From MonoBINOL to BisBINOL: Expanded Enantioselective Fluorescent Recognition of Amino Acids

Condensation of the methoxymethyl-protected (R)-3,3'-diformyl-1,1'-bi-2-naphthol (BINOL) with (pyridine-2,6-diylbis(methylene))bis(triphenyl phosphonium)dibromide in the presence of a base followed by deprotection gave a new bisBINOL-based fluorescent probe (R,R)-4. This compound showed expanded substrate scope in the recognition of amino acids with good enantioselective fluorescence responses toward 17 common amino acids. Two diastereomeric imines were synthesized from the condensation of (R,R)-4 with l- and d-valine, and the reactions of these imines with Zn(OAc)₂ were investigated by various spectroscopic methods for a better understanding of the enantioselective fluorescent recognition process.