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

Under very mild conditions heterocyclic secondary enamines react efficientl y with malonyl chloride to produce hydroxylated 2-pyridinone-fused heterocy cles. The reactions of enamines with oxalyl chloride, however, afford varie d 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, respective ly, the six-membered heterocyclic enamine gave 2-oxo-5,6,7,8-tetrahydro-2H- pyrido[3,2-b]. The reaction mechanisms are discussed. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.