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Hydroxylation of zl5-Steroids with N-Bromosuccinimide to 5a,6/3-Diols

A. M. Dawidar*

Chemistry Dept., Faculty of Science, Mansoura University, Mansoura, Arab Republic of Egypt

A. A. Saleh and M. M. Abdel-Malek

Laboratory of Natural Products, National Research Centre, Dokki, Cairo, A. R. Egypt

Z. Naturforsch. 35b, 102-106 (1980); received September 17, 1979

N-Bromosuccinimide, Zl5-Steroids, Diosgenin, Cholesterol, Stigmastane, Dehydroisoandrosterone The reaction of N-bromosuccinimide with Zl5-steroids such as, diosgenin, cholesterol,

stigmasterol and dehydroisoandrosterone were studied. It was found that NBS in a solvent mixture of acetone, water and acetic acid (8:1:0.1) oxidises the olefinic bond of all the previous steroidal compounds to the corresponding «ran-sglycollic, 5a,6/?-di-hydroxylic products in about 60% yield at room temperature.

Since the Corpus luteum hormone, progesterone was first isolated during the 1930s, considerable interest has been attached to the synthesis of steroidal hormone analogs. Amongst the more interesting variations are those having the 6-methyl group and 6,7-unsaturation linkage for their high progestational activity [1].

Introduction of the 6-methyl substituent at the 6-position was affected by epoxidation of one of the early intermediates from diosgenin route, pregneno-lone with alkaline hydrogen peroxide [2]. Petrow and his collaborators [3] have described several schemes that go back to diosgenin as the starting material. They reported a method for converting diosgenin into 3/?-acetoxy-6-methyl-25D-spirost-5-ene via (i) the 5a,6a-epoxide (ii) the 6/?-hydroxy-3,5-cyclo-25D-spirost-5-ene. Petrow also reported [4] the transformation of diosgenin to 6-methyl-25D-spirosta-3,6-diene-3-one employing the 3ß-acetoxy-5a-hydroxy-25D-spirostan-6-one as start-ing material. He obtained the 6-keto by first epoxidation of diosgenin followed by treatment of the epoxide by periodic acid in aqueous acetone and then oxidation of the triol.

In the present work we were seaking a more convenient method for preparation of the 5 a-

* Reprint requests to Dr. A. M. Dawidar. 0340-5087/80/0100-0102/$ 01.00/0

hydroxy-6-ketone derivative of diosgenin (1 c) which is important as a starting material for synthesis of the 6-methyl-6,7-dehydro progesterone [4]. Fieser et al. reported [5] that oxidation of cholesterol with N-bromosuccinimide and acetic acid in aqueous acetone afforded 5a-cholestane-3/?,5-diol-6-one (2 c) in a moderate yield. We thought to extend this reaction to diosgenin hoping thereby to develop a convenient route for the preparation of the desired 5a-hydroxy-6-keto derivative. When we used NBS for the oxidation of diosgenin under the same conditions reported by Fieser [5], we obtained a triol (la), C27H44O5, m.p. 281-283 °C. Its structure was established on the bases of spectral data (IR and MS) and by chemical conversion to 5a-25D-spirostane-3 /?,5-diol-6-one (1 c) with NBS in aqueous dioxane which is known to be specific for oxidation of 6 ̂ -hydroxy steroids [6]. Another proof for the triol structure (la) was arrived at by its oxidation with Jone's reagent [7] to the diketo derivative, 5a,25D-spirostane-5-ol-3,6-dione ( le) . This ac-counts for the presence of the two secondary hydroxyl groups at C-3 and C-6 positions. The tertiary hydroxyl group attached to C-5 was elimi-nated in a dehydration process when the diketonic derivative ( le ) was shaken in methanol containing 15% aqueous potassium hydroxide for 4 h at room temperature and 25D-spirost-4-ene-3,6-dione (I f ) was obtained as shown from its UV spectrum. The previous transformation reactions are illustrated in the following Scheme:

A. M. Dawidar et al. • Hydroxylation of Zl5-Steroids with N-Bromosuccinimide to 5a,6/?-Diols 103

N BS aq. dioxane

KOH Me OH

Schema 1

NBS/Me2C0 H20,Ac0H hq

Cr03/aq.H?S0^ Me2C0

From the foregoing data, it is clear that diosgenin when treated with NBS in a solvent mixture of acetone, water and acetic acid (8:1:0.1), the 5:6-double bond is oxidised to a trans glycollic system, 6ct,6/?-diol.

