SYMMETRY-MODIFIED CONFORMATIONAL MAPPING AND CLASSIFICATION OF THE MEDIUM RINGS FROM CRYSTALLOGRAPHIC DATA .2. EXO-UNSATURATED AND HETEROCYCLIC 7-MEMBERED RINGS

Crystallographic results derived from the Cambridge Structural Databas e (CSD) have been used to perform a systematic conformational analysis for methylenecycloheptane, oxocycloheptane, azocycloheptane, oxepane, azepane and for the epsilon-lactone and epsilon-lactam fragments. Con formational mappings based on symmetry-adapted deformation coordinates and principal component scores show that ring conformations in these fragments fall almost exclusively on the chair-twist-chair pseudorotat ion pathway. Symmetry-modified Jarvis-Patrick clustering has been used to generate conformational classifications in a semi-automated manner . A manual classification was preferred for the oxocycloheptanes due t o wide conformational diversity within a rather small data set. Energy calculations have also been carried out for these systems using force -field methods and a novel conformation-hunting algorithm that attempt s to locate all local minima without recourse to ring cleavage and ref ormation. There is good qualitative agreement between the crystallogra phic observations of conformation and the calculated features of the p otential energy hypersurface, despite the wide variety of substitution patterns covered by the crystallographic results.