Selective Demethylation of Polymethoxyxanthones with Aqueous Piperidine

Synthesis of Minor Xanthones from Garcinia mangostana

“…A similar separation (0.18-0.20 ppm) is also apparent in the spectra of other 4-(3-methylbut-2-enyl)xanthones (21)(22)(23). Although a variety of reagents have been used in the selective demethylation of polymethoxyxanthones (24,25), only aqueous piperidine or aqueous morpholine (5) is eminently suited to effect such reaction involving 3-methylbut-2-enylated poly- days gave, in addition to the expected gartanin [1], a trihydroxymethoxyxanthone which showed two chelated hydroxyl protons in its 'H-nmr spectrum. The third hydroxyl was placed in the 3 position because the uv spectrum showed a bathochromic shift of the long-wavelength band on addition of NaOAc.…”

“…1.3.8-Tribydroxy-7-methoxy-2-(3-metbylbut-2-tnyl)xanthom [17].-Yellow needles (85 mg): mp 195-197°(aqueous MeOH); uv X max (log e) 385(3.60), 327(4.20), 271(4.41), 264(4.39), 240(4.41); +NaOAc 372 (4.25), 271(4.44), 263 (4.41), 239 nm (4.46); 'H nmr (CDC13/DMSO-0 1.68, 1.80 (3H each, s, Me2C), 3.36 (2H, brd, CH2), 3.92(3H, s, 7-OMe), 5.27 (1H, brt, CH), 6.42(1H, s, H-4), 6.79, 7.23 (1H, each, d,/ = 9.1 Hz, H-5, H-6), 9.94 (1H, br s, 3-OH), 12.12, 12.29 (1H each, s, 8-OH, 1-OH); ms mlz 342 (84%), 327 (25), 299 (94), 287 (100). Anal, caled for C19H1806, C 66.66, H 5.30; found C 66.48, 5.07.…”

Synthesis of 1,2,3,8-Tetraoxygenated Xanthones

“…Selective demethylation at C-3 or C-6, of polymethoxy xanthones, when heated under reflux in aqueous piperidine, has been described in the literature (22,29,33). This selectivity has been explained (29) because the oxygen atom para to a carbonyl group is the least electronegative.…”