On the basis of the reported phytotoxic activity of sorgoleone and resorcinolic lipids identified from the root extracts of Sorghum bicolor , 8 resorcinolic lipid derivatives and 10 quinones with various side chain sizes were synthesized. The phytotoxicity of the compounds was tested against a monocot and a dicot species. The quinones were phytotoxic, whereas the resorcinolic lipids were not. Of the quinones, 2-hydroxy-5-methoxy-3-pentylcyclohexa-2,5-diene-1,4-dione, having a five-carbon side chain, showed phytotoxic activity similar to that of natural compound sorgoleone.