On the conformational preferability of 5,6-dihydro-4[H]-1,2-oxazines, their N-oxides and acyclic analogues

The most preferable conformation of 5,6-dihydro-4[H]-oxazines and their N-oxides is a distorted semi-chair, in which only the C(6) atom considerably deviates from the plane of the C = N bond, whereas ‘planar’ conformations, in which the O-alkyl group is at the cis position with respect to the free electron pair of the nitrogen atom or to the N→O fragment, are the most favourable for O-alkyl derivatives of acyclic oximes and alkyl nitronates, respectively.