Epoxidation stereospecificities

Peroxy acid oxidation of alkenes (Sections 6.18 and 16.9) Peroxy acids transfer oxygen to alkenes to yield epoxides. Stereospecific syn addition is observed. 

Various nucleophiles react with epoxides stereospecifically to give 1,2-difunc-tionalized products, establishing the stereochemistry of two vicinal carbons accordingly, optically active epoxides serve as useful synthetic intermediates for the construction of wide range of enantiopure compounds. Thus, the synthesis of optically active epoxides is the very important objective in organic synthesis... 

Compelling evidence for stepwise C-O bond formation in [Mn(salen)] -catalyzed epoxidation is found in the formation of both cis- and trans-epoxides as primary products from acyclic ds-olefins [68]. The extent of frans-epoxide formation depends strongly on the nature of the substrate. Whereas simple alkyl-substituted ds-olefins are epoxidized stereospecifically, aryl-substituted cis-olefins afford mixtures of cis- and trans-epoxides with the ds-isomers being formed selectively. Epoxidations of conjugated dienes and enynes also afford cis/trans mixtures, with the frans-epoxide product predominating. These observations may be interpreted according to a stepwise mechanism in which a discrete radical intermediate undergoes competitive collapse to ds-epoxide and rotation/collapse to frans-epoxide (Scheme 4). 

Approximately half of the papers on Ru porphyrins appearing during the last 5 years or so deal with olefin epoxidation. Epoxides are found in naturally occurring compounds and are important intermediates in organic synthesis in the case of asymmetric epoxidation, stereospecific ring-opening of the epoxide generates the chiral alcohols . 

Alkylation Reactions. DMF dialkyl acetals undergo a variety of reactions with 1,2-diols. For example, the reaction of trans-cyclohexane-l,2-diol with DMF dimethyl acetal leads to the formation of cyclohexane epoxide (eq 2) with inversion of configuration. Similarly, wej 0-l,2-diphenyl-l,2-ethanediol gives trans-stilbene epoxide stereospecifically (eq 3). This method has also been applied in the synthesis of cholestane epoxide from vicinal diols. If the intermediate 2-dimethylamino-1,3-dioxolane is treated with Acetic Anhydride, reductive elimination to the alkene occurs with retention of stereochemistry (eq 4). " ... 

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 (Section 6 18) Peroxy acids transfer oxygen to the double bond of alkenes to yield epoxides The reaction IS a stereospecific syn addition... 

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

Overall the stereospecificity of this method is the same as that observed m per oxy acid oxidation of alkenes Substituents that are cis to each other m the alkene remain CIS m the epoxide This is because formation of the bromohydrm involves anti addition and the ensuing intramolecular nucleophilic substitution reaction takes place with mver Sion of configuration at the carbon that bears the halide leaving group... 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

The remarkable stereospecificity of TBHP-transition metal epoxidations of allylic alcohols has been exploited by Sharpless group for the synthesis of chiral oxiranes from prochiral allylic alcohols (Scheme 76) (81JA464) and for diastereoselective oxirane synthesis from chiral allylic alcohols (Scheme 77) (81JA6237). It has been suggested that this latter reaction may enable the preparation of chiral compounds of complete enantiomeric purity cf. Scheme 78) ... 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

When the reaction of acetaldehyde with a 6-diazopenicillanate is catalyzed by BFs-EtiO, an epoxide of unknown stereochemistry is obtained (79H( 13)227). With ZnCh catalysis, however, the 6a-acetyl derivative is obtained, which can be stereospecifically reduced as the Mg chelate to the 6a-(/ -l-hydroxyethyl) derivative as part of an elegant synthesis of the carbapenem thienamycin (Scheme 40) (8UA6765). 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Sodium iodide in trifluoroacetic anhydride reacts with epoxides to form the corresponding alkenes in high yields [39] The reduction is stereospecific and generates olefins of the same geometry as the starting epoxides [39]... 

The reaction is easy to cany out, and yields are usually high. Epoxidation is a stereospecific syn addition. 

A method for achieving net anti hydroxylation of alkenes combines two stereospecific processes epoxidation of the double bond and hydrolysis of the derived epoxide. 

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