Enantioselective Lewis Acid Catalysis of Intramolecular Enone [2＋2] Photocycloaddition Reactions

摘要

Asymmetric catalysis of photochemical cycloadditions has been limited by the challenge of suppressing the unselective background reaction. Here, we report that the high cross-section ππ* transition of 5,6-dihydro-4-pyridones, a versatile class of enone substrates, undergoes a ＞50 nanometer (nm) bathochromic absorption shift upon Lewis acid coordination. Based on this observation, enantioselective intramolecular [2＋2] photocycloaddition reactions (82 to 90% enantiomeric excess) were achieved with these substrates using 0.5 equivalents of a chiral Lewis acid upon irradiation at a wavelength of 366 nm. One of the products was applied as a key intermediate in the total synthesis of (＋)-lupinine and the formal synthesis of (＋)-thermopsine. Several enones show similar bathochromic shifts in the presence of a Lewis acid, indicating that chiral Lewis acid catalysis may be a general approach toward enantioselective enone [2＋2] photocycloadditions.