Diels-Alder reactions with alkyl coumalates

Alkyl coumalates (methyl and ethyl 2-oxopyran-5-carboxylates) have been treated with a wide variety of dienophiles in Diels-Alder reactions. In all cases, extrusion of carbon dioxide occurred and multiple adducts were usually obtained; in one case a spontaneous loss of molecular hydrogen was observed. Thus methyl coumalate reacted with p-benzoquinone to give dimethyl anthraquinone-2,6-dicarboxylate (or the 2,7-analogue); with acetylenedicarboxylates, benzene-1,2,4-tricarboxylates were obtained; with benzyne, methyl naphthalene-2-carboxylate was formed. The adducts with maleic anhydride were 7-methoxycarbonylbicyclo[2,2,2]oct-7-ene-2,3;5,6-tetracarboxylic dianhydride and the 2-ethoxycarbonyl homologue. An analogous reaction with azodicarboxylate esters led (for example) to pentamethyl 2,3,5,6-tetra- azabicyclo[2,2,2]oct-7-ene-2,3,5,6,7-pentacarboxylate.