C-24 stereochemistry of marine Sterols: (22E)-24-Ethyl-24-methylcholesta-5,22-dien-3b-oβl and 24-Ethyl-24-methylcholest-5-en-3β-ol

SHORT REPORT

C-24 stereochemistry of marine Sterols: (22E)-24-Ethyl-24-methylcholesta-5,22-dien-3b-o β l and 24-Ethyl-24-methylcholest-5-en-3 β -ol

Shizue Echigo^I; Leonardo Castellanos^II; Carmenza Duque^II,*; Hidehiro Uekusa^I; Noriyuki Hara^I; Yoshinori Fujimoto^I

^IDepartment of Chemistry and Materials Science, Tokyo Institute of Technology, Meguro, 152-8551, Tokyo, Japan

^IIDepartamento de Química, Universidad Nacional de Colombia, AA 14490, Bogotá, Colombia

ABSTRACT

The C-24 configurations of (22E)-24-ethyl-24-methylcholesta-5,22-dien-3β-ol (1) and 24-ethyl-24-methylcholest-5-en-3β-ol (2), isolated from the Colombian Caribbean sponge Topsentia ophiraphidites, were determined to be R and S, respectively, by comparing their NMR data with those of stereodefined (24R)- and (24S)-samples that were synthesized in routes involving the orthoester Claisen rearrangement of Δ²³-22-allylic alcohols. This is the first synthetic study where the Claisen rearrangement is used to introduce a C-24 quaternary center in a stereospecific manner with acceptable yield. X-ray analysis of 1 confirmed these stereochemical assignments.

Keywords: 24-ethyl-24-methylcholesterol, 24-ethyl-24-methylcholesta-5,22-dien-3b-ol, Topsentia ophiraphidites, marine sterol, orthoester Claisen rearrangement

RESUMO

As configurações dos carbonos C-24 nos compostos (22E)-24-etil-24-metilcolesta-5,22-dien-3β-ol (1) e 24-etil-24-metilcolest-5-en-3β-ol (2), isolados da esponja do caribe colombiano Topsentia ophiraphidites foram determinadas como R e S, respectivamente, com base na comparação dos seus dados de RMN com aqueles de amostras de configurações já definidas 24R e 24S, as quais foram sintetizadas em rotas envolvendo ortoésteres provenientes do rearranjo de Claisen de álcoois Δ²³-22 alílicos. Este é primeiro estudo de síntese, em que o rearranjo de Claisen é utilizado para introduzir um centro quaternário em C-24, de maneira estereoespecífica e com rendimento aceitável. Análises por difração de raio X de 1 confirmaram essas atribuições estereoquímicas.

Introduction

It is well known that marine organisms such as sponges and corals are rich sources of uniquely modified sterols in the side-chain.¹ Among these sterols, C-24 dialkylated sterols are relatively limited. Djerassi's group reported the isolation of (22E)-Δ²²-24-ethyl-24-methylcholesterol (1), (22E)-Δ²²-24-ethyl-24-methylcholestanol, 24-ethyl-24-methylcholesterol (2), and (22E)-Δ^22,25-24-ethyl-24-methylcholesterol from a sponge, Pseudoaxinyssa sp.^2,3 Although they briefly mentioned the C-24 configurations of these sterols, details of the stereochemical assignment have not been reported.³ They also reported the preparation of stereochemically unestablished C-24 diastereomers of 1² and stereochemically defined, C-24 diastereomers of 24-ethyl-24-methylcholestanols.⁴ Recently, sterol 1 was isolated from the marine sponge Psammocinia bulbosa and its C-24 configuration was inferred as R without any discussion on the stereochemistry.⁵ Furthermore, 24-ethyl-24-methylcycloartane triterpenoids were isolated from higher plants, Coelogyne uniflora (Orchidaceae)⁶ and Skimmia wallichii (Rutaceae).⁷ In a previous paper we reported the isolation of a number of multiply alkylated sterols, including compounds 1 and 2, from the Colombian Caribbean sponge, Topsentia ophiraphidites.⁸ In particular, compound 1 was a major sterol constituent (ca. 40% of the total sterol). With sizeable amounts of compounds 1 and 2 in hand, we undertook a study to elucidate their C-24 configurations. In this paper we report a stereochemical determination at C-24 of 24-ethyl-24-methylsterols 1 and 2 (Figure 1), along with the ¹³C NMR data for the stereodefined C-24 epimers of 1 and 2.

