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Synthesis of C-5′-alkyl substituted 17-spirofuran 19-norsteroids
Steroids 66 (2001) 569 –579
Synthesis of C-5⬘-alkyl substituted 17-spirofuran 19-norsteroids Vladimir A. Khripacha,*, Vladimir N. Zhabinskiia, Dmitrii N. Tsavlovskiia, Olga A. Drachenovaa, Galina V. Ivanovaa, Olga V. Konstantinovaa, Margarita I. Zavadskayaa, Alexander S. Lyakhova, Alla A. Govorovaa, Marinus B. Groenb, Aede de Grootc a
Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, Kuprevich str., 5/2, 220141 Minsk, Belarus b Scientific Development Group, N.V. Organon, P.O. Box 20, 5340 BH Oss, The Netherlands c Wageningen University, Laboratory of Organic Chemistry and Research Center, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Received 31 May 2000; received in revised form 5 September 2000; accepted 12 September 2000
Abstract A number of new steroidal 17-spirofuran derivatives of the 19-nor series containing Me, Et or iPr-substituents in the heterocyclic moiety has been prepared, which are expected to have a strong progestagenic activity. The proposed approach made use of the 1,3-dipolar cycloaddition of low-molecular nitrile oxides with steroidal acetylenic alcohols followed by transformation of the isoxazole side chain. © 2001 Elsevier Science Inc. All rights reserved. Keywords: 19-Norsteroids; 1,3-Dipolar cycloaddition; Nitrile oxides; Acetylenic alcohols; Spirofurans
1. Introduction The 19-nor derivatives of progesterone occupy a special place among pharmaceutically important steroids. Many compounds in this series have been registered as medicines and others are under development [1]. A strong progestagenic activity has been demonstrated for 19-nor steroids containing a 3⬘-methylene-17R-spirodihydrofuran moiety 1 (R ⫽ H) [2]. Investigation of related compounds with various types of substitutions in the spirofuran moiety, especially those bearing an alkyl group at C-5⬘, could give better insight into structure - activity relationships and provide a good basis for the construction of new molecules with the desired properties. Synthetic intermediates, containing a keto group at C-3⬘, as well as compounds with different cyclic components, such as 3-OH and 3-OMe derivatives with an aromatic cycle A and 3-deoxo steroids, are interesting from this point of view. The nitrile oxide approach proved to be very fruitful for the construction of functionalized steroidal side chains in the synthesis of brassinosteroids [3–5], ecdysteroids [6], metabolites of vitamin D [7,8], and other compounds [9,10].
* Corresponding author. E-mail address: [email protected] (V.A. Khripach).
The data [11] on the preparation of certain steroids via this approach modified by the addition of a dihydrofuranone fragment in the side chain seemed to be promising for the synthesis of the title compounds 1 (R ⫽ alkyl), which are difficult to obtain in any other way [12]. The acetylenic alcohols 2 (mestranol) and 3 (norethisterone) were suitable starting compounds for the preparation of a variety of 19-norsteroids (Scheme 1). Mestranol 2 contains a stable ring A, that allowed for different reactions in the side chain. However, transformation of the aromatic ring A into the ⌬4-3-ketone system required a Birch reduction, which may not be compatible with many functional groups in the rest of the molecule. On the other hand, norethisterone 3, which already contains an enone in ring A, was rather vulnerable, and the enone moiety could not survive many reactions in the side chain. This is why the two approaches developed for the preparation of target compounds 1, starting from the acetylenic alcohols 2 and 3, were complementary. The most essential part of the synthetic routes toward the target compounds was the construction of the side chain. The general approach is shown in Scheme 2. 1,3-Dipolar cycloaddition of the acetylenic alcohol 4 to the corresponding nitrile oxide led to the hydroxy isoxazoles 5, which upon reductive cleavage via the intermediate enaminoketones 6, gave the furanones 7. The two subsequent stages of this
0039-128X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 0 3 9 - 1 2 8 X ( 0 0 ) 0 0 2 2 2 - 1
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Scheme 1. Starting compounds for the preparation of 5⬘-alkylspirofuran derivatives 1.
carried out on Kieselgel 60 (Merck Art. 7734). X-ray data collection (Nicolet R3 m diffractometer) was performed via the /2 scan mode. The structure was solved by direct methods (SHELXS 86) and refined by full-matrix least squares (SHELXL 93). The positions of hydrogen atoms were calculated, and refined using the riding model. 2.2. 1,3-Dipolar cycloaddition of nitrile oxides to acetylenic alcohols (general procedure)
approach were rather straightforward and included reduction of the double bond and Wittig olefination. The synthesis of three types of C-5⬘-substituted 17-spirofuran derivatives, where R ⫽ iPr, Et, and Me, has been studied and is reported here.
2. Experimental 2.1. General Melting points were taken on a Boetius micro-melting point apparatus and are uncorrected. IR spectra were recorded on a UR-20 spectrophotometer. 1H and 13C NMR spectra were taken on a Bruker AC-200 (200 MHz for 1H, 50 MHz for 13C) spectrometer using TMS as an internal standard. The exact mass measurements were carried out on a Finnigan MAT95 mass-spectrometer, operating in the 70 eV-EI mode and at a resolution of RP ⫽ 8000. Samples were introduced by direct probe for accurate mass measurement by peak matching. Reactions were monitored by TLC using aluminum or plastic sheets precoated with silica gel 60 F254 (Merck Art. 5715). Column chromatography was
Pyridine (0.02 ml) and oxime (5 mmol) were added to a stirred suspension of N-chlorosuccinimide (5 mmol) in CHCl3 (10 ml), and the mixture was stirred for 15 min. A solution of steroid (1 mmol) in CHCl3 (15 ml) was added to the mixture. After stirring the resulting mixture for 5–10 min, a solution of Et3N (5 mmol) in CHCl3 (10 ml) was added dropwise over a 3–10 h period. The mixture was washed with water and dried over Na2SO4. The solvent was evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (10:1 f 5:1) to give the isoxazole. 2.3. 3-Methoxy-17␣-(3⬘-isopropylisoxazol-5⬘-yl)-estra1,3,5(10)-trien-17-ol (10) The title compound was obtained from mestranol 2 and isobutyronitrile oxide in 99% yield. 1H NMR ␦: 1.03 (s, 3H, 18-Me), 1.30 (d, J ⫽ 7 Hz, 3⬘-iPr), 3.06 (sept, 1 H, J ⫽ 7 Hz, C3⬘-H), 3.78 (s, 3H, OMe), 6.05 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 5, 2 Hz, C2-H), 7.23 (d, 1H, J ⫽ 8.5 Hz, C1-H). 2.4. 3-Methoxy-17␣-(3⬘-methylisoxazol-5⬘-yl)-estra1,3,5(10)-trien-17-ol (16) The title compound was obtained from mestranol 2 and acetonitrile oxide in 99% yield. Mp 157–159°C (MeOHCH2Cl2). 1H NMR ␦: 1.03 (s, 3H, 18-Me), 2.32 (s, 3H, 3⬘-Me), 3.78 (s, 3H, OMe), 6.03 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8, 2 Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). IR (cm⫺1): 2935, 1605, 1590, 1495, 1460, 1440, 1415, 1375, 1350, 1250, 1240, 1145, 1100, 1040, 905. HRMS Calc. for C23H29NO3: 367.2147; Found: 367.2149. 2.5. Hydrogenation of the isoxazole (10)
Scheme 2. General approach to the preparation of the side chain of 5⬘-alkylspirofuran derivatives.
A suspension of Raney nickel (800 mg) was saturated with hydrogen for 1 h. Then, the isoxazole 10 (500 mg, 1.3 mmol) and boric acid (80 mg, 1.3 mmol) were added. The mixture was vigorously stirred under hydrogen at room temperature for 3 h. Then, the catalyst was filtered off, and the filtrate was evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (7:1 f 3:1) to give a mixture of (17S)-23-amino-17-hydroxy-3-methoxy-24methyl-19,21-bisnorchola-1,3,5(10),22-tetraen-20-one 11
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and (17S)-3-methoxy-5⬘-isopropylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘,3⬘-dihydrofuran)-3⬘-one] 12. The mixture was rechromatographed to give the enaminoketone 11 (298 mg, 62%). 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.78 (s, 3H, OMe), 5.21 (s, 1H, C22-H), 5.34 (br.s, 1H, NH), 10.0 (br.s, 1H, NH). 2.6. (17S)-3-Methoxy-5⬘-isopropylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘,3⬘-dihydrofuran)-3⬘-one] (12) 2.6.1. Variant A. A solution of the enaminoketone 11 (41 mg, 0.1 mmol) in acetic acid (2 ml) was stirred at 90°C for 2 h. Then, the solvent was evaporated, and the residue was chromatographed on SiO2 with hexane/EtOAc (7:1) to give an oil. The oil was crystallized from 0.5 ml of EtOH to afford furanone 12 (19.6 mg, 50%). Mp 150 –151°C. 1H NMR ␦: 1.02 (s, 3H, 18-Me), 1.24 (m, 6H, 5⬘-iPr), 2.72 (m, 1H, 5⬘-iPr), 3.78 (s, 3H, OMe), 5.29 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). IR (cm⫺1): 2940, 1685, 1605, 1580, 1500, 1465, 1450, 1355, 1285, 1240, 1035, 950. 2.6.2. Variant B. A mixture of the enaminoketone 11 (152 mg, 0.38 mmol), 5% HCl (4 ml) and ethanol (20 ml) was stirred at room temperature for 4 h. The usual work-up and crystallization from 3 ml of EtOH gave furanone 12 (132 mg, 92%). 2.7. Reduction of the furanone 12 with Li in liquid ammonia A solution of the furanone 12 (130 mg, 0.34 mmol) in ether-THF (1:1, 4 ml) was added to a stirred solution of Li (11 mg, 1.6 mmol) in liquid ammonia (6 ml) at ⫺50°C. Then, Li (28 mg) was added, and stirring was continued for 10 min. Excess Li was quenched with NH4Cl. The mixture was allowed to sit overnight for evaporation of ammonia. The product was isolated with CHCl3. The extract was dried, evaporated, and the residue was chromatographed on SiO2 with hexane/EtOAc (20:1 f 5:1) to give (17S)-3-methoxy-5⬘-isopropylspiro[estra-1,3,5(10)-triene-17,2⬘-tetrahydrofuran3⬘-one] 15 (78 mg, 60%). 2.8. Reduction of the furanone 12 with NaBH4 A mixture of EtOH (5 ml), the furanone 12 (27 mg, 0.07 mmol), and NaBH4 (18 mg, 0.47 mmol) was stirred at room temperature for 12 h. The usual workup and chromatography on SiO2 with hexane/EtOAc (5:1) gave (17S)-3methoxy-5⬘-isopropylspiro[estra-1,3,5(10)-triene-17,2⬘tetrahydrofuran-3⬘-ols] 13 and 14 (23 mg, 84%) as an inseparable mixture.
