Annular desmotropy of three pairs of seven-membered heterocycles confirmed by X-ray crystallographyCCDC reference numbers 756120-756126. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b925635kDedicated to the memory of Prof. Gábor Bernáth PhD, DSc, University of Szeged, who died on 1st October 2009

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. Desmotropy (isolated tautomers in solid state) is uniquely demonstrated by three pairs of 4,1-benzothiazepines. They are visibly distinguished by the puckering of the heterorings governed by the flopping (CH ↔ NH) hydrogen atoms.