A series of new N-(arylaminosulfanyl)- and N-(heteroarylaminosulfanyl)-phthalimide derivatives 3i–m and 3o–r, has been prepared by reaction of chlorosulfanylphthalimide with the trimethylsilyl derivative of the appropriate arylamine or heteroarylamine. On treatment with triethylamine at room temperature, most of these compounds 3 fragment to yield transient thionitroso species, Ar-NS and Het–NS, 4, which have been intercepted, generally in good yield, with conjugated dienes (2,3dimethylbuta-1,3-diene, isoprene, chloroprene and penta-1,3-diene) to yield cyclic 1,2-thiazine Diels–Alder adducts, and with alkenes (1-methylcyclohexene, α-pinene and β-pinene) to yield acyclic ene adducts. Competitive Diels–Alder and ene addition is observed with dimethylbutadiene and isoprene. The regiochemistry of addition of unsymmetrical dienes to thionitroso species has been elucidated. Sulfilimine 24 rearranges quantitatively to the dihydrothiophene derivative 27, thereby excluding compounds 24 as intermediates in the formation of 1,2-thiazine adducts 5.