Spectroscopic and chromatographic investigation of derivatized cyclodextrins

Abstract

Regioselective reactions of 1-(1-naphthyl)ethyl isocyanate (NEIC) with β-cyclodextrin (β-CD) were studied with and without NaH activation of β-CD in N,N-dimethylformamide (DMF) and pyridine. All six possible mono-substituted CD products were separated by HPLC and characterized by IH NMR. The primary substitution product predominates when the reaction is carried out under reflux conditions in pyridine without NaH activation. The C-2 substitution product predominates when the reaction is carried out in DMF. Conversion of 2-0-(1-(1-naphthyl)ethylcarbamoyl)-β- CD to 6-0-(1-(1-naphthyl)ethylcarbamoyl)-β-CD was observed when NaH was used to the activate hydroxyl groups of CD. 1H and 13C NMR spectra of 2-0-((R)-1-(1-naphthyl)ethylcarbamoyl)-β- cyclodextrin(RC-2) and 2-0-((S)-1-(1naphthyl) ethylcarbamoyl)-β-cyclodextrin(SC-2) were obtained in D20. NMR spectra indicate at least two conformers existing both in RC-2 (90:10) and SC-2 (61:39) at 25°C. Complete assignment of 1H and 13C spectra of the major conformer of RC-2 was accomplished using homo- and heteronuclear two dimensional NMR techniques. Correlations observed in 2D NMR (ROESY, HMBC) revealed complete glucose connectivity of RC-2 and SC-2. The inclusion state and orientation of the naphthyl substituent for both RC-2 and SC-2 was derived from the correlations observed between naphthyl and cyclodextrin (CD) protons in ROESY spectra, as was the various extents of anisotropic effects on the seven glucose units induced by the naphthyl group. The relative populations of the included and excluded orientation of the naphthyl substitutent in RC-2 and SC-2 were found to be solvent dependent in methanol/water systems. Computer modeling revealed one stable (included in the cavity) conformer in RC-2 and two possible excluded conformers in SC-2. Regiospecifically monosubstituted l-(l-naphthyl)ethylcarbamoylated β-cyclodextrins (NEC-β-CDs) were successfully employed as chiral additives to achieve chiral separation of N-(3,5-dinitrobenzoyl)-phenylglycine (3,5-DNB-PG), phenylalanine (3,5-DNB-PA), and homophenylalanine (3,5-DNBHPA). The enantioselectivity of the various site-substituted NEC-β-CDs in capillary zone electrophoresis (CZE) was compared with that of native β-CD. Complexation constants of the three 3,5-DNB- amino acids with β-CD were determined from the CZE results:3,β-DNB-L-HPA (473 ± 9 M-1) , 3,5-DNB-D-HPA (460 ± 10 M-1) , 3,5-DNB-L-PA (260 ± 4 M-1) , 3,5-DNB-D-PA (161 ± 3 M-1 ) , and 3,5-DNB-PG (43 ± 4 M-1) .