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2.9. (17S)-3-Methoxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘3-dihydrofuran)-3⬘-one] (17) A mixture of Raney nickel (150 mg), the isoxazole 16 (171 mg, 0.47 mmol), boric acid (50 mg, 0.8 mmol), and ethanol (20 ml) was stirred under hydrogen at room temperature for 48 h. The catalyst was filtered off, and the filtrate was evaporated. The residue was dissolved in ethanol (20 ml), and 5% HCl (5 ml) was added. The mixture was stirred at room temperature for 4 h, and then, an excess of saturated NaHCO3 was added. The product was isolated with EtOAc. The extract was dried and evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (10:1 f 3:1) to afford the furanone 17 (104 mg, 64%). 1H NMR ␦: 1.02 (s, 3H, 18-Me), 2.22 (s, 3H, 5⬘-Me), 3.78 (s, 3H, OMe), 5.31 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.69 (dd, 1H, J ⫽ 8.5, 2Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). 2.10. (17S)-3-Methoxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘-tetrahydrofuran-3⬘-ol] (18) NaBH4 (250 mg, 8.7 mmol) was added portionwise over 50 h to a stirred solution of the furanone 17 (281 mg, 0.6 mmol) in EtOH-THF (2:1, 15 ml). Excess reducing reagent was decomposed by addition of 5% AcOH. The mixture was extracted with EtOAc. The extract was dried over MgSO4 and evaporated. The residue was chromatographed on SiO2 with petroleum ether/EtOAc (20:1 f 5:1) to give the furanols 18 (273 mg, 96%). 1H NMR ␦: 0.95, 0.98 (s, 3H, 18-Me), 3.78 (s, 3H, OMe), 3.96 – 4.36 (m, 2H, C3⬘- and C5⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). 2.11. Birch reduction of (18) Isopropyl alcohol (4 ml) was added to a solution of 18 (217 mg, 0.6 mmol) in THF (5 ml), and the solution was cooled to ⫺60°C. Then, ammonia (30 ml) was condensed into the mixture, and Na (200 mg, 8.7 mmol) was added over 10 min. The stirring was continued at ⫺40°C for 45 min. After evaporation of the ammonia, the organic layer was separated. The solvent was removed in vacuo, and the residue was chromatographed on SiO2 with petroleum ether/EtOAc (10:1 f 5:1) to give (17S)-3-methoxy-5⬘methylspiro[estra-2,5(10)-diene-17,2⬘-tetrahydrofuran-3⬘ol] (19) (206 mg, 94%). 1H NMR ␦: 0.95, 0.98 (s, 3H, 18-Me), 3.55 (s, 3H, OMe), 3.94 – 4.34 (m, 2H, C3⬘- and C5⬘-H), 4.64 (t, 1H, J ⫽ 3 Hz, C2-H). 2.12. Hydrolysis of the enole ether (19) A mixture of 19 (17.5 mg, 0.049 mmol), EtOH (5 ml), and 5% HCl (2 ml) was heated at 60°C for 10 min. Then it was diluted with water and extracted with ether. The organic layer was washed successively with water and saturated
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NaHCO3 and then dried and evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (5:1 f 3:1) to give the (17S)-5⬘-methylspiro[estr-4-ene-17,2⬘-tetrahydrofuran-3⬘-ol] (20) (14.8 mg, 88%). 1H NMR ␦: 0.98, 1.00 (s, 3H, 18-Me), 3.94 – 4.34 (m, 2H, C2⬘- and C4⬘-H), 5.82 (s, 1H, C4-H). 2.13. Oxidation of (20) The steroidal alcohol 20 (207 mg, 0.6 mmol) in pyridine (2 ml) was added to a solution of chromium oxide (200 mg, 1.3 mmol) in pyridine (2 ml). The mixture was stirred at room temperature for 45 min, ether was added, the precipitate was filtered off, and the filtrate was evaporated. The obtained residue was chromatographed on silica gel to give: a) (5⬘R,17S)-5⬘-methylspiro[estr-4-ene-3-one-17,2⬘-tetrahydrofuran-3⬘-one] 22 (106 mg, 50%), mp 159 –161°C (ether). 1H NMR ␦: 0.96 (s, 3H, 18-Me), 1.38 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.16 (m, 1H, C5⬘-H), 5.82 (s, 1H, C4-H). HRMS Calc. for C22H30O3: 342.2195; Found: 342.2191; b) (5⬘S,17S)-5⬘-methylspiro[estr-4-ene-3-one-17,2⬘tetrahydrofuran-3⬘-one] 21 (84 mg, 42%) mp 140 –141°C (ether). 1H NMR ␦: 1.00 (s, 3H, 18-Me), 1.36 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.33 (m, 1H, C5⬘-H), 5.82 (br s, 1H, C4-H). HRMS Calc. for C22H30O3: 342.2195; Found: 342.2190. 2.14. (17S)-3-Methoxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘-tetrahydrofuran-3⬘-one] (23) LiAlH4 (0.076 g, 0.2 mmol) was added portionwise to a solution of furanone 17 (0.35 g, 0.1 mmol) in ether (40 ml) at room temperature. The reaction mixture was stirred at room temperature for 15 min, and then excess LiAlH4 was decomposed by addition of a 15% NaOH solution. The precipitate was filtered off and the filtrate was extracted with ether, dried over Na2SO4, and evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (25:1 f 18:1) to give the saturated furanone 23 (0.263 g, 75%). IR (cm⫺1): 2940, 1745, 1615, 1505, 1385, 1265, 1245, 1160, 1085, 1045. 1H NMR ␦: 0.94, 097 (s, 3H, 18-Me), 1.40 (s, 3H, 3⬘-Me), 3.77 (s, 3H, OMe), 4.17, 4.37 (m, 1H, C5⬘-H), 6.60 (d, 1H, J ⫽ 2 Hz, C4-H), 7.00 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.17, 7.19 (d, 1H, C1-H). 2.15. (17R)-17,23-Epoxy-3-methoxy-19-norchola1,3,5(10),20(21)-tetraene (24) A mixture of tBuOK (0.544 g, 4.8 mmol) and Ph3PCH3I (1.94 g, 4.8 mmol) in dry toluene (30 ml) was refluxed under argon for 1.5 h. Then, the furanone 23 was added, and the refluxing was continued for 3 h. The reaction mixture was cooled, and the precipitate was filtered off. The filtrate was washed with water and dried over CaCl2. The solvent was evaporated in vacuo. The residue was chromatographed on SiO2 with cyclohexane/benzene (15:1 f 3:1) and then cyclohexane/EtOAc (40:1 f 30:1) to give the olefin 24
(0.701 g, 86%). Mp 100 –102°C (hexane-ether). 1H NMR ␦: 0.91, 0.93 (s, 3H, 18-Me), 1.20 (d, 1H, J ⫽ 6 Hz, 23S-Me), 1.26 (d, 1H, J ⫽ 6 Hz, 23R-Me), 3.77 (s, 3H, OMe), 4.00 (m, 1H, C23R-H), 4.15 (m, 1H, C23S-H), 4.75 (m, 1H, C21H), 4.81 (m, 1H, C21-H), 5.01 (m, 2H, C21RS-H), 6.61 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.18 (d, 1H, J ⫽ 8.5 Hz, C1-H). After careful column chromatography only the (5⬘R)isomer of 24 could be isolated in pure state for identification purposes. IR (cm⫺1): 2935, 2880, 1620, 1505, 1460, 1385, 1275, 1105, 1050, 1025. 1H NMR ␦: 0.91 (s, 3H, 18-Me), 1.26 (d, 1H, J ⫽ 6 Hz, 23-Me), 3.76 (s, 3H, OMe), 4.00 (m, 1H, C23-H), 4.76 (m, 1H, C21-H), 5.01 (m, 1H, C21-H), 6.61 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.18 (d, 1H, J ⫽ 2 Hz, C1-H). 13C NMR ␦: 14.5, 22.0, 26.4, 26.7, 27.4, 29.8, 32.5, 36.4, 39.3, 43.5, 43.7, 46.4, 47.9, 55.2, 72.2, 94.8, 106.9, 111.4, 113.8, 126.3, 132.8, 138.0, 156.3, 157.4. 2.16. Birch reduction of furan (24) Liquid ammonia (47 ml) was added to a solution of furan 24 (0.6 g, 1.7 mmol) in dry THF (9 ml) and iPrOH (7 ml) at ⫺50°C. Then, sodium (0.6 g, 26 mmol) was added portionwise. The reaction mixture was stirred at ⫺45°C for 1 h, and then, an excess of NH4Cl was added. The mixture was allowed to warm for evaporation of the liquid ammonia. After warming to room temperature, the reaction mixture was diluted with saturated NH4Cl solution and extracted with ether. The extract was sequentially washed with water, 2.5% HCl, and saturated NaHCO3, and then dried over MgSO4. The solvent was evaporated in vacuo to give 0.55 g of an oil. This oil was dissolved in ethanol (50 ml), and 5% HCl (10 ml) was added. The mixture was stirred at 65°C for 10 min. After cooling to room temperature, a 5% KOH solution in ethanol (50 ml) was added, and the mixture was stirred for 3 min. Then, it was neutralized with 2.5% HCl solution and partially evaporated. The residue was extracted with EtOAc. The extract was dried over MgSO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane/EtOAc (40:1 f 12:1), and compounds 27, 28, and 29 were eluted in this sequence: a) (17R,23R)-17,23Epoxy-19-norchola-4,20(21)-dien-3-one 27 (72 mg, 12%). Mp 113–115°C (hexane-ether). IR (cm⫺1): 2940, 2880, 1695, 1635, 1460, 1385, 1275, 1210, 1105, 1085, 1040. 1H NMR ␦: 0.93 (s, 3H, 18-Me), 1.25 (d, 3H, J ⫽ 6 Hz, 23-Me), 3.98 (m, 1H, C23-H), 4.71 (d, 1H, J ⫽ 2.5 Hz, C21-H), 5.02 (d, 1H, J ⫽ 2 Hz, C21-H), 5.81 (s, 1H, C4-H). 13 C NMR ␦: (15.1, 22.2, 23.8, 26.8, 27.2, 31.5, 32.7, 36.2, 36.9, 37.2, 41.6, 43.2, 44.1, 46.8, 48.3, 50.0, 73.0, 95.3, 107.6, 125.1, 156.8, 167.5, 200.6. HRMS Calc. for C23H32O2: 340.2402; Found: 340.2396; b) (17R,23S)17,23-Epoxy-19-norchola-4,20-dien-3-one 28 (123 mg, 21%). Mp 119 –121°C (hexane-ether). IR (cm⫺1): 2940, 2875, 1690, 1630, 1460, 1390, 1275, 1210, 1110, 1070, 1035. 1H NMR ␦: 0.93 (s, 3H, 18-Me), 1.19 (d, 3H, J ⫽ 6
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Hz, 23-Me), 4.12 (m, 1H, C23-H), 4.78 (s, 1H, C21-H), 5.03 (s, 1H, C21-H), 5.83 (s, 1H, C4-H). 13C NMR ␦: 13.9, 22.0, 23.4, 26.4, 26.5, 29.7, 30.8, 35.5, 35.6, 36.5, 40.9, 41.1, 41.3, 46.6, 48.5, 49.2, 71.2, 95.0, 106.6, 124.5, 154.9, 166.7, 199.9. HRMS Calc. for C23H32O2: 340.2402; Found: 340.2397; c) (17R,23S)-17,23-Epoxy-19-norchol-4-en-3one 29 (232 mg, 39%). Mp 152–154°C (hexane-ether). IR (cm⫺1): 2960, 2885, 1685, 1630, 1460, 1385, 1275, 1210, 1130, 1085, 1015. 1H NMR ␦ 0.92, 0.95 (s, 3H, 18-Me), 4.10 (m, 1H, C23-H), 5.83 (s, 1H, C4-H). 13C NMR ␦: 15.0, 15.1, 16.3, 17.0, 22.9, 24.0, 24.4, 24.7, 27.1, 27.2, 31.6, 33.4, 34.7, 35.7, 36.2, 37.2, 37.5, 41.3, 42.0, 42.1, 43.2, 46.5, 47.6, 47.9, 49.0, 49.5, 49.96, 50.03, 50.31, 72.0, 73.4, 94.3, 95.0, 125.1, 167.45, 167.51, 200.6. HRMS Calc. for C23H34O2: 342.2559; Found: 342.2558. 2.17. 17␣-(3⬘-Methylisoxazol-5⬘-yl)-estra-1,3,5(10)-trien3,17-diol (31) Pyridine (0.05 ml) and acetaldoxime (0.24 ml, 16.9 mmol) were added to a suspension of N-chlorosuccinimide (2.25 g, 16.9 mmol) in chloroform (50 ml) at ⫺5– 0°C. The mixture was stirred for 10 min, and then, a solution of acetylenic alcohol 30 (0.5 g, 1.69 mmol) in chloroform (25 ml) was added. The cooling bath was removed, and a solution of triethylamine (2.3 ml, 16.9 mmol) in chloroform (5 ml) was added dropwise over 5 h. The reaction mixture was kept at 30°C for 24 h, and then, the solvent was removed in vacuo, and the residue was chromatographed on SiO2 with cyclohexane/ether (1:2) to give the isoxazole 31 (0.566 g, 95%). Mp 252–254°C (cyclohexane-EtOAc). IR (cm⫺1): 3540, 3245, 2930, 1620, 1585, 1500, 1360, 1290, 1250, 1140, 920, 820. 1H NMR ␦: 1.02 (s, 3H, 18-Me), 2.32 (s, 3H, 3⬘-Me), 5.00 (br.s, 1H, OH), 6.03 (s, 1H, C4⬘-H), 6.58 (m, 2H, Ph), 7.06 (d, 1H, J ⫽ 8 Hz, Ph). HRMS Calc. for C22H27NO3: 353.1991; Found: 353.1993. 2.18. (17S)-3-Hydroxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘,3⬘-dihydrofuran)-3⬘-one] (33) A mixture of the isoxazole 31 (0.1 g, 0.28 mmol) and Mo(CO)6 (0.08 g, 0.3 mmol) in acetonitrile (10 ml) and water (0.05 ml) was refluxed for 25 min. Then, 4 drops of HCl were added, and the refluxing was continued for 50 min. After cooling to room temperature the reaction mixture was diluted with saturated NaHCO3 and extracted with CHCl3. The extract was washed with water and dried over Na2SO4. The solvent was evaporated, and the residue was chromatographed on SiO2 with cyclohexane/EtOAc (2:1) to afford the furanone 33 (0.042 g, 44%). Mp 275–277°C (CHCl3-EtOAc). IR (cm⫺1): 3395, 2990, 2940, 2880, 1680, 1600, 1515, 1450, 1400, 1360, 1295, 1250, 1235, 1165, 980, 880. 1H NMR ␦: 1.00 (s, 3H, 18-Me), 2.24 (s, 3H, 5⬘-Me), 2.77 (t, 2H, J ⫽ 4.5 Hz), 5.35 (s, 1H, C4⬘-H), 6.17 (br. s, 1H, OH), 6.63 (m, 2H, Ph), 7.03 (d, 1H, J ⫽ 8 Hz, Ph). HRMS Calc. for C22H26O3: 338.1882; Found: 338.1880.
