Competitive [3+2] and [4+2] cycloaddition reactions of 2-furaldehyde phenylhydrazone with alkenes

Maleimides react with 2-furaldehyde phenylhydrazone 1 at the furan ring to give 4-(phenylhydrazono)methyl-1H-isoindole-1,3(2H)-diones when equimolar m ixtures are refluxed in benzene for shorter reaction times. Excess maleimid es and longer reaction times give mixtures of 4-(phenylhydrazono)methyl-1H- isoindole-1,3(2H)-diones and 3-(1,3-dihydro-1,3-dioxo-4-isoindolyl)-1,6a-di hydropyrrolo[3,4-c]pyrazolo-4,6(3aH,5H)-diones resulting from [4+2] and [32] double-cycloaddition. Reactions of 1 with dimethyl acetylene dicarboxyla te give the double-cycloaddition product dimethyl-3[4-hydroxy-2,3-bis(metho xycarbonyl)phenyl]-1-phenylpyrazole-4,5-dicarboxylate. Methyl acrylate and acrylonitrile gives only the [3+2] cycloaddition products, 3-(2-furyl)-1-ph enyl-4,5-dihydropyrazole-4-carboxylic methyl ester and 3-(2-furyl)-1-phenyl -4,5-dihydropyrazole-4-carbonitrile, respectively.