Furancarboxylic acids reactions

The dianions derived from furan- and thiophene-carboxylic acids by deprotonation with LDA have been reacted with various electrophiles (Scheme 64). The oxygen dianions reacted efficiently with aldehydes and ketones but not so efficiently with alkyl halides or epoxides. The sulfur dianions reacted with allyl bromide, a reaction which failed in the case of the dianions derived from furancarboxylic acids, and are therefore judged to be the softer nucleophiles (81JCS(Pl)1125,80TL505l). 

The reaction of benzo-2-furancarboxylic acid with thiosemicarbazide under microwave conditions enables the use of an equivalent amount of phosphorus oxychloride and a short reaction time <2003SC2891 >. Moreover, solvent-free conditions are achieved when acidic alumina is used as dehydrating agent under microwave heating in the reaction of alkyl carboxylic acids with thiosemicarbazide <2000SC3031>. 

Although the Heck reactions of heteroaryl halides are now commonplace [76], few examples are found using organohalide substrates possessing a carboxylic acid moiety [77]. However, 4,5-dibromo-2-furancarboxylic acid (86) underwent a Heck reaction with ethyl acrylate to afford diacrylate 87 [78]. 

Furan carboxylic acids are usually prepared by ring synthesis using the Feist-Benary and Paal-Knorr methods (Section 3.12.2.2). However, furancarboxylic acids can also be prepared by reaction of lithiofurans with carbon dioxide. A convenient source of furan-3-carboxylic acid (517) is the commercially available diethyl furan-3,4-dicarboxylate (518) (71S545). 

The top of the upright tube in the reaction flask is closed by a cork stopper holding a glass plunger which may be used for pushing back into the flask any sublimed 2-furancarboxylic acid. 

Bromopyrones 260, which are available from the bromination of pyrones 259, on treatment with base yield furancarboxylic acids 261 (Scheme 140) <1980JOC1524>. Related reactions include preparations of furans from pyrylium cations (Section 3.2.1.6.3.5). 

The reaction of furancarboxylic acids with sulfur tetrafluoride leads to the corresponding trifluoromethyl-substituted furans in good yields (see examples in Table 6). A number of furancarboxylic acids, mainly furan-2-carboxylic acids, and furan-2,5- and -3,4-dicarboxylic acids, show a different behavior. Here highly fluorinated dihydrofurans, e g. 25, are formed. This class of compounds is also obtained with the reagent system sulfur tetrafluoride/hydrogen fluoridc/chlorine or sulfur tetrafluoride/hydrogen fluoride/sulfuryl chloride. In some cases chlorine-containing products are found. 

This dismutation occurs to a small extent with most aldehydes in the presence of a strong base. It is the primary reaction only with aldehydes that lack an a-hydrogen atom and, therefore, cannot undergo a Idol condensation. The reaction has been reviewed (cf. method 81). It is used in the preparation of 2-furancarboxylic acid (63%), nitrobenzoic... 

SYNTHESIS OF FURANS VIA RHOOIUM(II) ACETATE-CATALYZED REACTION OF ACETYLENES WITH a-DIAZOCARBONYLS ETHYL 2-METHYL-5-PHENYL-3-FURANCAR BOXY LATE (3-Furancarboxylic acid, 2-methyl-5-phenyl-, ethyl ester)... 

A series of anti-inflammatory hydroxy butenolides was synthesized starting from 55. Thus heating 55 with ethyl phenyl-prop-1-ynoate 141 for 16 h at 210°C afforded 3-phenyl-4-furancarboxylic acid ethyl ester 142 (Fig. 3.42). Functionalization of the ester followed by reaction of the furan ring with singlet oxygen gave the biologically active butenolides, 143. 

For example, Cannizzaro reaction of formaldehyde gives methanol and formic acid. Methanol reacts with another molecule of formaldehyde (its hydrate) to form acetal 1,1-dimethoxymethane, also known as 2,4-dioxapentane. The reaction of methanol with formaldehyde hydrate oligomers yields, by analogy, higher dioxaalkanes. The Cannizzaro reaction of ben-zaldehyde also proceeds easily (especially in the presence of heavy metal traces), yielding benzyl alcohol and benzoic add. Benzoic acid is otherwise also formed by autoxidation of benzaldehyde. The Cannizzaro reaction of furan-2-carbaldehyde yields furfuryl alcohol and 2-furancarboxylic acid. 

Carboxylic esters, usually methyl or ethyl esters, can be converted into trifluoromethyl compounds by reaction with sulfur tetrafluoride in the presence of an excess of hydrogen fluoride. In comparison to carboxylic acids more vigorous conditions are needed." Substitution of the ester group carbonyl oxygen is observed with fluoroalkyP or phenyl esters " of polyfluorocarboxylic acids, fluorine-containing a-oxo esters,fluorine-containing a- and j -hydroxy esters, " and furancarboxylic esters. ... 

The customary process for making furoic acid starts with a Cannizzaro reaction between furfural and aqueous sodium hydroxide [74] to yield furfuryl alcohol and sodium 2-furancarboxylate ... 

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