A Novel Synthetic Approach from Diosgenin to a 17α-Hydroxy Orthoester via a Regio- and Stereo-Specific Rearrangement of an Epoxy Ester

Article abstract of DOI:10.1081/SCC-120028627

A cholesterol model compound, containing 16β-acetoxy, 17α-hydroxy, and (20S, 22R)-epoxy groups was synthesized from diosgenin in 13 steps and was rearranged regio- and stereo-specifically to an orthoester with BF₃-Et₂O.

Full text of DOI:10.1081/SCC-120028627

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A Novel Synthetic Approach from Diosgenin
to a 17α‐Hydroxy Orthoester via a Regio‐ and
Stereo‐Specific Rearrangement of an Epoxy Ester
Nisachon Chaosuancharoen a , Ngampong Kongkathip a & Boonsong Kongkathip a
a Natural Products and Organic Synthesis Research Unit, Department of Chemistry,
Faculty of Science , Kasetsart University , Chatuchak, Bangkok, 10900, Thailand
Published online: 21 Aug 2006.
To cite this article: Nisachon Chaosuancharoen , Ngampong Kongkathip & Boonsong Kongkathip (2004) A Novel Synthetic
Approach from Diosgenin to a 17α‐Hydroxy Orthoester via a Regio‐ and Stereo‐Specific Rearrangement of an Epoxy Ester,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 34:6,
961-983, DOI: 10.1081/SCC-120028627
To link to this article: http://dx.doi.org/10.1081/SCC-120028627
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 6, pp. 961–983, 2004
A Novel Synthetic Approach from Diosgenin
to a 17a-Hydroxy Orthoester via a Regio-
and Stereo-Specific Rearrangement of
an Epoxy Ester
Nisachon Chaosuancharoen, Ngampong Kongkathip,
*
and Boonsong Kongkathip
Natural Products and Organic Synthesis Research Unit, Department of
Chemistry, Faculty of Science, Kasetsart University, Chatuchak,
Bangkok, Thailand
ABSTRACT
A cholesterol model compound, containing 16b-acetoxy, 17a-hydroxy,
and (20S, 22R)-epoxy groups was synthesized from diosgenin in 13 steps
and was rearranged regio- and stereo-specifically to an orthoester with
.
BF3 Et2O.
Key Words: Orthoester; Epoxy ester; Regio-specific and stereo-specific
rearrangement; Diosgenin; Cholesterol.
*
Correspondence: Boonsong Kongkathip, Natural Products and Organic Synthesis
Research Unit, Department of Chemistry, Faculty of Science, Kasetsart University,
Chatuchak, Bangkok 10900, Thailand; Fax: 662-5793955; E-mail: fscibsk@yahoo.
com.
961
DOI: 10.1081/SCC-120028627
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Chaosuancharoen, Kongkathip, and Kongkathip
Figure 1. Structure of orthoesterol disulfate.
Biologically active compounds, isolated from marine sponges, include a
group of orthoesters that have interesting structures and activities, such as the
ergostanoids from Petunia inflata[1] and the antiviral orthoesterol disulfate A
(1), B (2) and C (3) (Fig. 1) from Petrosia weinbergi.[2,3] Compounds 1–3
showed in vitro activity against feline leukemia virus (FELV), influenza PR8
virus and murine coronavirus (A59).[2,3] The natural occurrence of epoxy
esters together with orthoesters suggested the possibility that the latter arise
biosynthetically via rearrangement of the former either under enzymatic or
mildly acidic conditions.[1] Recently, Giner et al.[4] reported a biomimetic
approach to the synthesis of a steroidal orthoester via an epoxy ester–
orthoester rearrangement using trifluoroacetic acid. This reaction also was
used to synthesize 2-methyl-D-erythritol via the formation of a [2.2.1] bicyclic
orthoester intermediate.[5]
The saponin OSW-1 has antitumor activity and has been the object of
many synthetic efforts.[6] During our synthetic approach to OSW-1 aglycone,
we noted that the two epoxy alcohols 17 and 19 obtained in 45% and 15%
yield, respectively, from the oxidation of allyl alcohol 16 that had been
synthesized from diosgenin (4) (Sch. 1), could serve as suitable compounds for
rearrangement to orthoesters.
Our synthetic approach was aimed at creating the cholesterol side chain
from the spiro-ring of the commercially inexpensive diosgenin (4) noting
that the number of carbon atoms in the side chains are the same. We
adapted the procedure reported by Basler et al.[7] for this purpose. As shown
in Sch. 1, after acetylation, the spiroketal ring-opening of diosgenin acetate

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
963
Scheme 1. Reagents and conditions: (a) Ac2O, Py, RT, 99%; (b) NaCNBH3, AcOH,
CH2Cl2, RT, 94%; (c) TsCl, Py, DMAP, CH2Cl2, RT, 86%; (d) LiAlH4, THF, RT,
78%; (e) K2Cr2O7, AcOH, 708C, 35%; (f) HClO4, Ac2O, benzene, RT, 76%; (g)
mCPBA, CH2Cl2, RT, 86%; (h) NaBH4, CH3OH, RT, 82% (13a : 13b ¼ 10 : 1); (i)
TBDMSCl, imidazole, DMF, CH2Cl2, RT, 92%; (j) Ac2O, Py, RT, 98%; (k) PPTS,
CH3OH, CH2Cl2, RT, 50%; (l) PDC, CH2Cl2, RT.

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Chaosuancharoen, Kongkathip, and Kongkathip
(5) was achieved by reduction with sodium cyanoborohydride in acetic acid
and dichloromethane to provide good yield of tetrahydrofuran 6. Tosylation
of 6 with p-toluenesulfonyl chloride in the presence of 4-(dimethylamino)
pyridine (DMAP) in pyridine and dichloromethane yielded tosylate 7. It was
found that the ratio of pyridine and dichloromethane was crucial for this
reaction. A ratio 3 : 5 of pyridine to dichloromethane was found to complete
the reaction in 12 hr and also to reduce the formation of an ether by-product.
Reduction of 7 with lithium aluminum hydride gave the desired 5-furosten-
3b-ol (8). After reacetylation, oxidative opening of the tetrahydrofuran
moiety of 8 to a diketone was accomplished by treatment with potassium
dichromate in acetic acid at 708C to give the 16,22-diketone 10 in moderate
yield. Allylic oxidation at C-7 was found to be a significant side-reaction,
but could be minimized by limiting the temperature to 708C and controlling
the volume of acetic acid. Under acidic conditions, the diketone 10 could be
converted to the furan 11, which then underwent autooxidation to provide
the ene-dione 12. The ene-dione 12 also could be produced from 11 with m-
chloroperbenzoic acid (mCPBA) in dichloromethane. Sodium borohydride
reduction of the ene-dione 12 in methanol gave two epimeric alcohols 13a
and 13b in the ratio of about 10 : 1 (Sch. 1), which were assigned as the
22R- and 22S-epimer, respectively. The absolute configuration of the 22S-
epimer was confirmed by x-ray crystallography. Treatment of the major
22R-epimer 13a with t-butyldimethylchlorosilane (TBDMSCl) provided a
selective protection of the hydroxyl group at C-22 in 14 in high yield.
Protection of the hydroxyl group at C-16 of 14 as the acetate, followed by
desilylation of the hydroxy at C-22 using pyridinium p-toluenesulfonate
(PPTS) in MeOH and CH2Cl2, provided the allyl alcohol 16. Oxidation of
16 was expected to provide the corresponding enone that could serve as an
intermediate for the synthesis of the OSW-1 aglycone. Neutral oxidation
condition with pyridinium dichromate (PDC) was chosen for this purpose.
After treatment of 16 with PDC in CH2Cl2 for 10 hr, the products included
the tertiary epoxy alcohol 17 (45%), the a,b-unsaturated ketone 18 (10%)
and the epoxy alcohol 19 (15%), as well as the epoxy ketone 20 (3%),
arising from 19. It appeared that oxidation of the allylic hydroxyl group of
16 occurred at a slower rate than did epoxidation of the double bond. Thus,
the enone 18 was formed in only a small amount (10%). The transformation
of 16 into 17 involved rearrangement of D17(20)-22R-ol to the isomeric
D20(22)-17a-ol system followed by epoxidation of the double bond; the
tertiary hydroxyl group was left untouched. The epoxidation of the double
bond presumably proceeded through formation of a chromate ester, followed
by the transfer of an oxygen atom from this ester to the double bond. Thus,
the epoxide moiety in 17 was formed on the same side of the molecule as
the 17a-hydroxyl group. The epoxidation of double bond in the allylic

