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-med iated allylic oxidation of glycals leading to 2,3-dihydro-4H-pyran-4-o nes is presented. Notable features are the isolation of ring-contracte d by-products 6 and 7, which are produced upon oxidation of per-O-benz ylated glycal 4, as well as the characterization of carbohydrate-deriv ed tetrahydrofurfurals 12a and 13a, which are formed by the conformati on-dependent oxidation of glycals 9a and 10b. In addition, the iodine( III)-mediated oxidation process has been studied by in situ NMR spectr oscopy of lyre-configured glycals 14a,b. Intermediate alkylphenyliodon ium species 19b,d and 2-enopyranosides 16a and 20a have been character ized by their NMR signals. These data support a plausible mechanism th at is initiated by electrophilic attack of the iodine(III) reagent on the electron-rich enol ether double bond of the glycal. This is follow ed by the breaking of a bond beta,gamma-positioned in relation to the carbohydrate-bound iodine and subsequent reductive elimination of iodo benzene. Thus, depending on the glycals employed, a number of diverse oxidation products may be formed.