Siu Chung Tsoi

Photochemical rearrangement of fused 1λ4,2-thiazines (2-azathiabenzenes); rapid migration of methylthio and phenylthio groups

Irradiation of fused 1λ4,2-thiazines (1) gives the isomeric fused pyrroles (2). The structures of the thienopyrroles (2a,b) are assigned by decoupling and n.O.e. experiments on the thieno[3,2-b]pyrroles (4a,b) formed from them by hydrolysis and decarboxylation. A mechanism is proposed for conversion of thiazines (1) into pyrroles (2)(Scheme): nitrenes (8) are formed and cyclise to the 2H-pyrroles (9) which aromatise by [1,5] SR and H shifts. Surprisingly, the SR groups migrate faster than ester, acetyl, or formyl.

Thermal rearrangements of azathiabenzenes; [1,4] shifts in cyclic sulphur–nitrogen ylides

The azathiabenzene derivatives (3), (7), and (11) rearrange on heating by a [1,4] shift of the sulphur substituent ot carbon rather than a [1,2] shift to nitrogen, even though for ylides (3) this rearrangement involves loss of thiophen and furan aromaticity.

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

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.

Thermal rearrangements of fused 1λ4,2-thiazines (2-azathiabenzenes); [1,4] shifts in cyclic sulphur–nitrogen ylides

When heated in xylene the fused 1λ4,2-thiazines (3), with the exception of the benzo compound (3c) which is thermally stable, rearrange to the 4aH-isomers (4). S-Phenyl groups migrate exclusively 1,4 to carbon with consequent disruption of the thiophene and furan aromaticity. This thermal [1,4] shift occurs more readily when aromaticity is not disrupted; thus thienothiazine (7) isomerises to (8) in toluene, and benzothiazine (11) isomerises to (12) spontaneously on formation in xylene. With the S- methyl compound (3g) this rearrangement is accompanied by proton tautomerism and [1,2] rearrangement to give the thienothiazepine (5) and, at higher temperature (156 °C), the thienopyrrole (6) is formed (Scheme 2). The 4aH-thienothiazine (4a) rearranges more extensively at higher temperatures to give, sequentially, the pyrrolo[1,2-b]isothiazole (13) at 156 °C (Scheme 5) and the thiophene (17) at 180 °C (Scheme 6).

Ring cleavage of 3-azidothiophens; a novel extrusion of acetylene

Mild thermal decomposition of the 3-azidothiophen (1) results in cleavage of the thiophen ring with extrusion of acetylene and formation of the isothiazole (3).

Ring cleavage of a 3-azidothiophene: novel extrusion of acetylene

Mild thermal decomposition of ethyl 2-azido-3-(3-azido-2-thienyl)propenote (8) results in the cleavage of the thiophene ring with extrusion of acetylene and formation of ethyl 5-cyanoisothiazole-3-carboxylate (10), together with ethyl thieno[3,2-c]pyridazine-3-carboxylate (9). The former reaction represents the first fragmentation of a five-membered heteroaromatic β-nitrene to extrude an acetylene, and the latter reaction is a rare example of formal intramolecular coupling of nitrenes in solution thermolysis. The vinyl azide group is necessary for this cleavage of the thiophene ring and a mechanism is proposed in which the derived nitrene co-ordinates with the thiophene sulphur atom to faciliate ring fragmentation. The analogous furan does not fragment similarly. Diels–Alder cycloadditions of 4-phenyl-1,2,4-triazole-3,5-dione to 2-vinylthiophenes were investigated as an independent approach to the thieno[3,2-c]pyridazine system, but the fused 4-phenyl-1,2,4-triazoline-3,5-dione ring proved resistant to hydrolytic cleavage.

Synthesis and X-ray structure of an azathiabenzene derivative, a cyclic sulphimide

Decomposition of azidothiophenes (3) gives the thieno[3,2-c][1λ4, 2]thiazines (4), which are stable cyclic sulphimides, the ylidic nature of (4a) being confirmed by its properties and X-ray structure determination; photolysis of the sulphimides (4) gives the thieno[3,2-b]pyrroles (6) suggesting that MeS and PhS undergo very rapid [1,5] sigmatropic shifts.