Tetrahydro-l,4,2-dioxazines

The conformational equilibria for tetrahydro-1,4,2-dioxazines (425) should combine the characteristic features of the equilibria for tetrahydro-l,3-oxa-zines (Section III,D,1) and tetrahydro-l,2-oxazines (Section III,C,1). Thus by analogy with the 3-methyltetrahydro-l,3-oxazine equilibrium (AG° N-Meeq N-Me -0.10 0.05 kcal mol 1 at -120°C, Table XX), in the 

Comparison of the conformational free energies for the (V-methyl group in Af-methylpiperidine (2.7 kcal mol- ) and in Af-methyltetrahydro-1,3-oxazine (AG° 0 kcal mol1) with the value of 1.0 kcal mol-1 for the 1,4,2-dioxazine just mentioned, suggests that the corresponding free energy of the methyl group in lV-methyltetrahydro-l,2-oxazine may be nearer 2.7 + 1.0 = 3.7 kcal mol-1. 

2-Ethyltetrahydro-1,4,2-dioxazine (425 R = Et) showed an increased preference for the axial N-Et conformation (AG 8r 0.72 kcal mol-1) (compare 

A/-methyl- and /V-ethyltetrahydro-1,3-oxazine in Table XXI).340 The 6-ethyl group also shows a greater axial preference than 6-methyl.341 In addition, in line with expectations based on similarly substituted 1,3-heterocyclic systems (Section III,D) the AC 82 of 0.61 kcal mol-1 for 2,3,3-trimethyltetrahydro- 

2-dioxazine (430 431) showed an increase in the A/-Meax conformation relative to 426 (R = Me).340 Two inversion processes characterized by AG  

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Tetrahydro-l,4,2-dioxazines 205 were prepared from aromatic aldehydes 204 and /3-ureidoxyalcohols 203 in dry benzene and/ -toluenesulfonic acid at reflux (Equation 58) <2000APF180>. 

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