Química Orgânica 2 · Condensação de Claisen e adição conjugada
Wieland–Miescher ketone by Robinson annulation
Condições: KOH, MeOH, Δ
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Deprotonate the 1,3-dione
Hydroxide (KOH in methanol) removes the hydrogen on the carbon between the two ketones of 2-methylcyclohexane-1,3-dione (pKa ≈ 10), giving a stabilized enolate.
As setas curvas
- A hydroxide lone pair takes the acidic C–H proton.
- The C–H pair stays on carbon, delocalized onto both ketones.
Michael addition to methyl vinyl ketone
The stabilized carbanion adds to the terminal CH2 of methyl vinyl ketone, creating the quaternary carbon; the C=C π pair moves next to the ketone.
As setas curvas
- The carbanion forms the C–C bond to the β-carbon.
- The C=C π pair moves onto the α-carbon.
Return the proton
Water protonates the enolate carbon, giving the Michael triketone and regenerating hydroxide.
As setas curvas
- The enolate carbon takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Form the methyl ketone enolate
Hydroxide removes a hydrogen from the terminal methyl of the side-chain ketone, the enolate that can reach a ring carbonyl to close a six-membered ring.
As setas curvas
- Hydroxide takes a methyl hydrogen.
- The C–H pair stays on carbon, giving the enolate.
Which enolate closes the ring?
Intramolecular aldol regiochemistry.
Intramolecular aldol addition
The enolate carbon attacks a ring ketone carbonyl carbon, closing the new six-membered ring and giving an alkoxide.
As setas curvas
- The enolate carbon forms the ring-closing C–C bond.
- The C=O π pair moves onto oxygen.
Protonate the alkoxide
Water protonates the alkoxide, giving the β-hydroxy ketone (aldol) and hydroxide.
As setas curvas
- The alkoxide takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Deprotonate α to the new ketone
Hydroxide removes a hydrogen on the carbon between the ring ketone and the carbinol carbon, giving an enolate.
As setas curvas
- Hydroxide takes the α-hydrogen.
- The C–H pair stays on carbon, giving the enolate.
E1cB dehydration
The enolate carbon forms the C=C bond and expels hydroxide from the neighbouring carbon, giving the conjugated enone.
As setas curvas
- The carbanion lone pair forms the C=C π bond.
- The C–O bond breaks and hydroxide leaves.
Wieland–Miescher ketone
The product is the Wieland–Miescher ketone (8a-methyl-3,4,8,8a-tetrahydronaphthalene-1,6(2H,7H)-dione), formed racemic at the quaternary carbon: Michael addition, intramolecular aldol and E1cB dehydration build the new cyclohexenone ring.
O que você vai praticar
Draw the Robinson annulation of 2-methylcyclohexane-1,3-dione with methyl vinyl ketone: Michael addition, intramolecular aldol ring closure and E1cB dehydration to the Wieland–Miescher ketone.
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