Porphyrins with exocyclic rings.: Part 10.: Synthesis of meso,β-propanoporphyrins from 4,5,6,7-tetrahydro-1H-indoles.

Benzyl (6) and tert-butyl 3-metyl-4,5,6,7-tetrahydro-1H-indole-2-carboxylates (28) were easily prepared from cyclohexanone using a variation of the Knorr pyrrole condensation. Regioselective oxidation with lead tetraacetate gave the corresponding 7-acetoxy derivatives, or related solvolysis products, and subsequent reaction with 5-unsubstituted pyrrole-2-carboxylates in the presence of p-toluenesulfonic acid in acetic acid gave a series of 7-pyrrolytetrahydroindoles 16 in excellent overall yields. Cleavage of the protective ester units, followed by acid-catalyzed condensation with diformyldipyrrylmethanes 19 under modified MacDonald "2 + 2" conditions gave good yields of meso,beta-propanoporphyrins 26. This chemistry was sufficiently versatile that a porphyrin with two six-membered exocyclic rings (34) could be prepared by the same methodology. On the other hand, attempts to cyclize an a,c-biladiene 37 incorporating a six-membered carbocyclic ring gave moderate to poor yields of the required meso,beta-propanoporphyrin 26a, probably due to a deleterious steric interaction between the carbocyclic ring and an adjacent alkyl substituent. Nonetheless, the results described below demonstrate the value of this approach for the synthesis of sedimentary cycloalkanoporphrins (C) 1997 Elsevier Science Ltd. All rights reserved.