Tripyrroles, and Tetrapyrroles

Some of the intermediate steps in the synthesis of a dipyrrylmethene have been studied chemically (36) and the spatial changes are similar to those postulated by Michaelis (93) for the triphenylmethane dyes. The condensation of the pyrrole-a-aidehyde (5) goes through an intermediate carbinol stage (4) in which the central carbon atom is bound tetrahedrally. The carbinol readily loses hydroxyl ion in the presence of very weak acid to form the planar carbonium ion 5), which represents one of the resonance forms of dipyrrylmethene usually written as (6), 

The course pursued in chemical preparations has been, for obvious reasons, the combination of two dipyrrylmethenes to form the porphin, and most biochemical postulates have favored such a scheme (25), although without particular justification. 

A mechanism studied by Corwin and Andrews (19), which may be of biological importance, is the condensation of a dip3u-rylmethene (9) on page 

332 with a pyrrole which has a free o-position (10). Here one pyrrole may exchange for another through the intermediate formation of a tripyrrole (11), which then splits to give the new dip3nrylmethene (12). 

One tripyrrylmethene has been found in nature it is the red pigment occurring in Serratia marcescens (Bacillus prodigiosus) which, according to Wrede (159), has the structure shown at the left above. 

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