The generality of the above mentioned method as a new way for trans hydroxylation of zl5 bond in steroids was demonstrated by its application for the preparation of 5a-cholestane-3/?,5,6/?-triol (2a), 5a-stigmastane-3ß,5,6ß-triol (3a) and 5a-andro-stane-3/?,5,6/?-triol-17-one (4a) from cholesterol, stigmasterol and dehydroisoandrosterone, respec-tively. The structures of all the formed triols were proved by their transformation to the corresponding 4-en-3,6-dione products.

As a result of this work, two new compounds were prepared and reported here for the first time which are 5a.25D-spirostane-5-ol-3,6-dione (le) and 5a-stigmastane-3/3,5-diol-6-one (3c).

Similar results were reported by Ueno [8] and Morita [9] who independently found that oxidation of cholesterol by N,N-dibromobenzene sulfonamide and isocyanor bromide gave 5a-cholestane-3 ß,5,6ß-triol.

Experimental Melting points were determined using a Büch 510

apparatus. IR spectra were recorded using Beck-mann spectrophotometer 4220. Mass spectral data were obtained with mass spectrophotometer MAT 112. Ultraviolet spectra were recorded on CE 595 double beam digital ultraviolet spectro-photometer.

5a,25D-Spirostane-3ß,5,6ß-triol (1 a) A suspension of diosgenin (1), (5 g, 0.012 mole) in

acetone (200 ml) and water (25 ml) was treated with NBS (2.68 g, 0.15 mole, 1.25equiv) and acetic acid (2.5 ml) and was shaken occasionally at room temp. In the course of 45 min the mixture

104 A. M. Dawidar et al. • Hydroxylation of Zl5-Steroids with N-Bromosuccinimide to 5a,6/?-Diols 104

became yellow then orange and finally colorless where all the solid material disappeared in the solution. The reaction mixture was left overnight, diluted with water, extracted with ether and processed as usual to give an amorphous solid (5.6 g, 65% yield). Crystallisation from MeOH gave product l a as shiny colorless plates, m.p. 282 to 284 °C. Reported [10] m.p. 281-283 °C. IR cm-1 : 3400 (broad, OH), 970, 920, 890 and 865 (spirostane side-chain). MS: m/e(%) : 448(20) (M+, C27H44O5), 430 (5) (M+-H2O), 412 (4) (M+-2H2O), 394 (5) (M+-3H20) and 139 (100) (due to ring F ions [11]).

5a,25D-Spirostane-3ß,5,6ß-triol-3,6-diacetate (lb) The triol (la), (200 mg) was dissolved in dry

pyridine (1 ml) and treated with AC2O (1 ml). The reaction mixture was left on a water-bath for 60 min and processed to give the diacetate material ( lb) as colorless needles (from acetone, 180 mg), m.p. 189-191 °C. Reported [10] m.p. 192-194 °C. I ß v ® r c m - i : 3410 (broad, OH), 1730, 1240 (acetate bands), 970, 920, 890 and 865 (spirostane side-chain). MS: mje ( % ) : 532 (15) (M+, C31H48O7), 514 (5) (M+-H2O), 472 (20) (M+-AcOH), 412 (40) (M+-2AcOH). Analysis: C31H48O7

Calcd C 69.92 H 9.02, Found C 69.73 H 9.56.