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2.19. Reduction of the furanone (33) with LiAlH4 LiAlH4 (0.2 g, 5.3 mmol) was added portionwise to a solution of the furanone 33 (0.4 g, 1.18 mmol) in THF (30 ml). The reaction mixture was stirred for 2 h at room temperature. Excess LiAlH4 was destroyed by addition of 15% NaOH solution. The resulting precipitate was filtered off and washed with CHCl3. The organic layer was dried over Na2SO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane/EtOAc (30:1 f 5:1) to give: a) (5⬘R,17S)-3-hydroxy-5⬘-methylspiro[estra1,3,5(10)-triene-17,2⬘-tetrahydrofuran-3⬘-one] 34 (88 mg, 22%). Mp 196 –198°C (toluene-cyclohexane). 1H NMR ␦: 0.94 (s, 3H, 18-Me), 1.40 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 2.57 (dd, 1H, J ⫽ 17, 5.5 Hz, C4⬘-H), 2.78 (t, 2H, J ⫽ 4.5 Hz), 4.19 (m, 1H, C5⬘-H), 5.30 (br.s, 1H, OH), 6.60 (m, 2H, Ph), 7.10 (d, 1H, J ⫽ 8.5 Hz, Ph); b) (5⬘S,17S)-3-hydroxy-5⬘methylspiro[estra-1,3,5(10)-triene-17,2⬘-tetrahydrofuran3⬘-one] 35 (92 mg, 23%). Mp 199 –201°C (toluenecyclohexane). 1H NMR ␦ 0.98 (s, 3H, 18-Me), 1.37 (d, 3H, J ⫽ 6.5 Hz, 5⬘-Me), 2.16 (dd, 1H, J ⫽ 18.5, 9 Hz, C4⬘-H), 2.61 (dd, 1H, J ⫽ 18.5, 6.5 Hz, C4⬘-H), 2.79 (t, 2H, J ⫽ 4.5 Hz), 4.38 (m, 1H, C5⬘-H), 5.01 (br.s, 1H, OH), 6.60 (m, 2H, Ph), 7.08 (d, 1H, J ⫽ 8 Hz, Ph). HRMS Calc. for C22H28O3: 340.2038; Found: 340.2042. 2.20. Reduction of the furanone (33) with lithium in liquid ammonia Furanone 33 (200 mg) in THF (5 ml) was added to a solution of lithium (50 mg) in liquid ammonia (30 ml) at ⫺72°C. The reaction mixture was stirred at ⫺72 to ⫺65°C for 15 min, and then excess lithium was destroyed by addition of NH4Cl. The reaction mixture was set aside for evaporation of the ammonia. After warming to room temperature the residue was diluted with saturated NaCl solution and extracted with CHCl3. The extract was dried over Na2SO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane/EtOAc (30:1 f 5:1) to give: a) furanone 34 (38 mg, 19%) and b) furanone 35 (73 mg, 36%). 2.21. (17R,23R)-17,23-Epoxy-3-hydroxy-19-norchola1,3,5(10),20(21)-tetraene (36) A suspension of tBuOK (0.1 g, 0.9 mmol) and Ph3PCH3I (0.37 g, 0.9 mmol) in dry toluene (6 ml) was refluxed under argon for 50 min. A solution of the furanone 34 (52 mg) in dry toluene (3 ml) was added, and the mixture was refluxed under argon for 3 h. Then, it was allowed to cool to room temperature, and the precipitate was filtered off. The filtrate was diluted with saturated NaCl solution and extracted with CHCl3. The extract was dried over Na2SO4, evaporated, and the residue was chromatographed on SiO2 with cyclohexane/EtOAc (10:1 f 5:1) to give the methylenefuran 36 (45
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mg, 87%). Mp 161–163°C (toluene-cyclohexane). IR (cm⫺1): 3425, 2980, 2935, 2880, 1625, 1515, 1455, 1390, 1255, 1020. 1H NMR ␦: 0.93 (s, 3H, 18-Me), 1.29 (d, 3H, 23-Me), 2.51 (dd, 1H, J ⫽ 14, 5 Hz, C22-H), 4.01 (m, 1H, C23-H), 4.67 (br.s, 1H, OH), 4.78 (d, 1H, J ⫽ 2.5 Hz, C21⬘-H), 5.07 (d, 1H, J ⫽ 2.5 Hz, C21-H), 6.60 (m, 2H, Ph), 7.13 (d, 1H, J ⫽ 8.5 Hz, Ph). 13C NMR ␦: 15.2, 22.3, 23.8, 27.1, 28.0, 30.4, 33.1, 37.1, 40.0, 44.2, 44.4, 47.1, 48.6, 72.9, 96.0, 107.6, 113.3, 115.9, 127.2, 133.6, 139.1, 153.9, 156.9. HRMS Calc. for C23H30O2: 338.2246; Found: 338.2245.
2.22. (17R,23S)-17,23-Epoxy-3-hydroxy-19-norchola1,3,5(10),20(21)-tetraene (37) The title compound was prepared from the furanone 35, in a similar manner as described above, in 86% yield. Mp 173–176°C (cyclohexane-EtOAc). IR (cm⫺1): 3400, 2935, 1625, 1515, 1460, 1295, 1260, 1110, 1070, 1025. 1H NMR ␦: 0. 94 (s, 3H, 18-Me), 1.20 (d, 3H, J ⫽ 6.5 Hz, 23-Me), 4.17 (m, 1H, C23-H), 4.84 (s, 1H, C21-H), 5.04 (s, 1H, C21-H), 6.60 (m, 2H, Ph), 7.11 (d, 1H, J ⫽ 8.5 Hz, Ph). 13C NMR ␦: 14.6, 22.7, 23.9, 27.3, 27.9, 30.4, 35.0, 36.3, 39.9, 42.0, 44.2, 47.6, 49.2, 71.9, 96.0, 107.3, 113.3, 115.9, 127.1, 133.3, 138.9, 154.1, 155.5. HRMS Calc. for C23H30O2: 338.2246; Found: 338.2245.
2.23. (5⬘R,17S)-5⬘-Methylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (39a) A mixture of the furanone 22 (0.29 g, 0.86 mmol), ethanedithiol (0.162 g, 1.7 mmol), and BF3.OEt3 (0.122 g, 0.86 mmol) in dry chloroform (15 ml) was stirred at room temperature for 10 h. Then, the mixture was washed with saturated NaHCO3 solution, and the organic layer was dried over Na2SO4 and evaporated. The residue was purified on SiO2 with cyclohexane/EtOAc (120:1 f 90:1) to give the ethylenethioketal 38a (0.252 g, 71%). Part of the obtained thioketal 38a (96 mg, 0.2 mmol) was dissolved in ethanol (25 ml), and Raney nickel was added. The mixture was stirred at room temperature for 4 h, and then, 0.03 g of Raney nickel was added again and stirring was continued for 6 h. The catalyst was filtered off and carefully washed with ethanol. The filtrate was evaporated, and the residue was chromatographed on SiO2 with cyclohexane/EtOAc (120:1 f 100:1) to afford the ⌬4-olefin 39a (60 mg, 78%) as an oil. IR (cm⫺1): 2935, 2875, 1760, 1465, 1390, 1155, 1095. 1H NMR ␦: 0.94 (s, 3H, 18-Me), 1.32 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.15 (m, 1H, C5⬘-H), 5.37 (s, 1H, C4-H). 13C NMR ␦: 14.0, 21.7, 22.1, 23.8, 25.4, 25.7, 28.8, 31.58, 31.64, 33.0, 35.5, 41.1, 41.9, 46.2, 47.1, 47.7, 49.7,
69.2, 93.3, 119.9, 140.2, 219.0. HRMS Calc. for C22H32O2: 328.2402; Found: 328.2405. 2.24. (5⬘S,17S)-5⬘-Methylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (39b) The title compound was prepared as an oil from the furanone 21, according to the procedure for 39a, in 52% overall yield. IR (cm⫺1): 2935, 2865, 1760, 1465, 1390, 1165, 1100, 1065. 1H NMR ␦: 0.95 (s, 3H, 18-Me), 1.35 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.34 (m, 1H, C5⬘-H), 5.36 (s, 1H, C4-H). 13C NMR ␦: 14.3, 22.5, 22.7, 24.5, 26.1, 26.8, 29.4, 32.2, 33.3, 35.1, 36.1, 41.8, 42.5, 45.3, 48.3, 49.5, 50.1, 70.6, 94.7, 120.7, 140.7, 219.7. HRMS Calc. for C22H32O2: 328.2402; Found: 328.2400. 2.25. (17␣)-(3⬘-Ethylisoxazol-5⬘-yl)-estr-4-en-3,17-diol (41) The title compound was obtained by the previously described procedure from compound 40 and propionitrile oxide in 83% yield, mp 155–157°C (ethyl acetate). 1H NMR ␦: 1.06 (s, 3H, 18-Me), 1.28 (t, 3H, J ⫽ 7.5 Hz, CH3CH2), 2.71 (q, 2H, J ⫽ 7.5 Hz, CH3CH2), 4.12 (m, 1H, C3-H), 5.40 (br s, 1H, C4-H), 6.00 (s, 1H, C4⬘-H). 2.26. (17S)-5⬘-Ethyl-3-hydroxyspiro[estr-4-ene-17,2⬘(2⬘,3⬘-dihydrofuran-3⬘-one] (42) The title compound was obtained from isoxazole 41 by the previously described procedure for 17 in 80% yield, mp 173–176°C (ethyl acetate). 1H NMR ␦: 1.00 (s, 3H, 18-Me), 1.24 (t, 3H, J ⫽ 7.5 Hz, CH3CH2), 2.48 (q, 2H, J ⫽ 7.5 Hz, CH3CH2), 4.16 (m, 1H, C3-H), 5.27 (s, 1H, C4⬘-H), 5.38 (br s, 1H, C4-H). 2.27. Reduction– oxidation of (42) Reduction of 450 mg of 42 with lithium in liquid ammonia followed by oxidation of the resulting mixture of alcohols gave 188 mg (42%) of (5⬘R,17S)-5⬘-ethylspiro[estr-4-ene-3-one-17,2⬘-tetrahydrofuran-3⬘-one] 44, mp 118 –120°C (EtOAc-hexane). 1H NMR ␦: 1.00 (s, 3H, 18 Me), 3.99 (m, 1H, C5⬘-H), 5.83 (s, 1H, C4-H); 13C NMR, ␦: 9.2, 14.0, 23.7, 25.9, 26.6, 28.9, 30.6, 31.4, 33.1, 35.5, 36.5, 40.5, 42.5, 43.9, 46.9, 47.5, 48.9, 74.2, 92.6, 124.6, 166.6, 199.6, 216.7; IR (cm⫺1): 1720, 1680, 1615, and 72 mg (16%) of (5⬘S,17S)-5⬘-ethylspiro[estr-4-ene-3-one-17,2⬘tetrahydrofuran-3⬘-one] 43, mp 113–115°C (ether-hexane). 1 H NMR ␦: 1.02 (s, 3H, 18-Me). 4.12 (m, 1H, C5⬘-H). 5.83 (s, 1H, C4-H). 13C NMR ␦: 9.5, 13.6, 23.8, 26.2, 26.6, 29.1, 30.6, 32.6, 34.1, 35.4, 36.5, 40.6, 42.5, 42.8, 47.4, 48.6, 48.7, 75.3, 93.4, 124.6, 166. 3, 199.7, 216.9; IR (cm⫺1): 1720, 1680, 1610.
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2.28. (17R,23R)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-dien-3-one (47b) A solution of 44 (0.22 g, 0.62 mmol), triethyl orthoformate (0.22 ml, 1.32 mmol), and p-TsOH (0.022 g, 0.115 mmol) in dry dioxane (10 ml) was stirred at room temperature for 6 h. The reaction mixture was poured into a 10% solution of pyridine in water (10 ml) at 0°C and left overnight in a refrigerator. The organic layer was dissolved in CHCl3, washed with water, dried over Na2SO4, and the solvent was evaporated in vacuo. The crude product 45b was obtained as an oil (0.22 g, 90%). 1H NMR ␦: 0.94 (s, 3H, 18-Me), 4.00 (m, 1H, C23-H), 5.20 (s, 1H, C4-H), 5.28 (d, 1H, J ⫽ 5 Hz, C6-H). The obtained product 45b was used directly in the next stage without further purification. A mixture of tBuOK (0.195 g, 1.72 mmol) and Ph3PCH3I (0.695 g, 1.72 mmol) in dry toluene (20 ml) was refluxed under argon for 1 h. Then, compound 45b from the previous stage in dry toluene (10 ml) was added. The mixture was stirred under reflux for 5 h. The precipitate was filtered off and washed with toluene and ether. The organic layer was washed with water and evaporated. The residue was treated with 60% AcOH (2 ml) for 20 min. Then, the reaction mixture was poured into a saturated solution of NaHCO3 and extracted with CHCl3. The extract was dried over Na2SO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane-THF (49:1) to give the enone 47b (0.13 g, 59%). Mp 110 –114°C (hexane-ether). IR (cm⫺1): 1710, 1680, 1620, 1460, 1440, 1380, 1260, 1210, 1090, 1030, 890. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.82 (m, 1H, C23-H), 4.72 (d, 1H, J ⫽ 3 Hz, C21-H), 5.04 (d, 1H, J ⫽ 3Hz, C21-H), 5.82 (s, 1H, C4-H). 13C NMR ␦: 10.5, 15.0, 23.7, 26.6, 27.2, 29.7, 31.4, 32.3, 36.2, 36.9, 37.1, 41.6, 41.6, 43.2, 46.8, 48.3, 50.0, 77.1, 94.9, 107.6, 125.1, 156.5, 167.6, 200.6. HRMS Calc. for C24H34O2: 354.2559; Found: 354.2560.
575
mmol) in dry chloroform (20 ml) was stirred at room temperature for 7 h. Then, the mixture was washed with a saturated solution of NaHCO3 and dried over Na2SO4. The solvent was removed in vacuo. The residue was dissolved in ethanol (80 ml), and Raney nickel was added portionwise (3 ⫻ 0.5 g). The reaction mixture was refluxed for 10 h. Then, the catalyst was filtered off, and the filtrate was evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (99:1) to give the olefin 49b (0.265 g, 55%). Mp 62– 65°C (EtOH). IR (cm⫺1): 1760, 1670, 1460, 1450, 1390, 1340, 1240, 1080, 1050. 1H NMR ␦: 0.92 (s, 3H, 18-Me), 3.98 (m, 1H, C5⬘-H), 5.38 (br.s, 1H, C4-H). 13C NMR ␦: 9.9, 14.2, 22.8, 24.5, 26.1, 26.5, 29.4, 29.7, 32.3, 32.4, 33.0, 36.2, 41.8, 42.6, 44.7, 47.9, 48.4, 50.4, 74.8, 93.6, 120.6, 140.9, 219.6. HRMS Calc. for C23H34O2: 342.2559; Found: 342.2558. 2.31. (5⬘S,17S)-5⬘-Ethylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (49a) The title compound was obtained as an oil from the enone 43, according to the same procedure described above, in 50% yield. IR (cm⫺1): 1750, 1680, 1460, 1440, 1390, 1350, 1240, 1070, 1040. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 4.14 (m, 1H, C5⬘-H), 5.38 (br.s, 1H, C4-H). 13C NMR ␦: 10.2, 14.3, 22.7, 24.5, 26.1, 26.8, 29.4, 29.7, 32.2, 33.2, 35.1, 36.1, 41.5, 42.5, 43.4, 48.4, 49.6, 50.1, 75.6, 94.4, 120.7, 140.7, 219.7. HRMS Calc. for C23H34O2: 342.2559; Found: 342.2558. 2.32. (17R,23R)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-diene (50b)
The title compound was prepared from ketone 43, according to the procedure described above, in 48% overall yield. Mp 80 – 83°C (hexane-ether). IR (cm⫺1): 1720, 1670, 1620, 1460, 1440, 1380, 1260, 1210, 1090, 1030, 890. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.90 (m, 1H, C23-H), 4.76 (t, 1H, J ⫽ 3 Hz, C21-H), 5.02 (t, 1H, J ⫽ 3 Hz, C21-H), 5.82 (s, 1H, C4-H). 13C NMR ␦: 10.5, 14.5, 23.9, 27.1, 27.2, 29.7, 31.4, 34.7, 36.0, 36.2, 37.1, 39.5, 41.5, 43.2, 47.2, 49.0, 49.8, 77.2, 95.4, 107.0, 125.1, 155.4, 167.4, 200.5. HRMS Calc. for C24H34O2: 354.2559; Found: 354.2561.
