Synthesis of 1,2-diazetidinones (aza-β-lactams) by photochemical ring contraction

Abstract

Irradiation of 4-diazopyrazolidine-3,5-diones (11) in the presence of alcohols, diethylamine, or water gives 1,2-diazetidinones (12) formed by photochemical Wolff rearrangement with ring contraction followed by reaction of the resulting ketene with the nucleophile. In the case of the bicyclic diazo compound (11d) a fragmentation reaction competes with ring contraction. The aza-β-lactams (12) show the expected high frequency carbonyl stretch in their i.r. spectra. The acid (12e) readily decarboxylates to give the 4-unsubstituted 1,2-dibenzyldiazetidinone (28), the four-membered ring of which is cleaved by alkaline hydrolysis, lithium aluminium hydride or diborane to give (29), (30), and (31), respectively (Scheme 9). Attempts to modify the carboxy substituent of (12e) into an acylamino group were unsuccessful.