Structural Characterization of a Minority Complex Triterpenoidal Saponin from the Methanolic Bark Extract of Albizia lebbeck (L.) Benth.

Albizia lebbeck (L.) Benth. (Fabaceae, Mimosoideae), is an arboreous species originating from Asia, widely distributed in tropical regions. Cultivated for ornamental and medicinal purposes, it is used in traditional medicine to treat several inflammatory conditions and is popularly known as a remedy to treat abdominal tumors1,2. In a previous investigation, a complex triterpenoid saponin was isolated as the main constituent of a methanolic extract of the barks of this species. Through chromatographic methods, spectroscopic techniques and chemical conversions, their chemical structure was determined3. We now report the investigation of a complex triterpenoid saponin present as a minor constituent of the methanolic bark extract of Albizia lebbeck (L.) Benth. Plant material was identified and collected at the at the Campus of the Federal University of Rio de Janeiro (UFRJ). Extraction was performed in methanol (315 g / 2.0L) at room temperature, followed by partitioning in n-butanol / water (1:1). The organic fraction was evaporated under reduced pressure and the dried residue was dissolved in methanol and subjected to open column chromatography (Sephadex LH-20) eluted with methanol. Based on TLC profiles, fractions were combined and chromatographed on a silica gel column eluted with CHCl3-MeOH-H2O (60: 40: 8) for further purification. Through data obtained by 1D NMR (1H, 13C and APT) and 2D NMR (COSY, HSQC and HMBC), a chemical structure was proposed. In the 1H NMR spectrum, seven anomeric hydrogen signals (δ 4.98, δ 5.04, δ 5.14, δ 5.34, δ 6.08, δ 6.16, δ 6.26) that correlate in the HSQC spectrum with the anomeric carbon signals (δ106.26, δ 104.76, δ 102.38, δ 105.76, δ 94.89, δ 101.64, δ 111.25) indicated the presence of seven carbohydrate monomers: one xylose (xyl), one N-acetylglucosamine (GlcNAc), one arabinopyranosyl unit (Arap), two glucoses (Glc 1 and Glc 2),one rhamnose (Rha) and one arabinofuranosyl unit (Araf), respectively. An unsaturated α, β carbonyl carbon signal (δ 167.83) along with olefinic carbons (δ 133.20, δ146.6, δ146.60, δ 111.75) and characteristic hydrogen signals (δ 7.24, δ 6.13, δ 5.17 / 5.57) indicated the presence of a monoterpene moiety . The signals of an ester carbonyl carbon at δ 176.01 (C28), olefinic carbons at δ144.24 (C 13) and δ 122.86 (C12), along with seven methyl hydrogen signals (δ 0.96, δ 1.04, δ 1.18, δ 1.19 , δ 1.46, δ 1.76, δ 1.88) indicated an acacic acid aglycone. Additional spectroscopic techniques and chemical conversions are necessary to confirm the chemical structure of this minority saponin and therefore perform an evaluation of its biological activities, in order to conduct an structure-activity relationship study .