Influence of carbocyclic rings on porphyrin cyclizations: synthesis of geochemically significant cycloalkanoporphyrins

Abstract

The synthesis of cycloalkanoporphyrins from cycloalkenopyrroles is described. Cycloalkeno[b]pyrroles were prepared in moderate to good yields from the corresponding cyclic ketones. Reaction with lead tetraacetate afforded the corresponding labile acetoxy derivatives. Condensation of the acetoxy compounds with α-free pyrroles in the presence of an acid catalyst gave pyrrolylcycloalkeno[b]pyrroles in good yields. These dipyrroles were the key intermediates in the total synthesis of porphyrins bearing exocyclic rings. Condensation of dipyrrole dicarboxylic acids with diformyldipyrrylmethanes (the MacDonald condensation) were carried out in the presence of p-toluenesulfonic acid. Good yields of the corresponding porphyrins were obtained when five-, six-, seven-, or sixteen-membered rings were present. The eight-membered ring slightly inhibited this reaction. a,c-Biladienes incorporating carbocyclic rings were also prepared. Poor yields of porphyrin were obtained when copper(II) salts were used to mediate the cyclization of the a,c-biladienes. When six-, seven-, or eight-membered rings were present, moderate to good yields of the corresponding porphyrins could be obtained when cyclizations were carried out with silver iodate-zinc acetate in dimethylformamide. Five-membered carbocyclic rings severely inhibited these cyclizations. This work provides the foundation for reliable syntheses of geochemically significant cycloalkanoporphyrins