Results and Discussion

In order to determine the C-24 configuration of 1 and 2, we decided to synthesize the stereochemically defined (24S)- and (24R)- (22E)-Δ²²-24-ethyl-24-methylcholesterols (1a and 1b), and (24R)- and (24S)-24-ethyl-24-methylcholesterols (2a and 2b). A method based on Claisen rearrangement appeared to be attractive for the stereoselective construction of the C-24 dialkyl side chains, although the application of this method to the construction of a C-24 quaternary center is unprecedented. The Johnson orthoester procedure and the Ireland variant of Claisen rearrangement are frequently employed for the synthesis of stereo-defined C-24 mono-alkylated sterol side-chains.^9-11 We also utilized the orthoester Claisen rearrangement for the stereoselective synthesis of (24R)- and (24S)- (22E)-24-isopropenyl-22-dehydrocholesterol and 24-isopropenylcholesterol, another multiply alkylated sterol of T. ophiraphidites.¹²

The synthetic route to 1a and 1b, and 2a and 2b is shown in Figure 2. The required starting materials, (22R)-allylic alcohol 3a and (22S)-allylic alcohol 3b, for the orthoester Claisen rearrangement were obtained stereoselectively from the corresponding enone^13-15 by reduction using DIBAL-H and L-selectride, respectively.¹⁴ Treatment of 3a with triethyl orthoacetate and propionic acid in refluxing xylene gave a mixture of the desired rearranged product 4a (not isolated as such) and by-products containing the 22-propionate ester of 3a. The C-24 epimer of 4a was not formed in the rearrangement reaction, since careful TLC and NMR analysis revealed that compounds 4a and 5a were free from 4b and 5b, respectively. The 24S configuration of 4a was assumed on the basis of the preferred transition state conformation for the rearrangement reaction (Figure S1) that is analogous to the conformation well established for the 3,3-sigmatropic reactions of steroidal 23-ene-22-allylic alcohols.¹⁶ The 24S configuration of 4a was ascertained by the eventual conversion of 4a to the (24S)-diastereomer 1a (vide infra). The product mixture containing 4a was reduced with LiAlH₄ and then the resulting alcohol mixture was acetylated to facilitate the separation. Hydride reduction of the purified acetate gave the primary alcohol 5a in 24% yield from 3a. Deoxygenation of the C-29 hydroxy group of 5a was achieved by super-hydride reduction of the corresponding mesylate 6a to give the i-methyl ether 7a. Removal of the i-methyl ether of compound 7a furnished the (24S)-diastereomer 1a. Hydrogenation of 7a gave the saturated (24R)-diastereomer 8a which was converted to the (24R)-diastereomer 2a upon deprotection of the i-methyl ether. Application of the same sequence of reactions to the (22S)-allylic alcohol 3b, furnished the (24R)-diastereomer 1b via the intermediates 4b, 5b, 6b and 7b. Hydrogenation of 7b gave 8b, which furnished the (24S)-diastereomer 2b by regeneration of the Δ⁵-3β-hydroxy system.

