Regio and stereoselective oxidations of unsaturated steroidal compounds with H2O2 mediated by CH3ReO3

We have investigated the oxidative behavior of sterols such as cholesteryl acetate (1), 7-dehydrocholesteryl acetate (2), ergosteryl acetate (3), cholecalciferol acetate (Vitamin D3 acetate) (4) and ergocalciferol acetate (Vitamin D2 acetate) (5) with the oxidant system methyltrioxorhenium/H2O2/pyridine in order to check potential parameters controlling the selectivity. The reactions, performed in CH2Cl2/H2O at 25 °C, have shown good regio- and stereoselectivity. All oxidation products were isolated by high-performance liquid chromatography (HPLC) and characterized by MS(EI) or FAB, 1H NMR, 13C NMR, APT, COSY, HSQC, HMBC, ROESY and NOEDS measurements. Seven new oxygenated compounds were also obtained. Under the experimental conditions adopted in this work, only the diene steroids, i.e. 7-dehydrocholesteryl acetate and ergosteryl acetate, undergo hydrolytic oxirane ring opening, whereas Vitamin D2 and D3 acetates, containing the triene system and cholesteryl acetate yield only epoxides. The selectivity seems to be controlled by the nucleophilicity of double bonds and by stereoelectronic and steric effects.