Diastereomeric resolution of 3-chloromandelic acid with threo-(1S,2S)-2-amino-l-p-nitrophenyl-1,3-propanediol

An optical resolution of 3-chloromandelic acid (3-ClMA) using threo-(1S,2S)-2-amino-l-p-nitrophenyl-1,3-propanediol ([S,S]-SA) as a resolving agent was presented. The effects of the type of solvents, the amount of solvent, molar ratio of the resolving agent to racemate and filtration temperature on resolution were investigated. Under the optimal resolution conditions, the content of less soluble salt reached 98%, and the resolution efficiency was as high as 94%. The weak intermolecular interactions (such as hydrogen bond, halogen bond, CH/π and van der Waals interactions) and molecular packing mode in crystal structure of the less soluble salt (R)-3-ClMA(S,S)-SA were investigated. A wall-like 2-D hydrogen-bonding network and hydrophobic structure between hydrogen-bonding walls were revealed. (S,S)-SA was also used to resolve 2-ClMA and 4-ClMA respectively and the corresponding less soluble salts (R)-2-ClMA·(R,R)-SA and (R)-4-ClMA·(R,R)-SA were obtained using threo-(1R,2R)-2-amino-l-p-nitrophenyl-1,3-propanediol ((R,R)-SA) as a resolving agent. In addition, two other resolving agents, (R)-ɑ-phenethylamine ((R)-PEA) and (R)-N-benzyl phenethylamine ((R)-BPA) reported in the literature for the resolution of 3-ClMA were examined along with the newly proposed resolving agent, (S,S)-SA. The crystal structures of the resulting less soluble salts (R)-3-ClMA·(S,S)-SA, (R)-3-ClMA·(R)-PEA and (R)-3-ClMA·(R)-BPA were compared and examined.

Resolution of Halogenated Mandelic Acids through Enantiospecific Co-Crystallization with Levetiracetam

The resolution of halogenated mandelic acids using levetiracetam (LEV) as a resolving agent via forming enantiospecific co-crystal was presented. Five halogenated mandelic acids, 2-chloromandelic acid (2-ClMA), 3-chloromandelic acid (3-ClMA), 4-chloromandelic acid (4-ClMA), 4-bromomandelic acid (4-BrMA), and 4-fluoromandelic acid (4-FMA), were selected as racemic compounds. The effects of the equilibrium time, molar ratio of the resolving agent to racemate, amount of solvent, and crystallization temperature on resolution performance were investigated. Under the optimal conditions, the resolution efficiency reached up to 94% and the enantiomeric excess (%e.e.) of (R)-3-chloromandelic acid was 63%e.e. All five halogenated mandelic acids of interest in this study can be successfully separated by LEV via forming enantiospecific co-crystal, but the resolution performance is significantly different. The results showed that LEV selectively co-crystallized with S enantiomers of 2-ClMA, 3-ClMA, 4-ClMA, and 4-BrMA, while it co-crystallized with R enantiomers of 4-FMA. This indicates that the position and type of substituents of racemic compounds not only affect the co-crystal configuration, but also greatly affect the efficiency of co-crystal resolution.