Chiral Stationary Phases Based on Aleuritic Acid as Chiral Spacer Arms: Middle and Terminal Chiral Factors vs. Chromatographic Separation Performance

Abstract

The enantiomeric monomer of aleuritic acid was used as a spacer arm to prepare chiral stationary phases (CSP1~3) for chiral selectors containing only aleuritic acid, S-phenylglycinol (S-PGL), and R,R-diphenylethylene-diamine (R,R-DPEDA). The Fourier transform infrared (FTIR), thermo-gravimetry (TG), elemental analysis, and solid state nuclear magnetic resonance (SSNMR) characterizations showed that the three stationary phases were coupled depending on the route design, and the grafting ratio was 12.4%, 78.2%, and 32.7%, respectively. The first specimen CSP1, containing only aleuritic acid, was able to separate 4 of the 9 target compounds. Separation of 6 compounds was achieved by the introduction of the single chiral factor S-PGL’s (CSP2). After introduction of the double chiral factor R,R-DPEDA (CSP3), separation of 8 compounds was achieved. Although the grafting ratio of R,R-DPEDA in CSP3 was only 32.7%, and the increase in carbon content was only 3.08% compared with that of CSP1, which was less than half of that of CSP2. Thus, CSP3 showed significantly higher separation selectivity. The difference in chiral factors led to the reverse resolution order of enantiomers under certain conditions for some compounds. The chiral spacer arm was shown to be important in separation and in the preparation methods of novel structural CSPs.