Mechanism of optical resolutions via diastereoisomeric salt formation. Part 7 - Role of the solvents in the optical resolution of alpha-phenylethylamine by 2R,3R-tartaric acid

The optical resolution of alpha-phenylethylamine (1) by 2R,3R-tartaric acid (2) was studied in different solvents, the precipitated salts were subject ed to thermoanalytical measurements and X-ray powder diffraction. The most efficient resolution can be accomplished by using methanol, the precipitate , an unsolvated salt containing the S-(-)-1.2R,3R-(+)-2 salt in abundance, is not a simple mixture of the diastereoisomeric salt pair, but a new modif ication. In water, ethanol and acetonitrile a reversed, but less effective resolution can be accomplished by precipitation of a hydrate which contains the R-(+)-1 in abundance. The precipitates from water and ethanol also pro ved to be new modifications. Only the precipitate from acetonitrile is the mixture of the diastereoisomeric salt pair, which can be expected in fracti onal crystallization of an eutectic forming diastereoisomeric salt pair. Th e number of different modifications found by changing the solvent in case o f this particular salt pair seems surprisingly high, but may occur for othe r diastereoisomeric salt pairs, too. The existing but not investigated poly morphism can be the explanation for the sometimes very poor reproducibility of the preparative resolutions.