10 mmol of 7-methoxy-3,4-dimethylcoumarin were stirred with 10 mmol
(4.04 g) of Lawesson's reagent (2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-
diphosphetane 2,4-disulfide) in 20 ml of toluene at 90°C for 1-2.5 h. After
cooling, the solvent was distilled off, the residue was taken up in DMF, and
water was added. The precipitate was filtered off with suction and the crude
3,4-dimethyl-7-(2-isopropyl-1,3,4-thiadiazol-5-ylmethoxy)-2-thiocoumarin was
purified by recrystallization from methanol or by column chromatography
(silica gel, methylene chloride). Yield: 98%, melting point: 134-135°C.
