Design and Synthesis of a Trifunctional Chiral Porphyrin with C₂ Symmetry as a Chiral Recognition Host for Amino Acid Esters

An intrinsic chiral recognition host, (R,R)- or (S,S)-[trans-5,15-bis(2-hydroxyphenyl)-10-{2,6-bis((methoxycarbonyl)methyl)phenyl}-2,3,17,18-tetraethylporphyrinato]zinc(II) (1), was synthesized by the coupling between (3,3',4,4'-tetraethyl-5,5'-bis(α-hydroxy-2-methoxybenzyl)-2,2'-dipyrryl)methane (8) and dimethyl 2-(bis(2-pyrryl)-methyl)-l,3-benzenediacetate (16). This pyrrylmethanol method made it possible to perform the regiospecific coupling between differently functionalized dipyrromethane units. Host 1 was designed to have three recognition elements: metal coordination, hydrogen bond donor, and hydrogen bond acceptor (and/or steric repulsion) groups. These groups are arranged in a convergent fashion, forming a chiral recognition pocket. Host 1 was resolved into two enantiomers, (+)-1 and (-)-1. The binding constants in CHC1₃ were determined by UV-vis titration. Host (+)-1 was found to show an enantioselectivity of 2.0-2.8 in respect to l- and d-enantiomers of I1e-OMe, Leu-OMe, Leu-OBzl, Val-OMe, Pro-OMe, and Phe-OMe. Host (+)-1 showed an enantioselectivity of 0.47 in respect to l- and d-enantiomers of serine benzyl ester, indicating that the enantioselectivity was reversed. Reference porphyrins 2-4, which lack some of recognition groups, were also synthesized by the pyrrylmethanol method to clarify the roles of the recognition groups of (+)-1 in thermodynamics of the binding processes. Total free energy change upon binding of l- and d-Ile-OMe to host (+)-l (ΔG°_total for L, -5.05, and d, -4.46 kcal/mol) was separated into three terms: metal coordination energy (ΔG°_Zn)>-4.15 kcal/mol; hydrogen bond energy (ΔΔG°_OH),-1.30 kcal/mol; and steric repulsion energy (ΔΔG°^L_COOMe or ΔΔG°^D_cooMe), +0.40 kcal/mol for l- and +0.99 kcal/mol for d-Ile-OMe. The third recognition group (CH₂CO₂Me) of (+)-l was found to destabilize the complexes due to steric repulsions. In contrast, the CH₂CO₂Me group was found to stabilize the complex between d-Ser-OBzl and (+)-l, suggesting that hydrogen bonding between the OH group of serine and the C=O group of (+)-l takes place. On the basis of these thermodynamic studies, chiral recognition was found to be achieved by cooperative functions of these three recognition groups.