Condensation reactions epoxides

Caution The. yield isolated from this reaction depends on the efficiency of this condenser the epoxidation is exothermic and methylenecyclopropane is volatile. 

In the condensation reaction between chloro- and bromo-methyl aryl sulfones and carbonyl compounds, a-sulfonyloxiranes were obtained. In this condensation reaction, bases such as potassium t-butoxides372, NaH373 and aqueous concentrated hydroxide with benzyltriethylammonium chloride under two-phase condensation were used374. In the reaction with aldehydes only the trans-epoxide isomers resulted, whereas lith-iofluoromethyl phenyl sulfone 289375 and 291376 were found to add to aldehydes affording /J-hydroxysulfones 290 and 292, respectively. 

Figure 2.6 Claisen-Schmidt condensation-asymmetric epoxidation reactions over nanocrystalline aerogelpre pared AP-MgO catalysts.

Oxidation is the first step for producing molecules with a very wide range of functional groups because oxygenated compounds are precursors to many other products. For example, alcohols may be converted to ethers, esters, alkenes, and, via nucleophilic substitution, to halogenated or amine products. Ketones and aldehydes may be used in condensation reactions to form new C-C double bonds, epoxides may be ring opened to form diols and polymers, and, finally, carboxylic acids are routinely converted to esters, amides, acid chlorides and acid anhydrides. Oxidation reactions are some of the largest scale industrial processes in synthetic chemistry, and the production of alcohols, ketones, aldehydes, epoxides and carboxylic acids is performed on a mammoth scale. For example, world production of ethylene oxide is estimated at 58 million tonnes, 2 million tonnes of adipic acid are made, mainly as a precursor in the synthesis of nylons, and 8 million tonnes of terephthalic acid are produced each year, mainly for the production of polyethylene terephthalate) [1]. 

The successful synthesis of 2-thienyl and substituted 2- and 3-thienyl-acetylenes in yields as high as 60-80% opened a wide variety of synthetic applications. Various addition reactions with carbonyl compounds or epoxides could be carried out with ease. Aliphatic as well as aromatic amine addition reactions, or condensation reactions with hydrazine or hydroxylamine could be easily performed. 

Usually, the formation of a new chiral centre involves the conversion of a prochiral sp carbon atom into one with sp hybridisation, the methods most generally used being the aldol and related condensations, pericyclic reactions (especially the Diels-Alder reaction), epoxidation, cyclopropanation and additions to double bonds (hydrogenation and hydroboration). Another possibility is the conversion of a prochiral sp carbon atom into a chiral centre, as for instance in the a-substitution (alkylation, halogenation, etc.) of a ketone. 

One recent publication from the group of Abu-Omar reports on a condensation reaction involving glycerol and furfural, both renewables, to produce dioxolanes, formally a dehydration reaction. Here, a cationic oxorhenium(V) oxazoline species is used as the catalyst for the formation of various 1,3-dioxalanes from furfural with diols or epoxides under mild conditions (Scheme 21). Especially interesting is the reaction of furfural with glycerol to obtain a 70 30 mixture of the corresponding 1,3-dioxolane and 1,3-dioxane in solvent-free conditions [125]. 

Figure 9.1 Chiral ligands and metal complexes with hard donor atoms. The complexes of these ligands with metal ions in relatively high oxidation states are used in asymmetric epoxidation, cyclopropanation, and nitroaldol condensation reactions.

Lignin condensation reactions include the formation of diphenylmethanes (57) and (52) and a-carbohydrate ether linkage (55) that may be derived from the quinonemethide (42) [32,277,336-340] or the epoxide (49) [342,343] intermediate. Coniferyl alcohol may be involved in the formation of p-7-linked condensed unit (54) [338,339]. 

There are currently two proposed mechanisms for the acyloin ester condensation reaction. In mechanism A the sodium reacts with the ester in a single electron transfer (SET) process to give a radical anion species, which can dimerize to a dialkoxy dianion. Elimination of two alkoxide anions gives a diketone. Further reduction (electron transfer from the sodium metal to the diketone) leads to a new dianion, which upon acidic work-up yields an enediol that tautomerizes to an acyloin. In mechanism B an epoxide intermediate is proposed. ... 

In the laboratory of P.G. Steel, a five-step synthesis of ( )-epiasarinin from piperonal was developed. The key steps in the sequence involved the Darzens condensation, aikenyi epoxide-dihydrofuran rearrangement and a Lewis acid mediated cyclization. The desired vinyl epoxide intermediate was prepared by treating the solution of ( )-methyl-4-bromocrotonate and piperonal with LDA, then quenching the reaction mixture with mild acid (NH4CI). 

Epoxides are isomerized to the corresponding aldehydes, ketones, or alcohols using a variety of crystalline aluminosilicates as catalysts. At 100-200° NaX and REX are effective catalysts for the conversions shown here. At higher temperatiu es further condensation reactions of... 

Ideal reactions for solution-phase parallel synthesis are those that are kinetically and thermodynamically favored, are tolerant of diverse functionality, and have a broad range of reactant tolerance. In this approach, capture resins and extraction procedures are often used for preliminary purification. The solution-phase reaction conditions must be validated in terms of scope and optimal reaction conditions over the range of reactants. Two common strategies for solution libraries involve derivatization of preformed functionalized scaffolds and multicomponent condensation reactions, for example, the Ugi reaction, the Passerini reaction, and the formation of hydroxyamininimides from an ester, a hydrazine, and an epoxide. 

In nonphenolic structures, the (3-0-4 linkage is cleaved via participation of the a- (or y-) hydroxyl group [74]. As shown in Figure 10.10, the reaction proceeds with formation of an epoxide, which can react further and be opened by attack of nucleophiles, such as NaSH and NaOH, or by nucleophilic sites in lignin or by carbohydrates [75,76]. In the latter cases, the reaction will again resnlt in condensation prodncts (the Condensation Reactions section). 

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