A detailed study of fostriecin, a potent and efficacious antitumor agent that inhibits the mitotic entry checkpoint through the selective inhibition of protein phosphatases 2A and 4, and related naturally-occurring and synthetic inhibitors of protein phosphatases (PP) will be conducted. This will include the implementation of an improved second-generation total synthesis of fostriecin, the total synthesis and stereochemical assignment of cytostatin, and the total synthesis and stereochemical assignment of sultriecin. Extensions of the studies to the definition of the fostriecin pharmacophore responsible for its potent and selective PP inhibition and the preparation of more stable and structurally simpler analogues that address issues currently limiting the clinical potential of fostriecin will be pursued.