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Background: The present report describes the semi-synthesis of a few O-prenylated phenolic derivatives and their in vitro antitumor activities. These compounds were prepared by modifying two naturally occurring antitumor phenols, 5,7-dihydroxy-3-(1'-hydroxy-1'-phenyl-methyl)-6-methoxy-chroman-4-one (A) and 2',4'-dihydroxy-3',6'-dimethoxychalcone (B), previously isolated from Polygonum limbatum Meisn. (Polygonaceae). The structures were elucidated by spectroscopic means and comparison with published data. The cytotoxicity of compounds was determined by using the resazurin assay in the parental drug-sensitive CCRF-CEM cell line and its multidrug-resistant P-glycoprotein-over-expressing subline, CEM/ADR5000., Results: We describe in the present paper four new semi-synthetic derivatives of A and B: 5-hydroxy-6-methoxy-7-O-(3'-methylbut-2'-enyl)chroman-4-one (1), trivially named metapchromone, 5-acetoxy-6-methoxy-7-O-[3'-methylbut-2'enyl]chroman-4-one (2), trivially named sargisin, 2'-hydroxy-3',6'-dimethoxy-4'-O-(3″-methylbut-2″-enyl)chalcone (3) trivially named limbachalcone A, and 2'-acetoxy-3',6'-dimethoxy-4'-O-(3″-methylbut-2″-enyl)chalcone (4) trivially named tsedengchalcone. Their preliminary cytotoxic activities have been determined. We also report herein the isolation of 1-methylhydantoin (C) and betulinic acid (D) from Polygonum limbatum for the first time., Conclusions: The study clearly suggests that semi-synthesis involving O-prenylation and acetylation of chalcones or other chromanones should be avoided in a search for anticancer drugs. This conclusion should be helpful when selecting substituents for the synthesis of potential anticancer drugs.