反应详情

2-hydroxy-3,4,5-trimethoxybenzoic acid (1.14 g, 0.005 mol) and 1,2,3,4-tetramethoxybenzene (0.99 g, 0.005 mol) and 25 ml of a 9% solution of P2O5 in methanesulfonic acid were shaken in a 50 ml cylindrical glass tube with a Teflon-lined screw-cap at room temperature for 54 hours. The dark orange mixture was then poured onto crushed ice (150 ml). After melting, the product was extracted with methylene chloride (3×40 ml). After removal of the solvent, the residue was chromatographed on silica gel (30 g) with CH2Cl2. Of the three fractions obtained (the eluent was monitored by thin-layer chromatography), the middle one was uniform and left pure 2-hydroxy-3,4,5,2′,3′,4′, 5′-heptamethoxybenzophenone (0.51 g, 25%) as a yellow oil upon evaporation of the solvent. This was dissolved in 100 ml 75% alcohol whereafter 5 ml of 10N NaOH were added and the mixture was heated to boiling in a beaker for three hours; the volume was kept at 100 ml by occasional addition of water. The mixture was then transferred to a 250 ml round bottom flask and refluxed for another 17 hours. After cooling, suction filtration yielded 0.36 g of 2,3,4,5,6,7-hexamethoxyxanthone as a white product (small needles, matted, 77%). In a deprotection procedure, similar to the one described above for pentamethoxyxanthone, 0.42 g of the hexamethoxyxanthone produced 0.296 g of hexahydroxyxanthone (91%) as a pale yellow powder. It was found advantageous to circumvent the need for centrifugation by stirring the methylene chloride-water mixture (from the quenching of the BBr3-solution) in a wide-mouthed container for several hours, leading to the evaporation of the methylene chloride; the mixture is then easily filterable.

1. customlined screw-cap at room temperature for 54 hours
2. additionThe dark orange mixture was then poured
3. extractionthe product was extracted with methylene chloride (3×40 ml)
4. customAfter removal of the solvent
5. customthe residue was chromatographed on silica gel (30 g) with CH2Cl2
6. customOf the three fractions obtained (the eluent
7. customupon evaporation of the solvent
8. dissolutionThis was dissolved in 100 ml 75% alcohol whereafter 5 ml of 10N NaOH
9. additionwere added
10. temperaturethe mixture was heated
11. additionthe volume was kept at 100 ml by occasional addition of water
12. customThe mixture was then transferred to a 250 ml round bottom flask
13. temperaturerefluxed for another 17 hours
14. temperatureAfter cooling
15. filtrationsuction filtration