Occurrence and Structure of Natural Corrinoids

Natural Bn-derivatives can occur either as complete corrinoids where a nucleotide fimction is attached to the propionic acid substituent at Cl7, or as incomplete corrinoids, which lack the nucleotide fimction and generally represent biosynthetic intermediates on the way to the complete corrinoids [23, 29]. 

In complete B12-analogues the constitution of the nucleotide base can vary and the known classes of nucleotide fimctionahties found are benzimidazoles, such as the 5,6-dimethylbenzimidazole (DMB) of the cobalamins, purines, such as adenine and 2-methyladenine found in pseudovitamin B12 (10) and factor A (11), respectively, and phenols, such as p-cresol found in dicyano-p-cresolylcobamide (52) [145]. Sewage sludge is a particular rich, classical source of corrinoids [23]. Recent studies on the corrinoids from anaerobic microorganisms have shown a wide range of purine bases and benzimidazoles [1,6,37]. 

In solution the intramolecular coordination of the nucleotide function to the lower a-axial coordination site of the corrin-bound cobalt center of complete corrins occurs with Httle build-up of strain [149]. This allows the (coordinating) nucleotide to steer the reactivity, as well as the face-selectivity, of certain organometalUc reactions involving the corrin-boimd cobalt center [75]. Experiments by Eschenmoser demonstrated that cobal-amins can self-constitute in solution from the Bi2-nucleotide portion and incomplete cobyrinic acid derivatives to show a remarkable kinetic and thermodynamic preference for the specific formation of the B 12-structure, and to a pre-enzymatic origin of the basic structural elements of the complete corrins [149]. 

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