Aldol reaction definition

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

Structures 1 and 1, with identical relative configuration at C-2. C-3, are not covered by Masamune s very scant directions which do not include a guideline on the definition of the backbone. Thus, an allylmetal expert , having prepared 1 as one member of a series with the substituent of the ally] group varied, will probably describe 1 as syn, but investigators working on aldol reactions or conjugate additions will certainly prefer the term anti for 1. ... 

Choudary et al. have used diamine-functionalized MCM-41 for the aldol reaction of a variety of aliphatic aldehydes with acetone. [47]. Conversions were good, although the ratio of aldol product to dehydrated product was quite variable, depending on the nature of the aldehyde. There does not seem to be any definite pattern in the selectivity achieved as a function of substitution of the aldehyde (Figure 8). 

No definitive conclusions can be drawn as to the course and mechanism of the reactions, but our present knowledge points to the occurrence of an aldol reaction of the aldehyde form of the monosaccharide with the activated methylene group of the /3-dicarbonyl compound. This is probably the first step in the reaction. The products of the aldol reaction can be isolated by protecting the hydroxyl group or amino group next to the reducing function of the monosaccharide. 

This dual enamine/iminium activation profile in cascade Michael/aldol reactions can also be found even in some early reports, mostly focused on the self-dimerization of enals catalyzed by proline or analogues derived thereof, which generally proceeded with low enantioselect vities. There is not a clear and definitive mechanistic pathway confirmed for these reactions, although the most widely accepted proposal for the dimerization of enals (Scheme 7.4) ° involved sequential activation of one molecule of the substrate as a dienamine (Michael donor) and another molecule as iminium ion (Michael acceptor). 

Shibasaki et al. first reported a chiral barium catalyst prepared from BINOL monomethyl ether and barium alkoxide, which was a good catalyst for asymmetric direct-type aldol reactions (Table 1) [21]. In the presence of a barium catalyst, several aliphatic aldehydes were tested, and the desired cross aldol products were obtained in good yields with moderate to good enantioselectivities. For the substrate BnOC(CH3)2CHO, the best enantioselectivity was observed (Table 1, entry 6). Although there is no definitive experimented proof, the barium catalyst was assumed to be monometallic and could be stored for several months under argon atmosphere. 

Various subsidiary catalysts have been found of value in pentaerytlu itol preparation. Kuzin reports that small proportions of sugars such as glucose or fmctose act as catalysts for the aldol reaction and have a definite accelerating effect on product fomiation. - s previously pointed out, Wyler claims that catalysts such as copper oxide, copper, and platinum accelerate and inipi o e the efficiency of pentaerythrose reduction. The use of quaternary ammonium compounds and amides as catalystft or subsidiary catalysts ha-5 also been patented bj - this investigator. 

An interesting pericyclic-anionic-pericyclic domino reaction showing a high stereoselectivity is the cycloaddition-aldol-retro-ene process depicted in scheme 20.1581 The procedure presumably starts with a [4+2]-cycloaddition of diene 98 and S02 in presence of a Lewis acid. After opening of the formed adduct reaction with (Z)-silyl vinyl ether 99 leads to a mixture of alk-2-enesulfinic acids 101. It follows a retro-ene reaction which affords a 7 3 mixture of the products 102 and 103. The reaction described by Vogel et al is a nice example for the efficient generation of polypropionate chains with the stereoselective formation of three stereogenic centers and one (0-double bond in a three-component domino reaction in its strict definition. 

An accessory proposal was Arthur Michael s hypothesis that many reactions proceed by addition, for example, a polymerization of acetaldehyde (CH3CH = O) in the presence of bases (OH) to an aldol (CH3CHOHCH2CHO), with subsequent loss of water to form crotonaldehyde (CH3CH = CHCHO). Michael, educated in America, Germany, and France, made use of Kekule s idea that two molecules may form a "polymolecule" or molecular compound, which, in turn breaks up to yield the final products.33 Lachman expressed fairly standard misgivings about this proposal of an intermediary and transition form "If we are going to explain reactions by means of addition products which we do not or cannot isolate, our explanation loses its definiteness. It becomes simply a possible explanation, and its conclusions are by no means binding."34... 

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). 

B Uoster 7 has pointed out the parallelism of the aldol and Darkens reactions -with respect to electronic effects. It is conceivable that steric effects can also play an important role in the reactivity of the carbonyl component, but there is insufficient evidence in the literature at present to warrant any definitive conclusion on this subject.183 ... 

No definitive evidence has appeared that identifies the source of the color generated during thermooxidation (95). However, two laboratories have postulated that the reactions leading to the formation of the color chromophores are aldol-type reactions, either via the reaction of aldehydes diiecdy (96)... 

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