Enantiomeric recognition of amino acid salts by macrocyclic crown ethers derived from enantiomerically pure 1,8,9,16-tetrahydroxytetraphenylenes

Asymmetric synthesis of (R,R)- and (S,S)-1,8,9,16- tetrahydroxytetraphenylenes was achieved from starting material (2R,3R)-butane-2,3-diol and (2S,3S)-butane-2,3-diol respectively by utilizing a center-to-axis strategy. A series of crown ether compounds 20, 24, and 25 and their corresponding enantiomers derived from chiral tetrahydroxytetraphenylene were synthesized in enantiomerically pure forms. Enantiomeric recognition properties of these hosts toward l- and d-amino acid methyl ester hydrochloride were studied by the UV spectroscopy titration. The tetramer hosts (S,S,S,S,S,S,S,S)-20 and (R,R,R,R,R,R,R,R)-20 exhibited the best enantioselectivities toward l- and d-alanine methyl ester hydrochloride salt with K_L/K_D = 4.1 and K_D/K_L = 3.9, respectively. The new chiral macrocyclic hosts would further enrich the host-guest chemistry.