Methyl alkene epoxidation

Finally, with the aim of discovering novel chiral oxomolybdenum catalysts able to perform enantioselective alkene epoxidations, Kuhn et al. have reported the exploration of the catalytic behaviour of a series of dioxomolybdenum(VI) complexes with chiral cw-8-phenylthiomenthol ligands derived from ( + )-pulegone. Therefore, the epoxidation of c -p-methylstyrene using t-butyl-hydroperoxide as the oxidant and performed in the presence of ( + )-(2i ,5i )-2-[1-methyl-l-(phenylthio)ethyl]-5-methylcyclohexanone oxime as the ligand, did not produce, however, a significant optical induction in these conditions. 

Racemic analogs of showdomycin have been synthesized from adducts (352 and 353) of methyl 2-nitroacrylate with furan. Synthesis of DL-2-ept-showdomycin involved treatment of the adducts with m-chloroperoxybenzoic acid, followed by nitrous acid elimination, to give the alkenic epoxide 379. Opening of the oxirane ring, and a sub-... 

Examination of the computer constructed cw-diphenylmetallaoxetanes and the tetramethylmetallaoxetanes showed very unfavorable steric interactions between the phenyl and methyl substituents and both the porphyrin ring and the o-bromo substituents. These severe steric effects do not depend on the porphyrin being planar or convexed. In order to avoid these steric interactions, the Fe-C and F-0 bond lengths of the metallaoxetane rings must be extended to unreasonable values of >3.2 A. It was concluded that metallaoxetanes could not be required intermediates in alkene epoxidation by hypervalent metal-oxo porphyrins. 

In the presence of methyl, tert-butyl, or (-)-menthyl esters of 2-oxocyclopentanecarboxylic acids, the cobalt(II)-salen type complexes and Jacobsen-type manganese(ni) complexes shown in Figure 17 are active catalysts for alkene epoxidation with dioxygen . 

The enol tautomers of 1-(2, 4, 6 -trialkylphenyl)-2-methyl-1,3-diketones form a range of alkene, epoxide, ether, and hydroperoxide products on reaction with singlet oxygen. The product distribution is substantially affected by the solvent, apparently owing to the disruption of intramolecular hydrogen bonding of the enols in polar solvents. 

Conjugated dienes can be epoxidized to provide vinylepoxides. Cyclic substrates react with Katsuki s catalyst to give vinylepoxides with high ees and moderate yields [17], whereas Jacobsen s catalyst gives good yields but moderate enantiose-lectivities [18]. Acyclic substrates were found to isomerize upon epoxidation (Z, )-conjugated dienes reacted selectively at the (Z)-alkene to give trans-vinylepoxides (Scheme 9.4a) [19]. This feature was utilized in the formal synthesis of leuko-triene A4 methyl ester (Scheme 9.4b) [19]. 

Episulfides can be converted to alkenes. " However, in this case the elimination is syn, so the mechanism cannot be the same as that for conversion of epoxides. The phosphite attacks sulfur rather than carbon. Among other reagents that convert episulfides to alkenes are Bu3SnH, certain rhodium complexes, LiAlH4 (this compound behaves quite differently with epoxides, see 10-85), and methyl iodide.Episulfoxides can be converted to alkenes and sulfur monoxide... 

Further variations on the epoxyketone intermediate theme have been reported. In the first (Scheme 9A) [78], limonene oxide was prepared by Sharpless asymmetric epoxidation of commercial (S)-(-)- perillyl alcohol 65 followed by conversion of the alcohol 66 to the crystalline mesylate, recrystallization to remove stereoisomeric impurities, and reduction with LiAlH4 to give (-)-limonene oxide 59. This was converted to the key epoxyketone 60 by phase transfer catalyzed permanganate oxidation. Control of the trisubstituted alkene stereochemistry was achieved by reaction of the ketone with the anion from (4-methyl-3-pentenyl)diphenylphosphine oxide, yielding the isolable erythro adduct 67, and the trisubstituted E-alkene 52a from spontaneous elimination by the threo adduct. Treatment of the erythro adduct with NaH in DMF resulted... 

We also note that some 2,2-disubstituted oxiranes have toxicological significance, as exemplified by 2,2-dimethyloxirane (2-methyl-l, 2-epoxypropane, 10.43, R = Me). This compound is the toxic metabolite of 2-methyl-prop-1-ene (isobutene), a gaseous alkene widely used as a monomer in the industrial production of adhesives, plastics, and other polymers. Interestingly, detoxification of this epoxide catalyzed by liver epoxide hydrolase was high in the human, intermediate in the rat, and low in the mouse [125], These activities were inversely correlated with the epoxide levels measured in vitro in liver tissue of these species. 

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