EXPECTED SELECTIVITY AND UNEXPECTED ISOMERIZATION IN THE INVERSE DIELS-ALDER REACTION OF AZOLYL DIENES

Hetaryl-substituted dienamines (la-e and 4a-c) of different geometry w hen treated with dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (2) reac ted selectively as enamines (i.e. with the Delta 3,4 double bond) to a fford the appropriate E or Z (hetarylvinyl)pyridazines (i.e. la-e gave 3a-c, whereas 4a-c afforded 5a-c). Study on the analogous transformat ion of the 1Z diene ether (6a) revealed that, in apolar solvent, isome rization of the olefinic side chain occurred (3a rather than 5a was ob tained). Formation of the trans product 3a was rationalized by a tauto meric equilibrium of the supposed intermediate 7 for which experimenta l evidence was provided.