Dihydro-1,4-thiazines conformation

Theoretical methods have been used to undetstand and predict charge densities and oxidation potentials, nuclear magnetic resonance (NMR) shifts and cycloconjugation in 1,4-thiazines, and the lowest-energy conformation in a dihydro-1,4-thiazine. 

The lowest energy conformations of dihydro-1,4-thiazine 56 (Figure 5) were calculated by computer using a molecular dynamics simulation and the information was used to calculate the geminal coupling constants <1995JFA2195>. 

Figure 5 Lowest-energy conformations of dihydro-1,4-thiazine 56.

The difference in conformational free energy of the N-Me group in N-methylpiperidine (2.7 kcal mol-1, favoring N-Meeq) and in N-methyltetra-hydro-l,3-thiazine (0.7 kcal mol-1, favoring lV-Meax at -120°C) suggests the predominance of the lV-Meax conformer 463 of 5-methyldihydro-1,3,5-dithiazine (462 R = Me). Indeed, H-NMR spectra and dipole-moment data show that NR axial is the predominant conformer for a series of AT-alkyl-dihydro-l,3,5-dithiazines (462 R = Me, Et, iPr, -Bu).356 This even applies to the Af-tert-butyl compound, which possesses an unconstrained axial tert-butyl group.357... 

Of the two possible tautomeric structures for monocyclic dihydro-1,4-oxazines and -thiazines the 3,4-dihydro-2/f-representation is preferred in which the ring has a half-chair conformation. 

The X-ray structures of two derivatives of 3,6-dihydro- 1,2-thiazine 1-oxide 137 indicate (cf. Scheme 51) (02AX165, 02AX198) the six-membered ring to be a half-chair conformer with the S=0 bond in an quasi-axial position. 

An investigation of the n.m.r. spectra of some 3-substituted 2,3-dihydro-6-methoxycarbonyl-l, 4-thiazines (67) led Dunn and Stoodley to suggest that the 3-methyl group shows no conformational preference in CDCI3 or C5D5N solution, but that OH, OAc, OMes, Cl, and I, in position 3, prefer to be axial in both solvents. A 3-methoxycarbonyl, they suggest, is axial in CsDsN but equatorial in CDCI3. 

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