A new route to (-)-agelastatin A is reported. The requisite nitrogen functionalities of the agelastatin core have been installed by intramolecular aziridination of an azidoformate and subsequent regioselective azidation, leading to net trans-diamination of the double bond. The present synthesis also demonstrates two new protocols for the preparation of an imidazolidinone hemiaminal motif from an oxazolidinone intermediate which comprise sequential N-tert-butoxycarbonylation, urea formation, hydrolysis, and oxidative cyclization, and direct aminolysis and subsequent oxidative cyclization.