Inherently chiral phosphonate cavitands as enantioselective receptors for mono-methylated L-amino acids
The properties of two inherently chiral cavitands in enantioselective recognition of N-methyl-leucine was studied in solution, through 1H and 31P NMR experiments, and in the solid state, through co-crystallisation experiments. The inherent chirality of the receptors was obtained through the desymmetrization of the rigid concave cavity by introducing three different bridging groups at the upper rim: two inward P=O groups, one inward P=S moiety and one methylene bridge in the AABC mode. For the solution studies, N-methyl-L-leucine was used as pure enantiomer and partial enantioselectivity was observed in the presence of an excess of a racemic mixture of cavitand receptor. Centrosymmetric crystals consisting of the complexes cR-cav1·N-methyl-D-leucine and cS-cav1·N-methyl-L-leucine were obtained from co-crystallisation of the racemic mixture of cR-cav/cS-cav 1 and N-methyl-D/L-leucine. Differently to the solution case, in the solid state the enantioselectivity of the chiral cavitand is complete and only the two enantiopure mirror complexes were obtained.