Total Syntheses of Phleghenrines A and C: A [4 + 2] Cycloaddition and Ring-Expansion Approach

The first total syntheses of Lycopodium alkaloids
phleghenrines A and C have been accomplished in 19 and 18 steps, respectively,
relying on three (hetero)-Diels–Alder ([4 + 2]) cycloaddition
reactions to forge the cyclic molecular backbone and two ring-expansion
reactions to manipulate the ring size. A chiral precursor is synthesized
through an auxiliary controlled Diels–Alder reaction, rendering
the asymmetric synthesis accessible. The established strategy provides
a general approach to the relevant novel Lycopodium alkaloids.