A New Systematization of the Conformational Behavior of Seven-Membered Rings. Isoclinal Anomeric and Related Orientations(1).

The chair and twist-chair conformations of seven-membered rings are classified as a function of the signs of their endocyclic torsion angles. The conformational analysis (MM3) of the methoxy- and methyloxepanes 1-6 used as patterns allows the study and classification of the different types of hydrogen orientation in the seven-membered saturated heterocycles. General principles are established, allowing the prediction of the stability of the different twist-chair conformations as a function of the substituent type, its position in the ring, and the type of hydrogen atom that has been substituted. These results are extended to the 1,4-dioxepane derivatives 7-13 and are compared with experimental data.