Studies on the synthesis of 2-phenylsulphonyl-3-styrylquinoxalines

o-Phenylenediamine 1 is condensed with pyruvic acid or sodium pyruvate to y ield 3-methylbenzo-1H-dihydropyrazine-2-one 2. The latter oil condensation with aromatic aldehydes give 3-styrylquinoxaline-1 H-2-one 3. Reaction of 3 with POCl3 in the presence of catalytic amount of DMF yields 2-chloro-3-st yrylquinoxaline it, which on reaction with thiophenol in the presence of tr iethylamine in methanol gives 2-phenylrhio-3-styrylquinoxaline 5. Oxidation of 5 with H2O2, in the presence of acetic anhydride affords 2-phenylsulpho nyl-3-styrylquinoxaline 6. 6 has also been prepared directly from 4 by reac tion with sodium benzenesulphinate under phase transfer conditions. In an a lternate approach to the synthesis of title compound, 2 is treated with POC l3 in DMF to obtain the known 2-chloro-3-methylquinoxaline 7. Reaction of 7 with thiophenol in the presence of triethylamine in methanol gives 2-methy l-3-phenylthioquinoxoline 8. Oxidation of 8 with H2O2 in Ac2O affords 2-met hyl-3-phenylsulphonylquinoxaline 9. 9 was also prepared in an alternate met hod from 7 by reaction with sodium benzenesulphinate. 9 on reaction with ar omatic aldehydes yields the title compounds 6.