= Solution
Disconnect the tertiary <amide> into \b[cyclohex-3-ene-1-carboxylic acid] and the secondary amine $\mathrm{EtNHCH(Me)CH_2CH_2Ph}$. The acid-derived ring is an immediate Diels–Alder disconnection: <buta-1,3-diene> plus the indicated \b[<methyl acrylate>] gives methyl cyclohex-3-ene-1-carboxylate. Heating, or suitable Lewis-acid activation of the <dienophile>, forms the two new <sigma bonds> and leaves the <diene>'s internal double bond as the ring alkene.
Hydrolyse this <ester> to the acid. Convert it to its acid <chloride> with oxalyl <chloride> or thionyl <chloride>, and react with the secondary amine in the presence of a base to capture HCl. The tertiary <amide> now contains the complete <cyclohexene> acyl fragment; direct conjugate addition of the amine to <methyl acrylate> before ring construction would give the wrong connectivity.
Prepare the amine by <reductive amination> of \b[4-phenylbutan-2-one, $\mathrm{MeCOCH_2CH_2Ph}$], with ethylamine, using <sodium triacetoxyborohydride> or another iminium-selective reducing system. This places <nitrogen> on $\mathrm{CH(Me)}$, not on a terminal methylene. If a route from simpler <carbon> compounds is wanted, crossed <aldol condensation> of acetone with <benzaldehyde> gives $\mathrm{MeCOCH=CHPh}$, and selective C=C reduction gives the required <ketone> before <reductive amination>.
The acid branch retains the acrylic <carbonyl> as the <amide> <carbonyl>; the amine branch supplies both N-ethyl and N-(4-phenylbutan-2-yl) substituents. The unlabelled stereogenic centres may be <racemic mixture>. This sequence separates ring formation from <amide> formation and avoids invoking uncontrolled alkylation of a tertiary <amide> <nitrogen>.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-1-q8-amide-route.png]
{title=Original two-branch synthesis combining a Diels–Alder acid fragment with a reductively aminated secondary amine}
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