Furans esters

The 3-oxo-2//-furans (260) obtained from the reaction of a-halogenoacyl halides with /3-ketonic esters at -20 °C show no tendency to enolize to the hydroxyfurans (261) but rather behave as unsaturated ketones (Scheme 71) (73RTC731). 2,4-Dialkylfurans (262) are obtained by condensation of acyl halides with allyl halides catalyzed by aluminum chloride (73KGS1434). The acid catalyzed condensation of 2-chlorocyclopentanone with dimethyl /3-oxoglutarate leads to the furan ester (263) (77JHC711). 

In a recent report, Padwa [64] disclosed a general method of wide applicability for constructing a variety of heterocyclic systems. It involved an amino furan ester as a more reactive four carbon component vis avis a pyrone in intramolecular Diels-Alder addition to an unactivated olefin. This method as applied to the synthesis of anhydrolycorinone consisted of the preparation of the tertiary amide 240 (Scheme 40) and its subsequent thermolysis to the N-o-bromoaroylindoline 241. It is believed that the initially formed cycloadduct 242 opened to the acyl iminium oxyanion 243, which by prototropy and dehydration generated 241. The latter, on photolysis in the presence of bis (tri-n-butyltin), furnished the tetracyclic ester 244, which was hydrolysed and decafboxylated to anhydrolycorinone as in the previous synthesis. 

Due to the electronic richness of the C-3 of benzo[4]furan 82, the Yb-catalyzed electrophilic substitution of benzo[7]furan 82 with glyoxalate led to a 3-a-hydroxybenzo[4]furan ester in a regioselective manner, as depicted in Equation (70) <2000JOC4732>. 

The addition of LiCl to zinc dust (activated with 1,2-dibromoethane plus TMSCl) has a dramatic effect on the reaction rates. The reaction is successful with iodo-heterocycles and some activated bromo-compounds, such as the furan ester shown below. 

A furan ester has been used in a similar way at 60oC with another bismaleimide (12.38) to make polymers with molecular weights as high as 30,000.193 The problem is that Diels-Alder adducts of furans reverse at relatively low temperatures, in this case at 90°C or more. Such polymers would have to be used at room temperature or not much higher. 

The intramolecular cycloaddition of a furan ester was reported several years ago to yield a tricyclic adduct [Eq. (21)].53... 

A simple synthesis of racemic allomuscarine via stereospecific intramolecular opening of the trans-substituted epoxide ring in butyl 4,5-epoxy-2-hydroxyhexanoate was reported by Chmielewski and Guzik (24,25) (Scheme 2). The furane ester intermediate was obtained in an ene reaction between butyl glyoxylate and 1-butene, followed by epoxidation of the... 

Furans and Benzofurans.—Formation. The acid-catalysed condensation of 2-chlorocyclopentanone with dimethyl jS-oxoglutarate leads to the furan ester (100). Compound (101) is obtained by the action of nitrotrichloroethylene on acetylacetone in the presence of bases. 1-Diethylaminopropyne, MeC=CNEt2, and benzoin undergo three competing reactions to give a mixture of the furans (102) and (103) and the butenolide (104). The sulphonium ylide (105) and... 

The oxidation of the cyclic enol ether 93 in MeOH affords the methyl ester 95 by hydrolysis of the ketene acetal 94 formed initially by regioselective attack of the methoxy group at the anomeric carbon, rather than the a-alkoxy ketone[35]. Similarly, the double bond of the furan part in khellin (96) is converted ino the ester 98 via the ketene acetal 97[l23],... 

Pd(II) salts promote the carbonylation of organomercury compounds. Reaction of phenylmercury chloride and PdCh under CO pressure affords benzophenone (429)[387]. Both esters and ketones are obtained by the carbonylation of furylmercury(Il) chloride in alcohol[388]. Although the yields are not satisfactory, esters are obtained by the carbonylation of aryl- and alkylmercuryfll) chlorides[389,390]. One-pot catalytic carbonylation of thiophene, furan, and pyrrole (430) takes place at the 2-position via mercuration and transmetallation by the use of PdCb, Hg(N03), and CuCl2[391]. 