A mixture of tBuOK (0.119 g, 1.053 mmol) and PhPCH3I (0.425 g, 1.053 mmol) in dry toluene (10 ml) was refluxed under argon for 1 h. Then, ketone 49b (0.13 g, 0.353 mmol) in dry toluene (10 ml) was added. The mixture was refluxed for 5 h under argon until the starting material disappeared. The precipitate was filtered off and washed with toluene and ether. The combined organic layer was washed with water and dried over Na2SO4. The solvent was removed in vacuo, and the residue was chromatographed on SiO2 with hexane/EtOAc (200:1) to afford the olefin 50b (0.056 g, 43%). IR (cm⫺1): 1670, 1460, 1440, 1380, 1070, 1030, 900. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.80 (m, 1H, C23-H), 4.70 (m, 1H, C21-H), 5.02 (m, 1H, C21-H), 5.38 (br. s, 1H, C4-H). 13C NMR ␦: 10.5, 15.1, 22.3, 24.0, 26.2, 26.6, 29.4, 29.8, 32.4, 33.1, 36.2, 37.0, 41.6, 42.05, 42.14, 42.6, 46.9, 48.6, 50.8, 77.0, 95.2, 107.3, 141.2, 156.1. HRMS Calc. for C24H36O: 340.2766; Found: 340.2760.
2.30. (5⬘R,17S)-5⬘-Ethylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (49b)
2.33. (17R,23S)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-diene (50a)
A mixture of the enone 44 (0.5 g, 1.4 mmol), ethanedithiol (0.215 ml, 2.8 mmol), and BF3.OEt2 (0.345 ml, 2.8
The title compound was prepared as an oil from the ketone 49a in 74% yield as described above. IR (cm⫺1):
2.29. (17R,23S)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-dien-3-one (47a)
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Scheme 3.
1670, 1460, 1440, 1380, 1070, 1030, 890. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.88 (m, 1H, C23-H), 4.76 (br.s, 1H, C21-H), 5.00 (br.s, 1H, C21-H), 5.38 (br.s, 1H, C4-H). 13C NMR ␦: 9.8, 13.9, 22.0, 23.3, 25.5, 26.3, 28.7, 29.0, 31.8, 34.3, 35.4, 35.5, 39.2, 41.5, 41.9, 46.8, 48.6, 50.0, 76.5, 95.0, 106.1, 119.8, 140.5, 155.0. HRMS Calc. for C24H36O: 340.2766; Found: 340.2762.
3. Results and discussion Initially, an approach for derivatives with a iPr-group at C-5⬘ was investigated using the addition of isobutyronitrile oxide because isobutyronitrile oxide is more stable than its lower homologs. Cycloaddition of isobutyronitrile oxide with mestranol 2 proceeded in quantitative yield with formation of the hydroxy isoxazole 10 as the only regioisomer (Scheme 3). Hydrogenolysis of the hydroxy isoxazole 10 could be easily affected under different conditions, but generally, the formation of furanone 12, was observed along with the expected enaminoketone 11. This product could be isolated in reasonable yield after column chromatography, but it proved to be more practical to convert the hydroxy isoxazole 10 directly into the furanone 12 via hydrogenation in acidic medium. Several variants of the transformation of enone 12 into ketone 15 were studied. Reduction of 12 with NaBH4 gave an unseparable mixture of alcohols 13 and 14, which upon Jones or Swern oxidation could be transformed into ketone 15. Treatment of enone 12 with lithium in liquid ammonia afforded ketone 15 directly. It is interesting that the reaction of enone 12, even with a large excess of LiAlH4, led mainly to ketone 15, probably because of the formation of a stable complex after the reduction of the double bond.
Scheme 4.
Synthesis of compounds with a small alkyl substituent (Me or Et) at C-5⬘ was performed next. The 1,3-dipolar cycloaddition of mestranol 2 to acetonitrile oxide gave a quantitative yield of hydroxy isoxazole 16 (Scheme 4). Its hydrogenolysis, followed by acid hydrolysis, gave furanone 17. Reduction of this heterocyclic product afforded a mixture of the alcohols 18, which were isomeric at C-3⬘ and C-5⬘. At this stage, the ⌬4-3-ketone could be formed, and Birch reduction of 18 followed by hydrolysis of the intermediate enol ether 19 gave enone 20. Oxidation of the secondary alcohol group produced ketones 21 and 22, which could be separated by column chromatography. Stereochemical assignment of the methyl group at C-5⬘ could not be done by spectroscopic methods, but fortunately, ketone 22 gave crystals suitable for X-ray analysis (Fig. 1). Thus, the problem of stereochemistry could be solved, not only for these two compounds, but also for the other 5⬘-alkyl steroids of this series, by analogy of their NMR spectra (Fig. 2). Synthesis of the 3⬘-methylene derivatives with a 5⬘methyl group was achieved as shown in Scheme 5. Reduction of enone 17 with LiAlH4 proceeded with formation of the ketones 23 in contrast to the reaction with NaBH4, where further reduction of the C-3⬘-carbonyl took place. Application of a large excess of LiAlH4 also allowed for detection of the diastereomeric mixture of the corresponding C-3⬘ alcohols, but even in this case, the ketones 23 were the main products of the reaction. The high temperature
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577
Fig. 1. ORTEP view of the furanone 22.
variant of the Wittig reaction gave the methylene derivatives 24 in good yield. Some problems arose in the next step because treatment of compound 24 with sodium in liquid ammonia led not only to the Birch reduction of ring A, but also gave a considerable amount of the side-chain reduced compound 25 with a methyl group at C-3⬘. The rate of the methylene group reduction was dependent on the configuration of the methyl group at C-5⬘. This was confirmed by identification of only 5⬘S-isomers 25 and 29 in the reaction products along with the expected methylene derivatives 27 and 28. Moreover, the ratio of 27 and 28 showed a lower amount of 28 than could be expected, based on the analysis of the NMR spectrum of the mixture of the C-5⬘-methyl derivatives 24. The acetylenic alcohol 30 provides a possible starting point for the preparation of the 3⬘-methylene derivatives 36 and 37, which differ from 24 by the presence of a hydroxy group instead of a methoxy group in ring A (Scheme 6). Ring A proved to be stable, and no problems arose in the course of the synthetic transformations performed. Cycloaddition of acetylenic alcohol 30 with acetonitrile oxide proceeded in nearly quantitative yield, but the ring opening of the resulting isoxazole 31 followed by hydrolysis of 32 and ring closure gave the unsaturated furanone 33 in moderate yield. Reduction of the double bond in 33 was investigated using LiAlH4 or lithium in liquid ammonia. In both cases, a
Fig. 2. Fragments of NMR spectra of the furanones 22 (above) and 21 (below).
Scheme 5.
moderate yield of a mixture of the reduced compounds 34 and 35 was obtained in slightly different ratios. Compounds 34 and 35 could be separated by chromatography, and each could be converted in good yield to the corresponding methylene compounds 36 and 37, respectively. Another variation in the structure of ring A of these 5⬘-methylene derivatives can be obtained when ring A is reduced to the corresponding 3-deoxo steroids. Although compounds 21 and 22 contain two keto groups, selective reduction of the conjugate keto group at C-3 proved to be straightforward (Scheme 7). Treatment of enone 21 or 22 with ethanedithiol in the presence of Lewis acid led almost exclusively to the corresponding thioketals 38, which could be smoothly reduced with Raney nickel to the deoxo derivatives 39. Our goal was to investigate various synthetic approaches toward spirofurans 1. That is why we chose a method starting from norethisterone 3 for the synthesis of compounds with a C-5⬘ ethyl group (Scheme 8). It is well known that the activity of dipolarophiles in 1,3-dipolar cycloaddition reactions with unsaturated compounds greatly depends on the number of substituents [13,14]. A monosubstituted double bond is much more reactive than a trisubstituted one. On the other hand, conjugation of the double bond with a
578
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Scheme 8. Scheme 6.
carbonyl group leads to a significant increase in activity. Norethisterone 3 contains a conjugated trisubstituted ⌬4double bond and a monosubstituted triple bond, both of which can react with nitrile oxide. Our investigation showed that the triple bond was a bit more reactive than the ⌬4double bond in ring A. Although the desired cycloaddition product could be obtained directly, better results were obtained when norethisterone 3 was first reduced to the allylic alcohols 40. The cyclic part of this compound was relatively stable toward nitrile oxides and allowed for selective cycloaddition at the triple bond in good yield. Reductive cleavage of the isoxazole 41 with Raney nickel in acetic acid gave the furanone 42. Reduction of the ⌬4⬘,5⬘-double bond followed by regeneration of the enone system in ring A led to a mixture of furanones 43 and 44, which could be separated by column chromatography. Synthesis of the 3⬘-methylene derivative 47 with a (5⬘R)ethyl group was accomplished as shown in Scheme 9. Treatment of enones 43 and 44 with triethyl orthoformate in the presence of TsOH led to the dienol ethers 45. The keto
Scheme 7.
group at C-3⬘ remained intact under these reaction conditions, and it could be transformed into a methylene function by a Wittig reaction. Final acid hydrolysis of the dienol ethers 46 gave the desired enones 47. Treatment of enones 43 or 44 with ethanedithiol (Scheme 10) in the presence of a Lewis acid gave the thioketals 48. Their desulfurization with Raney nickel gave the 3-deoxo derivatives 49. The Wittig olefination of 49 led to compounds 50 with a methylene group at C-3⬘. Thus, approaches toward steroids with a side chain containing a furan moiety and a 3⬘-methylene group and a C-5⬘ alkyl group have been achieved in several ways. A number of 3⬘-methylene-5⬘-alkyl steroids with different types of ring A substituents (⌬4-3-keto-, ⌬4-, 3-methoxy-1,3,5(10)-
Scheme 9.
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[3]
[4] [5]
[6]
[7] Scheme 10. [8]
triene-, and 3-hydroxy-1,3,5(10)-trienes) have been prepared. [9]
Acknowledgments [10]
The authors are indebted to Organon International for financial support of this research. We thank Dr. N.B. Khripach for recording of NMR spectra, and Mr. H. Jongejan for the exact mass measurements.
[11]
[12]
References [1] Zeelen FJ. Medicinal chemistry of steroids. Amsterdam: Elsevier, 1990. [2] Hamersma JAM, Orlemans EOM, Rewinkel JBM, inventors. AKZO NV, The Netherlands, assignee. Preparation of 17-spiro-
[13] [14]
579
methylene steroids. Eur Pat Appl EP 582,338. 1994 Feb 09 [CA 121;231157q]. Khripach VA, Zhabinskii VN, Litvinovskaya RP. Synthesis and some practical aspects of brassinosteroids. In: Cutler HG, Yokota T, Adam G, editors. Brassinosteroids. Chemistry, bioactivity, and applications. ACS Symposium series. Washington: American Chemical Society, 1991;474:43–55. Khripach VA, Zhabinskii VN, de Groot AE. Brassinosteroids—a new class of plant hormones. San Diego: Academic Press, 1999. Khripach VA, Zhabinskii VN, Pavlovskii ND, Lyakhov AS, Govorova AA. Synthesis and X-ray analysis of (20S)-22-benzamido-6methoxy-3␣,5-cyclo-26,27-bisnorcholest-22-en-24-one. Bioorg Khim 1998;24:856 – 61. Khripach VA, Litvinovskaya RP, Baranovskii AV. Synthesis of ecdysone side chains via isoxazoline derivatives. Mendeleev Commun 1992:117– 8. Khripach VA, Zhabinskii VN, Pavlovskii ND. Synthesis of steroidal 23-enamino-25-ketones—potential intermediates for preparation of vitamins D and analogs. Zh Org Khim 1998;34:59 – 63. Khripach VA, Zhabinskii VN, Pavlovskii ND. A nitrile oxide approach to the construction of the side chain of metabolites and analogs of vitamin D, functionalized at C-23. Zh Org Khim 1999;35: 390 –2. Litvinovskaya RP, Baranovskii AV, Khripach VA, Struchkov Yu, Ovchinnikov Yu. Synthesis and structure of (22R,24R)-6-acetoxy24-methyl-5␣-cholestane-3,5␣-22,24-tetrol. Mendeleev Commun 1994:89 –90. Laurent H, Schulz G. 17-Hydroxy-17␣-isoxazolyl steroids. Chem Ber 1969;102:3324 –32. Khripach VA, Zhabinskii VN, Olkhovick VK, Zavadskaya MI, Drachenova OA, Khripach NB. Stereoselective synthesis of 20-(hydrofuryl)steroids via isoxasole derivatives. Zhurn Org Khim 1996;32: 841– 4. Van der Louw J, Hamersma JAM, Groen MB. Radical cyclization to 3⬘alkylidene-17-spirodihydrofuran-substituted steroids observation of an unexpected intramolecular hydrogen transfer. 16th Conf. on Isoprenoids, 1995. p. 55. Grundman C, Gru¨nanger P. The nitrile oxides. Berlin: SpringerVerlag, 1971. Torsell KBG. Nitrile oxides, nitrones and nitronates. Weinheim: VCH Verlagsgeselschaft, 1988.
