Porphyrin synthesis through tripyrrins: an alternate approach

A new route for synthesis of unsymmetrically substituted porphyrins is described. The route follows the earlier approach through pyrromethanes, tripyrrin hydrobromides, and a,c-biladiene dihydrobromides, except that the pyrromethane intermediate is elongated in an initially “clockwise” direction to give a benzyl tripyrrincarboxylate. Various advantages over the tert-butyl tripyrrincarboxylates used in the earlier method (Scheme I) are discussed. The new route is demonstrated in the syntheses of five pure porphyrins required for other current studies.