The reaction of 3-thiazolines 1a-c with alpha- or beta-mercaptocarboxylic acids leads to bicyclic lactams 2-4. Depending on the reaction conditions diastereomeric mixtures 2b, 3, 4a,b are formed. The synthesis of the latter can be explained by a two-step mechanism. The configuration and conformation Of the diastereomers is deducted by H-1, C-13-NMR spectroscopy and NOE experiments.