Ring formation involving a 5-exo trig cyclization between a zinc enolate and a non activated double bond led to cis diastereoisomer of 3-substituted prolines. This cyclization was achieved with transfer of chirality onto the C-2 carbon when nitrogen was protected by an α- methylbenzyl group. Reprotonation of the lithium enolate of cis derivative yielded the trans diastereoisomer.
![(s)-[But-3-enyl-(1-phenyl-ethyl)-amino]-acetic acid ethyl ester Structure](https://image.chemsrc.com/caspic/261/186586-65-2.png)