Abstract
(2-Amino-5,6-dimethylbenzimidazolyl)cobamide is transformed to (2-hydroxy-5,6-dimethylbenzimidazolyl)cobamide by nitrous acid. Exchange of the NH2-group by hydrogen with nitrous acid/hypophosphorous acid yields vitamin B12. This reaction completes a cyclic vitamin B12 .fwdarw. [carboxy(2-cyanoamino-4,5-dimethylphenyl)amino]cobamide .fwdarw. (2-amino-5,6-dimethylbenzimidazolyl)cobamide .fwdarw. vitamin B12, which allows chemical 14C-labeling of vitamin B12. In this procedure cyanogen bromide, which is necessary for the first step, was labeled with [14C]cyanide. By the following reactions a vitamin B12 was formed in which C-2 of the 5,6-dimethylbenzimidazole moiety is labeled.

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