Stereospecific epoxidation

Epoxidation of 10 with m-chloroperbenzoic acid yielded the chrysene anti-1,2-diol-3,4-epoxide, whereas similar reaction of 11 gave a mixture of the corresponding anti and syn diol epoxides in a 5 3 ratio (57,59). These findings are in accord with previous observations that dihydrodiols free to adopt the diequatorial conformation undergo anti stereospecific epoxidation, whereas bay region diaxial dihydrodiols yield mixtures of anti and syn diastereomers. The syn-... 

The highly chemo-, regio-, and diastereoselective and stereospecific epoxidation of various allylic alcohols with only 1 equivalent of hydrogen peroxide in water solvent could be efficiently catalyzed by an isolated dinuclear peroxotungstate [W203(02)4(H20)2]2 [93,94] ... 

Park, J.B., Buehler, B., Hahicher, T., Hauer, B., Panke, S., Witholt, B. and Schmid, A., The efficiency of recombinant Escherichia coli as biocatalyst for stereospecific epoxidation. Biotechnol. Bioeng., 2006, 95, 501-512. 

An epoxytrichloroacetimidate was used as a key intermediate in the total synthesis of (+)-myiiocin. The intermediate diene 157 was constructed in several steps from 156. Stereospecific epoxidation of 157, followed by imidate formation gave 158. Treatment of 158 with Et2AlCl provided 159 for which the proper stereochemistry of the amino group is now set for the natural product (Scheme 8.46). 

In related work from the same group, dehydro-nucleoside 21 underwent stereospecific epoxidation with dimethyl-dioxirane (DMDO) to give epoxide 22. The ring opening of this compound with trimethylaluminium is presumably a predominantly S m1 process giving isomers 23 and 24 in a 5 1 ratio (Scheme 4) <2004JOC1831>. 

Stereospecific Epoxidation of 2-Butene. The hydroperoxide epoxidation reaction is stereospecific. Pure cis- and trans-2-butene were epoxi-dized separately by cumene hydroperoxide. The cis olefin gave exclusively cis epoxide, and the trans olefin gave exclusively trans epoxide. In both cases, the epoxide was the sole product formed from the olefin. They can be distinguished easily by their different retention times on a gas chromatography column of 20% diisodecyl phthalate on Chromosorb W(60-80 mesh). They were also identified by comparing their infrared spectra with authentic samples. 

Hikino et al. [153] have investigated by enzymatic means the stereospecific epoxidation reactions of olefinic double bonds in the plant Curcuma zedoaria Roscoe. They studied the bioconversion of germacrone (207), a constituent of C. zedoaria, by microorganisms in the hope of obtaining stereoselective epoxidation as in the case of the plant. Cuminghamella blakesleeana yielded three major products (208 - 210) from germacrone, Fig. (42). 

In this section, new methods for the stereospecific epoxidation of 2-hexene and the selective oxidation of naphthalene using Ti02 as photocatalyst under UV illumination are introduced. Both these reactions under similar conditions have been examined by several authors, but the conditions for the selective epoxidation and selective oxidation was optimized only recently. For the epoxidation of olefins, only molecular oxygen is needed for the reactant. It is emphasized that the epoxidation reaction proceeded stereospecifically in high yield. In the case of... 

Stereospecific Epoxidation of 2-hexene on Photoirradiated Ti02 Powders Using Molecular Oxygen as Oxidant... 

The reaction of cyclohexene with Mn (111 )(sal en )/t -B uOOH afforded cyclohexene oxide as the single product in the presence of radical inhibitors. When PhIO was used as the terminal oxidant, stereospecific epoxidation could be attained. Similar results were obtained with Co(II)(salen) (5) stereospecific epoxidation took place with PhIO as the... 

Several new examples for stereospecific epoxidation of compounds with more complicated structures can be seen in equations 31-38. 

A facile synthesis of 180 with a better yield has been developed as follows. O-Deacetylation of 51 with hydrochloric acid gave DL-(l,3/2)-3-bromomethyl-5-cyclo-hexene-1,2-diol (181). Stereospecific epoxidation of 181 with mCPBA and subsequent acetylation gave-the epoxide (182), which afforded the exocyclic methylene derivative (183) by dehydrobromination with silver fluoride [51]. Reductive cleavage of the oxirane ring of 183 with lithium aluminium hydride, followed by acetylation yielded... 

Further support for an iron-bound active oxidant comes from the study of the [Fe(II)(Tp3,5 Me2)(bf)] complex [237], This complex reacts with 02 to form a species capable of stereospecifically epoxidizing olefins. For example, epoxida-tion of cA-stilbene gives only cA-stilbene oxide as the product, but trara-stilbene cannot be epoxidized, suggesting that epoxidation occurs at a sterically congested transition state, i.e., near the iron center. More studies are needed to provide insight into the nature of the active oxidant. 

