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The initial nucleophile attacks silicon, not the nitrile or an alkene carbon. Each trimethylsilyl group has an electrophilic Si center. A methyl group from methyllithium attacks Si, passing through a pentacoordinate silicon arrangement; Si–O cleavage releases $\mathrm{SiM}{\mathrm{e}}_4$ and the lithium salt of the oxygen-containing fragment. Cleaving the silyl enol ether generates a lithium enolate; cleaving the aryl silyl ether generates a phenoxide. Neither operation needs a new deprotonation at a carbon stereocenter.Here $R-O^{-}$ represents either the phenoxide or the oxygen-centered enolate contributor. Excess reagent permits both cleavages. The pre-existing methyl/nitrile stereocenter retains its configuration: it has no hydrogen to remove, and neither bond attached to that center is broken.

After HMPA is added, each anionic oxygen attacks the carbonyl carbon of allyl chloroformate. The C=O pi electrons move to the carbonyl oxygen, giving a tetrahedral intermediate; collapse expels chloride. The enolate becomes an allyl enol carbonate, and the phenoxide becomes an aryl allyl carbonate, each containing $\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{OC}{\mathrm{H}}_2\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$. oxygen acylation accounts for preservation of the enol double bond and for the two newly installed carbonate groups. carbon attack would give a different carbon skeleton. The two oxygens are trapped because they were selectively unmasked by desilylation, rather than because every ether oxygen in the molecule is equally reactive.

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.

A silyl ether contains an O–Si bond, commonly protecting an alcohol or phenol. Cleavage at electrophilic silicon releases an oxygen-centered anion under suitable conditions.

The silylating reagent $\mathrm{tBuM}{\mathrm{e}}_2\mathrm{SiCl}$, commonly abbreviated TBSCl. nucleophilic substitution at silicon converts an alcohol into a silyl ether.