In organic chemistry, an aldol describes a structural motif consisting of a 3-hydroxy ketone, R−CH(OH)−CHR'−C(=O)−R", or 3-hydroxyaldehyde, R−CH(OH)−CHR'−CH=O. Both are composed of a hydroxy group (−OH) and either a ketone (>C=O) or an aldehyde (−CH=O, which is merely a ketone with a hydrogen substituent). An aldol consisting of a 3-hydroxy ketone is called a β-hydroxy ketone, and an aldol consisting of a 3-hydroxy aldehyde is called a β-hydroxy aldehyde. The term "aldol" may refer to 3-hydroxybutanal.[1][2]
Aldols are usually the product of aldol addition, i.e. the condensation of two aldehydes.[1] Stereoselective syntheses of aldols is an active area of asymmetric synthesis.
The chemistry of aldols is dominated by one reaction, dehydration:
When two molecules of aldehydes react to form an aldol (β-hydroxy aldehyde), the aldol usually produces secondary compounds since it is unstable.[1] Secondary compounds can be diols, unsaturated aldehydes, or alcohols.[1] The aldol 3-hydroxybutanal is a precursor to quinaldine, a precursor to the dye quinoline Yellow SS.[1]
Aldols are also used as intermediates in the synthesis of natural products and drugs.[3][4] The synthesis of Oseltamivir, an antiviral medicine used to treat the flu, involves the aldol reaction.[5]
The structural motif of aldols is often found in polyketide natural products, which can be used to manufacture antibiotics.[3]
Hydroxypivaldehyde is a rare example of a relatively robust aldol.[6]