AbstractThermal reactions of 1,2,3‐trisubstituted aziridines1with 1,3‐thiazole‐5(4H)‐thiones6in toluene yielded, in general, a mixture of two diastereoisomeric spirocyclic [2+3] cycloadducts. The formation of these products can be explained by a stereoselective electrocyclic ring opening of1to give an azomethine ylide2as the reactive intermediate, which is trapped immediately by6viaa stereoselective 1,3‐dipolar cycloaddition. Only in the case oftrans‐dimethyl 1‐(4‐methoxyphenyl)aziridine‐2,3‐dicarboxylate (trans‐1a), four diastereoisomeric cycloadducts were formed (Scheme 4). This result is rationalized by an isomerization of the intermediate azomethine