Abstract Quassinoids are naturally occurring compounds with phytotoxic and allelopathic activities isolated from several plant species of the Simaroubaceae family. There is relatively little information about the structural characteristics imparting biological activity to these diterpene lactones or their effects on plants. We studied the effects of the oxymethylene ring substitution on the biological activity and molecular conformation of several quassinoids. The presence of this functional group had a great effect on the three-dimensional conformation and biological activity of these natural products. In the absence of the oxymethylene ring, the quassinoids were more planar and had little phytotoxicity. In addition, this bridging function introduced a new reactive center that caused the terpene backbone to bend. Molecules with such conformation were highly phytotoxic, reducing root growth of lettuce ( Lactuca sativa) and affecting all stages of mitosis in onion ( Allium cepa) root tips.