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
965
alcohol 16 to 19 could be explained in the same manner. Further oxidation
of the secondary hydroxyl group of 19 provided the epoxy ketone 20. To
verify that the epoxy ketone 20 was formed by way of an epoxidation of the
double bond in the allylic alcohol 16, prior to the oxidation of the hydroxyl
group, both the isolated epoxy alcohol 19 and the enone 18 were treated
with PDC under the same condition as before. After 6 hr, only the epoxy
alcohol 19 was converted to the epoxy ketone 20 in ꢀ50% conversion as
sole product, whereas no reaction occurred for the enone 18. Similar
epoxidations of other steroidal allylic alcohols with chromium trioxide have
been reported by Glotter et al.[8] However, oxidation of 16 with
o-iodoxybenzoic acid in 1,2-dichloroethane provided a high yield of the
a,b-unsaturated ketone 18.
The tertiary epoxy alcohol 17 could serve as a very useful intermediate for
rearrangement to two classes of natural products: the protected OSW-1
aglycone and a hydroxy orthoester (Sch. 2). Treatment of 17 with boron
.
trifluoride etherate (BF3 OEt2) in dichloromethane at 08C for a few minutes
led to rearrangement to an [3.2.1] orthoester 22 in 80% yield with the same
configuration as that reported in nature[4] (Sch. 3). A [2.2.1] orthoester
23, which could be formed from the same intermediate, involving the
tertiary 17a-hydroxy group, was not detected. Thus, the cyclization occurred
Scheme 2.

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Chaosuancharoen, Kongkathip, and Kongkathip

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
967
Scheme 4.
regio- and stereo-specifically. The epoxy alcohol 17 probably cyclized
through energetically favored 6-exo route to form the dioxycarbocation 21,
thereby producing the more stable [3.2.1] bicyclic orthoester 22 as the only
product (Sch. 3). The epoxy ester 19, which could undergo a similar
rearrangement, did not react and remained unchanged upon treatment either
.
with BF3 OEt2 or trifluoroacetic acid under the same conditions. When the
reaction conditions were raised to room temperature, only complex mixtures
were obtained, indicating that a 6-endo cyclization is a disfavored process
(Sch. 4). The configurations at C-16 and C-22 of the orthoester 22 were
established based on a nuclear overhauser effect between H-16 and H-22 and
were found to have the reported natural configuration of such orthoesters. 1H
and 13C NMR data of the relevant portions of 22 were in good agreement with
those reported for orthoesterol B (2).[2]
Similar Lewis-acid-promoted rearrangement of a,b-epoxy acylates to the
corresponding ketones and orthoesters have been intensively studied by Kita
et al.[9] They have found that the dioxycarbocation intermediate was formed
by neighboring group participation of acyloxy groups and that formation of
such intermediates could be suppressed by a very strong electron-withdrawing
group, such as 4-nitrobenzoyl group. Therefore, in order to make the desired
epoxide-ketone rearrangement reaction applicable to OSW-1 system, the
acetoxy group at C-16 in our b,g-epoxy ester was replaced by a 4-nitrobenzoyl
group in an attempt to minimize orthoester formation (Sch. 5). Surprisingly,
even though containing a strong electron-withdrawing group, the epoxy
.
4-nitrobenzoate 26 rearranged into the orthobenzoate 27 with BF3 OEt2 in
a few minutes in high yield (95%). The epoxy 2,4-dinitrobenzoate 30 was
also investigated and after 1 hr, the orthobenzoate 31 was obtained (25%)
along with an unidentified product (20%), without any of the desired ketone
product.
In conclusion, we report herein the facile synthesis of a tertiary epoxy
alcohol from diosgenin, that provided a key intermediate which readily
rearranged regio- and stereo-specifically to yield a steroidal orthoester.

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Chaosuancharoen, Kongkathip, and Kongkathip

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
969
EXPERIMENTAL
1
The H and 13C NMR spectra were recorded on a Varian Gemini 300
spectrophotometer and on 400 MHz Brucker Advance DPX-400. Chemical
shifts were recorded as d values in ppm. Spectra were acquired in CDCl3
unless otherwise stated. The peak due to residual CHCl3 (7.26 ppm for 1H and
77.23 ppm for 13C) was used as the internal reference. Coupling constants (J)
are given in Hz. Infrared (IR) spectra were recorded in cm21 on a Perkin–
Elmer 2000 Fourier transform infrared spectrophotometer. Mass spectra (MS)
and accurate masses (HRMS) were obtained on a JEOL JMS-SX102 mass
spectrometer and on a GC-MS-QP-5050A spectrometer in electron impact
mode at 70 eV. Melting points (m.p.) were determined on a Mel-Temp
electrothermal apparatus and are reported uncorrected in 8C. Flash column
chromatography was conducted using Merck silica gel 60 (mesh size 0.040–
0.063 mm). Column chromatography was performed on Merck silica gel 60
(70–230 mesh), packed by the slurry method.
Diosgenin Acetate (5)
A solution of 4 (10 g, 0.024 mol), pyridine (110 mL), and acetic anhydride
(10 mL, 0.105 mol) was stirred overnight (14 hr) at room temperature. The
reaction was quenched by addition of ice and stirred for another 1 hr. The
white precipitate was filtered to afford crude product 5 (10.95 g, 99.4%),
which was used in the next step without further purification. Recrystallization
of the crude product from ethyl acetate gave needles; m.p. 188.5–1898C.
FTIR (KBr), nmax 2940, 1720, 1235, 1045 cm21. 1H NMR (CDCl3, 300 MHz)
d: 5.36 (d, J ¼ 4.5 Hz, 1H, C-6), 4.65–4.55 (m, 1H, C-3), 4.44–4.36 (m, 1H,
C-16), 3.49–3.33 (m, 2H, C-26), 2.02 (s, 3H, AcO), 1.03 (s, 3H, H-18), 0.96
(d, J ¼ 6.9 Hz, 3H, H-21), 0.78 (d, J ¼ 6.0 Hz, 3H, C-27), 0.78 (s, 3H, C-19).
13C NMR (CDCl3, 75 MHz) d: 170.7, 139.9, 122.6, 109.5, 81.0, 74.1, 67.0,
62.3, 56.6, 50.1, 41.8, 40.4, 39.9, 38.3, 37.2, 36.9, 32.2, 32.0, 31.6 (2C), 30.5,
29.0, 27.9, 21.6, 21.0, 19.5, 17.3, 16.5, 14.7 ppm. MS (FAB), m/z (relative
intensity): 457 (Mþ þ 1, 100), 397 (70), 313 (10), 282 (20), 253 (28), 154
(32), 91 (30), 55 (33). HRMS (FAB) calcd. for C29H45O4 [Mþ þ 1] 457.3318;
found 457.3315.
(22b,25R)-3b-Acetoxyfurost-5-ene-26-ol (6)
To a stirred solution of 5 (10 g, 0.022 mol) in glacial acetic acid (100 mL)
and dichloromethane (30 mL) was added sodium cyanoborohydride (4.2 g,

