Total synthesis and evaluation of cytostatin, its C10-C11 diastereomers, and additional key analogues: impact on PP2A inhibition

The total synthesis of cytostatin, an antitumor agent belonging to the fostriecin family of Natural Products, is described in full detail. The convergent approach relied on a key epoxide-opening reaction to join the two stereotriad units and a single-step late-stage stereoselective installation of the sensitive (Z,Z,E)-triene through a beta-chelation-controlled nucleophilic addition. The synthetic route provided rapid access to the C4-C6 stereoisomers of the cytostatin lactone, which were prepared and used to define the C4-C6 relative stereochemistry of the natural product. In addition to the natural product, each of the C10-C11 diastereomers of cytostatin was divergently prepared (11 steps from key convergence step) by this route and used to unequivocally confirm the relative and absolute stereochemistry of cytostatin. Each of the cytostatin diastereomers exhibited a reduced activity toward inhibition of PP2A (>100-fold), demonstrating the importance of the presence and stereochemistry of the C10-methyl and C11-hydroxy groups for potent PP2A inhibition. Extensions of the studies provided dephosphocytostatin, sulfocytostatin (a key analogue related to the natural product sultriecin), 11-deshydroxycytostatin, and an analogue lacking the entire C12-C18 (Z,Z,E)-triene segment, which were used to define the magnitude of the C9-phosphate (>4000-fold), C11-alcohol (250-fold), and triene (220-fold) contribution to PP2A inhibition. A model of cytostatin bound to the active site of PP2A is presented, compared to that of fostriecin, which is also presented in detail for the first time, and used to provide insights into the role of the key substituents. Notably, the alpha,beta unsaturated lactone of cytostatin, like that of fostriecin, is projected to serve as a key electrophile, providing a covalent adduct with Cys269 unique to PP2A, contributing to its potency (> or =200-fold for fostriecin) and accounting for its selectivity.