Cycloaddition‐elimination reactions of 4‐methyl‐5‐(substituted)‐imino‐Δ 2 ‐1,2,3,4‐thiatriazolines with isocyanates

4‐Methyl‐5‐(substituted)imino‐1,2,3,4‐thiatriazolines 1 (R 2 ≠ Me) undergo cycloaddition‐elimination reactions with isocyanates to yield 4‐methyl‐5‐(substituted)imino‐1,2,4‐thiadiazolidine‐3‐ones 5 via the thermodynamically less stable isomers 4 . The latter have not been isolated, except for 4q which was shown to iso‐merize rapidly into 5q with phenylsulfonyl isocyanate. The reactions of 1 are accelerated by using less bulky R 2 substituents and more electrophilic isocyanates, in accordance with the viewpoint that 1 reacts as a masked 1,3‐dipole. The products 4i‐n (= 5i‐n ), derived from 1b , add isocyanates reversibly to give 2,3,4,5‐tetrahydro‐6aλ 4 ‐thia‐1,3,4,6‐tetraazapentalene‐2,5‐diones 9i‐n , which have been isolated and characterized spectroscopically. Such compounds with a hypervalent sulfur atom thus occur as intermediates during the isomerization of 4 to 5 under the influence of isocyanates.