Synthesis of 1H-Imidazo[1, 2-a]pyrazolo[3, 4-c]pyridines

The reaction of nitrosyl chloride with 6- and 8-acetamido-7-methylimidazo[1, 2-a]pyridine (7g-i) reveal clear differences of reactivity of these isomeric structures. After bifunctionalization of the imidazolic moiety, the 6-acetamido derivatives do not yield the 1H-imidazo[1, 2-a]pyrazolo[4, 5-d]pyridine (4) system, but undergo a Gomberg-Bachman reaction complicated by Dimroth rearrangement. In contrast, upon similar treatment, the 8-acetamido compounds (17, 19) yielded the N-nitrosoacetamides (18a, b), which were converted into 1H-imidazo[1, 2-a]pyrazolo[3, 4-c]pyridines (20a, b) in 22 and 34% yields, respectively, without rearrangement.