The synthesis of the first members of a new class of cyclic peptide-containing hemicryptophanes is described. Synthesis was achieved through attachment of veratryl groups to the L-tyrosine side chains of a cyclic hexapeptide, c(YG)3, followed by intramolecular cyclodehydration to generate the CTV unit. The diastereomeric P- and M-hemicryptophanes were generated in a 2 : 1 ratio and were separated by chromatography. The enantioselective binding properties of the hemicryptophanes were investigated by complexation with carnitine. Both isomers were found to have significant selectivity for binding (R)-carnitine.
The first water-soluble hemicryptophane cage compound, 4, was synthesized in seven steps from commercially available products, and its complexation properties were studied. NMR and ITC experiments indicate that 4 is an efficient receptor for choline in water (ΔG° = -5.2 kcal mol(-1)). High substrate selectivity was achieved since no complexation was observed for its closely related substrates: glycine betaine and betaine aldehyde. Density functional theory calculations were performed to analyze the interactions that are involved in the formation of the inclusion complex.
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