Porphyrins with exocyclic rings. Part 3. A reassessment on the utility of cyclopenta[b]pyrroles in the synthesis of porphyrin molecular fossils. Preparation of three type II porphyrins related to deoxophylloerythroetioporphyrin (DPEP).

Abstract

The utility of cyclopenta[b]pyrroles in porphyrin synthesis has been reinvestigated. A 6-oxocyclopenta[b]pyrrole 18 was prepared by cyclization of the propanoyl chloride sidechain of an α-unsubstituted pyrrole 17d in the presence of tin(IV) chloride. Subsequent reduction with sodium borohydride afforded the corresponding 6-hydroxy compound 10 and further acid catalyzed condensation with α-unsubstituted pyrroles 11a and 11b gave the novel 6-pyrrolylcyclopenta[b]pyrroles 22a and 22b in excellent yields. Attempts to prepare deoxophylloerythroetioporphyrin (DPEP; 2), a widespread sedimentary porphyrin molecular fossil, from these dipyrrolic intermediates using the tripyrrene-a,c-biladiene route were unsuccessful. However, the synthesis of three related meso,β-ethanoporphyrins using the MacDonald condensation was successfully carried out in moderate to good yields. Retention of an sp 3 hybridized carbon bridge at the cyclopentene ring fusion site of the intermediary open chain tetrapyrroles appears to be crucial during macrocycle formation, as this diminishes the steric repulsion between the peripheral substituents and the carbocyclic ring.