Furan-carbaldehydes, carbonyl reactivity

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of iV-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive... 

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of Af-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive to hydroxide addition at the carbonyl carbon than A-methylpyrrole-2-carbaldehyde (76JOC1952). Pyrrole-2-aldehydes fail to undergo Cannizzaro and benzoin reactions, which is attributed to mesomerism involving the ring nitrogen (see 366). They yield 2-hydroxymethylpyrroles (by NaBH4 reduction) and 2-methylpyrroles (Wolff-Kishner reduction). The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

All derivatives of furan-2-carbaldehyde are reactive substances. They are easily oxidised by oxygen to the corresponding acids, which, together with the appropriate alcohols, are also formed from furan-2-carbaldehyde derivatives by Cannizzaro reaction. Derivatives of furan-2-carbaldehyde condense together or with other carbonyl compounds and enter into non-enzymatic browning reactions. Oxidation of 5-hydroxymethylfuran-2-carbaldehyde, for example, yields 5-hydroxymethylfuran-2-carboxylic acid (4-198). Condensation... 

The condensation of furo[3,2- ]pyrrole-type aldehydes 8g and 265-267 with hippuric acid was carried out in dry acetic anhydride catalyzed by potassium acetate as is shown in Scheme 26. The product methyl and ethyl 2-[( )-(5-oxo-2-phenyl-l,3-oxazol-5(4//)-ylidene)methyl]furo[3,2- ]pyrrol-5-carboxylates 268a-d were obtained. The course of the reaction was compared with the reaction of 5-arylated furan-2-carbaldehydes with hippuric acid. It was found that the carbonyl group attached at G-2 of the fused system 8 is less reactive than the carbonyl group in 5-arylated furan-2-carbaldehydes in this reaction <2004MOL11>. The configuration of the carbon-carbon double bond was determined using two-dimensional (2-D) NMR spectroscopic measurements and confirmed the (E) configuration of the products. 

Benzofuran-3(2/f)-ones (396) exist in the keto form but undergo ready enolization. Acetylation with acetic anhydride and sodium acetate affords 3-acetoxybenzo[6]furans, but reaction under acidic conditions usually supplies these products admixed with 3-acetoxy-2-acetylbenzo[6]furans. Alkylation usually furnishes a mixture of O- and C-alkylated products. 3-Acetoxy-6-methoxy-4-methylbenzo[6]furan, on Vilsmeier reaction, supplies the 3-chlorobenzo[6]furan-2-carbaldehyde, the product expected from an enolizable ketone (72AJC545). Benzofuran-3(2//)-ones react normally with carbonyl reagents. Grignard reagents react in the expected way and dehydration of the intermediate affords a 3-substituted benzo[6]furan. The methylene group is reactive so that self condensation, condensation with aldehydes and ketones and reaction with Michael acceptors all occur readily. 

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