Isolation and structure elucidation of four new pentacyclic diterpene polyesters from Euphorbia falcata L.

Abstract

In our continuing search for biologically active new compounds from Hungarian Euphorbiaceae species, the chemical constituents of Euphorbia falcata L. were investigated. E. falcata an annual herb, widely distributed in garbage places, crops, and fallow lands in Hungary. This plant has not been investigated previously. From the methanol extract of E. falcata four new pentacyclic euphoppin-type diterpene polyesters have been isolated. We report herein the isolation and structure determination of these compounds. The whole plants were extracted with MeOH at room temperature and, after concentration, the extract was partitioned between CHCl3 and H2O. The organic phase was fractionated by column chromatography on polyamide, then by vacuum liquid chromatography on silica gel. Selected fractions from these separations were further purified by RPC and preparative TLC, to yield pure compounds 1-4. The structure elucidation was performed by extensive spectroscopic analysis, including 1D and 2D NMR (1H-1H COSY, HSQC, and HMBC) and HRESIMS experiments. The stereochemistry of the compounds was studied by NOESY measurements. The isolated compounds were identified as di-, tetra-, penta- and hexaester derivatives of a polyfunctional euphoppin-related diterpene alcohol, acylated with acetic, propanoic, isobutanoic, n-hexanoic and benzoic acids. The 5–7-6–3 fused carbon skeleton indicates that compounds 1-4 biogenetically may be formed from a lathyrane precursor. These type of diterpenes occur rarely, and were only isolated previously from E. aleppica and E. decipiens [1,2,3].