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The terminal alkene of D first undergoes Wacker oxidation, giving the side chain $\mathrm{Ar}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{COC}{\mathrm{H}}_3$. Alkene coordination to Pd(II), water attack, proton transfers and beta-hydride elimination give the ketone. Copper and oxygen regenerate the active palladium oxidation state.

Base removes a proton from the terminal methyl of that ketone. Its enolate attacks the neighboring aromatic aldehyde intramolecularly, closing a six-membered ring. Dehydration gives a conjugated cyclohexadienone-type intermediate, and keto-enol tautomerism furnishes aromatic naphthol E. The retained methoxy and MOM-protected oxygen groups follow the original aromatic-carbon positions.

F is the O-allyl ether of E. Carbonate forms the naphthoxide and allyl bromide undergoes an SN2 reaction at its terminal carbon. Heating F then causes a [3,3]-Claisen rearrangement: the O–allyl bond breaks, the terminal allyl carbon bonds to the adjacent naphthalene C1, and the phenolic carbon temporarily becomes a ketone. Rearomatization gives G, with a free phenolic OH and a C1 allyl group. The C1 position gives the favored alpha-substituted naphthol and avoids the alternative less favorable annulation pattern.

The final step is a dearomative Michael addition, not a Nef conversion of the nitro group. Hydrogen bonding from the thiourea activates nitroethylene. The nucleophilic C1 resonance form of G attacks the terminal carbon of $\mathrm{C}{\mathrm{H}}_2=\mathrm{CHN}{\mathrm{O}}_2$, and proton transfer converts the nitronate to a 2-nitroethyl substituent. The phenolic carbon becomes a ketone.

H is the corresponding 1-allyl-1-(2-nitroethyl)naphthalen-2-one, retaining the methoxy and MOM substituents of G. The second ring now has a C2=O group and a C3=C4 bond, while the other ring remains aromatic. The reported ${}^{13}$C signal near $202$ ppm is consistent with that newly formed ketone. Related dearomative additions establish this reaction class.^[ref]

C1 is now an all-carbon quaternary stereocenter bearing both allyl and 2-nitroethyl groups. Both the drawn thiourea and the starting materials are achiral, so H is racemic. A chiral catalyst would be needed to select one absolute configuration.

A is the kinetic, less-hindered-side silyl enol ether. Its double bond lies away from the methyl substituent. Forming B requires removal of a proton from the more hindered $\alpha$ position adjacent to the methyl-bearing carbon.

The base strengths alone do not determine the ratios. trimethylsilyl triflate is strongly electrophilic: oxygen silylation activates the ketone towards deprotonation, and irreversible trapping of the resulting silyl enol ether can make the product ratio reflect competing deprotonation rates. A weak amine therefore need not first produce a substantial equilibrium concentration of the free ketone enolate.

DBU is the strongest and comparatively accessible base, and distinguishes the two sites least strongly, giving the smallest A:B ratio. Triethylamine gives greater preference for the less-hindered site. The more sterically hindered diisopropylethylamine discriminates most strongly and gives the largest proportion of A. Its slightly higher conjugate-acid $p{K}_a$ than that of triethylamine does not reverse this trend, because steric accessibility and the coupled silylation/deprotonation mechanism matter alongside basicity. The data do not establish a simple thermodynamic enolate distribution.

Dibutylboron triflate coordinates the imide carbonyl and triethylamine removes the propionyl $\alpha$ proton, giving predominantly the Z dibutylboron enolate. Its connectivity is $\mathrm{Aux}-\mathrm{N}-\mathrm{C}(\mathrm{O}-\mathrm{BB}{\mathrm{u}}_2)=\mathrm{CHMe}$; the imide auxiliary remains attached, rather than being displaced during enolization. Coordination of cinnamaldehyde's oxygen to boron organizes a six-membered Zimmerman–Traxler transition state.

A chair with the aldehyde's styryl group pseudoequatorial avoids the auxiliary's bulky benzyl side. Formation of the C–C bond and transfer of the aldehyde oxygen into the boron coordination sphere give the boron aldolate, which hydrolyses to D. A Z enolate gives the syn aldol relationship. In the printed product zigzag, the labelled $\alpha$-methyl bond should therefore be a solid wedge, on the same stereochemical face as the neighbouring solid-wedge hydroxyl bond. With the stated connectivity, the labelled carbon is R: priority is imide carbonyl, then $\mathrm{CH}(\mathrm{OH})\mathrm{CH}=\mathrm{CHPh}$, then methyl, then hydrogen. The adjacent hydroxyl centre is S. These assignments refer to the actual auxiliary and product orientation, not an arbitrary choice of an otherwise equivalent chair.