The ¹H and ¹³C NMR spectroscopic data of the synthetic diastereomers 1a and 1b are listed in Tables 1 and 2, respectively, together with those of the sterol 1 isolated from T. ophiraphidites. The ¹H and ¹³C signals of 1a and 1b were unambiguously assigned on the basis of 2-D (H-H COSY, HMQC and HMBC) NMR spectra. For example, in the HMQC spectrum of 1a, H-22 (δ 5.058) and H-23 (δ 5.144) were correlated with C-22 (δ 134.9) and C-23 (δ 134.6), respectively, while in the HMBC spectrum, correlation peaks were observed from 21-H₃ (δ 1.017) to C-17, C-20 and C-22, H₃-26/H₃-27 (δ 0.785 and 0.803) to C-24 and C-25, H₃-29 (δ 0.739) to C-24 and C-28, and H₃-30 (δ 0.795) to C-23, C-24, C-25 and C-28. The ¹H NMR data of the epimers were closely similar to each other. However, careful comparison of the spectra revealed that 1a or 1b showed an appreciable difference in the chemical shifts of 29-H₃ (Δδ 0.011 ppm), and natural 1 seemed to be identical with the (24R)-diastereomer 1b in this regard. As shown in Table 2, the ¹³C data were much more diagnostic for the identification of the diastereomers 1a and 1b. The C-24 signal showed the largest difference (0.19 ppm) between the diastereomers and the C-29 and C-28 signals were different by more than 0.10 ppm, and this much difference would be sufficient for the identification of the diastereomers. The ¹³C data of natural 1 met with those of the (24R)-diastereomer 1b. It was, therefore, concluded that the sterol 1 isolated from T. ophiraphidites has 24R configuration. The sterol 1 isolated from Pseudoaxinyssa sp.³ was also identified as the (24R)-diastereomer 1b, since the published ¹H NMR data (adjusted by the addition of 0.005 ppm) was much closer to those of 1b, rather than those of 1a. The sterol 1 isolated from P. bulbosa could be assigned as the (24R)-diastereomer 1b, since the reported ¹³C shifts of C-24, C-28 and C-29 resembled the values of 1b rather than those of 1a.

The ¹H and ¹³C NMR spectroscopic data of the sterol 2 isolated from T. ophiraphidites and the synthetic 24-diasteromers 2a and 2b are listed in Tables 1 and 2, respectively. The ¹H and ¹³C signals of 2a and 2b were unambiguously assigned as described for 1a and 1b. For example, in the HMBC spectrum of 2a, H₃-21 (δ 0.923) was correlated with C-17, C-20 and C-22, H₃-26/H₃-27 (δ 0.786 and 0.797) with C-24 and C-25, H₃-29 (δ 0.749) with C-24 and C-28, and H₃-30 (δ 0.674) with C-23, C-24, C-25 and C-28. The ¹H NMR signals, in particular H₃-26 and H₃-29, of 2 were in excellent agreement with those of the (24S)-diastereomer 2b, but not with those of the (24R)-diastereomer 2a. This was confirmed by comparison of the ¹³C NMR data. The C-30 signal showed the largest chemical shift difference (Δδ 0.11) between the diastereomers, and the C-20 and C-28 signals exhibited 0.09 and 0.07 ppm difference, respectively. Figure S2 illustrates a graphic representation of the systematic ¹³C shift comparison. It is obvious from Figure S2 that the sterol 2 is identical with 2b, rather than 2a. The minor deviation observed in some signals of 2 and 2b could be within an error level in the NMR measurements. Hence, the 24S configuration of the sterol 2 isolated from T. ophiraphidites was unequivocally established. Comparison of melting points and [α]_D values (see Experimental) also supported this stereochemical assignment. The C-24 configuration of 24-ethyl-24-methylcholesterol isolated from Pseudoaxinyssa sp.² was also assigned as S, since the reported ¹H NMR data (recorded in benzene-d₆) met with those (recorded in benzene-d₆) of (24S)-2b.

Finally, we carried out X-ray analysis for a crystalline sample of the sterol 1 isolated from T. ophiraphidites. The molecular structure and the C-24 stereochemistry of 1 were established as shown in Figure S3. The 24R configuration of 1 was inferred by assuming the absolute configuration of natural sterols. The results proved the 24R configuration of the rearranged product 4b (therefore, 4a should have 24S configuration) and correctness of the transition state conformation (Figure S1) proposed for the orthoester Claisen rearrangement employed for the stereoselective creation of the C-24 quaternary center.

Conclusions

The present study established the 24R configuration of the marine sterol 1, isolated from T. ophiraphidites, Pseudoaxinyssa sp., and Psammocinia bulbosa, and the 24S configuration of the marine sterol 2, isolated from T. ophiraphidites and Pseudoaxinyssa sp. The C-24 configurations for the sterols 1 and 2, originally proposed by Djerassi's group,³ was confirmed with full experimental details. The findings that sterols 1 and 2 possess the same C-24 orientation (24α_F for the methyl group) supported the view that the sterol 2 is a biosynthetic precursor of the corresponding Δ²²-sterol 1 in sponges. Djerassi and co-workers³ suggested that stigmasta-5,24-dien-3β-ol could be a progenitor of the unique sterols, 1 and 2. Stereochemical studies on the other stereochemically undefined multiply-alkylated sterols isolated from T. ophiraphidites are in progress in our laboratory.

