The bark of Magnolia officinalis Rehd. et Wils. was dried in the shade at room temperature and stored in a dark, cold room until use. The air-dried bark of Magnolia officinalis Rehd. et Wils. (3 kg) was cut into pieces and extracted twice with 95% (v/v) ethanol (four times as much as the weight of the dried plants) for 3 days at room temperature. After filtration through the 400-mesh filter cloth, the filtrate was filtered again through filter paper (Whatman Grade No. 5) and concentrated under reduced pressure. The combined extract (450 g) was suspended in H2O and the aqueous suspension was extracted with n-hexane, ethyl acetate, and n-BuOH, respectively. The n-hexane layer was evaporated to dryness to give a residue (70 g), which was chromatographed on silica gel with n-hexane:ethyl acetate (9:1) gradient to yield a crude fraction that included 4-O-methylhonokiol. This fraction was repeatedly purified by silica gel chromatography using n-hexane:ethyl acetate as the eluent to obtain pure 4-O-methylhonokiol (Fig. 1a). 4-O-methylhonokiol was identified by 1H-NMR and 13C-NMR. The results of the NMR data are as follows and are in agreement with previously published data [24 (link)]. 1H-NMR (400 MHz, CDCl3): δ 3.36 (2H, d, J = 7 Hz, H-7), 3.44 (2H, d, J = 7 Hz, 7′-H), 3.89 (3H, s, OMe), 5.05–5.14 (5H, m, H-9, H-9′, OH), 5.93–6.07 (2H, m, H-8, H-8′), 6.92 (1H, d, J = 7 Hz, Ar-H), 6.97 (1H, d, J = 8 Hz, Ar-H), 7.04–7.08 (2H, m, Ar-H), 7.24–7.31 (2H, m, Ar-H). 13C-NMR (100 MHz, CDCl3): δ 34.5 (C-7), 39.6 (C-7′), 55.8 (OMe), 111.2 (C-3′), 115.7 (C-4′), 115.8 (C-9), 116.1 (C-9′), 128.0 (C-1′), 128.1 (C-6), 129.0 (C-3), 129.2 (C-1), 130.0 (C-5), 130.4 (C-6′), 130.7 (C-2), 132.4 (C-5′), 136.7 (C-8), 138.0 (C-8′), 151.0 (C-2′), 157.2 (C-4). The ethanol extract of Magnolia officinalis contained 16.6% 4-O-methylhonokiol, followed by 16.5% honokiol and 12.9% magnolol, and 42–45% others.

Chemical structure of 4-O-methylhonokiol (a) and experimental scheme (b)