Application of the Helquist Annulation in Lycopodium Alkaloid Synthesis: Unified Total Syntheses of (−)-8-Deoxyserratinine, (+)-Fawcettimine, and (+)-Lycoflexine

A unified strategy for total synthesis of the Lycopodium alkaloids (−)-8-deoxyserratinine (7), (+)-fawcettimine (1), and (+)-lycoflexine (4) is detailed. The key features include a highly efficient Helquist annulation to assemble the cis-fused 6/5 bicycle, facile construction of the aza nine-membered ring system employing double N-alkylation strategy, providing access to the common tricyclic skeleton, asymmetric Shi epoxidation, delivering the desired β-epoxide stereospecifically to furnish (−)-8-deoxyserratinine (7), SmI₂ reduction of dihydroxylation derivative 35 to enable formation of (+)-fawcettimine (1), and a rapid biomimetic transformation of (+)-fawcettimine (1) into (+)-lycoflexine (4) via an intramolecular Mannich cyclization.