The vitamin B,, coenzyme has been partially synthesised from hydroxocobalamin (vitamin Blzb) by reduction followed by reaction with a 5'-tosyladenosine. Many analogues, including simple alkyl and acyl derivatives, have been obtained by similar methods. The reactive intermediate is postulated as a cobalt hydride on the basis of its addition to a variety of ethylenic and acetylenic compounds, yielding further analogues of the coenzyme. Reaction of the cobalt hydride with diazomethane gives the cobalt-methyl derivative. Certain reactions of the products are described.THE vitamin B,, coenzymes were first isolated by Barker and his colleagues,l and the chemical structure (I) of one of them, the 5,6-dimethylbenzimidazolylcobamide coenzyme was established by X-ray crystallographic and chemical analysis., The other coenzymes of the group differ only in the nature of the base contained in the nucleotide fragment. The novel cobalt-carbon bond linking the metal to the 5'-deoxyadenosine group of the coenzyme (I ; R = 5'-deoxyadenosyl) is easily broken by a variety of chemical reagents as well as by light, and many transformation products of the coenzyme have been characterised (see, e.g., refs. 3 and 4). Apart from the possible addition of hydrogen at C,-C,, in the corrin nucleus * the main ambiguity of structure (I; R = 5'-deoxyadenosyl) concerns the cobalt atom, which is probably tervalent on the basis of the observed chemical reactions, although there have been claims that the coenzyme is paramagnetic, which would suggest the presence of bivalent cobalt.* Until this ambiguity is resolved, the " cobalamin " nomenclature for the coenzymes and their analogues (I), e.g., " methylcobalamin " for (I; R = Me), has been avoided:
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