(2R,3R)-3-O-Benzoyl-N-benzyltartramide1

The title compound, C18H17NO6 [systematic name: (2R,3R)-4-benzylamino-2-benzoyloxy-3-hydroxy-4-oxobutanoic acid], is the first structurally characterized unsymmetrical monoamide–monoacyl tartaric acid derivative. The molecule shows a staggered conformation around the tartramide Csp3—Csp3 bond with trans-oriented carboxyl and amide groups. The molecular conformation is stabilized by an intramolecular N—H⋯O hydrogen bond. In the crystal, molecules are linked by O—H⋯O hydrogen bonds between the carboxyl and amide carbonyl groups, forming translational chains along [001]. Further O—H⋯O and N—H⋯O hydrogen bonds as well as weaker C—H⋯O and C—H⋯π intermolecular interactions extend the supramolecular assembly into a double-layer structure parallel to (100). There are no directional interactions between the double layers.

The title compound, C 18 H 17 NO 6 [systematic name: (2R,3R)-4benzylamino-2-benzoyloxy-3-hydroxy-4-oxobutanoic acid], is the first structurally characterized unsymmetrical monoamide-monoacyl tartaric acid derivative. The molecule shows a staggered conformation around the tartramide Csp 3 -Csp 3 bond with trans-oriented carboxyl and amide groups. The molecular conformation is stabilized by an intramolecular N-HÁ Á ÁO hydrogen bond. In the crystal, molecules are linked by O-HÁ Á ÁO hydrogen bonds between the carboxyl and amide carbonyl groups, forming translational chains along [001]. Further O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds as well as weaker C-HÁ Á ÁO and C-HÁ Á Á intermolecular interactions extend the supramolecular assembly into a double-layer structure parallel to (100). There are no directional interactions between the double layers.
Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009), PLATON (Spek, 2009) and publCIF (Westrip, 2010  The title molecule crystallizes in non-centrosymmetric C2 space group as R,R enantiomer. Molecular structure with the atom numbering scheme is presented in Fig. 1. Similarly to the previously characterized tartaric acid mono amides (Rychlewska et al., 1999;Rychlewska & Warżajtis, 2000, 2001, the title molecule adopts the staggered conformation around the C2-C3 bond (Fig. 2.). Thus, the carboxylic group is in trans (T) orientation with respect to the amide group, whereas the hydroxy and benzoyl substituents adopt the gauche counterclockwise orientation. The conformation on C-C* bond in amide fragment enables the formation of an intramolecular N-H···O hydrogen bond between N-H donor and the hydroxyl group. Hence the carbonyl group is on the opposite side of the proximal C*-O bond. Such orientation, called antiplanar (a) is also observed in the carboxylic fragment, and the overall conformation of molecules can be given as T(a,a). It is worth noting that in case of dibenzoyl tartaric mono amides (Rychlewska & Warżajtis, 2001) or those with unsubstituted OH groups (Rychlewska et al., 1999;Rychlewska & Warżajtis, 2000) the conformation of the acid fragment is such that the carbonyl group eclipses the nearest C-O bond (synplanar conformation, s), while the presence of at least one N-H bond forces the conformation of the amide fragment to be antiplanar with the intramolecular N-H···O bond.
The analysis of intermolecular interactions shows that the title molecules related by translation along [001] form infinite head-to-tail chains via hydrogen bonds between carboxylic OH donors and amide carbonyl groups. A topologically analogous chain motif was observed in the crystal structures of dibenzoyl mono amides by Rychlewska & Warżajtis (2001). Further, the O-H hydroxyl group acts as a donor to carboxyl C=O group joining the chains related by 2 1 screw axis into a double layer (Fig. 3). The layer is enhanced by N-H···O benzoyl carbonyl as well as weaker C-H···O and C-H···π intermolecular interactions. The neighbouring layers are held together by weak van der Waals forces only.

Experimental
A (1:2 mol/mol) mixture of O-benzoyl-L-tartaric anhydride (Bernaś et al., 2010) and benzylamine in acetonitrile was stirred at room temperature for 10 min. The mixture was then acidified with 10% HCl and filtered. The resulting white solid product was rinsed with water to give pure title compound with m.p. 465-467 K. Preparation and characterization of regioisomer of the title compound was described by Bell (1987)

Refinement
The position of the H atoms attached to O and N atoms were freely refined with U iso (H) = 1.2×U eq (N) and U iso (H) =1.5×U eq (O). Other H atoms were positioned geometrically and refined using a riding model with C-H = 0.95-0.99 Å and with U iso (H) = 1.2× U eq (C). The absolute structure was assigned on the basis of anomalous dispersion that confirmed the known chirality of the reagent. The estimated number of measured Friedel pairs amounts to 1321 with the fraction of 0.813.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.