Characterization of the β-cyclodextrin inclusion complexes with bichromophoric 1-benzyloxy-2-pyridone and related compounds

Abstract

The 1 : 1 complexation behaviour of β-cyclodextrin (β-CDx) with the title compounds has been investigated by measuring UV absorption, circular dichroism (CD) and ¹H NMR spectra of the β-CDx inclusion complexes formed. A methyl substituent introduced into the pyridone moiety of bichromophoric 1-benzyloxy-2-pyridone (1) was found to affect greatly the guest conformation within the cavity as well as the 1–β-CDx-complex stability. On the other hand, in addition to the inclusion-complex stability, the pyridone-ring conformation in the β-CDx complexes with 1-(1-naphthyl)methyloxy-2-pyridone (3) and its positional isomer, 1-(2-naphthyl)methyloxy-2-pyridone (5), does not undergo any methyl-substituent effects. An analysis of the induced CD bands originating from the naphthyl and pyridone residues of 3 and 5 revealed that the substitution mode of a naphthyl group, axially incorporated into the cavity, exerts dramatic effects both on the orientation of a planar pyridone ring to the cavity axis and on the stability of the obtained complex. Remarkable differences in guest conformation and inclusion-complex stability between the 3–β-CDx and 5–β-CDx systems were interpreted in terms of positional isomer effects caused by the preferential axial immersion of both 1-naphthyl and 2-naphthyl groups into the cavity. The NMR data substantiate the structure of the β-CDx complexes with 1 and its methyl derivative where both the phenyl and pyridone chromophores are situated inside the cavity, whereas the pyridone ring in the β-CDx complexes with 3 and 5 was shown by these data to be located around the rim of the secondary hydroxy-group side.