Tetrahydro-1,3,5-oxadiazines

Recently, a variable-temperature study of the 13C-NMR of 3,4-dimethyl-hexahydro-l,3,4-oxadiazine and of the trans-fixed derivative 434 have completely clarified the conformational equilibria and allowed a rather complete energy contour to be constructed for the monocyclic oxadiazine (Fig. 17).344 The conformational analysis of other methyl-substituted tetrahydro-1,3,4-oxadiazines345,346 is consonant with these results. 

Potekhin and Ye. A. Bogan Kova, Khim. Geterotsikl. Soedin., 1461 (1973). 

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Z,Z)-l,4-Di-tm-butoxy-l,3-butadiene in acetone gives two [4 + 2] adducts 10 and two acetone insertion products 11 (tetrahydro-1.3,4-oxadiazines) via a polar mechanism through intermediate cis- and fran.s-diazetidines26. 

Oxadiazine, tetrahydro-conformation, 3, 1054 ring-chain tautomerism, 3, 1056... 

By analogy with the tetrahydro-l,3,5-oxadiazine system, the ae conformer 466 of 3,5-dialkyltetrahydro-l,3,5-thiadiazines (465) should predominate in the conformational equilibrium. Investigation of a series of 3,5-dialkyl derivatives by H-NMR disclosed ring-inversion barriers that decrease with increasing size of the substituent (465 R = Me, AG 12.1 + 0.3 R = Et, AG 12.0 + 0.2 R = iPr, AG 10.5 + 0.4 kcal mol ). No further changes in the H-NMR spectra on cooling were found. Dipole-moment data indicated that all the compounds existed predominantly in the ae conformation.356 It is difficult to draw conclusions for Jtem in this series. 

The only preparation of a ring system of this type from thioureas, which has been reported in the literature, is the tetrahydro-1,3,5-oxadiazine series of Seidel and Boettner,302 who synthesized compounds 171 by treating thioureas with formaldehyde in the presence of p-toluenesulfonic acid. 

When the author reacted 2-chloro-5-chloromethyl-thiazole, (II), with 3-methyl-4-nitroamine-l,2,3,6-tetrahydro-5-(2-chloro-thiazole-4-yl)-methyl-l,3,5-oxadiazine, the Step 3 product, 3-methyl-4-nitroamine-l,2,3,6-tetrahydro-5-(2-chloro-thiazole-4-yl)-methyl-l,3,5-oxadiazine, (III), was isolated. 

H-l,2,5-Oxadiazine 2-Alkyl-5-phenyl-2,3,5,6-tetrahydro- EI6a. 311 (aus Imidazolidin)... 

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