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In a continuing search for neuroprotective stilbenoids, two uncommon diastereomeric stilbenoid glucosides, africanoside A (1) and B (2) as well as fifteen known stilbenes, E-resveratrol, E-gnetifolin E, E-piceid, E-resveratroloside, E-gnetifolin K, E-gnetol, E-isorhapontigenin, E-isorhapontin, E-isorhapontigenin-4′-O-glucopyranoside, E-gnetin C, E-bisisorhapontigenin B, E-gnemonoside A, E-gnemonoside C, E-gnemonoside D, and E-gnetin E, were isolated from rhizomes of Gnetum africanum, using a combination of centrifugal partition chromatography and preparative HPLC. The structure of these two stilbenoids was investigated by NMR, vibrational circular dichroism (VCD), and electronic circular dichroism (ECD) experiments. The absolute configurations of 1, and 2 were established by VCD as (7aS,8aS) and (7aR,8aR), respectively. Compound 1 significantly chelated Fe(II) at 100, 250 and 500 μM. The ability of stilbenoids to chelate ferrous ions which are implicated in physiopathological hallmark of Alzheimer’s disease provides useful data for potential treatment.

Dimeric stilbene glucosides 1–3 [two diastereomers of (−)-gnemonoside A (1a and 1b), (−)-gnemonoside C (2), and (−)-gnemonoside D (3)] as well as a mixture of the two enantiomers of gnetin C (4) were isolated from the rhizomes of Gnetum africanum. The two enantiomers of gnetin C, (+)-4 and (−)-4, were obtained from the aglycones of 1a and 1b, respectively. The configurations of these stilbenoids were investigated by NMR and vibrational circular dichroism (VCD) experiments. The absolute configurations of (−)-1a, (−)-2, (−)-3, and (−)-4 were established as 7aS,8aS by VCD spectroscopy in combination with density functional theory calculations. The antiamyloidogenic activity of the isolated stilbenes was also evaluated versus beta-amyloid fibrils. The four glucosides of gnetin C (1a, 1b, 2, and 3) were found to be the most active compounds, with inhibition percentages of 56, 56, 58, and 54 at 10 μM, respectively.