A new synthesis of zymosterol

Article abstract of DOI:10.1023/A:1015768623363

A modified scheme for the synthesis of zymosterol, one of the biosynthetically important yeast sterols, starting from 3-benzoyloxyergosta-8(14),22-dien-15-one has been suggested.

Full text of DOI:10.1023/A:1015768623363

Russian Journal of Bioorganic Chemistry, Vol. 28, No. 3, 2002, pp. 250–256. Translated from Bioorganicheskaya Khimiya, Vol. 28, No. 3, 2002, pp. 277–283.
Original Russian Text Copyright © 2002 by Baranovskii, Litvinovskaya, Khripach.
A New Synthesis of Zymosterol
1
A. V. Baranovskii, R. P. Litvinovskaya , and V. A. Khripach
Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus,
ul. Kuprevicha 5/2, Minsk, 220141 Belarus
Received July 03, 2001; in final form, August 13, 2001
Abstract—A modified scheme for the synthesis of zymosterol, one of the biosynthetically important yeast ste-
rols, starting from 3-benzoyloxyergosta-8(14),22-dien-15-one has been suggested.
Key words: steroids, synthesis of steroid side chains, zymosterol
INTRODUCTION
epimerization of the C20 chiral center by performing
this reaction at low temperature (–70°C). Similar
results (concerning the yield and the isomer ratio) were
also obtained in the case of other 22-aldehydes, e.g., upon
the synthesis of ∆22 analogues of cholic acids [7–10].
Zymosterol (I) is an important intermediate in the
biosynthesis of cholesterol and ergosterol from lano-
sterol. The isolation of zymosterol from the yeast ste-
rol fraction is the main way of obtaining zymosterol [1–
3]. However, this method is ineffective, because zymo-
sterol is a rare yeast sterol. A number of methods for the
chemical synthesis of this compound have been devel-
oped [4, 5], but all of them are multistep (up to 20 steps
from ergosterol), laborious, and only a low yield of the
product is obtained.
2
According to the literature data, the reduction of the
double bond in the side chain of a structurally similar
systems is not a serious problem. Various catalysts were
suggested for the hydrogenation [11–14], and their use
results in high yields. Obviously, the presence of the
tetrasubstituted ∆8(14) bond in (IV) should not affect the
selectivity of the ∆22 bond reduction. However, the use
of the Pd/C catalyst turned out to be ineffective. This
was likely due to the presence of negligible amounts of
phosphonate admixtures that are difficult to remove
[15]. The use of magnesium in methanol [10, 15]
resulted in the saturation of the conjugated double
bond, but was complicated by the significant hydrolysis
of benzoate. Raney nickel turned out to be an effective
catalyst, as its use helped obtain a high yield of (V)
(94%). The structure of (V) was confirmed by the
absence of the signals of olefin protons and the pres-
ence of the resonance of 7β proton in a low field
(I)
RO
RESULTS AND DISCUSSION
We suggest here a modified method for the synthesis
of zymosterol starting from 3-bensoyloxyergosta-
8(14),22-dien-15-one (II) [6] that reduces the number
of stages. The Horner–Emmons reaction with aldehyde
(III) [4], the ozonolysis product of (I), was chosen as
the main approach for the partial formation of the
zymosterol side chain. The treatment of aldehyde (III)
with triethyl phosphonoacetate anion proceeds in a
high (91%) yield and results in ester (IV) with the
Ö-geometry of the ∆22 bond (J22, 23 = 15 Hz). The less
polar Z-alkene was also found in the reaction mixture
by 1H NMR, but we did not isolate it because it was a
minor product and its reduction, like that of (IV), would
result in the same product. We successfully avoided the
1
(δ 4.14 ppm) of its H NMR spectrum. Note also that
the presence of resonances of C8 and C14 (δ 140.8 and
151.2 ppm, respectively) in the 13C NMR spectrum of
(V) confirms the presence of an intact ∆8(14) double
bond in the molecule.
For the formation of the 8(9) double bond, enone
(V) was transformed into diene (VII). This transforma-
tion was achieved through the intermediate enol ester
(VI), which was obtained by treatment of (V) with tri-
fluoromethanesulfonic anhydride in the presence of
2,6-di-tert -butyl-4-methylpyridine. Subsequent reduc-
tion of trifluoromethanesulfonyl enol ester (VI) on the
Pd-containing catalyst [4] resulted in diene (VII) (a
yield of 88%). The structure of (VII) was confirmed, in
particular, by the presence of the signal of the 15-olefin
1
Please send correspondence to: fax: (375-17) 264-8647; e-mail:
litvin@iboch.bas-net.by.
Abbreviations: AIBN, azodiisobutyronitrile; DIBAH, diisobutyl
aluminum hydride; and DMAP, 4,4-N,N-dimethylaminopyridine.
