ChemInform Abstract: Acceleration of the Dess-Martin Oxidation by Water.

Abstract

The Dess-Martin periodinane (DMP), l,l,l-triacetoxy-l,l-dihydro-l,2-benziodoxol-3(LH)-one (2),l is one of the mildest and most convenient reagents available for oxidation of alcohols (Figure 1). DMP enjoys increasing use despite suggestion that its behavior can be capricious.2 Recently, Ireland has addressed a problem of inconsistency in DMP batch quality by offering an improved procedure for synthesizing the reagenta2 Important as it is to have a reliable method of preparing pure DMP, we suspect that many D-M (Dess-Martin) oxidations in the literature have been effected not by DMP, but by an impurity commonly present in DMP samples. In our own laboratory, a crucial D-M oxidation in the total synthesis of rapamycin3 proceeded consistently only with a single 2-year old batch of the reagent, material that was largely insoluble in CDC13 and that exhibited a complex proton NMR spectrum. Several other samples of DMP, including relatively pure reagent prepared according to the Ireland method,2 failed to perform the desired transformation. Questions about the composition of the effective sample motivated the present Dess-Martin periodinane study. In attempts to obtain the impurities present in the effective DMP reagent, we explored Dess and Martin's preparation of the oxidant. However, pure, crystalline DMP was reproducibly obtained using their procedure by adhering to the following details: (1) In order to permit greater ease of stirring during oxidation of iodobenzoic acid, the initially viscous reaction mixture was more dilute than in the D-M preparation, which may ensure that oxidation proceeds to ~ompletion.~ (2) Iodinane oxide 1 was heated at 85 C in Ac20 and AcOH at least 1 h beyond the time that dissolution was complete. (3) After acetylation, the mixture was allowed to stand for 1-2 days to permit crystallization of DMP.5 (4) During isolation of the product, exposure to atmosphere was strictly avoided. The resulting DMP was crystalline and completely soluble in CH2Clz and CDC13. The reagent contained a small amount of AcOH, but otherwise its NMR spectra showed no significant (>3%) extraneous peaks. When pure DMP prepared as above was used under inert conditions (flame-dried glassware, dry solvents,