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970
Chaosuancharoen, Kongkathip, and Kongkathip
0.066 mol) in portions at room temperature. After stirring for 8 hr, the reaction
was quenched by addition of water and extracted with dichloromethane. The
combined organic layers were washed with saturated aqueous NaHCO3
solution, then water, dried over anhydrous Na2SO4, filtered, and concentrated
under reduced pressure. The residue was purified by flash column chroma-
tography (20% ethyl acetate/hexane) on silica gel to afford 6 (8.8 g, 88%);
1
m.p. 105–1068C. FTIR (KBr), nmax 3418, 2970, 1731, 1246, 1037 cm21. H
NMR (CDCl3, 300 MHz) d: 5.34 (d, J ¼ 4.2 Hz, 1H, C-6), 4.58–4.55 (m, 1H,
C-3), 4.31–4.24 (m, 1H, C-16), 3.49–3.36 (m, 2H, C-26), 3.33–3.27 (m, 1H,
C-22), 2.00 (s, 3H, AcO), 1.00 (s, 3H, C-18), 0.97 (d, J ¼ 6.6 Hz, 3H, C-21),
0.88 (d, J ¼ 6.6 Hz, 3H, C-27), 0.78 (s, 3H, C-19). 13C NMR (CDCl3,
75 MHz) d: 170.7, 139.8, 122.5, 90.5, 83.3, 74.0, 68.1, 65.2, 57.0, 50.1, 40.8,
39.5, 38.2, 38.0, 37.1, 36.8, 35.9, 32.4, 32.1, 31.7, 30.6, 30.3, 27.9, 21.6, 20.8,
19.5, 19.1, 16.8, 16.6 ppm. MS (FAB), m/z (relative intensity): 481
(Mþ þ Na, 13), 459 (Mþ þ 1, 55), 399 (25), 307 (25), 154 (100), 136 (75),
91 (25). HRMS (FAB) calcd. for C29H47O4 [Mþ þ 1] 459.3474; found
459.3481.
(22b,25R)-3b-Acetoxyfurost-5-ene-26-(p-toluenesulfonate) (7)
A solution of 6 (16.68 g, 0.036 mol), pyridine (60 mL), p-toluenesulfonyl
chloride (10.42 g, 0.054 mol) and 4-(dimethylamino) pyridine (2.28 g,
0.018 mol) in dichloromethane (100 mL) was stirred at room temperature
for 12 hr. The reaction was quenched by addition of water and extracted with
additional dichloromethane. The combined organic layers were washed with
10% HCl, saturated aqueous NaHCO3 solution, brine and water and dried over
anhydrous Na2SO4, filtered and concentrated under reduced pressure. The
residue was purified by flash column chromatography (10% ethyl acetate/
hexane) to provide 7 (20 g, 90%); m.p. 120–1218C. FTIR (KBr), nmax 2940,
1731, 1362, 1239, 1179 cm21 1H NMR (CDCl3, 300 MHz) d: 7.77
.
(d, J ¼ 8.4 Hz, 2H, C-20 and C-60), 7.33 (d, J ¼ 8.1 Hz, 2H, C-30 and C-50),
5.36 (d, J ¼ 4.5 Hz, 1H, C-6), 4.63–4.55 (m, 1H, C-3), 4.30–4.23 (m, 1H,
C-16), 3.90–3.78 (m, 2H, C-26), 3.26–3.20 (m, 1H, C-22), 2.44 (s, 3H, C-70),
2.02 (s, 3H, AcO), 1.03 (s, 3H, C-18), 0.96 (d, J ¼ 6.6 Hz, 3H, C-21), 0.88
(d, J ¼ 6.6 Hz, 3H, C-27), 0.77 (s, 3H, C-19). 13C NMR (CDCl3, 75 MHz) d:
170.7, 144.8, 139.9, 133.2, 130 (2C), 128.1 (2C), 122.6, 90.1, 83.4, 75.2, 74.1,
65.2, 57.1, 50.2, 40.8, 39.6, 38.3, 38.1, 37.2, 36.9, 33.2, 32.4, 32.2, 31.8, 30.8,
30.0, 27.9, 21.8, 21.6, 20.8, 19.5, 19.1, 16.6(2C) ppm. MS (FAB), m/z
(relative intensity): 635 (Mþ þ Na, 10), 613 (Mþ þ 1, 45), 307 (25), 154
(100), 136 (72), 91 (20). HRMS (FAB) calcd. for C36H53O6S [Mþ þ 1]
613.3563; found 613.3566.

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
(22b)-3b-Hydroxyfurost-5-ene (8)
971
To a cooled suspension of LiAlH4 (70 mg, 1.65 mmol) in dry tetrahydro-
furan was added a solution of 7 (340 mg, 0.55 mmol) in dry tetrahydrofuran.
The mixture was stirred under N2 at room temperature for 7 hr. The reaction
was quenched by careful addition of ethyl acetate and water and extracted with
dichloromethane. The combined organic layers were washed with water, dried
over anhydrous Na2SO4, filtered, and concentrated under reduced pressure.
The residue was purified by flash column chromatography (30% ethyl acetate/
hexane) to provide 8 (192 mg, 78%); m.p. 139–1408C. FTIR (KBr), nmax 3455,
1
2955, 1052 cm21. H NMR (CDCl3, 300 MHz) d: 5.34 (d, J ¼ 5.1 Hz, 1H,
C-6), 4.33–4.26 (m, 1H, C-16), 3.55–3.45 (m, 1H, C-3), 3.33–3.27 (m, 1H,
C-22), 1.02 (s, 3H, C-19), 0.99 (d, J ¼ 6.9 Hz, 3H, C-21), 0.88 (d, J ¼ 6.3 Hz,
6H, C-26 and C-27), 0.80 (s, 3H, C-19). 13C NMR (CDCl3, 75 MHz) d: 141.0,
121.7, 90.7, 83.3, 71.9, 65.4, 57.2, 50.3, 42.5, 40.9, 39.7, 38.1, 37.5, 36.8, 36.1,
32.5, 32.2, 31.8 (2C), 31.6, 28.5, 22.7, 22.7, 20.9, 19.6, 19.3, 16.6 ppm. MS
(FAB), m/z (relative intensity): 399 (Mþ 2 1, 15), 369 (100), 355 (12). HRMS
(FAB) calcd. for C27H43O2 [Mþ 2 1] 399.3263; found 399.3261.
(22b)-3b-Acetoxyfurost-5-ene (9)
A solution of 8 (2.0 g, 5.0 mmol), pyridine (20 mL) and acetic anhydride
(2 mL, 0.02 mol) was stirred overnight (14 hr) at room temperature. The
reaction was quenched by addition of ice and stirred for another 1 hr. The
white precipitate was filtered to afford crude product 9 (2.2 g, 99%), which
was used in the next step without further purification. Recrystallization of
the crude product from methanol gave fine needles; m.p. 133–1348C. FTIR
1
(KBr), nmax 2955, 1720, 1239, 1033 cm21. H NMR (CDCl3, 300 MHz) d:
5.36 (d, J ¼ 4.5 Hz, 1H, C-6), 4.61–4.57 (m, 1H, C-3), 4.33–4.26 (m, 1H,
C-16), 3.33–3.26 (m, 1H, C-22), 2.02 (s, 3H, AcO), 1.03 (s, 3H, C-18), 0.99
(d, J ¼ 6.9 Hz, 3H, C-21), 0.88 (d, J ¼ 6.3 Hz, 6H, C-26 and C-27), 0.80
(s, 3H, C-19). 13C NMR (CDCl3, 75 MHz) d: 170.7, 139.9, 122.6, 90.7,
83.3, 74.1, 65.4, 57.1, 50.2, 40.9, 39.6, 38.3, 38.1, 37.2, 36.9, 36.1, 32.4,
32.2, 31.8, 31.6, 28.5, 27.9, 22.8, 22.7, 21.6, 20.8, 19.5, 19.3, 16.6 ppm. MS
(FAB), m/z (relative intensity): 441 (Mþ 2 1, 45), 415 (100). HRMS (FAB)
calcd. for C29H45O3 [Mþ 2 1] 441.3369; found 441.3340.
3b-Acetoxycholest-5-ene-16,22-dione (10)
A solution of 9 (1.0 g, 2.26 mmol) and potassium dichromate (1.67 g,
5.68 mmol) in glacial acetic acid (10 mL) was stirred at 708C for 10 hr. The

