A photochemical route to 4-alkyltropones including nezukone

Abstract

An acetonitrile solution of acetylene and 3-methylcyclopent-2-enone (4) was irradiated with Pyrex-filtered light from a mercury lamp to give 5-methylbicyclo[3.2.0]hept-6-en-2-one (6) and 6-methyltricyclo[4.1.0.0^2,7]heptan-3-one (8). These were easily separated and when heated under reflux in Bu^tOH with SeO₂ gave the 4-methyltropone valence-tautomer 5-methylbicyclo[3.2.0]hept-3,6-dien-2-one (10) and 4-methyltropone (12), respectively, the latter in 20% overall, unoptimized yield. Similar reactions with 3-isopropylcyclopent-2-enone (5) in the place of (4) led to the natural troponoid 4-isopropyltropone (nezukone)(13).