Condensation between an aldehyde

The cyano-substituted five-ring oligomers were formed in good yields in a Knoe-venagel condensation between an aldehyde and a cyanomethyl-substituted compound, according to a procedure previously reported by Greenham el at. [6. The Knoevenagel condensation needs careful control of reaction conditions in order to... 

Comins et al. <20010L469> achieved the synthesis of the same allopumiliotoxin 267A 215 by a stereoselective alkynylation of the appropriately substituted 4-MeO-pyridine 226. The C-6 side chain is introduced by an aldol condensation between an aldehyde and indolizidinone 227 (Equation 17). 

The condensation between an aldehyde, an amine and an active methylene compound, named after Carl Mannich, was first published in 1912 [4]. The products of the reaction, a-amino ketones or Mannich bases are important compounds with numerous applications in the synthesis of pharmaceuticals and of natural products [7]. 

The aza-analogue of the SMS condensation has been reported [35, 45-47]. Veenstra and Schmid in 1997 [46] were the first to perform the three-component condensation between an aldehyde 6, an amine 92 and an allylsilane 1. A stoichiometric amount of Et20-BF3 was generally used and the yields were good (Scheme 13.37). 

The reaction is based upon the two components condensation between an aldehyde or ketone 6 (or their synthetic equivalents) and alcohol 95, which contains an allylsilane (or vinylsilane) moiety. The IMSC reaction is mediated by Lewis or Bronsted acids, which activate the carbonyl group of 6 towards nucleophilic attack. After addition of alcohol 95 on the activated carbonyl, the oxonium cation 96 is formed, which is intramolecularly captured by the pendant allylsilane function, leading to oxygen-containing rings 97 (Scheme 13.38). This process typically requires a stoichiometric (or more) amount of Lewis acid. 

Pd-containing aluminophosphate molecular sieves have been used to carry out crossed aldol condensations between an aldehyde and a ketone by using a 0.5 % Pd/ MnAPSO-31 catalyst in a vapour-phase fixed bed reactor.[14] Thanks to the excess of the ketone with respect to the aldehyde (4 1), it is possible to get high selectivity to the desired product, i.e. 70 % of heptan-2-one from n-butyraldehyde and acetone and 89 % of pentan-2-one from acetaldehyde and acetone, the major by-product being, in both cases, MIBK from acetone self-condensation. 

Because these reactions involve the direct acid-mediated condensation between an aldehyde and a pyrrolic precursor, they bear direct analogy to the Rothemund-Adler-type procedures used to prepare tetraarylporphyrins, as well as to the... 

Directed aldol condensation. Aldol condensation between an aldehyde and a ketone usually is not successful because self-addition of the aldehyde is the preferred reaction. Wittig and Reiff,1 however, showed that, if the aldehyde is first converted into a Schiff base (cyclohexylamine was used) and then metalated with lithium di-isopropylamide (chosen for obvious stcric reasons), aldol condensation can be achieved, usually in good yield.2 In the case of ketones, this route is superior to olefination via a phosphorylide. 

Analysis of enone (77) reveals a condensation between an aldehyde and the enol of a less reactive ketone, easily achieved by first making the enamine of the ketone—the cyclic secondary amine morpholine (78) is often used. 

A mixed aldol condensation between an aldehyde and a ketone can theoretically lead to four products. In practice, however, it proceeds in only one direction which is determined by the greater reactivity of the aldehyde than of the ketone as methylene component. For methyl ketones such as ethyl... 

Aldol-type reactions. The condensation between an aldehyde and a ketene silyl acetal proceeds in CH2CI2 with LiC104 as catalyst at —30°C. iy/i-Selectivity is observed when o-oxyaldehydes are used as reaction partners. 

In one synthesis of morachalcone A, an aldol condensation between an aldehyde and a ketone was used to prepare the... 

The Knoevenagel reaction is a cross-aldol condensation between an aldehyde and an active methylene compound, which produces water as sole side product (Figure 10.7). This reaction is usually catalyzed by bases in solution or by some sohds such as metal oxides [89]. 

The Strecker [1] and Bucherer-Bergs [2] reactions are the most important strategies and powerful tools for the synthesis of a-amino acids from a carbonyl compound and cyanide. The classical first approach uses ammonium hydroxide and potassium cyanide to obtain the corresponding aminonitrile compound 1, whereas the second one is a four-component condensation between an aldehyde or ketone derivative reacting with potassium cyanide and ammonium carbonate as source of ammonia and carbon dioxide affording hydan-toin componnds 2 [3], In both cases, final hydrolysis would deliver the corresponding o,a-disubstituted a-amino acid derivatives 3 (Scheme 10.1) [4],... 

The aldol or aldol-type reaction is well recognized as one of the most important carbon-carbon bond forming reactions in organic synthesis. As shown in Scheme 8.1, two stereogenic centers could be generated in this aldol reaction. The classical aldol condensation between an aldehyde and a ketone is often catalyzed by a base or an acid. Another approach is the acid-catalyzed cross-aldol reaction of silyl enol ethers with carbonyl compounds, the so-called Mukaiyama reaction. 

The intramolecular benzoin condensation between an aldehyde and ketone was virtually unknown until 2003, when Suzuki and co-workers established that thiazolylidine catalysts could effect this transformation in good to excellent yield for the synthesis of functionalized preanthraquinones such as 30. Subsequent to this work, reports of annulation reactions of a variety of ketoaldehyde substrates, as well as asymmetric variants, have appeared in the literature. ... 

The synthesis of thiazolo[5,4-d]pyrimidines 194 can be achieved from different 5-thiazolidinones, 2-butyl-lH-imidazole-5-carbaldehyde, and thiourea using microwave irradition within 5 min in the presence of HCl. These reactions study the Biginelli condensation reaction, in which the condensation between an aldehyde and urea has some similarities to the Mannich condensation. The generated iminium intermediate 193 (Scheme 85) acts as an electrophile for... 

Diketones are produced by a crossed aldol condensation between an aldehyde or ketone enolate and an ester. 

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