An in-depth study of the oxirane ring-opening reaction of a pivotal epoxypyrrolidine is reported. The introduction of various carbon- and heteroatom-centered nucleophiles at C4 has been achieved with high regiocontrol. The structures of the major products were assigned on the basis on an X-ray crystallographic study of six examples. A mechanistic study carried out at the B3LYP/6-31G** level of theory