Epoxides reaction with alkoxides

In Sect. 5.2.2, Example 5.12, we saw the electrophilic character of epoxide toward the alkoxide ion. The carbanion is another charged nucleophile that opens a three-membered ring in reaction with epoxides. When such an anion is unstabilized, its synthetic equivalent is an organometallic compound affording a-alkylated or arylated alcohols as products (Scheme 5.50a). 

Substitution of an additional nitrogen atom onto the three-carbon side chain also serves to suppress tranquilizing activity at the expense of antispasmodic activity. Reaction of phenothia zine with epichlorohydrin by means of sodium hydride gives the epoxide 121. It should be noted that, even if initial attack in this reaction is on the epoxide, the alkoxide ion that would result from this nucleophilic addition can readily displace the adjacent chlorine to give the observed product. Opening of the oxirane with dimethylamine proceeds at the terminal position to afford the amino alcohol, 122. The amino alcohol is then converted to the halide (123). A displacement reaction with dimethylamine gives aminopromazine (124). ... 

Zinc carbonate reacts with epoxide to form zinc alkoxide, which in turn reacts with carbon dioxide to regenerate zinc carbonate. The most effective catalyst systems were the reaction products between diethylzinc and polyhydroxy compounds such as water or pdyhydric phenols243,244. This copolymer is interesting as a biodegradable elastomer248. ... 

In principle, any reaction with a racemate using a chiral reagent can be used to effect a kinetic resolution. Kagan (56) has recently given an analysis of the relationship between the extent of reaction and die enantiomeric excess that can be achieved, while Sharpless (57) has applied kinetic resolution successfully to racemic allyl alcohols using his titanium alkoxide tartrate epoxidation reaction. 

The proposed mechanism of this reaction is based on the nucleophilic attack of the alkyllithium compound at the carbenoid carbon atom or at the a-lithiooxy carbene. The dilithium compound 102 gives the alkene 103 by the loss of lithium oxide (equation 56). When an alkoxy residue, which is a better leaving group than U2O, is offered in the a-position of the corresponding dilithium compound, the elimination of lithium alkoxide takes place instead of lithium oxide. This is illustrated by the reaction of epoxide 104 that delivers the allylic alcohol 105 upon treatment with n-butyllithium (equation The... 

It was proposed that propagation proceeds by reaction of carboxyl anion with epoxide to form an ester-alkoxide anion which can react in turn with anhydride to form ester and another carboxyl anion. The rate-determining step was thought to be the reaction of carboxyl anion with epoxide, which would lead to first-order kinetics with respect to epoxide concentration. 

The direct reaction between alcohols and epoxides is very slow so alkaline catalysts are employed and are presumed to increase the rate by converting the growing chain to the more nucleophilic alkoxide ion. Polymerizations of this kind have recently been carefully investigated by Gee and coworkers (21). The reactions are still not rapid even at elevated temperatures so the molecular weights obtained in this way are normally quite low (< 10,000). The most rapid reactions of epoxides are those catalysed by acids but even they must be regarded as of the nucleophilic type with acceleration due to the increased reactivity of the protonated monomer,... 

This value shows that the rate of the reaction between alkoxide anion and anhydride (Eq. (18)) is about five times higher than that of the reaction of carboxylate anion with epoxide (Eq. (19)). Since it holds for the stationary state of copolymerization ... 

According to this mechanism, initiation includes reaction between the tertiary amine and epoxide, and the primary active centre is represented by a zwitterion with an alkoxide anion and an irreversibly bound amine in the form of an ammonium cation (Eq. (69)). This zwitterion reacts in the next step with the anhydride (Eq. (70)) yielding a carboxylate anion. The growth reactions (Eqs. (71) and (72)) include interactions of the carboxylate anion with epoxide, and of the alkoxide anion with the anhydride. 

First, the hydrolytic condensation of cyclopentyltrichlorosilane in acetonitrile (Section 9.2.2) was repeated on a 125 ml scale (50 times up-scaling). Next, the silsesquioxane products were reacted with a titanium alkoxide, yielding a catalyst with the same epoxidation activity of the HTE lead and, therefore, confirming the applicability of HTE techniques to the synthesis of silsesquioxanes. The silsesquioxane products obtained prior to reaction with the titanium centre could be divided into two fractions one as a precipitate (A) and the other as solute in the reaction mixture (B). Fraction B was dried under reduced pressure and redissolved in tetrahydrofuran. 

In order that an alternating copolymer is produced, the metal alkoxide must undergo faster insertion of carbon dioxide than reaction with a second equivalent of epoxide. If the metal alkoxide reacts with a second epoxide or undergoes decarboxylation reactions, ether linkage(s) may be formed. Ether linkages are undesirable as they compromise the properties of the polymer and reduce the carbon dioxide uptake. 

Why does the stabilized ylid prefer to react with the double bond In order to understand this, let s consider first the reaction of a simple, unstabilized ylid with an unsaturated ketone. The enone 1 has two electrophilic sites, but from Chapters 10 and 23, in which we discussed the regioselectivity of j attack of nucleophiles on Michael acceptors like this, you would expect that direct 1,2-attack on the i ketone is the faster reaction. This step is irreversible, and subsequent displacement of the sulfide i leaving group by the alkoxide produces an epoxide. It s unimportant whether a cyclopropane prod- uct would have been more stable ihe epoxide forms faster and is therefore the kinetic product. 

A highly useful twofold reaction of silyl dithioacetals with epoxides was described by Tietze and coworkers (Scheme 2.107) [249]. Treatment of 2.2equiv. of enan-tiopure epoxides 2-463 with lithiated silyldithiane 2-458b in the presence of a crown ether led to 2-467 after aqueous work-up. It can be assumed that by attack of the lithium compound 2-462 at the sterically less-hindered side of the epoxide 2-463, the alkoxide 2-464 is formed which in a subsequent Brook rearrangement produces the lithium dithioacetal 2-465. This reacts again with an epoxide to give 2-466 and furthermore 2-467. Treatment with NaF then leads to the diol 2-468 which can be converted into the dihydroxy ketones 2-469 and the corresponding 1,3,5-triols, respectively. 

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