Product analysis in the iodine(III)-promoted oxidation of carbohydrate-derived cyclic enol ethers: A mechanistic study

A study on the mechanism of the well-documented hypervalent iodine-mediated allylic oxidation of glycals leading to 2,3-dihydro-4H-pyran-4-ones is presented. Notable features are the isolation of ring-contracted by-products 6 and 7, which are produced upon oxidation of per-O-benzylated glycal 4, as well as the characterization of carbohydrate-derived tetrahydrofurfurals 12a and 13a, which are formed by the conformation-dependent oxidation of glycals 9a and 10b. In addition, the iodine(III)-mediated oxidation process has been studied by in situ NMR spectroscopy of lyre-configured glycals 14a,b. Intermediate alkylphenyliodonium species 19b,d and 2-enopyranosides 16a and 20a have been characterized by their NMR signals. These data support a plausible mechanism that is initiated by electrophilic attack of the iodine(III) reagent on the electron-rich enol ether double bond of the glycal. This is followed by the breaking of a bond beta,gamma-positioned in relation to the carbohydrate-bound iodine and subsequent reductive elimination of iodobenzene. Thus, depending on the glycals employed, a number of diverse oxidation products may be formed.