Synthesis of methyl (20R,22E)- and (20S,22E)-3-oxochola-1,4,22-trien-24-oate

Article abstract of DOI:10.1002/1522-2675(200204)85:4<1096::AID-HLCA1096>3.0.CO;2-U

Methyl (22E)-3-oxochola-1,4,22-trien-24-oate (4;C₂₅H₃₄O₃) is a naturally occurring steroid with unknown configuration at C(20). Starting from the (20S)-3-oxo-23,24-dinorchol-4-en-22-al (1a), we prepared both diastereoisomeric methyl esters 4a and 4b by a three-step procedure (Scheme). In the case of 4b, the initial epimerization of aldehyde 1a was followed by completion of the sequence and then separation via fractional crystallization to afford pure (20R)-methyl ester 4a and its (20S)-diastereomer 4b. Only the analytical data of the (20S)-compound 4b were in good agreement with those reported for the natural product.

Full text of DOI:10.1002/1522-2675(200204)85:4<1096::AID-HLCA1096>3.0.CO;2-U

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Helvetica Chimica Acta Vol. 85 (2002)
Synthesis of Methyl (20R,22E)- and (20S,22E)-3-Oxochola-1,4,22-
trien-24-oate
by Manuela Linker and Wolfgang Kreiser*
Naturstoffchemie Universit‰t Dortmund, Otto-Hahn-Str. 6, D-44221 Dortmund
(e-mail: kreiser@chemie.uni-dortmund.de, linker@chemie.uni-dortmund.de)
Dedicated to the memory of V. Cerny
Methyl (22E)-3-oxochola-1,4,22-trien-24-oate (4; C25H34O3) is a naturally occurring steroid with unknown
configuration at C(20). Starting from the (20S)-3-oxo-23,24-dinorchol-4-en-22-al (1a), we prepared both
diastereoisomeric methyl esters 4a and 4b by a three-step procedure (Scheme). In the case of 4b, the initial
epimerization of aldehyde 1a was followed by completion of the sequence and then separation via fractional
crystallization to afford pure (20R)-methyl ester 4a and its (20S)-diastereomer 4b. Only the analytical data of
the (20S)-compound 4b were in good agreement with those reported for the natural product.
Introduction.
In pursuing their program designed for the discovery of new
antifouling substances from marine benthic invertebrates, Tomono et al. [1] reported in
1999 the isolation of four unknown steroids from an octocoral Dendronephthya sp. of
the order Alcyonacea. These compounds showed no antifouling activity against
barnacle (Balanus amphitrite) larvae, but, instead, lethality to the barnacle larvae at a
concentration of 100 mg/ml (LD100). Tomono et al. identified one of the new
compounds as methyl 3-oxochola-1,4,22-trien-24-oate (4) without specifying the
configuration at C(20). No melting point was reported, but an optical rotation of
[a]2D2 ˆ ‡53.6 (CHCl3, c ˆ 0.28) and detailed NMR data were given.
In this paper, we wish to report the preparation of both the (20R)- and (20S)-
epimers 4a and 4b, their separation via fractional crystallization, their unambiguous
proof of configuration by X-ray analysis, and the comparison of their spectral data with
the naturally occurring steroid derivative.
Results and Discussion. For the synthesis of the (20R)-methyl ester (4a), we
started from (20S)-3-oxo-23,24-dinorchol-4-en-22-al (1a) (Scheme). Horner-Wittig
reaction with (diethoxyphosphinyl)acetate [2] led to the corresponding ethyl ester 2a,

Helvetica Chimica Acta Vol. 85 (2002)
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Scheme. Synthesis of (20R,22E)- and (20S,22E)-3-Oxochola-1,4,22-trien-24-oate (4a and 4b, resp.)
i) (EtO)2P(O)CH2COOEt, NaH, THF. ii) TsOH ¥ H2O, MeOH. iii) DDQ, toluene. iv) H2SO4, H2O, EtOH. v)
Crystallization from cyclohexane.
and after transesterification in the presence of TsOH in MeOH, the methyl ester 3a was
obtained. Its corresponding acid had already been isolated from different marine
organisms and characterized as 3a [3][4]. Dehydration of 3a by DDQ (ˆ4,5-dichloro-
3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile) [5] finally gave methyl (20R,22E)-3-
oxochola-1,4,22-trien-24-oate (4a). Appropriate crystals of 4a were grown from
cyclohexane, and its configuration (20R) has been determined by X-ray analysis [6].
In the Table, some relevant NMR chemical-shift data of 4a are listed and compared
to those of the natural product. As for the side-chain shift values, there are considerable
and significant differences. Also the sign and value of the optical rotation of 4a and of
the natural product 4 are different [1]. So far, these observations indicate a
configurational deviation in the natural product×s side chain.
To get a reliable structure proof, the synthesis of the (20S)-methyl ester 4b was
undertaken. Since the acidity of HÀC(20) of 4a is low (no reaction with NaH at reflux