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Chaosuancharoen, Kongkathip, and Kongkathip
reaction was quenched by addition of H2O and the aqueous layer was
extracted with dichloromethane. The combined organic layers were washed
with saturated aqueous NaHCO3 solution, then water and dried over
anhydrous Na2SO4, filtered and concentrated under reduced pressure. The
residue was purified by column chromatography (5% ethyl acetate/hexane) to
afford recovered 9 (300 mg, 30%) and the product 10 (413 mg, 40%, 57%
based on recovered 9) as white solid; m.p. 151.5–152.58C. FTIR (KBr), nmax
1
2955, 1731, 1709, 1239, 1033 cm21. H NMR (CDCl3, 300 MHz) d: 5.37
(d, J ¼ 4.8 Hz, 1H, C-6), 4.66–4.55 (m, 1H, C-3), 2.61 (d, J ¼ 4 Hz, 1H),
2.75–2.50 (m, 3H) 2.03, (s, 3H, AcO), 1.04 (s, 3H, C-21), 1.04 (d, J ¼ 6.9 Hz,
3H, C-18), 0.90 (d, J ¼ 5.7 Hz, 6H, C-26 and C-27), 0.79 (s, 3H, C-19). 13C
NMR (CDCl3, 75 MHz) d: 218.2, 214.2, 170.7, 140.1, 122.0, 73.9, 66.3, 51.3,
49.8, 43,5, 41.9, 40.6, 38.8, 38.2, 37.4, 36.9 (2C), 32.4, 31.9, 31.1, 27.8 (2C),
22.7 (2C), 21.6, 20.7, 19.5, 15.6, 13.2 ppm. MS (DCI), m/z (relative intensity):
456 (Mþ, 2), 441 (10), 396 (35), 340 (100), 325 (30), 297 (35). HRMS (FAB)
calcd. for C29H45O4 [Mþ þ 1] 457.3318; found 457.3300.
3b-Acetoxyfurost-5,16,20(22)-triene (11)
To a stirred solution of 10 (835 mg, 1.83 mmol) in benzene (6 mL) was
added 1.5 mL of a solution of acetic anhydride (3 mL) and HClO4 (2 drops).
After 0.5 h, the mixture was quenched by addition of water and extracted with
dichloromethane. The combined organic layers were washed with saturated
aqueous NaHCO3 solution, then water, dried over anhydrous Na2SO4, filtered
and concentrated under reduced pressure. The residue was purified by column
chromatography (10% ethyl acetate/hexane) to provide 11 (568 mg, 71%).
FTIR (KBr), nmax 2947, 1735, 1254, 1033 cm21. 1H NMR (CDCl3, 300 MHz)
d: 5.40 (d, J ¼ 4.8 Hz, 1H, C-6), 4.61 (m, 1H, C-3), 2.04, (s, 3H, AcO), 1.90
(s, 3H, C-21), 1.08 (s, 3H, C-18), 0.91 (s, 3H, C-19), 0.91 (d, J ¼ 6.0 Hz, 6H,
C-26 and C-27). 13C NMR (CDCl3, 75 MHz) d: 170.7, 155.0, 153.9, 140.2,
136.8, 122.4, 111.4, 74.0, 60.6, 50.8, 41.2, 38.3, 38.1, 37.2, 37.0, 35.6, 31.9,
30.4, 27.9, 27.9, 26.8, 24.6, 22.6, 22.6, 21.6, 20.6, 19.4, 18.1, 9.0 ppm. MS
(DCI), m/z (relative intensity): 438 (Mþ, 100), 423 (60), 381 (25), 363 (10),
91 (10), 43 (25). HRMS (FAB) calcd. for C29H42O3 [Mþ]438.3134; found:
438.3154.
3b-Acetoxycholesta-5,17(20)-diene-16,22-dione (12)
To a stirred solution of 11 (447 mg, 1.02 mmol) in CH2Cl2 (10 mL) was
added mCPBA (184 mg, 1.22 mmol). After 1 hr, the mixture was quenched by

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
973
addition of aqueous 0.5 M sodium thiosulfate solution (5 mL) and extracted
with dichloromethane. The combined organic layers were washed with
saturated aqueous NaHCO3 solution, then water, dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. The residue was
purified by column chromatography (10% ethyl acetate/hexane) to provide 12
(390 mg, 84%) as white solid with m.p. 136–1378C. FTIR (KBr), nmax 2947,
1
1731, 1716, 1697, 1242, 1030 cm21. H NMR (CDCl3, 300 MHz) d: 5.37
(d, J ¼ 4.5 Hz, 1H, C-6), 4.60 (m, 1H, C-3), 2.03, (s, 3H, AcO), 1.93 (s, 3H,
C-21), 1.10 (s, 3H, C-18), 1.06 (s, 3H, C-19), 0.89 (d, J ¼ 6.0 Hz, 6H, C-26
and C-27). 13C NMR (CDCl3, 75 MHz) d: 211.5, 205.7, 170.7, 145.6, 142.3,
140.1, 122.0, 73.9, 50.8, 49.8, 43.5, 39.2, 38.2, 37.9, 36.9, 36.8, 36.2, 32.5,
31.7, 30.9, 27.9, 27.8, 22.6, 22.6, 21.6, 20.9, 19.5, 17.2, 16.0 ppm. MS (CDI),
m/z (relative intensity): 454 (Mþ, 90), 439 (20), 394 (100), 383 (50), 323 (45),
183 (20), 43 (20). MS (FAB), m/z (relative intensity): 477 (Mþ þ Na, 15),
455 (Mþ þ 1, 100), 395 (20). HRMS (FAB) calcd. for C29H42O4 [Mþ]
454.3083; found: 454.3093.
(22R)-3b-Acetoxycholesta-5,17(20)-diene-16b,22-diol (13a)
and (22S)-3b-Acetoxycholesta-5,17(20)-diene-16b,22-diol (13b)
To a solution of 12 (417 mg, 0.92 mmol) in methanol (8 mL) was added
NaBH4 (70 mg, 1.85 mmol) and the mixture was stirred at room temperature
for 0.5 hr. The reaction was quenched by addition of water to destroy the
excess NaBH4. To the mixture was added saturated aqueous NaCl solution
followed by extraction with dichloromethane. The combined organic layers
were washed with water, dried over anhydrous Na2SO4, filtered and concen-
trated under reduced pressure. The residue was purified by flash column
chromatography (15% ethyl acetate/hexane) to provide 13a (317 mg, 75%)
and 13b (30 mg, 7%) and a mixture of 13a and 13b (25 mg, 6%); 13a and
13b were white solid with m.p. 137–1388C and 180.5–181.58C,
respectively.
13a: FTIR (KBr), nmax 3402, 3305, 2947, 1723, 1261, 1205, 1037 cm21
.
1H NMR (CDCl3, 400 MHz) d: 5.40 (d, J ¼ 4.7 Hz, 1H, C-6), 4.76
(t, J ¼ 7.2 Hz, 1H, C-16), 4.67–4.59 (m, 1H, C-3), 4.55 (dd, J ¼ 4.1, 8.8 Hz,
1H, C-22), 2.06 (s, 3H, AcO), 1.77 (s, 3H, C-21), 1.07 (s, 3H, C-18), 1.06
(s, 3H, C-19), 0.92 (d, J ¼ 6.6 Hz, 6H, C-26 and C-27). 13C NMR (CDCl3,
100 MHz) d: 170.8, 148.6, 139.9, 133.0, 122.4, 74.0, 73.9, 72.2, 52.1, 49.8,
44.7, 38.2, 37.7, 37.0, 36.8, 33.5, 31.8, 31.2, 29.9, 28.3, 27.9, 23.0, 22.7, 21.6,
21.2, 19.4, 17.2, 13.1 ppm. MS (EI), m/z (relative intensity): 440 (Mþ 2 H2O,
1), 167 (22), 109 (26), 95 (23), 43 (100). HRMS (FAB) calcd. for C29H46O4
[m]þ 458.3396; found: 458.3372.

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974
Chaosuancharoen, Kongkathip, and Kongkathip
13b: FTIR (KBr), nmax 3298, 2940, 1727, 1466, 1436, 1365, 1242, 1048,
1026 cm21. 1H NMR (CDCl3, 400 MHz) d: 5.40 (d, J ¼ 4.8 Hz, 1H, C-6), 4.87
(t, J ¼ 7.4 Hz, 1H, C-16), 4.67–4.59 (m, 1H, C-3), 4.50 (t, J ¼ 7.1 Hz, 1H,
C-22), 2.07 (s, 3H, AcO), 1.74 (s, 3H, C-21), 1.10 (s, 3H, C-19), 1.06 (s, 3H,
C-18), 0.92 (d, J ¼ 6.6 Hz, 6H, C-26 and C-27). 13C NMR (CDCl3, 100 MHz)
d: 171.3, 151.9, 140.4, 133.1, 122.8, 74.5, 73.1, 73.0, 52.8, 50.4, 44.6, 38.7,
37.8, 37.4, 37.2, 35.8, 35.7, 32.2, 32.0, 31.4, 28.8, 28.3, 23.4, 23.2, 22.1, 21.6,
19.9, 17.3, 12.7 ppm. MS (EI), m/z (relative intensity): 440 (Mþ 2 H2O, 1),
167 (18), 109 (17), 95 (17), 43 (100).
(22R)-3b-Acetoxy-22-tert-butyldimethylsilyloxycholesta-
5,17(20)-diene-16b-ol (14)
A solution of 13a (420 mg, 0.92 mmol), t-butyldimethylchlorosilane
(420 mg, 2.76 mmol) and imidazole (155 mg, 2.3 mmol) in N,N-dimethyl-
formamide (4.2 mL) and dichloromethane (8 mL) was stirred at room
temperature for 1 hr. The reaction was quenched by addition of water and
extracted with dichloromethane. The combined organic layers were washed
with water, dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. The residue was purified by flash column chromatog-
raphy (2.5% ethyl acetate/hexane) to provide 14 (495 mg, 94.4%) as
amorphous white solid; m.p. 63–648C. FTIR (KBr), nmax 3477, 2947, 1735,
1720, 1240, 1030, 832, 722 cm21 1H NMR (CDCl3, 400 MHz) d: 5.41
.
(d, J ¼ 4.7 Hz, 1H, C-6), 4.69 (t, J ¼ 7.0 Hz, 1H, C-16), 4.67–4.58 (m, 1H,
C-3), 4.46 (dd, J ¼ 9.8, 2.2 Hz, 1H, C-22), 2.06 (s, 3H, AcO), 1.73 (s, 3H,
C-21), 1.06 (s, 3H, C-18), 1.05 (s, 3H, C-19), 0.92 (d, J ¼ 6.6 Hz, 6H,
C-26 and C-27), 0.90 (s, 9H, –C(CH3)3), 0.06 (s, 3H –SiCH3), 0.00
(s, 3H, –SiCH3). 13C NMR (CDCl3, 100 MHz) d: 171.2, 146.9, 140.4,
135.6, 122.8, 74.5, 74.0, 73.0, 52.5, 50.2, 44.9, 38.7, 37.9, 37.4, 37.2, 36.3
(2C), 35.0, 32.2, 31.5, 28.5, 28.3, 26.4 (3C), 23.5, 23.1, 22.0, 21.6, 19.9,
18.8, 17.7, 13.3, 23.8, 24.2 ppm. MS (EI), m/z (relative intensity): 572
(Mþ, 1), 515 (2), 501 (28), 213 (7), 167 (20), 75 (100), 57 (24), 43 (78).
(22R)-22-tert-Butyldimethylsilyloxy-3b,16b-
diacetoxycholesta-5,17(20)-diene (15)
A solution of 14 (100 mg, 0.17 mmol), acetic anhydride (0.1 mL,
0.10 mmol) in pyridine (1 mL) was stirred at room temperature for 12 hr. The
reaction was poured into ice and stirred for the additional 1 hr. The pre-
cipitated white solid was filtered and washed with water for several times to

