Mechanism of optical resolutions via diastereoisomeric salt formation. Part 6 - Thermoanalytical investigation of the optical resolution of racemic mandelic acid with S-(+)-2-benzylamino-butanol

The optical resolution of racemic mandelic acid (I) by S-2-benzylamino-buta nol (II) was performed in water, ethyl acetate, and water saturated ethyl a cetate. It was found that the efficiency of the resolution is three times h igher in water saturated ethyl acetate than in either water or ethyl acetat e. The salt mixtures produced during the resolutions and the pure diastereo isomeric salts were analyzed by TG, DSC and X-ray powder diffraction measur ements. The R-(-)-I-S-(+)-II salt has the higher melting point and heat of fusion value which indicates that this is the more stable salt. Though the general assumption is that diastereoisomeric salt pairs of successful optic al resolutions form eutectic systems, the R-mandelic acid-S-2-benzylamino-b utanol and the S-mandelic acid-S-2-benzylamino-butanol system was found to behave in a different way. Melting did not start at or near the estimated e utectic temperature. The difference can be explained either by miscibility in the solid phase (solid solution) or by a blocked interaction between the crystals of the two solid salts. This unusual behaviour of the salt pair s hould be responsible for the unusual difference in the efficiency of the re solutions performed in different solvents.