Dioxirane-catalyzed epoxidations

FIGURE 35.6. First chiral ketones used for dioxirane-catalyzed epoxidation. 

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

Teeth whiteners, percarbamide, 623 Temperature, reaction rates, 903-12 Terminal olefins, selenide-catalyzed epoxidation, 384-5 a-Terpinene, peroxide synthesis, 706 a-Terpineol, preparation, 790 Terrorists, dialkyl peroxide explosives, 708 Tertiary amines, dioxirane oxidation, 1152 Tertiary hydroperoxides, structural characterization, 690-1... 

Many attempts have been made to use hydrogen peroxide as the final oxidizing agent in ketone-catalyzed epoxidations. Because hydrogen peroxide itself does not convert ketones to dioxiranes, in-situ activation of the oxidant is necessary. Shi et al. have achieved this goal by using acetonitrile as a component of the solvent mixture... 

One highly attractive feature of ketone-catalyzed epoxidation via chiral dioxir-anes is that reliable models can be developed to rationalize the observed enantio-selectivities. For the reaction of a dioxirane with an alkene, two extreme transition states can be envisaged the so-called spiro and planar modes (Fig. 12.3). 

Moreover, in the same work, the 0-labeling experiment confirmed chiral dioxiranes to be the intermediates in chiral ketone-catalyzed epoxidation reactions. Murray et al. reported the synthesis and structural characterization of cyclooctatetraene tetraepoxides 90 and 91 through the oxidation of cyclooctatetraene with excess of DMDO lb... 

Epoxidations. Further utilities for the dioxirane derived from fructose is the regio-and enantioselective epoxidation of conjugated dienes and enynes. Continuing explorations of the (salen)manganese(III) systems have uncovered the catalyzed epoxidation of 2-sulfonyl-1,3-dienes at the nondeactivated sites, and the advantage of added carboxylate salt as cocatalyst. ... 

Denmark has recently disclosed results that help address the question of whether dioxirane (A) or Criegee-type (B) intermediates are involved in ketone-catalyzed epoxidation reactions (Fig. 14) [98]. [ 0]-Labeling experiments using ketone 40 showed 80% of the expected isotope label was incorporated into the epoxide product, providing compelling evidence that dioxiranes are indeed the active oxidizing species [99]. 

Epoxides are very versatile intermediates, and asymmetric epoxidation of olefins is an effective approach to the synthesis of enantiomericaUy enriched epoxides [1-3]. Great success has been achieved for the epoxidation of allyhc alcohols [1], the metal-catalyzed epoxidation of unfunctionalized olefins (particularly conjugated cis- and tri-substituted) [2], and the nucleophilic epoxidation of electron-deficient olefins [3]. In recent years, chiral dioxiranes have been shown to be powerful agents for asymmetric epoxidation of olefins. Dioxiranes can be isolated or generated in situ from Oxone (potassium peroxymonosulfate) and ketones (Scheme 3.1) [4,5]. When the di-oxirane is used in situ, the corresponding ketone is regenerated upon epoxidation. Therefore, in principle, a catalytic amount of ketone can be used. When a chiral ketone is used, asymmetric epoxidation should also be possible [6]. Extensive studies have been carried out in this area since the first chiral ketone was reported by Curd in 1984 [7]. This chapter describes some of the recent progress in this area. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Carbenes, dioxirane preparation, 1132 Carbocations, antimalarial endoperoxides, 1309 Carbohydrate hydroperoxides, Mo-catalyzed olefin epoxidation, 432, 436 Carbohydrates, TBARS assay, 669 Carbonate esters, oxidative ozonolysis, 737, 738... 

Sugar-derived ketones, catalyzed dioxirane epoxidation, 1147 Sulfanilamide, TEARS assay, 667 Sulfate ion radical, peroxydisulfate organic salts, 1014 Sulfides... 

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