Antioxidant phenolics from the leaves of Alstonia boonei de Wild

Alstonia boonei De Wild (Apocynaceae) leaves are used traditionally in West Africa for the treatment of various ailments including rheumatic and muscular pains as well as hypertension and malaria¹. Chemical investigation of the ethyl acetate fraction of the leaf extract led to the isolation of 10 phenolic compounds. The structures of these compounds were determined using UV, HPLC-MS, 1 D and 2 D NMR spectroscopy and identified as follows: Quercetin-3-O- [α-L-rhamnopyranosyl (1→6)] β-D-glucopyranoside (1); quercetin-3-O- [α-L-rhamnopyranosyl (1→6)] β-D-galactopyranoside (2); kaempferol 3-O- [α-L-rhamnopyranosyl (1→6)] β-D-glucopyranoside (3); kaempferol -3-O- [α-L-rhamnopyranosyl (1→6)] β-D-galactopyranoside (4); quercetin-3-O- [α-L-rhamnopyranosyl (1→4)] β-D-glucocpyranoside (5); quercetin-3-O- [α-L-rhamnopyranosyl (1→4)] β-D-galactopyranoside (6); quercetin-3-O- [α-L-rhamnopyranosyl (1→2)] β-D-glucopyranoside (7); quercetin-3-O- [α-L-rhamnopyranosyl (1→2)] β-D-galactopyranoside (8); chlorogenic acid (9) and 3 – 4-dicaffeioylcinnamic acid (10). These compounds are being reported for the first time from Alstonia spp. Compounds 1, 2, 5, 6 and 9 showed high antioxidant activity on DPPH free radical scavenging model with IC₅₀ values of 52, 54, 39, 65 and 49 µg/mL respectively. The two kaempferol derivatives (3 and 4) did not show good antioxidant activity (IC₅₀ > 100 µg/mL). The antioxidant activity of some of the isolated quercetin derivatives and chlorogenic acid may explain the ethnomedicinal use of the plant extracts in the management of diseases associated with oxidative stress.

References:

[1] Iwu, M.M. 1993. Handbook of African Medicinal Plants. CRC Press, Boca Raton, FL, pp. 116 – 118