Enantioseparations by subcritical fluid chromatography at cryogenic temperatures

Two chiral stationary phases, the Whelk-O 1 and its polysiloxane-borne analog, the polyWhelk-O, have been used to separate enantiomers using subcritical carbon dioxide at cryogenic temperatures. As the temperature is decreased, greater enantioselectivity and resolution is obtained, usually without significant loss in column efficiency. Indeed, an increase in chromatographic efficiency with the reduction of temperature has been observed in several instances. The enantiomers of several compounds that show low enantioselectivity at room temperature have been cleanly separated at temperatures as low as -47 degrees C. Additionally, the enantiomers of atropisomers having low configurational stability at ambient temperature have been fully separated under similar cryogenic conditions. The ability to retain good chromatographic efficiency at low temperature is attributed to the rapid adsorption-desorption kinetics of this type of brush-type stationary phase. (C) 1997 Elsevier Science B.V.