Stereodivergent Synthesis of Chiral Hydrobenzofuranpyrrolidines by Catalytic Asymmetric Dearomative Cyclization and Controlled Epimerization

Abstract

A methodology to access stereoisomeric sets of products bearing multiple stereogenic centers is still a significant challenge in asymmetric catalysis. We present herein our experimental studies on the stereodivergent synthesis of chiral hydrobenzofuran‐fused pyrrolidines with three stereogenic centers via organocatalytic asymmetric dearomative cyclization and epimerization process. Chiral bifunctional thiourea catalyst could successfully promote the enantioselective dearomatization cyclization of 2‐nitrobenzofurans with o‐hydroxy aromatic aldimines, which enabled the synthesis of (3S,3aR,8bR)‐hydrobenzofuran[3.2]pyrrolidines in 79–92% yields with >20:1 stereoselectivities and 93–>99% enantio‐selectivities. While catalytic amount of DBU could induce the direct intramolecular epimerization of (3S,3aR,8bR)‐hydrobenzofuran[3.2] pyrrolidines to its diastereomers (3R,3aR,8bR)‐hydrobenzofuran[3.2] pyrrolidines in 72–87% yields without loss of stereoselectivities. The mechanistic pathways of the epimerization process were investigated by a series of control experiments study. This work provides an alternative and forward solution for the stereodivergent preparation of functionalized pyrrolidines with potential bioactivities.