= Solution
The initial <nucleophile> attacks \b[<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 $\mathrm{SiMe_4}$ 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>.
$$
\mathrm{R{-}O{-}SiMe_3+MeLi\longrightarrow R{-}O^-Li^++SiMe_4}.
$$
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>. \b[The <enolate> becomes an allyl <enol carbonate>, and the <phenoxide> becomes an aryl allyl carbonate], each containing $\mathrm{O{-}C(O){-}OCH_2CH{=}CH_2}$. <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.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-1-oxygen-trapping.png]
{title=Silicon cleavage and oxygen acylation in the two-site trapping sequence}
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