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

The molecular packing modes of a series of mono- and diamides of (R,R)-tart aric acid are discussed on the basis of their crystal structures. Derivativ es include combinations of methylester, amide, N-methylamide and N,N-dimeth ylamide groups, both symmetrically and asymmetrically substituted. The symm etrically 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 b onds. On the other hand, in derivatives containing methylester and/or methy lamide groups OH ... OH ... O=C hydrogen-bond patterns seem to dominate. Ty pes 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 chai ns aligned in a manner resembling the helical arrangement of peptides. The helices are formed along the screw axis with an identity period of approxim ately 5 Angstrom. In tartramic acids, containing in one molecule both carbo xyl and amide functions, in competition between the two groups to control t he molecular arrangement, the latter dominates, unless it is N-substituted tartramide, in which case the carboxyl group predominates. Problems with pa cking, which occur in some of the structures owing to the steric bulk of th e methyl groups, are overcome by changes in conformation (esters) or by co- crystallization with solvent water molecules (methylamides and dimethylamid es). These derivatives are also more likely to crystallize with multiple as ymmetric units.