The reaction of methyl-5-acetylsalicylate (I) with alpha-bromo-gamma-butyrolactone (II) by means of K2CO3 in refluxing acetone gives alpha-[(2-methoxycarbonyl-4-acetylphenyl)oxy]-gamma-butyrolactone (III), which is hydrolyzed with NaOH in methanol yielding alpha-[(2-carboxy-4-acetylphenyl)oxy]-gamma-butyrolactone (IV). The cyclization of (IV) with refluxing acetic anhydride affords 5-acetyl-4',5'-dihydrospiro[benzofuran-2(3H),3'(2'H)-furan]-2',3-dione (V), which is finally treated at 155 C in dimethylsulfoxide containing NaCl.