Stereoselective synthesis of 4-substituted azetidine-2,3-diones by ring opening of 1,3-thiazolidine-derived spiro-β-lactams

New 3-heterocycle substituted 1,3-thiazolidine-derived 4-spiro-β-lactams with a relative trans-configuration were stereoselectively synthesised by means of a Staudinger ketene-imine reaction between the ketene generated from the (2S,4R)-1,3-thiazolidine-2,3,4-tricarboxylic acid 3-(1,1-dimethylethyl) 4-methyl ester 1 and imines 2b-e. The 1,3-thiazolidine-derived 4-spiro-β-lactams were transformed into the corresponding enantiomerically pure 4-heterocycle substituted azetidine-2,3-diones by means of an oxidative cleavage of the 1,3-thiazolidine ring. The opening of the 1,3-thiazolidine ring was studied under different experimental conditions and a consistent mechanism is proposed.