Epoxidations with peroxy acids

The stereochemistry of the epoxidation of A-norsteroids is less predictable than that of the corresponding steroids in view of the flattened nature of ring A and the preferred cis fusion of the hydrindane it has been shown that epoxidation with peroxy acids proceeds predominantly from the fi-face in some norsteroids294. 

Of special interest are olefm epoxidation with peroxy acids at the moment of their formation from acid anhydrides and hydrogen peroxide. Among the catalysts of olefin oxide formation from olefins interacting with H202 in the presence of metal (Ti, W, V, Ta, Ge, Mo, U, Ru) oxides or acids are tungstic acid and its salts [47], Epoxidation is described by the following mechanism ... 

The pattern you saw for epoxidation with peroxy-acids (more substituted alkenes react faster) is followed by bromination reactions too. The bromonium ion is a reactive intermediate, so the rate-determining step of the brominations is the bromination reaction itself. The chart shows the effect on the rate of reaction with bromine in methanol of increasing the number of alkyl substituents from none (ethylene) to four. Each additional alkene substituent produces an enormous increase in rate. The degree of branching (Me versus n-Bu versus t-Bu) within the substituents has a much smaller, negative effect (probably of steric origin) as does the geometry (E versus Z) and substitution pattern (1.1- S... 

Hydroxyl-Directed Epoxidation with Peroxy Acids ° ... 

Dihydroxylation usually occurs from the less hindered side of the double bond, but appears to be less susceptible to steric hindrance than epoxidation with peroxy acids. 

There is an exception to the poor reactivity exhibited by hydrogen peroxide. When 158 was treated with 30% hydrogen peroxide, in the presence of benzonitrile and potassium hydrogen carbonate, epoxide 159 was formed in 86% yield, in what is known as the Payne epoxidatlon. l The reagents shown generate per-oxybenzimidic acid in situ. This transformation was taken from Smith s synthesis of (+)-calyculin A.250 Payne epoxidation is known to give a diastereofacial preference that is opposite to epoxidations with peroxy-acids. In this particular example, the epoxide 159 was a 3 1 (a P) mixture of isomers at the epoxy carbon. 

Epoxidations with peroxy-acids are highly stereoselective and take place by cis addition to the double bond of the alkene. For example, oleic acid 26 gave cis-9, 0-epoxystearic acid 27, whereas elaidic acid 28 gave the isomeric traw -epoxide 29 (5.41). 

Route A involves the protection of the hydroxyl groups of ntj/o-inositol at C-1 and C-2 as the cyclohexanon ketal, and acetylation of the remaining hydroxyl groups followed by removal of the protecting group from C-1 and C-2. The resulting 3,4,5,6-tetra-O-acetyl-myo-inositol is converted to the 1,2-thionocarbonate, from which tetra-O-acetyl conduritol B is obtained by the removal of sulfur and carbon dioxide with trimethyl phosphite. Deacetylation and epoxidation with peroxy acids give the desired product. 

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