Packing modes of (R,R)-tartaric acid esters and amides

The molecular packing modes of a series of mono- and diamides of (R,R)-tartaric acid are discussed on the basis of their crystal structures. Derivatives include combinations of methylester, amide, N-methylamide and N,N-dimethylamide groups, both symmetrically and asymmetrically substituted. The symmetrically substituted derivatives do not utilize their C-2 symmetry in the crystal. The packing of primary tartramides seems to be driven by NH ... O=C hydrogen bonds and supplemented by strong OH ... O=C and weak NH ... OH bonds. On the other hand, in derivatives containing methylester and/or methylamide groups OH ... OH ... O=C hydrogen-bond patterns seem to dominate. Types of aggregates, characteristic for the investigated derivatives, include cyclic dimers and ring systems analogous to the dimers, but formed by two different although complementary functional groups, as well as sets of chains aligned in a manner resembling the helical arrangement of peptides. The helices are formed along the screw axis with an identity period of approximately 5 Angstrom. In tartramic acids, containing in one molecule both carboxyl and amide functions, in competition between the two groups to control the molecular arrangement, the latter dominates, unless it is N-substituted tartramide, in which case the carboxyl group predominates. Problems with packing, which occur in some of the structures owing to the steric bulk of the methyl groups, are overcome by changes in conformation (esters) or by co-crystallization with solvent water molecules (methylamides and dimethylamides). These derivatives are also more likely to crystallize with multiple asymmetric units.