Ethers epoxides formation

Fig. 2-19. Internal ether (epoxide) formation. Inversion occurs at the carbon atom involved, if the leaving group L is not at a primary position.

Some cleavage takes place even if the phenoHc hydroxyl is blocked as an ether link to another phenylpropane unit and quinonemethide formation is prevented. If the a- or y-carbon hydroxyl is free, alkaH-catalyzed neighboring-group attack can take place with epoxide formation and P-aryloxide elimination. In other reactions, blocked phenoHc units are degraded if an a-carbonyl group is present. 

Although sodium methoxide is also generally used for epoxide formation, it cleaves the epoxide ring only after heating at reflux temperatures for several hours (to give methyl ether derivatives), and no difficulty is experienced in controlling these separate reactions. 

Where several cyclic ethers can be formed by intramolecular tosyl te ion displacement, epoxide formation usually occurs In j>ref r ence to formation of larger other rings. This is particularly true whi-u the alternatives axe four- or six-memberecL There are often formed, however, tive-membeted oxides by attack of the C( hydroxyl un. 1 2,3-anhydro function,842-l 7 -1818 according to tho general scheme depicted in Eq- (252). Prolonged exposure of 2,3-anhydro sugars containing free hydroxyl functions at Ctn> to alkaline conditions > obviously undesirable for tliis reason. 

Cyclization of allylic alcohols to form epoxides has been particularly problematical, and the reactions have been more of mechanistic than of synthetic interest. For reactions conducted under basic conditions, it is possible that epoxide formation involves initial halogen addition followed by nucleophilic displacement to form the epoxide. Early examples of direct formation of epoxides from allylic alcohols with sodium hypobromite," bromine and 1.5 M NaOH,12 and r-butyl hypochlorite13 have been reviewed previously.fr Recently it has been shown that allylic alcohols can be cyclized effectively with bis(jym-collidine)iodine(I) perchlorate (equation 3).14 An unusual example of epoxide formation competing with other cyclization types is shown in equation (4).15 In this case, an allylic benzyl ether competes effectively with a -/-hydroxyl group as the nucleophile. 

The ether linkage ( C—0—C ) is normally prepared by one of the three following procedures. The Williamson synthesis is the most general method, while the diazomethane procedure and the epoxide formation procedure are useful for specific types of... 

The plant bufadienolide scillarenin (500) has been synthesized. The starting material was 15a-hydroxycortexone (501), which was converted into the diketone ketal (502) by cupric acetate oxidation at C(21), followed by selective ketalization and tosylate elimination. Protection at C(3) as the dienol ether, oxiran formation at C(20) with dimethylsulphonium methylide, and regeneration of the C(3)- and C(21)-oxo-groups by acid hydrolysis then provided (503). Selective reaction at C(21) with the sodium salt of diethyl methoxycarbonyl-methylphosphonate, and boron trifluoride rearrangement of the epoxide ring to the aldehydo-unsaturated ester (504), was followed by enol lactonization to the bufadienolide (505). This was converted, in turn, to scillarenin (500) via the 14,15-bromohydrin, by standard reactions. Unsubstituted bufadienolides have also been prepared by the same method. 

Epoxide Formation (Internal Williamson Ether Synthesis) (3)OC-cyc/o-Alkoxy-de-halogenation... 

The homologation of ketones by the addition of diazoalkanes complements the Tiffeneau-Demjanov rearrangement. Epoxide formation is a side reaction which can be minimized if polar aprotic solvents are avoided (Scheme 7). Rearrangement i.e. homologation) is maximized in ether solvents or by Lewis acid catalysis. The reaction is most effective in the ring expansion of cyclic ketones. 

Epoxidations. Formation of chiral epoxides from enol ethers derived from protected glucopyranosyl derivatives has been reported. 1,2-Epoxyalkylphosphonates are obtained from epoxidation of vinylphosphonates. Generation of dimethyldioxirane at high pH (10.5-11.5) is advantageous. ... 

Allenes possessing a hydroxyl group at the allylic position undergo arylation which is accompanied by epoxide formation. A 3-aryl group is readily introduced to 1,2-cyclo-hexanedione and 2-ethoxy-2-cyclohexenone. Ethyleneacetals of aryl methyl ketones are obtained on arylation of hydroxyethoxy vinyl ether. ... 

The alcohol was protected as its TMS ether, and the C-15,16 alkene stereospecifically dihydroxylated to give compound 50. The diol was then converted to its cyclic sulfate derivative according to the Sharpless protocol.29 Attempted base-catalyzed elimination of the sulfate to introduce the C-14,15 alkene was plagued by side-reactions involving epoxide formation by displacement of the sulfate by the adjacent TMS ether, perhaps aided by enolization of the methyl ketone. Instead, displacement of the sulfate by iodide ion occurred uneventfully to provide 51 as its tetrabutylammonium salt. 

Although widely used as a dehydration reagent, Martin s sulfurane is also known to facilitate amide cleavage reactions,7 cyclic ether (including epoxide) formations,8 and sulfilimine syntheses.9 In the 1970s Martin demonstrated all of these transformations in his series of papers outlining the reactivity of the title sulfurane. 

Octyl epoxy tallate. See Epoxidized octyl tallate n-Octyl ester of 3,4,5-trihydroxybenzoic acid. See Octyl gallate Octyl ether. See Dioctyl ether Octyl formate... 

The synthesis of dichloronorcarane from cyclohexene by the chloroform-base-PTC method has been improved further as has the preparation of a-halogeno-aP-unsaturated ketones via em-dihalogenocyclopropanes by employing trimethylsilyl vinyl ethers rather than ethyl vinyl ethers. The formation of gem-difluorocyclo-propanes proceeds in high yield (60— 90%) when chlorodifluoromethane is treated with halide ion and an epoxide in the presence of an olefin. The epoxide-halide ion combination is employed to produce a base of sufficient strength, and in sufficient concentration, to maximize the production of difluorocarbene oxiran and chloro-methyloxiran afford the most suitable bases when treated with chloride ion (Scheme 4). 

It is the chief constituent of the poisonous American wormseed oil and is found in amounts of up to 40% in oE of Cheno podium amhrosioides, although not found in other oils of the same type [52]. Ascaridole, as an inner peroxide, is found on chromatograms at a higher position than the hydroperoxides which have been repeatedly detected as intermediates during epoxide formation. Using n-hexane-diethyl ether (87 + 13), the three limonene peroxides can also be separated under standard conditions (hRf 22, 27 and 33) [85]. 

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