A convenient synthesis of pyrazolidine and 3‐amino‐6,7‐dihydro‐1H,5H‐pyrazolo[1,2‐a] pyrazol‐1‐one

A convenient four‐step synthesis of the aminobicyclopyrazolone hydrochloride 1 ·HC1 was achieved starting from di‐tert‐butyl hydrazodiformate (8) . The route entails cyclization of 8 with 1,3‐dibromopropane under phase transfer conditions followed by deprotection of 1,2‐di‐tert‐butoxycarbonylpyrazolidine (9) to give pyrazolidine hydrochloride ( 2 ·HC1). Cyanoacetylation of the latter and ring closure of the resulting cyanoacetyl pyrazolidine (7) gave 1·HC1. This novel synthesis circumvents distillation of pyrazolidine (2) and flash chromatography to provide the hydrochloride of 1 in 35–46% overall yields compared to 6% yield for the patent procedure.