Enantioselectivity of azalanstat and its ketal tosylate intermediate in chiral high performance liquid chromatography separations

Separation of the stereoisomers of azalanstat (RS-21607-197), a substituted imidazolyl-1,3-dioxolane which acts as a potent inhibitor of lanosterol 14 alpha-demethylase in cholesterol biosynthesis, and its ketal tosylate intermediate was achieved by chiral high performance liquid chromatography. Resolution of all four stereoisomers of azalanstat was accomplished by normal phase separation of the diastereomers on a short silica gel column coupled on-line to a Chiralpak AS amylose carbamate phase chiral column for resolution of the enantiomers. A reversal in elution order of the cis-(2S,4S) and cis-(2R,4R) enantiomers of the ketal tosylate intermediate of azalanstat was exhibited by an Ultron ES-OVM ovomucoid protein bonded-phase column with a change in organic modifier from ethanol to acetonitrile. This unusual enantioselectivity in the chiral reverse phase separation of the ketal tosylate was attributed to a putative change in binding domains or recognition sites on the ovomucoid protein as a function of the organic modifier of the mobile phase. Resolution of all four stereoisomers of the ketal tosylate was achieved by use of a Chiralpak AD amylose carbamate phase column in the normal-phase mode.