Reaction of epoxidations

Although the chemical reactions of epoxides will not be covered m detail until the fol lowing chapter we shall introduce their use m the synthesis of alcohols here... 

There is an important difference in the regiochemistry of ring opening reactions of epoxides depending on the reaction conditions Unsymmetncally substituted epoxides tend to react with anionic nucleophiles at the less hindered carbon of the ring Under conditions of acid catalysis however the more highly substituted carbon is attacked... 

The reactions of carboxyUc acids and anhydrides with epoxy resins have been extensively studied in a variety of investigations, particularly References 27—31. The general reaction of epoxide resins and anhydrides is... 

Group of plastics composed of resins produced by reactions of epoxides or oxiranes with compounds such as amines, phenols, alcohols, carboxylic acids, acid anhydrides and unsaturated compounds. 

Reaction of Epoxides with Boron Trifluoride-Etherate... 

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]... 

The (3-elimination of epoxides to allylic alcohols on treatment with strong base is a well studied reaction [la]. Metalated epoxides can also rearrange to allylic alcohols via (3-C-H insertion, but this is not a synthetically useful process since it is usually accompanied by competing a-C-H insertion, resulting in ketone enolates. In contrast, aziridine 277 gave allylic amine 279 on treatment with s-BuLi/(-)-spar-teine (Scheme 5.71) [97]. By analogy with what is known about reactions of epoxides with organolithiums, this presumably proceeds via the a-metalated aziridine 278 [101]. 

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. 

Synthesis of Complex Molecules by Rearrangement Reactions of Epoxides... 

A completely different way of preparing isocyanides involves the reaction of epoxides or oxetanes with trimethylsilyl cyanide and zinc iodide, for example, ... 

By studying the NMR spectra of the products, Jensen and co-workers were able to establish that the alkylation of (the presumed) [Co (DMG)2py] in methanol by cyclohexene oxide and by various substituted cyclohexyl bromides and tosylates occurred primarily with inversion of configuration at carbon i.e., by an 8 2 mechanism. A small amount of a second isomer, which must have been formed by another minor pathway, was observed in one case (95). Both the alkylation of [Co (DMG)2py] by asymmetric epoxides 129, 142) and the reduction of epoxides to alcohols by cobalt cyanide complexes 105, 103) show preferential formation of one isomer. In addition, the ratio of ketone to alcohol obtained in the reaction of epoxides with [Co(CN)5H] increases with pH and this has been ascribed to differing reactions with the hydride (reduction to alcohol) and Co(I) (isomerization to ketone) 103) (see also Section VII,C). 

We have already discussed the regioselectiviLy of Lhe reactions of epoxide with nucleophiles and devised strategies (p 64-5) to achieve Lhe synthesis of compounds (32). 

Based on the above results and previous works [3,9] on the reaction of epoxides and CO2, we tentatively propose the plausible mechanism for the copolymerization of GMA and CO2 (schane 1), Alkyhnethyl imidazolim salt (QX) and epoxide (GMA) r cted to synthesize an active spedes followed by chain propagation involving a < ncerted insertion of th e epoxide. However, more detailed mechanistic studies are needed to clairly understand the polymerization steps. 

The enantioselectivity was significantly influenced by the steric factor of the thiols employed. When p-MeC6H4SH and PhSH were used, the optical yields decreased to 69% and 3%, respectively. Shibasaki et al. have reported that gallium-lithium-bi-naphthoxide (GLB) 51 became a good catalyst for the enantioselective ring opening reaction of epoxide for the production of 52 (Eq. 7.39) [46]. 

Scheme 2.52. Reactions of epoxide 2-221 with 1,3-dicarbonyl dianions.

A general method for the preparation of 2-hydroxyalkyl-l,4,7-triazacyclodecane macrocycles 202, 203, 204 with a single pendant arm < 1999J(P 1)1211 > through reaction of epoxides has been achieved from the ortho amide derivative l,4,7-triazatricyclo[5.2.1.04 10]decane 40 (Scheme 30) <1999J(P1)1211>. 

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