Journal of Organic Chemistry 2005-10-14

New asymmetric approach to natural pyrrolizidines: synthesis of (+)-amphorogynine A, (+)-amphorogynine D, and (+)-retronecine.

Caroline Roche, Katarína Kadlecíková, Amaël Veyron, Philippe Delair, Christian Philouze, Andrew E Greene, David Flot, Manfred Burghammer

Index: J. Org. Chem. 70 , 8352-8363, (2005)

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Abstract

Three natural pyrrolizidines, (+)-amphorogynines A and D and (+)-retronecine, have been prepared from a common lactam intermediate. This central compound, in turn, was synthesized in diastereomerically enriched form through a highly selective [2 + 2]-cycloaddition of dichloroketene with a chiral enol ether, followed by Beckmann ring expansion and reduction. Subsequent stereocenters were then cleanly introduced through internal induction.