Regioselective formation of novel functionalized 1-aza-9-oxafluorenes

A small series of novel 1-aza-9-oxafluorenes have been prepared by regiosel ective cycloaddition reaction of p-benzoquinone to the sterically unhindere d side of unsymmetrically substituted N-acyl-l,4-dihydropyridines (1). The stereochemistry of the products is discussed on the basis of X-Ray crystal structure analysis of one starting structure (la). The formation of pyridin e derivatives by possible redox reaction of the adducts was not observed. R otameric properties of the N-acyl-1,4-dihydropyridines are demonstrated by H-1 and C-13 NMR spectroscopy, thus stabilizing the 1,4-dihydropyridine sys tem towards competing oxidation by the quinone.