Structure and reactions of a thiazolino[3,2‐ a ]pyrimidine carbinolamine

Ethyl 4‐chloroacetoacetate condenses with 4‐amino‐6‐hydroxy‐2‐pyrimidinethiol to yield ethyl 3‐hydroxy‐5‐oxo‐7‐aminothiazolino[3,2‐ a ]pyrimidin‐3‐acetate ( 3 ), and not ethyl 3‐hydroxy‐5‐amino‐7‐oxothiazolino[3,2‐ a ]‐pyrimidin‐3‐acetate, as previously reported. This compound reacts with ammonia to produce ethyl 4‐(6′‐amino‐4′‐oxo‐2′‐pyrimidinethiol)‐3‐aminocrotonate ( 8 ), the open chain structure of which accounts for the cyclization of 3 to the poly‐cyclic lactam, 6a‐hydroxy‐5,6,6a,7‐tetrahydro‐8‐thia‐1,4‐diazacycl[3.3.2]azine‐2,5‐dione, rather than the sterically more favorable lactone, when 3 was treated with triethylamine. Some other reactions of 3 are described, and the structures of 3 and 8 were comfirmed by single crystal X‐ray diffraction analysis.