Sulfur extrusion from 1,2,6-thiadiazines

There are several examples of sulfur extrusion from 1,3,4-thiadiazines (144) either under basic conditions via the 877-anion (31) <75AG(E)581, 77S196), or by heating in acetic acid (70LA(741)45, 76JPR971). Both sets of conditions give pyrazoles in good yield. 

Arylamino-3-bromo-l-aza-1,3-butadienes (281 R = Br), prepared by the action of A-bromo-succinimide on the 1-azadienes (281 R = H), on treatment with thionyl chloride in cold pyridine furnish the 4-bromo-l,2,6-thiadiazine 1-oxides (282) in high yields (72-82%) (Equation (39)) <83JCS(Pl)2273>. In contrast, the 3-chloro-azabutadiene (281 R =C1) remains unchanged with thionyl chloride at room temperature, whereas at 70°C the enamino ketone (281 0=0 in place of C=NH) is formed rather than the chlorothiadiazine 1-oxide. At higher temperatures (100°C) the bromo- and chloro-1-azadienes yield 4-chloropyrazoles by thermal extrusion of sulfur monoxide from the initially formed 1,2,6-thiadiazine 1-oxides (Section 6.16.6.1). 

Kinetic parameters for the chalcogen extrusion from 1,3,4-thiadiazines and 1,3,4-selenadiazines 29 to pyrazoles have been determined <1993PHA732, 2006UP1>. It was found that the activation energy and enthalpy of activation are higher for 1,3,4-selenadiazines and the kinetic investigations indicate that the selenium derivatives are more stable than the sulfur-containing compounds (Table 1). 

Benzofuran 332 reacts with 1-phenylthiosemicarbazide to give the benzofuran derivative 333, which undergoes ring cleavage to the intermediate 334 and subsequent cyclization to the 1,3,4-thiadiazine 335 <2005H(65)1569>. The reactions of benzofuran 332 with 4,4-dialkylthiosemicarbazides proceed with extrusion of a sulfur atom from the 1,3,4-thiadiazine intermediate and the formation of pyrazole derivatives (Scheme 46) <2005H(65)1569>. 

Giordano and Belli have formed imidazoles from 2,4,6-triaryl-4/f-l,3,5-thiadiazines by a base-catalyzed extrusion of sulfur [77], Schmidt et al. also refer to an analogous extrusion of oxygen from the 4/f-l,3-oxazine [78]. Scheme 4 depicts two extrusion reactions used to form imidazoles. 

Diimines 1 react with sulfur dichloride or disulfur dichloride to give the 1,2,6-thiadiazines 2aj.23 When compounds 1 are treated with thionyl chloride, the 1,2,6-thiadiazine S-oxides 3a-l are formed.23,24 On heating in toluene to 90°C, compounds 2a-j are converted into the pyrazoles 4 by extrusion of sulfur. The pyrazoles 4 can also be obtained by heating the diimines 1 in pyridine for 8 hours with sulfur dichloride, disulfur dichloride, or thionyl chloride. In contrast, the tetrahydro-l,2,6-thiadiazines23 obtained from 1,3-diamines and sulfur dichloride cannot be converted to pyrazolidine derivatives by extrusion of sulfur. When compounds 2 a j or 3a-l are heated at 60 °C in tetrahydrofuran containing 6M potassium hydroxide, the starting materials 1 are recovered. 

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