3-{[(Benzyloxy)carbonyl]amino}butanoic acid

In the title compound, C12H15NO4, the butyric acid group has a stretched trans conformation. The dihedral angle between the phenyl ring and the oxycarboxyamino N—(C=O)—O—C plane is 56.6 (2)°. In the crystal, an inversion dimer is formed by a pair of O—H⋯O hydrogen bonds. The dimers are further linked by N—H⋯O hydrogen bonds between amide groups, forming a tape along the b axis.

Fortunately, the title compound, 3-benzyloxycarbonylaminobutanoic acid (Cbz-BABA), (I), was crystallized, and we herein report on the crystal structure. The molecular structure of (I) is shown in Fig. 1. The bond lengths (Allen et al., 1987) and angles are within normal ranges except for the O-H bond length at the carboxy dimer. The part of BABA is essentially similar with that reported by Gross & Vahrenkamo (2005). The butyric acid group owns a stretched trans-conformation (O1-C1-C2-C3-C4). At the β-position the benzyloxycarboxyamino group is attached perpendicular to the butyric acid group.
In the crystal structure, an enantiomer makes a planar structure with the intermolecular hydrogen bond (N1-HN1···O3) along the b axis. The planar structure is stacked to the enantiopure layer along the c axis. The carboxy dimer is made from the enantiomeric isomers with the intermolecular hydrogen bond (O1-H1···O2). The H atom is shared by carboxy dimer then the bond distance O1-H1 is longer than that of general carboxy group. The hydrophobic and hydrophilic layers are well separated along the a axis. The structure shows a herring bone stacking mode (Fig. 2).

Experimental
The title compound was purchased from Aldrich-Sigma Co. Ltd. Rod-like colourless crystals suitable for X-ray diffraction were obtained by vapour-phase diffusion of an ethanol and chloroform mixture solution at 297 K.

Refinement
All H atoms were located in a difference-Fourier map. H atoms bonded to N and O atoms were then refined isotropically.
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 (1.5 for methyl groups) times U eq (C).
supplementary materials sup-2 Figures Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 30% probability level.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and tor-

sion angles
Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted Rfactors wR and all goodnesses 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 observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq