Urea-tetrahydrobenzoxanthene receptors for carboxylic acids

Hydrogen-bonding receptors for carboxylic acids have been prepared based on a cis tetrahydrobenzoxanthene skeleton. X-ray diffraction study of one of these compounds revealed that the cleft is suitable for establishing strong linear hydrogen bonds with the oxygen of a water molecule. Complexes that set only three H-bonds with the guests showed no chiral recognition with amino acid derivatives. However, suitable functionalization of the receptor provided a fourth H-bond with certain amino acid derivatives, leading to significant enantioselective complexation in this case.