Experimental

Melting points were determined on a Yazawa BY-1 hot-stage micro melting point apparatus and are uncorrected. NMR spectra were obtained on a JEOL JNM LA-400 (400 MHz for ¹H, 100 MHz for ¹³C) spectrometer in CDCl₃ solution (4-10 mg sample per test tube) with tetramethylsilane as an internal reference at ca. 25 ºC. ¹³C chemical shifts are referred to the solvent signal (δ 77.00). EI and FAB MS spectra were recorded with a JEOL JMS-AX700 spectrometer. Optical rotations were measured on a JASCO DIP-360 polarimeter.

(22R,23E)-6β-Methoxy-3α,5-cyclo-5α-ergost-23-en-22-ol (3a) and (22S,23E)-6β- methoxy-3α,5-cyclo-5α-ergost-23-en-22-ol (3b)

DIBAL-H (1.01 mol L^-1 in toluene, 0.70 mL, 0.71 mmol) was added to a solution of 6β-methoxy-3α,5-cyclo-5α-ergost-23-en-22-one¹⁴ (490 mg, 1.15 mmol) in dry THF (10 mL) at -78 ºC under N₂, and the mixture was stirred at the same temperature for 30 min. Addition of ether and extractive work-up gave a crude product which was chromatographed on silica gel with hexane-EtOAc (15:1) to give the (22R)-alcohol 3a (308 mg, 63%) as a white solid, mp 128-129 ºC. ¹H NMR: δ 0.44 (dd, J 8.0, 5.1 Hz, 4α-H), 0.65 (dd, J 5.1, 4.4 Hz, 4β-H), 0.73 (s, H₃-18), 0.96 (d, J 6.4 Hz, H₃-21), 1.01 (d, J 6.8 Hz, H₃-26, H₃-27), 1.03 (s, H₃-19), 1.62 (d, J 1.2 Hz, H₃-28), 2.23 (sep, J 6.8 Hz, H-25), 2.78 (brt, J 2.8 Hz, H-6), 3.33 (s, OMe), 4.47 (dd, J 7.8, 1.2 Hz, H-22), 5.33 (brd, J 7.8 Hz, H-23). ¹³C NMR: δ 12.16, 12.29, 13.07, 14.16, 19.28, 21.27, 21.37, 21.46, 22.77, 24.18, 24.94, 27.92, 30.56, 33.34, 35.09, 35.23, 36.68, 40.20, 41.88, 42.73, 43.37, 47.97, 52.67, 56.38, 56.56, 70.54, 82.41, 124.85, 142.72. Analysis calc. for C₂₉H₄₈O₂: C, 81.25; H, 11.29. Found: C, 81.31; H, 11.56.

L-Selectride (1.0 mol L^-1 in THF, 8.64 mL, 8.64 mmol) was added to a solution of 6β-methoxy-3α,5-cyclo-5α-ergost-23-en-22-one (1.23 g, 2.88 mmol) in dry THF (25 mL) at -78 ºC under N₂, and the mixture was stirred at the same temperature for 5 h. Addition of ether and extractive work-up gave a crude product which was chromatographed on silica gel with hexane-ether (4:1) to give the (22S)-alcohol 3b (784 mg, 64%) as a colorless oil. ¹H NMR: δ 0.43 (dd, J 8.1, 5.1 Hz, H-4α), 0.65 (dd, J 5.1, 4.4 Hz, H-4β), 0.74 (s, H₃-18), 1.00-1.02 (H₃-21, H₃-26, H₃-27, H₃-19), 1.70 (brs, H₃-28), 2.26 (sep, J 6.8 Hz, H-25), 2.77 (brt, J 2.3 Hz, H-6), 3.32 (s, OMe), 4.42 (dd, J 9.5, 3.7 Hz, H-22), 5.28 (brd, J 9.3 Hz, H-23). ¹³C NMR: δ 12.20, 12.53, 13.04, 14.07, 19.25, 21.35, 21.43, 21.50, 22.72, 24.35, 24.92, 27.87, 30.45, 33.32, 34.97, 35.22, 37.04, 40.15, 42.04, 43.03, 43.33, 47.97, 53.24, 56.08, 56.52, 69.55, 82.34, 120.42, 146.45. Analysis calc. for C₂₉H₄₈O₂: C, 81.25; H, 11.29. Found: C, 81.44; H, 11.51.

