Alcohols, allylic with peroxy acids

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

Based on these calculations, one can conclude that TSs for epoxidation of alkenes and allylic alcohols with peroxy acids, dioxiranes, and Re-peroxo complexes share a spiro geometry in which the plane of the attacking peroxo... 

The epoxidation of allylic alcohols can be carried out with peroxy acids in aqueous medium. In the case of polyolefinic alcohols, regioselectivity results from control of the pH of the reaction the proton of the allylic hydroxy group plays a fundamental role when the oxidation is carried out at a high pH296. 

Allylic alcohols can be converted to epoxy-alcohols with tert-butyUiydro-peroxide on molecular sieves, or with peroxy acids. The addition of an appropriate chiral ligand to the metal-catalyzed hydroperoxide epoxidation of allylic alcohols leads to high enantioselectivity. This important modification is... 

The oxidation of allylic alcohols has been studied thoroughly using a variety of catalysts. The reactivity of the vanadium-tert-butyl hydroperoxide reagents towards the double bond of allylic alcohols makes possible selecfive epoxidation. Thus, reaction of geraniol with t-BuOOH and vanadium acetylacetonate [VO(acac)2] gave the 2,3-epoxide 33 (5.44). With peroxy-acids, reaction takes place preferentially at the other double bond. 

With one exception—when the substituent is a hydroxyl group. When an allylic alcohol is epoxidized, the peroxy-acid attacks the face of the alkene syn to the hydroxyl group, even when that face is more crowded. For cyclohexenol the ratio of syn epoxide to anti epoxide is 24 1 with m-CPBA and it rises to 50 1 with CF3CO3H. 

Allylic iodides with excess amounts of peroxy acids in a two-phase system in the presence of an inorganic base give the corresponding rearranged dlylic alcohols in good yields (Scheme 3). ° It has been demonstrated that the reaction proceeds under milder conditions than the Mislow-Evans reaction. 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

M. Freccero, R. Gandolfi, M. Sarzi, A. Rastelli, Competition behs een peroxy acid oxygens as hydrogen bond acceptors in B3LYP transition structures for epoxidations of allylic alcohols with peroxyformic acid, J. Org. Chem. 64 (1999) 3853. 

Peroxy-acids work for expoxidizing allylic alcohols syn to the OH group, but another reagent is better when the OH group is further from the alkene. 4-Hydroxycyclopentene, for example, can be converted into either diastereomer of the epoxide. If the alcohol is protected with a large group such as TBDMS t-butyl-... 

Epoxidation of alkenes normally occurs with approach of the peroxy-acid from the less-hindered side of the double bond. However, where there is a polar substituent, particularly in the allylic position, this may influence the direction of attack by the peroxy-acid. Thus, whereas 2-cyclohexenyl acetate gives a mixture consisting predominantly of the trans-epoxide (as expected with attack from the less-hindered side of the double bond), the free alcohol gives almost exclusively the cw-epoxide under the same conditions (5.42). The stereoselectivity and the faster rate of reaction with the hydroxy compound result from hydrogen bonding of the reactants. 

Henbest showed that in the absence of severe steric interference, allylic cyclohexenols are epoxidized stereoselectively by organic peroxy acids to furnish c/s-epoxy alcohols a large number of c/s-epoxy alcohols have been prepared by epoxidizing aUylic cyclohexenols. A mixture (5 1) of labile bisaUylic alcohols (19) and (20) was reacted with m-CPBA (eq 5) from the reaction mixture diepoxide (21) was isolated as a single isomer. Epoxidation of (Z)-cyclooct-2-en- l-ol (22) furnishes exclusively (99.8%) the trans-epoxide (23) (eq 6). Similar observations have been made subsequently. This result, as well as the stereoselectivity observed during the epoxidation of other allylic alcohols, both cyclic and acylic, has been rationalized on the basis of transition state models. ... 

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