Porphyrins with Exocyclic Rings. 2. Synthesis of Geochemically Significant Tetrahydrobenzoporphyrins from 4,5,6,7-Tetrahydro-2H-isoindoles

Article abstract of DOI:10.1021/jo00044a013

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 15percent 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.