Secondary metabolites isolated from Rhigozum madagascariensis rhizome with antituberculosis properties

Abstract

Rhigozum madagascariensis Drake is an endemic plant from the Toliara region of Madagascar [1], which belongs to the Bignoniaceae family or Trumpet Creeper Family. The Bignoniaceae family comprising about 650 – 750 species in 116 – 120 genera. Members of the family are mostly trees and lianas, shrubs and more rarely herbaceous plants, some of them are used as ornamental plants due to their large and often colourful flowers, and others in horticulture as the calabash tree (Crescentia cujete). The family has a cosmopolitan distribution and is present in both the Old World and the New World, with Catalpa the only genus common to both. Members are distributed mostly in the tropics and subtropics, with a great diversity in South America. Thirteen species in eight genera are present in Southern Africa, of which ten species from five genera are particularly found in Madagascar. Only few phytochemical studies from Madagascar Bignoniaceae have been reported until now. Air-dried and finely powdered Rhigozum madagascariensis rhizome (2.17 kg) was extracted with n-hexane in a Söxhlet system for 8h to yield 12.9 g of extract. The extract was kept at -20 °C for 24h and the soluble fraction concentrated under vacuum (˜ 4.9 g) re-dissolved in MeOH and kept at -20 ° C for 48h, to remove waxes. The methanol soluble part after concentration (˜ 4.6 g) was treated with hexane/acetone (2:1) and kept at 5 °C during one week to remove chlorophylls yielding a final residue of 3.2 g. The residue was chromatographed over silica gel with hexane/ethyl acetate (8:2) and ethyl acetate to yield eight fractions. Fractions were re-chromatographed and/or crystallized to isolated and identified the naphtoquinone: dehydro-a-lapachone [2], and dimeric quinones: tecomaquinone I [3], II – V, in addition to fatty acids (palmitic, oleic, linoleic and myristic acids) and the sterols stigmasterol and β-sitosterol.