ORDER
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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
975
afford the crude product 15 in quantitative yield, which could be used in the
next step without purification. Recrystallization from hexane gave needles;
m.p. 195–1968C. FTIR (KBr), nmax 1731, 1250, 1235, 1033, 832, 769 cm21
.
1H NMR (CDCl3, 400 MHz) d: 5.65 (t, J ¼ 7.3 Hz, 1H, C-16), 5.40
(d, J ¼ 4.4 Hz, 1H, C-6), 4.66–4.59 (m, 1H, C-3), 4.03 (dd, J ¼ 9.9, 2.0 Hz,
1H, C-22), 2.07 (s, 3H, AcO), 2.04 (s, 3H, AcO), 1.75 (s, 3H, C-21), 1.06
(s, 3H, C-18), 1.03 (s, 3H, C-19), 0.90 (d, J ¼ 6.6 Hz, 6H, C-26 and C-27),
0.89 (s, 9H, –C(CH3)3), 0.04 (s, 3H, –SiCH3), 0.00 (s, 3H, –SiCH3). 13C
NMR (CDCl3, 100 MHz) d: 171.2, 171.1, 141.3, 140.3, 136.6, 122.9, 74.9,
74.5, 74.0, 52.6, 50.2, 44.9, 38.6, 38.0, 37.4, 37.2, 36.7, 34.6, 33.6, 32.0, 31.6,
29.2, 28.3, 26.4 (3C), 23.5, 23.4, 22.1, 21.9, 21.6, 19.9, 18.8, 16.9, 12.8, 23.8,
24.3 ppm. MS (EI), m/z (relative intensity): 557 (Mþ 2 C4H9, 1), 117 (37),
73 (43), 55 (15), 43 (100).
(22R)-3b,16b-Diacetoxycholesta-5,17(20)-diene-22-ol (16)
A solution of 15 (100 mg, 0.16 mmol) and pyridinium p-toluenesulfonate
(35 mg, 0.14 mmol) in methanol (6 mL) and dichloromethane (3 mL) was
stirred at room temperature for 24 hr. The reaction was quenched by addition
of saturated aqueous NaCl solution and extracted with dichloromethane. The
combined organic layers were washed with water, dried over anhydrous
Na2SO4, filtered and concentrated under reduced pressure. The residue was
purified by flash column chromatography (5% and 10% ethyl acetate/hexane)
to provide both recovered 15 (39 mg, 39%) and the product 16 (40 mg, 49%,
80% based on recovered 15) as a white solid; m.p. 154.5–155.58C. FTIR
(KBr), nmax 3498, 2944, 1730, 1712, 1465, 1373, 1251, 1240, 1030 cm21. 1H
NMR (CDCl3, 400 MHz) d: 5.63 (t, J ¼ 7.0 Hz, 1H, C-16), 5.39 (d, J ¼ 5.0 Hz,
1H, C-6), 4.66–4.57 (m, 1H, C-3), 4.18–4.15 (m, 1H, C-22), 2.05 (s, 3H,
AcO), 2.02 (s, 3H, AcO), 1.78 (s, 3H, C-21), 1.04 (s, 3H, C-19), 1.03 (s, 3H,
C-18), 0.91 (d, J ¼ 6.5 Hz, 6H, C-26 and C-27). 13C NMR (CDCl3, 100 MHz)
d: 170.5, 170.4, 142.8, 139.7, 134.3, 122.2, 74.2, 73.8, 72.8, 52.0, 49.6, 44.4,
38.0, 37.4, 36.8, 36.5, 35.7, 33.0, 31.4 (2C), 31.1, 28.6, 27.7, 22.8, 22.7, 21.4,
21.3, 21.0, 19.2, 16.2, 11.5 ppm. MS (EI), m/z (relative intensity): 482
(Mþ 2 H2O, 1), 440 (1), 380 (1), 298 (2), 95 (12), 81 (15), 43 (100).
Oxidation of (22R)-3b,16b-Diacetoxycholesta-5,17(20)-diene-
22-ol (16)
A solution of 16 (117 mg, 0.23 mmol) and pyridinium dichromate
(264 mg, 0.70 mmol) in CH2Cl2 (4 mL) was stirred at room temperature for

