Enones

Enones are a class of organic compounds in which a carbonyl group is conjugated to a carbon–carbon double bond. The defining motif is an alpha,beta-unsaturated carbonyl compound, typically represented as R1-CO-CH=CH-R2, where the C=C bond is adjacent to the carbonyl. The term enone is most often applied to alpha,beta-unsaturated ketones; the related alpha,beta-unsaturated aldehydes are called enals, and related diaryl enones include chalcones.

Structure and properties: Conjugation extends the system, lowering the C=O stretching frequency and giving characteristic UV absorption that shifts toward longer wavelengths. Enones are often more reactive toward nucleophilic conjugate (1,4-) additions than nonconjugated ketones, due to the electron-poor beta-carbon.

Reactivity: The beta-carbon is highly electrophilic, enabling Michael additions (conjugate additions) of nucleophiles such as enolates, thiols, and amines. They also participate as dienophiles in Diels–Alder reactions. Hydrogenation can selectively reduce either the C=C or the C=O bond under suitable conditions; many enones undergo either 1,2- or 1,4-additions depending on the reagents and reaction conditions.

Synthesis: Enones are commonly prepared by aldol condensation of an aldehyde and a ketone followed by dehydration; the oxidation of allylic alcohols or the dehydration of beta-hydroxy ketones are also used routes. Cross-aldol condensations, particularly with aryl aldehydes, can yield chalcones (diaryl enones).

Applications and occurrence: Enones appear in natural products and fragrances and are versatile intermediates in organic synthesis and medicinal chemistry, serving as building blocks for diverse transformations and complex molecules.