Synthesis of C-5⬘-alkyl substituted 17-spirofuran 19-norsteroids Vladimir A. Khripacha,*, Vladimir N. Zhabinskiia, Dmitrii N. Tsavlovskiia, Olga A. Drachenovaa, Galina V. Ivanovaa, Olga V. Konstantinovaa, Margarita I. Zavadskayaa, Alexander S. Lyakhova, Alla A. Govorovaa, Marinus B. Groenb, Aede de Grootc a
Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, Kuprevich str., 5/2, 220141 Minsk, Belarus b Scientific Development Group, N.V. Organon, P.O. Box 20, 5340 BH Oss, The Netherlands c Wageningen University, Laboratory of Organic Chemistry and Research Center, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Received 31 May 2000; received in revised form 5 September 2000; accepted 12 September 2000
Abstract A number of new steroidal 17-spirofuran derivatives of the 19-nor series containing Me, Et or iPr-substituents in the heterocyclic moiety has been prepared, which are expected to have a strong progestagenic activity. The proposed approach made use of the 1,3-dipolar cycloaddition of low-molecular nitrile oxides with steroidal acetylenic alcohols followed by transformation of the isoxazole side chain. © 2001 Elsevier Science Inc. All rights reserved. Keywords: 19-Norsteroids; 1,3-Dipolar cycloaddition; Nitrile oxides; Acetylenic alcohols; Spirofurans
1. Introduction The 19-nor derivatives of progesterone occupy a special place among pharmaceutically important steroids. Many compounds in this series have been registered as medicines and others are under development [1]. A strong progestagenic activity has been demonstrated for 19-nor steroids containing a 3⬘-methylene-17R-spirodihydrofuran moiety 1 (R ⫽ H) [2]. Investigation of related compounds with various types of substitutions in the spirofuran moiety, especially those bearing an alkyl group at C-5⬘, could give better insight into structure - activity relationships and provide a good basis for the construction of new molecules with the desired properties. Synthetic intermediates, containing a keto group at C-3⬘, as well as compounds with different cyclic components, such as 3-OH and 3-OMe derivatives with an aromatic cycle A and 3-deoxo steroids, are interesting from this point of view. The nitrile oxide approach proved to be very fruitful for the construction of functionalized steroidal side chains in the synthesis of brassinosteroids [3–5], ecdysteroids [6], metabolites of vitamin D [7,8], and other compounds [9,10].
* Corresponding author. E-mail address: [email protected] (V.A. Khripach).
The data [11] on the preparation of certain steroids via this approach modified by the addition of a dihydrofuranone fragment in the side chain seemed to be promising for the synthesis of the title compounds 1 (R ⫽ alkyl), which are difficult to obtain in any other way [12]. The acetylenic alcohols 2 (mestranol) and 3 (norethisterone) were suitable starting compounds for the preparation of a variety of 19-norsteroids (Scheme 1). Mestranol 2 contains a stable ring A, that allowed for different reactions in the side chain. However, transformation of the aromatic ring A into the ⌬4-3-ketone system required a Birch reduction, which may not be compatible with many functional groups in the rest of the molecule. On the other hand, norethisterone 3, which already contains an enone in ring A, was rather vulnerable, and the enone moiety could not survive many reactions in the side chain. This is why the two approaches developed for the preparation of target compounds 1, starting from the acetylenic alcohols 2 and 3, were complementary. The most essential part of the synthetic routes toward the target compounds was the construction of the side chain. The general approach is shown in Scheme 2. 1,3-Dipolar cycloaddition of the acetylenic alcohol 4 to the corresponding nitrile oxide led to the hydroxy isoxazoles 5, which upon reductive cleavage via the intermediate enaminoketones 6, gave the furanones 7. The two subsequent stages of this
0039-128X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 0 3 9 - 1 2 8 X ( 0 0 ) 0 0 2 2 2 - 1
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Scheme 1. Starting compounds for the preparation of 5⬘-alkylspirofuran derivatives 1.
carried out on Kieselgel 60 (Merck Art. 7734). X-ray data collection (Nicolet R3 m diffractometer) was performed via the /2 scan mode. The structure was solved by direct methods (SHELXS 86) and refined by full-matrix least squares (SHELXL 93). The positions of hydrogen atoms were calculated, and refined using the riding model. 2.2. 1,3-Dipolar cycloaddition of nitrile oxides to acetylenic alcohols (general procedure)
approach were rather straightforward and included reduction of the double bond and Wittig olefination. The synthesis of three types of C-5⬘-substituted 17-spirofuran derivatives, where R ⫽ iPr, Et, and Me, has been studied and is reported here.
2. Experimental 2.1. General Melting points were taken on a Boetius micro-melting point apparatus and are uncorrected. IR spectra were recorded on a UR-20 spectrophotometer. 1H and 13C NMR spectra were taken on a Bruker AC-200 (200 MHz for 1H, 50 MHz for 13C) spectrometer using TMS as an internal standard. The exact mass measurements were carried out on a Finnigan MAT95 mass-spectrometer, operating in the 70 eV-EI mode and at a resolution of RP ⫽ 8000. Samples were introduced by direct probe for accurate mass measurement by peak matching. Reactions were monitored by TLC using aluminum or plastic sheets precoated with silica gel 60 F254 (Merck Art. 5715). Column chromatography was
Pyridine (0.02 ml) and oxime (5 mmol) were added to a stirred suspension of N-chlorosuccinimide (5 mmol) in CHCl3 (10 ml), and the mixture was stirred for 15 min. A solution of steroid (1 mmol) in CHCl3 (15 ml) was added to the mixture. After stirring the resulting mixture for 5–10 min, a solution of Et3N (5 mmol) in CHCl3 (10 ml) was added dropwise over a 3–10 h period. The mixture was washed with water and dried over Na2SO4. The solvent was evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (10:1 f 5:1) to give the isoxazole. 2.3. 3-Methoxy-17␣-(3⬘-isopropylisoxazol-5⬘-yl)-estra1,3,5(10)-trien-17-ol (10) The title compound was obtained from mestranol 2 and isobutyronitrile oxide in 99% yield. 1H NMR ␦: 1.03 (s, 3H, 18-Me), 1.30 (d, J ⫽ 7 Hz, 3⬘-iPr), 3.06 (sept, 1 H, J ⫽ 7 Hz, C3⬘-H), 3.78 (s, 3H, OMe), 6.05 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 5, 2 Hz, C2-H), 7.23 (d, 1H, J ⫽ 8.5 Hz, C1-H). 2.4. 3-Methoxy-17␣-(3⬘-methylisoxazol-5⬘-yl)-estra1,3,5(10)-trien-17-ol (16) The title compound was obtained from mestranol 2 and acetonitrile oxide in 99% yield. Mp 157–159°C (MeOHCH2Cl2). 1H NMR ␦: 1.03 (s, 3H, 18-Me), 2.32 (s, 3H, 3⬘-Me), 3.78 (s, 3H, OMe), 6.03 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8, 2 Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). IR (cm⫺1): 2935, 1605, 1590, 1495, 1460, 1440, 1415, 1375, 1350, 1250, 1240, 1145, 1100, 1040, 905. HRMS Calc. for C23H29NO3: 367.2147; Found: 367.2149. 2.5. Hydrogenation of the isoxazole (10)
Scheme 2. General approach to the preparation of the side chain of 5⬘-alkylspirofuran derivatives.
A suspension of Raney nickel (800 mg) was saturated with hydrogen for 1 h. Then, the isoxazole 10 (500 mg, 1.3 mmol) and boric acid (80 mg, 1.3 mmol) were added. The mixture was vigorously stirred under hydrogen at room temperature for 3 h. Then, the catalyst was filtered off, and the filtrate was evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (7:1 f 3:1) to give a mixture of (17S)-23-amino-17-hydroxy-3-methoxy-24methyl-19,21-bisnorchola-1,3,5(10),22-tetraen-20-one 11
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and (17S)-3-methoxy-5⬘-isopropylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘,3⬘-dihydrofuran)-3⬘-one] 12. The mixture was rechromatographed to give the enaminoketone 11 (298 mg, 62%). 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.78 (s, 3H, OMe), 5.21 (s, 1H, C22-H), 5.34 (br.s, 1H, NH), 10.0 (br.s, 1H, NH). 2.6. (17S)-3-Methoxy-5⬘-isopropylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘,3⬘-dihydrofuran)-3⬘-one] (12) 2.6.1. Variant A. A solution of the enaminoketone 11 (41 mg, 0.1 mmol) in acetic acid (2 ml) was stirred at 90°C for 2 h. Then, the solvent was evaporated, and the residue was chromatographed on SiO2 with hexane/EtOAc (7:1) to give an oil. The oil was crystallized from 0.5 ml of EtOH to afford furanone 12 (19.6 mg, 50%). Mp 150 –151°C. 1H NMR ␦: 1.02 (s, 3H, 18-Me), 1.24 (m, 6H, 5⬘-iPr), 2.72 (m, 1H, 5⬘-iPr), 3.78 (s, 3H, OMe), 5.29 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). IR (cm⫺1): 2940, 1685, 1605, 1580, 1500, 1465, 1450, 1355, 1285, 1240, 1035, 950. 2.6.2. Variant B. A mixture of the enaminoketone 11 (152 mg, 0.38 mmol), 5% HCl (4 ml) and ethanol (20 ml) was stirred at room temperature for 4 h. The usual work-up and crystallization from 3 ml of EtOH gave furanone 12 (132 mg, 92%). 2.7. Reduction of the furanone 12 with Li in liquid ammonia A solution of the furanone 12 (130 mg, 0.34 mmol) in ether-THF (1:1, 4 ml) was added to a stirred solution of Li (11 mg, 1.6 mmol) in liquid ammonia (6 ml) at ⫺50°C. Then, Li (28 mg) was added, and stirring was continued for 10 min. Excess Li was quenched with NH4Cl. The mixture was allowed to sit overnight for evaporation of ammonia. The product was isolated with CHCl3. The extract was dried, evaporated, and the residue was chromatographed on SiO2 with hexane/EtOAc (20:1 f 5:1) to give (17S)-3-methoxy-5⬘-isopropylspiro[estra-1,3,5(10)-triene-17,2⬘-tetrahydrofuran3⬘-one] 15 (78 mg, 60%). 2.8. Reduction of the furanone 12 with NaBH4 A mixture of EtOH (5 ml), the furanone 12 (27 mg, 0.07 mmol), and NaBH4 (18 mg, 0.47 mmol) was stirred at room temperature for 12 h. The usual workup and chromatography on SiO2 with hexane/EtOAc (5:1) gave (17S)-3methoxy-5⬘-isopropylspiro[estra-1,3,5(10)-triene-17,2⬘tetrahydrofuran-3⬘-ols] 13 and 14 (23 mg, 84%) as an inseparable mixture.
