Furfural, aldol condensation

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. 

Based on these results and the disadvantageously high volatility of hexane, Huber et al. decided to develop another approach utilizing 5-hydroxymethylfurfural (5-HMF) or furfural, compounds that can be obtained from the dehydration of glucose or xylose, respectively [54-57]. The production of Cg-Ci5 alkenes from these furans is achievable by sequential aldol condensation with acetone followed by hydrogenation... 

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. 

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 . 

The first total synthesis of the intricate Stemona alkaloid (+ /—)-isostemofoline (224) was reported by Kende and coworkers 81) starting from 1,2-hexanediol (225) which was straightforwardly converted to 227 (Scheme 22) 82). Reductive cycUzation with sodium hydrosulfite in refluxing aqueous ethanol, and protection of the unstable pyrrole as tert-butyl carbamate, afforded 228 in five steps with 12% overall )deld. The key bicyclic ketone 231 was assembled by [4 + 3] cycloaddition of pyrrole 228 and diazoester 229 promoted by rhodium octanoate dimer, followed by enol silane deprotection, exo-specific hydrogenation, and nucleophilic decarboxylation (47% overall yield). Sodium methoxide-catalyzed aldol condensation of ketone 231 and furfural provided the Q-j/i-unsaturated ketone 232 whose olefin configuration was established by nOe studies. Allylation of 232 provided a 2.4 1 mixture of ketone 234 and the corresponding allylic enol ether 233, which could be converted to the former via a stereoselective Claisen rearrangement. 

The formation scheme of furfural is speculated as shown in Figure 3. An aldehyde group of furfural reacted with an amino group of lysine to form an imine. The double bond was migrated to form Amadori type compound, which was hydrolyzed to form alpha-keto carboxylic acid. The formed keto carboxylic acid reacted with an intra-molecular amino group to form tetrahydro-pyridine carboxylic acid or reacted with another furfural by aldol condensation, from which furpipate was formed by dehydration of 2 molecules of water. Another possible scheme is that lysine was converted to tetrahydropyridine carboxylic acid, which reacted with furfural to form furpipate. 

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]. 

Similar to 5-HMF, furfural can also undergo aldol-condensation with external carbonyl-containing molecules using base or acid catalyst. Further hydrogenation of aldol products can produce high-quality longer-chain alkanes. 

Faba, L., Diaz, E., Ordonez, S., 2012. Aqueous-phase furfural-acetone aldol condensation over basic mixed oxides. Applied Catalysis B Environmental 2012 (113), 201—211. 

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. ... 

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]. 

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