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 hydrochlorid e 1 . HCI was achieved starting from di-tert-butyl hydrazodiformate (8). Th e route entails cyclization of 8 with 1,3-dibromopropane under phase transf er conditions followed by deprotection of 1,2-di-tert-butoxycarbonylpyrazol idine (9) to give pyrazolidine hydrochloride (2 . HCl). Cyanoacetylation of the latter and ring closure of the resulting cyanoacetyl pyrazolidine (7) gave 1 . HCl. This novel synthesis circumvents distillation of pyrazolidine (2) and flash chromatography to provide the hydrochloride of 1 in 35-46% o verall yields compared to 6% yield for the patent procedure.