Crystal structure of β-benzyl dl-aspartate N-carboxyanhydride

In the title racemic compound, the benzyl ring is almost normal to the oxazolidine ring, with a dihedral angle of 80.11 (12)°. In the crystal, inversion dimers are formed between the l- and d-enantiomers via pairs of N—H⋯O hydrogen bonds.


Chemical context
N-Carboxyanhydrides (NCAs) of amino acids are used extensively as monomers for the preparation of high molecular weight polypeptides (Kricheldorf, 2006). Amino acid NCAs are easily soluble but the resulting polypeptides are not soluble in general organic solvents. Only a few amino acid ester NCAs such as -benzyl l-glutamate NCA and -benzyl l-aspartate NCA are polymerized in solutions, because the resulting polypeptides are soluble in them. Thus, the polymerization of these amino acid ester NCAs has been investigated by many researchers. On the other hand, we found that every amino acid NCA crystal is polymerized in the solid state in hexane by the initiation of amines, and we have studied the solid-state polymerization of amino acid NCAs with reference to their crystal structures (Kanazawa, 1992(Kanazawa, , 1998Kanazawa et al., 1978Kanazawa et al., , 2006. We have studied the polymerization of -benzyl l-aspartate NCA (BLA NCA) initiated by butyl amine in solution and the solid state (Kanazawa & Sato, 1996), and determined the crystal structure of BLA NCA (Kanazawa & Magoshi, 2003), to consider the high reactivity in the solid state. In addition, we have attempted the preparation of single crystals of the title compound, -benzyl dl-aspartate NCA (BDLA NCA). The BDLA NCA single crystals were obtained by a slow crystallization in solutions. The polymerization of BDLA NCA was carried out both in dioxane solution and in the solid state in hexane, using butyl amine as initiator. BDLA NCA is not so reactive in solutions; the existence of l-and denantiomers in solution seems unfavourable for fast polymerization. On the other hand, the compound is very reactive in the solid state. It is therefore important to determine its crystal structure in order to consider the difference in the reactivity between the solution and the solid state.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The oxazolidine ring is planar, with a maximum deviation of 0.027 (2)Å for atom C1. The side chain has an extended conformation with the torsion angles C3-C4-C5-O5 and C4-C5-O5-C6 being 178.29 (14) and À179.29 (17) , respectively. The benzyl ring is almost normal to the oxazolidine ring, making a dihedral angle of 80.11 (12) .

Supramolecular features
In the crystal, -benzyl l-aspartate NCA and -benzyl daspartate NCA molecules form a dimer structure around a crystallographic center of symmetry via a pair of N1-H1Á Á ÁO1 i hydrogen bonds ( Fig. 2 and Table 1). The dimers are linked by C-HÁ Á ÁO hydrogen bonds, forming layers parallel to the ab plane ( Fig. 2 and Table 1). The five-membered oxazolidine rings are packed in a layer and the -CH 2 COOCH 2 C 6 H 5 groups are packed in another layer; these two different layers are stacked alternately. This sandwich structure is one of the important requirements for high reactivity in the solid state, because the five-membered rings can react with each other in the layer.

Synthesis and crystallization
The synthesis of BDLA was carried out by the reaction of dl-aspartic acid with benzyl alcohol in a manner similar to that for -benzyl l-glutamate (BLG) (Kanazawa, 1992). The title compound was obtained by the reaction of BDLA with triphosgene in tetrahydrofuran, as reported previously for BLA NCA (Kanazawa & Magoshi, 2003). The reaction product was recrystallized slowly in a mixture of ethyl acetate and hexane (1:50 v/v), avoiding moisture contamination, giving colourless prismatic crystals.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The N-bound H atom was located in a difference Fourier map and refined with U iso (H) = 1.2U eq (N). The C-bound H atoms were positioned geometrically (C-H = 0.93-0.98 Å ) and treated as riding with U iso (H) = 1.2U eq (C). Crystal packing of the title compound, viewed along the c axis, showing the hydrogen bonds as dashed lines (see Table 1). Table 1 Hydrogen-bond geometry (Å , ).

Benzyl 2-(2,5-dioxooxazolidin-4-yl)acetate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.