Mesoionic purinone analogs. VIII. Synthesis and properties of mesoionic 5‐substituted‐6‐methylimidazo[1,2‐c]pyrimidine‐2,7‐diones

Mesoionic imidazo[1,2‐c]‐pyrimidine‐2,7‐diones 1a‐c, analogs of purine‐2,8‐dione, were prepared from 4‐amino‐l‐methylpyrimidin‐6‐ones 6a‐c. These mesoionic purinone analogs were found to exist predominantly in the C3‐H tautomeric form 1 and to undergo hydrolytic ring‐opening reactions to produce 2‐(4‐imidazol‐idon‐2‐ylidenyl)acetamides. Reaction of 1c with dimethyl acetylene dicarboxylate produced triazacyclopent‐[cd]indene 25 via 1,3‐dipolar cycloaddition.