This article is part of a series on Indolizidines and quinolizidines: natural products and beyond, edited by Prof Joseph P Michael, University of Witwatersrand.

Full Research Paper

Knorr-Rabe partial reduction of pyrroles: Application to the synthesis of indolizidine alkaloids

Brendon S Gourlay¹, John H Ryan² and Jason A Smith¹

¹School of Chemistry, University of Tasmania, Hobart, Australia

²CSIRO Division of Molecular and Health Technologies, Bag 10, Clayton South, Victoria, Australia

Beilstein Journal of Organic Chemistry 2008, 4:3doi:10.1186/1860-5397-4-3

Published:	15 January 2008

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Abstract

Background

The Birch reduction of electron rich pyrroles does not occur readily. However, dissolving metal reduction with zinc under acidic conditions gives 3-pyrrolines (2,5-dihydropyrroles) in reasonable yield. This dissolving metal reduction was first reported by Knorr and Rabe in 1901 but since then has only been reported for the reduction of electron rich pyrroles.

Results

The partial reduction of bicyclic α-ketopyrrole derivatives has been performed under dissolving metal conditions with zinc and hydrochloric acid to give excellent yields of hexahydroindolizidines. This reduction method has been utilised for the diastereoselective synthesis of 5-alkylindolizidines and the stereoselectivity obtained is opposite to that of catalytic hydrogenation.

Conclusion

An efficient stereoselective synthesis of indolizidine alkaloids has been developed from α-ketopyrrole intermediates using a modified version of Knorr and Rabe's pyrrole reduction.

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