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2.9. (17S)-3-Methoxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘3-dihydrofuran)-3⬘-one] (17) A mixture of Raney nickel (150 mg), the isoxazole 16 (171 mg, 0.47 mmol), boric acid (50 mg, 0.8 mmol), and ethanol (20 ml) was stirred under hydrogen at room temperature for 48 h. The catalyst was filtered off, and the filtrate was evaporated. The residue was dissolved in ethanol (20 ml), and 5% HCl (5 ml) was added. The mixture was stirred at room temperature for 4 h, and then, an excess of saturated NaHCO3 was added. The product was isolated with EtOAc. The extract was dried and evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (10:1 f 3:1) to afford the furanone 17 (104 mg, 64%). 1H NMR ␦: 1.02 (s, 3H, 18-Me), 2.22 (s, 3H, 5⬘-Me), 3.78 (s, 3H, OMe), 5.31 (s, 1H, C4⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.69 (dd, 1H, J ⫽ 8.5, 2Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). 2.10. (17S)-3-Methoxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘-tetrahydrofuran-3⬘-ol] (18) NaBH4 (250 mg, 8.7 mmol) was added portionwise over 50 h to a stirred solution of the furanone 17 (281 mg, 0.6 mmol) in EtOH-THF (2:1, 15 ml). Excess reducing reagent was decomposed by addition of 5% AcOH. The mixture was extracted with EtOAc. The extract was dried over MgSO4 and evaporated. The residue was chromatographed on SiO2 with petroleum ether/EtOAc (20:1 f 5:1) to give the furanols 18 (273 mg, 96%). 1H NMR ␦: 0.95, 0.98 (s, 3H, 18-Me), 3.78 (s, 3H, OMe), 3.96 – 4.36 (m, 2H, C3⬘- and C5⬘-H), 6.62 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.16 (d, 1H, J ⫽ 8.5 Hz, C1-H). 2.11. Birch reduction of (18) Isopropyl alcohol (4 ml) was added to a solution of 18 (217 mg, 0.6 mmol) in THF (5 ml), and the solution was cooled to ⫺60°C. Then, ammonia (30 ml) was condensed into the mixture, and Na (200 mg, 8.7 mmol) was added over 10 min. The stirring was continued at ⫺40°C for 45 min. After evaporation of the ammonia, the organic layer was separated. The solvent was removed in vacuo, and the residue was chromatographed on SiO2 with petroleum ether/EtOAc (10:1 f 5:1) to give (17S)-3-methoxy-5⬘methylspiro[estra-2,5(10)-diene-17,2⬘-tetrahydrofuran-3⬘ol] (19) (206 mg, 94%). 1H NMR ␦: 0.95, 0.98 (s, 3H, 18-Me), 3.55 (s, 3H, OMe), 3.94 – 4.34 (m, 2H, C3⬘- and C5⬘-H), 4.64 (t, 1H, J ⫽ 3 Hz, C2-H). 2.12. Hydrolysis of the enole ether (19) A mixture of 19 (17.5 mg, 0.049 mmol), EtOH (5 ml), and 5% HCl (2 ml) was heated at 60°C for 10 min. Then it was diluted with water and extracted with ether. The organic layer was washed successively with water and saturated
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NaHCO3 and then dried and evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (5:1 f 3:1) to give the (17S)-5⬘-methylspiro[estr-4-ene-17,2⬘-tetrahydrofuran-3⬘-ol] (20) (14.8 mg, 88%). 1H NMR ␦: 0.98, 1.00 (s, 3H, 18-Me), 3.94 – 4.34 (m, 2H, C2⬘- and C4⬘-H), 5.82 (s, 1H, C4-H). 2.13. Oxidation of (20) The steroidal alcohol 20 (207 mg, 0.6 mmol) in pyridine (2 ml) was added to a solution of chromium oxide (200 mg, 1.3 mmol) in pyridine (2 ml). The mixture was stirred at room temperature for 45 min, ether was added, the precipitate was filtered off, and the filtrate was evaporated. The obtained residue was chromatographed on silica gel to give: a) (5⬘R,17S)-5⬘-methylspiro[estr-4-ene-3-one-17,2⬘-tetrahydrofuran-3⬘-one] 22 (106 mg, 50%), mp 159 –161°C (ether). 1H NMR ␦: 0.96 (s, 3H, 18-Me), 1.38 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.16 (m, 1H, C5⬘-H), 5.82 (s, 1H, C4-H). HRMS Calc. for C22H30O3: 342.2195; Found: 342.2191; b) (5⬘S,17S)-5⬘-methylspiro[estr-4-ene-3-one-17,2⬘tetrahydrofuran-3⬘-one] 21 (84 mg, 42%) mp 140 –141°C (ether). 1H NMR ␦: 1.00 (s, 3H, 18-Me), 1.36 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.33 (m, 1H, C5⬘-H), 5.82 (br s, 1H, C4-H). HRMS Calc. for C22H30O3: 342.2195; Found: 342.2190. 2.14. (17S)-3-Methoxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘-tetrahydrofuran-3⬘-one] (23) LiAlH4 (0.076 g, 0.2 mmol) was added portionwise to a solution of furanone 17 (0.35 g, 0.1 mmol) in ether (40 ml) at room temperature. The reaction mixture was stirred at room temperature for 15 min, and then excess LiAlH4 was decomposed by addition of a 15% NaOH solution. The precipitate was filtered off and the filtrate was extracted with ether, dried over Na2SO4, and evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (25:1 f 18:1) to give the saturated furanone 23 (0.263 g, 75%). IR (cm⫺1): 2940, 1745, 1615, 1505, 1385, 1265, 1245, 1160, 1085, 1045. 1H NMR ␦: 0.94, 097 (s, 3H, 18-Me), 1.40 (s, 3H, 3⬘-Me), 3.77 (s, 3H, OMe), 4.17, 4.37 (m, 1H, C5⬘-H), 6.60 (d, 1H, J ⫽ 2 Hz, C4-H), 7.00 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.17, 7.19 (d, 1H, C1-H). 2.15. (17R)-17,23-Epoxy-3-methoxy-19-norchola1,3,5(10),20(21)-tetraene (24) A mixture of tBuOK (0.544 g, 4.8 mmol) and Ph3PCH3I (1.94 g, 4.8 mmol) in dry toluene (30 ml) was refluxed under argon for 1.5 h. Then, the furanone 23 was added, and the refluxing was continued for 3 h. The reaction mixture was cooled, and the precipitate was filtered off. The filtrate was washed with water and dried over CaCl2. The solvent was evaporated in vacuo. The residue was chromatographed on SiO2 with cyclohexane/benzene (15:1 f 3:1) and then cyclohexane/EtOAc (40:1 f 30:1) to give the olefin 24
(0.701 g, 86%). Mp 100 –102°C (hexane-ether). 1H NMR ␦: 0.91, 0.93 (s, 3H, 18-Me), 1.20 (d, 1H, J ⫽ 6 Hz, 23S-Me), 1.26 (d, 1H, J ⫽ 6 Hz, 23R-Me), 3.77 (s, 3H, OMe), 4.00 (m, 1H, C23R-H), 4.15 (m, 1H, C23S-H), 4.75 (m, 1H, C21H), 4.81 (m, 1H, C21-H), 5.01 (m, 2H, C21RS-H), 6.61 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.18 (d, 1H, J ⫽ 8.5 Hz, C1-H). After careful column chromatography only the (5⬘R)isomer of 24 could be isolated in pure state for identification purposes. IR (cm⫺1): 2935, 2880, 1620, 1505, 1460, 1385, 1275, 1105, 1050, 1025. 1H NMR ␦: 0.91 (s, 3H, 18-Me), 1.26 (d, 1H, J ⫽ 6 Hz, 23-Me), 3.76 (s, 3H, OMe), 4.00 (m, 1H, C23-H), 4.76 (m, 1H, C21-H), 5.01 (m, 1H, C21-H), 6.61 (d, 1H, J ⫽ 2 Hz, C4-H), 6.70 (dd, 1H, J ⫽ 8.5, 2 Hz, C2-H), 7.18 (d, 1H, J ⫽ 2 Hz, C1-H). 13C NMR ␦: 14.5, 22.0, 26.4, 26.7, 27.4, 29.8, 32.5, 36.4, 39.3, 43.5, 43.7, 46.4, 47.9, 55.2, 72.2, 94.8, 106.9, 111.4, 113.8, 126.3, 132.8, 138.0, 156.3, 157.4. 2.16. Birch reduction of furan (24) Liquid ammonia (47 ml) was added to a solution of furan 24 (0.6 g, 1.7 mmol) in dry THF (9 ml) and iPrOH (7 ml) at ⫺50°C. Then, sodium (0.6 g, 26 mmol) was added portionwise. The reaction mixture was stirred at ⫺45°C for 1 h, and then, an excess of NH4Cl was added. The mixture was allowed to warm for evaporation of the liquid ammonia. After warming to room temperature, the reaction mixture was diluted with saturated NH4Cl solution and extracted with ether. The extract was sequentially washed with water, 2.5% HCl, and saturated NaHCO3, and then dried over MgSO4. The solvent was evaporated in vacuo to give 0.55 g of an oil. This oil was dissolved in ethanol (50 ml), and 5% HCl (10 ml) was added. The mixture was stirred at 65°C for 10 min. After cooling to room temperature, a 5% KOH solution in ethanol (50 ml) was added, and the mixture was stirred for 3 min. Then, it was neutralized with 2.5% HCl solution and partially evaporated. The residue was extracted with EtOAc. The extract was dried over MgSO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane/EtOAc (40:1 f 12:1), and compounds 27, 28, and 29 were eluted in this sequence: a) (17R,23R)-17,23Epoxy-19-norchola-4,20(21)-dien-3-one 27 (72 mg, 12%). Mp 113–115°C (hexane-ether). IR (cm⫺1): 2940, 2880, 1695, 1635, 1460, 1385, 1275, 1210, 1105, 1085, 1040. 1H NMR ␦: 0.93 (s, 3H, 18-Me), 1.25 (d, 3H, J ⫽ 6 Hz, 23-Me), 3.98 (m, 1H, C23-H), 4.71 (d, 1H, J ⫽ 2.5 Hz, C21-H), 5.02 (d, 1H, J ⫽ 2 Hz, C21-H), 5.81 (s, 1H, C4-H). 13 C NMR ␦: (15.1, 22.2, 23.8, 26.8, 27.2, 31.5, 32.7, 36.2, 36.9, 37.2, 41.6, 43.2, 44.1, 46.8, 48.3, 50.0, 73.0, 95.3, 107.6, 125.1, 156.8, 167.5, 200.6. HRMS Calc. for C23H32O2: 340.2402; Found: 340.2396; b) (17R,23S)17,23-Epoxy-19-norchola-4,20-dien-3-one 28 (123 mg, 21%). Mp 119 –121°C (hexane-ether). IR (cm⫺1): 2940, 2875, 1690, 1630, 1460, 1390, 1275, 1210, 1110, 1070, 1035. 1H NMR ␦: 0.93 (s, 3H, 18-Me), 1.19 (d, 3H, J ⫽ 6
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Hz, 23-Me), 4.12 (m, 1H, C23-H), 4.78 (s, 1H, C21-H), 5.03 (s, 1H, C21-H), 5.83 (s, 1H, C4-H). 13C NMR ␦: 13.9, 22.0, 23.4, 26.4, 26.5, 29.7, 30.8, 35.5, 35.6, 36.5, 40.9, 41.1, 41.3, 46.6, 48.5, 49.2, 71.2, 95.0, 106.6, 124.5, 154.9, 166.7, 199.9. HRMS Calc. for C23H32O2: 340.2402; Found: 340.2397; c) (17R,23S)-17,23-Epoxy-19-norchol-4-en-3one 29 (232 mg, 39%). Mp 152–154°C (hexane-ether). IR (cm⫺1): 2960, 2885, 1685, 1630, 1460, 1385, 1275, 1210, 1130, 1085, 1015. 1H NMR ␦ 0.92, 0.95 (s, 3H, 18-Me), 4.10 (m, 1H, C23-H), 5.83 (s, 1H, C4-H). 13C NMR ␦: 15.0, 15.1, 16.3, 17.0, 22.9, 24.0, 24.4, 24.7, 27.1, 27.2, 31.6, 33.4, 34.7, 35.7, 36.2, 37.2, 37.5, 41.3, 42.0, 42.1, 43.2, 46.5, 47.6, 47.9, 49.0, 49.5, 49.96, 50.03, 50.31, 72.0, 73.4, 94.3, 95.0, 125.1, 167.45, 167.51, 200.6. HRMS Calc. for C23H34O2: 342.2559; Found: 342.2558. 2.17. 17␣-(3⬘-Methylisoxazol-5⬘-yl)-estra-1,3,5(10)-trien3,17-diol (31) Pyridine (0.05 ml) and acetaldoxime (0.24 ml, 16.9 mmol) were added to a suspension of N-chlorosuccinimide (2.25 g, 16.9 mmol) in chloroform (50 ml) at ⫺5– 0°C. The mixture was stirred for 10 min, and then, a solution of acetylenic alcohol 30 (0.5 g, 1.69 mmol) in chloroform (25 ml) was added. The cooling bath was removed, and a solution of triethylamine (2.3 ml, 16.9 mmol) in chloroform (5 ml) was added dropwise over 5 h. The reaction mixture was kept at 30°C for 24 h, and then, the solvent was removed in vacuo, and the residue was chromatographed on SiO2 with cyclohexane/ether (1:2) to give the isoxazole 31 (0.566 g, 95%). Mp 252–254°C (cyclohexane-EtOAc). IR (cm⫺1): 3540, 3245, 2930, 1620, 1585, 1500, 1360, 1290, 1250, 1140, 920, 820. 1H NMR ␦: 1.02 (s, 3H, 18-Me), 2.32 (s, 3H, 3⬘-Me), 5.00 (br.s, 1H, OH), 6.03 (s, 1H, C4⬘-H), 6.58 (m, 2H, Ph), 7.06 (d, 1H, J ⫽ 8 Hz, Ph). HRMS Calc. for C22H27NO3: 353.1991; Found: 353.1993. 2.18. (17S)-3-Hydroxy-5⬘-methylspiro[estra-1,3,5(10)triene-17,2⬘(2⬘,3⬘-dihydrofuran)-3⬘-one] (33) A mixture of the isoxazole 31 (0.1 g, 0.28 mmol) and Mo(CO)6 (0.08 g, 0.3 mmol) in acetonitrile (10 ml) and water (0.05 ml) was refluxed for 25 min. Then, 4 drops of HCl were added, and the refluxing was continued for 50 min. After cooling to room temperature the reaction mixture was diluted with saturated NaHCO3 and extracted with CHCl3. The extract was washed with water and dried over Na2SO4. The solvent was evaporated, and the residue was chromatographed on SiO2 with cyclohexane/EtOAc (2:1) to afford the furanone 33 (0.042 g, 44%). Mp 275–277°C (CHCl3-EtOAc). IR (cm⫺1): 3395, 2990, 2940, 2880, 1680, 1600, 1515, 1450, 1400, 1360, 1295, 1250, 1235, 1165, 980, 880. 1H NMR ␦: 1.00 (s, 3H, 18-Me), 2.24 (s, 3H, 5⬘-Me), 2.77 (t, 2H, J ⫽ 4.5 Hz), 5.35 (s, 1H, C4⬘-H), 6.17 (br. s, 1H, OH), 6.63 (m, 2H, Ph), 7.03 (d, 1H, J ⫽ 8 Hz, Ph). HRMS Calc. for C22H26O3: 338.1882; Found: 338.1880.
