Epoxidations chiral reagents

With this epoxidation procedure it is possible to convert the achiral starting material—i.e. the allylic alcohol—with the aim of a chiral reagent, into a chiral, non-racemic product in many cases an enantiomerically highly-enriched product is obtained. The desired enantiomer of the product epoxy alcohol can be obtained by using either the (-1-)- or (-)- enantiomer of diethyl tartrate as chiral auxiliary ... 

Of course, the key limitation of the ylide-mediated methods discussed so far is the use of stoichiometric amounts of the chiral reagent. Building on their success with catalytic asymmetric ylide-mediated epoxidation (see Section 1.2.1.2), Aggarwal and co-workers have reported an aza version that provides a highly efficient catalytic asymmetric synthesis of trans-aziridines from imines and diazo compounds or the corresponding tosylhydrazone salts (Scheme 1.43) [68-70]. 

These reagents may be considered to be one of the elusive aza-analogues of peroxyacids, and there are significant mechanistic similarities between the Rees-Atkinson reaction and the Bartlett epoxidation. Chiral Q-reagents have been used to effect highly stereoselective aziridination of alkenes (Scheme 4.13) [1],... 

Sulfur ylides are a classic reagent for the conversion of carbonyl compounds to epoxides. Chiral camphor-derived sulfur ylides have been used in the enantioselective synthesis of epoxy-amides <06JA2105>. Reaction of sulfonium salt 12 with an aldehyde and base provides the epoxide 13 in generally excellent yields. While the yield of the reaction was quite good across a variety of R groups, the enantioselectivity was variable. For example benzaldehyde provides 13 (R = Ph) in 97% ee while isobutyraldehyde provides 13 (R = i-Pr) with only 10% ee. These epoxy amides could be converted to a number of epoxide-opened... 

In principle, any reaction with a racemate using a chiral reagent can be used to effect a kinetic resolution. Kagan (56) has recently given an analysis of the relationship between the extent of reaction and die enantiomeric excess that can be achieved, while Sharpless (57) has applied kinetic resolution successfully to racemic allyl alcohols using his titanium alkoxide tartrate epoxidation reaction. 

Enantioselective deprotonations of meso substrates such as ketones or epoxides are firmly entrenched as a method in asymmetric synthesis, although the bulk of this work involves stoichiometric amounts of the chiral reagent. Nevertheless, a handful of reports have appeared detailing a catalytic approach to enantioselective deprotonation. The issue that ultimately determines whether an asymmetric deprotonation may be rendered catalytic is a balance of the stoichiometric base s ability... 

Epoxidation of [22.10] (105b) and [26.10]betweenanenes (105c) with this chiral reagent was quenched at 50 % conversion, and the optically active [22.10] and [26.10] betweenanenes, isolated by SiOz gel chromatography, exhibited [a]n2 —32.4° (hexane) and —24.7° (hexane) respectively60. ... 

More examples are found for varied oxidation processes mainly for various epoxidations carried out by metal catalysts bearing F-modified ligands, such as porphyrins,139 Ru perfluoroacetylacetonate salt,140 or salen complexes,141 142 or using the 3 selenium compound as catalyst.143 The potential for enantioselective transformations offering an easy way to recover precious chiral reagents and catalysts was demonstrated in enantioselective epoxidation using fluorous chiral salen... 

Let s take these three chiral synthons in turn. First, the simplest one the central epoxide. The reagent we need here will carry a leaving group, such as a tosylate, and it can easily be made from the epoxy-alcohol. This gives a very good way of making this compound as a single enantiomer—a Sharpless asymmetric epoxidation of ally] alcohol. 

If we use a chiral reagent to synthesize an amino acid, however, it is possible to favor the formation of the desired enantiomer over the other, without having to resort to a resolution. For example, single enantiomers of amino acids have been prepared by using enantioselective (or asymmetric) hydrogenation reactions. The success of this approach depends on finding a chiral catalyst, in much the same way that a chiral catalyst is used for the Sharpless asymmetric epoxidation (Section 12.15). 

Applying these methods for the epoxidation of prochiral olefins without additional measures racemic epoxides are obtained. In most cases, the idea is to make the method stereoselective and thus obtain pure or enriched enantiomers of epoxides by using chiral reagents or by addition of optically active auxiliaries. Some of the results obtained by various groups will be discussed. 

AE reactions of simple olefins. The Sharpless AE reaction has been supplemented by other approaches to asymmetric epoxide synthesis the most evident goal being to obviate the need for an allylic alcohol. Attempts to carry out asymmetric epoxidation reactions on simple olefins have utilized transition-metal-containing catalysts such as porphyrins as well as stoichiometric chiral reagents (peroxides, dioxiranes, and oxaziridines). These approaches have been summarized [19]. 

In the total synthesis, the most noteworthy aspect is the philosophy of the approach. Also, the important point is not just the total yield, but, primarily the completion of the synthesis, as artists never exhibit their unfinished work and since a synthesis cannot be an Unfinished Symphony. Considering the many kinds of useful reactions which have been developed to serve as synthetic key steps, such as asymmetric epoxidation, asymmetric reduction and enantiospeci-fic aldol reaction using chiral reagents, and, considering the many kinds of enantiomerically pure materials such as carbohydrates which are available, any complex antibiotic may be synthesized. Consequently, a philosophy of synthesis is more urgently required than ever before. 

Tartrate esters also feature as the chiral reagents in the Sharpless asymmetric epoxidation, a reaction applied in the synthesis of deoxy-sugars. Two groups have epoxidized divinyl carbinol (33) and converted the resulting mono-epoxlde (3 ) into the 2,6-dideoxy-sugars D-ollvose (35), its 3- -niethyl ether (D-oleandrose) and 4-0 -benzyl ether, and Into D-digltoxose (36) and Its 4- -benzyl ethers (Scheme 7). Epoxidation of the meso-... 

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