Alkene epoxidation regioselectivity

A series of 2-vinyl-3-silyloxybicyclo[3.2.0]heptan-6-ones has also been converted to prostanoid lactones in excellent yield but variable regioselectivity. Some of the best regioselectivity was obtained using H202 in trifluoroethanol (see p. 1097).241 The strained cyclobutanone ring and the relatively unreactive terminal vinyl group favor the desired reaction in preference to alkene epoxidation. 

Several efficient procedures for alkene epoxidation using Oxone were reported, such as Oxone/aqueous NaOH, Oxone/acetone, Oxone/water , Oxone/PTC/benzene/ aqueous buffer solution or Oxone/2-butanone system. Thus, sorbic acid can be regioselectively oxidized using Oxone/aqueous NaOH to 4,5-epoxy-2-hexenoic acid in 84% yield. Similarly, cyclooctene is oxidized to cyclooctene oxide in 81% yield, just by stirring it with Oxone in water . 1-Dodecene is epoxidized in good yield by Oxone/PTC in benzene aqueous buffer solution. It is otherwise difficult to epoxidize 1-dodecene by other oxidizing reagents. ... 

Swem dimer, hydrogen bonding, 104, 105 Synchronous transition states, peracid alkene epoxidation, 50-8 syn selectivity, regioselective aUyUc... 

A very interesting way to control alkene epoxidation was introduced by Breslow and Meresca. In the steroid diene, 77, epoxidation takes place exclusively at the 4,5 double bond using Mo(CO)6 and TBHP. However, by attaching a template, as in 78, to the alcohol, the regioselectivity could be inverted so that epoxidation takes place only at the 17,20 double bond. It was concluded that the appendage did not act as a steric shield, but the remote tertiary alcohol moiety was transformed in situ to a hydroperoxide resulting in the observed selectivity by intramolecular epoxidation. This approach was then extended to other functionalized polyenes, such as famesol and geranylgeraniol. ... 

The development of the oxazolidinone catalyst has allowed Shi to further increase the scope of the epoxidation to a range of new substrates. Using ketone catalyst 5 styrene is epoxidized with 24% ee, while the use of oxazolidinone 13a for the same transformation gave 86% ee, with further improvement in enantioselectivity seen with the inclusion of electron-donating substituents on the aromatic ring of the styrene substrates [43]. ds/tra s-Dienes and trienes were epoxidized regioselectivity at the ds-alkene and no isomerization or further epoxidation of the products were observed [44], while epoxidation of conjugated dx-enynes resulted in chemoselective epoxidation of the alkene [45]. 

The regioselectivity of alkene epoxidation leading to diol formation has not been examined in sufficient detail for predictive generalizations to be made, but for linear terpenoid substrates a preference for the terminal olefinic bond is often demonstrated, as discussed in Sec. III.C [8]. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Epoxidation of conjugated dienes can be regioselective when one double bond is more electron-rich than the other otherwise mixtures of mono- and diepoxides will be obtained. When the alkene contains an adjacent stereocenter, the epoxidation can be diastereoselective [2]. Hydroxy groups can function as directing groups, causing the epoxidation to take place syn to the alcohol [2, 3]. 

Gas electron diffraction (GED) acyl peroxides, 702 peroxynitrates, 743-4 see also Electron diffraction Gas-hqiud chromatography (GLC) hydroperoxide determination, 684-5 ozonide determination, 719 Gas-phase addition, hydroperoxides, 157 Gas-phase epoxidation, alkenes, 58 gauche arrangement, acyclic organic peroxides, 102, 104-5 GED (gas electron diffraction), 743-4 Geminal regioselectivity... 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

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