The<unk>  is an important carbon-carbon bond formation reaction in organic chemistry. In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β-hydroxy ketone, or<unk> " (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals. Sometimes, the<unk>  addition product loses a molecule of water during the reaction to form an α,β-unsaturated ketone. This is called an<unk>  condensation. The<unk>  was discovered independently by Charles-Adolphe Wurtz and by Alexander Porfyrevich Borodin in 1872. Borodin observed the<unk>  dimerization of 3-hydroxybutanal from acetaldehyde under acidic conditions. The<unk>  is used widely in the large scale production of commodity chemicals such as pentaerythritol
and in the pharmaceutical industry for the synthesis of optically pure drugs. For example, Pfizer's initial route to the heart disease drug Lipitor (INN: atorvastatin), approved in 1996, employed two<unk> s, allowing access to multigram-scale quantities of the drug.
The<unk>  structural motif is especially common in polyketides, a class of natural products from which many pharmaceuticals are derived, including the potent immunosuppressant FK506, the tetracycline antibiotics, and the antifungal agent amphotericin B. Extensive research on the<unk>  has produced highly efficient methods which enable the otherwise challenging synthesis of many polyketides in the laboratory.
This is important because many polyketides, along with other biologically active molecules, occur naturally in quantities impractically small for further investigation. The synthesis of many such compounds, once considered nearly impossible, can now be performed routinely on the laboratory scale, and is approaching economic viability on a larger scale in some cases, such as the highly active anti-tumor agent discodermolide. In biochemistry, the<unk>  is one of the key steps of glycolysis, where it is catalyzed by enzymes called<unk> ases.
The<unk>  is particularly valuable in organic synthesis because it produces products with two new stereogenic centers (on the α- and β-carbon of the<unk>  adduct, marked with asterisks in the scheme above). Modern methods, described below, now allow the relative and absolute configuration of these centers to be controlled. This is of particular importance when synthesizing pharmaceuticals, since molecules with the same structural connectivity but different stereochemistry often have vastly different chemical and biological properties.
A variety of nucleophiles may be employed in the<unk> , including the enols, enolates, and enol ethers of ketones, aldehydes, and many other carbonyl compounds. The electrophilic partner is usually an aldehyde, although many variations, such as the Mannich reaction, exist. When the nucleophile and electrophile are different (the usual case), the reaction is called a crossed<unk>  (as opposed to dimers formed in an<unk>  dimerization).
Aldol reaction
Chichibabin reaction
Fleming-Tamao oxidation
Bucherer-Bergs reaction
One-electron reduction
SN2 reaction
Fischer esterification
Baeyer-Villiger oxidation