Synthesis of quinoxaline derivatives bearing the styryl and phenylethynyl groups and application to a fluorescence derivatization reagent

The cross-coupling of 2-chloro-6-methoxycarbonyl-3-methylquinoxaline (2) and 3-chloro-7-methoxy-1-methylquinoxalin-2(1H)-one (7) with phenylacetylene in the presence of Pd(PPh3)(4) gave 6-methoxycarbonyl-3-methyl-2-phenylethynylquinoxaline (3) and 7-methoxy-1-methyl-3-(4-methoxycarbonyl)phenylethynylquinoxalin-2(1H)-one (9), respectively. Compounds (3 and 9) were further converted into the corresponding olefinic compounds, 6-methoxycarbonyl-3-methyl-2-styrylquinoxaline (4) and 7-methoxy-1-methyl-3-(4-methoxycarbonyl)styrylquinoxalin-2(1H)-one (10), by partial hydrogenation on palladium catalysts such as Lindlar catalyst and Pd/BaSO4-quinoline, but the conformation of the resulting olefins was unexpectedly E-form. These quinoxaline derivatives showed fluorescent emission bands at a range from 398 to 467 nm in MeCN when the excitation wavelength of 353-405 nm was applied. Further, 3-(4-chlorocarbonyl)phenylethynyl-7-methoxy-1-methylquinoxalin-2(1H)-one (12) was demonstrated to be applicable to a fluorescence derivatization reagent for amines.