A new and efficient synthesis of (−)-indolizidine 167B by tandem metathesis

An enantioselective synthesis of the natural alkaloid (−)-indolizidine 167B is described. From an easily accessible chiral cycloheptene derivative a 2,5-dihydropyrrolidine was formed via a ruthenium-catalysed tandem ring-rearrangement metathesis. Annellation of the second ring was effected by an intramolecular reductive amination step under complete stereocontrol. (−)-Indolizidine 167B was obtained in 35% overall yield and high optical purity.