The conformational analysis of tetrahydro-1,4,2-dioxazines

Abstract

Abstract A conformational study of some tetrahydro-1,4,2-dioxazines by use of 1 H and 13 C NMR spectroscopy is reported. The conformational characteristics of this ring are compared to those of the related systems, tetrahydro-1,2- and 1,3-oxazine. A study of model compounds allows the assignment of ring and nitrogen inversion processes in the variable temperature NMR spectra. Ring inversion is found to be a lower energy process than nitrogen inversion. The barriers to these processes are measured in several derivatives and the implications of the results for studies of nitrogen inversion in other 6-membered rings are pointed out. The conformational free energy differences of N-Me and N-Et groups are measured and discussed. It is somewhat easier to put an N-Et group axial than N-Me ( ca .0.25 kcal mole −1 ). It also appears than an Et group at C-6 goes axial more readily than does a Me.