A CHEMICAL EVIDENCE SUPPORTING THE FORMATION OF DIHYDROPYRAN ADDUCTS FROM THE REACTION OF 5-ARYLIDENERHODANINES WITH 1-MORPHOLINOCYCLOHEXENE THROUGH ZWITTERIONIC INTERMEDIATE

Hydrolytic cleavage of dihydropyran adducts, under mild conditions lea ds to the corresponding 1,5-dicarbonyl compounds. The presence of the zwitterionic intermediate in the formation of adducts through a two st ep pathway is demonstrated by captured of it by TCNE to give spirocycl ohexane derivatives. These results can be regarded as clear evidence f or the formation of dihydropyran adducts via zwitterionic intermediate through 1,4-cycloaddition reaction.