Epoxidation with dioxiranes

Enantioselective epoxidation of unfunctionalized alkenes was until recently limited to certain ds-alkenes, but most types of alkenes can now be successfully epoxi-dized with sugar-derived dioxiranes (see Section 9.1.1.1) [2]. Selective monoepox-idation of dienes has thus become a fast route to vinylepoxides. Functionalized dienes, such as dienones, can be epoxidized with excellent enantioselectivities (see Section 9.1.2). 

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. 

Vinyl epoxides are highly useful synthetic intermediates. The epoxidation of dienes using Mn-salen type catalysts typically occurs at the civ-olefin. Epoxidations of dienes with sugar-derived dioxiranes have previously been reported to react at the trans-olefin of a diene. A new oxazolidinone-sugar dioxirane, 9, has been shown to epoxidize the civ-olefin of a diene <06AG(I)4475>. A variety of substitution on the diene is tolerated in the epoxidation, including aryl, alkyl and even an additional olefin. All of these substitutions provided moderate yields of the mono-epoxide with good enantioselectivity. 

Wang and Shi have published a detailed study of their fructose-based dioxirane epoxidation catalyst system with hydroxyalkene substrates. The ees obtained were highly pH dependent. The lower enantioselectivity obtained at low pH is attributed to the substantial contribution of direct epoxidation by Oxone. The results obtained with... 

A high catalyst loading (typically 20-30 mol%) is usually required for the epoxidation with ketone 26 because Baeyer-Vilhger oxidation presumably decomposes the catalyst during the epoxidation. The fused ketal moiety in ketone 26 was replaced by a more electron-withdrawing oxazohdinone (32) and acetates (33) with the anticipation that these replacements would decrease the amount of decomposition via Baeyer-Villiger oxidation (Fig. 8) [71, 72]. Only 5 mol% (1 mol% in some cases) of ketone 32 was needed to get comparable reactivity and enantioselectivity with 20-30 mol% of ketone 26 [71]. Since dioxiranes are electrophilic reagents, they show low reactivity toward electron-deficient olefins, such as a, 3-unsaturated esters. Ketone 33, readily available from ketone 26, was found to be an effective catalyst towards the epoxidation of a, 3-unsaturated esters [72]. 

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). 

This controversy concerning the use of MP2 calculations for epoxidation reactions was rather short-lived since more efficient density functional calculations (DFT) came into general use and generally produced symmetrical spiro transition structures. Consequently, the use of MP2 theory for 0—0 bond cleavage reactions has been largely discontinued. Most have assumed that the question of symmetrical versus asymmetrical approach of the peracid had been resolved. Recall that this same problem with MP2 calculations existed for the early calculations for dioxirane epoxidation (see Section V.D). 

As already mentioned, the dioxirane epoxidation of an alkene is a stereoselective process, which proceeds with complete retention of the original substrate configuration. The dioxirane epoxidation of chiral alkenes leads to diastereomeric epoxides, for which the diastereoselectivity depends on the alkene and on the dioxirane structure. A comparative study on the diastereoselectivity for the electrophihc epoxidants DMD versus mCPBA has revealed that DMD exhibits consistently a higher diastereoselectivity than mCPBA however, the difference is usually small. An exception is 3-hydroxycyclohexene, which displays a high cis selectivity for mCPBA, but is unselective for DMD . ... 

The results of the dioxirane epoxidation of some 3-alkyl-substituted cyclohexenes and of 2-menthene indicate that the diastereoselectivity control is subject to the steric interactions of the dioxirane with the substituents of the substrate, while the size of the dioxirane substituents has only a minimal effect . In the favored transition structure, the alkyl groups of the dioxirane cannot interact effectively with the substituents at the stereogenic center of the chiral alkene . ... 

The dioxirane epoxidation of a prochrral alkene will produce an epoxide with either one new chirality center for terminal alkenes, or two for internal aUcenes. When an optically active dioxirane is nsed as the oxidant, expectedly, prochiral alkenes should be epoxi-dized asymmetrically. This attractive idea for preparative purposes was initially explored by Curci and coworkers in the very beginning of dioxirane chemistry. The optically active chiral ketones 1 and 2 were employed as the dioxirane precursors, but quite disappointing enantioselectivities were obtained. Subsequently, the glucose-derived ketone 3 was used, but unfortunately, this oxidatively labile dioxirane precursor was quickly consumed without any conversion of the aUcene . After a long pause (11 years) of activity in this challenging area, the Curci group reported work on the much more reactive ketone... 

In related work from the same group, dehydro-nucleoside 21 underwent stereospecific epoxidation with dimethyl-dioxirane (DMDO) to give epoxide 22. The ring opening of this compound with trimethylaluminium is presumably a predominantly S m1 process giving isomers 23 and 24 in a 5 1 ratio (Scheme 4) <2004JOC1831>. 

Figure 6B.2. Transition-state models for epoxidation with optically active oxaziridines and dioxiranes.

Dioxiranes react with cholesterol and its acetate to give ca 1 1 mixtures of 5a,6a- and 5/i,6/i-epoxides (in contrast to peroxy acids, that are known to produce ca 5 1 mixture)318. 

Dioxiranes epoxidize different compounds with C=C stereospecifically and electrophili-cally (equation 25) in high yield (90-100%). 

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