Six new 2-alkylthio-5-(3,4,5-trimethoxyphenyl)-1,3,4-thiadiazole derivatives were synthesized from gallic acid by six steps: etherification, esterification, hydrazidation, salt formation, cyclization, and thioetherification. The synthesis of the title compounds catalyzed by indium in aqueous media was easy in process with high yield and environmental-friendliness. The products were characterized by elemental analysis, IR, ¹H NMR and ¹³C NMR spectra. The crystal structure of compound 8a was determined by X-ray diffraction analysis. 8a belongs to the monoclinic system with space group C2/c and cell dimensions of a＝1.5730(19) nm, b＝0.5477 nm, c＝4.2244 nm, β＝92.015(6)°, V＝3.6370(2) nm³, Z＝8, D_c＝1.497 g/cm³, ＝0.463 mm^－1, F(000)＝1696, R₁＝0.0783, wR₂＝0.1832. It was found that the title compounds 8 possess good antitumor activity to PC3 and BGC-823 cells in vitro by MTT method. For example, the antiproliferation activity of compound 8e to PC3 cells at the concentration of 5 μmol•L^－1 at 48 h was 55.71% and the antiproliferation activity of compound 8b to BGC-823 cells at the concentration of 5 μmol•L^－1 at 48 h was 66.21%.

Key words: 1,3,4-thiadiazole, 3,4,5-trimethoxyphenyl, crystal structure, antitumor activity