Structural characterization of L-proline and morpholine appended chiral calix[4]resorcinarene molecules

Calix[4]resorcinarenes serve as host molecules for small guest molecules. Recently calixarenes have been appended to chiral molecules in an attempt to promote chiral recognition. To take advantage of both cavity host and chiral substituent properties the position of the chiral moiety is important. We report the synthesis and structural characterization of two calix[4]resorcinarene based molecules that have helical chirality in the solid state. The calix[4]resorcinarene 1 has chiral 1-proline ethyl ester substituents positioned perpendicular to the cavity whereas the calix[4]resorcinarene 2 has morpholines positioned parallel to the cavity which extend the depth of the cavity. Compound 1 is one of the first compounds to show the position of chiral centers with respect to the calixarene cavity. H-1 and C-13 NMR spectroscopy indicate that the helical chirality of 2 is retained at low temperature in nonpolar solvents.