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

A series of new C2-symmetric 2,2′-bipyridine-contaning crown macrocycles 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 4.83×105 M−1 and −ΔG0 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 () with K(S)/K(R) up to 2.10 (ΔΔG0=−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.