NOVEL SYNTHESIS OF POLYFUNCTIONALIZED 1,4,4A,9A-TETRAHYDRO-1-AZA-9-OXAFLUORENES BY UNEXPECTED CYCLOADDITION OF 4-(4-METHOXYPHENYL)-1,4-DIHYDRO-PYRIDINES AND P-BENZOQUINONE

A surprising acid-catalysed cycloaddition reaction occurs between 5-di acetyl-4-(4-methoxyphenyl)-1,4-dihydropyridine (4) and p-benzoquinone in dioxane/HClO4 (5%) yielding novel polyfunctionalized 1,4,4a,9a-tetr ahydro-1-aza-9-oxafluorenes (9) as exclusive products. On the other ha nd the corresponding 4-unsubstituted 1,4-dihydropyridine (1) is oxidiz ed by p-benzoquinone. Novel structures are characterized by standard s pectroscopy and, furthermore, confirmed by acetylation. The different reactivity of the 4-substituted and 4-unsubstituted derivatives is dic ussed on the basis of semiempirical MNDO calculations and redox potent ials.