Typical geoporphyrins with a skeleton of 32 carbons are accounted for by a mechanism involving decarboxylation of chlorophylls, but the presence has been shown of geoporphyrins with carbon number greater than 32. It was proposed that these high carbon number porphyrins were derived from chlorophylls with extended alkyl substituents, e.g. bacteriochlorophylls, or from a random process such as transalkylation. In this study, to investigate the formation process of these high carbon number geoporphyrins, heating experiments of four vinylporphyrins, which possess 1 to 4 3-methyl-4-vinylpyrrole units, were performed. Chromic acid oxidation of the heating products of vinylporphyrins afforded a characteristic profile of C8 maleimide formation by one-carbon extension at a side chain of model substrates. We provided the first evidence that a 3-methyl-4-vinylpyrrole unit afforded 2-methyl-3-n-propylmaleimide as the predominant product among the side-chain extension products formed. On the other hand, a 3-ethyl-4-methylpyrrole unit afforded 2,3-diethylmaleimide as the major extension product.
