Crystal structure of (3R)-3-benzyl-4-[(tert-butoxycarbonyl)amino]butanoic acid

The characteristic feature of the title molecule, C16H23NO4, is the syn configuration of the partially double amide C—N bond [C—N—C—O torsion angle = −14.8 (2)°]. The crystal packing is determined by intermolecular O—H⋯O and N—H⋯O hydrogen bonds, which link the molecules into a double-chain structure extending along [010].

The molecular structure is shown in Figure 1. The crystal packing is determined by intermolecular N5-H···O2 and O1-H···O6 hydrogen bonds, which organize the molecules into infinite double chains parallel to the [010] direction (Fig.2).
The geometrical parameters of the hydrogen bonds are listed in Table 1.
Acids were recovered from ethyl acetate solution using 1M NaHSO 4 solution.
The enantiomeric purity was determined according to the known procedure using N α -(2,4-dinitro-5-fluorophenyl)-Lvalinamide as derivating reagent (Marfey, 1984). Sample of enantiomer (5 mg) was dissolved in TFA -dichloromethane (1:1), the solution was shaken for 10 min, then solvents were removed by evaporation. The residue was dissolved in CH 2 Cl 2 and the solvent was removed again. This procedure was repeated five times to remove TFA completely. The dry residue was dissolved in 0.2 M NaHCO 3 to obtain 0.05 M solutions (0.5 mL) of (3R)-4-amino-3-benzyl-butanoic acid. Single crystals were obtained by recrystallization from acetonitrile at room temperatute.

S3. Refinement
All H atoms were located in difference Fourier maps but finally their positions were determined geometrically, except H5 that was freely refined. H atoms were refined as riding on their carriers with C-H= 0.95 Å for aromatic CH groups, 0.97 Å for CH 2 groups, 0.96 Å for methyl groups and N-H = 0.86 Å for the amide group, and with U iso (H) = 1.2U eq (C,N), except for methyl group where U iso (H) = 1.5U eq (C). The absolute structure was known from the synthetic procedure and is confirmed by the Flack parameter of 0.05 (15).

Figure 1
The molecular structure with displacement ellipsoids drawn at the 50% probability level. Packing of the title compound viewed along the [101] direction. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.16 e Å −3 Δρ min = −0.18 e Å −3 Absolute structure: Flack (1983), 1138 Friedel pairs Absolute structure parameter: 0.05 (15) 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.