Furan-3-carbaldehyde, reaction with

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. 

Structural changes affect seriously an electrophilic substitution orientation in pyrroles owing to their low positional selectivity in reactions with electrophiles. Thus, in contrast to thiophene, selenophene, and, especially, furan analogues, even a relatively weak type 11 substituent in position 2 of the pyrrole ring is capable of overcoming the a-oiienting effect of the heteroatom and directs an electrophile preferably to the position 4 (68JCS(B)392). N-(p-Nitrophenyl)pyrrole-2-carbaldehyde... 

Problem 9.1. Predict the product(s) of the treatment of furan-2-carbaldehyde (furfural) with base under the conditions of the Cannizzaro reaction. 

A-Vinylic phosphazenes, are useful building blocks, that have been used in Aza-Wittig reactions with unsaturated aldehydes to form 3-azatrienes through a [2 -I- 21-cycloaddition-cycloreversion sequence. " The presence of an alkyl substituent in position 3 of A-vinylic phosphazenes increases the steric interactions, and [4 + 2] periselectivity (1,4 addition) is observed." Other Aza-Wittig reactions include the reaction of iminophosphorane (60) with aromatic isocyanates to obtain, inter alia, useful carbodiimides for the selective synthesis of pyrimidones." Also the iminophosphorane (61) was reacted with furan-2-carbaldehyde, thiophene-2-carbaldehyde, furan-3-carbaldehyde, and thiophene-3-carbaldehyde to give, depending on temperature and aldehyde, trans imines or mixtures of trans and cis imines." " The... 

MaiUard reaction. In neutral solutions, for example, thiamine reacts with reducing sugars and other carbonyl compounds (Figure 5.14). Its reaction with glucose yields glucothiamine as a primary product. Reaction with furan-2-carbaldehyde, which is one of the main degradation products of ascorbic acid in acidic solutions, explains the decomposition of thiamine in the presence of vitamin C. 

Aldehydes and ketones react with ammonia and primary amines to form imines. Reaction with aldehydes yields aldimines, and ketimines arise in reactions with ketones (Figure 8.28). Imines derived from aliphatic carbonyl compounds are generally unstable and are transformed to more stable products, such as amines, diamines and others. Secondary amines react with aldehydes and ketones with the formation of enamines (Figure 8.29). Imines are similarly flavour-active compounds derived from furan-2-carbaldehyde. For example, the aromas of Ar-furfuryl(isobutylidene)amine, N-(furfuryl)isopentyhdeneamine (8-155) and N-(furfurylidene)isobutylamine (8-156) reportedly resemble chocolate. 

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... 

Liquid-phase photolysis of furan atroom temperature occurred in very low yields (1 % conversion), giving a mixture of Diels-Alder adducts deriving from the reaction of cyclopropene-3-carbaldehyde and formylallene with furan (85JOC3034). 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

Vilsmeier reaction of 2-oxindole (830) afforded 2-chloroindole-3-carbaldehyde (891). Suzuki cross-coupling of 891 with furan-3-boronic acid (1124), followed by protection of the indole nitrogen with benzyloxymethyl (BOM) chloride, led to... 

The reactions of substituted furo[3,2- ]pyrrole-5-carbohydrazides with 5-arylfuran-2-carbaldehydes, 4,5-disubsti-tuted furan-2-carbaldehydes, and thiophene-2-carbaldehyde have been studied <2005CEC622>. The advantage of microwave (MW) irradiation on some of these reactions was reflected in the reduced reaction time and increased yields (Table 8). The series of substituted hydrazones 241-246 was obtained from these... 

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. 

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. 

Side chain furan functionalization has been accomplished by acetalization (Section 1.11.4.4.3) of poly(vinyl alcohol) (180) with furan-3-carbaldehyde (181 Scheme 87) (72MI11106). The adduct (182) was utilized as the diene reactant in a Diels-Alder reaction to bind vinyl-functional siloxane oligomers to the polymer. 

The reaction of furans with ammonia and its derivatives is of considerable synthetic utility (B-73MI31 too). Substituted furan-2-carbaldehydes and 2-acylfurans on heating with ammonia and ammonium salts, often under pressure, yield 3-hydroxypyridines. The mechanism of this reaction is thought to involve nucleophilic attack of ammonia at the 2-position. Ring opening affords an amino aldehyde or ketone and thence, by reclosure, the 3-hydroxy-pyridine (Scheme 29). A wide range of substitutents is tolerated. Primary amines with furan-2-carbaldehydes yield A-substituted pyrroles, the closure of the intermediate... 

To conclude this section two compound specific reactions are worthy of mention. Firstly, the interaction of atomic carbon with benzene has been found to provide 1 albeit in 11% yield and as one of eleven hydrocarbon products46. Secondly, 4,5,7-tri-t-butylisobenzofuran has been reported to undergo furan-cyclopropenylaldehyde photorearrangement rather than An electrocyclization to the Dewar furan47. Steric factors undoubtedly dominate in yielding 2,3,5-tri-/-butylcyclopropabenzene-l-carbaldehyde the product was obtained in solution and characterized by H NMR only. 

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