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

In addition to the parent compounds, nine methyl‐substituted 1,2‐dihydro‐4H‐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)‐4H‐3,1‐benzoxazine in solution. The configurational and conformational assignments were based on 1H and 13C 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‐4H‐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‐4H‐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 13C chemical shift correlations with those of 3,4‐dihydro‐2H‐1,3‐benzoxazines also improves the structural knowledge about the latter series. Whereas the N‐methyl groups in 3,4‐dihydro‐2H‐1,3‐benzoxazines are axially orientated, model calculations reveal that the N‐methyl substitutions in the title compounds can be orientationally heterogeneous.