Many insect pheromones are derivatives of simple alkenes. Disparlure 56, an attractant for the gypsy moth, is an epoxide derived by stereospecific epoxidation from the Z-alkene 57. As neither substituent is anion-stabilising, a simple Wittig should give the right geometry. 

Stereospecific epoxidation of hydroxy enones.1 Epoxidation of the racemic i>droxy enones 1 with H202 in a basic medium shows only a slight preference for the... 

Sharpless and Flood682 observed reactions characteristic of monooxygenases, such as stereospecific epoxidation of olefins and arene hydroxylation, with oxotransition metal (M=0) oxidants. For example, Cr02X2 (X=C1, OAc) oxidized tritium labeled naphthalene to naphthoquinone accompanied by tritium migration. Rearrangement is suggestive of arene oxide transients. 

The peroxyacid epoxidation is quite general, with electron-rich double bonds reacting fastest. The following reactions are difficult transformations made possible by this selective, stereospecific epoxidation procedure. The second example uses magnesium monoper-oxyphthalate (MMPP), a relatively stable water-soluble peroxyacid often used in large-scale epoxidations. These aqueous MMPP epoxidations, carried out at neutral pH to avoid opening the epoxide, avoid the large-scale use of hazardous chlorinated solvents. 

Before leaving the area of oxene chemistry, there is one further system worthy of mention the manganese Schiff-base complexes. The Schiff-base complexes were prepared in response to the Katsuki-Sharpless system for stereospecific epoxidation (Figure 2.19).57 The Katsuki-Sharpless system consists of titanium(IV) isopropoxide and ( + )- or (—)-diethyl tartrate with... 

Some stereospecific epoxidations such as the Katsuki-Sharpless system only function with alkyl hydroperoxides. 

Figure 3.22 Stereospecific epoxidation of olefins with hydrogen peroxide.

Anhydrous Fe Cl3 catalyzes the stereospecific epoxidation of norbomene, the demethylation of A, A-dimethylaniline, and the oxidative cleavage of PhCMe(OH)CMe(OH)Ph (and other a-diols) by hydrogen peroxide (Table 11 and Scheme 4). For each class of substrate, the products parallel those that result from their enzymatic oxidation by cytochrome P-450. The close congruence of the prodncts indicates that the reactive oxygen in the Fe Cl3/HOOH model system and in the active form of cytochrome P-450 is essentially the same, with strong electrophilic oxene character (stabilized singlet atomic oxygen). 

Several other examples of stereoselective and stereospecific epoxidations can be found in the literature16-19. 

Epoxidation of acyclic allyl alcohols with peracid and Mo/TBHP displays an opposite stereospecificity to that for the V/TBHP system. Trimethylsilyl-substituted allylic alcohols give t/zreo-epoxyalcohols with MCPBA and erythro-alcohols with VO(acac)a-TBHP, with high stereoselectivity. In the stereospecific epoxidation of cis- and trans-allyl alcohols, formation of a transition state is assumed with the development of two H bonds between the hydrogen atom of the hydroxy group of the allyl alcohol and the oxygen of the peracid, and between the hydrogen of the peracid OH and the oxygen of the ether 10. An analysis of the diastereometric transition-state interactions for stereoselective epoxidation of acyclic allylic alcohols has been published. A conformational effect may be responsible for the unexpected cis major product in Eq. 2. 

Stereospecific epoxidation of olefins with radical 62, produced in a mixture of tetramethyltetrazene-ZnCl2 and oxygen, is a radical-catalyzed process. ... 

In contrast to the stereospecific epoxidation of acyclic alkenes with peroxy acids, oxidation of acyclic a,(3-unsaturated ketones with alkaline hydrogen peroxide is stereoselective in that only one stereoisomer is formed from cis- and trans-enones ... 

For the competitive epoxidation of cis- and fra s-2-octenes with 4a, the ratio of the formation rate of czs-2,3-epoxyoctane to that of the trans isomer is >3x10 , which is much larger than the ratios (1.3-11.5) reported for other stereospecific epoxidation systems. The epoxidation of 3-substituted cyclohexenes, such as 3-methyl-l-cyclohexene and 2-cycIohexen-l-ol, showed an unusual diastereoselectivity the corresponding epoxides were formed highly diastereoselectively with the oxirane ring trans to the substituents anti configuration) (Eq. 4.6). In addition, the more accessible but less nucleophilic double bonds in noncon-jugated dienes, such as frfl is-l,4-hexadiene, (R)-(-F)-limonene, 7-methyl-l,... 

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