Sterpurene, a sesquiterpene hydrocarbon metabolite of Chondrostereumpurpureum, a plant pathogen and potential mycoherbicide, was synthesized by a six-step sequence, in 33% overall yield. The key steps were a thermal [4â€‚+â€‚2] (Dielsâ€“Alder) cycloaddition of a ketene acetal diene with a conjugated ester dienophile, and a remarkably stereoselective [2â€‚+â€‚2] photocycloaddition of ethylene with the resulting conjugated ketone. Several related, more highly oxygenated, metabolites isolated from culture filtrates of C. purpureum (cf. Ayer) were tested for toxicity to cells of a hybrid aspen, Populusdeltoides X nigra. Their phytotoxicity, at concentrations as low as 10â€‚ppm, suggests that, like the endo-polygalacturonase produced by the fungus, these sesquiterpenes may be partially responsible for the foliar lesions associated with infection of deciduous species by the pathogen. Keywords: Chondrostereumpurpureum, mycoherbicide, metabolites, sterpurenes, synthesis, Dielsâ€“Alder, photocycloaddition, pyramidalization, phytotoxicity.