Furane-carboxaldehyde

An alternative route to 5-nitro-2-furancarboxaldehyde requires nitration of 2-furancarboxaldehyde oxime [1121 -47-7] with mixed acid to give the nitrated oxime [555-15-7] and concomitant hydrolysis (22). Furthermore, 2-furan-carboxaldehyde derivatives with the R-substituent in place have been nitrated to the desired product (23). 

Nitrovin. 2- 3-(5-Nitro-2-furanyl)-l-[2-(5-nitro-2-furanyl)ethenyl]-2-propenyhdene hydrazinecarboximidamide hydrochloride has been marketed for both human and veterinary use as an antibacterial agent. The product, which has also seen use as a veterinary food additive (32), is prepared from 5-nitro-2-furan-carboxaldehyde and acetone followed by treatment of the resulting dione with aininoguariidine (33). 

Problem 31.5 Predict the products from the treatment of furfural (2-furan-carboxaldehyde) with concentrated aqueous NaOH. 

FIGURE 10.12 Pyrogram of oak wood pyrolyzed at 600°C. Peaks 1 = acetic acid, 2 = furan carboxaldehyde, 3 = 2-methoxy -methyl phenol, 4 = 2,6-dimethoxy phenol, 5 = 2,6-dimethoxy-4-propenyl phenol. 

A versatile and regioselective synthesis of benzo[b]furans, naphthalenes, indoles and benzothiophenes was achieved by reaction of o-alkynylarene and heteroarene carboxaldehyde derivatives in the presence of iodonium ions. The reaction mechanism was also discussed <06CEJ5790>. 

El Haji, T. Masroua, A. Martin, J.-C. Descotes, G. Synthesis of 5-(hydroxymethyl)-furan-2-carboxaldehyde and its derivatives by acid treatment of sugars on ion-exchange resins, Bull Soc. Chim. Fr., 1987, 855-860. 

The preparation of 5-arylfurfurals (and arylthiophene-2-carboxaldehydes) via Pd-catalyzed C-C bond formation in aqueous media was carried out <990L965>. Full details have been reported for the Pd-catalyzed cycloisomerization of (Z)-2-en-4-yn-l-ols (a facile synthesis of a variety of substituted furans) <99JOC7687>... 

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... 

Dielectric absorption on furan-2-carboxaldehyde has been measured (78JCS(F2)727 81ZPC147). Even in a polymer matrix, the energy barrier obtained for internal rotation is close to that determined with other experimental techniques, suggesting a low influence of the surrounding medium on the torsional process. 

As examples of the conformational behavior of these derivatives, a number of selected experimental results have been gathered in Table I Those referring to furan-2-carboxaldehyde show the wide range of conformational composition found in different solvents. 

Thus, aside from furan-2-carboxaldehyde, where the equilibrium between conformers is rather sensitive to external effects, the 2-formyl derivatives of five-membered heterocycles are rather rigid from a conformational point of view. 

The situation appears decisively different in 3-formyl derivatives. Theoretical calculations (77JCS(P2)1601 79NJC473) reveal a small energy difference between X,0-cis and X,0-trans conformations in furan-3-carboxaldehyde (4, X = O), and pyrrole-3-carboxaldehyde (4, X = NH), and the X,0-cis form is... 

The energetics relative to the conformational problem of pyridine-2-carboxaldehyde thus appear qualitatively similar to that of furan-2-carbox-aldehyde the less polar conformer is more stable as an isolated molecule and polar media are likely to reverse the relative stability. Nevertheless, while the energy difference between cis and trans forms of 2-formyl derivative of furan becomes nearly zero in solvents of relatively low polarity (72T3015), the same seems to occur for pyridine derivative in more polar solvents. 

In mobile equilibria, the increase in the dielectric constant of the solution should enhance the population of the conformer with higher polarity (SOM 18), as is also proved by theoretical calculations. The solvation energy may lead to inversion in isomer stability on going from the vapor phase to a solution of a certain polarity, as occurs for furan-2-carboxaldehyde (72T3015). Inversion in conformer stability may also occur on going from the vapor to the liquid or solid state. 

Pyrroles are found in the volatiles of most heated foods [29], although they have received less attention than some other classes of aroma volatiles. Some pyrroles may contribute desirable aromas, e.g. 2-acetylpyrrole has a caramel-like aroma, and pyrrole-2-carboxaldehyde is sweet and corn-like, but alkylpyrroles and ac-ylpyrroles have been reported to have unfavourable odours [22]. Many more volatile pyrroles have been found in coffee than in other foods [30], and they are common products of amino acid-sugar model systems. Pyrroles are closely related in structure to the furans, and they are probably formed in a related manner from the reaction of a 3-deoxyketose with ammonia or an amino compound followed by dehydration and ring closure (cf Scheme 12.2). 

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