Stereospecific reactions epoxide formation from

The most studied example of 0 -3 participation is probably the base-catalyzed hydrolysis of 2-chloroethanol to produce ethylene oxide. As mentioned above, the reaction is catalyzed by hydroxide but not by water. Studies of the solvent isotope effect along with spectroscopic and conductivity measurements have confirmed the postulated two-step process. The reaction is often used as a stereospecific route to the more hindered epoxide derived from an olefin (via initial halohydrin addition). The carbohydrate field serves as a rich field for the application of epoxide opening and closing reactions. In this respect, Cerny and co-workers have observed that (193), (194), and (195) undergo hydroxide-catalyzed epoxide formation with relative rates of 1 23.3 180, respectively. 

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. 

Finally, carbenoid species can be used as the carbon donor in aldehyde epoxidations. Thus, the rhodium carbenoid derived from the cyclic diazoamide 49 and rhodium(II) acetate reacts stereo selectively with aryl aldehydes to provide spiro-indolooxiranes 50 with Z-stereochemistry. The reaction is believed to proceed via the formation of a carbonyl ylide 51, which undergoes stereospecific thermal conrotatory electrocyclization to form the observed epoxide <04SL639>. 

Thus, a large range of alkenes were epoxidised with success (Scheme 7.3). The stereospecificity of the reaction was shown by the exclusive formation of ds-epoxides from (Z)-alkenes. In contrast, ( )-alkenes were poorly active, giving low yields and enantiomeric excesses. Simple aliphatic alkenes were also good substrates, which is of primary importance, since few methods are available for these substrates. Moreover, the reaction worked well with [Z) enol esters, giving the corresponding epoxides with high enantioselectivity (up to >99% enantiomeric excess). ... 

Multiple forms of cytochrome P-450 have differing catalytic activities with respect to formation of primary metabolites of BaP. Six different forms of cytochrome P-450 isolated from rabbit liver microsomes catalyzed the oxidation of BaP at different rates and produced different ratios of individual phenolic and quinone metabolites (572, 491). Different forms of cytochrome P-450 also showed differing activities and stereospecificity in metabolism of the —)-tranS 7,8-dihydrodiol to diol epoxides 101). The specificity of differing inducible forms of cytochrome P-450 in the metabolism of BaP is important since the predominance of a given form may influence the balance between activation and detoxification reactions. 

Synthesis of Epoxides from Haiohydrins (Section 11.8C) Formation of the halohydrin and the following intramolecular Sf 2 reaction are both stereoselective (the configuration of the alkene is retained in the epoxide) and stereospecific (for alkenes that show ds, trans isomerism, the configuration of the epoxide depends on the configuration of the alkene). 

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