Unsaturated Oxiranes

Rearrangements to unsaturated ketones with hydrogen or alkyl migration have been observed in the acetone-sensitized reactions of cyclic and alicyclic 3,4-epoxy-olefins (Eq. 331). °  

A similar rearrangement and geometric isomerization occur in the case of 2,4-hexadienemonooxirane. Irradiation of the spirooxirane 135 leads to cumulene, 135a, allyl alcohol 135b, and oxetane derivatives 135c. ° Cumulene formation is interpreted as occurring via a cyclopropylcarbene. 

In acetonitrile, exo-epoxynorbornene derivatives undergo intramolecular (27t + 2a) photocycloaddition (Eq. 332).  

In other solvents (ether, cyclohexane, acetone), the cycloaddition does not take place, only reduction of the double bond is observed. As regards the two monooxiranes of ewdo-bicyclopentadiene, in the course of acetone-sensitized photolysis only enrio-4-oxatetracycloundec-9-ene 136 is changed. Photoreduction of the double bond proceeds by a free-radical mechanism there is no interaction between the triplet-state excited olefin and the oxirane part.  

Photoinduced valence isomerization has been observed. An example is the process given in Eq. 333.  

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Preformed polymers containing reactive functionalities other than vinyl unsaturation (oxirane, hydroxyl, carboxyl, etc.)... 

A comparative analysis of the IR spectra of monomers under study and known model compounds (tetraalkylstannanes and unsaturated oxiranes) indicates corresponding shifts of absorption bands for the oxirane ring and the Sn—C bond. 

Theoretical evidence [Hartree-Fock (RHF) calculations and density functional theory] has been obtained for a concerted mechanism of oxirane cleavage and A-ring formation in oxidosqualene cyclization. A common concerted mechanistic pathway has been demonstrated for the acid-catalysed cyclization of 5,6-unsaturated oxiranes in chemical and enzymic systems. For example, the conversion of (24) into (26) proceeds via (25) and not via a discrete carbocation (27). Kinetic studies and other evidence are presented for various systems. 

The sulfonimidoyl group acts in the asymmetric synthesis of the unsaturated oxiranes shown in Schemes 1.3.20 and 1.3.21 as both a chiral auxiliary and a nucleofuge. These results suggested the application of the sulfonimidoyl group as a chiral linker in solid-phase synthesis. 

Related photochemical transformations have been reported in unsaturated oxirans. The formation of /(,y-unsaturaled ketones (58) from the a,/i-un-saturated oxirans (59) is believed to proceed via hydrogen migration in the allylic radical (60).51 Carbon-oxygen bond homolysis, followed by two... 

Dialkyl vinylmethylidene tellurium compounds reacted with aliphatic and aromatic aldehydes to form dialkyl telluriums and a,/ -unsaturated oxiranes (cis/trans mixtures with the c/ i -isomer as the predominant component) in yields as high as 94%2. 

Studies have been made of the rearrangement of oxiranes containing an acetylenic side-chain, isoprene oxides, and 7,5-unsaturated oxiranes. ... 

Considerable research effort has been directed towards the generation, detection, and isolation of oxirenes, a highly strained class of antiaromatic heterocycles <83CRV519>. Although the unsaturated oxiranes are called oxirenes, much confusion is generated by the fact that Chemical Abstracts denotes oxiranes fused onto a polycyclic aromatic compound as oxirenes also (e.g., compound (16), Figure 9 <91JHC473 . 

The invention of Heath et al. relates to novel phosphonate allyl monomers, made from reaction of an unsaturated oxirane with amine- or hy-drojyl-functionalized phosphonic acids (Scheme 3.21). These monomers were copolymerized with other unsaturated species (acrylic acid, maleic acid, acrylamide, or monomer derivatives of sulfonic acid, etc), yielding phosphonate polymers or oligomers. Phosphorus-containing monomers were incorporated at a ratio of 0.1-30% and polymerized in aqueous media. The final polymers had a molecular weight of 800-30 000 g mol . With their phosphonic acids groups (free acid or salts forms), they are of particular use as oilfield scale inhibitors. 

The thermal fragmentation of unsaturated bicyclic 1,4-peroxides, often readily made from 1,4-dienes (Scheme 84), has become an important route to novel bis(oxiranes) (80T833, 81CRV91). 

Group of plastics composed of resins produced by reactions of epoxides or oxiranes with compounds such as amines, phenols, alcohols, carboxylic acids, acid anhydrides and unsaturated compounds. 

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. 

Formation of oxiranes on the sterically more hindered side of the steroid ring system is usually carried out via /raw -halohydrins which afford oxiranes on treatment with base (c -Halohydrins yield ketones on exposure to base). Two general methods are available for the synthesis of tm s-halohydrins (1) the reduction of a-halo ketones and (2) the addition of a hypohalous acid to unsaturated steroids. 

The lack of strong ultraviolet absorption by oxiranes is particularly useful for monitoring the epoxidation of a, -unsaturated ketones. 

The use of an unsaturated anionic initiator—such as potassium p-vinyl benzoxide—is possible for the ring opening polymerization of oxirane [43]. Although initiation is generally heterogenous, the polymers exhibit the molecular weight expected and a low polydispersity. In this case, the styrene type unsaturation at chain end cannot get involved in the process, as the propagating sites are oxanions. 

The procedure is outlined in Scheme 8.33, starting from the generic allylic alcohol 125. SAE on 125 would provide epoxide 126, which could easily be transformed into the unsaturated epoxy ester 127 by oxidation/Horner-Emmonds olefmation (two-carbon extension). This operation makes the oxirane carbon adjacent to the double bond more susceptible to nucleophilic attack by a hydride, so reductive opening (DIBAL) of 127 provides, with concomitant ester reduction, diol 128. Pro-... 

Uses. It is used to prep oxiranes and has been used for the epoxidation of over 600 unsaturated compds(Ref 7)... 

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