Cycloaddition of diphenylnitrilimine to coumarins. The synthesis of 3a,9b-dihydro-4-oxo-1H-benzopyrano [4 ,3-c]pyrazole derivatives

Abstract

The cycloaddition of diphenylnitrilimine to a series of coumarins occurs with the same regioselectivity in all cases, regardless the nature of the substituent present, to yield the corresponding 3a,9b-dihydro-4-oxo-lH-benzopyrano [4,3-c]pyrazoles 4 a–f. Dehydrogenation of 4a yields 4-oxo-1H-benzopyrano [4,3-c]-pyrazole 6 . In ethanolic sodium ethoxide solution, the reaction between coumarin and diphenylnitrilimine precursor, namely N-phenylbenzohydrazidoyl chloride 1 , affords no cycloadducts, but yields o-(β-ethoxycarbonylvinyl)phenyl N-phenylbenzohydrazidate 10 . The regiochemistry of the cycloadducts 4 a–f and the reaction sequence leading to 10 are outlined. The structures of the products have been established by spectroscopic methods and independent syntheses wherever possible.