Synthesis of fused 1λ4,2-thiazines(2-azathiabenzenes)

Abstract

By analogy with the formation of acyclic sulphimides from sulphides and azides, mild thermolysis of aryl, hetaryl, and vinyl azides with a 1,6-related, conjugated thioether function gives the cyclic sulphimides, 1λ⁴,2-thiazines. Thus in boiling toluene the azidothiophenes (9), (12) and (18), the azidobenzenes (14), and the azidofuran (16) give the fused 1λ⁴,2-thiazines (10), (13), (19), (15), and (17) respectively. Yields are high when the products are stabilised by delocalisation of the ylidic (N^––S⁺) charges, but low when they are not. The S-allyl sulphimide (10c) is not isolable however since it rearranges spontaneously to the thienopyrrole (11). In contrast with azidothiophene (12), the ‘reversed’ thiophenes (20) do not give the corresponding 3,4-fused thienothiazines; the nitrene from the azide (20a) cyclises preferentially to carbon to give thienopyrrole (21) and that from the azide (20b) inserts into a methyl group to give the thienopyridine (22). Preferential cyclisation onto carbon rather than sulphur is also observed in the benzene series; thus azides (24) give 4-thio substituted indole-2-carboxylates (25) in good yield. The spectroscopic properties and thermal stabilities of all the 1λ⁴,2-thiazines prepared show them to be pyramidal cyclic S–N ylides rather than planar aromatic systems.