INTRAMOLECULAR DIELS-ALDER REACTIONS .5. APPROACHES TO THE PYRROLO[3,4-C]CARBAZOLE AND PYRIDO[4,3-C]CARBAZOLE SYSTEMS

Two methods for the preparation of indole-2,3-quinodimethanes are reported. Treatment of 1,2-dimethyl-alpha-oxo-N-(phenylmethyl)-N-2-propenyl-1H-indole-3-acetamide (3) with sodium bis(trimethylsilyl)amide in refluxing THF gave 2-(phenylmethyl)-10c-hydroxy-6-methyl-3,3a,4,5,6,10c-hexahydropyrrolo[3,4-c]-carbazol-1(2H)-one (5) in 18% yield by intramolecular Diels-Alder reaction of the intermediate enolate 4. LiBH4-reduction of amide 3 and thermolysis of the resulting alpha-hydroxyamide 8 at 190 degrees C gave a 63:37 mixture of the cis and trans isomers of 2-(phenylmethyl)-6-methyl-3,3a,4,5,6,10c-hexahydropyrrolo[3,4-c]carbaxol-1(2H)-one (9a and 9b) in 64% yield. The corresponding N-3-butenylamide 10 at 205 degrees C led to a 67:33 mixture of cis- and trans-7-methyl-2,3,4,4a,5,6,7,11c-octahydro-2-(phenylmethyl)-1H-pyrido[4,3-c]carbazol-1-one (11a and 11b) in 53% yield. Thermolysis of 1,2-dimethyl-alpha-hydroxy-N-(phenylmethyl)-N-2-propynyl-1H-indole-3-acetamide (15) gave an equimolar mixture of lactam 9a and the aromatized product, 3,6-dihydro-6-methyl-2-(phenylmethyl)pyrrolo[3,4-c]carbazol-1(2H)-one (17) in 80% yield by disproportionation of the intermediate lactam 16. Reaction of 1,2-dimethyl-1H-indole-3-carboxaldehyde with methyl acrylate and sodium bis(trimethylsilyl)amide produced methyl 1,2-dihydro-9-methyl-9H-carbazole-3-carboxylate (20) in 26% yield, most likely by a sequence of Michael addition to the enolate of the aldehyde and intramolecular aldol condensation.