One-pot reductive coupling of N-acylcarbamates with activated alkenes: application to the asymmetric synthesis of pyrrolo[1,2-a]azepin-5-one ring system and (?)-xenovenine?

Organic & Biomolecular Chemistry Pub Date: 2011-11-17 DOI: 10.1039/C1OB06697H

Abstract

The one-pot reductive coupling of N-acylcarbamates with activated alkenes is described. The method is based on partial reduction of N-acylcarbamates with DIBAL-H, followed by N-acyliminium ion formation and SmI2-mediated radical coupling with activated alkenes. Both acyclic and cyclic N-acylcarbamates can be used as stable substrates, and a range of activated alkenes serve as effective radical receptors. The reductive coupling of L-N-acylcarbamates 12/13 gave 2,5-disubstituted pyrrolidine derivatives in high trans-diastereoselectivities. The reductive coupling with penta-2,4-dienoate proceeded exclusively in a 1,6-addition fashion, producing a single non-conjugated E-isomer. On the basis of this method, a three-step construction of pyrrolo[1,2-a]azepin-5-one 16, the skeleton of many stemona alkaloids and lehmizidine alkaloids, and a seven-step synthesis of (?)-xenovenine (pyrrolizidinecis-223H, ent-6), the unnatural enantiomer of the frog/ant venom alkaloid possessing potent inhibitory activity towards nAChR channel, were achieved starting from L-12.

Graphical abstract: One-pot reductive coupling of N-acylcarbamates with activated alkenes: application to the asymmetric synthesis of pyrrolo[1,2-a]azepin-5-one ring system and (?)-xenovenine
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