49 (m, 1H, 3-H), 2.65 (t, 1H, 17-H), 0.99 (s, 3H, 19-H3), 0.90 (s, 9H, SiCMe3), 0.61 (s,
26

ACCEPTED MANUSCRIPT
13
3H, 18-H3), 0.07 (s, 6H, SiMe2). C NMR (50 MHz, CDCl3) δ = 156.3 (C), 141.6 (C), 141.3
(CH), 137.1 (C), 121.0 (CH), 110.8 (CH), 110.3 (CH2), 106.0 (CH), 72.6 (CH), 56.9 (CH), 52.0
(CH), 50.4 (CH), 43.1 (C), 42.8 (CH2), 38.6 (CH2), 37.4 (CH2), 36.7 (C), 32.5 (CH), 32.0 (CH2),
31.9 (CH2), 26.2 (CH2), 25.9 (3 x CH3), 24.3 (CH2), 21.1 (CH2), 19.4 (CH3), 18.3 (C), 12.8
(CH3), -4.6 (2 x CH3). MS (LCMS) m/z: 503.3 (M+Na). Anal. Calcd for C31H48O2Si: C, 77.44;
H, 10.06; Found: C, 77.31; H, 9.87.
4.9. 3β-tert-Butyldimethylsilyloxy-20-(5-methyl-2-furyl)-5α-pregn-20(21)-ene (19b):
Compound 19b was prepared from 16b using the same procedure for synthesis of 19a from 16a.
1
Yield: 91%. Mp: 123-124 °C. H NMR (200 MHz, CDCl3) δ = 7.33 (d, J = 2 Hz, 1H, furan-H),
6.29-6.35 (m, 2H, furan-H), 5.67 (s, 1H, 21-Ha), 5.04 (s, 1H, 21-Hb), 3.54 (m, 1H, 3-H), 2.63 (t,
1H, 17-H), 0.88 (s, 9H, SiCMe3), 0.78 (s, 3H, 19-H3), 0.58 (s, 3H, 18-H3), 0.07 (s, 6H, SiMe2).
13C NMR (100 MHz, CDCl3) δ = 156.4 (C), 141.3 (CH), 137.2 (C), 110.8 (CH), 110.3 (CH2),
106.0 (CH), 72.0 (CH), 56.6 (CH), 52.1 (CH), 45.1 (CH), 43.4 (C), 38.8 (CH2), 38.7 (CH2), 37.2
(CH2), 36.2 (CH), 35.6 (C), 32.1 (CH), 31.9 (CH2), 28.8 (CH2), 26.3 (CH2), 25.9 (3 x CH3), 24.2
(CH2), 21.3 (CH2), 18.3 (C), 13.0 (CH3), 12.4 (CH3), -4.6 (2 x CH3). MS (LCMS) m/z: 503.3
(M+Na). Anal. Calcd for C31H50O2Si: C, 77.12; H, 10.44; Found: C, 77.01; H, 9.96.
4.10. 3β-Hydroxy-20-(2-furyl)-pregn-5,20(21)-diene (20a):
To a solution of 3β-tert-Butyldimethylsilyloxy-20-(2-furyl)-pregn-5,20(21)-diene 19a (0.48 g, 1
mmol) in anhydrous THF (10 mL) and n-tetrabutylammonium fluoride in THF (2 mL, 2 mmol)
was added and the reaction mixture was stirred at 30 °C for 18 h and then quenched with
aqueous ammonium chloride. THF was removed under vacuuo and extracted with ethyl acetate
(2 × 150 mL). The combined organic extracts were washed with brine (25 mL), dried over
Na2SO4, concentrated under reduced pressure afforded 0.41 g of crude compound 20a. Column
chromatographic purification of crude product over silica gel using ethyl acetate-petroleum ether
(20:80, Rf 0.4, 20% EA/PE) as eluent afforded 20a (0.33 g, 91%) as a colourless solid. Mp: 102-
1
103 °C. H NMR (200 MHz, CDCl3) δ = 7.34 (d, J = 2 Hz, 1H, furan-H), 6.27-6.36 (m, 2H,
furan-H), 5.69 (s, 1H, 21-Ha), 5.37 (d, J = 6 Hz, 6-H), 5.06 (s, 1H, 21-Hb), 3.53 (m, 1H, 3-H),
13
2.65 (t, 1H, 17-H), 1.00 (s, 3H, 19-H3), 0.61 (s, 3H, 18-H3). C NMR (100 MHz, CDCl3) δ =
27

ACCEPTED MANUSCRIPT
156.3 (C), 141.3 (CH), 140.8 (CH), 137.1 (C), 121.5 (CH), 110.8 (CH), 110.3 (CH2), 106.0
(CH), 71.7 (CH), 56.8 (CH), 51.9 (CH), 50.3 (CH), 43.0 (C), 42.2 (CH2), 38.5 (CH2), 37.2
(CH2), 36.5 (C), 31.8 (CH), 31.5 (CH2), 26.2 (CH2), 24.2 (CH2), 21.1 (CH2), 19.4 (CH3), 12.7
(CH3). MS (LCMS) m/z: 389.3 (M+Na). Anal. Calcd for C25H34O2: C, 81.92; H, 9.35; Found: C,
81.76; H, 9.18.
