3, 4-Disubstituted piperidines were synthesized through anti SN2′ allylation of 4- substituted 2-cyclopentenyl esters with reagents based on RMgX and CuX, thus allowing equal access to both trans-and cis-isomers. As an application, the paroxetine intermediate was synthesized efficiently. During the investigation, the MeOCH2CO2 group was found to show high reactivity in the pivotal anti SN2′ type reaction using the reagent derived from ...