Abstract: The synthesis and antibacterial activity of a series of 2,6-substituted 5,8-dihydro-8-ethyl-5-oxopyrido (2,3-d)pyrimidine are reported. The product from ethyl 2-chloro-4-(2-methoxycarbonyldiethylamino) pyrimidine-5-carboxylate (Ⅳ) through Dieckmann cyclization is determined as methyl 2-chloro-7,8-dihydro-8-ethyl-5-hydroxypyrido (2,3-d) pyrimidine-6-carboxylate (Ⅴ). It exists in the enol form, not the keto form (Ⅵ) as reported by Pesson.