DBU removes the proton $\alpha$ to sulfonium, generating the propargyl-substituted sulfur ylide $\mathrm{M}{\mathrm{e}}_2{\mathrm{S}}^{+}-\mathrm{C}{\mathrm{H}}^{-}\mathrm{C}\equiv \mathrm{CMe}$. Its carbon attacks the enone $\beta$ carbon bearing indolyl, shifting the conjugated electrons into an enolate. The new intermediate has the carbon sequence ${\mathrm{O}}^{-}-\mathrm{C}(\mathrm{Ph})=\mathrm{C}(\mathrm{CN})-\mathrm{CH}(\mathrm{indolyl})-\mathrm{CH}(\mathrm{SM}{\mathrm{e}}_2^{+})(\mathrm{C}\equiv \mathrm{CMe})$.

Intramolecular attack by the enolate oxygen on the sulfonium-bearing carbon displaces dimethyl sulfide and closes a five-membered ring. This conjugate addition–O-alkylation sequence gives the dihydrofuran, retaining the C≡CMe substituent and placing the original carbonyl oxygen in the ring. Enolate carbon alkylation would instead give a cyclopropane; the observed oxygen-containing ring identifies the alternative closure. An approach and betaine conformation minimizing crowding between indolyl and alkynyl substituents, followed by stereoinverting displacement at the sulfonium carbon, rationalizes the depicted trans product. The result is racemic in the absence of an external chiral influence.

A suitable sequence is selective alpha-bromination, then enol O-methylation. Use one equivalent of Br${}_2$ 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.

Manganese(III) removes one electron from the enol/enolate of H, and loss of its hydroxyl proton gives a carbon-centred radical at the tether-bearing alpha carbon of the resulting 1,3-dicarbonyl system. The enolic OH becomes the second carbonyl oxygen. This radical attacks the terminal CH${}_2$ of the tethered isopropenyl group, forming a new C–C bond and a six-membered ring. The residual radical is on the substituted carbon bearing methyl, which is more stable than the primary radical from the alternative regiochemical closure.

Dioxygen traps that radical as a peroxyl intermediate. reduction/proton transfer and O–O cleavage in the ensuing oxygenation/workup sequence give the new tertiary alcohol on the methyl-bearing carbon. The drawn net transformation specifies that oxygenated product, not a persistent hydroperoxide; individual electron/proton transfers require the reaction medium and workup and cannot be fixed uniquely from the abbreviated reagent line alone. The rigid cage and folded tether select the closure face and therefore the relative configuration of the new ring junction.

Finally the new hydroxyl oxygen attacks silicon of TBSCl, with imidazole assisting deprotonation and chloride departure, giving the tertiary silyl ether I. The hindered pre-existing hydroxyls remain as shown. Track the two key atom changes: the terminal alkene carbon becomes a methylene in the new ring, and the former substituted alkene carbon acquires oxygen and retains its methyl group. Calling the first step a two-electron aldol reaction would miss both the single-electron oxidant and the role of dioxygen.

The ketone $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{C}-\mathrm{C}(=\mathrm{O})-\mathrm{C}{\mathrm{H}}_3$. Only the methyl side supplies alpha hydrogens for ordinary enolate formation.

Conjugate reduction followed by carbon-electrophile trapping of the resulting enolate. The original enone alpha position determines the alkylation site before any later equilibration.

A silyl enol ether has the connectivity $\mathrm{C}=\mathrm{C}-\mathrm{O}-\mathrm{Si}{\mathrm{R}}_3$. Nucleophilic attack at silicon can cleave the Si–O bond and generate an enolate without adding to the original carbonyl carbon.

An O-silylated ester enolate, with two oxygen substituents on one carbon of the enol double bond.

Transfer of one electron between species, often converting an enol or enolate into a radical. It must be distinguished from transfer of a two-electron hydride.

The ionic compound $\mathrm{NaN}{\mathrm{H}}_2$, commonly used as a strong base. ketone deprotonation produces an enolate and ammonia.

conjugate addition generates an enolate that is intercepted by an aldehyde or ketone.

An enolate favoured after equilibration under the stated conditions. It is often more substituted, but aggregation, solvent and counterion can change the preference.

Aldol-type carbonyl addition at a more remote carbon of a conjugated enolate.

An enolate whose nucleophilic carbon is conjugated to a carbonyl through an additional unsaturated unit.

A six-membered chair-like organization of a metal enolate and carbonyl compound used to rationalize aldol stereoselectivity.