Crystallization-based optical resolution of 1,1′-binaphthalene-2,2′-dicarboxylic acid via 1-phenylethylamides: control by the molecular structure and dielectric property of solvent

In the recrystallization of a diastereomeric mixture of amides (RS_a,S)-1 formed from racemic 1,1′-binaphthalene-2,2′-dicarboxylic acid and (S)-1-phenylethylamine, either of the diastereomers crystallizes out as a diastereomerically pure form, depending on the solvent employed; sterically undemanding solvents, acetone, dichloromethane, and acetonitrile, afford crystals formulated as (S_a,S)-1·solvent with an exception of ethanol, which affords (R_a,S)-1·EtOH, while sterically bulkier solvents afford (R_a,S)-1 including no solvent. The stereoselectivity can be rationalized by the crystal structures. A dielectrically controlled resolution (DCR) can also be carried out by using mixed solvents, which contain, for example, 25 vol % of acetone and varying ratios of hexane and 1-propanol in total 75 vol %; (S_a,S)-1·acetone is deposited as crystals from the solvents with a dielectric constant (ε) range 8.9 ≤ ε ≤ 10.2, while (R_a,S)-1 is deposited from the solvents with 14.8 ≤ ε ≤ 20.3.