SolutionsUnsaturated Aldehydes and Ketones
1. Reactivity of the conjugated system
Write the resonance forms of 3-buten-2-one and indicate which atoms are attacked by nucleophiles.
Besides the neutral form, there is a form with negative charge on oxygen and positive charge on the carbonyl carbon (C2), and another with the positive charge on the β carbon (C4). Nucleophiles can therefore attack the carbonyl carbon (1,2-addition) or the β carbon (1,4- or conjugate addition).
2. Michael and Robinson
Which product is obtained from the reaction of 2-methylcyclohexane-1,3-dione with 3-buten-2-one in the presence of base, and then on heating?
The highly stabilized enolate of the dione adds in a conjugate fashion (Michael addition) to methyl vinyl ketone. The resulting triketone then undergoes an intramolecular aldol condensation and, on heating, dehydrates: this is the Robinson annulation, which gives the Wieland-Miescher ketone, a bicyclic system with a cyclohexenone.