Metallation, of furan

There are cases known in which the use of different precursors of 221 was not equally successful in the synthesis of a certain cydoadduct. Thus, when the bromo-fluorocyclopropane 239 and 1,3-cyclopentadiene were employed as source and trapping reagent for 221, respectively, the diene was deprotonated to a large extent [66, 139], whereas this process did not play a major part on utilization of the dibromocy-clopropane 254 (Scheme 6.55). Further, an experiment to prepare the furan adducts 231-233 (Scheme 6.53) from 239 failed [66], probably because of the metalation of furan by MeLi or the carbenoid resulting from 239. 

Thiophene can be di-metallated in the 2- and 5-positions using two equivalents of BuLi TMEDA in hexane [ l 26]. Similar di-metallations of furan and 1-methyl-pyrrole have been reported [126], but only that of thiophene seems synthetically attractive. 

Metallation with alkyllithiums proceeds selectively at an a-position, indeed metallation of furan is one of the earliest examples of the now familiar practice of aromatic ring-metallation. The preference for a-deprotonation is nicely illustrated by the demonstration that 3-lithiofuran, produced from 3-bromofuran by metal/ halogen exchange at —78°C, equilibrates to the more stable 2-lithiofuran if the temperature rises to > —40°C more forcing conditions can bring about 2,5-dilithiation of furan. ... 

Catalytic hydrogenation of furan to tetrahydrofuran is accompHshed in either Hquid or vapor phase. Hydrogenation of the double bonds is essentially quantitative over nickel catalysts but is generally accompanied by hydrogenolysis over the noble metals. 

Directive effects on lithiation have also been studied. The regiospecific /3-metallation of A-methylpyrrole derivatives and 2-substituted furans has been effected by employing the directive effect of the oxazolino group (82JCs(Pl)1343). 2-Substituted furans and thiophenes are metallated in the 5-position. The formation of 2-lithio-3-bromofuran on treatment of... 

Stoichiometric closure of furan and pyrrole cycles on McMurry reaction induced by low-valent transition metals 98PAC1071. 

The thiophene ring can be elaborated using standard electrophilic, nucleophilic, and organometallic chemistry. A variety of methods have been developed to exploit the tendency for the thiophene ring (analogous to that of furan and pyrrole) to favor electrophilic substitution and metallation at its a-carbons. Substitution at the p-carbons is more challenging, but this problem can also be solved by utilizing relative reactivity differences. 

In developed countries, e-waste is collected to recover some materials of value and to be safely rid of the lead, cadmium, mercury, dioxins, furans, and such toxic materials as they contain. In developing countries, e-waste is collected principally to recover a few metals of value. 

This article aims to review papers that were published in 2006 on reactions and syntheses of furans, benzofurans and their derivatives. Two reviews have summarized the syntheses of furans <06OBC1627> and tetrahydrofurans <06EJO1627>. Another review <06001613> records the progress of transition metal-catalyzed asymmetric ring opening of oxabenzonorbornadienes. 

Direct metallation of 2-substituted furan can be realized when 2-furancarboxamide is allowed to react with f-BuLi in the presence of TMEDA. Treatment of the lithiated 2-furancarboxyamide with acetaldehyde provides the corresponding 2,3-disubstituted furan in 52% yield <00SL1788>. 3-TrialkyIsilyl-2-furancarboxamide can also be afforded in high yield if 2-... 

Incineration of domestic waste is also a contributor to environmental pollution. One more source of dioxins is pulp-and-paper industry. Comparatively new ways of forming polychlorinated dibenzo-p-dioxins and furans are high-temperature processes like copper melting in electric arc furnaces, and production of magnesium, nickel and, possibly, other metals of their chlorides. 

Furylstannanes may be prepared by a number of methods, one of which involves direct metalation of a furan followed by quenching with tin chloride [49]. The second method for furylstannane preparation uses halogen-metal exchange to generate the lithiofuran species, which is then quenched by tin chloride [50]. The third method, more suitable to base-sensitive substrates, is Pd-catalyzed coupling of a halofuran or halobenzofuran with hexabutylditin [51]. 

Regioselactive g-metallation of ir-excessive five ring heterocycles is not a novel reaction. Oxazoline and pyridine as well as carboxylate- and carboxamide -substituted heterocycles have been lithiated. From the point of synthetic utility thiophenes have been shown to be useful substrates after careful optimization of reaction conditions furans have been of less utility. 

Lithiation of thiazolo[5,4-b]pyridine-N-oxides (503) by n-butyllithium at -65°C is selectively directed by the pyridine N-oxide moiety, whereas lithiation of the parent heterocycle by LDA at -78°C exclusively occurs at the C-4 position (89TL183). Interestingly, no metalation of the furan ring occurred (Scheme 152). 

Directive effects on lithiation have also been studied. The regiospecific (3-metallation of N-methylpyrrole derivatives and 2-substituted furans has been effected by employing the directive effect... 

The bicyclic ketone (44), obtained from the Fe2(CO)9-promoted [3 + 4] cycloaddition reaction of a,a,a, a -tetrabromoacetone and 2-isopropylfuran followed by Zn-Cu couple reduction, has been converted to the naturally occurring troponoid, /3-thujaplicin (46) (75JOC806). Hydrogenation of (44), ether cleavage, bromination and dehydrobromination gave the tropone (45), an intermediate easily converted into the tropolone (46) by a standard procedure (Scheme 10). A related [3 + 4] cycloaddition reaction of oxyallyl metallic with furan has been used to assemble the antibiotic C-nucleosides (78JA2561). 

An anionic technique by indirect grafting was proposed for N-metallation of Nylon by Yamaguchi (153-155), in which alcali metals dissolved in liquid ammonia displace the amidic hydrogen atoms. Nylon derivatives and graft copolymers can be synthetized from the N-metallated Nylon (153). For ethylene oxide as grafting monomer, the metallated fibers were soaked in a tetrahydro-furan solution of the monomer, at 60° C (154). Methyl methacrylate is grafted on Nylon with a conversion over 90% by this technique (155). Other procedures involve the use of sodium methoxide in methanol solution and subsequent anionic graft copolymerization of acrylonitrile in a tetrahydrofuran solution (156). 

Furans can be converted into N- alkylpyrroles by heating with primary amines and alumina. Similar thermal conversions of furans and benzo[6]furans to their sulfur analogues in the presence of alumina or other metal oxide catalysts and hydrogen sulfide are also known. l,3-Diphenylbenzo[c]furan is converted into the thiophene by heating with phosphorus pentasulfide. The mechanism of these reactions is obscure. 

Ring opening is common in the alkali metal and liquid ammonia reduction of furans unless an anion stabilizing group is present, so most work has been done with derivatives of furancarboxylic acids. Treatment of furan-2-carboxylic acid with lithium and ammonia at -78 °C followed by rapid addition of ammonium chloride affords 2,5-dihydrofuran-2-carboxylic acid (80%). Reductive alkylation similarly gives 2-alkyl-2,5-dihydrofuran-2-carboxylic acids. This method has been used in a synthesis of rosefuran, the intermediate dihydrofuran (66) being converted into the product (67) by oxidative decarboxylation with... 

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