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Table. Comparison of the NMR Data of Both C(20)-Epimers 4a and 4b with Those Reported for the Natural
Product 4 [1]. d in ppm.
4b
0.66
1.17
0.94
6.81
5.74
3.69
Natural product 4
4a
0.73
1.18
1.03
6.77
5.70
3.67
1H-NMR:
13C-NMR:
Me(18)
Me(19)
Me(21)
HÀC(22)
HÀC(23)
MeO
0.69
1.19
0.96
6.82
5.75
3.71
C(12)
C(13)
C(14)
C(16)
C(17)
C(20)
C(21)
C(22)
C(24)
38.4
42.6
55.1
27.5
55.7
39.9
20.0
155.3
167.2
38.5
42.6
55.1
27.5
55.8
39.9
20.0
155.3
163.6
39.2
42.9
54.7
27.9
55.2
39.6
19.1
154.5
167.3
temperature), an epimerization of this compound is not suitable for this purpuse.
Aldehyde 1a should be a more promising candidate for inversion of configuration at
C(20) by synthesis of the enolate or the corresponding enamine. After acid hydrolysis
[7] of aldehyde 1a, only a 1:2 mixture 1a/1b ((R/S); determined by 1H-NMR) could be
obtained. The same ratio was determined after acid hydrolysis of the enamine 22-
(piperidin-1-yl)-23,24-dinorchola-4,20(22)-dien-3-one, prepared from 1a according to
Herr and Heyl [7].
Since a chromatographic separation of the diastereoisomers 1a/1b failed and
crystallization resulted in enrichment of the starting (20S)-aldehyde 1a, the epimer
mixture 1a/1b was used as starting material for the same sequence as described above
(Scheme). Crystallization of the resulting methyl esters 4a/4b from cyclohexane led to
the pure (20R)-methyl ester 4a. Then, after evaporation, the mother liquor consisted of
the diastereomers 4b/4a ((S/R)) in a ratio of ca. 5 :1. Further crystallizations enriched
the predominant epimer 4b, leading to pure (20S)-methyl ester 4b. The crystallizations
1
were monitored by H-NMR spectroscopy (s of MeO), as illustrated in the Figure.
The relevant NMR data of 4b are very similar to those of natural product 4 (see
Table), and the optical rotation measured for 4b is of the same order as the one
reported for 4 [1]. These data disclose that the (20S)-epimer is identical to the natural
steroid (no m.p. is available for the natural product 4).
To demonstrate the configuration at C(20), an X-ray-analysis of 4b has been
undertaken [8], too, which undoubtedly established its (20S) configuration. The
tendency of the epimers to crystallize in different space groups (4a: P21, monoclinic;
4b: P212121, orthorhombic) might serve as an explanation for their simple separation
via crystallization.

