trans-Benzoxanthene receptors for enantioselective recognition of amino acid derivatives

Neutral cleft-type hydrogen-bonding receptors based on a trans-benzoxanthene skeleton have shown good stereoselective association towards carbamate derivatives of amino acids. Among these, the best results corresponded to the commercially available benzyloxycarbamate (Cbz) while the t-butyloxycarbamate (Boc) protecting group afforded disappointing results. Preparative TLC impregnated in ethoxycarbonyl proline provided a rapid way to resolve the receptor racemic mixture. X-Ray analysis and Overhauser effects allow us to suggest a structure for these complexes and the reasons for the observed chiral discrimination.