Electrochemical bromination of cholest-5-enes

Abstract

Four 5,6-unsaturated steroids—3β-chlorocholest-5-ene ( 1a), cholesterol ( 1b) and its acetate ( 1c) and benzoate ( 1d)—were subjected to constant current electrolysis (50 mA, 2 F mol −1) in an electrolytic cell divided by a ceramic membrane, using a platinum foil as the anode and a graphite stick as the cathode. When electrolysis was carried out in a solution of tetraethylammonium bromide in aprotic solvents (dichloromethane, acetonitrile or acetic anhydride), the addition of electrochemically-generated elemental bromine onto the double bond of the cholesterol derivatives gave their corresponding 5α,6β-dibromosteroids—3β-chloro-5α,6β-dibromocholestane ( 2a), 5α,6β-dibromocholestan-3β-ol ( 2b), 5α,6β-dibromocholestan-3β-yl acetate ( 2c) and 5α,6β-dibromocholestan-3β-yl benzoate ( 2d)—as the sole products, and in good yields (58–91%). However, the electrolysis of steroids 1a– c in a solution of tetraethylammonium bromide with methanol as the solvent proceeded to give, in addition to dibromides 2a– c, the corresponding diastereomeric pairs of 5-bromo-6-methoxysteroids: 5α-bromo-3β-chloro-6β-methoxycholestane ( 3a) and 5β-bromo-3β-chloro-6α-methoxycholestane ( 4a), 5α-bromo-6β-methoxycholestan-3β-ol ( 3b) and 5β-bromo-6α-methoxycholestan-3β-ol ( 4b) and 5α-bromo-6β-methoxycholestan-3β-yl acetate ( 3c) and 5α-bromo-6β-methoxycholestan-3β-yl acetate ( 4c). The benzoate 1d was not soluble enough in methanol, even with heating. The products were characterized by physical and spectral data (IR, 1H NMR and 13C NMR). Single crystal X-ray structure determinations of compounds 2a and 3a are also reported.