1
2
proton at δ 5.38 ppm) in its H NMR spectrum. We
should note that all attempts to hydroborate enol ester
1068-1620/02/2803-0250$27.00 © 2002 MAIK “Nauka /Interperiodica”

A NEW SYNTHESIS OF ZYMOSTEROL
251
CO2Et
O
i
ii
O
O
O
BzO
(II)
(III)
(IV)
iii
CO2Et
CO2Et
CO2Et
vi
iv
OR
R
O
(V)
(VI) R = OTf
(VII) R = H
(VIII) R = H
(IX) R = Ac
vii
v
viii
(X) R = C(S)OPh
ix
CO2Me
CO2Et
CO2Me
x
xi
tBuMe2Si
BzO
HO
(XI)
(XII)
(XIII)
xii
R
xiv
tBuMe2Si
RO
(XIV) R = CHO
(XV) R = CH2OH
(XVI) R = tBuMe2Si
(I) R = H
xiii
xv
Reagents, conditions, and yields: (i) (1) O , Sudan III, CH Cl , –78°C, 75% and (2) Me S; (ii) (EtO) P(O)CH CO Et, NaH, THF,
3
2
2
2
2
2
2
–78°C, 91%; (iii) Ra/Ni, H , EtOH, 20°C, 94%; (iv) Tf O, 2,6-di-tert-butyl-4-methylpyridine, CH Cl , 20°C, 99%; (v) Pd(OAc) ,
2
2
2
2
2
Ph P, Bu N, HCO H, DMF, 70°C, 88%; (vi) (1) BH · Me S, THF, 15°C and (2) Et N, H O , H O, THF, 20°C, 48%; (vii) Ac O,
3
3
2
3
2
3
2
2
2
2
DMAP, CH Cl , 20°C, 92%; (viii) PhOCSCl, DMAP, CH Cl , 20°C, 99%; (ix) Bu SnH, AIBN, PhMe, 90°C, 75%; (x) KOH,
2
2
2
2
3
MeOH, THF, 20°C, 86%; (xi) tBuMe SiCl, imidazole, DMF, 20°C, 93%; (xii) DIBAH, PhMe, –78°C, 87% of (XIII);
2
(xiii) (COCl) , DMSO, Et N, CH Cl , –78°C, 67%; (xiv) BuLi, iPrPh PI, THF, 0°C, 87%; and (xv) nBu NF, THF, 20°C, 97%.
2
3
2
2
3
4
(VI) for the subsequent obtainment of 8(9)-ene steroids tion, we used a two-step procedure of hydroboration–
deoxygenation [4, 16] for the synthesis of the ∆8-deriv-
ative from the 8(9),14(15)-diene (VII). As we learned
that the ester groups undergo partial hydrolysis upon
oxidation of the intermediate boron derivative, we
replaced the alkaline hydrogen peroxide with a neutral
failed.
The catalytic hydrogenation or hydroboration of
(VII) followed by treatment with propionic acid led us
(and other authors who studied the relative diene struc-
tures [4, 5]) to a nearly inseparable mixture of ∆8(9)- and
∆
8(14)-steroids in comparable amounts. In this connec- oxidizer. We used triethylamine oxide generated in situ,
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 28 No. 3 2002

252
BARANOVSKII et al.
which was more effective than trimethylamine oxide to 67% and, hence, increase its total yield. The signal of
under these conditions. The alcohol (VIII) was thus 24-proton (δ 9.74 ppm) as a broadened singlet in the
1H NMR spectrum and the bands of oscillations of the
aldehyde carbonyl group at 1750 and 2720 cm–1 in the
IR spectrum are the most characteristic parameters that
confirm the structure of aldehyde (XIV).
Aldehyde (XIV) readily enters the Wittig reaction
[10, 25–27] with the corresponding ylide and gives
diene (XVI). Its 3O-protective group was hydrolyzed
with tetrabutylammonium fluoride to give alcohol (I),
all the physicochemical characteristics of which coin-
cided with those of natural zymosterol [2, 4, 5, 28–31].
Thus, we have developed a new scheme for the syn-
thesis of zymosterol starting from 3-benzoyloxyer-
gosta-8(14),22-dien-15-one, which allowed us to
decrease the number of stages in comparison with that
suggested previously [4].
obtained in a yield of 48%. The starting diene was also
isolated from the reaction mixture, and its conversion
was 89%. The signal of the 15β proton (δ 4.11 ppm)
appeared in the 1H NMR spectrum of (VIII), whereas
the band corresponding to the valent stretching oscilla-
tions of the hydroxyl group (3550 cm–1) appeared in its
IR spectrum. To additionally confirm its structure,
(VIII) was acetylated with acetic anhydride in the pres-
ence of DMAP to give the corresponding derivative
(IX). This exhibited the signal from the protons of
acetyl group in its 1H NMR spectrum, a downfield shift
of the signal of 15-proton, and the disappearance of the
band corresponding to the oscillations of the hydroxyl
group from its IR spectrum.
