Abstract: In only two steps and in 65% overall yield, natural trioxane artemisinin (I) was converted on gram scale into C-10-carba trioxane dimer (3). This new, very stable dimer was then transformed easily in one additional step into four different dimers (4-7). Alcohol and diol dimers (4 and 5) and ketone dimer (7) are 10 times more antimalarially potent in vitro than artemisinin (1), and alcohol and diol dimers (4 and 5) are strongly inhibitory but not cytotoxic toward several human cancer cell lines. Water-soluble carboxylic acid derivatives (8a-10c and 12) were easily prepared from dimers (4-6); they are thermally stable even at 60° C. for 24 hours, are more orally efficacious as antimalarials than either artelinic acid or sodium artesunate, and have potent and selective anticancer activities.