Química Orgánica 2 · Condensación de Claisen y adición conjugada
Wieland–Miescher ketone by Robinson annulation
Condiciones: KOH, MeOH, Δ
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El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 10 pasos escritos
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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Qué vas a practicar
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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