To perform the Barton deoxygenation, alcohol
(VIII) was converted into xanthate (X), and the latter
was subjected to reduction with tri-n-butyltin hydride
in the presence of AIBN. The standard procedure [17]
was ineffective, as diene (VII) was the main product;
that is, the Chugaev reaction prevailed. After a number
of attempts, we found the conditions under which
reduction prevailed over elimination. Thus, the addition
of substrate (X) and the reagents to the reaction flask
preliminarily heated to 90°C [18] resulted in a 75%
yield of (XI). Ester (XI) has the completely formed
cyclic moiety of zymosterol, which was most clearly
proved by its 13C NMR spectrum (the signals of C8 and
C9 at 128.3 and 135 ppm).
EXPERIMENTAL
Melting points were determined on a Kofler hot
1
plate. H and 13C NMR spectra were registered on a
Bruker A-200 (200 MHz) spectrometer in CDCl3
(unless otherwise specified) using Me4Si as the internal
standard. The values of chemical shifts (δ, ppm) and
spin coupling constants (J, Hz) are given. IR spectra
were taken on a UR-20 instrument in films or in KBr
pellets. Mass spectra were measured on a Hewlett–
Packard 5890 spectrometer using linear temperature
programming from 40 to 280°ë at a rate of 10°/min and
The completion of zymosterol synthesis required an accelerating voltage of 70 eV. The solvents were
the construction of its side chain by the introduction of purified according to standard procedures [32]. All
isopropenyl moiety. This could be achieved by the reactions were performed in a nitrogen atmosphere.
reduction of the side chain ester to the corresponding The reactions were monitored by TLC on Silica gel
aldehyde followed by the Wittig reaction. However, it 60 F254 precoated plates (Merck). The reaction mix-
was necessary to preliminarily replace the protective
group in position 3, because it was impossible to reduce
the ester group in the side chain by DIBAH selectively
in the presence of the benzoate moiety. In one attempt,
steroid (XI) gave a mixture of all four possible products
upon reduction even at a very low temperature (–78°C).
The alkaline hydrolysis of benzoate with potassium
hydroxide in methanol was accompanied by a transes-
terification and resulted in a 86% yield of alcohol (XII);
some C24-acid was also observed. A similar situation
was also retained with sodium methylate or Ba(OH)2 in
tures were chromatographically separated on Silica gel
60 (40–60 µm, Merck).
The starting (II) was synthesized in seven steps
starting from ergosterol [6]; mp 174–176°ë (åÂéç–
CH2Cl2).
Aldehyde (III), mp 182–185°ë (methanol), was
obtained from (II) in 75% yield by the procedure
reported in [4].
(22E)-3b-Benzoyloxy-15-oxo-5a-chola-8(14),22-
dien-24-oic acid ethyl ester (IV). Sodium hydride
place of KOH. Alcohol (XII) was converted into silyl (80% suspension, 1.75 g, 58 mmol) was added in small
ester (XIII), and the latter was reduced with DIBAH portions to a stirred solution of triethyl phosphonoace-
[10, 19]. The reduction resulted in two products, labile tate (13.2 g, 11.7 ml, 59 mmol) in THF (70 ml) at room
aldehyde (XIV) (87%) and alcohol (XV) (10%). We temperature. After the gas liberation ceased, the solu-
should mention that 24-alcohols like (XV) are often tion was cooled to –70°ë and a solution of aldehyde
used to obtain 24-aldehydes, which are the starting (III) (17.6 g, 39.3 mmol) in THF (150 ml) was added.
products in the synthesis of a number of ∆24-sterols
(e.g., desmosterol) using the Wittig reaction. Oxidation
to the aldehyde is usually achieved by pyridinium chlo-
The resulting solution was stirred for 30 min at –70°ë
and for 1 h at –20°ë and then treated with saturated
aqueous NH4Cl solution (100 ml) and diethyl ether
rochromate [20] or oxalyl chloride in DMSO according (150 ml). The aqueous phase was separated and
to Swern [21–24]. The use of the Swern oxidation extracted with ether. The combined organic extract was
allowed us to increase the yield of (XIV) from (XV) up washed with water and saturated aqueous NaCl solu-
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 28 No. 3 2002

A NEW SYNTHESIS OF ZYMOSTEROL
253
tion, dried over Na2SO4, and evaporated in a vacuum.
