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Joined 2026-09-21 Message
Incoming links: Enol ether
Enol ether 2026-10-20
An ether with oxygen directly attached to an alkene carbon. Electrophilic activation and hydrolysis can convert an appropriate enol ether back to a carbonyl compound.
Enol ether hydrolysis 2026-10-20
Past exam of the chemistry B course of the University of Cambridge 2021 Paper 1 B4 c Solution 2026-10-20
The first step is a vinylogous aldol condensation at C5 of the unsaturated lactam. Number nitrogen N1, the lactam carbonyl C2, and then the methoxy-substituted alkene C3=C4 and the methylene group C5.
hydroxide reversibly removes a C5 proton. The resulting vinylogous enolate is delocalized through C5–C4–C3–C2–O: it is not necessary to create an unstabilized isolated carbanion beside nitrogen. Its C5 nucleophilic contributor attacks acetone. protonation gives a C5-linked tertiary alcohol, . Further removal of the remaining C5 hydrogen and elimination through an aldol-dehydration sequence forms C5=C(Me)2, leaving the original methoxy enol ether in place.
In the second step, aqueous acid hydrolyses that enol ether to a ketone at C4. protonation at C3 gives an oxygen-stabilized cation at C4. water attacks C4, proton transfers form a hemiacetal-like intermediate, and loss of methanol followed by deprotonation produces C4=O and the C3 methylene group.
The first equivalent of titanocene methylidene methylenates the ester: the carbonyl oxygen and titanium form an oxatitanacyclobutane, whose fragmentation gives and a titanium–oxygen product. The second equivalent adds across this electron-rich double bond. The initial adduct is a titanacyclobutane with cyclic connectivity , closing from the last methylene to titanium. The ester's phenyl-substituted carbon lies opposite Ti; the two new methylenes originate from the two methylidene equivalents.
Beta-methoxy elimination opens that metallacycle to B, , the allyltitanium reagent. The C–OMe bond breaks, OMe transfers to Ti, one Ti–C bond becomes the new C=C bond, and the other Ti–C bond remains as the allylic nucleophile. The cyclic adduct is therefore a precursor to the opened reagent B; treating B as merely the one-equivalent enol ether loses the second reagent carbon.
The aldehyde oxygen coordinates to titanium. Allylic carbon transfer to its carbonyl carbon, accompanied by movement of the C=O pi electrons to oxygen, gives the titanium alkoxide . Acidic aqueous workup cleaves O–Ti and protonates oxygen, giving . A six-membered allyl-transfer arrangement is a convenient mechanistic model. No enantioselectivity is imposed by these achiral reagents.
A suitable sequence is selective alpha-bromination, then enol O-methylation. Use one equivalent of Br under controlled acidic enolization conditions to introduce bromine at the enolizable alpha carbon that becomes the brominated vinyl carbon. The other alpha site is a constrained bridgehead and cannot form the required ordinary enol double bond. Then O-alkylate the ketone with trimethyloxonium tetrafluoroborate and remove the brominated alpha proton with a nonnucleophilic base such as triethylamine. Collapse of the O-methylated carbonyl oxonium gives the alpha-bromo methyl enol ether E.
The methyl group belongs on oxygen, not on the alpha carbon. Simply writing methyl iodide with an unspecified enolate would not guarantee this outcome because competitive C-alkylation is possible. Use conditions designed for carbonyl O-alkylation followed by deprotonation, and limit bromine to avoid overbromination or damage to the protecting group.
Trimethyloxonium tetrafluoroborate 2026-10-20
The oxonium salt , a strong methylating reagent. carbonyl O-methylation followed by alpha-deprotonation can form an enol ether.