A Convenient Procedure for the Synthesis of 3β-Hydroxy-6-oxo-5α-steroids. Application to the Synthesis of Laxogenin

5-steroids is reported; the methodology was applied to the synthesis of laxogenin (7), substance that behaves as a plant growth hormone. This is an alternative way to produce an important functionality found in many examples of naturally occurring steroids. The developed procedure uses inexpensive reagents and can be carried out in four steps. The oxidizing and acidic steps used in this methodology did not affect the labile spiroketal side chain present in diosgenin (16).

Recently, we have synthesized laxogenin (7), a steroidal sapogenin isolated from Smilax sieboldi 8 and its C-23 substituted derivatives 8 and 9 and reported that they have shown plant growth promoting activity similar of that of brassinosteroids ( Figure 2). 8 In that report, a protocol based on Brown´s hydroboration for the introduction of the 3βhydroxy-6-oxo moiety was developed.
The 5β,6β-epoxy moiety has been previously used for the preparation of 6-oxosteroids. In particular, Henbest and Wrigley reported 9 that treatment of 5,6β-epoxy-5β-cholestan-3β-ol acetate with BF 3 ·Et 2 O led to corresponding 3β-acetoxy-5-fluoro-6β-hydroxysteroid which was oxidized to 3β-acetoxy-5-fluorocholestan-6-one using Jones reagent. More recently extension of this procedure to stigmasterol has been u to obtain brassinosteroid analogues bearing the 5α-flu 6-oxo moiety. 9 Owing our interest on steroids bearing s functionality, and after some reports on the hig stereoselective β-epoxidation of ∆ 5 -steroids using biph systems involving potassium permanganate and m salts, 10 we envisaged the β-epoxidation of the C5 double bond followed by the regioselective oxirane opening as the key steps for the introduction of an oxy atom at position C-6. This led us to an alternative prot for the synthesis of 3β-hydroxy-6-oxo steroids.

Results and Discussion
Treatment of cholesteryl acetate, in a mixture Fe 2 (SO 4 ) 3 resulted in the highly diasteroselective β-epoxidation of the C5-C6 double bond; only traces of the α-epoxide could be detected in the 1 H NMR spectra. Regioselective opening of the β-oxirane ring with aq. HBr led to the bromohydrin 11 as sole product.
Treatment of the bromohydrin 12 with Jones reagent, supported on silica gel, led the bromoketone 13 (Scheme 1). In this way, manipulation of the reaction was rapidly effectuated and permitted a better treatment for wastes.
We also studied this three-reaction sequence without  isolation of the intermediate epoxide 11 and bromohydrin 12; this resulted in a very fast and convenient protocol for the conversion of cholesteryl acetate into the bromoketone 13 (75% overall yield for the consecutive three steps). Treatment of 13 with zinc in refluxing acetic acid yielded the acetylated ketone 14, which on saponification afforded the desired 3β-hydroxy-5α-cholestan-6-one (15). The same one-pot synthetic sequence was applied to diosgenin acetate (16) to produce laxogenin (7). Under these strong oxidizing and acidic media, the labile spiroketal side chain of diosgenin resulted unchanged (Scheme 2).

Experimental
NMR spectra were registered in CDCl 3 on a Varian Mercury spectrometer at 400 MHz for 1 H or 100 MHz for 13 C. Chemical shifts (δ) are expressed on ppm downfield from TMS. Melting points were obtained on a Gallenkamp MFB 595 apparatus and were not corrected.

5-Bromo-5α-cholestan-3β,6β-diol 3-acetate (12)
A solution of the epoxide 11 (445 mg, 1 mmol CH 2 Cl 2 (15 mL) was vigorously shaken for 5 min separatory funnel with 5 mL of 48% HBr. The org layer was washed with H 2 O (5x 15 mL), dried (anh. Na 2 S and evaporated to afford 510 mg (97%) of the bromohy 12; mp 174-175 ºC, lit. 12  was stirred until an homogeneous orange powder was formed. CH 2 Cl 2 (15 mL) was added followed by the addition of a solution of the bromohydrin 12 (526 mg, 1 mmol) in CH 2 Cl 2 (10 mL); the mixture was stirred for 20 min, filtered through an small pad of silica gel and the eluent was evaporated to afford 414 mg (79%) of the bromoketone 13; mp 157-158 ºC (decomp.), lit. 13  3β-Acetoxy-5-bromo-5α-cholestan-6-one (13) via a three steps procedure: β-epoxidation, oxirane-opening, alcoholoxidation KMnO 4 (6 g) and Fe 2 (SO 4 ) 3 .nH 2 O (3 g) were finely grounded in a mortar, H 2 O (0.6 mL) was added and the mixture was placed in a round bottom flask containing CH 2 Cl 2 (45 mL). A solution of cholesteryl acetate (10) (1.284 g, 3 mmol) in CH 2 Cl 2 (45 mL) was added followed by addition of tert-butyl alcohol (1.5 mL). After 20 min of stirring at room temperature, the mixture was filtered through a pad of celite and eluted with 30 mL of CH 2 Cl 2 . The crude filtrate was washed with H 2 O (5x25 mL) and vigorously shaken for 5 min in a separatory funnel with 15 mL of 48% HBr, washed with H 2 O (5x 30 mL) and added to a stirred mixture of silica gel-supported Jones Reagent (prepared from 2.5 mL of Jones reagent and 5 g silica gel as described for the oxidation of 12) and CH 2 Cl 2 (30 mL). The mixture was stirred for 20 min, filtered through a small pad of silica gel and the eluent was evaporated to afford 1.178 mg (75%) of the bromoketone 13, identical as described above.