Regioselectivity in the photochemical ring contraction of 4-diazopyrazolidine-3,5-diones to give aza-β-lactams

Abstract

Irradiation of 4-diazopyrazolidine-3,5-diones (15) in the presence of alcohols or water gave mixtures of the isomeric 1,2-diazetidinones (16) and (17), formed by competing photochemical Wolff rearrangement of the two nitrogen groups, followed by reaction of the resulting ketenes with the nucleophile. Some regioselectivity is observed in the ring contraction process, and the relative order of migration of nitrogen groups is NPh > NCHPh₂∼ NCH₂Ph ∼ NMe > NCH₂CO₂Et. The structures of the 1,2-diazetidinones (17c) and (24) were confirmed by X-ray crystallography, and a crystal structure of the diazo compound (15g) was also obtained. Possible reasons for the regioselectivity in the ring contraction are discussed.