Protecting-Group-Free Total Synthesis of (−)-Lycopodine via Phosphoric Acid Promoted Alkyne Aza-Prins Cyclization

A protecting-group-free route for the total synthesis of (−)-lycopodine was demonstrated in only 8 steps from Wade’s fawcettimine enone (12 steps from commercial availiable (R)-(+)-pulegone). The key core of this alkaloid was constructed through a phosphoric acid promoted and highly stereocontrolled alkyne aza-Prins cyclization reaction, synchronously establishing the bridged B-ring and the C13 quaternary stereocenter. Importantly, the synthesis further features a new efficient approach for the preparation of other lycopodine-type alkaloids.