A practical enantioselective fluorescent sensor for mandelic acid

A novel optically active bisbinaphthyl fluorescent sensor, (S,S)- or (R,R)-1, is designed for the recognition of chiral alpha-hydroxycarboxylic acids. A convenient method has been developed to synthesize this compound. It is observed that (S)-mandelic acid enhances the fluorescence intensity of the (S,S)-sensor significantly more than (R)-mandelic acid does. The enantioselective fluorescent response is confirmed with the observation of a mirror image relationship for the interaction of (S,S)- and (R,R)-sensors with mandelic acid. The enantioselectivity in fluorescence response [(I(S) - I(0))/(I(R) - I(0)) = 2.49] is quite high, which makes the sensor useful for practical application. The fluorescence intensity change of the sensor is found to be linearly related to the enantiomeric composition of mandelic acid. This sensor is potentially useful for the combinatorial search of chiral catalysts for the asymmetric synthesis of alpha-hydroxycarboxylic acids.