Epoxides with nucleophilic reagents

Chart II summarizes the reactions of these epoxides with nucleophilic reagents. In all but one reaction, the product is very preponderantly, if not exclusively, that derived by diaxial opening of the epoxide ring. The absence of the alternative, diequatorial isomer argues against the likelihood, in the general case, of a mechanism... 

Write equations for the reaction of ethylene oxide or other epoxides with nucleophiles such as H+, and H20, H+ and alcohols, ora Grignard reagent. 

The presence of a C-C-OH moiety adjacent to a potential nucleophilic site in a TM, as exemplified below, points to a reaction of an epoxide with a nucleophilic reagent in the forward synthesis. The facile, regioselective opening of epoxides by nucleophilic reagents provides for efficient two-carbon homologation reactions. 

When triptolide 1 was reacted with nucleophillic reagents HBr, CH3CH2CH2SH, HOAc, MeOH, ammonium thiocyanide or 4-methoxybenzenesulfonyl chloride, the same C12-C13 epoxide reacted to form compounds 69-73 [70-72], Refluxing 1 for 96 hrs in phosphate buffer solution at pH 4 generated triptriolide 10 in 43% yield [72], In contrast, when 1 was stirred with sodium cyanoborohydride followed by dropwise addition of neat boron trifluoride diethyl etherate at room temperature for 16 hrs, the epoxide ring at C-7-C-8 opened and the compound 74 was obtained. Subsequently, reaction of compound 74 with HC1 at room... 

Treatment of pristinamycin IIa with meta-chloroperbenzoic acid afforded a compound to which the structure (79) was initially assigned, resulting from epoxidation of the more substituted double bond (12,13-C). This material did not display chemical properties characteristic of an epoxide as the assumed epoxide moiety remaining after treatment with nucleophilic reagents. Michael-type addition products on the dehydroproline ring were observed after treatment with thiols or amines (see Sect. 5.4.5). 2D-NMR analysis of the product from reaction of pristinamycin IIa with mCPBA showed that a transannular oxidative cyclization had taken place leading to formation of (80). The reaction can be considered to involve initial epoxidation of the 12,13-double bond followed by an intramolecular nucleophilic attack by the 37-hydroxy of the enol ether (Scheme 19). A similar transannular oxidative cyclization reaction has been reported for the reaction of l,5-dimethylcyclooct-4-en-l-ol with meta-chloroperbenzoic acid [125]. 

Abstract Polybutadiene is a versatile starting material for polymer-analogous reactions because of the high cmitent of easily accessed double bonds. It is a large scale polymeric product with relatively low costs. Polybutadiene may be tuned in its properties by consecutive chemical functionalizations to expand its range of applications. The polarity decreases content double bond hydrogenation and may be increased by the addition of heteroatoms to the olelinic entities. The functionalized of double bonds (e.g. to epoxides, aldehydes, carboxylates, hydroxyls or amines) opens the option of subsequent reactions in particular with nucleophilic reagents. This article focuses on post-modifications of polybutadiene homo-polymers by such sequential reactions and shows their relevance to applications. 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

The most striking chemical property of epoxides is their far- greater reactivity toward nucleophilic reagents compared with that of simple ethers. Epoxides react rapidly with nucleophiles under conditions in which other ethers are inert. This enhanced reactivity results from the angle strain of epoxides. Reactions that open the ring relieve this strain. 

Although several interesting nitrogen-centered nucleophiles have been developed with ARO reactions of epoxides (vide supra), kinetic resolutions with such reagents are unlikely to be of practical value for the recovery of enantioenriched terminal epoxides. This is due to the fact that these nucleophiles are too valuable to be discarded in a by-product of the resolution, are generally not atom-economical, and, particularly in the case of azide, may represent safety hazards. 

The reaction of allenes with peracids and other oxygen transfer reagents such as dimethyldioxirane (DM DO) or hydrogen peroxide proceeds via allene oxide intermediates (Scheme 17.17). The allene oxide moiety is a versatile functionality. It encompasses the structural features of an epoxide, an olefin and an enol ether. These reactive intermediates may then isomerize to cyclopropanones, react with nucleophiles to give functionalized ketones or participate in a second epoxidation reaction to give spirodioxides, which can react further with a nucleophile to give hydroxy ketones. 

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