Epoxides formation reactions

Some cleavage takes place even if the phenoHc hydroxyl is blocked as an ether link to another phenylpropane unit and quinonemethide formation is prevented. If the a- or y-carbon hydroxyl is free, alkaH-catalyzed neighboring-group attack can take place with epoxide formation and P-aryloxide elimination. In other reactions, blocked phenoHc units are degraded if an a-carbonyl group is present. 

The great reactivity of the sulfurane prepared by this procedure toward active hydrogen compounds, coupled with an indefinite shelf life in the absence of moisture, makes this compound a useful reagent for dehydrations,amide cleavage reactions, epoxide formation, sulfilimine syntheses, and certain oxidations and coupling reactions. 

In general, epoxidation of steroids with trans-anti-trans ring fusions leads to exclusive formation of the a-oxirane. Steroid Reactions lists examples of exclusive a-epoxide formation from 2-, 4-, 6-, 7-, 8(9)-, 14-, 16- and 17(20)-unsaturated steroids. Further examples of a-epoxidation of steroid 1-enes, 3-enes, 8-enes, 9(ll)-enes, 8(14)-enes and 16-enes have been reported. The preferred attack by the reagent on the a-side of the steroid nucleus can be attributed to shielding of the -side of the molecules by the two angular methyl groups. 

Ester functions are not saponified under these ring opening conditions. However, a trans-a-acetoxy function hinders the epoxide opening reaction and a noticeable decrease in yield is observed in comparison to the cw-a-acetoxy isomer. The ring opening reaction is also dependent on the concentration of sulfuric acid. Polymer formation results when the acid concentration is too low and the reaction is markedly slower with excessive concentrations of acid. A 0.5% (vol./vol.) concentration of acid in DMSO is satisfactory. Ring opening does not occur when ethanol, acetone, or dioxane are used as solvent. 

It is appropriate at this juncture to address some of the more useful transformations of 2,3-epoxy alcohols.913 A 2,3-epoxy alcohol such as compound 14 possesses two obvious electrophilic sites one at C-2, and the other at C-3. But in addition, C-l of a 2,3-epoxy alcohol also has latent electrophilic reactivity. For example, exposure of 14 to aqueous sodium hydroxide solution results in the formation of triol 19 in 79% yield (see Scheme 5). In this interesting transformation, hydroxide ion induces the establishment of an equilibrium between 2,3-epoxy-l-ol 14 and the isomeric 1,2-epoxy-3-ol 18. This reversible, base-induced epoxide migration reaction is a process known as the Payne rearrangement.14... 

In this oxidative degradation, MTO decomposes into catalytically inert perrhenate and methanol. The decomposition reaction is accelerated at higher pH, presumably through the reaction between the more potent nucleophile H02- and MTO. The decomposition of MTO under basic conditions is rather problematic, since the selectivity for epoxide formation certainly profits from the use of nonacidic conditions. 

Epoxide formation may be a side reaction occurring during initiation by t-butylperoxy radicals. The mechanism proposed for this process is as follows (Scheme 3,831, 1... 

The degradation of vinyl chloride and ethene has been examined in Mycobacterium sp. strain JS 60 (Coleman and Spain 2003) and in Nocardioides sp. strain JS614 (Mattes et al. 2005). For both substrates, the initially formed epoxides underwent reaction with reduced coenzyme M and, after dehydrogenation and formation of the coenzyme A esters, reductive loss of coenzyme M acetate resulted in the production of 5-acetyl-coenzyme A. The reductive fission is formally analogous to that in the glutathione-mediated reaction. 

Dimethylsulfonium methylide is both more reactive and less stable than dimethylsulfoxonium methylide, so it is generated and used at a lower temperature. A sharp distinction between the two ylides emerges in their reactions with a, ( -unsaturated carbonyl compounds. Dimethylsulfonium methylide yields epoxides, whereas dimethylsulfoxonium methylide reacts by conjugate addition and gives cyclopropanes (compare Entries 5 and 6 in Scheme 2.21). It appears that the reason for the difference lies in the relative rates of the two reactions available to the betaine intermediate (a) reversal to starting materials, or (b) intramolecular nucleophilic displacement.284 Presumably both reagents react most rapidly at the carbonyl group. In the case of dimethylsulfonium methylide the intramolecular displacement step is faster than the reverse of the addition, and epoxide formation takes place. 

