Basic forms of supramolecular self-assembly organized by parallel and antiparallel hydrogen bonds in the racemic crystal structures of six disubstituted and trisubstituted cyclopentane derivatives.

A selection of stereoisomeric 2-hydroxy-1-cyclopentanecarboxamides, a 4-tert-butyl derivative and three tert-butyl derivatives of the respective carboxylic acid were subjected to X-ray crystallography. The optically active molecules (I)--(VI) form racemic crystals. Each racemic structure is basically determined by two intermolecular hydrogen bonds of O--H...O==C--XH and O==C--X--H...OH types (X = O, NH). The partially similar patterns of close packing observed reflect five basic forms of supramolecular self-assembly. In the racemic crystals of chiral molecules, there are homo- and heterochiral chains of molecules formed by the principal (O--H...O==C) hydrogen bonds. These chains assemble either in a parallel or antiparallel mode. The parallel homochiral chains (hop) observed in structure (II), (1R*,2R*)-2-hydroxy-1-cyclopentanecarboxamide, demand the polar space group Pca2(1), while the parallel heterochiral chains (hep) are organized in antiparallel layers with space group P2(1)/n in structure (VI), (1R*,2S*,5R*-5-tert-butyl-2-hydroxy-1-cyclopentanecarboxylic acid). Heterochiral chains in an antiparallel array (hea) are found in (I), (1R*,2S*)-2-hydroxy-1-cyclopentanecarboxamide, and (V) [(1R*,2S4S)-4-tert-butyl-2-hydroxy-1-cyclopentanecarboxylic acid, space group P2(1)/c]. Structures (IV), (1R*,2S*,4R*)-4-tert-butyl-2-hydroxy-1-cyclopentanecarboxylic acid, and (III), (1R*,2R*,4S*)-4-tert-butyl-2-hydroxy-1-cyclopentanecarboxamide, reveal that homochiral chains in an antiparallel array (hoa; cross-linked by heterochiral dimers held together by the second hydrogen bonds) can be formed by either translation (space group P1) or a screw axis (space group P2(1)/c). These alternatives are denoted hoa1 and hoa2. Similarly, within each pattern (hea, hep and hop) two slightly different alternatives can be expected. The partial similarities in the identified five patterns of hydrogen bonding are described by graph-set notations. Structures (I), (IV) and (V) can be characterized by a common supramolecular synthon, while the highest degree of similarity is shown by the isostructurality of (I) and (V).