Organic Chemistry 2 · Aldol addition and condensation
Wieland–Miescher ketone by intramolecular aldol
Conditions: NaOH, MeOH, Δ
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The mechanism, step by step
Next plays each step's curved arrows and explains them. The written steps are below.
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Hydroxide forms the side-chain methyl enolate
Hydroxide removes a hydrogen from the terminal methyl of the 3-oxobutyl side chain. The ring diketone has no α-hydrogen between its carbonyls (that carbon is quaternary), and this methyl enolate is the one that can reach a ring carbonyl through a six-membered ring.
The curved arrows
- Hydroxide takes a hydrogen of the side-chain methyl group.
- The C–H pair forms the terminal enolate C=C bond.
- The side-chain ketone π pair moves onto oxygen.
Six-membered ring closure
The enolate carbon attacks one of the two ring ketone carbonyls. The new ring holds the enolate carbon, the side-chain ketone, two CH2 groups, the quaternary carbon and the attacked carbonyl carbon, which becomes a tertiary alkoxide at the ring fusion.
The curved arrows
- The enolate oxygen lone pair re-forms the side-chain C=O.
- The enolate π pair forms the ring C–C bond to a ring ketone carbon.
- That ring ketone π pair moves onto its oxygen.
Water protonates the alkoxide
Water protonates the ring-fusion alkoxide, giving the bicyclic aldol (a ketol) and regenerating hydroxide.
The curved arrows
- The tertiary alkoxide takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Enolization next to the ring fusion
On heating, hydroxide removes the hydrogen on the new ring carbon between the ketone and the carbinol carbon at the ring fusion.
The curved arrows
- Hydroxide takes the α-hydrogen next to the ring fusion.
- The C–H pair forms the enolate C=C bond.
- The ketone π pair moves onto oxygen.
E1cB dehydration
The enolate re-forms the C=O and expels hydroxide from the ring-fusion carbon. The new C=C sits at the ring fusion, conjugated with the ketone, giving the bicyclic enone.
The curved arrows
- The oxygen lone pair rebuilds the C=O.
- The enolate π pair forms the ring-fusion C=C.
- The C–O pair leaves as hydroxide.
Wieland–Miescher ketone
The Wieland–Miescher ketone forms as the racemate, with water as the by-product; its quaternary methyl-bearing carbon is a stereocentre set by which ring carbonyl was attacked.
What you'll practice
Draw the aldol ring closure of 2-methyl-2-(3-oxobutyl)cyclohexane-1,3-dione: methyl enolate formation, six-membered ring closure onto a ring ketone, protonation, second enolization and E1cB dehydration to the Wieland–Miescher ketone.
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