Solvent-Induced Reversed Stereoselectivity in Reciprocal
Resolutions of Mandelic Acid and <i>erythro</i>-2-Amino-1,2-diphenylethanol
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Abstract
Solvent-induced
chirality switching in reciprocal optical resolution
between mandelic acid (<b>1</b>) and <i>erythro</i>-2-amino-1,2-diphenylethanol (<b>2</b>) has been demonstrated.
The stereochemistry of the deposited salts was controlled by changing
the crystallization solvent from 1-PrOH or 1-BuOH to 1,4-dioxane.
It was revealed from <sup>1</sup>H NMR spectra, thermogravimetric
analysis, and X-ray crystallography of the salts that an equimolar
amount of the crystallization solvent was incorporated in each diastereomeric
salt. On the basis of the crystal structures, it was found that both
the hydrogen-bonding ability and the size of the solvent molecule
played an important role. Differences in the formed hydrogen-bonding
networks (columnar or sheetlike structure) and their packing manner
were found to be crucial for the reversed stereoselectivity. Furthermore,
pseudopolymorphic salt crystals that incorporated 1,4-dioxane were
obtained during the enantioseparation of racemic <b>2</b>, and
their solid-state properties were examined by measurement of their
IR spectra. This solvent-induced dual stereocontrol technique was
successfully applied to the successive resolution process, eliminating
the need to change the resolving agent for access to both enantiomers
of <b>1</b> and <b>2</b>