Synthesis And Properties Of Chiral Cyclohexanediamine Schiff Base Macrocycle

Chirality is a common phenomenon in the biological world.Homochirality is the basic characteristics of biological macromolecules,such as L-amino acids and D-sugars,which are the basic building blocks of proteins and nucleic acids.Respectively,they are chiral enantiomers.A large number of drug molecules have a chiral structure containing both D-and L-enantiomers,leading to the mirror image arrangements of the two forms eliciting quite different physiological responses.Researchers have been making continuous efforts to achieve rigorous analysis and discrimination of enantiomers.The structure of chiral Schiff base（Salen）complex are similar to many functional enzymes in living organs,thereby leading to their structural advantages for selective binding and structure and function of biologically orangic molecule in living systems.Chiral Schiff base macrocyclic compounds have better synergy due to convergence of donor/acceptor functional groups on the ring margin,resulting in further enhanced selective recognition and separation of chiral biomolecules and their pharmaceutical molecules.In this paper,chiral Schiff base metal macrocyclic complexes R,R-Zn₂（H₂O）（salen）₂(M_R,R)and S,S-Zn₂（H₂O）（salen）₂(M_S,S)were synthesized from R,R-and S,S-chiral cyclohexanediamines,and two enantiomers of the mono-Schiff base macrocycle containing chiral NH moiety in the cyclic structure（C_R and C_S）,and their molecular structures were obtained via the single-crystal X-diffraction technique.On this basis,on the one hand,the host-guest roles of between the Schiff base metallacyclic host with enantimer ofα-amino acid are investigated.These results demonstrate that chiral host M_R,R exhibits the highest enantioselectivity for enantimers ofα-Arg,and M_S,Sexhibits the highest enantioselectivity for enantimers ofα-Lysine respectively.Further,CD spectra studies indicate that the configuration of the central chelate ring Zn N₂O₂ remains unchanged.However,the CD couplet signs are inverted in the higher-energies while the same chiral host binding interaction with the D-/L-isomers respectively,demonstrating that sign of the Cotton effect in CD spectra can refect the structure and chirality of the guest,which can be used as discriminate forα-Arginine andα-Lysine enantiomers.Furthermore,DTF calculation shows that the binding affinity and enantioselectivity of metallacyclic host toward enantimers of the amino acids are attributed to the hydrogen-bonding interactions between the amino acid molecule with the bridging H₂O located in binuclear zinc（Ⅱ）metallacyclic complexes.On the other hand,the recognition effect of C_R and C_S on penicillamine enantiomers was investigated.Comparison of bonding constants based on the chiral macrocycle binds with the enantiomers of penicillamine,the results show that the chiral C_R exhibits higher enantioselectivity for L-Pen,while C_S exhibits the higher enantioselectivity for D-Pen,respectively.The interactions between different enantiomers of the chiral macrocycle and penicillamine are attributed to the intermolecular hydrogen bonding of enantiomers by the asymmetrical chiral NH moiety in the mono-Schiff base macrocycle.Further,investigation of CD spectroscopic titration indicate that the enantiomer of penicillamine with the same chirality as the host macrocycle binds comparatively stronger with the host as compared to its enantiomer.