STUDIES ON THE BENZOXAZINE SERIES .3. PREPARATION AND C-13 NMR STRUCTURAL STUDY OF GAMMA-EFFECTS OF SOME N-SUBSTITUTED 3,4-DIHYDRO-2H-1,3-BENZOXAZINES

Seventeen N‐substituted 3,4‐dihydro‐2H‐1,3‐benzoxazines [N‐substituent = Et, Pri, But, CH2C6H5 or CH(CH3)C6H5] were prepared and their structures studied in the light of 13C chemical shifts. The γ effects caused by C(α)‐methyl or C(α)‐phenyl substitution at the heterocyclic ring carbons were found to be valuable structural parameters. By using N‐tert‐butyl derivatives as models, and by dividing γtot effects into their components, the rotamer populations due to the rotation around the NC(α) bond could be evaluated. The method also allows the configurational assignment of diastereomeric N‐α‐methylbenzyl derivatives. The effect of the half‐chair structure on the 13C NMR parameters is discussed. Copyright © 1990 John Wiley & Sons, Ltd.