3,4-Dimethoxy-3-cyclobutene-1,2-dione (compound 30) was synthesized using a modified Liu et al. method [7]. The procedure was as follows: 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid, 2.053 g, 18 mmol) was dissolved in anhydrous methanol (18 mL), followed by addition of trimethyl orthoformate (4 mL, 36.5 mmol) to the solution. The reaction mixture was refluxed at 56 °C for 24 hours. Upon completion of the reaction, the crude product was concentrated under reduced pressure. The resulting light yellow solid was dissolved in dichloromethane and purified by silica gel column chromatography (eluent ratio of ethyl acetate: hexane = 1:2) to afford 3,4-dimethoxycyclobut-3-ene-1,2-dione (2.29 g, 89.4% yield) as a final white solid. The melting point of the compound is 55-57 °C (literature value 56-58 °C) [7,8]. NMR hydrogen spectrum (300 MHz, CD2Cl2) δ 4.34 (6H, s, OCH3); NMR carbon spectrum (75 MHz, CD3OD) δ 189.35 (C2, C=O), 189.35 (C1, C=O), 184.35 (C3), 184.35 (C4), 60.29 (OCH3), 60.29 ( OCH3). Low resolution mass spectra (EI): m/z 142.03 (100%), 114.03 (18%), 99.01 (16%), 86.01 (54%), 67.99 (8%), 56.25 (7%).