(22E,24S)-24-(2-Hydroxyethyl)-6β-methoxy-24-methyl-3α,5-cyclo-5α-cholest-22-ene (5a)

A solution of the (22R)-alcohol 3a (155 mg, 0.362 mmol), triethyl orthoacetate (0.23 mL, 1.25 mmol) and propionic acid (12 µL, 0.161 µmol) in xylene (5.0 mL) was heated at reflux under N₂ for 3 h. The mixture was directly subjected to silica gel column chromatography. Elution with hexane-EtOAc (15:1) gave the crude rearranged product (140 mg) which was mainly composed of the desired 4a and 22-O-propionate side-product. The mixture was dissolved in dry THF (2.0 mL) and LiAlH₄ (24.0 mg, 0.632 mmol) was added and after 30 min stirring, it was worked up in the usual manner. The resulting mixture (62.4 mg) was mixed with Ac₂O (0.25 mL) and pyridine (0.50 mL) and allowed to stand overnight at room temperature. Extractive (ether) work-up gave a crude product which was chromatographed over silica gel with hexane-EtOAc (20:1) to give 29-acetate (40.5 mg) as an oil. LiAlH₄ (6.0 mg, 158 µmol) was added to a solution of the acetate in THF (1.0 mL) and the mixture was stirred for 10 min at room temperature. Extractive (ether) work-up gave the primary alcohol 5a (36.0 mg, 24% from 3a) as a colorless oil. ¹H NMR: δ 0.43 (dd, J 8.1, 5.1 Hz, H-4α), 0.65 (dd, J 5.1, 4.4 Hz, H-4β), 0.73 (s, H₃-18), 0.81 (d, J 7.1 Hz, H₃-26), 0.83 (d, J 7.1 Hz, H₃-27), 0.89 (s, H₃-30), 1.01 (d, J 6.8 Hz, H₃-21), 1.02 (s, H₃-19), 2.06 (m, H-20), 2.77 (m, H-6), 3.32 (s, OMe), 3.63 (m, H₂-29), 5.14 (dd, J 15.9, 8.6 Hz, H-22), 5.27 (d, J 15.9 Hz, H-23). ¹³C NMR: δ 12.40, 13.06, 17.20, 17.73, 18.41, 19.26, 21.01, 21.43, 22.72, 24.21, 24.92, 29.02, 30.45, 33.32, 35.05, 35.20, 37.13, 40.14, 40.30, 40.73, 41.85, 42.68, 43.35, 48.01, 56.08, 56.52, 56.57, 60.28, 82.37, 134.77, 135.27. HREIMS m/z: 456.3967 calc. for C₃₁H₅₂O₂ [M⁺]. Found: 456.4011.

(22E,24S)-24-Ethyl-6β-methoxy-24-methyl-3α,5-cyclo-5α-cholest-22-ene (7a)

