Biomimetic Organocatalytic Approach to 4-Arylquinolizidine Alkaloids and Application in the Synthesis of (-)-Lasubine II and (+)-Subcosine II.

An enantioselective, biomimetic organocatalytic synthesis of 4-arylquinolizidin-2-ones, key intermediates in the synthesis of several Lythraceae alkaloids, was developed. The methodology features S-proline-mediated Mannich/aza-Michael reactions of readily available arylideneacetones and Δ1-piperideine. The total syntheses of (-)-lasubine II and (+)-subcosine II as well as the formal syntheses of structurally related Lythraceae alkaloids were achieved. The use of Δ1-pyrroline in the Mannich/aza-Michael reaction provides enantiomerically enriched 5-arylindolizidin-7-ones, which are precursors to nonopiate antinociceptive agents.