Asymmetric synthesis of 1,3-thiazolidine-derived spiro-beta-lactams via a Staudinger reaction between chiral ketenes and imines

Enantiomerically pure 1,3-thiazolidine-derived spiro-beta-lactams were stereoselectively synthesised by means of a Staudinger ketene-imine reaction starting from optically active N-Boc-1,3-thiazolidine-2-carboxylic acid derivatives and imines. The reactions were stereoselective and afforded spiro-beta-lactams with a relative trans-configuration. The absolute configuration of the new stereocentres was assigned on the basis of the well-accepted mechanism and confirmed by means of the X-ray crystal structure analysis. The spiro-beta-lactams were transformed into enantiomerically pure chiral monocyclic beta-lactams by opening the thiazolidine ring and recovering the chiral auxiliary.