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Incoming links: Enolization
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.
A route from the supplied cyclohexenone starts with conjugate methyl addition. Form an organocuprate from methylmagnesium bromide and Cu(I), or use catalytic Cu(I) with controlled methylmagnesium addition, to obtain 3-methylcyclohexanone after protonation. copper directs 1,4 rather than direct 1,2 addition, so the ketone remains intact.
Generate its less hindered kinetic enolate at the alpha carbon opposite the methyl-bearing side, using LDA at low temperature, and trap with phenylselenyl chloride. oxidation of the resulting alpha-selenyl ketone with hydrogen peroxide produces a selenoxide, whose syn elimination introduces the enone double bond on that side. Renumbering the double bond from the carbonyl gives 5-methylcyclohex-2-en-1-one A. The selenium-containing phenyl group is a removable reagent fragment, not a carbon building block incorporated into A; the only new product carbon is the allowed methyl group. Regioselective kinetic enolization is important: enolizing the other alpha site would regenerate an enone with the methyl in the wrong position. This route produces racemic A unless a separate asymmetric step is introduced, which is sufficient for the stereochemically unspecified starting ketone.
Tin tetrachloride coordinates the enone carbonyl, lowering the dienophile pi-star energy. Butadiene reacts in an endo-oriented Diels–Alder cycloaddition: its terminal carbons bond to the enone's two alkene carbons, its central bond becomes the new cyclohexene double bond, and the enone C=C pi electrons become a new sigma bond. All three bond reorganizations occur in a suprafacial six-electron cycle.
Approach from the face opposite the methyl substituent avoids steric crowding and gives the indicated relationship between methyl and the non-enolizable junction hydrogen. The immediate stereospecific cycloaddition gives a cis-fused junction relationship. The junction carbon next to the ketone can subsequently enolize; reprotonation on either face epimerizes this centre without breaking either newly formed C–C bond. This accounts for the mixture at the wavy-bond position while the other junction and methyl relationship remains fixed. The modest 1.5:1 ratio should not be mistaken for failure of suprafaciality in the cycloaddition itself. Later oxime formation removes the ketone's ordinary enolization pathway and furnishes the depicted isolated junction diastereomer.