Abstract
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.