Organic Chemistry 2 · Sigmatropic shifts, cycloadditions and electrocyclic reactions
Claisen precursor for 4-methylhex-5-en-2-one
Conditions: Target: 4-methylhex-5-en-2-one; one Claisen [3,3] rearrangement on heating.
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The question
Which allyl vinyl ether gives 4-methylhex-5-en-2-one on heating?
Reveal the answer and explanation
Answer and explanation
Correct answer (E)-but-2-enyl isopropenyl ether (crotyl isopropenyl ether)
The isopropenyl CH2 bonds to the methyl-bearing allyl terminus, giving the 4-methyl branch next to a terminal vinyl group.
Not this one but-3-en-2-yl isopropenyl ether
Allylic transposition puts the new bond at the CH2= end, giving hept-5-en-2-one with an internal alkene.
Not this one allyl isopropenyl ether
This gives hex-5-en-2-one, which lacks the 4-methyl group.
Not this one allyl but-1-en-2-yl ether
The ethyl on the vinyl carbon ends up beside the carbonyl, giving hept-6-en-3-one.
Worked solution
A Claisen product has C=O, then two sp3 carbons, then the C=C (γ,δ). Break the bond between the α-carbon and the β-carbon (C3–C4) and reattach the oxygen: C2 becomes an isopropenyl ether carbon, and C4–C5=C6 becomes the allyl group. Because the allyl group is transposed in a [3,3] shift, the methyl-bearing C4 must be the far end of the allyl group, so the precursor is (E)-but-2-enyl isopropenyl ether, with oxygen on the CH2 end.
What you'll practice
Identify the allyl vinyl ether whose Claisen rearrangement gives the γ,δ-unsaturated ketone 4-methylhex-5-en-2-one, accounting for allylic transposition.
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