Gas-phase enantioselectivity of chiral N-linked peptidoresorc[4]arene isomers toward dipeptides

The gas-phase enantioselectivity of cone N-linked peptidoresorc[4]arenes (generally symbolized as M) toward the homologue dipeptides (generally symbolized as A) has been evaluated by measuring the kinetics of the A release from the diastereomeric [M·H· A]+ complexes induced by (R)-(-)-2-butylamine (B). In most cases investigated, the heterochiral [M·H· A]+ complexes, namely those wherein the configuration of the A guest is opposite to that of the host M pendants, react faster (up to 5 times) than the homochiral analogues, wherein guest A guest has the same configuration of the host M pendants. The kinetic results, discussed in the light of previous MS and NMR evidence, indicate that both the efficiency and the enantioselectivity of the guest exchange reaction depend essentially on the structure and the relative stability of the diastereomeric [M·H· A]+ complexes. These, in turn, depend on the functional groups and the configuration of both the guest and the host pendants. The absence of any significant effects of the B configuration indicates that, in all systems investigated, the dipeptide guest A is predominantly located outside the host chiral cavity.