Mesoionic 1,2,4-triazolo pyrimidines

Heating the mesoionic l-amino-2-thioxo-l,2,4-triazolo[l,5-c]quinazo-lines 59 with aromatic aldehydes and ethanolic hydrochloric acid resulted in the formation of Schiff bases and simultaneous pyrimidine ring cleavage... 

A number of mesoionic l,2,4-triazolo[4,3-a]pyrimidines (15) were obtained upon desulfurization of l-methyl-l-(4,6-dimethylpyrimidin-2-yl)thiosemicarbazides (13) with dicyclohexylcarbodiimide (DCC) [88JCS(CC)506 93JCS(P1)705] or by cyclization of l-alkyl-l-(4,6-dimethylpyrimidin-2-yl)hydrazines (14) with carbon disulfide or phosgene [88JCS(P1)351] (Scheme 8). 

The mesoionic l,2,4-triazolo[l,5-c]quinazolines 59 were obtained upon cyclization of the 4-thioxo-l,3-benzothiazines 57 with thiocarbohydra-zine through the intermediate 3-(4-thiosemicarbazido)pyrimidines 58 (86-JHC43) (Scheme 19). 

The mesoionic l-alkyl-3-alkylamino-l,2,4-triazolo[4,3-c]pyrimidines 37 were obtained by cyclization of the l,4-dialkyl-l-(pyrimidin-4-yl)thio-... 

The mesoionic 1-alkyl-1,2,4-triazolo[4,3-c]pyrimidinium-3-aminides 37 were incapable of undergoing Dimroth rearrangement the 5-alkenyl-l,2,4-triazolo[4,3-c]pyrimidines 78, formed as a consequence of pyrimidine ring lysis with water, methanol, or ethanol at room temperature, were incapable of recyclization [97JCS(P2)49] (Scheme 33). 

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