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 temperat ure is decreased, greater enantioselectivity and resolution is obtaine d, usually without significant loss in column efficiency. Indeed, an i ncrease in chromatographic efficiency with the reduction of temperatur e has been observed in several instances. The enantiomers of several c ompounds that show low enantioselectivity at room temperature have bee n cleanly separated at temperatures as low as -47 degrees C. Additiona lly, the enantiomers of atropisomers having low configurational stabil ity at ambient temperature have been fully separated under similar cry ogenic conditions. The ability to retain good chromatographic efficien cy at low temperature is attributed to the rapid adsorption-desorption kinetics of this type of brush-type stationary phase. (C) 1997 Elsevi er Science B.V.