Cyclic Ethers Epoxides

For the most part, cyclic ethers behave like acyclic ethers. The chemistry the ether functional group is the same, whether it s in an open chain or j a ring. Common cyclic ethers such as tetrahydrofuran and dioxane, for ex pie, are often used as solvents because of their inertness, yet they canj cleaved by strong acids. 

The only cyclic ethers that behave differently from open-chain ethers the three-membered-ring compounds called epoxides, or oxiranes. strain of the tbree-meml red ring gives epoxides unique chemical reach Ethylene oxide, the simplest epoxide, is an intermediate in the mam ture of both ethylene glycol, used for automobile antifreeze, and polyester mers. More than 4 million tons of ethylene oxide are produced each year the United States by air oxidation of ethylene over a silver oxide catalyst a SOO°C. This process is not useful for other epoxides, however, and is of littb value in the laboratory. Note that the name ethylene, oxide is not a systematie one because the -ene ending implies the presence of a double bond in the mol eculc. The name is frequently used, however, because ethylene oxide is deriva from ethylene by addition of an oxygen atom. Other simple epoxides are name similarly. The systematic name for ethylene oxide is 1,2-epoxyethane. 

In the laboratory, epoxides are prepared by treatment of an alkene with a peroxyacid, RCO3H. Many different peroxyacids can be used to accomplish epoxidation, but m-chloroperoxybenzoic acid is the most common choice. 

Peroxyacids transfer oxygen to the alkene with syn stereochemistry through a one-step mechanism without intermediates. Studies have shown that the oxygen atom farthest from the carbonyl group is the one transferred. 

Another method for the synthesis of epoxides is through the use of halo-hydrins, prepared by electrophilic addition of HO-X to alkenes (Section 7.3). en halohydrins are treated with base, HX is eliminated, and an epoxide is produced. 

The one group of cyclic ethers that behaves differently from open-chain ethers contains the three-membered-ring compounds called epox.id.es, or oxiranes, 

This formation of an epoxide by treatment of a halohydrin with base is just an intramolecular Williamson ether synthesis. The nucleophilic alkox-ide ion and the electrophilic alkyl halide are in the same molecule. 

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The difference between the most kinetically favoured cyclisations is easily seen in the type of base needed to cyclise chloro-alcohols to three- and five-membered cyclic ethers (epoxides and THFs). Chloroethanol 19 cyclises only as the oxyanion specific base is needed, i.e. a strong enough base to remove the OH proton completely. By contrast, 4-chlorobutanol cyclises by general base catalysis the proton is removed during the cyclisation 22 and weak bases will do.1, 2... 

In addition to step and chain polymerizations, another mode of polymerization is of importance. This is the ring-opening polymerization of cyclic monomers such as cyclic ethers, esters (lactones), amides (lactams), and siloxanes. Examples of commercially important types are given in Table 10.1. Of those listed, only the polyalkenes are composed solely of carbon chains. Those that have enjoyed the longest history of commercial exploitation are polyethers prepared from three-membered ring cyclic ethers (epoxides), polyamides from cyclic amides (lactams), and polysiloxanes from cyclic siloxanes. 

Epoxides are three-membered cyclic ethers. Epoxides can be synthesized by the oxidation of corresponding alkenes with peroxy acid. 

There is a wide variety of reactants in this reaction. Suitable unsaturated starting materials or unsaturated compound-forming components are acetylenes, olefins, alcohols, cyclic and non cyclic ethers, epoxides, acetals, esters, saturated aldehydes, lactones and halides. Water, alcohols, ammonia, amines, mercaptans and carboxylic acids, e. g. are used as nucleophilic compounds with mobile hydrogen atoms. 

Figure 16.2 shows examples of naturally occurring, physiologically active cyclic ethers. Tetrahydrocannabinol (THC), the principal active ingredient in marijuana, includes a six-membered ring ether. Morphine contains a five-membered ring ether. Three-membered cyclic ethers (epoxides) are rare in nature. The juvenile hormone of some insects contains a cir-substituted epoxide. This hormone controls the rate insect maturation. 

Bromodimethylborane is also effective for the cleavage of cyclic ethers. Epoxides react at —78 °C while the analogous four- to seven-membered ring heterocycles react between 0 °C and rt. In contrast to other boron-containing Lewis acids, Me2BBr reacts via a predominantly Sn2 mechanism. Tetrahydrofuran derivatives which are substituted at the 2-position give rise to primary bromides as the major or exclusive products. The nature of the... 

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