Vitamin B12, in the form of its coenzyme, is a cofactor for a series of eleven enzyme catalyzed transformations. Of these, three enzymes: Beta-methylaspartate mutase, methylmalonyl-SCoA mutase and methylitaconate mutase are of special interest because they catalyze a carbon-skeleton rearrangement, for which there has been no analogue in organic chemistry. In this proposal, we present new chemical model reactions in which methylitaconate and methylmalonate, attached through their methyl groups to the cobalt atom of vitamin B12, rearrange spontaneously to alpha-methyleneglutarate and succinate, respectively. This is the first observation of a B12 rearrangement occurring in the absence of enzyme. Synthesis of a model for the Beta-methylaspartate rearrangement is described. A novel series of experiments, designed to explore the hydrogen abstraction reaction through model studies, is delineated.