Selective reduction of oxo bile acids: synthesis of 3 beta-, 7 beta-, and 12 beta-hydroxy bile acids

Preparation of some biologically important keto bile acids is described. Advantage is taken of the preferential ketalization of 3-oxo group in bile acids over 7- and 12-oxo groups for the selective reduction of these keto groups. The method was found to be specially useful for preparation of 7 beta-, 12 alpha, and 12 beta-3H]-3-oxo bile acids. Improved methods are also described for the preparation of epimers of naturally occurring bile acids at C-3, C-7, and C-12. 3 beta-Hydroxy bile acids (iso-bile acids) were prepared with the use of diethylazodicarboxylate/triphenylphosphine/formic acid. Iso-bile acids were obtained in excellent yields (80-95%) except during synthesis of isoursodeoxycholic acid (yield, 50%). Isoursodeoxycholic acid was, however, prepared in very good yield via epimerization of 3 alpha-hydroxyl group in 7-oxolithocholic acid followed by stereoselective reduction of 7-oxo group. A highly efficient method for the reduction of 7-oxo and 12-oxo groups was developed. Thus, 7-oxolithocholic acid and 7-oxoisolithocholic acid on reduction with potassium/tertiary amyl alcohol yielded ursodeoxycholic acid and isoursodeoxycholic acid in yields of 96% and 94%, respectively, while reduction of 7-oxodeoxycholic acid resulted in ursocholic acid in 93% yield. In a similar manner, reduction of 12-oxolithocholic acid and 12-oxochenodeoxycholic acid yielded 3 alpha, 12 beta-dihydroxy-5 beta-cholanoic acid (lagodeoxycholic acid; 92% yield) and 3 alpha, 7 alpha, 12 beta-trihydroxy-5 beta-cholanoic acid (lagocholic acid, 86% yield).