A solution of 20% lithiumhexamethyldisilazane in tetrahydrofuran (217.5 mL, 260 mmol) was added to a well-stirred solution of 2,4-dihydroxy acetophenone, 14 (9.88 g, 65 mmol) in tetrahydrofuran (150 mL) under nitrogen atmosphere at −78° C. for 30 min. The reaction mixture was stirred at −78° C. for 1 h and at −30° C. for 2 h. It was again cooled to −78° C. and a solution of ethyl-3,5-dibenzyloxy-benzoate 8 (23.5 g, 65 mmol) in tetrahydrofuran (100 mL) was added over a period of 30 min. It was further stirred at room temperature for overnight and poured into ice water containing hydrochloric acid. It was extracted with chloroform, dried over sodium sulphate, filtered and evaporated on rotavapor to afford 1-(3,5-bis-benzyloxy-phenyl)-3-(2,4-dihydroxy-phenyl)-propane-1,3-dione (16). The residue, 16 (37.8 g) was refluxed with Dowex-H+ (8 g) in 2-propanol (300 mL) for 34 h. 2-Propanol was evaporated on rotavapor, residue suspended in hot dimethyl formamide and filtered. The filtrate was 3,4-Dimethoxy-benzoic acid (10). Yield 2 g (8%); mp 179-181° C.; MS (FAB) 183 (M++1); IR (KBr) 2939, 2836, 1867, 1678; 1H NMR (200 MHz, CDCl3) δ 7.79 (dd, J=8.4 Hz, 1.9 Hz, 1H), 7.60 (d, J=1.8 Hz, 1H), 6.93 (d, J=8.5 Hz, 1H), 3.96 (s, 6H).
