Asymmetric Synthesis of Pyrrolizidines, Indolizidines and Quinolizidines via a Double Reductive Cyclisation Protocol

Abstract

This account describes an overview of the asymmetric syntheses of pyrrolizidines, indolizidines and quinolizidines via a common double reductive cyclisation protocol. The highly diastereoselective conjugate addition of an enantiopure lithium amide to an α,β-unsaturated ester incorporating a terminal C=C bond installed the nitrogen-bearing stereogenic centre and was followed by enolate functionalisation to introduce the second olefinic functionality. Alternatively, conjugate addition to the corresponding α-alkenyl α,β-unsaturated ester followed by α-protonation of the intermediate enolate may also be used to access the cyclisation precursor. After oxidation of the two terminal olefinic units to give the corresponding dialdehyde, tandem hydrogenolysis/hydrogenation was employed to efficiently construct the azabicyclic core of each target molecule. This double reductive cyclisation strategy was successfully utilised in the syntheses of 13 azabicyclic alkaloids or closely related analogues.1 Introduction2 Asymmetric Syntheses of (–)-Isoretronecanol and (–)-Trachelanthamidine3 Asymmetric Syntheses of (+)-Trachelanthamidine [(+)-Laburnine], (+)-Tashiromine and (+)-epi-Lupinine4 Asymmetric Syntheses of (–)-Hastanecine, (–)-Turneforcidine and (–)-Platynecine5 Asymmetric Syntheses of (–)-Macronecine, (–)-Petasinecine, (–)-1-epi-Macronecine, (+)-1-epi-Petasinecine and (+)-2-epi-Rosmarinecine6 Conclusion