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‐4 H ‐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 lyxo ‐configured glycals 14a,b . Intermediate alkylphenyliodonium species 19b,d and 2‐enoyranosides 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 β,γ‐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.