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2.19. Reduction of the furanone (33) with LiAlH4 LiAlH4 (0.2 g, 5.3 mmol) was added portionwise to a solution of the furanone 33 (0.4 g, 1.18 mmol) in THF (30 ml). The reaction mixture was stirred for 2 h at room temperature. Excess LiAlH4 was destroyed by addition of 15% NaOH solution. The resulting precipitate was filtered off and washed with CHCl3. The organic layer was dried over Na2SO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane/EtOAc (30:1 f 5:1) to give: a) (5⬘R,17S)-3-hydroxy-5⬘-methylspiro[estra1,3,5(10)-triene-17,2⬘-tetrahydrofuran-3⬘-one] 34 (88 mg, 22%). Mp 196 –198°C (toluene-cyclohexane). 1H NMR ␦: 0.94 (s, 3H, 18-Me), 1.40 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 2.57 (dd, 1H, J ⫽ 17, 5.5 Hz, C4⬘-H), 2.78 (t, 2H, J ⫽ 4.5 Hz), 4.19 (m, 1H, C5⬘-H), 5.30 (br.s, 1H, OH), 6.60 (m, 2H, Ph), 7.10 (d, 1H, J ⫽ 8.5 Hz, Ph); b) (5⬘S,17S)-3-hydroxy-5⬘methylspiro[estra-1,3,5(10)-triene-17,2⬘-tetrahydrofuran3⬘-one] 35 (92 mg, 23%). Mp 199 –201°C (toluenecyclohexane). 1H NMR ␦ 0.98 (s, 3H, 18-Me), 1.37 (d, 3H, J ⫽ 6.5 Hz, 5⬘-Me), 2.16 (dd, 1H, J ⫽ 18.5, 9 Hz, C4⬘-H), 2.61 (dd, 1H, J ⫽ 18.5, 6.5 Hz, C4⬘-H), 2.79 (t, 2H, J ⫽ 4.5 Hz), 4.38 (m, 1H, C5⬘-H), 5.01 (br.s, 1H, OH), 6.60 (m, 2H, Ph), 7.08 (d, 1H, J ⫽ 8 Hz, Ph). HRMS Calc. for C22H28O3: 340.2038; Found: 340.2042. 2.20. Reduction of the furanone (33) with lithium in liquid ammonia Furanone 33 (200 mg) in THF (5 ml) was added to a solution of lithium (50 mg) in liquid ammonia (30 ml) at ⫺72°C. The reaction mixture was stirred at ⫺72 to ⫺65°C for 15 min, and then excess lithium was destroyed by addition of NH4Cl. The reaction mixture was set aside for evaporation of the ammonia. After warming to room temperature the residue was diluted with saturated NaCl solution and extracted with CHCl3. The extract was dried over Na2SO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane/EtOAc (30:1 f 5:1) to give: a) furanone 34 (38 mg, 19%) and b) furanone 35 (73 mg, 36%). 2.21. (17R,23R)-17,23-Epoxy-3-hydroxy-19-norchola1,3,5(10),20(21)-tetraene (36) A suspension of tBuOK (0.1 g, 0.9 mmol) and Ph3PCH3I (0.37 g, 0.9 mmol) in dry toluene (6 ml) was refluxed under argon for 50 min. A solution of the furanone 34 (52 mg) in dry toluene (3 ml) was added, and the mixture was refluxed under argon for 3 h. Then, it was allowed to cool to room temperature, and the precipitate was filtered off. The filtrate was diluted with saturated NaCl solution and extracted with CHCl3. The extract was dried over Na2SO4, evaporated, and the residue was chromatographed on SiO2 with cyclohexane/EtOAc (10:1 f 5:1) to give the methylenefuran 36 (45
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mg, 87%). Mp 161–163°C (toluene-cyclohexane). IR (cm⫺1): 3425, 2980, 2935, 2880, 1625, 1515, 1455, 1390, 1255, 1020. 1H NMR ␦: 0.93 (s, 3H, 18-Me), 1.29 (d, 3H, 23-Me), 2.51 (dd, 1H, J ⫽ 14, 5 Hz, C22-H), 4.01 (m, 1H, C23-H), 4.67 (br.s, 1H, OH), 4.78 (d, 1H, J ⫽ 2.5 Hz, C21⬘-H), 5.07 (d, 1H, J ⫽ 2.5 Hz, C21-H), 6.60 (m, 2H, Ph), 7.13 (d, 1H, J ⫽ 8.5 Hz, Ph). 13C NMR ␦: 15.2, 22.3, 23.8, 27.1, 28.0, 30.4, 33.1, 37.1, 40.0, 44.2, 44.4, 47.1, 48.6, 72.9, 96.0, 107.6, 113.3, 115.9, 127.2, 133.6, 139.1, 153.9, 156.9. HRMS Calc. for C23H30O2: 338.2246; Found: 338.2245.
2.22. (17R,23S)-17,23-Epoxy-3-hydroxy-19-norchola1,3,5(10),20(21)-tetraene (37) The title compound was prepared from the furanone 35, in a similar manner as described above, in 86% yield. Mp 173–176°C (cyclohexane-EtOAc). IR (cm⫺1): 3400, 2935, 1625, 1515, 1460, 1295, 1260, 1110, 1070, 1025. 1H NMR ␦: 0. 94 (s, 3H, 18-Me), 1.20 (d, 3H, J ⫽ 6.5 Hz, 23-Me), 4.17 (m, 1H, C23-H), 4.84 (s, 1H, C21-H), 5.04 (s, 1H, C21-H), 6.60 (m, 2H, Ph), 7.11 (d, 1H, J ⫽ 8.5 Hz, Ph). 13C NMR ␦: 14.6, 22.7, 23.9, 27.3, 27.9, 30.4, 35.0, 36.3, 39.9, 42.0, 44.2, 47.6, 49.2, 71.9, 96.0, 107.3, 113.3, 115.9, 127.1, 133.3, 138.9, 154.1, 155.5. HRMS Calc. for C23H30O2: 338.2246; Found: 338.2245.
2.23. (5⬘R,17S)-5⬘-Methylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (39a) A mixture of the furanone 22 (0.29 g, 0.86 mmol), ethanedithiol (0.162 g, 1.7 mmol), and BF3.OEt3 (0.122 g, 0.86 mmol) in dry chloroform (15 ml) was stirred at room temperature for 10 h. Then, the mixture was washed with saturated NaHCO3 solution, and the organic layer was dried over Na2SO4 and evaporated. The residue was purified on SiO2 with cyclohexane/EtOAc (120:1 f 90:1) to give the ethylenethioketal 38a (0.252 g, 71%). Part of the obtained thioketal 38a (96 mg, 0.2 mmol) was dissolved in ethanol (25 ml), and Raney nickel was added. The mixture was stirred at room temperature for 4 h, and then, 0.03 g of Raney nickel was added again and stirring was continued for 6 h. The catalyst was filtered off and carefully washed with ethanol. The filtrate was evaporated, and the residue was chromatographed on SiO2 with cyclohexane/EtOAc (120:1 f 100:1) to afford the ⌬4-olefin 39a (60 mg, 78%) as an oil. IR (cm⫺1): 2935, 2875, 1760, 1465, 1390, 1155, 1095. 1H NMR ␦: 0.94 (s, 3H, 18-Me), 1.32 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.15 (m, 1H, C5⬘-H), 5.37 (s, 1H, C4-H). 13C NMR ␦: 14.0, 21.7, 22.1, 23.8, 25.4, 25.7, 28.8, 31.58, 31.64, 33.0, 35.5, 41.1, 41.9, 46.2, 47.1, 47.7, 49.7,
69.2, 93.3, 119.9, 140.2, 219.0. HRMS Calc. for C22H32O2: 328.2402; Found: 328.2405. 2.24. (5⬘S,17S)-5⬘-Methylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (39b) The title compound was prepared as an oil from the furanone 21, according to the procedure for 39a, in 52% overall yield. IR (cm⫺1): 2935, 2865, 1760, 1465, 1390, 1165, 1100, 1065. 1H NMR ␦: 0.95 (s, 3H, 18-Me), 1.35 (d, 3H, J ⫽ 6 Hz, 5⬘-Me), 4.34 (m, 1H, C5⬘-H), 5.36 (s, 1H, C4-H). 13C NMR ␦: 14.3, 22.5, 22.7, 24.5, 26.1, 26.8, 29.4, 32.2, 33.3, 35.1, 36.1, 41.8, 42.5, 45.3, 48.3, 49.5, 50.1, 70.6, 94.7, 120.7, 140.7, 219.7. HRMS Calc. for C22H32O2: 328.2402; Found: 328.2400. 2.25. (17␣)-(3⬘-Ethylisoxazol-5⬘-yl)-estr-4-en-3,17-diol (41) The title compound was obtained by the previously described procedure from compound 40 and propionitrile oxide in 83% yield, mp 155–157°C (ethyl acetate). 1H NMR ␦: 1.06 (s, 3H, 18-Me), 1.28 (t, 3H, J ⫽ 7.5 Hz, CH3CH2), 2.71 (q, 2H, J ⫽ 7.5 Hz, CH3CH2), 4.12 (m, 1H, C3-H), 5.40 (br s, 1H, C4-H), 6.00 (s, 1H, C4⬘-H). 2.26. (17S)-5⬘-Ethyl-3-hydroxyspiro[estr-4-ene-17,2⬘(2⬘,3⬘-dihydrofuran-3⬘-one] (42) The title compound was obtained from isoxazole 41 by the previously described procedure for 17 in 80% yield, mp 173–176°C (ethyl acetate). 1H NMR ␦: 1.00 (s, 3H, 18-Me), 1.24 (t, 3H, J ⫽ 7.5 Hz, CH3CH2), 2.48 (q, 2H, J ⫽ 7.5 Hz, CH3CH2), 4.16 (m, 1H, C3-H), 5.27 (s, 1H, C4⬘-H), 5.38 (br s, 1H, C4-H). 2.27. Reduction– oxidation of (42) Reduction of 450 mg of 42 with lithium in liquid ammonia followed by oxidation of the resulting mixture of alcohols gave 188 mg (42%) of (5⬘R,17S)-5⬘-ethylspiro[estr-4-ene-3-one-17,2⬘-tetrahydrofuran-3⬘-one] 44, mp 118 –120°C (EtOAc-hexane). 1H NMR ␦: 1.00 (s, 3H, 18 Me), 3.99 (m, 1H, C5⬘-H), 5.83 (s, 1H, C4-H); 13C NMR, ␦: 9.2, 14.0, 23.7, 25.9, 26.6, 28.9, 30.6, 31.4, 33.1, 35.5, 36.5, 40.5, 42.5, 43.9, 46.9, 47.5, 48.9, 74.2, 92.6, 124.6, 166.6, 199.6, 216.7; IR (cm⫺1): 1720, 1680, 1615, and 72 mg (16%) of (5⬘S,17S)-5⬘-ethylspiro[estr-4-ene-3-one-17,2⬘tetrahydrofuran-3⬘-one] 43, mp 113–115°C (ether-hexane). 1 H NMR ␦: 1.02 (s, 3H, 18-Me). 4.12 (m, 1H, C5⬘-H). 5.83 (s, 1H, C4-H). 13C NMR ␦: 9.5, 13.6, 23.8, 26.2, 26.6, 29.1, 30.6, 32.6, 34.1, 35.4, 36.5, 40.6, 42.5, 42.8, 47.4, 48.6, 48.7, 75.3, 93.4, 124.6, 166. 3, 199.7, 216.9; IR (cm⫺1): 1720, 1680, 1610.
V.A. Khripach et al. / Steroids 66 (2001) 569 –579
2.28. (17R,23R)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-dien-3-one (47b) A solution of 44 (0.22 g, 0.62 mmol), triethyl orthoformate (0.22 ml, 1.32 mmol), and p-TsOH (0.022 g, 0.115 mmol) in dry dioxane (10 ml) was stirred at room temperature for 6 h. The reaction mixture was poured into a 10% solution of pyridine in water (10 ml) at 0°C and left overnight in a refrigerator. The organic layer was dissolved in CHCl3, washed with water, dried over Na2SO4, and the solvent was evaporated in vacuo. The crude product 45b was obtained as an oil (0.22 g, 90%). 1H NMR ␦: 0.94 (s, 3H, 18-Me), 4.00 (m, 1H, C23-H), 5.20 (s, 1H, C4-H), 5.28 (d, 1H, J ⫽ 5 Hz, C6-H). The obtained product 45b was used directly in the next stage without further purification. A mixture of tBuOK (0.195 g, 1.72 mmol) and Ph3PCH3I (0.695 g, 1.72 mmol) in dry toluene (20 ml) was refluxed under argon for 1 h. Then, compound 45b from the previous stage in dry toluene (10 ml) was added. The mixture was stirred under reflux for 5 h. The precipitate was filtered off and washed with toluene and ether. The organic layer was washed with water and evaporated. The residue was treated with 60% AcOH (2 ml) for 20 min. Then, the reaction mixture was poured into a saturated solution of NaHCO3 and extracted with CHCl3. The extract was dried over Na2SO4 and evaporated. The residue was chromatographed on SiO2 with cyclohexane-THF (49:1) to give the enone 47b (0.13 g, 59%). Mp 110 –114°C (hexane-ether). IR (cm⫺1): 1710, 1680, 1620, 1460, 1440, 1380, 1260, 1210, 1090, 1030, 890. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.82 (m, 1H, C23-H), 4.72 (d, 1H, J ⫽ 3 Hz, C21-H), 5.04 (d, 1H, J ⫽ 3Hz, C21-H), 5.82 (s, 1H, C4-H). 13C NMR ␦: 10.5, 15.0, 23.7, 26.6, 27.2, 29.7, 31.4, 32.3, 36.2, 36.9, 37.1, 41.6, 41.6, 43.2, 46.8, 48.3, 50.0, 77.1, 94.9, 107.6, 125.1, 156.5, 167.6, 200.6. HRMS Calc. for C24H34O2: 354.2559; Found: 354.2560.
575
mmol) in dry chloroform (20 ml) was stirred at room temperature for 7 h. Then, the mixture was washed with a saturated solution of NaHCO3 and dried over Na2SO4. The solvent was removed in vacuo. The residue was dissolved in ethanol (80 ml), and Raney nickel was added portionwise (3 ⫻ 0.5 g). The reaction mixture was refluxed for 10 h. Then, the catalyst was filtered off, and the filtrate was evaporated. The residue was chromatographed on SiO2 with hexane/EtOAc (99:1) to give the olefin 49b (0.265 g, 55%). Mp 62– 65°C (EtOH). IR (cm⫺1): 1760, 1670, 1460, 1450, 1390, 1340, 1240, 1080, 1050. 1H NMR ␦: 0.92 (s, 3H, 18-Me), 3.98 (m, 1H, C5⬘-H), 5.38 (br.s, 1H, C4-H). 13C NMR ␦: 9.9, 14.2, 22.8, 24.5, 26.1, 26.5, 29.4, 29.7, 32.3, 32.4, 33.0, 36.2, 41.8, 42.6, 44.7, 47.9, 48.4, 50.4, 74.8, 93.6, 120.6, 140.9, 219.6. HRMS Calc. for C23H34O2: 342.2559; Found: 342.2558. 2.31. (5⬘S,17S)-5⬘-Ethylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (49a) The title compound was obtained as an oil from the enone 43, according to the same procedure described above, in 50% yield. IR (cm⫺1): 1750, 1680, 1460, 1440, 1390, 1350, 1240, 1070, 1040. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 4.14 (m, 1H, C5⬘-H), 5.38 (br.s, 1H, C4-H). 13C NMR ␦: 10.2, 14.3, 22.7, 24.5, 26.1, 26.8, 29.4, 29.7, 32.2, 33.2, 35.1, 36.1, 41.5, 42.5, 43.4, 48.4, 49.6, 50.1, 75.6, 94.4, 120.7, 140.7, 219.7. HRMS Calc. for C23H34O2: 342.2559; Found: 342.2558. 2.32. (17R,23R)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-diene (50b)
The title compound was prepared from ketone 43, according to the procedure described above, in 48% overall yield. Mp 80 – 83°C (hexane-ether). IR (cm⫺1): 1720, 1670, 1620, 1460, 1440, 1380, 1260, 1210, 1090, 1030, 890. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.90 (m, 1H, C23-H), 4.76 (t, 1H, J ⫽ 3 Hz, C21-H), 5.02 (t, 1H, J ⫽ 3 Hz, C21-H), 5.82 (s, 1H, C4-H). 13C NMR ␦: 10.5, 14.5, 23.9, 27.1, 27.2, 29.7, 31.4, 34.7, 36.0, 36.2, 37.1, 39.5, 41.5, 43.2, 47.2, 49.0, 49.8, 77.2, 95.4, 107.0, 125.1, 155.4, 167.4, 200.5. HRMS Calc. for C24H34O2: 354.2559; Found: 354.2561.
