[3+2] versus [4+2] Cycloadditions of Quinone Monoimide with Azadienes:  A Lewis Acid-Free Access to 5-Amino-2,3-dihydrobenzofuranes

Abstract

The reaction between p-quinone monoimide 1a and various azadienes 2 is described in the absence of a Lewis acid promoter. When alpha,beta-unsaturated hydrazones are substituted by proton or alkyl groups, 2,3-dihydrobenzofuranes 4, a motif that is present in numerous biologically active products, are obtained in moderate to excellent yields. The regio- and stereoselectivity of this reaction has been proved by a complete NMR study, including 1H-15N correlations.