Porphyrins, Chlorophyll a, and Corrins

The reaction sequence is successful because reverse, ring-contraction reactions are unlikely and because only the final product contains a secondary lactam group, which is depro-tonated under the reaction conditions. 

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

If one heats acetone and pyrrole in the presence of catalytic amounts of acid, so-called acetone pyrrole is formed in over 80%i yield. This colorless, macrocyclic compound contains four pyrrole units which are connected by dimethylmethylene bridges, ft is formed by electrophilic-a-substitution of pyrrole by acetone, acid-catalyzed oligomerization, and spontaneous. 

R phenyl, e -deficient aryl 30-40% R p-tolyl, e -rich aryl 10% 

This apparently extremely simple synthesis of symmetrical porphyrins from aldehydes and pyrrole has also been used to produce porphyrins with interesting stereochemical proper- 

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