Epoxidations, directed

Henbest Epoxidation- epoxidation directed by a polar group... 

Conversion of an epoxide directly to an acetonide is accomplished with acetone and SnCl4 (81-86% yield) or with A-(4-methoxybenzyl)-2-cyan- opyridinium hexafluoroantimonate [A-(4-MeOC6H4CH2)-2-CN-PyrSbFJ (59-100% yield). ... 

The synthesis in Scheme 13.40 features a catalytic asymmetric epoxidation (see Section 12.2.1.2). By use of me30-2,4-dimethylglutaric anhydride as the starting material, the proper relative configuration at C(4) and C(6) is ensured. The epoxidation directed by the (+)-tartrate catalyst controls the configuration established at C(2) and C(3) by the epoxidation. Although the epoxidation is highly selective in... 

Alkenes can also be epoxidized directly with a variety of organic peroxy acids or related reagents such as peroxy carboximidic acid, RC(NH)OOH, which is readily available through an in situ reaction of... 

Notwithstanding the possible divergent reaction mechanisms for the two types of epoxides direct displacement vs elimination-Michael addition sequence, the polarity alternation rule reveals the correct regiochemistry. The same can be said for analysis of aromatic substitutions based on PAR or resonance considerations. 

This is a general method of preparing hexaalkylphosphorous triamides from the corresponding dialkylamines. The procedure is simple, and the yields are high. Hexaalkylphosphorous triamides are powerful nucleophiles.8 This feature can be used in a rather unique way to synthesize epoxides directly from aldehydes.8, 6... 

The importance of 1,2- rather than 1,3-allylic strain in the vanadium(V)-catalysed BufOOH epoxidation directed by hydroxy groups has been assessed using (Z)-3-en-2-ols as model substrates344. 

Epoxides directly attached to the oxetane ring in l,5-dioxaspiro[3.2]hexanes (e.g., 71) can be ring-opened by certain nucleophiles without attack on the less-strained oxetane ring <19990L825, 2003JOC1480>. However, such reactions can alternatively lead to the formation of a-substituted-/3 -hydroxyketones. It was reported that the pAla of the nucleophile generally determines the reaction outcome (Scheme 13). 

There is currently significant debate about the mechanism of substrate oxidation by Q [62, 80, 81, 89]. Studies examining the MMO-catalyzed oxidation of nor-carane, of which the products derived from radical and cationic rearrangements clearly differ, indicated that both radical and cationic species are involved in product formation with a radical lifetime on the order of 20-150 ps [79, 90]. There is, furthermore, evidence suggesting that compound P may be able to effect alkene epoxidation directly [91]. Thus, in analogy with P450, multiple mechanisms and oxidants may be involved in the oxygenation of different substrates by MMO. 

Enantiopure allylic alcohols are employed widely as building blocks for asymmetric synthesis, and particularly as substrates for various diasteroselective allcene functionalization reactions such as cyclopropanation and epoxidation directed by the hydroxyl group [129]. 

The Mitsunobu reaction has been successfully applied to the synthesis of carbohydrate epoxides directly from diols (Scheme 3.15c).84 The more accessible and nucleophilic hydroxyl is converted into an alkoxyphos-phonium ion, which in turn is intramolecularly displaced by a hydroxyl to give an epoxide. Again it is of critical importance that a coplanar SN2 transition state be attained. 

It should be noted that thiiranes polymerize under these basic conditions much more rapidly than the corresponding epoxides. Direct comparison of the rates is not possible because the same temperature range could not be used. 

By the means of epoxide ring opening, thiosugars can also be obtained. Sometimes with nucleophiles such as thiocyanate or thiourea a more complex reaction occurs to convert epoxides directly into episulfides, as illustrated by the conversion of the 5,6-anhydro-L-/ (o compound 147 to the D-gZMCO-5,6-episulfide 148 (O Scheme 66). Also, episulfides can be ring opened with nucleophiles [109]. 

A powerful class of epoxidation reagents in this category are the anions derived from A/-p-tolylsulfo-nylsulfoximines (equation 16). These reagents form epoxides directly upon addition to carbonyl compounds and are available in a number of alkyl substitution patterns. Their chemistry is similar to that of dimethyloxosulfonium methylide, to which they often provide a practical alternative due to their greater nucleophilicity. 

Interestingly, it has also been shown that the Caldariomyces fumago chloroperoxidase is able to yield epoxides directly from olefins in the absence of halide... 

Selective Intramolecular Epoxidations Directed by Removable Tethers... 

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