Cercosporamide, a potent antifungal agent and phytotoxin from the fungus Cercosporidium henningsii, was characterized as a heterodimer related to usnic acid. Cassava (Manihot esculenta), a tuber of the spurge family Euphorbiaceae, is a primary root crop for human consumption in semitropical to tropical regions. The plant is extensively cultivated for domestic consumption in west tropical Africa, South America, and throughout the South Pacific regions and is an important export in the form of tapioca. "Brown leaf spot" is a major leaf disease of cassava caused by Cercosporidium henningsii. Infection of leaves by Cercosporidium henningsii causes necrotic lesions symptoms reminiscent of those caused by known phytotoxins. The development of these lesions in affected leaves generally results in leaf drop, leading to a direct loss in tuber yield. Few chemically directed phytotoxin studies on C. henningsii or other disease-causing pathogens of cassava have appeared despite the importance of this plant as a food crop in less developed regions of the world where it is grown in soils of marginal quality and diseases can dramatically decrease yields. If the C. henningsii phytotoxins could be identified, they could be used in cassava tissue culture systems to find toxin-insensitive cassava biotypes. Our studies on C. henningsii, including the isolation and characterization of an unusual bioactive amide, are the subject of this report. Transformation of enantiomerically enriched propargyl esters 5 into dihydrofurans 6 with complete enantiospecificity is achieved by Ag(1)-catalyzed rearrangement and cyclization, and the sequence is successfully applied to the enantioselective synthesis of an antitumor protective and hypolipidemic antibiotic, (SI-(-)-ascofuranone).