MsCl (15 µL, 194 µmol) was added to a solution of 5a (30.6 mg, 67 µmol) in pyridine (0.30 mL) and the mixture was stirred for 10 min at room temperature. Extractive (ether) work-up gave a crude mesylate 6a as an oil. A solution of the mesylate in dry THF was treated with Super-hydride (1.0 mol L^-1 in THF, 370 µL, 370 µmol) at room temperature under N₂ for 30 min. Extractive (ether) work-up gave a crude product which was chromatographed on silica gel with hexane-EtOAc (20:1) to yield the 22-ene 7a (14.9 mg, 50%) as an oil. ¹H NMR: δ 0.43 (dd, J 8.1, 5.1 Hz, H-4α), 0.65 (dd, J 5.1, 4.4 Hz, H-4β), 0.73 (s, H₃-18), 0.74 (d, J 7.4 Hz, H₃-29), 0.79 (d, J 7.6 Hz, H₃-26), 0.80 (s, H₃-30), 0.81 (d, J 5.8 Hz, H₃-27), 1.01 (d, J 6.6 Hz, H₃-21), 1.03 (s, H₃-19), 2.07 (m, H-20), 2.77 (brt, 2.8 Hz, H-6), 3.32 (s, OMe), 5.06 (dd, J 15.7, 8.4 Hz, H-22), 5.14 (d, J 15.9 Hz, H-23). ¹³C NMR: δ 8.52, 12.46, 13.08, 17.33, 17.99, 18.25, 19.30, 21.36, 21.50, 22.77, 24.26, 24.97, 29.03, 30.50, 31.62, 33.36, 35.09, 35.29, 35.87, 40.22, 40.87, 41.09, 42.68, 43.41, 48.09, 56.28, 56.55, 56.67, 82.45, 134.59, 134.93. HREIMS m/z: 440.4018 calc. for C₃₁H₅₂O [M⁺]. Found: 440.4067.

(22E,24S)-24-Ethyl-24-methylcholesta-5,22-dien-3β-ol (1a)

A solution of the 22-ene 7a (7.3 mg, 16.6 µmol) in dioxane (0.5 mL) and H₂O (1.15 mL) containing p-TsOH•H₂O (12 mg, 63.1 µmol) was heated at 105 ºC for 3 h. Extractive (ether) work-up gave a crude product which was crystallized from MeOH to yield the (24S)-22-ene 1a (5.4 mg, 76%) as white needles, mp 159-161 ºC. [α]_D²⁵ -41.6º (c, 0.50, CHCl₃). ¹H and ¹³C NMR spectroscopic data are listed in Tables 1 and 2, respectively. EIMS m/z: 426 [M⁺, trace], 383 [M-isopropyl], 367, 273, 257, 231, 213. HRFABMS m/z: 409.3834 calc. for C₃₀H₄₉ [MH⁺-H₂O]. Found: 409.3784.

(24R)-24-Ethyl-6β-methoxy-24-methyl-3α,5-cyclo-5α-cholestane (8a)

A solution of the 22-ene 7a (10.5 mg, 23.8 µmol) in AcOEt (1.0 mL) was hydrogenated in the presence of 10% Pd/C (5.8 mg) overnight. Dilution with hexane and filtration through a pad of silica gel afforded an oily saturated product 8a (7.4 mg, 70%). ¹H NMR : δ 0.43 (dd, J 8.1, 5.1 Hz, H-4α), 0.65 (dd, J 5.1, 4.4 Hz, H-4β), 0.67 (s, H₃-30), 0.72 (s, H₃-18), 0.75 (t, J 7.6 Hz, H₃-29), 0.79 (d, J 7.1 Hz, H₃-26), 0.80 (d, J 7.1 Hz, H₃-27), 0.92 (d, J 6.6, H₃-21), 1.02 (s, H₃-19), 2.77 (brt, J 2.8 Hz, H-6), 3.33 (s, OMe). ¹³C NMR: δ 7.95, 12.22, 13.07, 17.05, 17.15, 18.87, 19.29, 20.25, 21.50, 22.77, 24.21, 24.97, 28.37, 28.69, 29.05, 30.48, 32.26, 33.31, 33.36, 35.06, 35.30, 36.75, 36.82, 40.28, 42,77, 43.39, 48.03, 56.19, 56.54, 56.55, 82.45. HREIMS m/z: 442.4172 calc. for C₃₁H₅₄O [M⁺]. Found: 442.4156.

(24R)-24-Ethyl-24-methylcholest-5-en-3β-ol (2a)

Compound 8a (5.9 mg, 13.3 µmol) was treated as described for the conversion of 7a to 1a. Crystallization of a crude product from MeOH gave the (24R)-epimer 2a (4.7 mg, 82%) as white needles, mp 159-161 ºC. [α]_D²⁵ -30.8º (c, 0.42, CHCl₃). ¹H and ¹³C NMR spectroscopic data are listed in Tables 1 and 2, respectively. EIMS m/z: 428 (M⁺), 413, 410, 395, 385, 367, 343, 317, 273, 255, 231, 213. HREIMS m/z: 428.4018 calc. for C₃₀H₅₂O [M⁺]. Found: 428.4020.

