What Are Epoxides

An epoxide is a cyclic ether in which oxygen is one atom of a three-membered ring  

Although epoxides are technically classed as ethers, we discuss them separately because of their exceptional chemical reactivity compared with other ethers. 

Common names for epoxides are derived by giving the common name of the alkene from which the epoxide might have been derived, followed by the word oxdde, an example is ethylene oxide. 

Ethylene oxide, one of the few epoxides manufactured on an industrial scale, is prepared by passing a mixture of ethylene and air (or oxygen) over a silver catalyst  

In the United States, the annual production of ethylene oxide by this method is approximately 10 kg. 

---

What are epoxides In what sense might they be regarded as ethers Is there any way that epoxides may be formed from other kinds of compounds in the body How might this occur ... 

Few non.chemists know exactly what an epoxide is. but practically everyone has used an "epoxy glue for household repairs or an epoxy resin for a protective coating. Epoxy resins and adhesives generally consist of two components that are mixed just prior to use. One component is a liquid "prepolymer/ and the second is a "curing agent" that reacts with the prepolymer and causes it to solidify. 

Analyze the following data on the product ratios obtained in the epoxidation of 3-substituted cyclohexenes by dimethyldioxirane. What are the principal factors that determine the stereoselectivity ... 

What are the causes behind the recurring gradation of this nucleophilicity series Nucleophilicity obviously measures the ability of the nucleophile to make an electron pair available to the electrophile (i.e., the alkylating agent or the epoxide). With this as the basic idea, the experimentally observable nucleophilicity gradations can be interpreted as follows. 

The hallmark of titanium tartrate catalyzed asymmetric epoxidation is the high degree of enantiofacial selectivity seen for a wide range of allylic alcohols. The question naturally arises as to what is the mechanism of this reaction and what are the structural features of the catalyst that produce these desirable results. These questions have been studied extensively and the results have been the subject of considerable previous discussion. - d36 pg purpose of this chapter, we wish to review those aspects of the mechanistic-structural studies that may helpful in devising synthetic applications of this reaction. 

Figure 2.1. Brevetoxins are based on two different structural backbones, based on what are perceived to be the two parent molecules, PbTx-2 (brevetoxin B) and PbTx-1 (brevetoxin A). All other known derivatives are based on alteration of the R-side chain, epoxidation across the double bond in the H-ring of PbTx-2, or derivatization at the C-37 hydroxyl in PbTx-2. PbTx-8, the chloromethyl ketone derivative of PbTx-2, is an artifact of chloroform extraction and subsequent phosgene conversion of PbTx-2. Common features include trans-fused polyether ring systems consisting of five- to nine-membered rings. denotes likely chemical artifact from extraction (Baden et al. 2005).

In a lithium amide promoted deprotonation, one lithium amide molecule is consumed for each deprotonated epoxide molecule. Since chiral hthium amides are expensive reagents, there is a strong desire to develop less costly synthetic procedures for stereoselective deprotonations. Catalysis has the potential to solve the problem. What are needed are bulk bases capable of regenerating the chiral hthium amide from the chiral diamine produced in the deprotonation reaction. There have been some attempts along this line, e.g., by Asami and co-workers, who used the non-chiral hthium amide LDA as bulk base and the chiral hthium amide 4 as catalyst [9,12,39-41]. However, the stereoselectivity was considerably lower than what had been achieved in absence of the bulk base, i.e., under stoichiometric conditions. Most likely, the decreased stereoselectivity in the presence of bulk LDA is due to competing deprotonation by LDA to yield racemic product alcohol. The situation is illustrated in Scheme 9. 

Some of what are known as barrier resins (see Section 7.3) also belong to the class of rubber-reinforced polymers. Even thermosets can also be reinforced by rubbers, for example, epoxides by telechelic oligobutadienes with carboxyl end groups. 

These unpleasant side effects are eliminated by using what are known as reactant resins. These are predominantly methylol compounds of cyclic urea derivatives with tertiary nitrogen, compounds with epoxide groups, or substances with activated ethylene double bonds ... 

Ammonia and amines react with epoxides with the same stereospecificity as anionic nucleophiles. Draw a sawhorse or Newman projection formula for the product of the reaction shown, clearly showing the stereochemistry at both chirality centers. What are the Cahn-lngold-Prelog R,S descriptors for these chirality centers in the reactant and the product ... 

So we are still left with two models of the stereochemistry of DNA alkylated by a PAH diol epoxide the PAH either lies in a groove of DNA or else tries to intercalate between the bass of DNA. Since it is covalently bonded to a base it must cause considerable distortion if it tries to lie between the bases. However, the stacking observed in the crystalline state seems to argue for partial intercalation. We will need crystal structures of at least one appropriately alkylated polynucleotide before this problem can be resolved. And when this is done it will be just the beginning of the answer to the problem of alkylation of DNA by activated carcinogens. The subsequent question is, what is the lesion in DNA that is important in carcinogenesis, and then what does it cause to happen so that tumor formation is initiated ... 

The simplicity of this assay makes it highly attractive for further development. Some of the limitations include the need for the preparation of relatively large amounts of appropriately modified DNA for the initial immunization. This may be reduced with the development of in vitro immunization techniques in which as little as 5ng of antigen can be effective. The antibodies must also be characterized once prepared. For example, a variety of monoclonal antibodies against B[a]PDE-modified DNA have been prepared (42) which show varying specificity. At one extreme, some require the full structure of the adduct bound to DNA, at the other, BtalP tetraol will effectively compete. What is not clear at the moment is how specific such antisera are for a particular PAH. Will these antisera recognize only B[a]P tetraol structures or those of any diol epoxide modified DNA ... 

---