The resulting oily residue was separated on a silica gel
column eluted with 5 : 95 EtOAc–toluene to yield
18.52 g (91%) of ester (IV), mp 165–167°ë (metha-
nol), αD +74° (c 1.30, CHCl3); 1ç NMR: 0.79 (3 H, s,
18-Me), 1.02 (3 H, s, 19-Me), 1.18 (3 H, d, J 6.5,
21-Me), 1.29 (3 H, t, J 7.1, ëç3ëç2), 4.14 (1 H, m,
H7β), 4.14 (2 H, q, J 7.1, ëç3ëç2), 5.00 (1 H, m,
H3α), 5.79 (1 H, d, J 15, H23), 6.82 (1 H, dd, J 9 and
15, H22), 7.40 (2 H, m, m-H in Ph), 7.53 (1 H, m, p-H
3b-Benzoyloxy-5a-chola-8,14-dien-24-oic acid
ethyl ester (VII). Compound (VI) (10.1 g), Pd(OAc)2
(188 mg, 0.84 mmol), triphenylphosphine (438 mg,
1.94 mmol), tributylamine (15.6 ml, 57.2 mmol), and
formic acid (1.7 ml) were successively dissolved in
DMF (30 ml). The resulting solution was stirred for
40 min at 70°ë and then 25 ml of the solvent was
removed in a vacuum. The residue was cooled and
diluted with water (100 ml). The organic material was
extracted with ethyl acetate. The extract was washed
with 1 N HCl, saturated aqueous NaHCO3, and satu-
rated aqueous NaCl, dried over Na2SO4, and evapo-
rated. The residue was chromatographed on a silica gel
13
in Ph), and 8.02 (2 H, d, J 7.3, o-H in Ph); C NMR:
13.5 q, 14.9 q, 19.7 q, 20.1 t, 20.5 q, 27.9 t, 28.1 t, 29.7
t, 34.3 t, 36.9 t, 37.5 t, 39.3 s, 39.4 d, 42.9 s, 43.6 t, 44.6
d, 50.6 d, 51.3 d, 60.9 t, 74.3 d, 120.8 d, 128.8 two d,
column eluted with 5 : 95 EtOAc–toluene to yield 6.65 g
1
130.1 two d, 131.4 s, 133.4 d, 140.2 s, 151.6 s, 153.2 d, (88%) of diene (VII), mp 138–140°ë (methanol); H
166.7 s, 167.2 s, and 207.1 s; IR (ν, KBr, cm–1): 1720,
NMR: 0.83 (3 H, s, 18-Me), 0.96 (3 H, d, J 5.5, 21-Me),
1.06 (3 H, s, 19-Me), 1.26 (3 H, t, J 7.1, ëç3ëç2), 4.13
(2 H, q, J 7.1, ëç3ëç2), 4.97 (1 H, m, H3α), 5.38
((1 H, s, H15), 7.40 (2 H, m, m-H in Ph), 7.52 (1 H, m,
p-H in Ph), and 8.05 (2 H, d, J 7.3, Ó-H in Ph); 13C
NMR: 15.7 q, 17.2 q, 19.8 q, 20.0 q, 23.3 t, 26.7 t, 28.0 t,
29.3 t, 32.4 t, 32.8 t, 35.2 d, 35.7 t, 36.6 t, 37.3 t, 39.1 t,
39.3 s, 42.3 d, 46.6 s, 58.4 d, 61.7 t, 75.4 d, 118.9 d,
124.7 s, 129.7 two d, 131.0 two d, 132.3 s, 134.2 d,
142.0 s, 152.3 s, 167.6 s, and 175.7 s; IR (ν, KBr, cm–1):
.
1630, 1450, 1280, and 1120; MS, m/z: 518 [M]+ , 472,
429, 396, 251, and 105.
3b-Benzoyloxy-15-oxo-5a-chol-8(14)-en-24-oic
acid ethyl ester (V). Raney nickel (6 g) was added to a
solution of (IV) (47 g, 90.7 mmol) in ethanol (1 l). The
flask was evacuated and filled with hydrogen. The mix-
ture was hydrogenated with vigorous stirring for 24 h.
The catalyst was filtered off and washed with CH2Cl2.
The filtrates were combined and concentrated. The
resulting oily residue was dissolved in EtOAc and fil-
tered through a silica gel layer to give 44.3 g (94%) of
saturated ester (V); mp 125–127°ë (methanol); αD
.
1750, 1730, 1280, and 1120; MS, m/z: 504 [M]+ , 459,
367, 253, and 105.
3b-Benzoyloxy-15a-hydroxy-5a-chol-8-en-24-oic
acid ethyl ester (VIII). A solution of (VII) (10.2 g,
20.2 mmol) in THF (60 ml) was cooled to 0°C and
treated with 16.2 ml (32.3 mmol) of 2 M borane–
methyl sulfide complex in THF under stirring. The
resulting solution was stirred for 2 h at 15°C, cooled to
0°C, and carefully treated with water (2 ml). After the
gas liberation ceased, triethylamine (5.7 ml) and then
30% H2O2 (2.2 ml) were added to the solution. The
mixture was stirred for 2 h at 15°C, treated with
Na2S2O3 and NaCl solutions, the organic phase was
separated, and the aqueous phase was extracted with
ethyl acetate. The extract was dried over Na2SO4 and
concentrated in a vacuum. The residue was chromato-
graphed on a silica gel column eluted with 1 : 9 EtOAc–
toluene to give 4.7 g (46%) of the starting diene (VII)
and 5.0 g (48%) of alcohol (VIII) as an oil; 1H NMR:
0.64 (3 H, s, 18-Me), 0.92 (3 H, d, J 5.5, 21-Me), 1.01
(3 H, s, 19-Me), 1.24 (3 H, t, J 7.1, ëç3ëç2), 4.11
(2 H, q, J 7.1, ëç3ëç2), 4.11 (1 H, m, H15β), 4.96 (1
H, m, H3α), 7.42 (2 H, m, m-H in Ph), 7.54 (1 H, m, p-
H in Ph), and 8.03 (2 H, d, J 7.3, Ó-H in Ph); 13C NMR:
12.5 q, 14.2 q, 17.6 q, 18.0 q, 21.4 d, 22.6 t, 25.3 t, 27.1
t, 27.6 t, 30.7 t, 31.1 t, 34.1 t, 34.8 t, 35.3 d, 35.8 s, 37.0
t, 40.3 t, 43.1 s, 52.6 d, 59.4 d, 60.1 t, 71.7 d, 74.0 d,
126.9 s, 128.1 two d, 129.4 two d, 130.8 s, 132.6 d,
135.7 s, 166.0 s, and 174.0 s; IR (ν, film, cm–1): 3550,
1740, 1720, 1610, 1280, and 1120; MS, m/z: 504 [M –
H2O]+, 459, 367, 253, and 105.
