Regioselective 1,3‐Dipolar Cycloadditions of a ‘Thiocarbonyl‐methanide’ ((Alkylidenesulfonio)methanide) with Aromatic SulfinesReaction of the spirocyclic 2,5‐dihydro‐1,3,4‐thiadiazole7and thiobenzophenoneS‐oxide (6a) in THF at 45° yielded the spirocyclic 1,3‐dithiolane 1‐oxide8, thiirane9, and the diazane derivative10in a ratio of 61:15:23 (Scheme 2). The formation of8is rationalized by a 1,3‐dipolar cycloaddition of ‘thiocarbonyl‐methanide’1, generated from7by thermal elimination of N2, and the CS bond of sulfine6a. Cyclization of intermediate1leads to thiirane9. Under the same conditions,7and adamantane‐2‐thioneS‐oxide (6b) or 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanoneS‐oxide (4) reacted to give only9and10but no cycloadduct of type8(Scheme 4). With the aim to favor the formation of8, a mixture of6aand 1.1 equiv. of7was heated to 45° without any solvent in a sealed tube. The ratio of products was only slightly different from that of the thermolysis in THF. An analogous experiment with7and 9H‐fluorene‐9‐thioneS‐oxide (6c) yielded cycloadduct13and9(Scheme 5). It is most interesting that the 1,3‐dipolar cycloadditions of1and the sulfines6aand6cproceeded with different regioselectivity. A reaction mechanism for the unexpected formation of10is proposed inScheme 7. The key step is the base‐catalyzed ring opening of7and the nucleophilic addition