Helvetica Chimica Acta Vol. 85 (2002)
1099
1
Figure. Shift of the MeO singlet in the H-NMR spectrum during crystallization of 4a/4b
Experimental Part
General. All reagents and solvents were commercially available and used as provided. Aldehyde 1a was
provided by Schering Co. THF and toluene were distilled from Na and stored over molecular sieves (4 ä),
MeOH was distilled from Mg. All solns. were dried over anh. Na2SO4. TLC: Merck silica gel 60 F254 plates,
detection with UV and phosphoromolybdic acid. Column chromatography (CC): silica gel 60 (230 400 mesh),
Merck. M.p.: B¸chi 510; uncorrected. Optical rotations: Perkin-Elmer 241 polarimeter; c in g/100 ml. IR
Spectra: Nicolet Impact-400-D or Perkin-Elmer 457 spectrometer; nÄ in cmÀ1. NMR Spectra: Bruker DRX 400;
1H at 400.13 and 13C at 100.6 MHz; d in ppm rel. to internal standard Me4Si (ˆ0 ppm), J in Hz. MS: Finnigan
MAT-8230 or TSQ-7000 (70 eV); in m/z.
Ethyl (20R,22E)-3-Oxochola-4,22-dien-24-oate (2a). A soln. of ethyl (diethoxyphosphinyl)acetate (10.31 g,
45 mmol) in dry THF (80 ml) is added dropwise at 08 to a stirred suspension of NaH (1.80 g, 45 mmol; 60% in
mineral oil) in dry THF (150 ml). At the end of gas evolution, 1a (9.85 g, 30 mmol) in dry THF (100 ml) is added
dropwise, and the soln. is stirred overnight. After addition of H2O (30 ml), the mixture is extracted with AcOEt
(2 Â 60 ml), the org. layer washed with sat. NaHCO3 soln. (2 Â 70 ml) and brine (70 ml), dried (Na2SO4), and
evaporated, and the residue dried in vacuo: 2a (11.85 g, 99%), sufficiently pure for the next step. Rf
(cyclohexane/AcOEt 2 :1) 0.41. M.p. 155 1578. [a]2D1 ˆ ‡54.6 (CHCl3, c ˆ 0.28). IR: 2940, 2908, 2819, 2850,
1
1237, 1721, 1676, 1652, 1614. H-NMR (400.13 MHz, CDCl3): 0.70 (s, Me(18)); 1.04 (d, 3J ˆ 6.8, Me(21)); 1.13
(s, Me(19)); 1.23 (t, 3J ˆ 7.0, MeCH2O); 0.7 2.4 (m, 21 H); 4.12 (q, 3J ˆ 7.0, MeCH2O); 5.66 (s, HÀC(4)); 5.68
(d, 3J ˆ 15.1, HÀC(23)); 6.77 (dd, 3J ˆ 8.8, 15.6, HÀC(22)). 13C-NMR (100.6 MHz, CDCl3): 12.1 (q, C(18)); 14.2
(q, MeCH2O); 17.3 (q, C(19)); 20.9 (t, C(11)); 24.1 (t, C(15)); 28.0 (t, C(16)); 31.8 (t, C(1)); 32.8 (t, C(6)); 33.9
(t, C(7)); 35.5 (d, C(8)); 35.6 (t, C(2)); 38.5 (s, C(10)); 39.3 (t, C(12)); 39.6 (d, C(20)); 42.6 (s, C(13)); 53.6
(d, C(9)); 54.7 (d, C(14)); 55.6 (d, C(17)); 60.0 (t, MeCH2O); 119.0 (d, C(23)); 123.7 (d, C(4)); 154.3 (d, C(22));
166.9 (s, C(24)); 171.3 (s, C(5)); 199.4 (s, C(3)). MS: 398.
Methyl (20R,22E)-3-Oxochola-4,22-dien-24-oate (3a). A soln. of 2a (11.96 g, 30 mmol) and TsOH ¥ H2O
(1.14 g, 6 mmol) in dry MeOH (300 ml) is heated under reflux for 2 days. The MeOH is evaporated and the
residue treated with AcOEt (250 ml) and H2O (100 ml). The combined org. extract is washed with sat. NaHCO3
(3 Â 100 ml), H2O (100 ml), and brine (100 ml), dried, and evaporated: 3a (11.46 g, ca. 100%), sufficiently pure
for the next step. Purification is possible by CC (cyclohexane/AcOEt 2 :1, Rf 0.38). M.p. 146.5 148.58. [a]D21
ˆ
‡54.5 (CHCl3, c ˆ 0.28). IR: 2945, 2921, 2908, 2890, 2869, 2851, 1731, 1672, 1654, 1612, 1236. 1H-NMR
(400.13 MHz, CDCl3): 0.71 (s, Me(18)); 0.87 1.24 (m, 7 H); 1.05 (d, 3J ˆ 6.5, Me(21)); 1.15 (s, Me(19)); 1.38
2.43 (m, 14 H); 3.68 (s, MeO); 5.69 (s, HÀC(4)); 5.71 (d, 3J ˆ 15.6, HÀC(23)); 6.79 (dd, 3J ˆ 9.0, 15.6,
HÀC(22)). 13C-NMR (100.6 MHz, CDCl3): 12.1 (q, C(18)); 17.3 (q, C(19)); 19.1 (q, C(21)); 20.9 (t, C(11));

