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.