Ring epoxides

The intramolecular addition of sulfur ylides to imines (e.g. 72) has proven to be an excellent route to fused-ring aziridines (e.g. 73) <06AG(I)7066>. The addition of a sulfonamide to a vinylsulfonium salt leads to the formation of the sulfur ylide 72. The ylide then undergoes an intramolecular addition to form the product fused-ring aziridine 73. This method has also been used for the synthesis of fused-ring epoxides. 

When aldonolactones are dissolved in strong aqueous base the lactone ring is immediately opened to give the acid salt. If bromodeoxylactones are treated similarly, besides opening of the lactone ring, epoxides are also formed. Depending on the pH and the structure of the substrate different reactions may occur. 

Cleavage of epoxides (6, 16-17 8, 10-12). Cleavage of epoxides catalyzed by Woelm neutral alumina is particularly useful in the case of medium-ring epoxides, which are generally rather unreactive and which are liable to undergo transannular reactions. For example, rw-cyclooctene oxide supported on neutral alumina is converted to rran.r-2-acetoxycyclooctanol in 78% yield by reaction with acetic acid at... 

Epoxides (also called oxiranes) are three-membered cyclic ethers. They are more reactive titan typical ethers due to the strain created by the small ring. Epoxides react with water in the presence of an acid catalyst to form diols, commonly called glycols. This is an anti-addition. 

The target compound contains a reactive epoxide ring. Epoxides react with acids and bases, and the epoxide might not survive the crucial ether-forming reaction just shown. Perhaps the epoxide is best added after formation of the ether. That gives us the following final two steps in the synthesis ... 

PbTx-2, CH2C(=CH2)CH0 PbTx-3, CH2C(=CH2)CH20H PbTx-5, [PbTx-2], C-37 OAc PbTx-6, [PbTx-2],H-ring epoxide PbTx-8, CH2COCH2CI PbTx-9, CHjCHCCHjtCHjOH... 

Table 4 Representative examples of the conversion of fused-ring epoxides into their thiirane derivatives...

Epoxides are ethers having the oxygen atom in a three-membered ring. Epoxides are also called oxiranes. 

Small rings (epoxides, cyclopropanes) Biphenyl Fast opening of the anion radical Fast cleavage of C— N and C—S [39, 45]... 

Diols can be prepared by direct hydroxylation of an alkene with OSO4 followed by reduction with NaHSOs (Section 7.8). The reaction takes place readily and occurs with syn stereochemistry. We ll see in the next chapter that 1,2-diols can also be prepared by acid-catalyzed hydrolysis of epoxides—compounds with a three-membered, oxygen-containing ring. Epoxide opening is complementary to direct hydroxylation because it occurs with anti stereochemistry. 

Isomerization of cyclooctene oxide. Isomerization of cyclooctene oxide (1) with lithium diethylamide gives the bicyclic alcohol (2) and 3-hydroxycyclooctene (3) in the approximate ratio of 4 1 (2, 247). The abnormal product (2) is obtained only with medium-ring epoxides large-ring epoxides give exclusively allylic alcohols. Sheng1 finds that cyclooctene oxide can be isomerized exclusively to 3-hydroxycyclooctene (3) if... 

The products from the oxidation of alkylbenzenes under simulated atmospheric conditions have been noted earlier. Both ring epoxides that were highly functionalized and aliphatic epoxides from ring fission were tentatively identified (Yu and Jeffries 1997), and formation of the latter, many of which are mutagenic, may cause further concern over transformation products from monocyclic aromatic hydrocarbons in the atmosphere. 

Iodination of the 12-membered ring epoxide 79 gave an [8.2.1] oxabicyclic product, Eq. 60 [87b]. 

There are three major types of epoxy resins cycloaliphatic epoxy resins (R and R are part of a six-membered ring), epoxidized oils (R and R are fragments of an unsaturated fatty acid, such as oleic acid in soybean oil), and glycidated resins (R is hydrogen and R can be a polyhydroxyphenol or a polybasic acid). The first two types of epoxy resins are obtained by the direct oxidation of the corresponding olefin with a peracid as illustrated by the following ... 

We solved the problem by using a three-membered ring (epoxide) for (2) and an a-halo carbonyl compound for (3) two apparently different devices which in fact rely on the same principle. The atoms marked + in (2) and (3) are easily made nucleophilic —by enolisation (i) for example—and the common principle is to use a preliminary nucleophilic attack on a heteroatom to reverse the natural polarity of the atom from nucleophilic to electrophilic. Halogenation (i, E = Br) of a ketone provides a reagent for (3) and epoxidation... 

Support for the new mechanism was provided by trapping the intermediate with methyl iodide to give epoxide 133. The structure of this compound was indicated by IR absorption at 1646 cm- (2-quinolone carbonyl) and by the NMR spectrum. The epoxide was converted readily into the diol balfourolone (135) on alumina chromatography or by treatment with 2 N sodium hydroxide at 20° the mild conditions and the failure of 2,4-dimethoxyquinoline epoxides (e.g., 109) to react under similar conditions suggest that this reaction occurs through formation of O-methylbalfourodinium salt (134) and subsequent nucleophilic attack at the 2-position rather than by direct reaction of hydroxide ion on the epoxide ring. Epoxide 133 is the presumed intermediate in reaction of 4-methoxy-... 

Epoxides are highly reactive due to the ease of opening of the highly strained three-membered ring. Epoxides can undergo base and acid-catalyzed cleavage. Reactions in (a) - (c) are examples of such cleavages. 

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