Aldol condensation definition

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). 

Few definitive studies have been reported for the reduced catalyst. One of the earliest studies which was made attempted to relate activity for aldol condensation of n-butylaldehyde to Mo5+ concentration on reduced Mo/Al catalysts (39). The catalysts contained varying amounts of Mo and were prereduced under identical conditions. Aldol conversions were approximately linear with Mo5+ signals (by ESR) up to about 10% Mo. Morris et al. (101) also report a correlation between Mo5+ and catalytic activity for ethylene polymerization over reduced Mo/Al catalysts. Unfortunately, in both studies, tests were not done on a catalyst having the same Mo content but prereduced under different conditions (to give different Mo5+ values). Therefore, the correlation obtained was not unequivocal since other parameters besides Mo5+ might also be proportional to Mo level. 

It was actually in the course of a synthesis of Mori that the true absolute configuration of ( + )-lineatin (296) was definitely confirmed. This started with a [2 + 2] cycloaddition between isoprene and dichloroketene, giving a mixture of two isomeric dichlorocyclobutanones 333 and 334 (3.3 1). After reduction with zinc and acetic acid, the major cyclobutanone 335 was separated. Aldol condensation of the anion of 335 with acetone enabled the isopropyl side chain to be introduced. The later stages of the synthesis of ( )-lineatin are shown in Scheme 27, and the overall yield was 3.8%. In the course of diis synthesis, however, the alcohol 336 was resolved using the chiral hydroxy lactone 252, available fix>m commercial (lR,3/f)-c s-chrysanthemic acid. The ether 337 was shown by X-ray crystallography to have the absolute configuration demonstrated. Since it was related to natural (+)-lineatin, the absolute configuration of the latter was established. ... 

But you should be familiar with the proper terminology. When we go all the way to the a.fi-unsaturated ketone, we call the reaction an aldol condensation. By definition, a condensation is any reaction where two molecules come together, and in the process, a small molecule is liberated. The small molecule can be N2 or CO2 or H2O, etc. In this case, we have two molecules of ketone coming together, and in the process, a molecule of water is liberated ... 

Unfortunately, there is more than one definition for threo- and erythro. Winstein and co-workers gave the following definition of these diastereomers In a compound with two asymmetric carbons that has two common ligands and a third that differs, the isomers that would be meso if the third ligand were identical are erythro diastereomers.An alternative definition is If two asymmetric carbons have only one ligand in common, then the other four ligands are paired in the same commonsense way and isomers that would have equal pairs eclipsed in any conformation are erythro. Eliel, Mislow and their co-workers defined erythro and threo in terms of Fischer projections. The aldol products (see sec. 9.4.A) 68 and 69, and the ester-aldehyde condensation products 70 and 71.30 shown with the erythro and threo notation. 

Among alkali metal enolates, those derived from ketones are the most robust one they are stable in etheric solutions at 0 C. The formation of aldehyde enolates by deprotonation is difficult because of the very fast occurring aldol addition. Whereas LDA has been reported to be definitely unsuitable for the generation preformed aldehyde enolates [15], potassium amide in Hquid ammonia, potassium hydride in THE, and super active lithium hydride seem to be appropriate bases forthe metallation of aldehydes [16]. In general, preformed alkali metal enolates of aldehydes did not find wide application in stereoselective synthesis. Ester enolates are very frequently used, although they are more capricious than ketone enolates. They have to be formed fast and quantitatively, because otherwise a Claisen condensation readily occurs between enolate and ester. A complication with ester enolates originates from their inherent tendency to form ketene under elimination... 

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