Dioxirane, oxidation with

The barrier for ethylene epoxidation with DMDO was calculated at the QClSD(T)//QClSD(ftill)/6-31G level. The barrier for ethylene oxidation with the parent dioxirane is lower (16.6 kcalmoL at the same level). 

Should the substrate to be oxidized and its oxidation product be hydrolytically resistant and thermally persistent, the oxidation with in-situ-generated dioxirane is recommended. The advantages include that more ketone catalyst is available, the generation of the... 

SCHEME 5. Dioxirane oxidation of alkenes with both electron-donor and electron-acceptor groups... 

SCHEME 10. Dioxirane oxidation of substrates with heteroatom functionaUties... 

Substrates with doubly bonded nitrogen-atom functionalities, e.g. the C=N-R (imino, oxime) group, are usually cleaved by dioxirane to give the corresponding carbonyl product" . A particular case represents the DMD oxidation of the nitronate ions, generated from nitroalkanes" or nitroarenes. For example, the nitronate anion 16 (equation 13) affords initially the cyclohexadienone on oxidation with DMD, which subsequently tau-tomerizes to the phenol as the final product. An exception is the DMD oxidation of an... 

The cycloaddition of an oxygen atom to the CN double bond, analogous to the epox-idation of CC double bonds, is rare. For example, when the imine 17 is oxidized with in-situ-generated dioxirane (equation 15), the oxaziridine 18 is obtained as the major... 

Sulfur compounds with divalent sulfur functionalities are much more prone to dioxirane oxidation on account of their higher nucleophilicity compared to the above-presented oxygen-type nucleophiles. Examples of this type of dioxirane oxidation abound in the literature. Such a case is the oxidation of thiols, which may be quite complex and afford a complex mixture of oxidation products, e.g. sulfinic acids, sulfonic acids, disulfides, thiosulfonates and aldehydes , and is, therefore, hardly useful in synthesis. Nevertheless, the oxidation of some 9i/-purine-6-thiols in the presence of an amine nucleophile produces n >( -nucleoside analogs in useful yields (equation 19). This reaction also displays the general chemoselectivity trend that divalent sulfur functionalities are more reactive than trivalent sp -hybridized nitrogen compounds P. 

The by far more common and preparatively valuable dioxirane oxidation of divalent sulfur substrates is that of sulfides, to produce either sulfoxides or sulfones . Since sulfoxides are considerably less reactive than sulfides, the reaction outcome may be conveniently controlled by the stoichiometry of the oxidant For example, in the low-temperature oxidation of thiophene by an excess of DMD, the corresponding 1,1-dioxide (sulfone) has been obtained, albeit in low yield (equation 20). This is the first preparatively useful method for isolating this elusive sulfone, which also accentuates the importance of the neutral and anhydrous conditions under which the oxidations with the isolated DMD may be conducted. 

The enantioselective oxidation of prochiral sulfides with DMD has been achieved by using bovine serum albumin (BSA) as the chiral inductor Moderate to good enan-tioselectivities have been reported in the presence of this protein, for which a typical example is shown in equation 22 . As yet, however, no enantioselective oxidation of a prochiral sulfide has been documented by employing an optically active dioxirane. We have tried the enantioselective oxidation of methyl phenyl sulfide with the dioxirane generated from the ketone 7 (Shi s ketone), but an ee value of only ca 5% was obtained. One major hurdle that needs to be overcome with such enantioselective dioxirane oxidations is the suppression of the background oxidation of the sulfide substrate by Caroate, an unavoidable feature of the in-situ mode. 

The dioxirane oxidation of the C=S and N=S double bonds usually leads to the corresponding 5-oxides. In the latter case, A-oxidation may compete with 5-oxidation , and the experimental results indicate that the chemoselectivity depends on the electron density of these heteroatoms . 

Unlike epoxidations, the enantioselective CH oxidation is still a virgin field in dioxirane chemistry. The first (and to date only ) enantioselective C—H oxidation has been reported for vic-diols. Thus, the oxidation with the fructose-derived dioxirane of ketone 7 (Shi s ketone) yields the optically active a-hydroxy ketones in ee values of up to 75% °. A typical example of this asymmetric CH oxidation is shown in equation 32 °. 

Preservatives, pharmaceutical preparations, 623 PRESS technique, NIR spectrophotometry, 624 Primary amines, dioxirane oxidation, 1151 Primary ozonides (POZ), 716, 717 dialkyl peroxide formation, 706 IR spectroscopy, 718, 719-20 microwave spectroscopy, 721 molecular model, 750 NMR spectroscopy, 709, 723-4 octaUn ozonation, 165 ozone water disinfection, 606 7r-complexes with ozone, 732... 

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