[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.