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REPRINTS
976
Chaosuancharoen, Kongkathip, and Kongkathip
10 hr. Then the reaction mixture was filtered through Celite and eluted with
dichloromethane. The filtrate was washed with saturated aqueous NaHCO3
solution, and then water, dried over anhydrous Na2SO4, filtered, and concen-
trated under reduced pressure. The residue was purified by column chroma-
tography (5% ethyl acetate/hexane) to give 17 (53 mg, 45%), 18 (12 mg,
10%), 19 (18 mg, 15%) and 20 (4 mg, 3%).
1
17: FTIR (KBr), nmax 3500, 2947, 1735, 1256, 1033 cm21. H NMR
(CDCl3, 400 MHz) d: 5.40 (d, J ¼ 5.2 Hz, 1H, C-6), 4.91 (dd, J ¼ 8.0, 5.1 Hz,
1H, C-16), 4.63-4.61 (m, 1H, C-3), 3.14 (t, J ¼ 6.2, 1H, C-22), 2.74 (s, 1H,
OH), 2.07 (s, 3H, AcO), 2.05 (s, 3H, AcO), 1.45 (s, 3H, C-21), 1.09 (s, 3H,
C-18), 1.06 (s, 3H, C-19), 0.92 (d, J ¼ 6.6 Hz, 6H, C-26 and C-27). 13C NMR
(CDCl3, 100 MHz) d: 170.5, 169.6, 139.6, 122.3, 83.2, 82.2, 73.8, 62.3, 60.2,
49.5, 48.3, 47.1, 38.1, 36.9, 36.6, 35.4, 33.5, 31.6, 31.4, 31.1, 27.9, 27.7, 25.6,
22.6, 22.3, 21.4, 21.3, 20.1, 19.3, 17.3, 15.8 ppm. MS (EI), m/z (relative
intensity): 456 (Mþ 2 HOAc, 1), 296 (2), 214 (4), 105 (7), 81 (12), 55 (15), 43
(100).
1
18: FTIR (KBr), nmax 2940, 1735, 1727, 1690, 1231 cm21. H NMR
(CDCl3, 400 MHz) d: 5.66–5.61 (m, 1H, C-16), 5.40 (d, J ¼ 4.9 Hz, 1H, C-6),
4.66–4.58 (m, 1H, C-3), 2.60–2.30 (m, 6H), 2.06 (s, 3H, AcO), 1.99 (s, 3H,
AcO), 1.98 (d, J ¼ 1.6 Hz, 3H, C-21), 1.09 (s, 3H, C-18), 1.05 (s, 3H, C-19),
0.92 (d, J ¼ 6.5 Hz, 3H, C-26), 0.91 (d, J ¼ 6.5 Hz, 3H, C-27). 13C NMR
(CDCl3, 100 MHz) d: 208.3, 170.5, 170.4, 148.7, 139.6, 133.3, 122.2, 74.4,
73.8, 51.3, 49.6, 44.7, 39.1, 38.0, 36.8, 36.6 (2C), 32.6, 32.2, 31.4, 30.7, 27.8,
27.7, 22.5, 22.4, 21.4, 20.9 (2C), 19.3, 16.1, 15.2 ppm. MS (EI), m/z (relative
intensity): 498 (Mþ, 1), 438 (1), 378 (2), 181 (3), 91 (7), 81 (11), 55 (11), 43
(100).
19: FTIR (KBr), nmax 3462, 2947, 1735, 1246, 1231, 1037 cm21. 1H NMR
(CDCl3, 400 MHz) d: 5.39 (d, J ¼ 5.2 Hz, 1H, C-6), 4.89 (dd, J ¼ 8.4, 5.4 Hz,
1H, C-16), 4.66–4.68 (m, 1H, C-3), 3.16 (m, 1H, C-22), 2.05 (s, 6H, 2(AcO)),
1.44 (s, 3H, C-21), 1.07 (s, 3H, C-18), 1.04 (s, 3H, C-19), 0.93 (d, J ¼ 6.3 Hz,
3H, C-26), 0.92 (d, J ¼ 6.3 Hz, 3H, C-27). 13C NMR (CDCl3, 100 MHz) d:
170.5, 169.4, 139.6, 122.0, 80.1, 79.2, 76.0, 73.7, 67.9, 50.9, 49.2, 42.3, 38.0,
36.8, 36.4, 35.9, 34.0, 31.4, 31.1 (2C), 30.9, 28.7, 27.6, 22.8, 22.5, 21.4, 21.1,
20.4, 19.3, 13.9, 12.4 ppm. MS (EI), m/z (relative intensity): 456
(Mþ 2 HOAc, 9), 296 (6), 270 (19), 105 (15), 81 (20), 55 (20), 43 (100).
1
20: FTIR (KBr), nmax 2932, 1735, 1709, 1246, 1041 cm21. H NMR
(CDCl3, 400 MHz) d: 5.39 (d, J ¼ 4.7 Hz, 1H, C-6), 4.93 (dd, J ¼ 8.0, 6.6 Hz,
1H, C-16), 4.66–4.58 (m, 1H, C-3), 2.05 (s, 3H, AcO), 1.95 (s, 3H, AcO), 1.60
(s, 3H, C-21), 1.10 (s, 3H, C-18), 1.04 (s, 3H, C-19), 0.88 (d, J ¼ 5.7 Hz, 6H,
C-26 and C-27). 13C NMR (CDCl3, 100 MHz) d: 210.4, 171.1, 170.2, 140.3,
122.5, 78.9, 78.4, 74.3, 67.9, 51.8, 49.9, 43.0, 38.6, 37.4, 37.3, 37.1, 34.6, 34.0,
32.7, 31.8, 31.7, 28.3, 28.2, 23.0, 22.9, 22.0, 21.0, 20.9, 19.9, 16.8, 14.5 ppm.

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
977
MS (EI), m/z (relative intensity): 454 (Mþ 2 HOAc, 1), 295 (2), 295 (3), 157
(5), 91 (7), 81 (12), 55 (11), 43 (100).
3b,16b-Diacetoxycholesta-5,17(20)-diene-22-one (18)
A solution of 16 (30 mg, 0.06 mmol) and o-iodoxybenzoic acid (50 mg,
0.18 mmol) in 1,2-dichloroethane (2 mL) was refluxed for 1.5 hr. After cooling
down, the reaction mixture was filtered through Celite and eluted with ethyl
acetate. The filtrate was dried over anhydrous Na2SO4, filtered and concen-
trated under reduced pressure. The residue was purified by column chroma-
tography (5% ethyl acetate/hexane) to give the desired product 18 (23.6 mg,
80%).
(20S,22R)-3b-Acetoxy-17a-hydroxycholesta-5-ene-16b,20,
22-orthoacetate (22)
To a solution of 17 (50 mg, 0.10 mmol) in CH2Cl2 (2 mL) at 08C was
.
added BF3 OEt2 (2 drops). After stirring at 08C for a few minutes, the reaction
mixture was quenched by addition of water and extracted with dichloro-
methane. The combined organic layers were washed with saturated aqueous
NaHCO3 solution, then water, dried over anhydrous Na2SO4, filtered and
concentrated under reduced pressure. The residue was purified by column
chromatography (2.5% ethyl acetate/hexane) to give 22 (40 mg, 80%) as
white solid with m.p. 232–232.58C. FTIR (KBr), nmax 3537, 2947, 1723,
1
1246, 1033 cm21. H NMR (CDCl3, 300 MHz) d: 5.38 (d, J ¼ 4.5 Hz, 1H,
C-6), 4.65–4.55 (m, 1H, C-3), 4.16 (dd, J ¼ 7.9, 3.7 Hz, 1H, C-16), 3.52 (dd,
J ¼ 9.7, 3.6 Hz, 1H, C-22), 2.03 (s, 3H, AcO), 1.38 (s, 3H, C-21), 1.13 (s, 3H,
C-18), 1.06 (s, 3H, C-19), 0.90 (d, J ¼ 6.4 Hz, 6H, C-26 and C-27). 13C NMR
(CDCl3, 75 MHz) d: 170.6, 139.6, 122.3, 116.7, 90.2, 83.7, 78.5, 77.8, 73.8,
49.8, 49.5, 47.4, 38.0, 37.4, 37.0, 36.6, 32.4, 31.8 (2C), 31.5, 28.3, 28.0, 27.7,
22.6, 22.5 (2C), 21.4, 20.3, 19.3, 17.2, 15.2 ppm. MS (EI), m/z (relative
intensity): 456 (Mþ 2 HOAc, 1), 396 (1), 214 (45), 139 (53), 91 (22), 55
(100), 69 (72), 79 (20).
(22R)-3b-Acetoxy-22-tert-butyldimethylsilyloxy-16b-4-
nitrobenzoylcholesta-5,17(20)-diene (24)
To a stirred solution of 14 (163 mg, 0.28 mmol), 4-nitrobenzoic acid
(62 mg, 0.37 mmol) and 4-(dimethylamino) pyridine (10.4 mg, 0.09 mmol) in

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978
Chaosuancharoen, Kongkathip, and Kongkathip
dichloromethane was added dicyclohexylcarbodiimide (76.5 mg, 0.37 mmol)
at room temperature. After a few minutes, a white solid precipitated from the
solution. The suspension was stirred at room temperature for 24 hr. The
precipitated white solid was filtered and the filtrate was extracted with
dichloromethane. The combined organic layers were washed with saturated
aqueous NH4Cl solution, then water, dried over anhydrous Na2SO4, filtered
and concentrated under reduced pressure. The residue was purified by column
chromatography (hexane) to provide 24 (94 mg, 45%, 73% based on recovered
14) as white solid; m.p. 128–1298C. FTIR (KBr), nmax 2952, 1731, 1720,
1
1530, 1276, 1242, 1074, 1030, 832, 716 cm21. H NMR (CDCl3, 400 MHz)
d: 8.25 (d, J ¼ 8.8 Hz, 2H, C-30 and C-50), 8.15 (d, J ¼ 8.8 Hz, 2H, C-20 and
C-60), 5.99 (t, J ¼ 7.1 Hz, 1H, C-16), 5.36 (d, J ¼ 4.2 Hz, 1H, C-6), 4.65–4.50
(m, 1H, C-3), 4.02 (d, J ¼ 8.8 Hz, 1H, C-22), 2.50–2.57 (m, 1H, C-15), 2.01
(s, 3H, AcO), 1.74 (s, 3H, C-21), 1.10 (s, 3H, C-18), 1.02 (s, 3H, C-19), 0.84
(s, 9H, –C(CH3)3), 0.54 (d, J ¼ 6.5 Hz, 3H, C-26), 0.32 (d, J ¼ 6.5 Hz, 3H,
C-27), 0.00 (s, 3H, –SiCH3), 20.04 (s, 3H, –SiCH3). 13C NMR (CDCl3,
100 MHz) d: 171.1, 164.4, 151.1, 141.0, 140.4, 137.7, 136.5, 131.3 (2C), 124.1
(2C), 122.7, 76.1, 74.4, 74.0, 52.8, 50.2, 45.1, 38.7, 38.0, 37.4, 37.2, 36.4,
34.8, 33.7, 32.0, 31.8, 28.7, 28.3, 26.4 (3C), 23.0, 22.9, 22.0, 21.6, 19.9, 18.7,
17.1, 12.8, 23.8, 24.3 ppm.
(22R)-3b-Acetoxy-16b-4-nitrobenzoylcholesta-5,17(20)-diene-
22-ol (25)
A solution of 24 (87 mg, 0.12 mmol) and pyridinium p-toluenesulfonate
(23 mg, 0.09 mmol) in methanol (6 mL) and dichloromethane (2 mL) was
stirred at room temperature for 20 hr. The reaction was quenched by addition
of saturated aqueous NaCl solution and extracted with dichloromethane. The
combined organic layers were washed with water, dried over anhydrous
Na2SO4, filtered and concentrated under reduced pressure. The residue was
purified by column chromatography (hexane and 10% ethyl acetate/hexane)
to provide both recovered 24 (35.8 mg, 41%) and the product 25 (31.4 mg,
43%, 73% based on recovered 24) as white solid; m.p. 170–1718C. FTIR
(KBr), nmax 3564, 2952, 1731, 1716, 1528, 1277, 1244, 1030 cm21. 1H NMR
(CDCl3, 300 MHz) d: 8.26 (d, J ¼ 8.8 Hz, 2H, C-30 and C-50), 8.14
(d, J ¼ 8.8 Hz, 2H, C-20 and C-60), 5.99 (t, J ¼ 7.0 Hz, 1H, C-16), 5.35
(d, J ¼ 4.8 Hz, 1H, C-6), 4.65–4.50 (m, 1H, C-3), 4.16 (d, J ¼ 7.7 Hz, 1H,
C-22), 2.56–2.47 (m, 1H, C-15), 2.01 (s, 3H, AcO), 1.79 (s, 3H, C-21), 1.11
(s, 3H, C-18), 1.02 (s, 3H, C-19), 0.58 (d, J ¼ 6.5 Hz, 3H, C-26), 0.40
(d, J ¼ 6.5 Hz, 3H, C-27). 13C NMR (CDCl3, 75 MHz) d: 170.7, 163.9, 150.6,
142.5, 139.8, 135.8, 135.3, 130.6 (2C), 123.6 (2C), 122.1, 75.6, 73.8, 72.8,

