Mononuclear heterocyclic rearrangement: Synthesis of [5:5] bicyclic [c]-fused 3-aminopyrazoles via the N-N bond formation strategy

The formation of [5:5] bicyclic heterocyclic ring systems containing [c]pyrazoles, i.e. imidazo[4,5-c]pyrazole, pyrazolo[3,4-c]pyrazole, pyrrolo-[2,3-c]pyrazole, and pyrazolo[3,4-d][1,2,3]triazole, was accomplished by mononuclear heterocyclic rearrangement (MHR). The core pyrazole ring was formed based on a N-N bond formation strategy. The ring transformation of 5-substituted 3-(2-aminoaryl)-1,2,4-oxadiazoles (14, 15a-b, 16b and 33) under thermal conditions to the corresponding [5:5] bicyclic [c]-fused 3-aminopyrazole ring systems (17a, 18a-b, 20 and 34 respectively) was promoted by sodium hydride in DMF or DMSO. The ring transformation by MHR has provided a practical and general synthetic method for the derivatives of 3-aminoimidazo-[4,5-c] pyrazole (4), 3-aminopyrazolo[3,4-c]pyrazole (5), 3-aminopyrrolo[2,3-c]-pyrazole (6) and 6-aminopyrazolo[3,4-d][1,2,3]triazole (7).