N,3,4-Trisubstituted pyrrolidines by electron transfer-induced oxidativecyclizations of N-allylic β-amino ester enolates

摘要

Oxidative radical cyclizations starting from easily accessible N-allylic β-alanine esters are reported. De-protonation generates the corresponding enolates, which are transformed efficiently into a-ester radicals bysingle electron transfer mediated by ferrocenium hexafluorophosphate. The stereochemistry of the radical5-exo cyclization can be switched by the configuration of the enolate precursor. First examples of asym-metric oxidative radical cyclizations using N-(1-phenylethyl)-substituted β-amino esters are reported. Onlytwo of the four possible diastereomers are formed from the (E)-enolate with high cis-selectivity. From (Z)-enolates, an additional diastereomer is formed, which is likely to be formed only under chelation control.