A mixture of tBuOK (0.119 g, 1.053 mmol) and PhPCH3I (0.425 g, 1.053 mmol) in dry toluene (10 ml) was refluxed under argon for 1 h. Then, ketone 49b (0.13 g, 0.353 mmol) in dry toluene (10 ml) was added. The mixture was refluxed for 5 h under argon until the starting material disappeared. The precipitate was filtered off and washed with toluene and ether. The combined organic layer was washed with water and dried over Na2SO4. The solvent was removed in vacuo, and the residue was chromatographed on SiO2 with hexane/EtOAc (200:1) to afford the olefin 50b (0.056 g, 43%). IR (cm⫺1): 1670, 1460, 1440, 1380, 1070, 1030, 900. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.80 (m, 1H, C23-H), 4.70 (m, 1H, C21-H), 5.02 (m, 1H, C21-H), 5.38 (br. s, 1H, C4-H). 13C NMR ␦: 10.5, 15.1, 22.3, 24.0, 26.2, 26.6, 29.4, 29.8, 32.4, 33.1, 36.2, 37.0, 41.6, 42.05, 42.14, 42.6, 46.9, 48.6, 50.8, 77.0, 95.2, 107.3, 141.2, 156.1. HRMS Calc. for C24H36O: 340.2766; Found: 340.2760.
2.30. (5⬘R,17S)-5⬘-Ethylspiro[estr-4-ene-17,2⬘tetrahydrofuran-3⬘-one] (49b)
2.33. (17R,23S)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-diene (50a)
A mixture of the enone 44 (0.5 g, 1.4 mmol), ethanedithiol (0.215 ml, 2.8 mmol), and BF3.OEt2 (0.345 ml, 2.8
The title compound was prepared as an oil from the ketone 49a in 74% yield as described above. IR (cm⫺1):
2.29. (17R,23S)-17,23-Epoxy-24a-homo-19-norchola4,20(21)-dien-3-one (47a)
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Scheme 3.
1670, 1460, 1440, 1380, 1070, 1030, 890. 1H NMR ␦: 0.96 (s, 3H, 18-Me), 3.88 (m, 1H, C23-H), 4.76 (br.s, 1H, C21-H), 5.00 (br.s, 1H, C21-H), 5.38 (br.s, 1H, C4-H). 13C NMR ␦: 9.8, 13.9, 22.0, 23.3, 25.5, 26.3, 28.7, 29.0, 31.8, 34.3, 35.4, 35.5, 39.2, 41.5, 41.9, 46.8, 48.6, 50.0, 76.5, 95.0, 106.1, 119.8, 140.5, 155.0. HRMS Calc. for C24H36O: 340.2766; Found: 340.2762.
3. Results and discussion Initially, an approach for derivatives with a iPr-group at C-5⬘ was investigated using the addition of isobutyronitrile oxide because isobutyronitrile oxide is more stable than its lower homologs. Cycloaddition of isobutyronitrile oxide with mestranol 2 proceeded in quantitative yield with formation of the hydroxy isoxazole 10 as the only regioisomer (Scheme 3). Hydrogenolysis of the hydroxy isoxazole 10 could be easily affected under different conditions, but generally, the formation of furanone 12, was observed along with the expected enaminoketone 11. This product could be isolated in reasonable yield after column chromatography, but it proved to be more practical to convert the hydroxy isoxazole 10 directly into the furanone 12 via hydrogenation in acidic medium. Several variants of the transformation of enone 12 into ketone 15 were studied. Reduction of 12 with NaBH4 gave an unseparable mixture of alcohols 13 and 14, which upon Jones or Swern oxidation could be transformed into ketone 15. Treatment of enone 12 with lithium in liquid ammonia afforded ketone 15 directly. It is interesting that the reaction of enone 12, even with a large excess of LiAlH4, led mainly to ketone 15, probably because of the formation of a stable complex after the reduction of the double bond.
Scheme 4.
Synthesis of compounds with a small alkyl substituent (Me or Et) at C-5⬘ was performed next. The 1,3-dipolar cycloaddition of mestranol 2 to acetonitrile oxide gave a quantitative yield of hydroxy isoxazole 16 (Scheme 4). Its hydrogenolysis, followed by acid hydrolysis, gave furanone 17. Reduction of this heterocyclic product afforded a mixture of the alcohols 18, which were isomeric at C-3⬘ and C-5⬘. At this stage, the ⌬4-3-ketone could be formed, and Birch reduction of 18 followed by hydrolysis of the intermediate enol ether 19 gave enone 20. Oxidation of the secondary alcohol group produced ketones 21 and 22, which could be separated by column chromatography. Stereochemical assignment of the methyl group at C-5⬘ could not be done by spectroscopic methods, but fortunately, ketone 22 gave crystals suitable for X-ray analysis (Fig. 1). Thus, the problem of stereochemistry could be solved, not only for these two compounds, but also for the other 5⬘-alkyl steroids of this series, by analogy of their NMR spectra (Fig. 2). Synthesis of the 3⬘-methylene derivatives with a 5⬘methyl group was achieved as shown in Scheme 5. Reduction of enone 17 with LiAlH4 proceeded with formation of the ketones 23 in contrast to the reaction with NaBH4, where further reduction of the C-3⬘-carbonyl took place. Application of a large excess of LiAlH4 also allowed for detection of the diastereomeric mixture of the corresponding C-3⬘ alcohols, but even in this case, the ketones 23 were the main products of the reaction. The high temperature
V.A. Khripach et al. / Steroids 66 (2001) 569 –579
577
Fig. 1. ORTEP view of the furanone 22.
variant of the Wittig reaction gave the methylene derivatives 24 in good yield. Some problems arose in the next step because treatment of compound 24 with sodium in liquid ammonia led not only to the Birch reduction of ring A, but also gave a considerable amount of the side-chain reduced compound 25 with a methyl group at C-3⬘. The rate of the methylene group reduction was dependent on the configuration of the methyl group at C-5⬘. This was confirmed by identification of only 5⬘S-isomers 25 and 29 in the reaction products along with the expected methylene derivatives 27 and 28. Moreover, the ratio of 27 and 28 showed a lower amount of 28 than could be expected, based on the analysis of the NMR spectrum of the mixture of the C-5⬘-methyl derivatives 24. The acetylenic alcohol 30 provides a possible starting point for the preparation of the 3⬘-methylene derivatives 36 and 37, which differ from 24 by the presence of a hydroxy group instead of a methoxy group in ring A (Scheme 6). Ring A proved to be stable, and no problems arose in the course of the synthetic transformations performed. Cycloaddition of acetylenic alcohol 30 with acetonitrile oxide proceeded in nearly quantitative yield, but the ring opening of the resulting isoxazole 31 followed by hydrolysis of 32 and ring closure gave the unsaturated furanone 33 in moderate yield. Reduction of the double bond in 33 was investigated using LiAlH4 or lithium in liquid ammonia. In both cases, a
Fig. 2. Fragments of NMR spectra of the furanones 22 (above) and 21 (below).
Scheme 5.
moderate yield of a mixture of the reduced compounds 34 and 35 was obtained in slightly different ratios. Compounds 34 and 35 could be separated by chromatography, and each could be converted in good yield to the corresponding methylene compounds 36 and 37, respectively. Another variation in the structure of ring A of these 5⬘-methylene derivatives can be obtained when ring A is reduced to the corresponding 3-deoxo steroids. Although compounds 21 and 22 contain two keto groups, selective reduction of the conjugate keto group at C-3 proved to be straightforward (Scheme 7). Treatment of enone 21 or 22 with ethanedithiol in the presence of Lewis acid led almost exclusively to the corresponding thioketals 38, which could be smoothly reduced with Raney nickel to the deoxo derivatives 39. Our goal was to investigate various synthetic approaches toward spirofurans 1. That is why we chose a method starting from norethisterone 3 for the synthesis of compounds with a C-5⬘ ethyl group (Scheme 8). It is well known that the activity of dipolarophiles in 1,3-dipolar cycloaddition reactions with unsaturated compounds greatly depends on the number of substituents [13,14]. A monosubstituted double bond is much more reactive than a trisubstituted one. On the other hand, conjugation of the double bond with a
578
V.A. Khripach et al. / Steroids 66 (2001) 569 –579
Scheme 8. Scheme 6.
carbonyl group leads to a significant increase in activity. Norethisterone 3 contains a conjugated trisubstituted ⌬4double bond and a monosubstituted triple bond, both of which can react with nitrile oxide. Our investigation showed that the triple bond was a bit more reactive than the ⌬4double bond in ring A. Although the desired cycloaddition product could be obtained directly, better results were obtained when norethisterone 3 was first reduced to the allylic alcohols 40. The cyclic part of this compound was relatively stable toward nitrile oxides and allowed for selective cycloaddition at the triple bond in good yield. Reductive cleavage of the isoxazole 41 with Raney nickel in acetic acid gave the furanone 42. Reduction of the ⌬4⬘,5⬘-double bond followed by regeneration of the enone system in ring A led to a mixture of furanones 43 and 44, which could be separated by column chromatography. Synthesis of the 3⬘-methylene derivative 47 with a (5⬘R)ethyl group was accomplished as shown in Scheme 9. Treatment of enones 43 and 44 with triethyl orthoformate in the presence of TsOH led to the dienol ethers 45. The keto
Scheme 7.
group at C-3⬘ remained intact under these reaction conditions, and it could be transformed into a methylene function by a Wittig reaction. Final acid hydrolysis of the dienol ethers 46 gave the desired enones 47. Treatment of enones 43 or 44 with ethanedithiol (Scheme 10) in the presence of a Lewis acid gave the thioketals 48. Their desulfurization with Raney nickel gave the 3-deoxo derivatives 49. The Wittig olefination of 49 led to compounds 50 with a methylene group at C-3⬘. Thus, approaches toward steroids with a side chain containing a furan moiety and a 3⬘-methylene group and a C-5⬘ alkyl group have been achieved in several ways. A number of 3⬘-methylene-5⬘-alkyl steroids with different types of ring A substituents (⌬4-3-keto-, ⌬4-, 3-methoxy-1,3,5(10)-
Scheme 9.
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[3]
[4] [5]
[6]
[7] Scheme 10. [8]
triene-, and 3-hydroxy-1,3,5(10)-trienes) have been prepared. [9]
Acknowledgments [10]
The authors are indebted to Organon International for financial support of this research. We thank Dr. N.B. Khripach for recording of NMR spectra, and Mr. H. Jongejan for the exact mass measurements.
[11]
[12]
References [1] Zeelen FJ. Medicinal chemistry of steroids. Amsterdam: Elsevier, 1990. [2] Hamersma JAM, Orlemans EOM, Rewinkel JBM, inventors. AKZO NV, The Netherlands, assignee. Preparation of 17-spiro-
[13] [14]
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methylene steroids. Eur Pat Appl EP 582,338. 1994 Feb 09 [CA 121;231157q]. Khripach VA, Zhabinskii VN, Litvinovskaya RP. Synthesis and some practical aspects of brassinosteroids. In: Cutler HG, Yokota T, Adam G, editors. Brassinosteroids. Chemistry, bioactivity, and applications. ACS Symposium series. Washington: American Chemical Society, 1991;474:43–55. Khripach VA, Zhabinskii VN, de Groot AE. Brassinosteroids—a new class of plant hormones. San Diego: Academic Press, 1999. Khripach VA, Zhabinskii VN, Pavlovskii ND, Lyakhov AS, Govorova AA. Synthesis and X-ray analysis of (20S)-22-benzamido-6methoxy-3␣,5-cyclo-26,27-bisnorcholest-22-en-24-one. Bioorg Khim 1998;24:856 – 61. Khripach VA, Litvinovskaya RP, Baranovskii AV. Synthesis of ecdysone side chains via isoxazoline derivatives. Mendeleev Commun 1992:117– 8. Khripach VA, Zhabinskii VN, Pavlovskii ND. Synthesis of steroidal 23-enamino-25-ketones—potential intermediates for preparation of vitamins D and analogs. Zh Org Khim 1998;34:59 – 63. Khripach VA, Zhabinskii VN, Pavlovskii ND. A nitrile oxide approach to the construction of the side chain of metabolites and analogs of vitamin D, functionalized at C-23. Zh Org Khim 1999;35: 390 –2. Litvinovskaya RP, Baranovskii AV, Khripach VA, Struchkov Yu, Ovchinnikov Yu. Synthesis and structure of (22R,24R)-6-acetoxy24-methyl-5␣-cholestane-3,5␣-22,24-tetrol. Mendeleev Commun 1994:89 –90. Laurent H, Schulz G. 17-Hydroxy-17␣-isoxazolyl steroids. Chem Ber 1969;102:3324 –32. Khripach VA, Zhabinskii VN, Olkhovick VK, Zavadskaya MI, Drachenova OA, Khripach NB. Stereoselective synthesis of 20-(hydrofuryl)steroids via isoxasole derivatives. Zhurn Org Khim 1996;32: 841– 4. Van der Louw J, Hamersma JAM, Groen MB. Radical cyclization to 3⬘alkylidene-17-spirodihydrofuran-substituted steroids observation of an unexpected intramolecular hydrogen transfer. 16th Conf. on Isoprenoids, 1995. p. 55. Grundman C, Gru¨nanger P. The nitrile oxides. Berlin: SpringerVerlag, 1971. Torsell KBG. Nitrile oxides, nitrones and nitronates. Weinheim: VCH Verlagsgeselschaft, 1988.