New C2-symmetric 2,2′-bipyridine crown macrocycles for enantioselective recognition of amino acid derivatives

A series of new C-2-symmetric 2,2'-bipyridine-contaning crown macrocycles 1-4 has been developed for enantiomeric recognition of amino acid derivatives. These new macrocycles have been showed to be strong complexing agents for primary organic ammonium salts (with K up to 4.83 X 10(5) M-1 and -Delta G(0) up to 32.4 kJ mol(-1)) and also useful chromophores for UV-vis titration studies. These macrocyclic hosts exhibited enantioselective binding towards the (S)-enantiomer of phenylglycine methyl ester hydrochloride (Aml) with K-(S)/K-(R) up to 2. 10 (Delta Delta G(0) = - 1.84 kJ mol(-1)) in CH2Cl2 with 0.25% CH3OH. The structure-binding relationship studies showed that the aromatic subunit and the ester group of the ammonium guests are both important for good enantioselectivity. In addition, the host-guest complexes have been studied using various NMR experiments. (c) 2005 Elsevier Ltd. All rights reserved.