Conformational analysis. XXVII—A 1H and 13C NMR conformational study of cis‐ and trans‐annelated 2‐p‐nitrophenyl‐4,5‐ and ‐5,6‐tetramethyleneper‐hydro‐ and 2‐p‐chlorophenyl‐4,5‐ and ‐5,6‐tetramethylenedihydro‐1,3‐oxazines

The 400 MHz 1H NMR spectra of cis‐ and trans‐annelated 2‐p‐nitrophenyl‐4,5‐ and ‐5,6‐tetramethyleneperhydro‐ and 2‐p‐chlorophenyl‐4,5‐ and ‐5,6‐tetramethylenedihydro‐1,3‐oxazines were recorded. The AMKX subspectra due to the protons at C‐4, C‐5 and C‐6 were analysed. Both cis‐ and trans‐annelated an equatorial 2‐p‐nitrophenyl group, as concluded from the values of their vicinal coupling constants and those in some simple model compounds, in agreement with the information given by 13C NMR and low‐temperature experiements. Correspondingly, trans‐2‐p‐chlorophenyl‐4,5‐ and ‐5,6‐tetramethylenedihydro‐1,3‐oxazines exist in anancomeric conformations; the cis isomers, however, are equilibrium mixtures of the N‐ or O‐eq and N‐ or O‐ax forms where the latter predominate, as shown by vicinal H, H coupling constants, 13C chemical shifts and low‐temperature experiments.