A novel synthesis of (-)-huperzine A via tandem intramolecular aza-Prins cyclization-cyclobutane fragmentation.

The acetylcholinesterase inhibitor (-)-huperzine A was synthesized from (S)-4-hydroxycyclohex-2-enone in 17 steps by a route that involved two cyclobutane fragmentations. The first of these employed a retro-aldol cleavage to generate the α-pyridone ring of huperzine A, and the second invoked a novel intramolecular aza-Prins reaction in tandem with stereocontrolled scission of a cyclobutylcarbinyl cation to create the aminobicyclo[3.3.1]nonene framework of the natural alkaloid.