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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
979
52.1, 49.6, 44.6, 38.0, 37.2, 36.8, 36.6, 35.6, 33.3, 33.0, 31.5, 31.2, 28.2, 27.8,
22.3, 22.2, 21.4, 21.0, 19.3, 16.5, 11.6 ppm.
(20R)-3b-Acetoxy-16b-4-nitrobenzoyl-20,22-epoxycholesta-5-
ene-17a-ol (26)
A solution of 25 (23 mg, 0.04 mmol) and pyridinium dichromate (43 mg,
0.12 mmol) in dichloromethane (2 mL) was stirred at room temperature for
3 hr. Then the reaction mixture was filtered through Celite and eluted with
dichloromethane. The filtrate was washed with saturated aqueous NaHCO3
solution, then water, dried over anhydrous Na2SO4, filtered and concentrated
under reduced pressure. The residue was purified by column chromatography
(5% ethyl acetate/hexane) to give 26 (9 mg, 38%) as white solid; m.p. 174–
1758C. FTIR (KBr), nmax 3440, 3104, 2955, 1731, 1705, 1522, 1268, 1238,
1033 cm21. 1H NMR (CDCl3, 400 MHz) d: 8.35 (d, J ¼ 9.0 Hz, 2H, C-30 and
C-50), 8.28 (d, J ¼ 9.0 Hz, 2H, C-20 and C-60), 5.41 (d, J ¼ 4.8 Hz, 1H, C-6),
5.26 (dd, J ¼ 8.1, 4.9 Hz, 1H, C-16), 4.68–4.60 (m, 1H, C-3), 3.14
(t, J ¼ 6.1 Hz, 1H, C-22), 2.57–2.50 (m, 2H, C-15), 2.06 (s, 3H, AcO), 1.43
(s, 3H, C-21), 1.18 (s, 3H, C-18), 1.07 (s, 3H, C-19), 0.82 (d, J ¼ 6.6 Hz, 3H,
C-26), 0.81 (d, J ¼ 6.6 Hz, 3H, C-27). 13C NMR (CDCl3, 100 MHz) d: 171.2,
164.3, 151.2, 140.3, 136.3, 131.4 (2C), 124.4 (2C), 122.8, 84.6, 83.8, 74.4,
63.0, 60.4, 50.1, 49.3, 47.5, 38.7, 37.6, 37.2, 36.0, 34.1, 32.2 (2C), 31.7, 28.4,
28.3, 26.3, 23.1, 22.8, 22.0, 20.8, 19.9, 17.8, 15.9 ppm.
(20S,22R)-3b-Acetoxy-17a-hydroxycholesta-5-ene-16b,20,22-
ortho-4-nitrobenzoate (27)
To a solution of 26 (9 mg, 0.014 mmol) in CH2Cl2 (1 mL) at 08C was
added BF3 OEt2 (1 drop), followed by stirring at 08C for a few minutes. The
reaction mixture was quenched by addition of water and extracted with
dichloromethane. The combined organic layer were washed with saturated
aqueous NaHCO3 solution, then water, dried over anhydrous Na2SO4, filtered
and concentrated under reduced pressure. The residue was purified by column
chromatography (10% ethyl acetate/hexane) to give 27 (8.3 mg, 95%) as
white solid; m.p. 234–2358C. FTIR (KBr), nmax 3522, 2947, 1731, 1522,
1
1257, 1033 cm21. H NMR (CDCl3, 400 MHz) d: 8.25 (d, J ¼ 9.0 Hz, 2H,
C-30 and C-50), 7.84 (d, J ¼ 9.0 Hz, 2H, C-20 and C-60), 5.42 (d, J ¼ 4.5 Hz,
1H, C-6), 4.68–4.60 (m, 1H, C-3), 4.42 (dd, J ¼ 8.0, 3.4 Hz, 1H, C-16), 3.77
(dd, J ¼ 9.8, 3.7 Hz, 1H, C-22), 2.07 (s, 3H, AcO), 1.50 (s, 3H, C-21), 1.25
(s, 3H, C-18), 1.10 (s, 3H, C-19), 0.96 (d, J ¼ 6.4 Hz, 6H, C-26 and C-27). 13
C

ORDER
REPRINTS
980
Chaosuancharoen, Kongkathip, and Kongkathip
NMR (CDCl3, 100 MHz) d: 171.2, 149.1, 144.2, 140.3, 127.9 (2C), 123.9
(2C), 122.8, 115.7, 91.8, 85.7, 79.4 (2C), 74.4, 50.0, 50.2, 48.2, 38.7, 38.0,
37.6, 37.2, 33.0, 32.4 (2C), 32.2, 28.9, 28.4, 28.3, 23.2, 23.1, 22.0, 20.9, 19.9,
17.8, 15.9 ppm.
(22R)-3b-Acetoxy-22-tert-butyldimethylsilyloxy-16b-2,4-
dinitrobenzoylcholesta-5,17(20)-diene (28)
To a stirred solution of 27 (280 mg, 0.49 mmol), 2,4-dinitrobenzoic acid
(210 mg, 0.99 mmol) and 4-(dimethylamino) pyridine (30 mg, 0.25 mmol) in
dichloromethane was added dicyclohexylcarbodiimide (200 mg, 0.98 mmol)
at room temperature. After a few minutes, a white solid precipitated from the
solution. The suspension was stirred at room temperature for 9 hr. The
precipitated white solid was filtered and the filtrate was extracted with
dichloromethane. The combined organic layers were washed with saturated
aqueous NH4Cl solution, then water, dried over anhydrous Na2SO4, filtered
and concentrated under reduced pressure. The residue was purified by flash
column chromatography (10% ethyl acetate/hexane) to provide 28 (215 mg,
56%). FTIR (KBr), nmax 3105, 2947, 1731, 1548, 1541, 1280, 1242, 1074,
1030, 832, 772 cm21. 1H NMR (CDCl3, 400 MHz) d: 8.72 (d, J ¼ 2.2 Hz, 1H,
C-30) 8.52 (dd, J ¼ 8.4, 2.2 Hz, 1H, C-50), 8.02 (d, J ¼ 8.4 Hz, 1H, C-60), 5.90
(t, J ¼ 7.2 Hz, 1H, C-16), 5.42 (d, J ¼ 4.5 Hz, 1H, C-6), 4.67–4.59 (m, 1H,
C-3), 4.08 (dd, J ¼ 9.9, 2.5 Hz, 1H, C-22), 2.62–2.55 (m, 1H, C-15), 2.07
(s, 3H, AcO), 1.74 (d, J ¼ 0.9 Hz, 3H, C-21), 1.07 (s, 3H, C-18), 1.04 (s, 3H,
C-19), 0.89 (s, 9H, C(CH3)3), 0.64 (d, J ¼ 6.6 Hz, 3H, C-26), 0.52
(d, J ¼ 6.6 Hz, 3H, C-27), 0.07 (s, 3H, -SiCH3), 0.01 (s, 3H, -SiCH3). 13C
NMR (CDCl3, 100 MHz) d: 171.2, 163.3, 149.7, 149.5, 140.5, 140.2, 138.2,
132.9, 132.3, 127.5, 122.7, 120.0, 78.6, 74.4, 73.8, 52.6, 50.2, 45.0, 38.6, 37.9,
37.4, 37.2, 36.5, 34.6, 32.4, 32.0, 31.6, 28.9, 28.3, 26.4 (3C), 23.1, 23.0, 22.0,
21.6, 19.9, 18.8, 16.7, 12.8, 23.8, 24.3 ppm.
(22R)-3b-Acetoxy-16b-2,4-dinitrobenzoylcholesta-5,17(20)-
diene-22-ol (29)
A solution of 28 (194 mg, 0.25 mmol) and pyridinium p-toluenesulfonate
(49 mg, 0.20 mmol) in methanol (15 mL) and dichloromethane (5 mL) was
stirred at room temperature for 36 hr. The reaction was quenched by addition
of saturated aqueous NaCl solution and extracted with dichloromethane. The
combined organic layers were washed with water, dried over anhydrous
Na2SO4, filtered and concentrated under reduced pressure. The residue was
purified by column chromatography (5% ethyl acetate/hexane and 10% ethyl