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Helvetica Chimica Acta Vol. 85 (2002)
24.1 (t, C(15)); 28.0 (t, C(16)); 31.8 (t, C(1)); 32.8 (t, C(6)); 33.9 (t, C(7)); 35.5 (d, C(8)); 35.6 (t, C(2)); 38.5
(s, C(10)); 39.3 (t, C(12)); 39.6 (d, C(20)); 42.6 (s, C(13)); 51.3 (q, MeO); 53.6 (d, C(9)); 54.7 (d, C(14)); 55.5
(d, C(17)); 118.6 (d, C(23)); 123.7 (d, C(4)); 154.6 (d, C(22)); 167.3 (s, C(24)); 171.2 (s, C(5)); 199.3 (s, C( 3)).
‡
MS: 385.0 ([ M ‡ 1] ).
Methyl (20R,22E)-3-Oxochola-1,4,22-trien-24-oate (4a). A soln. of 3a (6.0 g, 15.6 mmol) and DDQ (4.53 g,
20.0 mmol) in dry toluene (175 ml) is heated under reflux for 24 h. After cooling to r.t. and filtration, the mixture
is washed several times with 1% (w/w) KOH soln. (50 ml), H2O ( 3Â 50 ml), and brine (2 Â 50 ml), dried, and
evaporated. Purification by CC (cyclohexane/AcOEt 2 :1, Rf 0.30) yields 4a (3.82 g, 64%) as a light yellow solid.
After crystallization from cyclohexane, with addition of active charcoal, crystals suitable for X-ray-analysis are
obtained. M.p. 167.5 168.58. [a]2D1 ˆ À16.8 (CHCl3 , c ˆ 0.279). 1H-NMR (400.13 MHz, CDCl3): 0.73
(s, Me(18)); 1.03 (d, 3J ˆ 6.5, Me(21)); 1.18 (s, Me(19)); 0.98 1.25 (m, 7 H); 1.53 2.47 (m, 10 H); 3.67
(s, MeO); 5.70 (d, 3J ˆ 15.8, HÀC(23)); 6.02 (s, HÀC(4)); 6.18 (d, 3J ˆ 10.0, HÀC(2)); 6.77 (dd, 3J ˆ 9.0, 15.8,
HÀC(22)); 7.00 (d, 3J ˆ 10.0, HÀC(1)). 13C-NMR (100.6 MHz, CDCl3): 12.2 (q, C(18)); 18.6 (q, C(19)); 19.1
(q, C(21)); 22.7 (t, C(11)); 24.3 (t, C(15)); 27.9 (t, C(16)); 32.8 (t, C(6)); 33.5 (t, C(7)); 35.4 (d, C(8)); 39.2
(t, C(12)); 39.6 (d, C(20)); 42.9 (s, C(13)); 43.5 (s, C(10)); 51.3 (q, MeO); 52.2 (d, C(9)); 54.7 (d, C(14)); 55.2
(d, C(17)); 118.7 (d, C(23)); 123.8 (d, C(4)); 127.4 (d, C(2)); 154.5 (d, C(22)); 155.8 (d, C(1)); 167.3 (s, C(24));
‡
169.2 (s, C(5)); 186.3 (s, C(3)). MS: 383.0 ([ M ‡ 1] ).
3-Oxo-23,24-dinorchol-4-en-22-al (1a/1b). (20S)-Aldehyde 1a (6.57 g, 20 mmol) is dissolved in a mixture of
EtOH (320 ml), conc. H2SO4 soln. (64 ml), and H2O (64 ml) and heated under reflux for 20 min. The mixture is
poured onto ice (400 g) and extracted with Et2O ( 1Â 350 ml, 2 Â 150 ml). The combined org. extract is washed
with H2O ( 3Â 150 ml) and brine (150 ml), dried, and evaporated: 1a/1b ((S/R)) ca. 1:2. Colorless solid. M.p.
124 1268. Rf (cyclohexane/AcOEt 2 :1) 0.36. [a]2D1 ˆ ‡94.8 (CHCl3, c ˆ 0.28). IR: 2934, 1727, 1668. H-NMR
1
(400.13 MHz, CDCl3)1): 0.66 (s, 4 H; *Me(18)); 0.71 (s, 2 H, Me(18)); 0.94 (d, 3J ˆ 6.8, 4 H, *Me(21)); 1.07
(d, 3J ˆ 6.8, 2 H, Me(21)); 1.11 (s, 4, *Me(19)); 1.13 (s, 2 H, Me(19)); 0.66 1.23 (m, 4 H); 1.26 2.45 (m, 18 H);
5.67 (s, 2 H, HÀ*C(4), HÀC(4)); 9.47 (d, 3J ˆ 5.0, 1.3 H, HÀ*C(22)); 9.51 (d, 3J ˆ 3.5, 0.7 H, HÀC(22)).
