Conformational analysis. XXVII —A 1 H and 13 C 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 1 H 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 13 C 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, 13 C chemical shifts and low‐temperature experiments.