Enantioselective photochemical reactions of 2-pyridones in solution

reaction: see text] Two key photochemical reactions of prochiral 2-pyridones were studied in the presence of a chiral host. The 4 + 4]-photocycloaddition with cyclopentadiene (CpH) proceeded smoothly and with high enantioselectivity (84-87% ee). The absolute configuration of the endo-diastereoisomer was established by X-ray crystallography. The electrocyclic 4pi]-ring closure to 3-oxo-2-azabicyclo2.2.0]-5-hexenes occurred with lower enantioselectivity (20-23% ee at -20 degrees C). The velocity of the latter reaction slowed significantly with decreasing temperature.