Intramolecular Cyclization of 2‐Alkynylphenylcarbonyls With a Pendant Double Bond under Copper Catalysis: A General Approach to Norabietane Core Structure

Synthetic routes leading to [6,6,6]‐tricyclic compounds were reported using metal catalysts from ene‐yne‐aldehydes in our group. Based on this result, the synthesis of various [6,6,6]‐tricyclic compounds is reported using transition metal‐catalyzed cyclization from enyne‐carbonyl substrates containing H, Me, Ph, OMe, CO2Me, and CO2i‐Pr groups. Tricyclic compounds containing the CO2Me group would be the prospective precursors of the natural product Lepenine. Synthesis of [6,6,6]‐tricyclic compounds using copper‐catalyzed cycloisomerization is reported starting from 2‐alkynylphenylcarbonyls containing H, Me, Ph, OMe, CO2Me, and CO2i‐Pr groups. The tricyclic compounds containing CO2Me group are building blocks for the synthesis of lepenine's prospective precursor belonging to the denudatine alkaloids and Norabietane core skeleton compounds.