title: Predictable close-packing similarities between cis- and trans-2-hydroxy-1-cyclooctanecarboxylic acids and trans-2-hydroxy-1-cyclooctanecarboxamide.
abstract: In order to extend the experimental data already available on the close packing of cyclopentanes substituted with vicinal COX (X = OH, NH(2)) and OH groups to the analogous cyclohexanes, cycloheptanes and cyclooctanes, (1R*,2S*)-cis-2-hydroxy-1-cyclooctanecarboxylic acid (8C), (1R*,2R*)-trans-2-hydroxy-1-cyclooctanecarboxylic acid (8T) and (1R*,2R*)-trans-2-hydroxy-1-cyclooctanecarboxamide (8T*) were subjected to X-ray crystal structure analysis. In 8T and 8T*, the hydrogen bonds form infinite ribbons of dimers joined by R(2)(2)(12) rings with C(i) symmetry. Two types of dimer alternate along each ribbon. The dimers differ in the donor and acceptor roles of the functional groups. This pattern was previously deduced topologically among the possible forms of association for heterochiral dimers [Kálmán et al. (2002). Acta Cryst. B58, 494-501]. As they have the same pattern of hydrogen bonds, 8T and 8T* are isostructural. The additional donor (i.e. the second hydrogen of the NH(2) group) present in 8T* links the adjacent ribbons so as to form smaller R(2)(2)(8) rings between them. The crystals of the cis stereoisomer 8C are built up from antiparallel hydrogen-bonded helices. The topology and symmetry of this structure are the same as for the close packing of (1R*,2R*,4S*)-4-tert-butyl-2-hydroxy-1-cyclopentanecarboxamide [Kálmán et al. (2001). Acta Cryst. B57, 539-550]; only the hydrogen-bond donors and acceptors are interchanged, in the same way as in the two dimer types of 8T and 8T* ribbons. This analogy suggests that helices may originate as homochiral dimers with C(2) symmetry and polymerize into helices during crystal formation. The conformational characteristics of the heterochiral dimers observed in the title compounds and in closely related structures are discussed.