1,2,3-Trimethoxybenzene → 3,4,5-Trimethoxybenzoic Acid

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Reaction Steps

A mixture of 1,2,3-Trimethoxybenzene (1.48 g) and 2-hydroxy-3,4,5-trimethoxybenzoic acid (2.00 g) is stirred in 40 ml of ≈9% solution of P2O5 in methanesulfonic acid at room temperature in a stoppered flask for 4 hours. The 2-hydroxy-3,4,5-trimethoxybenzoic acid was obtained by the method of Mayer and Fikentscher (Mayer and Fikentscher (1956) Chem. Ber. 89:511) from 3,4,5-trimethoxybenzoic acid by bromination and then copper-catalyzed replacement of bromine (by OH) of 2-bromo-3,4,5-trimethoxybenzoic acid. The resultant orange mixture is poured onto crushed ice (500 ml) producing an unfilterable gummy precipitate. This crude product is then subjected to base-catalyzed ring closure by heating in a beaker in 100 ml of 40% ethanol and 10 ml of 10N NaOH just below boiling point. As the mixture reaches 80° C., a white flocculent product appears. The temperature is maintained just below the boiling point and the volume is kept constant by addition of water. After 5 hours, the supernatant is bright yellow and a mass of the precipitate has formed. Heating is continued for 4 more hours. Cooling, filtering (by suction) and washing with water afforded 1.37 g of analytically pure 2,3,4,5,6-pentainethoxyxanthone (yield approximately 45% relative to benzoic acid). This base-catalyzed ring closure is illustrated below:

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