DESIGN AND PREPARATION OF SERINE-THREONINE PROTEIN PHOSPHATASE INHIBITORS BASED UPON THE NODULARIN AND MICROCYSTIN TOXIN STRUCTURES .2. SYNTHESIS OF A FUNCTIONALIZED NODULARIN MACROCYCLE AND A STRIPPED-DOWN MICROCYSTIN MACROCYCLE

Nodularins and microcystins are complex natural isopeptidic hepatotoxi ns that serve as subnanomolar inhibitors of the eukaryotic serine-thre onine protein phosphatases, PP1 and PP2A, In Part 1 (A, P. Mehrotra, K , L, Webster and D, Gani, J. Chem. Sec., Perkin Trans. 1, 1997, preced ing paper) each of the key structural or potentially reactive motifs w ithin each macrocycle type was assessed as a contributor towards phosp hatase inhibitory efficacy and a stripped-down nodularin-type macrocyc le was identified as a suitable precursor to potentially active synthe tic inhibitors. Subsequently, synthetic routes to the 19-membered nodu larin macrocyclic system were developed, using solution-phase chemistr y, which demonstrated that only certain cyclisation protocols were via ble, Here we describe an extension of this chemistry to provide a 19-m embered nodularin macrocycle, ha-OMe-gamma-Sar-(R)-Asp-alpha-OMe-beta- (S)-Phe-], appropriately functionalised with a hydroxymethyl group for the incorporation of lipophilic side-chains, We also demonstrate that the 25-membered microcystin macrocycle, -(R)-Ala-(S)-Leu-(R)-Asp-alph a-OMe-beta-(S)-Phe-], can be prepared in good yield using similar prot ocols in which macrocyclisation is effected through the reaction of th e amino group of the (2S)-phenylalanine residue with the beta-pentaflu orophenyl ester of the (2R)-aspartic acid residue.