INFLUENCE OF CARBOCYCLIC RINGS ON PORPHYRIN CYCLIZATIONS - SYNTHESIS OF GEOCHEMICALLY SIGNIFICANT CYCLOALKANOPORPHYRINS

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 di-formyldipyrrylmethanes (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. © 1990, American Chemical Society. All rights reserved.