[Development of novel oxidation reactions using hypervalent iodine reagents and their application to total synthesis of biologically active natural products]

Recently, hypervalent Iodine (III) reagents have been used extensively in organic synthesis. In particular, phenyliodine (III) diacetate (PIDA) and phenyliodine (III) bis(trifluoroacetate) (PIFA) have received a great deal of attention due to low toxicity, ready availability, easy handling, and reactivities similar to that of heavy metal reagents or anodic oxidation. In contrast to the widely explored phenolic oxidation using PIDA or PIFA, reactions of phenol ethers with hypervalent iodine reagents have been limited and have yielded mostly iodonium salts. We found unexpectedly that the reaction of p-substituted phenol ethers with PIFA in the presence of some nucleophiles in polar and poorly nucleophilic solvents such as 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoro-2-propanol caused a novel and straightforward nucleophilic substitution reaction on the aromatic ring via a cation radical intermediate [ArH+.]. Successful application of PIFA-induced umpolung of electron-rich aromatic ring reactivity allowed intramolecular cyclization reactions of phenol ethers leading to novel and efficient preparations of synthetically useful products such as biaryl compounds, quinone-imines, and sulfur-containing heterocycles. Using these methods, the first total synthesis of potent cytotoxic marine alkaloid, makaluvamine F was also achieved.