Natural corrin, porphyrin and related systems

Because of its immense scope, a detailed description of corrins (and vitamin B12) will not be presented here. The reader is instead referred to reviews of B12 chemistry and its biosynthesis that have appeared recently. Further, because they are more directly related to the corrins than are the corroles, the chemistry of the dehydrocorrins will not be discussed here. Also not included in this review are the so-called artificial porphyrins of Floriani, et al. (e.g., 2.5), since it is deemed by these authors in their review that these macrocycles are more dehydrocorrin-like than corrole-like in their nature. Other systems omitted here include the spiro porphyrins of Battersby and coworkers, the tetraphosphole macrocycles of Mathey and coworkers and the tetrapyrrolic systems of Bartczak and Smith and co-workers. Thus, the emphasis will be on those contracted porphyrins that are most removed, in structural and chemical terms, from the macrocyclic unit found in coenzyme B12 and its analogs. 

The corrin ligand of coenzyme and vitamin B12 is structurally and biosyn-thetically related to the natural porphyrins (6-8). Besides its linearly conjugated chromophoric system, which comprises the four cobalt-coordinating (inner) nitrogen atoms, the corrin ligand exhibits a uniquely functionalized periphery and... 

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