+92° (c 0.77, CHCl3); 1ç NMR: 0.78 (3 H, s, 18-Me),
0.99 (3 H, s, 19-Me), 1.02 (3 H, d, J 6.3, 21-Me), 1.26
(3 H, t, J 7.1, ëç3ëç2), 4.12 (2 H, q, J 7.1, ëç3ëç2),
4.14 (1 H, m, H7β), 4.99 (1 H, m, H3α), 7.40 (2 H, m,
m-H in Ph), 7.52 (1 H, m, p-H in Ph), and 8.04 (2 H, d,
J 7.3, Ó-H in Ph); 13C NMR: 13.5 q, 14.9 q, 19.5 q,19.5
q, 20.2 t, 27.9 t, 28.1 t, 29.7 t, 31.3 t, 31.8 t, 34.3 d, 34.7
t, 36.9 t, 37.5 t, 39.4 s, 42.9 s, 43.5 t, 44.6 d, 51.3 d, 51.3
d, 61.0 t, 74.3 d, 128.7 two d, 130.1 two d, 131.4 s,
133.6 d, 140.8 s, 151.2 s, 166.8 s, 174.4 s, and 208.5 s;
IR (ν, KBr, cm–1): 1730, 1720, 1630, 1280, and 1120;
.
MS, m/z: 520 [M]+ , 475, 383, 251, and 105.
3b-Benzoyloxy-15-trifluoromethylsulfonyloxy-5a-
chola-8,14-dien-24-oic acid ethyl ester (VI). A solu-
tion of (V) (7.8 g, 15 mmol) and 2,6-di-tert-butyl-4-
methylpyridine (4 g, 19.5 mmol) in CH2Cl2 (40 ml) was
cooled to 0°ë and trifluoromethanesulfonic anhydride
(4.9 g, 2.9 ml, 17.3 mol) was added under stirring. The
resulting solution was stirred for 8 h at room tempera-
ture and then diluted with hexane (140 ml). The precip-
itate was filtered off and washed with hexane. The com-
bined extract was evaporated, and the resulting oily
product (10.1 g) was used in the next step without addi-
tional purification; 1H NMR: 0.91 (3 H, s, 18-Me), 0.96
(3 H, d, J 5.5, 21-Me), 1.07 (3 H, s, 19-Me), 1.27 (3 H,
t, J 7.1, ëç3ëç2), 4.14 (2 H, q, J 7.1, ëç3ëç2), 4.97
(1 H, m, H3α), 7.40 (2 H, m, m-H in Ph), 7.52 (1 H, m,
p-H in Ph), and 8.05 (2 H, d, J 6.7, Ó-H in Ph).
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 28 No. 3 2002

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BARANOVSKII et al.
3b-Benzoyloxy-15a-acetoxy-5a-chol-8-en-24-oic (2 H, q, J 7.1, ëç3ëç2), 4.94 (1 H, m, H3α), 7.40 (2 H,
acid ethyl ester (IX). Acetic anhydride (0.02 ml,
0.2 mmol) and DMAP (24 mg, 0.2 mmol) were added
to a solution of (VIII) (50 mg, 0.095 mmol) in methyl-
ene chloride (2 ml). The solution was stirred for 12 h,
diluted with methylene chloride, washed with water,
NaHCO3 solution, and saturated aqueous NaCl, dried
over Na2SO4, and evaporated. The residue was chro-
matographed on a silica gel column. Elution with 5 : 95
EtOAc–toluene gave 49 mg (92%) of (IX), mp 126–
128°C (methanol); 1H NMR: 0.67 (3 H, s, 18-Me), 0.92
(3 H, d, J 5.5, 21-Me), 1.00 (3 H, s, 19-Me), 1.24 (3 H,
t, J 7.1, ëç3ëç2), 2.01 (3 H, s, OAc), 4.10 (2 H, q, J
7.1, ëç3ëç2), 4.93 (2 H, m, H3α and H15β), 7.41
(2 H, m, m-H in Ph), 7.52 (1 H, m, p-H in Ph), and 8.02
(2 H, d, J 7.3, Ó-H in Ph); 13C NMR: 13.9 q, 15.7 q, 19.1
q, 19.5 q, 22.8 q, 24.1 t, 26.7 t, 28.1 t, 29.1 t, 32.2 t, 32.8
t, 35.6 t, 36.4 t, 36.8 d, 37.4 s, 38.4 t, 39.2 t, 41.9 d, 43.9
s, 54.3 d, 56.8 d, 61.7 t, 75.5 d, 76.0 d, 127.8 s, 129.7
two d, 131.0 two d, 132.3 s, 134.2 d, 137.7 s, 167.6 s,
172.5 s, and 175.4 s; IR (ν, film, cm–1): 1740, 1720,
1280, and 1120.
