Oxygen ring opening

The proposed mechanism of the isomerization of polyfluorooxiranes to ketones does not involve a 1,2-fluorine shift, but rather coordination of the Lewis acid with the epoxide oxygen, ring opening to the carbocation, and intermolecular addition and elimination of fluoride.29 45... 

Photochemical oxidation of 4,5-dibromo(chloro)-l,3-dimethyl-2-imidazolone converts it into 1,3-dimethyl-2,4,5-imidazolidinetrione. The bromo derivatives are quite unstable to light and moisture <81AG(E)612>. Singlet oxygen ring opens l,3-disubstituted-4,5-diphenyl-2-imidazolones to form diacylureas. It has been suggested that this photooxidation may involve dioxetanes (78) rather than... 

This domino reaction starts with protonation of the endocyclic oxygen, ring opening and formation of the oxonium ion 59 (Scheme 4.8), migration of the acetyl group... 

The experimental observations combine with the principles of nucleophilic substi tution to give the picture of epoxide ring opening shown m Figure 16 5 The nucleophile attacks the less crowded carbon from the side opposite the carbon-oxygen bond Bond... 

Step 2 Nucleophilic attack by water on carbon of the oxonium ion The carbon-oxygen bond of the ring is broken in this step and the ring opens... 

How do the bond distances of 1 2 epoxypropane change on protonation of the nng oxygen" Assum ing that the longer C—O bond is the weaker of the two do the bond distances in the protonated form correlate with the regioselectivity of acid catalyzed ring opening ... 

Rea.ctlons, Butyrolactone undergoes the reactions typical of y-lactones. Particularly characteristic are ring openings and reactions in which ring oxygen is replaced by another heteroatom. There is also marked reactivity of the hydrogen atoms alpha to the carbonyl group. 

Tocotrienols differ from tocopherols by the presence of three isolated double bonds in the branched alkyl side chain. Oxidation of tocopherol leads to ring opening and the formation of tocoquinones that show an intense red color. This species is a significant contributor to color quaUty problems in oils that have been abused. Tocopherols function as natural antioxidants (qv). An important factor in their activity is their slow reaction rate with oxygen relative to combination with other free radicals (11). 

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

Carbenes of type (113), generated by thermal decomposition of the appropriate tosyl-hydrazone salts, undergo ring opening more readily when the ring heteroatom is oxygen rather than when it is sulfur (78JA7927). 

S-Substituted thiiranium ions react with water and alcohols to give trans ring opening (Scheme 72). A report that oxygen nucleophiles attack sulfur as well as carbon has been shown to be incorrect (79ACR282). The intermediate thiiranium ion (57) in the presence of lithium perchlorate readily yields the carbenium ion which undergoes a transannular hydride... 

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