Annular desmotropy of three pairs of seven-membered heterocycles confirmed by X-ray crystallography

The tautomers of 3-ethoxycarbonyl-2-phenyl-4,1-benzothiazepine and its 2-(4-methylphenyl) and 2-(4-chlorophenyl) derivatives, identified in solution by spectroscopic means, could be separated in the form of single-crystals suitable for X-ray diffraction. This is at present a unique situation in the literature on desmotropy. For the three desmotropic pairs, the bond lengths and angles present in the puckered seven-membered rings are fairly consistent within each group of desmotropes, but differ between the imines and the enamines. The structural similarity within the imines is revealed best by the isostructurality of the methyl and chloro derivatives, while the puckering differences between the heterorings of the desmotropic groups are reminiscent of conformational polymorphism.