Heterocyclic syntheses with malonyl chloride. Part XI. Reactions of 2-alkyl-(or -aryl-)thio-7-chloropyrano[3,4-e][1,3]oxazine-4,5-diones with water and with alcohols

Abstract

When heated with water in dioxan, the title compounds lost carbon dioxide and thiol in undergoing a complex degradation to 4-chloro-6-hydroxy-2-pyridone. Under milder conditions, S-alkyl (or -aryl) 6-amino-4-chloro-2-oxopyran-5-thiocarboxylates were obtained. With an excess of hydrogen chloride and 1 molar proportion of water in boiling dioxan, S-alkyl (4-halogeno-2-oxopyran-6-yl)thiocarbamates resulted. An excess of boiling ethanol produced ethyl 4-chloro-1,2-dihydro-6-hydroxy-2-oxopyridine-3-carboxylate, whilst under milder conditions there was addition to the 2,3-double bond or substitution of the 2-thio-group, or both, followed by elimination opening of the oxazine ring between the 1- and 2-positions. With 1 molar proportion of ethanol in boiling dioxan, S-alkyl (6-chloro-3-ethoxycarbonyl-2-oxopyran-4-yl)thiocarbamates resulted. Spectroscopic and chemical evidence for the constitutions of the products is described, and modes of formation are discussed.