New acylated triterpenoid saponins from the roots of Acacia polyacantha Willd. (Fabaceae)

Abstract

In our continuing search of saponins from the plants of Fabaceae family, phytochemical investigation of the roots of Acacia polyacantha, led to the isolation and structural characterization of six undescribed triterpenoid saponins, named polyacosides A-F (1-6). Their structures were established, using extensive analysis by NMR techniques, mainly 1D NMR (¹H, ¹³C, and DEPT) and 2D NMR (COSY, NOESY, HSQC, TOCSY and HMBC) experiments, HRESIMS and by comparison with the literature data, as 3-O-[β-d-xylopyranosyl-(1 → 4)- [β-d-galactopyranosyl-(1 → 2)-β-d-xylopyranosyl-(1 → 2)]-α-l-arabinopyranosyl]-21-O-[Cis-2-methoxycinnamoyl] machaerinic acid (1), 3-O-[β-d-xylopyranosyl-(1 → 4)- [β-d-galactopyranosyl-(1 → 2)-β-d-xylopyranosyl-(1 → 2)]-α-l-arabinopyranosyl]-21-O- [Cis-3,4-dimethoxycinnamoyl] machaerinic acid. (2), 3-O- [β-d-galactopyranosyl-(1 → 2)-β-d-xylopyranosyl-(1 → 2)-α-l-arabinopyranosyl]-21-O- [Trans-4-methoxycinnamoyl] machaerinic acid (3), 3-O- [β-d-glucopyranosyl-(1 → 2)-β-d-glucopyranosyl] -21-O- [Cis-3,4-dimethoxycinnamoyl] machaerinic acid (4), 3-O- [β-d-glucopyranosyl-(1 → 2)-β-d-glucopyranosyl] -21-O- [Cis-2-methoxycinnamoyl] machaerinic acid (5) and 3-O- [β-d-glucopyranosyl-(1 → 2)-β-d-glucopyranosyl] -21-O- [Trans-4-methoxycinnamoyl] machaerinic acid (6). Our findings highlight the presence of methoxycinnamoyl group linked to C-21 of the machaerinic acid aglycone moiety as first report of 21-methoxycinnamoyl-machaerinic acid derivative from the plants of Acacia genus (Fabaceae). This represents therefore a valuable contribution to the chemotaxonomy of the Acacia genus of Fabaceae family, which is known to be a rich source of triterpenoid saponins.

Keywords: Acacia polyacantha; Acylated triterpenoid saponins; Fabaceae; NMR; Polyacosides.