Evaluation of norbornene-beta-cyclodextrin-based monomers and oligomers aschiral selectors by means of nonaqueous capillary electrophoresis

Norbornen-5-yl carboxylic acid and norbornen-5-ylmethylsilyl ether-based be ta -cyclodextrins (beta -CDs) containing up to three norbornene ester and u p to five norbornene silyl ether units have been prepared from beta -CD and norbornen-5-carboxylic chloride and norbornen-5-ylmethyldichlorosilane, re spectively. Oligomers (n = 2-4) were prepared therefrom using ring-opening metathesis polymerization (ROMP). Monomeric and oligomeric substituted beta -CDs were evaluated as chiral selectors in nonaqueous capillary zone elect rophoresis using 35 mM sodium bicarbonate in N-methylformamide (NMF) as bac kground electrolyte. Both monomeric and oligomeric norbornene ester- and no rbornene silyl ether-type selectors showed good enantioresolution for dansy lated (DNS-) amino acids using concentrations of the chiral selector of up to 4% w/v. A significant improvement in resolution was observed upon the in troduction of up to five norbornene silyl ether units into a beta -CD molec ule, whereas higher degrees of substitution with norbornen-5-yl-carboxyl gr oups read to a reduction in enantioresolution of DNS-amino acids. Thus, pen takis(norbornen-5-ylmethyihydroxysiloxyl)-beta -CD turned out to be superio r to mono(norbornen-5-ylmethylhydroxysiloxyl)-beta -CD in terms of enantior esolution. Moreover, norbornene silyl ether-type selectors were found to be more efficient than norbornene ester-type selectors. Finally, oligomeric s electors were found to possess superior or at least comparable enantioselec tivity in the separation of DNS- amino acids compared to the parent monomer s. A maximum in enantioresolution was obtained with oligo(pentakis(norborne n-5-ylmethylhydroxysiloxyl)beta -CD).