Alkynyl)oxiranes also react with carbon nucleophiles to afford furan derivatives. Furanes of different types are obtained depending on the structure of the substrates. 7-Methyl-2-ethynyloxirane (95) reacts with acetoacetate to give the furan 97 by the elimination of formaldehyde from the cyclized product 96. The hydroxy ester of the alkylidenefuran 98 and the corresponding lactone 99 are obtained by the reaction of i-methyl-2-(2-propynyI)oxirane[40, 42]. 

Alkylfurans, halofurans, alkoxyfurans, furfuryl ester and ethers, and furfural diacetate [613-75-2] behave similarly. Furans containing electron withdrawing constituents, for example, furfural, 2-furoic acid, and nitrofurans, fail as dienes even with very strong dienophiles. 

Aqueous mineral acids react with BF to yield the hydrates of BF or the hydroxyfluoroboric acids, fluoroboric acid, or boric acid. Solution in aqueous alkali gives the soluble salts of the hydroxyfluoroboric acids, fluoroboric acids, or boric acid. Boron trifluoride, slightly soluble in many organic solvents including saturated hydrocarbons (qv), halogenated hydrocarbons, and aromatic compounds, easily polymerizes unsaturated compounds such as butylenes (qv), styrene (qv), or vinyl esters, as well as easily cleaved cycHc molecules such as tetrahydrofuran (see Furan derivatives). Other molecules containing electron-donating atoms such as O, S, N, P, etc, eg, alcohols, acids, amines, phosphines, and ethers, may dissolve BF to produce soluble adducts. 

Z-Furan. 3-(5-Nitro-2-furanyl)-2-propenamide, is prepared by condensation of 5-nitro-2-furancarboxaldehyde diacetate with malonic ester followed by PCl chlorination and amination (29). The product was marketed in Japan as a food preservative. 

Furan and thiophene undergo addition reactions with carbenes. Thus cyclopropane derivatives are obtained from these heterocycles on copper(I) bromide-catalyzed reaction with diazomethane and light-promoted reaction with diazoacetic acid ester (Scheme 41). The copper-catalyzed reaction of pyrrole with diazoacetic acid ester, however, gives a 2-substituted product (Scheme 42). 

Benzo[b]furan-2-carboxylic acid, 3,6-dimethoxy-, methyl ester UV, 4, 589 <71AJC1883> Benzo[b]furan-2-carboxylic acid, 5,6-dimethyl-, methyl ester... 

H NMR, 4, 561 <65MI31002> Benzo[b]furan-2-carboxylic acid, methyl ester UV, 4, 589 Benzo[b]furan-3-carboxylic acid, 6-chloro-4,5,7-trifluoro-2-methyl-, ethyl ester C NMR, 4, 568 <72JCS(P2)1733> Benzo[b]furan-3-carboxylic acid, 6-cyano-4,5,7-trifluoro-2-methyl-, ethyl ester C NMR, 4, 568 <72JCS(P2)1733>, 569 <72JCS(P2)1733>... 

F NMR, 4, 569 <72JCS(P2)1733> Benzo[b]furan-3-carboxylic acid, 2-methyl-, methyl ester... 

H NMR, 4, 561 <720MR(4)343> Benzo[b]furan-4-carboxylic acid, methyl ester UV, 4, 589 (73AJC1059)... 

H-Cyclopenta[7,8]naphtho[2,3-6]furan-7-yl glyoxylate, 2,3,6,10-tetrahydro-6-hydroxy-6-methyl-3,10-dioxo-, methyl ester H NMR. 4, 560 (66JCS(C)743) Cyclopenta[6]quinoline, 6,7,8,9-tetrahydro- C NMR, 2, 122 (77JOC300, 77JOC2742)... 

Furan-3-carboxamide, N,fV-dimethyl-AG, 4, 34 <76JOC3591, 77TI337) Furan-2-carboxylate, 2-nitro-, ethyl ester UV, 4, 588 <50JA753>... 

Furan-2-carboxylic acid, 2,5-dihydro-, ethyl ester H NMR, 4, 573 <75JA5I60)... 

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