m, m-H in Ph), 7.52 (1 H, m, p-H in Ph), and 8.02 (2 H,
d, J 7.3, Ó-H in Ph); C NMR (C6D6): 11.5 q, 14.3 q,
13
17.8 q, 18.5 q, 23.0 t, 24.0 t, 25.6 t, 27.4 t, 28.0 t, 28.9 t,
31.3 t, 31.4 t, 34.6 d, 35.1 t, 35.9 s, 36.1 d, 37.3 t, 40.7 d,
42.4 s, 52.1 d, 54.9 d, 60.0 t, 74.1 d, 128.3 s, 128.5 two d,
129.9 two d, 131.7 s, 132.7 d, 135.0 s, 165.8 s, and
173.3 s; IR (ν, KBr, cm–1): 1740, 1730, and 1290; MS,
.
m/z: 506 [M]+ , 461, 384, 255, 213, and 105.
3b-Hydroxy-5a-chol-8-en-24-oic acid methyl
ester (XII). Potassium hydroxide (0.4 g, 7.1 mmol)
was added to a solution of (XI) (5.3 g, 10.47 mmol) in
anhydrous 4 : 1 methanol–THF mixture (400 ml). The
solution was stirred for 5 h, neutralized with 1 N HCl,
and then poured out to water (0.5 l). The precipitate was
filtered off, washed with water, dried in a vacuum, and
chromatographed on a silica gel column. Elution with
1 : 9 EtOAc–toluene gave 3.49 g (86%) of (XII); mp
1
139–141°ë (methanol); αD +27° (Ò 0.45, CHCl3); ç
NMR: 0.58 (3 H, s, 18-Me), 0.90 (3 H, d, J 6.1, 21-Me),
0.92 (3 H, s, 19-Me), 3.59 (1 H, m, H3α), and 3.64
(3 H, s, OMe); 13C NMR: 11.2 q, 17.8 q, 18.3 q, 22.7 t,
23.7 t, 24.5 t, 27.1 t, 28.6 t, 30.9 t, 31.1 t, 31.6 t, 35.1 t,
35.7 s, 35.8 d, 36.9 t, 38.3 t, 40.7 d, 42.1 s, 51.4 q,
51.8 d, 54.5 d, 71.1 d, 128.1 s, 135.0 s, and 174.7 s; IR
(ν, KBr, cm–1): 3500, 1730, 1310, and 1110; MS, m/z:
3b-Benzoyloxy-15a-phenyloxythiocarbonyloxy-
5a-chol-8-en-24-oic acid ethyl ester (X). Phenyl chlo-
rothionoformate (2.36 g, 13.68 mmol, 1.9 ml) was
added at 0°ë to a stirred solution of (VIII) (5.1 g,
9.77 mmol) and DMAP (3.34 g, 27.36 mmol) in meth-
ylene chloride (35 ml). The solution was stirred for 12 h
and then poured onto ice–diethyl ether. The organic
phase was separated; washed with a CuSO4 solution
(6 × 40 ml), water, aqueous NaHCO3 (3 × 40 ml), and
saturated aqueous NaCl; dried over Na2SO4; and then
evaporated. The resulting oily product (ï) (6.4 g) was
used at the next stage without additional purification;
1H NMR: 0.73 (3 H, s, 18-Me), 0.95 (3 H, d, J 5.5,
21-Me), 1.03 (3 H, s, 19-Me), 1.26 (3 H, t, J 7.1,
ëç3ëç2), 4.13 (2 H, q, J 7.1, ëç3ëç2), 4.97 (1 H, m,
H3α), 5.39 (1 H, m, H15β), 7.34 (8 H, m, Ar), and 8.05
(2 H, d, J 7.3, Ó-H in Ph); IR (ν, film, cm–1): 1750,
1725, 1280, and 1210.
.
388 [M]+ , 370, 355, 273, and 213.
