Definition
Robinson Annulation is a tandem organic reaction that builds a new six-membered carbocyclic ring bearing an α,β-unsaturated ketone (cyclohex-2-enone) in a single pot. It combines two classic reactions:
The term annulation means ring formation. Robinson Annulation specifically refers to the one-pot sequence that appends a new cyclohexenone ring onto a donor ketone.
History & Nobel Prize
Sir Robert Robinson (1886–1975) published the reaction in 1935 as part of his work on the synthesis of natural products. He recognised that the combination of Michael addition with aldol condensation provided a powerful, general method for constructing six-membered rings found in steroids and terpenes.
Robinson was awarded the 1947 Nobel Prize in Chemistry “for his investigations on plant products of biological importance, especially the alkaloids.” The annulation named after him is considered one of the most elegant ring-forming strategies in synthetic organic chemistry.
Starting Materials
α-Enolisable Ketone
Must have at least one α-hydrogen. Common examples: cyclohexanone, acetone, 2-methyl-1,3-cyclohexanedione.
Methyl Vinyl Ketone
MVK (but-3-en-2-one). The β-carbon is electrophilic. Often delivered from a Mannich base to prevent polymerisation.
Mechanism Overview (4 Steps)
Enolate Formation
Base removes α-H from the donor ketone to give an enolate.
Michael Addition
Enolate attacks the β-carbon of MVK → 1,5-diketone.
Intramolecular Aldol
New enolate closes the ring → β-hydroxy ketone.
Dehydration
Loss of water → conjugated cyclohex-2-enone product.
Product & Applications
The product is a cyclohex-2-enone: a six-membered carbocyclic ring with an α,β-unsaturated ketone. This structural motif is the backbone of:
- Steroids (testosterone, cortisol, cholesterol)
- Sesquiterpenes and diterpenes
- Alkaloids and polycyclic natural products
- The Wieland–Miescher ketone — key steroid synthesis intermediate