Novel synthesis of polyfunctionalized 1,4,4a,9a-tetrahydro-1-aza-9-oxafluorenes by unexpected cycloaddition of 4-(4-methoxyphenyl)-1,4-dihydropyridines and p-benzoquinone

Article abstract of DOI:10.3987/COM-98-8189

A surprising acid-catalysed cycloaddition reaction occurs between 3,5-diacetyl-4-(4-methoxyphenyl)-1,4-dihydropyridine (4) and p-benzoquinone in dioxane/HClO₄ (5%) yielding novel polyfunctionalized 1,4,4a,9a-tetrahydro-1-aza-9-oxafluorenes (9) as exclusive products. On the other hand the corresponding 4-unsubstituted 1,4-dihydropyridine (1) is oxidized by p-benzoquinone. Novel structures are characterized by standard spectroscopy and, furthermore, confirmed by acetylation. The different reactivity of the 4-substituted and 4-unsubstituted derivatives is dicussed on the basis of semiempirical MNDO calculations and redox potentials.