Spacer length effect of a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

A new chiral stationary phase (CSP) containing 11 methylene-unit spacer was prepared by bonding (+)-(18-crown-6)-2,3,11,12-carboxylic acid to aminoundecylsilica gel. The new CSP was superior to the one containing three methylene-unit spacer in the resolution of alpha-amino acids, beta-amino acids, amines, and amino alcohols in terms of both the separation (alpha) and the resolution factors (R(S)). In the resolution of alpha-amino acids on the new CSP containing a long spacer, the retention factors (k(1)) were quite small compared to those on the CSP containing a short spacer. However, in the resolution of relatively more lipophilic beta-amino acids, amines, and amino alcohols, the retention factors (k(1)) were generally greater on the CSP containing a long spacer than on the CSP containing a short spacer. All of these resolution behaviors have been rationalized by the effective competition of the ammonium ions (R-NH(3)(+)) generated by the residual undecylamino groups of the new CSP under acidic condition with the ammonium ions (R-NH(3)(+)) of analytes for the complexation inside the cavity of the crown ether ring of the CSP and the effective lipophilic interaction between the CSP and the relatively more lipophilic analytes.