PORPHYRINS WITH EXOCYCLIC RINGS .2. SYNTHESIS OF GEOCHEMICALLY SIGNIFICANT TETRAHYDROBENZOPORPHYRINS FROM 4,5,6,7-TETRAHYDRO-2H-ISOINDOLES

Benzo- and tetrahydrobenzoporphyrins are widespread constituents of oil shales and petroleum. Although the origins of these materials are not known, a case is made for divinylchlorophyll a, a widespread pigment in marine algae, being the precursor to many of these geoporphyrins. Total syntheses of four tetrahydrobenzoporphyrins related to etioporphyrin III are described. Tetrahydroisoindoles were prepared by condensation of isocyanoacetates with 1-nitrocyclohexene in the presence of DBU or by reaction of aminomalonates with 2-formylcyclohexanone. Condensation of 3-unsubstituted 4,5,6,7-tetrahydro-2H-isoindoles 23c and 23d with (acetoxymethyl)pyrroles in the presence of Montmorillonite clay gave dipyrrylmethanes 28a and 36a in excellent yield. Hydrogenolysis of the benzyl esters and subsequent acid-catalyzed condensation with pyrrole aldehydes 37a and/or 37b gave a series of a,c-biladiene dihydrobromides. Copper(II) mediated cyclization of the a,c-biladienes 32, 33, 35, and 38, followed by demetallation with 15% sulfuric acid-trifluoroacetic acid, gave four isomeric tetrahydrobenzoporphyrins 10-13 in unusually high yield. This work provides a general route for the synthesis of these important porphyrin molecular fossils.