ORDER
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Synthetic Approach from Diosgenin to 17a-Hydroxy Orthoester
981
acetate/hexane) to provide both recovered 28 (110 mg, 56.7%) and the
product 29 (42 mg, 26%, 60% based on recovered 28), the latter as a white
solid; m.p. 134–1358C. FTIR (KBr), nmax 3447, 3104, 2947, 1727, 1544,
1
1283, 1246, 1030 cm21. H NMR (CDCl3, 300 MHz) d: 8.73 (d, J ¼ 2.1 Hz,
1H, C-30), 8.54 (dd, J ¼ 8.4, 2.2 Hz, 1H, C-50), 8.02 (d, J ¼ 8.4 Hz, 1H, C-60),
5.91 (t, J ¼ 7.2 Hz, 1H, C-16), 5.42 (d, J ¼ 4.6 Hz, 1H, C-6), 4.68-4.60 (m, 1H,
C-3), 4.21 (dd, J ¼ 9.1, 3.0 Hz, 1H, C-22), 2.63-2.57 (m, 1H, C-15), 2.07
(s, 3H, AcO), 1.78 (d, J ¼ 0.8 Hz, 3H, C-21), 1.08 (s, 3H, C-18), 1.04 (s, 3H,
C-19), 0.65 (d, J ¼ 6.4 Hz, 3H, C-26), 0.55 (d, J ¼ 6.5 Hz, 3H, C-27). 13C
NMR (CDCl3, 100 MHz) d: 171.2, 163.4, 149.7, 149.4, 142.5, 140.4, 136.2,
133.0, 132.2, 127.6, 122.7, 120.1, 78.6, 74.4, 73.3, 52.6, 50.2, 45.2, 38.6, 38.0,
37.4, 37.2, 36.2, 33.6, 32.3, 32.0, 31.7, 28.9, 28.3, 23.0, 22.9, 22.1, 21.6, 19.9,
16.7, 12.0 ppm.
(20R)-3b-Acetoxy-16b-2,4-dinitrobenzoyl-20,22-
epoxycholesta-5-ene-17a-ol (30)
A solution of 29 (30 mg, 0.05 mmol) and pyridinium dichromate (52 mg,
0.14 mmol) in dichloromethane (2 mL) was stirred at room temperature for
3 hr. Then the reaction mixture was filtered through Celite and eluted with
dichloromethane. The filtrate was washed with saturated aqueous NaHCO3
solution, then water, dried over anhydrous Na2SO4, filtered and concentrated
under reduced pressure. The residue was purified by column chromatography
(7.5% ethyl acetate/hexane) to give 30 (15 mg, 47%) as white solid; m.p.
165–1668C. FTIR (KBr), nmax 3440, 3106, 2947, 1738, 1705, 1544, 1280,
1
1250, 1030 cm21. H NMR (CDCl3, 400 MHz) d: 8.83 (d, J ¼ 2.1 Hz, 1H,
C-30), 8.58 (dd, J ¼ 8.4, 2.2 Hz, 1H, C-50), 8.04 (d, J ¼ 8.4 Hz, 1H, C-60), 5.27
(dd, J ¼ 8.2, 5.3 Hz, 1H, C-16), 5.42 (d, J ¼ 4.8 Hz, 1H, C-6), 4.68–4.60
(m, 1H, C-3), 3.10 (dd, J ¼ 6.9, 5.4 Hz, 1H, C-22), 2.54–2.48 (m, 1H, C-15),
2.07 (s, 3H, AcO), 1.37 (s, 3H, C-21), 1.07 (s, 3H, C-18), 0.98 (s, 3H, C-19),
0.88 (d, J ¼ 6.5 Hz, 3H, C-26), 0.86 (d, J ¼ 6.5 Hz, 3H, C-27). 13C NMR
(CDCl3, 100 MHz) d: 171.2, 163.2, 149.6, 148.6, 140.2, 133.5, 131.9, 128.8,
122.8, 120.2, 85.6, 84.6, 74.4, 63.5, 60.8, 50.1, 48.8, 47.5, 38.6, 37.5, 37.2,
36.2, 33.0, 32.3, 32.2, 31.7, 28.5, 28.3, 26.4, 23.2, 22.9, 22.0, 20.8, 19.9, 17.6,
15.6 ppm.
(20S,22R)-3b-Acetoxy-17a-hydroxycholesta-5-ene-16b,20,22-
ortho-2,4-dinitrobenzoate (31)
To a solution of 30 (10 mg, 0.015 mmol) in CH2Cl2 (1 mL) at 08C was
.
added BF3 OEt2 (1 drop), followed by stirring at 08C for 1 hr. The reaction

ORDER
REPRINTS
982
Chaosuancharoen, Kongkathip, and Kongkathip
mixture was quenched by addition of water and extracted with dichloro-
methane. The combined organic layers were washed with saturated aqueous
NaHCO3 solution, then water, dried over anhydrous Na2SO4, filtered, and
concentrated under reduced pressure. The residue was purified by column
chromatography (5% ethyl acetate/hexane) to give 31 (2.5 mg, 25%) as white
solid. FTIR (KBr), nmax 3514, 3104, 2947, 1731, 1716, 1548, 1254,
1
1045 cm21. H NMR (CDCl3, 400 MHz) d: 8.38 (dd, J ¼ 8.7, 2.2 Hz, 1H,
C-50), 8.33 (d, J ¼ 2.2 Hz, 1H, C-30), 8.09 (d, J ¼ 8.7 Hz, 1H, C-60), 5.41
(d, J ¼ 4.8 Hz, 1H, C-6), 4.68–4.60 (m, 1H, C-3), 4.39 (dd, J ¼ 8.0, 3.4 Hz,
1H, C-16), 3.67 (dd, J ¼ 10.2, 3.4 Hz, 1H, C-22), 2.25–2.29 (m, 1H, C-15),
2.07 (s, 3H, AcO), 1.50 (s, 3H, C-21), 1.18 (s, 3H, C-18), 1.09 (s, 3H, C-19),
0.94 (d, J ¼ 6.6 Hz, 3H, C-26 and C-27).
ACKNOWLEDGMENTS
N.C. is a Ph.D student under the Royal Golden Jubilee Program, the
Thailand Research Fund (TRF). We are grateful for the financial support from
the TRF, through the Royal Golden Jubilee Program. Part of the research was
conducted by N.C. as a guest worker in the Laboratory of Bioorganic
Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases
(NIDDK), National Institutes of Health (NIH). We thank Dr. John W. Daly
and Dr. Thomas F. Spande for helpful suggestions and discussions. Some 1H,
13C NMR and mass spectral data were kindly provided by Prof. Walter
C. Taylor, University of Sydney, Australia, Mr. Wesley White, Dr. H. J. C.
Yeh and Mr. Noel Whittaker, NIDDK, NIH. We also are indebted to Assistant
Professor Palangpon Kongsaeree, x-ray Crystallographic Laboratory, Mahidol
University for x-ray crystallography of compound 13b.
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Received in Japan July 25, 2003

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