Cyclodextrins as chiral additives for chromatographic separation of some mandelic acid esters into enantiomers

Chromatographic behaviour of mandelic acid and its (methyl, ethyl,n-propyl and n-butyl) esters have been studied in RP-HPLC system modified with α-, β-, γ-cyclodextrins and permethylated β-cyclodextrin. It has been found that although native cyclodextrins form with esters a relatively stable complexes of 1:1 stoichiometry, which do not recognize enantiomers. On the contrary, permethylated β-cyclodextrin permits efficient separation of optical isomers. Improved separations have been observed with the lengthening of aliphatic chain in an ester molecule. The enantiomers of esters have been always eluted with the same sequence: first R(-) enantiomer, second the S(+) one, while the order was inverse for free mandelic acid optical isomers. This difference may suggest dissimilar manner of attachment. The attempts to discuss the results in the light of Dalgliesh three point attachment concept have been undertaken. It was assumed that hydrophobic interactions between methyl groups of permethylated β-cyclodextrin and alkyl group of ester produce diversity in attachment of enantiomers. In optimization procedure i.e. to obtain base line separation at the shortest time of analysis three parameters have been taken into account; temperature, methanol concentration and native β-cyclodextrin addition. In effect the use of addition of β-cyclodextrin is worth to recommendation.