A New Stereochemical Model from NMR for Benzoylated Cyclodextrins, Promising New Chiral Solvating Agents for the Chiral Analysis of 3,5-Dinitrophenyl Derivatives

Abstract

Hexakis(2,3-di-O-benzoyl)-α-cyclodextrin and hexakis(2,3,6-tri-O-benzoyl)-α-cyclodextrin have been employed as chiral solvating agents (CSAs) for the NMR determination of the enantiomeric composition of derivatives of chiral amines, amino alcohols, alcohols, carboxyl acids, and amino acids bearing a 3,5-dinitrophenyl moiety. The conformational features of the two cyclodextrins have been carefully analyzed by NMR spectroscopy, and the origin of the symmetry change (C 6 → C 3), detected by NMR for hexakis(2,3-di-O-benzoyl)-α-cyclodextrin in CDCl3, has been clarified.

NMR Detection of the Conformational Distortion Induced in Cyclodextrins by Introduction of Alkyl or Aromatic Substituents

“…This kind of tilting can lead to a lowering of the global symmetry of the cyclodextrin if a hydrogen bond acceptor group is present at the secondary site (cyclodextrin 3), allowing hydrogen bond interactions with the primary hydroxy group of the adjacent unit. [11] This is also the only case in which a glucopyranose ring distortion is detected, as reflected by the change in the vicinal coupling constants. [11] For 6, which contains a benzyl group instead of a benzoyl one, this interaction is not allowed.…”

“…[11] This is also the only case in which a glucopyranose ring distortion is detected, as reflected by the change in the vicinal coupling constants. [11] For 6, which contains a benzyl group instead of a benzoyl one, this interaction is not allowed.…”

“…Synthetic procedures and characterization data for cyclodextrins 3؊7 are reported in refs. [10] [11] …”

“…[11] [12] (1) An accurate NMR investigation [11] of the stereochemistry and dynamics of cyclodextrin 4 revealed that it retains the C 6 symmetry but it is characterized by a closed structure where the benzoyl groups occlude both the wider and narrower torus rims. In the case of 3, benzoylation reduced the global symmetry to C 3 , at least in CDCl 3 as solvent, and the presence of two different kinds of glucopyranoside residues was revealed, undistorted and distorted.…”

“…In the case of 3, benzoylation reduced the global symmetry to C 3 , at least in CDCl 3 as solvent, and the presence of two different kinds of glucopyranoside residues was revealed, undistorted and distorted. [11] We report now the results of an NMR investigation on the conformational features of the cyclodextrins chemically modified by means of the introduction of alkyl groups (Omethylation), 1؊2, and aromatic moieties (O-benzoylation and/or O-benzylation), 3؊7. Of the latter, cyclodextrins selectively substituted at the primary hydroxy groups (5) or at the secondary ones (3, 6) and persubstituted cyclodextrins (4, 7) were studied.…”