Scheme 2.
Synthetic procedure of compounds 20 and 21. Reagents and conditions: a) Na2SO4, BF3·Et2O, toluene, N2, −78 °C, 1 h, 12%; b) DDQ, ClCH2CH2Cl, rt, 2 min, 74%; c) NaOH, CH3OH/H2O, rt, 1 h, 85%.
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