Phytotoxic activity and conformational analysis of thymol analogs from Hofmeisteria schaffneri

Abstract

Bioassay-guided fractionation of two phytotoxic extracts (a CH 2Cl 2–MeOH (1:1) and an aqueous) prepared from the aerial parts of Hofmeisteria schaffneri led to isolation of thymol analogs 3– 5, along with seven known compounds, 1, 2 and 6– 10. Compounds 3– 5 were identified by spectroscopic methods as 1,4-bis(2′-hydroxy-4′-methylphenyl)butane-1,4-dione ( 3), 2-isopropyl-5-methylphenyl (2 Z)-2-methylbut-2-enoate ( 4) and 2-hydroxy-2-(2-hydroxy-4-methylphenyl)propane-1,3-diyl (2 Z,2′ Z)bis(2-methylbut-2-enoate) ( 5) and designated trivial names of hofmeisterins II–IV, respectively. Their conformational behavior was also studied by molecular modeling using density functional theory calculations at the B3LYP/DGDZVP level. Compounds 1– 4 and 6– 10 significantly inhibited radicle growth of seedlings of Amaranthus hypochondriacus and Echinochloa crus-galli in the Petri dish bioassay with IC 50’s ⩽ 10 −4 M. Furthermore, the northymol analog 3 provoked significant bleaching of seedlings of A. hypochondriacus. However, none of the isolates affected either seedling growth or germination of Medicago sativa.