Chiral sugar recognition by a diboronic-acid-appended binaphthyl derivative through rigidification effect

To obtain a 'rigid' saccharide receptor with a high chiral discrimination ability two 2-boronylphenylmethyl groups were directly introduced into OH groups in (R)-2,2'-dihydroxy-1,1'-binaphthyl. This compound (R)-5 increased its fluorescence intensity upon the formation of intramolecular 1:1 complexes with saccharides, mainly because of the 'rigidification effect'. The association constants (K) could be determined from plots of the fluorescence increase against the saccharide concentration. It was found that when the K difference between enantiomers is relatively small (K(D)/K(L)= 1.1/1.0~ 1.0/1.7) the enantiomer with the larger fluorescence increase shows the smaller K. On the other hand, xylose and talose, which resulted in a chiral discrimination ability of 8.7-fold and 2.0-fold, respectively, showed the reverse fluorescence-K relationship. It is noteworthy that the chiral discrimination factor for xylose is the highest among those achieved so far.