Paper
Crystal Structures of (R)-2-Phenoxypropionic acid-2-Aminobutanoic acid Diastereomers
Published 2006 ·
Analytical Sciences: X-ray Structure Analysis Online
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Abstract
great importance in the pharmaceutical industry. Optically active 2-phenoxypropionic acid (PPA), being a useful chiral resolution agent, directly separates such racemic amino acids into their enantiomers without any substitutions.1,2 The title compounds, (S)-2-aminobutanoic acid (S-ABA) and R-PPA complex, (I), and R-ABA and R-PPA complex, (II), are given in a separating procedure. In the present study, we differentiated the molecular-recognition system in (I) from that in (II), and correlated with its higher optical separation. The compounds were prepared as follows. Rod-shaped crystals of (I) were obtained primarily with up to 96.4%ee in HPLC from an aqueous solution of racemic-ABA and R-PPA in a 2:1 molar ratio with slow cooling. After filtration, the addition of R-PPA and recrystallization, the residual solution was slowly evaporated, and some rod-shaped crystals of (II) were obtained secondarily with 97.0%ee. Crystal and experimental data are given in Table 1. The crystal structures were solved by direct methods (SAPI91),3 and refined by fullmatrix least squares. The absolute configuration was chosen to correspond to that of R-PPA. ORTEP drawings of (I) and (II) are shown in Fig. 2. Selected geometric data are given in Table 2. The crystal structures demonstrate the molecular recognitions between ABA and RPPA. The R-PPA molecules have an almost identical structure, as we have already reported. The enantio-ABA molecules also display a common inner-salt structure, which is novel in the Cambridge Structural Database, and has a restricted gauche–C1B–C2B–C3B–C4B wedged between the amino and carboxy groups. In the crystal packings of (I) and (II), the hydrophobic and x75 ANALYTICAL SCIENCES 2006, VOL. 22
The crystal structures of (R)-2-phenoxypropionic acid-2-aminobutanoic acid diastereomers reveal molecular recognitions between ABA and RPPA, highlighting their importance in pharmaceutical industry.
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