Oxiranes epoxide opening

Nucleophilic opening of oxiranes to give ultimately 1,2-diols is usually effected without isolation of the oxirane oxiranation (epoxidation) of alkenes with unbuffered peroxy-ethanoic acid or hydrogen peroxide in methanoic acid (Section 5.05.4.2.2(/)) tends to give monoesters of 1,2-diols (e.g. 53), which can be hydrolyzed to the diols (Scheme 46). 

Epoxyketone 245 is readily available from 16-dehydropregnenolone via several steps, including a crucial microbiological 11a-hydroxylation. Dehydration of 245 gives the 9,11-olefin 246. The alcohol at C-21 is then converted to the mesylate (247), and this is reduced to give the methyl ketone (248). The olefin is then converted to the 9a-fluoro-llp-hydroxy array (250) by the standard sequence [addition of HOBr, closure to the oxirane (249), opening with HF]. Note that the reactivity of the epoxides in 249 is... 

Since acidity (Lewis or Brpnsted) impacts adversely on the yield of epoxides, Clerici and Ingallina (204) added basic compounds in low concentrations to TS-1 catalysts during epoxidation of alkenes to inhibit the oxirane ring opening and enhanced the epoxide yields. A comprehensive investigation of the influence of pH on product selectivity in epoxidation of allylalcohol, allylchloride, and styrene catalyzed by various titanosilicates was reported recently by Shetti et al (205). 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxirane ring. Catalytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for frans-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicyclic tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

Oxetanes The least common cyclic ethers are the four-membered oxetanes. Because these four-membered rings are strained, they are more reactive than larger cyclic ethers and open-chain ethers. They are not as reactive as the highly strained oxiranes (epoxides), however. 

Nitriles can react with functionalized oxiranes in a regioselective manner in a tandem epoxide-opening Ritter reaction (Scheme 39) <2005JOC7447>. 

Anti hydroxylation of an alkene is readily achieved with peroxycarboxylic adds. Add-catalyzed ring opening of the irutial produd, an oxirane (epoxide), forms the monoester of a 1,2-diol, hydrolysis of which affords the parent diol. Alternative reagents whi ate often used for anti hydroxylation of alkenes are hydrogen peroxide with oxides of tungsten > 4.2i gj. selenium, 444i and iodine-silver benzoate (Pr6vost reaction).. ... 

Functionality adjacent to the epoxide can modify its reactivity. For example, 2,3-epoxy sulfides can be converted to a thiiranium species upon treatment with TMS triflate. This intermediate reacts with 0-silyl amides regiospeci-fically to form l-substituted-3-hydroxy-2-thioethers. Simple primary amines undergo polyalkylation, but imines can be used as an indirect amine equivalent <1996T3609>. Nitriles react with functionalized oxiranes in a regioselective manner in a tandem epoxide opening-Ritter reaction (Equation 17) <2005JOC7447>. 

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