3b-tert-Butyldimethylsilyloxy-5a-chol-8-en-24-
oic acid methyl ester (XIII). tert-Butyldimethylsilyl
chloride (1.49 g, 9.88 mmol) and imidazole (1.34 g,
19.76 mmol) were added to a solution of (XII) (2.95 g,
7.8 mmol) in DMF (25 ml). The solution was stirred for
12 h at room temperature, poured out into water, and
the organic material was extracted with hexane. The
extract was washed with water and saturated NaCl,
dried over Na2SO4, and then evaporated. The residue
was chromatographed on a silica gel column. Elution
with 5 : 95 EtOAc–toluene led to (XIII), yield 3.55 g
(93%); mp 125–127°ë (methanol); αD +30° (Ò 0.98,
1
CHCl3); ç NMR: 0.02 (6 H, s, Me2Si), 0.57 (3 H, s,
3b-Benzoyloxy-5a-chol-8-en-24-oic acid ethyl
ester (XI). A degassed solution of (X) (6.4 g,
9.77 mmol) and tributyltin hydride (5.61 g, 19.28 mmol,
5.1 ml) solution in toluene (28 ml) were added in 5-ml
portions to a flask heated to 90°C while simultaneously
(with each portion) adding AIBN (30 mg, 0.18 mmol).
The solution was stirred for 10 min after each addition.
After the reduction was over, the solution was cooled,
aqueous KF solution was added, and the reaction mix-
ture was stirred for 12 h. The organic layer was sepa-
rated, washed with saturated solution of NaCl, dried
over Na2SO4, and evaporated. The residue was chro-
matographed on a silica gel column. Elution with 5 : 95
EtOAc–toluene gave 3.70 g (75%) of (X), mp 137–
139°ë (methanol); αD +33° (Ò 0.6, CHCl3); 1ç NMR:
0.59 (3 H, s, 18-Me), 0.92 (3 H, d, J 6.1, 21-Me), 0.99
(3 H, s, 19-Me), 1.23 (3 H, t, J 7.1, ëç3ëç2), 4.10
18-Me), 0.86 (9 H, s, t-BuSi), 0.90 (3 H, d, J 6.1, 21-
Me), 0.91 (3 H, s, 19-Me), 3.52 (1 H, m, H3α), and 3.64
(3 H, s, OMe); 13C NMR (C6D6): –4.3 q, 11.4 q, 18.0 q,
18.3 s, 18.5 q, 23.1 t, 24.1 t, 26.0 t, 26.1 q, 27.6 t, 28.9
t, 31.2 t, 31.3 t, 32.6 t, 35.5 t, 36.0 s, 36.1 d, 37.4 t, 39.4
t, 41.1 d, 42.4 s, 51.0 q, 52.2 d, 54.9 d, 72.2 d, 128.1 s,
135.5 s, and 173.7 s; IR (ν, KBr, cm–1): 1730 and 1110;
.
MS, m/z: 502 [M]+ , 445, 369, 353, and 213.
3b-tert-Butyldimethylsilyloxy-5a-chol-8-en-24-al
(XIV) and 3b-tert-butyldimethylsilyloxy-5a-chol-8-
en-24-ol (XV). A solution of DIBAH (0.206 M,
25.2 ml) in toluene was added over 20 min to a stirred
solution of (XIII) (2.6 g, 5.18 mmol) in toluene (50 ml)
at –78°C. The resulting solution was stirred for an addi-
tional 40 min and diluted with ethyl acetate (5 ml). It
was then treated with 0.5% acetic acid (10 ml) and the
RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY Vol. 28 No. 3 2002

A NEW SYNTHESIS OF ZYMOSTEROL
255
resulting emulsion was stirred for 1 h at room tempera- 1.944 g (87%); mp 103–105°ë (methanol); αD +32°
1
ture. The aqueous phase was separated and extracted
with ethyl acetate. The extracts were combined, washed
with saturated NaCl solution, dried over Na2SO4, and
evaporated. The residue was chromatographed on a sil-
ica gel column eluted with toluene to give 2.12 g (87%)
of aldehyde (XIV) and 0.25 g (10%) of the more polar
alcohol (XV).
(Ò 0.60, CHCl3); ç NMR: 0.02 (6ç, s, åÂ2Si), 0.57
(3 H, s, 18-Me), 0.85 (9 H, s, t-BuSi), 0.91 (3 H, s,
19-Me), 1.56 and 1.65 (2 × 3 H, 2 s, 26-Me and 27-Me),
3.54 (3 H, m, H3α), and 5.06 (1 H, br. s, H24); 13C
NMR: –4.6 q, 11.2 q, 17.6 q, 17.9 q, 18.2 s, 18.6 q, 22.8
t, 23.8 t, 24.8 t, 25.5 t, 25.8 q, 25.9 q, 27.2 t, 28.7 t, 32.1
t, 35.3 t, 35.7 s, 36.0 d, 36.0 t, 37.0 t, 38.8 t, 40.9 d, 42.1
s, 51.9 d, 54.8 d, 72.1 d, 125.2 d, 128.0 s, 130.8 s, and
1
(XIV): mp 120–122°ë (hexane); H NMR: 0.03
(6 H, s, åÂ2Si), 0.58 (3 H, s, 18-Me), 0.86 (9 H, s, 135.2 s; IR (ν, KBr, cm–1): 1640, 1480, 1270, 1110,
.
