alpha-Amanitin is an extremely toxic bicyclic octapeptide isolated from the death-cap mushroom, Amanita phalloides. As a potent inhibitor of RNA polymerase II, alpha-amanitin is toxic to eukaryotic cells. Recent interest in alpha-amanitin arises from its promise as a payload for antibody drug conjugates. For over 60 years, A. phalloides has been the only source of alpha-amanitin. Here we report a synthesis of alpha-amanitin, which surmounts the key challenges for installing the 6-hydroxy-tryptathionine sulfoxide bridge, enantioselective synthesis of (2S,3R,4R)-4,5-dihydroxy-isoleucine, and diastereoselective sulfoxidation.