Preparation of the Reduced Forms of Vitamin B12 and of Some Analogs of the Vitamin B12 Coenzyme Containing a Cobalt-Carbon Bond "Vitamins and Coenzymes," Vol. XVIII, Part C,
D.B. McCormick and L.D. Wright, Eds.
Reduction of the cobalamins, which contain trivalent cobalt, proceeds in two distinct steps. The first one-electron reduction product is vitamin B12r which undergoes a further one-electron reduction to B12s which may be considered as either containing monovalent cobalt or being a trivalenl cobalt hydride. B12s is the most nucleophilic species known to exist in aqueous solution,and this high nucleophilicity results in the ease of preparation of the B 12 coenzyme and its analogs, since B12s undergoes both rapid substitution and addition reactions with a variety of electrophiles. Chemical, electrochemical and catalytic reductions are discussed for the preparation of vitamin B12r and B12s and their conversion to alkyl, vinyl and ethynyl cobalamins along with methods of purification.