4.11. 3β-Hydroxy-20-(2-furyl)-5α-pregn-20(21)-ene (20b):
Compound 20b was prepared from 19b using the same procedure for synthesis of 20a from 19a.
1
Yield: 93%. Mp: 113-115 °C. H NMR (200 MHz, CDCl3) δ = 7.34 (d, J = 2 Hz, 1H, furan-H),
6.30-6.35 (m, 2H, furan-H), 5.68 (s, 1H, 21-Ha), 5.05 (s, 1H, 21-Hb), 3.60 (m, 1H, 3-H), 2.64 (t,
1H, 17-H), 0.80 (s, 3H, 19-H3), 0.59 (s, 3H, 18-H3). 13C NMR (100 MHz, CDCl3) δ = 156.3 (C),
141.3 (CH), 137.2 (C), 110.8 (CH), 110.3 (CH2), 106.0 (CH), 71.3 (CH), 56.6 (CH), 54.5 (CH),
52.0 (CH), 44.9 (CH), 43.3 (C), 38.7 (CH2), 38.1 (CH2), 37.0 (CH2), 36.1 (CH), 35.5 (C), 32.0
(CH2), 31.4 (CH2), 28.7 (CH2), 26.2 (CH2), 24.2 (CH2), 21.3 (CH2), 12.9 (CH3), 12.4 (CH3). MS
(LCMS) m/z: 391.3 (M+Na). Anal. Calcd for C25H36O2: C, 81.47; H, 9.85; Found: C, 81.51; H,
9.79.
4.12. 3β-Acetoxy-20-(2-furyl)-pregn-5,20(21)-diene (21a):
Compound 21a was prepared from 20a using the same procedure for synthesis of 18a from 17a.
Yield: 94%. 1H NMR (200 MHz, CDCl3) δ = 7.34 (d, J = 2 Hz, 1H, furan-H), 6.27-6.36 (m, 2H,
furan-H), 5.69 (s, 1H, 21-Ha), 5.37 (d, J = 6 Hz, 6-H), 5.06 (s, 1H, 21-Hb), 4.67 (m, 1H, 3-H),
13
2.64 (t, 1H, 17-H), 2.01 (s, 3H, OCOCH3), 1.00 (s, 3H, 19-H3), 0.62 (s, 3H, 18-H3). C NMR
(50 MHz, CDCl3) δ = 170.7 (C), 156.3 (C), 141.3 (CH), 140.8 (CH), 137.2 (C), 121.4 (CH),
110.8 (CH), 110.3 (CH2), 106.0 (CH), 73.7 (CH), 56.7 (CH), 51.9 (CH), 50.3 (CH), 43.0 (C),
42.2 (CH2), 38.5 (CH2), 37.2 (CH2), 36.5 (C), 31.8 (CH), 31.4 (CH2), 26.2 (CH2), 24.2 (CH2),
21.4 (CH3), 21.2 (CH2), 19.4 (CH3), 12.8 (CH3). MS (LCMS) m/z: 431.3 (M+Na). Anal. Calcd
for C27H36O3: C, 79.37; H, 8.88; Found: C, 79.16; H, 8.67.
