Chiral recognition in salts of trans-1,2-diaminocyclohexane and optically active tartaric acids:: crystal structure of 1:2 salt of (S,S)-diaminocyclohexane with (R,R)-tartaric acid.

X-ray results for the highly organised superstructure (space group P6(5)22) of the 1:2 salt of (1S,2S)-diaminocyclohexane with (R,R)-tartaric acid (1) are compared with the previously reported [1] crystal structures of two diastereomeric 1:1 salts of (R,R)-diaminocyclohexane with (R,R)- and (S,S)-tartaric acid. The asymmetric unit of the unit cell of 1 consists of one molecule of (S,S)-diaminocyclohexane, one molecule of (R,R)-hydrogentartrate, half a molecule of (R,R)-tartaric acid, half of an (R,R)-tartrate anion, and a water molecule. The presence of all three ionisation states of (R,R)-tartrate in one crystal is most unusual, as is the anti conformation of the carboxyl group in (R,R)-tartaric acid. Association of (S,S)-diaminocyclohexane and (R,R) hydrogentartrate involves three-centre hydrogen bonds between ammonium groups as donors and alpha-hydroxycarboxylate and alpha-hydroxycarboxyl moieties as accepters. The remaining hydrogen bonds stabilise this motif and expand it to three dimensions. The results provide some insights into the enantioselective association of trans-1,2-diaminocyclohexane with optically active tartaric acids, the main driving force being the hydrogen bonding. While in a homochiral 1:1 salt the two ions combine by a scissors-shaped hydrogen bonding motif formed by two ammonium functions as donors and vicinal hydroxyl groups as accepters, in heterochiral salts the cation and anion species join together via three-centre hydrogen bonds between ammonium functions and planar or nearly planar alpha-hydroxycarboxylate moieties. (C) 1998 Elsevier Science B.V. All rights reserved.