(22E,24R)-24-(2-Hydroxyethyl)-6β-methoxy-24-methyl-3α,5-cyclo-5α-cholest-22-ene (5b)

Treatment of the (22S)-alcohol 3b (414 mg) as described for 3a gave a product (377 mg) which was composed of the desired 4b and 22-O-propionate side-product. The mixture was reduced with LiAlH₄ (28.7 mg) in ether (5 mL) and the product was chromatographed on silica gel with hexane-ether (10:1) to give a primary alcohol 5b (105 mg, 24% from 3b) as an oil. ¹H NMR: δ 0.43 (dd, J 8.0, 5.1 Hz, H-4α), 0.65 (dd, J 5.1, 4.4 Hz, H₃-4β ), 0.73 (s, H₃-18), 0.80 (d, J 7.6 Hz, H₃-26), 0.83 (d, J 7.6 Hz, H₃-27), 0.88 (s, H₃-30), 1.01 (d, J 6.6 Hz, H₃-21), 1.03 (s, H₃-19), 2.07 (m, H-20), 2.77 (brt, J 2.6 Hz, H-6), 3.32 (s, OMe), 5.13 (dd, J 15.6, 8.3 Hz, H-22), 5.25 (brd, J 15.6 Hz, H-23). ¹³C NMR: δ 12.42, 13.06, 17.15, 17.61, 18.58, 19.28, 21.16, 21.47, 22.74, 24.25, 24.95, 28.99, 30.47, 33.35, 35.07, 35.26, 36.97, 40.14, 40.19, 40.78, 42.04, 42.71, 43.39, 48.05, 56.11, 56.55, 56.59, 60.41, 82.40, 134.62, 135.37. HREIMS m/z: 456.3967 calc. for C₃₁H₅₂O₂ [M⁺]. Found: 456.3948.

(22E,24R)-24-Ethyl-6β-methoxy-24-methyl-3α,5-cyclo-5α-cholest-22-ene (7b)

The primary alcohol 5b (100 mg) was converted to the 22-ene 7b (47.5 mg, 49%) via the mesylate 6b as described for the conversion of 5a to 7a. 7b: an oil. ¹H NMR: δ 0.43 (dd, J 8.0, 5.1 Hz, H-4α), 0.65 (dd, J 5.1, 4.4 Hz, H-4β ), 0.73 (s, H₃-18), 0.78 (d, J 7.1 Hz, H₃-26), 0.80 (s, H₃-30), 0.80 (d, J 6.8 Hz, H₃-27), 1.01 (d, J 7.1 Hz, H₃-21), 1.02 (s, H₃-19), 2.77 (brt, J 2.6 Hz, 6-H), 3.32 (s, OMe), 5.05 (dd, J 15.6, 8.0 Hz, H-22), 5.25 (brd, J 15.6 Hz, H-23). ¹³C NMR: δ 8.67, 12.46, 13.08, 17.26, 17.88, 18.29, 19.29, 21.29, 21.49, 22.77, 24.24, 24.97, 28.96, 30.49, 31.76, 33.36, 35.09, 35.29, 35.70, 40.22, 40.84, 41.07, 42.68, 43.41, 48.09, 56.23, 56.54, 56.67, 82.44, 134.61, 134.88. HREIMS m/z: 440.4018 calc. for C₃₁H₅₂O [M⁺]. Found: 440.4014.

(22E,24R)-24-Ethyl-24-methylcholesta-5,22-dien-3β-ol (1b)

The 22-ene 7b (25.8 mg) was treated as described for the conversion of 7a to 1a. The crude product was chromatographed on silica gel with hexane-EtOAc (7:1) and crystallized from MeOH to afford 1b (15 mg, 60%) as white needles, mp 164-165 ºC [mp of natural 1, 165-166.5 ºC (from methanol)⁸]. [α]_D²⁵ -49.8º (c, 2.0, CHCl₃) (lit.⁸ -45.4º for 1). The EI-MS data were essentially identical with those of 1a. ¹H and ¹³C NMR data are listed in Tables 1 and 2, respectively. Analysis calc. for C₃₀H₅₀O: C, 84.44; H, 11.81. Found: C, 84.68; H, 11.49.