An explosion was experienced dining work up of an epoxide opening reaction involving acidified sodium azide in a dichloromethane/dimethyl sulfoxide solvent. The author ascribes this to diazidomethane formation from dichloromethane [1]. A second report of an analoguous accident, also attributed to diazidomethane, almost certainly involved hydrogen azide for the cold traps of a vacuum pump on a rotary evaporator were involved this implies an explosive more volatile than dichloromethane. It is recommended that halogenated solvents be not used for azide reactions [2]. 

Scheme 7.27. Domino ester reduction/epoxide formation/reductive epoxide-opening reaction.

The mechanism of epoxide formation (Scheme 7) has not been established but the intermediacy of nickel enolates and ensuing aldol type reactions are suspected28 (cf. Zn-mediated formation of furans from a-bromoketones29). A limitation on the synthesis is that R cannot be aryl for these cases, the products are 2,4-diarylfurans (see Section IV,B,1).30... 

Some related reactions are worth mentioning in this context. Addition of allylnickel bromide to styrene oxide to give an alcohol has been reported (example 7, Table IV). Tsutsumi has described the Darzens-type reaction of two molecules of a-bromoketones to give dimethylfurans (example 8, Table IV). This reaction consists of the addition of the ketomethylenic group to the carbonyl group of another molecule, followed by epoxide formation and bromide elimination. A subsequent rearrangement leads to a dialkylfuran. 

There are two channels of the reaction of acylperoxyl radicals with olefins hydrogen atom abstraction and addition to the double bond with epoxide formation [5,35] ... 

Sajus et al. [243,244] synthesized the peroxo complex of molybdenum(VI) and studied its reaction with a series of olefins. This peroxo complex M0O5 was proved to react with olefins with epoxide formation. The selectivity of the reaction increases with a decrease in the complex concentration. It was found to be as much as 95% at epoxidation of cyclohexene by M0O3 in a concentration 0.06 mol L-1 at 288 K in dichloroethylene [244], The rate of the reaction was found to be... 

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

An alternative to the synthesis of epoxides is the reaction of sulfur ylide with aldehydes and ketones.107 This is a carbon-carbon bond formation reaction and may offer a method complementary to the oxidative processes described thus far. The formation of sulfur ylide involves a chiral sulfide and a carbene or carbenoid, and the general reaction procedure for epoxidation of aldehydes may involve the application of a sulfide, an aldehyde, or a carbene precursor as well as a copper salt. This reaction may also be considered as a thiol acetal-mediated carbene addition to carbonyl groups in the aldehyde. 

When the ketone (280) was heated at reflux with pTsOH in benzene, the product (281) was isolated 95). The mechanism of this intriguing rearrangement may involve 1,3-hydride shift or epoxide formation 9S), This reaction appears to be an efficient method for the synthetis of [3.3.3]propellane. 

Hargreaves has suggested that the insolubilization of some closely related polymers is due to photolytic homolysis of the endoperoxide 0-0 bond and subsequent generation of carbon-centered radicals from the O radicals (19). There are several facts that make this an extremely unlikely explanation for the data described here these include the quantitative insufficiency of the maximum amount of endoperoxide reaction obtainable with a few hundred mJ/cm2 dose (homolysis quantum yield <0.5 (46), and extinction coefficient 1 (M cm)-1 (47)), and the synthetic utility of such homolysis reactions in related molecules in the presence of good hydrogen atom donors (implying facile epoxide formation) (48). Clearly the crosslinking observed under N2 is not accounted for by this mechanism. 

Optically pure P-ethanolamines react with dichlorocarbene under phase-transfer catalytic conditions to produce epoxides of high configurational retention [30]. Initial reaction occurs at the tertiary nitrogen centre (Scheme 7.29) with subsequent cleavage of the C-N bond. The reaction is configurationally controlled, as shown by the reaction of the conformationally rigid cyclic systems epoxide formation occurs with the equatorial hydroxyl system (50%), but not with the axial hydroxyl compound. 

The use of 0(3P) atoms produced by microwave irradiation of He-Oa mixtures has shown that alkenes react with atomic oxygen in solution or neat to give predominantly epoxides. Unlike reactions in the gas phase, at low temperature these produce useful product yields and distributions. Similar yields suggest that epoxide formation and 1,2-H/1,2-C shifts/ring contractions compete. 

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