Furfural aldol reaction

Examples include acetaldehyde, CH CHO paraldehyde, (CH CHO) glyoxal, OCH—CHO and furfural. The reaction is usually kept on the acid side to minimize aldol formation. Furfural resins, however, are prepared with alkaline catalysts because furfural self-condenses under acid conditions to form a gel. 

Mixed or crossed aldol condensation Aldol condensations between different carbonyl reactants are called crossed (or mixed) reactions. Crossed aldol condensation works well if one carbonyl compound has no a-hydrogen(s). For example, acetone reacts with furfural in a crossed-aldol reaction to give the corresponding a,P-unsaturated ketone 3.15. 

The trivial name of the reaction was applied by Wurtz in 1872, and stems from the trivial name of the dimer resulting from the acid-catalyzed self-reaction of acetaldehyde (equation 1). In time, the term came to be applied to the analogous self-condensation reactions of ketones, the first known example of which was the acid-mediated dimerization of acetone, discovered in 1838. The first use of a base as a catalyst for the aldol reaction was in the reaction of furfural with acetaldehyde or acetone (equation 2). This example also illustrates the first example of a mixed aldol reaction, a process that came to be known as the Claisen-Schmidt condensation. ... 

Draw the product of the crossed aldol reaction between furfural and cyclohexanone and the product formed by its base-catalyzed dehydration. 

Left) Xylulose and furfural from isomerization and dehydration reaction together with by-products derived from the condensation and the resinification reactions. (Right) Low-molecular-mass organic acids, giycolaidehyde or pyruvaldehyde, from different retro-aldol reactions [148]. 

Hayashi and co-workers applied the asymmetric direct aldol reaction for the synthesis of chiral building block 75 for (+)-cytotrienin A 76," which is a microbial antitumor secondary metabolite. The diol 75 was synthesized with high enantioselectivity by means of the organo-catalytic aldol reaction of furfural 55 and propanal 39 in the presence of a catalytic amount of 4-acyloxy proline 74 (Scheme 27.13). The original procedure, i.e., with proline as a catalyst, was not effective for large-scale synthesis of 75 because of low yield and low diastereoselec-tivity. Total synthesis of (+)-cytotrienin A 76 was achieved in another 32 steps from diol 75. 

In addition, heteroaromatic aldehyde such as furfural can also serve as a substrate in this reaction, giving the corresponding aldol in a moderate yield (Table 20, entry 9). Conjugated aldehydes were also good substrates (Table 20, entry 10). Aliphatic aldehydes lead to a poor yield of the aldol due to incomplete conversion (Table 20, entries 11 and 12). 

Contrary to some reports, electrophilic addition reactions may occur in other multiple-bond systems. In many of the reactions of aldehydes and ketones the first stage involves the addition of some entity across the carbon-oxygen bond, e.g., the formation of oximes, semicarbazones, hydrazones, hydrates (1,1-diols) and their ethers, and the aldol condensation. Most of these reactions entail a subsequent loss (elimination) of a small molecule e.g. water, ammonia, ethanol) and, while one must be careful to determine whether the rate-determining stage involves attack on the carbonyl compound or elimination from the adduct , there are some systems in which it is evident that electrophilic attack is involved in the slow stage of the reaction sequence. Examples of such reactions are the acid-catalysed formation of oximes of aliphatic - and aromatic carbonyl compounds, of furfural semi-carbazone , and of 1,1-diols from aldehydes or ketones . 

Aldehydes that lack an a-hydrogen and therefore cannot undergo an aldol condensation undergo the Cannizzaro reaction in the presence of a strong base, giving the alcohol and the corresponding carboxylic acid. Furfural in the presence of sodium hydroxide yields 72-76% furfuryl alcohol and 73-76% furoic acid upon acidifying [7]. 

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