1,2,6-Thiadiazine 1,1-dioxides aromaticity

Decarboxylation of P-keto-esters is facile in aqueous DMSO, but in less reactive cases decarboxylation is accelerated by sodium chloride. Condensation reactions of dibenzocycloheptadienone (334) with aromatic aldehydes have been reported. Syntheses of the heterocyclic carboxylic acids (335) have been described. No effect of the heterocycle s ring current on the chemical shifts of the methylene protons could be detected in the n.m.r. spectrum of the nonamethylene-thiadiazine dioxide (336), which was prepared from cyclododecane-l,3-dione. ... 

For the benzo-fused derivative of the parent 1,2,6-thiadiazine 1,1-dioxide, the aromatic tautomer 125a (Scheme 35) was found to be the most stable (82JOC536). 

Substituted 2 f-l,2,4-benzothiadiazin-3(4//)-one 1,1-dioxides are available by a one-pot intramolecular Friedel-Crafts reaction involving the action of chlorosulfonyl isocyanate on a secondary aromatic amine in the presence of aluminum chloride <85IJC(B)1295>. Chlorosulfonyl isocyanate is also employed in the synthesis of oxazolo- and thiazolo[2,3-z(][l,2,4]-thiadiazin-3(2//)-ones (196 X = O and S) from 2-ethyloxazolines (195 X = O) and 2-ethylthiazolines (195 X = S) respectively (Scheme 33) <93SC121>. 

A detailed comparative study of the chemistry of 1,2,6-thiadiazine 1,1-dioxides and related pyrazoles has been carried out <82JOC536>. Useful comparisons can be made regarding tautomeric equilibria, C NMR chemical shifts, and the general reactivity of the 4-positions. As expected however, the lack of aromaticity of the 1,2,6-thiadiazine 1,1-dioxide system (Section 6.16.4.1) imparts substantial differences to their general chemistry. 

Ab initio calculations on the three tautomeric forms of 2//-1,2,6-thiadiazine 1,1-dioxide reveals that for the 2H- (17) and 4H- (20) tautomers the planar conformers are less stable than the nonplanar boat conformers by 2.7 and 1.2 kcal moU, respectively. For the 3,5-dimethyl- derivative there is reasonable agreement between the calculated bond angles and those obtained by x-ray crystallographic measurements <90JP0470>. In contrast, for the fully conjugated tautomer (18) theoretical calculations reveal that the planar form is the more stable and that aromatic character is to be... 

In contrast, on the basis of a new aromaticity index (Ig) for six-membered heterocycles, 1,2,6-thiadiazin-4-one 1,1-dioxide (60) has been assigned an value of 54.2 (benzene 1 = 100) and is reckoned to have moderate to low aromatic character and is deemed to be more aromatic than pyranones but less aromatic than 4-pyridones <86T89>. 

Thiadiazine 1,1-dioxides (389) have been synthesized by Ochoa s group and a comparative study of their physiochemical properties with that of pyrazoles 390 has been made371,372. Rough parallels are observed in the tautomeric equilibria of (389) and (390) with 13C chemical shifts and the reactivity of the 4-position while differences in their aromaticity have been observed. The 1,2,6-thiadiazine-1,1-dioxides 389 are prepared either by reaction of sulphamide or substituted sulphamides with 1,3-dicarbonyl compounds or their acetal derivatives or by AT-alkylation of the unsubstituted derivatives. 

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