Reaction Steps
10 g (24 mmol) of podophyllotoxin are dissolved in 60 ml of trifluoroacetic acid. 5.4 ml (72 mmol) of dimethyl sulphide and 47 ml (72 mmol) of methanesulphonic acid are successively added. Stirring is maintained for 9 hours, 5.4 ml (72 mmol) of dimethyl sulphide are again added and stirring is maintained for 9 hours. The medium is run quickly onto ice (600 ml) and extracted with ethyl acetate (3×300 ml). The organic phases are washed with water and then with a NaHCO3 solution to neutrality. After drying over sodium sulphate, filtering and evaporating, 6.3 g of crude demethylation product are obtained. Flash chromatography on silica (elution: CH2Cl2/acetone 9/1) makes it possible to isolate 550 mg of 5-(4-hydroxy-3,5-dimethoxyphenyl)-9-hydroxy-5,8,8a,9-tetrahydro-5aH-furo[3′,4′:6,7]naphtho[2,3-d][1,3]dioxol-6-one, that is to say 4′-demethylepipodophyllotoxin of formula (Va). 1.10·g of 5-(3,4-dihydroxy-5-methoxyphenyl)-9-hydroxy-5,8,8a,9-tetrahydro-5aH-furo[3′,4′:6,7]naphtho[2,3-d][1,3]dioxol-6-one (Analysed: C20H18O8.0.15H2O; calculated: C %, 61.74; H %, 4.74. found: C %, 61.67; H %, 4.68), and then 1.9 g of 9-hydroxy-5-(3,4,5-trihydroxyphenyl)-5,8,8a,9-tetrahydro-5aH-furo[3′,4′:6,7]naphtho[2,3-d][1,3]dioxol-6-one are also isolated. The latter compound has a proton NMR spectrum with the following characteristics: 1H NMR (d6-DMSO) δ 8.65 (m, 2H), 7.95 (m, 1H), 6.71 (s, 1H, H5), 6.47 (s, 1H, H8), 5.98 (d, 2H, J=2 Hz, OCH2O), 5.93 (s, 2H, H2′, H6′), 4.68 (d, 1H, J=3.2 Hz, H4), 4.34 (t, 1H, J=8 Hz, H11a), 4.29 (d, 1H, J=5.2 Hz, H1), 4.16 (dd, 1H, J=8 Hz, J′=10 Hz, H11b), 3.17 (dd, 1H, J=5.2 Hz and J′=14 Hz, H2), 2.76 (m, 1H, H3).