(24S)-24-Ethyl-6β-methoxy-24-methyl-3α,5-cyclo-5α-cholestane (8b)

The 22-ene 7b (24.8 mg) was hydrogenated as described for 8a to afford the saturated compound 8b (24.4 mg, 98%) as an oil. ¹H NMR: δ 0.43 (dd, J 7.8, 5.1 Hz, H-4α), 0.65 (dd, J 5.1, 4.4 Hz, H-4β), 0.67 (s, H₃-30), 0.71 (s, H₃-18), 0.75 (t, J 7.7 Hz, H₃-29), 0.77 (d, J 7.8 Hz, H₃-26), 0.79 (d, J 7.1 Hz, H₃-27), 0.92 (d, J 6.3 Hz, H₃-21), 1.02 (s, H₃-19), 2.77 (brt, J 2.7 Hz, H-6), 3.32 (s, OMe). ¹³C-NMR: δ 7.94, 12.22, 13.07, 17.12, 17.15, 18.91, 19.29, 20.37, 21.50, 22.77, 24.21, 24.97, 28.38, 28.62, 29.03, 30.49, 32.27, 33.29, 33.36, 35.08, 35.31, 36.67, 36.81, 40.28, 42.78, 43.39, 48.05, 56.18, 56.55, 82.45. HRFABMS m/z: 443.4253 calc. for C₃₁H₅₅O [MH⁺]. Found: 443.4277.

(24S)-24-Ethyl-24-methylcholest-5-en-3β-ol (2b)

Compound 8b (22.9 mg) was converted to 2b as described for the conversion of 8a to 2a. Recrystallization of the product from MeOH gave 2b (15 mg, 68%) as white needles, mp 147-148 ºC (mp of natural 2, 145-146 ºC (from methanol)⁸), [α]_D²⁵ -35.9º (c, 1.7, CHCl₃) (lit.⁷ -34.6º for 2). The EIMS data were essentially identical with those of 2a. ¹H and ¹³C NMR data are listed in Tables 1 and 2, respectively. Analysis calc. for C₃₀H₅₂O: C, 84.04; H, 12.23. Found: C, 84.04; H, 12.53.

X-ray study of natural sterol (1)

Crystals for X-ray analysis were obtained by allowing to stand a warmed MeOH solution of 1 with slow evaporation of the solvent through a small hole in the cap. C₃₀H₅₂O•1/2MeOH•1/2H₂O, orthorhombic, P2₁2₁2₁, Z = 8, a = 70.8949(17), b = 7.7590(2), c = 10.0415(2) Å, V = 5523.6(2) ų, T = 123 K, Dx = 1.086 gcm^-3, λ(CuKα) = 1.54184 Å, µ = 0.490 mm^-1. A number of 9298 reflections were collected by the oscillation photograph method using Rigaku R-AXIS RAPID Imaging Plate camera and Lp and semi-empirical absorption corrections were applied. The structure was solved by direct methods using SIR-97. The positional and anisotropic thermal parameters were refined by full-matrix least squares SHELXL-97. The positions of hydrogen atoms were calculated geometrically and refined using the riding model. Final R = 0.090 for 6737 unique reflections with |I_o| > 2σ(I_o). One methyl group of the terminal isopropyl group has a disordered structure in both independent molecules. The occupancy factors are 0.60:0.40 and 0.52:0.48, respectively. Sterol molecules are linked by hydrogen bonding directly or via methanol and water molecules.

Supplementary Information

Supplementary data associated with this paper are available free of charge at http://jbcs.sbq.org.br as a PDF file.

Acknowledgments

This work was supported in part by a Grant from Colciencias to L. C. and C. D.

Submitted: March 19, 2010

Published online: February 8, 2011

Supplementary Information

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Publication Dates

• Publication in this collection
  16 May 2011
• Date of issue
  May 2011

History

• Accepted
  08 Feb 2011
• Received
  19 Mar 2010