Synthesis and reactions of [1,2,4]triazolo[1,5-a]pyrimidinium betaines

A number of 3-substituted [1,2,4]triazolo[1,5-a]pyrimidinium betaines have been synthesized and the reactions of these novel compounds has been investigated. Treatment of 2-alkylaminopyrimidines with phosgene gave 2-(N-alkyl-N-chlorocarbonylamino)pyrimidines (2f–i) which reacted with azidotrimethylsilane to yield 3-substituted [1,2,4]triazolo[1,5-a]pyrimidinium-2-olates (3a–d). Analogous [1,2,4]triazolo[1,5-a]pyrimidinium-2-thiolates (3f–i) were obtained from the 2-olates by treatment with 2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphosphetane 2,4-disulphide (Lawesson's reagent). The 3-hydroxypropyl derivatives (3e) and (3j) were prepared by deprotection of the benzyl ethers (3d) and (3i) using boron trichloride in dichloromethane at –10 °C. The 2-olates (3a) and (3b) reacted with dimethyl acetylenedicarboxylate to give the pyridine derivatives (7a) and (7b) and the 2-olate (3b) reacted regioselectively with methyl propiolate to provide the analogue (7c). Reaction of the 2-olates (3a) and (3b) with diphenylcyclopropenone gave 1,3-oxazin-6-ones (12), Evidence for the structure of one compound (12b) was adduced, in part, by transformation with ammonia into the pyrimidone (13) and with sodium methoxide into two separable isomers of the urea derivative (14).