Efficient syntheses and resolutions of inherently chiral calix[4]quinolines in the cone and partial-cone conformation

The syntheses of five pairs of novel inherently chiral calix[4]arenes are described. Two synthetic routes were adopted to generate racemic 3-carboxylic or 2-carboxylic group substituted calix[4]-quinolines in the cone or the partial-cone conformation, respectively. The chiral products were thoroughly characterized by various spectroscopic methods. The optical resolutions of chiral calix[4]-quinolines 5, 6, 11, and 17 were successfully achieved through the separation of their diastereomers using common column chromatography or preparative TLC. The chirality of compound 20 was proven by the splitting of the H-1 NMR signals in the presence of Pirkle's reagent. The H-1 NMR features of the diastereomers are discussed. The CD spectra of each pair of enantiomers showed excellent mirror images. The experimental results disclose that 3-carboxylic calix[4]-quinolines can be resolved more easily than the 2-carboxylic ones in both the cone conformation and the partial-cone conformation.