THE ROLE OF WATER-SOLVATION IN THE OPTICAL RESOLUTION OF DL-LEUCINE WITH (S)-(-)-2-PHENYLETHANESULFONIC ACID - CHARACTERIZATION AND X-RAY CRYSTAL-STRUCTURES OF THEIR DIASTEREOMERIC SALTS

In the optical resolution by diasteromeric salt formation of DL-leucin e with (S)-(-)-1-phenylethanesulfonic acid (PES), the less-soluble L-L eu .(-)-PES salt preferentially crystallized from acetonitrile-methano l and, in contrast the same resolution from acetonitrile-water afforde d the monohydrated D-salt [D-Leu .(-)-PES . H2O]. The role of the wate r of crystallization formed during this resolution system has been exp lored through a comparison of the crystalline characteristics of D-, m onohydrated D-, and L-Leu .(-)-PES by thermoanalysis (DSC) and spectro scopy. The DSC indicated that D-Leu .(-)-PES had a polymorphic form wi th the water of crystallization; that is, D-Leu .(-)-PES . H2O was tho roughly dehydrated at 60-90 degrees C and simultaneously converted int o the D-Leu .(-)-PES form. However, L-Leu .(-)-PES does not have the w ater of crystallization even in 75% relative humidity. Detailed compar isons among the X-ray crystal structures of three salts have revealed that in all of the salts the conformations of the PES anion and the Le u cation, respectively, were approximately the same, and their crystal structures comprised specific interaction modes having characteristic hydrogen-bond networks. In particular, in D-Leu .(-)-PES . H2O the PE S and Leu ions are cross-linked to each other through water molecules. These observations suggested that the water of crystallization, toget her with the formation of a stable crystal-packing mode by cross-linki ng the PES and Leu ions, presumably plays an important role in the sel ective crystallization in the resolution.