13C-NMR (100.6 MHz, CDCl3)1): 12.2 (q, C(18)); 12.8 (q, *C(18)); 13.3 (q, C(21)); 13.5 (q, *C(21)); 17.3
(q, (C(19), *C(19)); 20.6 (t, *C(11)); 20.8 (t, C(11)); 23.7 (t, *C(15)); 24.4 (t, C(15)); 26.3 (t, *C(16)); 26.9
(t, C(16)); 31.8 (t, C(1), *C(1)); 32.7 (t, C(6), *C(6)); 33.8 (t, C(7), *C(7)); 35.4 (d, *C(8)); 35.5 (d, C(8)); 35.6
(t, C(2), *C(2)); 38.1 (s, *C(10)); 38.5 (s, C(10)); 39.2 (t, C(12), *C(12)); 42.1 (s, *C(13)); 42.9 (s, C(13)); 48.7
(d, *C(17)); 49.3 (d, C(17)); 50.8 (d, C(20)); 51.6 (d, *C(20)); 53.6 (d, C(9), *C(9)); 55.0 (d, C(14)); 55.2
(d, *C(14)); 123.8 (d, C(4), *C(4)); 171.1 (s, C(5), *C(5)); 199.5 (s, C(3)); 199.5 (s, *C(3)); 204.8 (d, C(22));
‡
205.6 (d, *C(22)). MS: 328.8 ([M ‡ 1] ).
Methyl (20S,22E)-3-Oxochola-1,4,22-trien-24-oate (4b). As described for (20R)-epimer 4a, starting from 1a/
1b. The crude product 4a/4b is purified by CC (cyclohexane/AcOEt 2 :1; Rf 0.30). Fractional crystallization
from cyclohexane leads to pure (20R)-methyl ester 4a. The mother liquor is evaporated, and after 4
crystallizations from cyclohexane (once with addition of active charcoal), pure (20S)-methyl ester 4b is obtained
after standing overnight at r.t. Fine needles. M.p. 138 1398. These crystals are suitable for X-ray analysis.
[a]2D1 ˆ ‡90.7 (CHCl3, c ˆ 0.279). IR: 2972, 2942, 2907, 2887, 2868, 2849, 1730, 1667, 1654, 1623, 1601, 1268, 1235.
1H-NMR (400.13 MHz, CDCl3): 0.66 (s, Me(18)); 0.94 (d, 3J ˆ 6.5, Me(21)); 1.17 (s, Me(19)); 0.93 1.25
(m, 6 H); 1.50 2.46 (m, 11 H); 3.69 (s, MeO); 5.74 (d, 3J ˆ 15.5, HÀC(23)); 6.02 (s, HÀC(4)); 6.17 (d, 3J ˆ 10.0,
HÀC(2)); 6.81 (dd, 3J ˆ 10.0, 15.5, HÀC(22)); 6.98 (d, 3J ˆ 10.0, HÀC(1)). 13C-NMR (100.6 MHz, CDCl3): 12.3
(q, C(18)); 18.6 (q, C(19)); 20.0 (q, C(21)); 22.5 (t, C(11)); 24.1 (t, C(15)); 27.5 (t, C(16)); 32.8 (t, C(6)); 33.5
(t, C(7)); 35.4 (d, C(8)); 38.4 (t, C(12)); 39.9 (d, C(20)); 42.6 (s, C(13)); 43.5 (s, C(10)); 51.4 (q, MeO); 52.3
(d, C(9)); 55.1 (d, C(14)); 55.7 (d, C(17)); 118.7 (d, C(23)); 123.8 (d, C(4)); 127.4 (d, C(2)); 155.3 (d, C(22));
‡
155.9 (d, C(1)); 167.2 (s, C(24)); 169.2 (s, C(5)); 186.4 (s, C(3)). MS: 383.0 ([ M ‡ 1] ).
REFERENCES
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[2] E. G. Baggiolini, J. A. Iacobelli, B. M. Hennessy, A. D. Batcho, J. F. Sereno, M. R. Uskokovic, J. Org.
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[4] A. D. Guerriero, M. D×Ambrosio, H. Zibrowius, F. Pietra, Helv. Chim. Acta 1996, 79, 982.
1
)
The asterisks denote C-atoms of 1b.

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[5] J. Fried, J. A. Edwards, −Organic Reactions in Steroid Chemistry×, Van Nostrand Reinhold Company, New
York, 1972, Vol. 1, p. 317.
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Received November 22, 2001