Molecular recognition of chiral porphyrins with amino acid esters

Association constants between α,α,α,β- ZnT (o- BocTyr ) TAPP (1), α,α,α,β- ZnT (o- BocAla )- TAPP (2), and a series of amino acid esters were determined in chloroform by means of titrations monitored by UV-vis spectra. Association constants increased in the order K( AlaOMe ) < K( ValOMe ) < K( LeuOMe ), showing the same preference for the electronic effect of bulky amino acid esters. For the guest SerOMe and ThrOMe , the association constants of two hosts showed different orders; the order of 1 is K( SerOMe ) < K( ThrOMe ), and that of 2 is the reverse. The host 2 shows a D -/ L -selectivity of 6.9 for leucine methyl ester at 293 K in CHCl 3, which is the highest enantioselectivity achieved in our study. The host-guest system was studied by 1 H NMR spectra. On association, every proton experiences a small upfield shift. The intensity of circular dichroism (CD) spectra of 1 in the presence of optically active guests were reduced as compared with that of free 1, while that of 2 with optically active guests was of the induced split type. Molecular modeling of the host-guest systems were studied by simulated annealing on the basis of the Tripos force field. The minimal energies of the complexes were calculated, and the results show that the recognition behavior of hosts 1 and 2 toward amino acid esters is very different. The change of the interaction energy of 2 with guest molecules is larger than that of 1.