Synthesis and characterization of 4-ethoxymethylene-2-[1]-naphthyl-5(4H)-oxazolone and its fluorescent amino acid derivatives

4-Ethoxymethylene-2-[1]-naphthyl-5(4H)-oxazolone 1b was synthesized from 1-naphthoyl-glycine and triethyl orthoformate as a fluorescent analogue of the hapten 4-ethoxymethylene-2-phenyl-5(4H)-oxazolone. A simple and general method has been devised to prepare conjugates of type 6 between 1b and amines and amino acids by direct reaction. Compounds 6 are stable in solid form and in aqueous solution at 20°C, but are a mixture of two isomers. 1H- and 13C-NMR experiments on model compounds have identified the more stable isomer. Comparative analysis of fluorescence properties of 1b and of amino acid derivatives has identified the spectral properties of two chromophores. An intense and pH-sensitive emission band centered at λ=460 nm (λex=360 nm) in aqueous solution has been discovered. This feature might be useful for monitoring cellular compartments of different pH.