5 a-25 D-Spirostane-3ß,5-diol-6-one (1 c) A solution of the triol ( la), (0.5 g, 0.0011 mole)

in a mixture of pure dioxane (45 ml) and water (5 ml) was treated with NBS (0.42 g, 0.0023 mole, 2 equiv) under stirring. After 15 min the color changed to yellow and then to deep orange. A solution of sodium metabisulfite (10%) was added to the reaction mixture after 2 h. Water was added followed by extraction with CHCI3. Processing as usual afforded the diolone product ( lc) , (0.46 g, 91% yield), m.p. 270-273 °C (from CHCI3). I R r ^ cm-1 : 3400 (broad, OH), 1700 ( > C = 0 ) , 970, 920, 890 and 865 (spirostane side-chain). MS: m / e ( % ) : 446 (82) (M+, C27H4205), 428 (100) (M+-H2O) and 410 (70) (M+-2H20).

5a,25D-Spirostane-3ß,5-diol-6-one-3-acetate (1 d) Compound 1 c was acetylated with pyridine/Ac20

to yield the monoacetate derivative (Id) as colorless needles (from CHCl3/MeOH), m.p. 270-272 °C. Reported [12] m.p. 273-275 °C. IR cm"1: 3430 (broad, OH), 1730 ( - « 0 - ) and 1705) six membered ring > C = 0 ) . MS: m % ) : 488 (42) (M+, C29H44O6), 470 (10) (M+-H20), t' 8 (79) (M+-AcOH) and 410 (90) (M+-H20 + Ac0H). Analysis: C29H44O6

Calcd C 71.31 H 9.01, Found C 71.75 H 9.68.

5a,25D-Spirostane-5-ol-3,6-dione (le) A solution of compound 1 a (0.5 g, 0.0011 mole) in

pure acetone (100ml) was cooled to 0 °C, treated with 2.3 ml of Jone's reagent [7] and processed as usual to give the dione material (le) in 96% yield (0.48 g). Crystallization from CHCl3/MeOH gave colourless needles, m.p. 270-272 °C. IR 4 B x r cm"1 : 3950 (broad, OH), 1710 (C=0 ) , 970, 920, 890 and 865 (spirostane side-chain). Analysis: C27H40O5

Calcd C 72.97 H 9.00, Found C 77.21 H 9.24.

25D-Spirost-4-en-3,6-dione (I f ) To the dione product (le) (0.5 g, 0.0011 mole) in

MeOH (150 ml), was added KOH (7.5 ml from 15% aqueous solution) and the reaction mixture w as left on a shaker for 4 h. After concentrating under vacuum (to 50 ml) water was added and the precipitated material was filtered off, where a yellowish white solid (0.45 g, 90% yield) was ob-tained. Crystallization from EtOH gave a yellowish white needles (If) , m.p. 198-200 °C. Reported [10] for 25D-spirost-4-en-3,6-dione, m.p. 194-195 °C. UV: AgtoH 250 nm (log £ = 4.04), reported [10] Amax = 253 nm (log £ = 4.03). IR %ax cm"1 : 1690, 1710, 970, 920, 890 and 865. MS: mje 300 (M+, C27H38O4).

5a-Cholestane-3ß,5,6ß-triol (2 a) It was prepared in the same manner as with the

triol ( la) in 80% yield, m.p. 235-237 °C (from acetone), reported [13] m.p. 237-239 °C. IR v cm-1 : 3410 (broad, OH). Analysis: C27H48O3

Calcd C 77.14 H 11.42, Found C 76.50 H 11.86.

5 a-Cholestane-3ß,5,6 ß-triol-3,6-diacetate (2b) This derivative was prepared from compound 2 a

in 90% yield, m.p. 160-163 °C (from acetone), reported [13] m.p. 167 °C. IR v ™ cm-1 : 3395 (broad, OH) and 1730, 1250 (acetate groups). Analysis: C31H52O5

Calcd C 73.80 H 10.30, Found C 73.00 H 10.39.

