The synthesis of cobyrinic acid derivatives by reduction of dehydrocobyrinates is largely unexplored. It is, however, a rational path to B12 analogues that lack specific substituents of the corrin moiety of natural B12 derivatives. The partial syntheses of four epimeric 7-decarboxymethyl-cobyrinates is described, which is achieved by reduction of Δ7-dehydro-7-de[carboxymethyl]-cobyrinate with zinc or with the ‘prebiotic’ reducing agent formic acid. A direct and remarkably efficient route was found to 7-decarboxymethyl-cobyrinates, which are cobyrinic acid derivatives in which the c-side chain at ring B of vitamin B12 is missing. The structures of the hexamethyl-7-decarboxymethyl-cobyrinates were characterized and the stereochemical and conformational properties at their newly saturated ring B were analyzed. The stereochemical outcome of the reduction was found to depend strongly on the reaction conditions. In 7-decarboxymethyl-cobyrinates, both peripheral carbon centres of ring B carry a hydrogen atom, and the characteristic quaternary carbon centre at C7 of the cobyrinic acid moiety of vitamin B12 is lacking. The still highly substituted 7-decarboxymethyl-cobyrinates are readily dehydrogenated in the presence of dioxygen, furnishing 7-de[carboxymethyl]-Δ7-dehydro-cobyrinate as the common, unsaturated oxidation product. The noted stability of vitamin B12 and of other CoIII-cobyrinates in the presence of air is a consequence of their highly substituted corrin macrocycle, a finding of interest in the context of chemical rationalizations of the B12 structure.
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