A dimeric triterpenoid glycoside and flavonoid glycosides with free radical-scavenging activity isolated from Rubus rigidus var. camerunensis

The aerial part of Rubus rigidus var. camerunensis (Rosaceae) is used to treat respiratory and cardiovascular disorders in the Cameroonian traditional medicine. The ethanol extract exhibited more potent antioxidant activity (Emaxs of 119% and 229% activity on DPPH and β-carotene test) than aqueous extract. Bioactivity-guided fractionation of the ethanol extract based on free radical-scavenging assay (DPPH assay) afforded five flavonoid glycosides (four flavonol glycosides and an anthocyanin) and three glucosides of 19α-hydroxyursane-type triterpenoid (two monomeric and one dimeric triterpenoids). The flavonoids were identified as kaempferol 3-O-(2″-O-E-p-coumaroyl)-β-D-glucopyranoside (1), kaempferol-3-O-β-D-glucopyranoside (astragalin, 2), kaempferol-3-O-α-L-arabinofuranoside (juglanin, 3), quercetin-3-O-β-D-glucopyranoside (isoquercitrin, 4), pelargonidin-3-O-β-D-glucopyranoside (callistephin, 5). The three triterpenoids were 2α, 3β, 19α, 23-tetrahydroxyurs-12-ene-28-O-β-D-glucopyranosyl ester (nigaichigoside F1, 6), 2α, 3β, 19α-trihydroxyurs-12-ene-23-carboxyl-28-O-β-D-glucopyranosyl ester (suavissimoside R1, 7) as monomeric triterpenoids and coreanoside F1 (8) as a dimeric triterpenoid. The flavonoids exhibited potent antioxidant activities (66 to 93.56% against DPPH radical) and they were also active on β-carotene test. Coreanoside F1 exhibited a 63% antioxidant activity, meanwhile the other two triterpenoids showed a weak activity. Three important facts on structure-activity relationship were observed: Compound 8, a dimeric triterpenoid glycoside, strongly enhanced antioxidant activity of its monomers, compound 3 with 3-O-α-L-arabinofuranyl has much more potent activity than compound 2 with 3-O-β-D-glucopyranosyl, and antocyanin (5) is more potent than its corresponding flavonol glycosides.