Bufadienolides. 25. Direct Conversion of 14-Dehydrobufalin to Bufalin

The easily prepared iodo- (3b), bromo- (3c), and chlorohydrin (3d) derivatives of 14-dehydrobufalin (la), when subjected to careful hydrogenolysis using Urushibara nickel A or Raney nickel, readily yield bufalin (3a). This experimentally simple route from 14-dehydrobufalin to bufalin obviates the prior necessity of proceeding via resibufogenin (2a). In related experiments, treatment of resibufogenin (2a) with hydrogen chloride was found to yield chlorohydrin 3d, and the reverse reaction was achieved using hot α-collidine. A new procedure for selective hydrolysis of buf adienolide 3β-acetates was realized employing an acidic ion exchange resin.