1,3-Dipolar Cycloaddition Reactions of Thiazolo(5,4-d)pyrimidine 1-Oxides with Acetylenic Esters Involving New Ring Transformations of the Thiazole Nucleus.

The reaction of 2- (methoxycarbonyl) -4, 6-dimethylthiazolo [5, 4-d] pyrimidine-5, 7 (4H, 6H) -dione 1-oxide (1) with dimethyl acetylenedicarboxylate resulted in a new ring transformation of the thiazole nucleus to give 5- [2-methoxalyl-1, 2-bis (methoxycarbonyl) vinyl] imino-1, 3-dimethyl-6-thiobarbituric acid (4), 6, 7-bis (methoxycarbonyl) -1, 3-dimethylpyrimido [4, 5-b] [1, 4] thiazine-2, 4- (1H, 3H) -dione (8), 6, 7-bis (methoxycarbonyl) -1, 3-dimethylpyrrolo [3, 2-d] pyrimidine-2, 4 (1 H, 3H) -dione (9), and anhydro-2- [1-methoxalyl-1- (O-s-odiummethoxycarbonyl)] methyl-4, 6-dimethylthiazolo [5, 4-d] pyrimidine-5, 7 (4H, 6H) -dione hydroxide (11). Analogously, the reaction of 1 with ethyl phenylpropiolate gave 5 [2-benzoyl-2- (ethoxycarbonyl) -1- (methoxycarbonyl) vinyl] imino-1, 3-dimethy1-6-thiobarbituric acid (22), 5-benzoyl-7- (ethoxycarbonyl) -6- (methoxycarbonyl) -1, 3-dimethylpyrimido [4, 5-b] [1, 4] thiazine-2, 4 (1H, 3H) -dione (23), and 7- (ethoxycarbonyl) -6- (methoxycarbonyl) -1, 3-dimethylpyrrolo [3, 2-d] pyrimidine-2, 4 (1H, 3H) -dione (24). The structures of these products were supported by their spectral [infrared (IR), mass (MS), ultraviolet (UV), proton nuclear magnetic resonance (1H-NMR) or carbon-13 nuclear magnetic resonance (13C-NMR)] data or by single-crystal X-ray diffraction analysis as well as by chemical transformations. The ring transformation of the thiazole nucleus can be best explained by taking into account the initial involvement of a 1, 3-dipolar cycloaddition reaction.