5a-Cholestane-3ß,5-diol-6-one (2 c) It was prepared from the triol (2a) in 95% yield,

m.p. 236-238 °C (from acetone), reported [5] m.p, 231 ° C . I R v £ £ c m - i : 3400 (OH) and 1710 ( > C = 0 . ) MS: mje\ 418 (M+, C27H46O3).

5a-Cholestane-3ß,5-diol-6-one-3-acetate (2d) This derivative was obtained from product 2 c by

mild acetylation in 80% yield, m.p. 233-235 °C (from acetate), reported [5] m.p. 230 °C. IRvm!kr

A . M. Dawidar et al. • Hydroxylation of Zl5-Steroids with N-Bromosuccinimide to 5a,6/?-Diols 105

cm-1 : 3400 (OH), 1730, 1270 (acetate) and 1710 ( > C = 0 ) . MS: mje {%): 460 (1) (M+, C29H48O4), 400 (68) (M+-AcOH).

5 a-Cholestane-5-ol-3.6-dione (2 e) Oxidation of the triol (2 a) by Jone's reagent gave

the dione (2e) in 95% yield, m.p. 253-256 °C (from acetone). Reported [14] m.p. 241-243 °C IR cm-1 : 3300 (OH) and 1710 (> C = 0 ) . Analysis: C27H44O3

Calcd C 77.88 H 10.57, Found C 77.50 H 10.79.

Cholest-4-ene-3,6-dione (2f) Dehydration of compound 2 e gave product 2 f in

90% yield, m.p. 118-119 °C. Reported [15] 122 to 124 °C. IR v ™ cm-1 : 1690 (C-C=C-CO) and 1600 (C=C). UV: Amax 250 nm (EtOH), log £ 4.25. MS: m/e: 398 (M+, C27H42O2).

5a-Stigmastane-3ß,5,6ß-triol (3 a) It was obtained from stigmastan-5-en-3/?-ol (3)

by its reaction with NBS in 60% yield, m.p. 250 to 252 °C (CHCI3). Reported [16] m.p. 252 °C. I P f max cm-1 : 4420 (OH). Analysis: C29H5203

Calcd C 77.67 H 11.60, Found C 76.49 H 11.88.

5a-Stigmastane-3ß,5,6ß-triol-3,6-diacetate (3 b) Mild acetylation of the triol (3 a) gave the di-

acetate derivative (3b) in 90% yield, m.p. 157 to 159 °C. Reported [16] m.p. 138-139 °C. IR v^ax cm-1 : 3430 (OH) and 1730, 1250 (acetate groups). Analysis: C33H56O5

Calcd C 74.47 H 10.52, Found C 74.50 H 10.89.

5a-Stigmastane-3ß-5-diol-6-one (3 c) Oxidation of the triol (3 a) with NBS in dioxane

gave the monoketonic product (3c) in 85% yield, m.p. 2 5 1 - 2 5 3 °C (acetone). I R v™ c m - 1 : 3 4 0 0 (OH) a n d 1710 ( > C = 0 ) . M S : m / e : 446 (M+, C 2 9H 5 o0 3 ) .

5a-Stigmastane-3ß,5-diol-6-one-3-acetate (3d) It was prepared in 80% yield by mild acetylation

of product 3c, m.p. 243-247 °C (acetone). Reported [14] m.p. 251 °C. Analysis: C3iH5204

Calcd C 76.23 H 10.67, Found C 76.88 H 10.77.

5 a-Stigmastane-5-ol-3,6-dione (3 e) Oxidation of product 3 a with Jone's reagent gave

the dione (3e) in 95% yield, m.p. 256-257 °C (acetone). Reported [16] m.p. 256 °C. IR c m - i : 3330 (OH) and 1715 ( > C = 0 ) .

Analysis: C29H48O3 Calcd C 78.37 H 10.81, Found C 78.5 H 11.06.

