Enantioselective recognition of an inherently chiral calix[4]arene crown-6 carboxylic acid cone conformer towards chiral aminoalcohols

Abstract

A comparative study was conducted to evaluate the chiral recognition ability of several inherently chiral calixcrown carboxylic acids towards chiral aminoalcohols. 1H NMR titration experiments indicated that inherently chiral calixcrown carboxylic acids each having a phenolic hydroxyl group possess superior chiral recognition ability to the completely alkylated calixarene derivatives. Particularly, inherently chiral calix[4]crown-6 carboxylic acid cone conformer 10 demonstrated significant enantioselectivity in its chiral recognition towards 2-amino-3-methyl-1-butanol (G3) and 2-amino-2-phenylethanol (G6), with the (cS)-10 preferably binding the (S)-guests. Job plots confirmed that 10 and both enantiomers of G3/G6 form 1 : 1 instantaneous complexes. DFT calculations revealed the existence of multiple hydrogen bonds in the host–guest complexes. The enantioselectivity of the recognition is ascribed to the stronger hydrogen bonding between (cS)-10 and (S)-guests than between (cS)-10 and (R)-guests, based on the calculation results.