[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 la-c by means of Mukaiyama's reagent, and then reacted with imines 2a - c to the new, isomeric spiro-beta-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-beta-lactams with a relative trans configuration. The spiro-beta-lactams could be transformed into the corresponding monocyclic beta-lactams by means of thiazolidine ring opening or into substituted thiazolidines via hydrolysis of the beta-lactam ring.