SolutionsRobinson annulation
Exercise 1
Write the product of the Robinson annulation of 2-methylcyclohexanone with methyl vinyl ketone under basic conditions. Why is the new ring formed on the carbon bearing the methyl group?
4a-Methyl-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one (an octalone with an angular methyl at the ring junction) is obtained. Under equilibrium conditions (base in a protic solvent) the thermodynamic, more substituted enolate forms, which is the one bearing the methyl; the Michael addition takes place at that carbon and creates the quaternary centre.
Exercise 2
Why is its precursor 4-(diethylamino)butan-2-one, or the corresponding quaternary ammonium salt, often used instead of methyl vinyl ketone?
Methyl vinyl ketone polymerizes easily under basic conditions. The Mannich base (or, better, its methylammonium salt) releases the enone in the medium by Hofmann elimination as it is consumed, so that its concentration stays low and polymerization is reduced.