Fused thiazolidine 11 and thiazoline derivatives 19a–c are obtained by intramolecular cycloaddition-elimination reactions of appropriately substituted 5-iminothiatriazolines 10 and 18a–c. The incorporation of a sidechain phenyl group excercises a favorable effect on cyclization since no reaction is observed with 25. Comparable intermolecular reactions also fail to occur.