 Solution

Disconnect the tertiary amide into cyclohex-3-ene-1-carboxylic acid and the secondary amine $\mathrm{EtNHCH}(\mathrm{Me})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{Ph}$. The acid-derived ring is an immediate Diels–Alder disconnection: buta-1,3-diene plus the indicated 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 4-phenylbutan-2-one, $\mathrm{MeCOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{Ph}$, with ethylamine, using sodium triacetoxyborohydride or another iminium-selective reducing system. This places nitrogen on $\mathrm{CH}(\mathrm{Me})$, not on a terminal methylene. If a route from simpler carbon compounds is wanted, crossed aldol condensation of acetone with benzaldehyde gives $\mathrm{MeCOCH}=\mathrm{CHPh}$, 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.