Epoxidation reagents

A very common oxidizing reagent is peroxytrifluoroacetic acid, which is usually generated in situ from trifluoroacetic acid [29, 30, 31] or trifluoroacetic anhydride [32, 33, 34] and hydrogen peroxide Peroxytrifluoroacetic acid is one of the most efficient epoxidizing reagents [35] It can be used to prepare epoxides... 

It should be noted that epoxidation of a dienone with mCPBA or other electrophilic epoxidation reagents proceeds with complementary regioselectivity, yielding y,8-epoxy enones instead of the ot,P-epoxy ketones discussed above. This feature has been utilized in several natural product syntheses Scheme 9.8 demonstrates... 

Toxicity. The peroxy acid is highly toxic and may cause death or permanent injury after very short exposure to small quants (Ref 12) Uses. Peroxyacetic acid is the most important epoxidation reagent used today because of its economical availability and ease of use with a wide variety of reactants (Ref 10). It also finds wide usage as an organic oxidizing agent (Ref 11) Refs 1) Beil 2, 169, (78), [174] [379 ... 

It is a powerful explosive [1] produced when xenon tetrafluoride or xenon hexafluoride are exposed to moist air and hydrolysed. Some tetrafluoride is usually present in xenon difluoride, so the latter is potentially dangerous. Although safe to handle in small amounts in aqueous solution, great care must be taken to avoid solutions drying out, e.g. around ground stoppers [2], Full safety precautions have been discussed [2,3,4], Precautions necessary for use of aqueous solutions of the trioxide as an epoxidation reagent are detailed [5,6], A safe method of preparation,... 

Jackson and coworkers have used a new approach to the synthesis of P-hydroxy-a-amino acids using (arylthio)nitrooxiranes. D-Isopropylideneglyceraldehyde is converted into the corresponding 1-arylthio-l-nitroalkene, which is a key material for stereoselective synthesis of P,y-dihydroxyamino acids (Scheme 4.6). The key step is stereoselective nucleophilic epoxida-tion of the 1-arylthio-l-nitroalkene. Syn and anti epoxides are selectively obtained by appropriate choice of epoxidation reagent.51... 

The rationalization of stereoselectivity is based on two assumptions. (1) The 1-arylthio-1-nitroalkenes adopt a reactive conformation in which the ally lie hydrogen occupies the inside position, minimizing 1,3-allylic strain. (2) The epoxidation reagent can then either coordinate to the ally lie oxygen (in the case of Li), which results in preferential syn epoxidation or in the absence of appropriate cation capable of strong coordination (in the case of K) steric and electronic effects play a large part, which results in preferential anti epoxidation (Scheme 4.7).52... 

The vanadium-promoted epoxidation method has proved to be valuable in synthetic routes leading to dl-Ci8 Cecropia juvenile hormone20 and lasalocid A.24 The mechanism of vanadium-mediated epoxidation has been elucidated25 but an evaluation of the use of other organometallic compounds as epoxidation reagents is required.26... 

Allyl amides (enamides), for example, 225, 228, and 230 cyclize to oxazolines, for example, 226, 229, and 231 when the double bond is activated by an electrophile. The double bond can also be conjugated to a ketone, or present as an allylic epoxide. Reagents commonly used to promote the cyclization include acids,iodine,selenium reagents,and trimethylsilyl triflate (Scheme 8.63). ... 

A recent report (J. Org. Chem. 2004, 69, 8510) by Paul G. Williard of Brown University and Ruggero Curci of University di Bara of the oxidation of 5 to 6 serves as a timely reminder that the widely-used epoxidation reagent dimethyl dioxirane is also useful for the oxidation of secondary alcohols to ketones. 

Several solvent systems have been utilized for epoxide titration. Desirable properties for a solvent in this connexion are 1 that it be easy to purify and store (2) that it be unreactivc towards both tin-epoxide and the epoxide reagent and 3 that it not be excessively volatile, noxious, or toxic. 

The transformation of carboxylic acids and their functional derivatives to the corresponding peroxycarboxylic acids or diacyl peroxides are generally known reactions.200-201 Among the hydroperoxy derivatives, trifluoroperacetic acid is a frequently used epoxidizing reagent in organic chemistry and is usually prepared in situ.200-201... 

Chromyl nitrate, Cr02(N03)2, was found by Miyaura and Kochi to be a much better epoxidizing reagent than Cr02Cl2.277 This reaction is solvent dependent. In basic solvents such as DMF or pyridine, epoxide is the major product (equation 100), whereas in an acetone solution alkene ketal, resulting from the addition of acetone to epoxide, is predominantly produced (equation 101). 

Epoxidation of C=X double bonds is a very active field in organic chemistry. It seems that for almost every type of double bond a special epoxidation reagent has been developed. 

The following examples use m-chloroperoxybenzoic acid (MCPBA), a common epoxidizing reagent, to convert alkenes to epoxides having the same cis or trans stereochemistry. MCPBA is used for its desirable solubility properties The peroxyacid dissolves, then the spent acid precipitates out of solution. 

If the OH group is not blocked at all but left free, and the epoxidation reagent is the vanadium complex VO(acac)2 combined with f-BuOOH, the syn epoxide is formed instead. The vanadyl group chelates reagent and alcohol and delivers the reactive oxygen atom to the same face of the alkene. 

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