4.13. 3β-Acetoxy-20-(2-furyl)-5α-pregn-20(21)-ene (21b):
Compound 21b was prepared from 20b using the same procedure for synthesis of 18a from 17a.
1
Yield: 96%. Mp: 80-82 °C. H NMR (200 MHz, CDCl3) δ = 7.33 (d, J = 2 Hz, 1H, furan-H),
28

ACCEPTED MANUSCRIPT
6.29-6.35 (m, 2H, furan-H), 5.67 (s, 1H, 21-Ha), 5.04 (s, 1H, 21-Hb), 4.68 (m, 1H, 3-H), 2.64 (t,
1H, 17-H), 0.81 (s, 3H, 19-H3), 0.58 (s, 3H, 18-H3). 13C NMR (100 MHz, CDCl3) δ = 170.7 (C),
156.3 (C), 141.3 (CH), 137.2 (C), 110.8 (CH), 110.3 (CH2), 106.0 (CH), 73.7 (CH), 56.5 (CH),
54.4 (CH), 52.0 (CH), 44.7 (CH), 43.3 (C), 38.7 (CH2), 36.7 (CH2), 36.1 (CH), 35.5 (C), 34.0
(CH2), 31.9 (CH2), 28.5 (CH2), 27.4 (CH2), 26.2 (CH2), 24.2 (CH2), 21.5 (CH3), 21.2 (CH2), 12.9
(CH3), 12.2 (CH3). MS (LCMS) m/z: 433.3 (M+Na). Anal. Calcd for C27H38O3: C, 78.98; H,
9.33; Found: C, 78.83; H, 9.29.
4.14. General procedure for the ionic hydrogenation of compound 21a and 21b:
To the solution of compound 21a or 21b (1 mmol) in dichloromethane (10 mL), triethylsilane
(0.16 mL, 1 mmol) and trifluoroacetic acid (0. 39 mL, 5 mmol) were sequentially added. The
resulting red solution was stirred at 30 °C for 1 h, saturated sodium bicarbonate solution was
added and the reaction was worked up with dichloromethane (2 × 75 mL). The combined organic
extracts were washed with brine (2 × 25 mL), dried over Na2SO4, concentrated under reduced
pressure afforded crude product. Flash chromatographic purification over silica using ethyl
acetate-petroleum ether as a eluent afforded compound 18a or 18b. Yield: 18a (97%); 18b
(99%).
4.15. 3β-Hydroxy-20(S)-20-(5-methyl-2-furyl)-pregn-5-ene (24a):
Compound 24a was prepared from 23a (obtained from using the same procedure for synthesis of
17a from 16a). Yield: 96%. Mp: 184-186 °C (Methanol). IR (Nujol, cm-1): 3504 (-OH). 1H NMR
(200 MHz, CDCl3) δ = 5.75-5.82 (m, 2H, furan-H), 5.34 (d, 1H, J = 5.1 Hz, 6-H), 3.54 (m, 1H,
3-H), 2.65 (m, 1H, 20-H), 2.24 (s, 3H, furan-CH3), 1.24 (d, J = 6.8 Hz, 3H, 21-H3), 1.02 (s, 3H,
13
19-H3), 0.74 (s, 3H, 18-H3). C NMR (50 MHz, CDCl3 + CD3OD) δ = 159.5 (C), 149.1 (C),
140.6 (C), 121.3 (CH), 105.2 (CH), 103.6 (CH), 71.1 (CH), 56.5 (CH), 54.8 (CH), 44.9 (CH),
42.0 (C), 41.7 (CH2), 39.4 (CH2), 37.0 (CH2), 36.3 (C), 36.0 (CH), 31.7 (CH), 31.6 (CH2), 31.0
(CH2), 28.1 (CH2), 23.9 (CH2), 20.9 (CH2), 19.8 (CH3), 19.1 (CH3), 13.2 (CH3), 11.5 (CH3). MS
(LCMS) m/z: 405.3 (M+Na). Anal. Calcd for C26H38O2: C, 81.62; H, 10.01; Found: C, 81.53; H,
9.87.
29