Structural Studies of Self-Folding Cavitands

Abstract

Resorcinarene-based cavitands 1a – c fold into a deep open-ended cavity by means of intramolecular hydrogen bonds in both apolar solutions and the solid state. The X-ray crystal-structure analysis of cavitand 1a features a seam of secondary amide C=O⋅⋅⋅H−N interactions that bridge adjacent rings and are held in place by intra-annular hydrogen bonds. This results in a cavity of 9.2×7.0 Å dimensions. The arrangement of the amides in 1a – 1c is cycloenantiomeric, with clock- and counterclockwise orientation of the head-to-tail amide sequence. Interconversion rates of the two enantiomers are controlled by solvent polarity: the rate is slow on the NMR time-scale in aromatic solvents and CDCl3, but fast in (D6)acetone. The 1H- and 13C-NMR-spectral analysis is in agreement with the crystallographic data. Chiral cavitand 1b with eight HN−C(O)−C*HMeEt ((+)-(S)) groups on its upper rim exists as two cyclodiastereoisomers (in a ca. 3 : 1 ratio) in apolar solution. A `library' of 512 diastereoisomeric cavitands 1c is obtained as a mixture by using the corresponding racemic acid chloride.