1.3.5- Oxadiazines, tetrahydro-, conformation

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

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. 

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. 

There have been no further reports on the conformation of 1,3,4-oxadiazine and 1,3,4-thiadiazine systems since the early studies on tetrahydro-3,4-dimethyl-1,3,4-oxadiazine <76JCS(P2)I86I, 77JCS(P2)1816> and the corresponding dimethyl 1,3,4-thiadiazines <78CHE1281,80JCS(P2)279>. 

The X-ray crystal structures for 2-(4-bromophenyl)-4-(2-fluoroethyl)-5,6-dihydro-4//-l,3,4-oxadiazine hydrochloride 19 <1995CJC853>, 3-A -propionyl-4-A -bornyl-5-methyl-4-phenyl-3,4,5,6-tetrahydro-l,3,4-oxadiazin-4-one 20 (R = A -bor-nyl) <2005TA1047>, 2-phenylimino-3-methyl-perhydropyrido[l,2][l,3>4]-oxadiazme 21 <1997H(45)927> and 4,5-dimethyl-6-phenyl-3-phenylacetyl-3,4,5,6-tetrahydro-2//-l,3,4-oxadiazin-2-one 22 <2001T9789> have been recorded. The X-ray crystal structure of 3,5-dimethyl-6-phenyl-3,4,5,6-tetrahydro-l,3,4-oxadiazin-2-one derivative 23 has been reported <2004JOC727>. The structure of 23 may be described as a twisted boat conformation. 

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