Stigmast-4-en-3,6-dione (3f) Dehydration of the diketonic compound (3 e) by

NaOH gave product 3f in 80% yield, m.p. 160 to 162 °C. Reported [16] m.p. 166 °C. IR v ™ cm-1 : 1690 and 1605 ( -C=C-CO). UV: Amax 250 nm (EtOH), log £4.18. MS: m/e: 426 (M+, C29H46O2).

Androstane-3ß,5 a,6ß-triol-17-one (4 a) It was prepared from androst-5-ene-3/?-ol-17-one

(4) in 40% yield, m.p. 270-272 °C (CHCl3/MeOH). IR v ™ cm-1 : 3575, 3450, 3370 (OH) and 1730 (five-membered > C = 0 ) . MS: m / e ( % ) : 322 (2) (M+, C19H30O4), 304 (5) (M+-H2O), 286 (32) (M+-2H20), 271 (12) (M+-2H2O + CH3) and 268 (M+-3H20). Analysis: C19H30O4

Calcd C 70.80 H 9.31, Found C 70.13 H 9.24.

Androstane-3ß,5 a,6ß-triol-17-one-3,6-diacetate (4b) Mild acetylation of the triol (4 a) gave the di-

acetate derivative (4b) in 90% yield, m.p. 180 to 182 °C (acetone). Reported [17] m.p. 184-185 °C. I R f K B r c m - i : 3430 (OH), 1740 ( > C = 0 ) , 1710, 1270 and 1240 (acetate groups). Analysis: C23H34O6

Calcd C 67.98 H 8.37, Found C 67.48 H 8.98.

Androstane-3ß,5a-diol-6,17-dione (4 c) Oxidation of compound 4 a with NBS in dioxane

gave the dione (4c) in 90% yield, m.p. 285-289 °C (CHCI3). Reported [17] m.p. 282-284 °C. IR v ™ cm-1 : 3420, 3310 (OH), 1720 ( > C = 0 in five membered ring) and 1705 (> C = 0 in six membered ring). MS: m/e ( % ) : 320 (22) (M+, C19H28O4), 302 (5) (M+-18), 284 (1) M+-2H20) and 269 (M+-2H20 + CH3).

Androstane-3ß,5a-diol-16,17-dione-3-acetate (4d) Mild acetylation of compound 4 c gave the acetate

derivative (4d) in 90% yield, m.p. 190-192 °C (acetone). Reported [17] m.p. 197.5-199.5 °C. IR cm-1 : 3420 (OH), 1710 ( > C = 0 ) and 1740, 1270 (acetate). MS: m/e ( % ) : 362 (8) (M+, C21H30O5), 302 (53) (M+-AcOH) and 269 ( M + - A c 0 H + H 2 0 + CH3).

Androstane-5a-ol-3,6,17-trione (4 e) This trione was prepared from compound 4 a by

oxidation with Jone's reagent in 95% yield, m.p. 240-242 °C (acetone). Reported [18] m.p. 248 to 249 °C. IR f&Br c m _ i . 3420 (OH), 1740 and 1715

106 A. M. Dawidar et al. • Hydroxylation of Zl5-Steroids with N-Bromosuccinimide to 5a,6/?-Diols 106

( > C = 0 ) . MS: ra/e ( % ) : 318 (5) M+ C19H26O4) and 300 (1) (M+-H2O).

Analysis: C19H26O4 Calcd C 71.69 H 8.17, Found C 72.00 H 8.78,

A?idrost-4-ene-3,6,17-trione (4f) It was prepared by dehydration of product 4e in

80% yield, m.p. 222-224 °C. Reported [17] m.p. 223-225 °C. IR cm"1: 1740 ( ? C = 0 ) , 1670 and 1600 ( -C=C-CO). UV: Amax 250 nm (EtOH), l o g £ 4 . 2 2 . M S : mje ( % ) : 3 0 0 ( 3 0 ) ( M + , C19H24O3).

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