= Solution
One valid asymmetric route uses \b[N-Boc-(R)-proline as a <chiral> starting material]. Convert its acid to the <Weinreb amide> with N,O-dimethylhydroxylamine and a coupling reagent such as EDC, then add phenylmagnesium bromide at low temperature. <chelation> of the magnesium tetrahedral intermediate prevents a second addition; aqueous workup releases the phenyl <ketone>. Reduce it with <sodium borohydride>, separate the resulting <alcohol> <diastereomers>, and retain the one with \b[$R$ configuration at both pyrrolidine C2 and the benzylic carbinol]. This is an asymmetric synthesis through a pre-existing enantiopure centre, even though the final <reduction> need not be perfectly diastereoselective. If the other protected <alcohol> is isolated, Mitsunobu esterification with a carboxylic acid followed by <ester> <hydrolysis> inverts just the carbinol centre. Keeping <nitrogen> Boc-protected during that inversion avoids neighbouring-amine participation. Mild Boc removal followed by reductive benzylation with benzaldehyde and <sodium triacetoxyborohydride> gives \b[D, the $(R,R)$ amino <alcohol>]. Use conditions that avoid epimerizing the amino <ketone>.
For ring expansion, activate D's <alcohol> with methanesulfonyl chloride and a <tertiary amine> base. Its own tertiary <nitrogen> attacks the benzylic <carbon> from the back, displacing mesylate with inversion and making a fused <aziridinium ion>. Acetate, supplied as a soluble acetate salt, attacks the \b[original pyrrolidine C2] from the opposite face and breaks the old C2–N bond. The new N–benzylic-C bond is retained, incorporating that side-chain <carbon> into the six-membered ring. Inversion at the two successively attacked <carbons> gives \b[the cis acetate/phenyl piperidine E] in the displayed orientation. Attack at the benzylic <carbon> instead would simply open the three-membered <ion> back to a five-membered-ring derivative; specifying the attacked <carbon> is the crucial regiochemical step.
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