Cyclobutane Synthesis and Fragmentation. A Cascade Route to the Lycopodium Alkaloid (−)-Huperzine A

An asymmetric total synthesis of the nootropic alkaloid (−)-huperzine A was completed using a cascade sequence initiated by an intramolecular aza-Prins reaction and terminated by a stereoelectronically guided fragmentation of a cyclobutylcarbinyl cation as the key step in assembling the bicyclo[3.3.1]nonene core of the natural product. Intramolecular [2 + 2]-photocycloaddition of the crotyl ether of (S)-4-hydroxycyclohex-2-enone afforded a bicyclo[4.2.0]octanone containing an embedded tetrahydrofuran in which the cyclohexanone moiety was converted to a triisopropylsilyl enol ether and functionalized as an allylic azide. The derived primary amine was acylated with α-phenylselenylacrylic acid, and the resulting amide was reacted with trimethylaluminum to give a [2 + 2]-cycloadduct, which underwent retroaldol fission to produce a fused α-phenylselenyl δ-lactam. Periodate oxidation of this lactam led directly to an α-pyridone, which was converted to a fused 2-methoxypyridine. Reductive cleavage of the activat...