A NEW SYSTEMATIZATION OF THE CONFORMATIONAL BEHAVIOR OF 7-MEMBERED RINGS - ISOCLINAL ANOMERIC AND RELATED ORIENTATIONS

The chair and twist-chair conformations of seven-membered rings are cl assified 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 diffe rent types of hydrogen orientation in the seven-membered saturated het erocycles. General principles are established, allowing the prediction of the stability of the different twist-chair conformations as a func tion of the substituent type, its position in the ring, and the type o f hydrogen atom that has been substituted. These results are extended to the 1,4-dioxepane derivatives 7-13 and are compared with experiment al data.