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Simple and efficient synthesis of a novel fused bicyclic heterocyclic compound 3 by the reaction of ethyl 2-cyano-3,3-bis(methylthio)acrylate and thiourea in the presence of potassium carbonate in dimethylformamide under reflux conditions is reported. The optimized molar ratio of the substrates is 2:1. The parent compound was used for further derivatization by using a synthetic strategy that is based on suitability of the substituted pyrimido[2,1-b][1,3]thiazine system to react as a bis-electrophilic species with various nucleophiles. Products 4a–e and 5a–e were obtained in good yields (65–76%).

3,8-Dichloro-4H-[1,2,4]triazino[3,4-b][1,3]benzothiazole-4-one on reaction independently with sodium hydroxide solution and sodium azide undergo novel ring contraction reactions to give 7-chloro[1,2,4]triazolo[3,4-b][1,3] benzothiazole and 8-chloro tetrazolo[1′,5′:1,5][1,2,4]triazolo[3,4-b][1,3]benzothiazole respectively.

The 2-amino-7-methoxypyrimido[4,5-b]quinoline (1) on treatment with bis(methylthio)methylene malononitrile (2) in ethyl alcohol and catalytic amount of TEA gives 3-cyano-4-imino-9-methoxy-2-methythio-4H-pyrimido[2,1-b]pyrimido[4,5-b]quinoline (3). The latter were further reacted with selected N-, O-, and C-nucleophiles such as aryl amines, hetryl amines, substituted phenols, and compounds containing an active methylene group.