Conformation Analyses, Dynamic Behavior, and Amide Bond Distortions of Medium-sized Heterocycles. 2. Partially and Fully Reduced 1-Benzazocines, Benzazonines, and Benzazecines

Partially and fully reduced forms of benzo-fused eight- to ten-membered nitrogen heterocycles (1-benzazecines, 1-benzazonines and 1-benzazecines) have been prepared. Conformational features, transannular distances and dynamic behavior were studied using X-ray crystallography and VT NMR spectroscopy. The amide moiety in the nine-membered benzazonine ring <b>5b</b> favors <i>N</i>-pyramidization, whereas the ten-membered benzazecine <b>5c </b>adopts an amide twist. Molecular mechanics calculations reveals a correlation between the amide twist (τ) and ring stability. The dynamic behavior of the heterocycles in solution were also found to be dependent on the extent and nature of the amide distortion. We thus conclude that ring strain of these medium-sized heterocyclic rings is relieved through amide distortion, which leads to a more stable structure.