Dioxiranes. ketones converted

Various ketones can be converted into the corresponding three-membered cyclic peroxides, i.e. dioxiranes by treatment with buffered aqueous solutions of Oxone (equation 44). 

Chiral dioxirane that was also generated in situ from the corresponding ketone and Oxone was first used for catalytic asymmetric epoxidation by Curd et al., although enantioselectivity was low [7], Later, Yang et al. disclosed that this approach had a bright prospect if used with a combination of Oxone and chiral ketone 3 [8]. Ketone 3 is converted into the corresponding dioxirane in situ, which epoxidizes olefins (Scheme 6B.5). 

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... 

In the metal-free epoxidation of enones and enoates, practically useful yields and enantioselectivity have been achieved by using catalysts based on chiral electrophilic ketones, peptides, and chiral phase-transfer agents. (E)-configured acyclic enones are comparatively easy substrates that can be converted to enantiomeri-cally highly enriched epoxides by all three methods. Currently, chiral ketones/ dioxiranes constitute the only catalyst system that enables asymmetric and metal-free epoxidation of (E)-enoates. There seems to be no metal-free method for efficient asymmetric epoxidation of achiral (Z)-enones. Exocyclic (E)-enones have been epoxidized with excellent ee using either phase-transfer catalysis or polyamino acids. In contrast, generation of enantiopure epoxides from normal endocyclic... 

Ketones can be converted to dioxiranes by Oxone (2KHSO5 KHSO4 K2SO4) under shghtly alkaline conditions (pH 7-8) (400). The dioxirane of 1,1,1-trifluoroacetone is a powerful yet selective oxidant under mild conditions, typically at temperatures below 313 K (10). Exemplary reactions are stereospecific olefin epoxidation and hydroxylation of tertiary C-H groups, or ketonization of CH2 groups. With chiral ketones, even enantioselective reactions are possible (401). Although the reactions are often performed in excess ketone, it is actually possible to use the ketone in a catalytic fashion, for example, for 1,1,1-trifluoroacetone (Scheme 5). 

The method is also successful for carboxyhc esters , and A,A-disubstituted amides, and can be made enantioselective by the use of a chiral oxaziridine. Dimethyldioxirane also oxidizes ketones (through their enolate forms) to a-hydroxy ketones. Titanium enolates can be oxidized with tert-butyl hydroperoxide or with dimethyl dioxirane and hydrolyzed with aqueous ammonium fluoride to give the a-hydroxy ketone. Ketones are converted to the a-oxamino derivative (0=C CH2- 0=C CHONHPh) with excellent enantioselectivity using... 

These ketone precursors can also serve as chiral auxiliaries. The dioxirane from the fructose-derived ketone 20 converts trisubstituted and rram-disubstitued alkenes (e.g., 22) to the corresponding epoxides in very good yields and enantioselectivities, but is less effective for terminal and cis-disubstituted substrates. Fortunately, the oxazolidino analog 21 exhibits a complementary scope, providing high enantioselectivities for these latter olefins (e.g., 24). The stereochemical outcome of the reaction has been rationalized on the basis of a spiro transition... 

Dimethyldioxirane converts A(Wdimethylhydrazones to the corresponding nitriles in 94-98% yields in 2-3 min.272 It has also been used to convert cycloctatetraene to tetraepox-ides.273 A dioxirane analogue, a perfluorinated dialkylox-aziridine, has been used to oxidize alcohols to ketones.274... 

Many variations of the Rubottom oxidation employ oxidants other than m-CPBA in order to execute the transformation under mild conditions or to allow for enantioselective synthesis. Use of dimethyl dioxirane (DMDO) for the oxidation of enolsilanes has become a popular alternative to traditional conditions for Rubottom oxidations. This mild oxidant has been used to facilitate the isolation of 2-silyloxyoxiranes, which are stable under the essentially neutral reaction conditions." For example, treatment of 26 with DMDO at -40 °C afforded 27 in 99% yield.1 Ib These compounds can subsequently be converted to 2-hydroxyketones, as described above, or can be used in other transformations.12 Chiral dioxiranes generated in situ from chiral ketones and oxone have also been employed in enantioselective Rubottom oxidations developed independently by Shil3a and Adam.13b As shown above, enolsilane 28 was transformed to a-hydroxyketone 29 in 80% yield and 90% ee.l3a... 

Ketones and Other Oxygen Functions. Various ketones can be converted to the corresponding dioxiranes by treatment with buffered aqueous solutions of Oxone (eq 1). Of particular interest are dimethyldioxirane (R = R = Me) and methyl(trifluoromethyl)dioxirane (R = Me, R = CF3) derived... 

Ketones and Other Oxygen Functions. Various ketones can be converted to the corresponding dioxiranes by treatment with buffered aqueous solutions of Oxone (eq 1). Of particular interest are dimethyldioxirane (R = R = Me) and methyl(trifluoro-methyl)dioxirane (R = Me, R = CF3) derived from acetone and l,l,l-trifluoro-2-propanone, respectively. The discovery of a method for the isolation of dilute solutions of these volatile dioxiranes in the parent ketone by codistillation from the reaction mixture has opened an exciting new area of oxidation chemistry. Solutions of dioxiranes derived from higher molecular weight ketones have also been prepared. ... 

Fluorinated alcohols, and in particular HFIP, have also proven beneficial for epoxidations of alkenes using persulfate (Oxone) as the terminal oxidant and fluoroketones as catalysts [9, 10]. Under these conditions, the fluorinated ketones are converted to dioxiranes, which are the active epoxidizing spedes. Typical catalysts... 

Dioxiranes for alkene epoxidation may be prepared in situ from a catalytic amount of a ketone and Oxone (potassium peroxymonosulfate triple salt). )V,)V-Dimethyl-and A, A -dibenzylalloxans (20a) and (20b) (Figure 3) have been prepared and used as novel dioxirane catalysts for the epoxidation of a range of di- and tri-substituted alkenes in good to excellent yield. H2O2 (rather than the usual Oxone) has been successfully used as primary oxidant in asymmetric epoxidations with Shi s fructose-derived ketone (21) in acetonitrile. The ketone is converted into the dioxirane, which is responsible for epoxidation and the active oxidant responsible for dioxirane formation is proposed to be peroxyimidic acid formed by combination of H2O2 with acetonitrile. ... 

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