Oxirane, three-membered ring opening

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,... 

Epoxides. Epoxides or oxiranes are quite reactive metabolically. These three-membered rings will readily open in the presence of most nucleophiles. They will react with most biopolymers, including proteins and nucleic acids. Compounds with epoxides have very short half-lives and tend to be toxic. 

Oxacyclopropane (oxirane), the simplest cyclic ether, is an outstanding exception to the generalization that most ethers are resistant to cleavage. Like cyclopropane, the three-membered ring is highly strained and readily opens under mild conditions. Indeed, the importance of oxacyclopropane as an industrial chemical lies in its readiness to form other important compounds. The major products derived from it are shown in Figure 15-5. 

Oxetanes are generally much more stable to nucleophilic attack than the more strained three-membered ring oxiranes. However, activation of the oxygen atom by a Lewis acid increases the electronegativity of the adjacent carbon atom and renders oxetanes susceptible to attack from a nucleophile accompanied by subsequent ring opening. 

The ring-opening reaction of three-membered rings (cyclopropanes and oxiranes, respectively) adjacent to the carbonyl group is a straightforward process for the... 

The strain in the three-membered ring is one of its most important properties. It is also the basis of the explanation of many of the features of the molecule, for example, the high reactivity in ring-opening reactions and the low electron-donor ability. These properties can be explained in terms of the hybridization of the bonding and nonbonding electron orbitals and the angular strain in the transition state. A short account of the MO model with reference to the oxiranes is found in the work of Rosowsky. ... 

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. 

The reaction of an oxirane with a nucleophilic carbanion or an ylide is a versatile method for the synthesis of cyclopropanes. Oxiranes and stabilized carbanions or ylides bearing different substituents are readily prepared from a variety of starting materials. - The inherent polarity and strain of the three-membered ring makes oxiranes particularly susceptible to transformation to cyclopropanes by intermolecular ring opening. 

An oxirane, also known as an epoxide (77), is a three-membered ring ether. The highly strained three-membered ring, analogous to a cyclopropane ring (Chapter 8, Section 8.5.3), makes an epoxide highly reactive. Contrary to most other ethers, they are easily opened by nucleophiles. How are epoxides prepared An important method for the preparation of epoxides is the oxidation of an alkene. 

PROBLEM 10.12 If hydroxide is such a poor leaving group, one would expect RO would also be a bad leaving group, and it is. In fact, simple ethers (R—O—R) are not generally cleaved in strong base. Why, then, do oxiranes open so easily in base (Fig. 10.32) What is special about the three-membered ring that facilitates the displacement ... 

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