1090, and 1080. MS, m/z: 498 [M]+ , 441, 365, 255,
t-BuSi), 0.91 (3 H, d, J 6.1, 21-Me), 0.91 (3 H, s,
19-Me), 3.54 (1 H, m, H3α), and 9.74 (1 H, br. s, H24);
and 213.
13C NMR (C6D6): –4.3 q, 11.4 q, 18.0 q, 18.4 s, 18.5 q,
3b-Hydroxy-5a-cholesta-8,24-diene (I) (zymo-
23.1 t, 24.1 t, 26.0 t, 26.1 q, 27.6 t, 28.0 t, 28.9 t, 32.6 t,
sterol). A 1 M solution of tetra-n-butylammonium flu-
35.6 t, 36.0 d, 36.0 s, 37.4 t, 39.4 t, 40.9 d, 41.1 d, 42.4
oride (7.5 ml) in THF was added to a solution of (XVI)
s, 52.2 d, 54.9 d, 72.2 d, 128.1 s, 135.5 s, and 200.7 d;
(0.935 g, 1.88 mmol) in THF (3 ml). The resulting solu-
IR (ν, KBr, cm–1): 2720, 1750, and 1110; MS, m/z: 472
tion was stirred for 24 h at room temperature, diluted
.
[M]+ , 445, 415, 387, 339, 255, and 213.
with diethyl ether, washed with water, 0.1 N HCl, satu-
rated aqueous NaHCO3, and saturated aqueous NaCl;
dried over Na2SO4; and evaporated. The residue was
chromatographed on a silica gel column. Elution with
toluene led to (XVI); yield 0.702 g (97%); mp 107–
109°ë (methanol); αD +48° (Ò 0.58, CHCl3) (lit.: mp
107–109°ë, αD +50° [31]; mp 110°ë, αD +49° [30]; mp
(XV): mp 149–151°ë (hexane); αD +21° (Ò 0.62,
1
CHCl3); ç NMR: 0.02 (6 H, s, Me2Si), 0.5 (3 H, s,
18-Me), 0.86 (9 H, s, t-BuSi), 0.91 (3 H, s, 19-Me), 0.92
(3 H, d, J 5.8, 21-Me), and 3.57 (3 H, m, H3α and H24);
13C NMR: –4.6 q, 11.2 q, 17.9 q, 18.2 s, 18.7 q, 22.8 t,
23.7 t, 25.5 t, 25.9 q, 27.2 t, 28.6 t, 29.4 t, 31.8 t, 32.1 t,
35.3 t, 35.7 s, 36.0 d, 37.0 t, 38.8 t, 40.9 d, 42.1 s, 51.9
d, 54.7 d, 63.5 t, 72.1 d, 128.0 s, and 135.2 s; IR
1
110–112°ë, αD +52° [2, 28]); ç NMR (C6D6): 0.69
(3 H, s, 18-Me), 0.91 (3 H, s, 19-Me), 1.04 (3 H, d,
J 6.1, 21-Me), 1.61 and 1.69 (2 × 3 H, 2 s, 26-Me and
27-Me), 3.04 (1 H, s, OH), 3.40 (3 H, m, H3α), and
5.27 (1 H, br. t, J 7.0, H24); 13ë NMR (C6D6): 11.5 q,
17.7 q, 18.0 q, 18.9 q, 23.2 t, 24.2 t, 25.3 t, 25.9 q, 26.0
t, 27.6 t, 29.1 t, 32.1 t, 35.6 t, 36.0 s, 36.5 d, 36.5 t, 37.4
t, 38.8 t, 41.1 d, 42.4 s, 52.3 d, 55.2 d, 71.0 d, 125.7 d,
(ν, KBr, cm–1): 3500, 1270, 1110, and 1090; MS, m/z:
.
474 [M]+ , 417, 341, 255, and 213.
Oxidation of alcohol (XV) to aldehyde (XIV).
DMSO (215 mg, 2.76 mmol) in methylene chloride
(2 ml) was added to a solution of (COCl)2 (175 mg,
1.38 mmol) in methylene chloride (3 ml) at –78°ë
under stirring. After 5 min, when the solution again
became transparent, alcohol (XV) (293 mg, 0.62 mmol)
in methylene chloride (3 ml) and, after 15 min, triethyl-
amine (368 mg, 3.61 mmol) were added. The resulting
solution was stirred for 40 min at –20°ë; diluted with
ethyl acetate; successively washed with 1% HCl, aque-
ous NaHCO3, and saturated NaCl solution, dried over
Na2SO4, and then evaporated. The residue was chro-
matographed on a silica gel column eluted with toluene
to give 196 mg (67%) of aldehyde (XIV).
128.2 s, 130.8 s, and 135.5 s; IR (ν, KBr, cm–1): 3500,
.
1640, 1470, and 1380; MS, m/z: 384 [M]+ , 369, 297,
271, and 213.
ACKNOWLEDGMENTS
We are grateful to Steraloids (United States) for the
gift of the starting steroids.
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