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The Robinson annulation is an organic reaction used to create a six-membered ring α,β-unsaturated ketone using a ketone (or aldehyde) and methyl vinyl ketone. It is named after Sir Robert Robinson, the British chemist who discovered it while he was at the University of Oxford.
The Wieland-Miescher ketone is the Robinson annulation product of 2-methyl-1,3-cyclohexanedione and methyl vinyl ketone while the Hajos-Parrish ketone is the product of 2-methyl-1,3-cyclopentanedione and methyl vinyl ketone.
Methyl vinyl ketone (or variants thereof) are essential for the annulation as they are simultaneously a Michael acceptor and able to take part in an aldol condensation. The first step in the Robinson annulation (also spelt annelation) is a Michael addition followed by an aldol reaction as the annulation step in the process. The reaction then proceeds as an aldol condensation to make the desired cyclohexenone ring.
Asymmetric Robinson annulation
The organocatalyst Proline has been used to resolve the enantiomeric isomers of Robinson annulations in asymmetric synthesis. . A proline derivative was employed in an asymmetric annulation of a geranial :
The reaction sequence in the related Hauser annulation is michael addition - Dieckman condensation - elimination . The Hauser donor is an aromatic methylene sulfoxide or sulfone with a carboxylic ester group in the ortho position. The Hauser acceptor is also a Michael acceptor. In the original Hauser publication ethyl 2-carboxybenzyl phenyl sulfoxide reacts with 3-pentene-2-one with LDA as a base in THF at -78°C :
The original reaction product still contains the sulfoxide group but it is lost on heating in an elimination reaction. The ultimate reaction product is a napthalene derivative. The dual purpose of the sulfoxide group is as stabilizing group for the carbanion in the first reaction step and as leaving group in the second.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Robinson_annulation". A list of authors is available in Wikipedia.|