Annulation of heterocyclic secondary enamines with dicarboxylic acid dichlorides, an unexpected ring size effect

Under very mild conditions heterocyclic secondary enamines react efficiently with malonyl chloride to produce hydroxylated 2-pyridinone-fused heterocycles. The reactions of enamines with oxalyl chloride, however, afford varied products depending upon the heterocyclic structure of the enamine. Whilst a C-acylated enamine and a lactam-fused heterocycle were obtained from the reaction of the five- and seven-membered heterocyclic enamines, respectively, the six-membered heterocyclic enamine gave 2-oxo-5,6,7,8-tetrahydro-2H-pyrido[3,2-b]pyran. The reaction mechanisms are discussed.