[2+2] Cycloaddition Reactions of Imines with Cyclic Ketenes: Synthesis of 1,3‐Thiazolidine Derived Spiro‐ β ‐lactams and Their Transformations

Unsymmetric cyclic ketenes were generated from N ‐acyl‐1,3‐thiazolidine‐2‐carboxylic acids 1a – c by means of Mukaiyama 's reagent, and then reacted with imines 2a – c to the new, isomeric spiro‐ β ‐lactams 3 and 4 via [2+2] cycloaddition ( Staudinger ketene–imine reaction; Scheme 1 ). The reactions were stereoselective ( Table 1 ) and mainly afforded the spiro‐ β ‐lactams with a relative trans configuration. The spiro‐ β ‐lactams could be transformed into the corresponding monocyclic β ‐lactams by means of thiazolidine ring opening or into substituted thiazolidines via hydrolysis of the β ‐lactam ring.