Studies on the benzoxazine series. 2 —Preparation and 1 H and 13 C NMR structural study of some substituted 1,2‐dihydro‐4 H ‐3,1‐benzoxazines

In addition to the parent compounds, nine methyl‐substituted 1,2‐dihydro‐4 H ‐3,1‐benzoxazines with and without N ‐methyl substitution were prepared. The chain tautomer could only be detected in the case of 1,2‐dihydro‐2‐( p ‐nitrophenyl)‐4 H ‐3,1‐benzoxazine in solution. The configurational and conformational assignments were based on 1 H and 13 C NMR data. The calculations of the methyl substituent effects on the heterocyclic ring (half‐chair structure) carbons gave a good fit between the observed and calculated shifts and made in possible to estimate that 1,2‐dihydro‐4‐methyl‐4 H ‐3,1‐benzoxazine is a 75:25 and its N ‐methyl derivative a 69:31 mixture of the 4eq′‐ and 4ax′‐ forms. 1,2‐Dihydro‐ trans ‐1,2,4‐trimethyl‐4 H ‐3,1‐benzoxazine is also not conformationally homogeneous, but is a 43:57 mixture of the 2eq,4ax′‐ and 2ax,4eq′‐ forms. A comparison of the 13 C chemical shift correlations with those of 3,4‐dihydro‐2 H ‐1,3‐benzoxazines also improves the structural knowledge about the latter series. Whereas the N ‐methyl groups in 3,4‐dihydro‐2 H ‐1,3‐benzoxazines are axially orientated, model calculations reveal that the N ‐methyl substitutions in the title compounds can be orientationally heterogeneous.