SINGLET AND TRIPLET PHOTOCYCLOADDITION REACTIONS OF 2-PYRIDONES WITH PROPENOATE AND 2,4-PENTADIENOTES, AND THE FRONTIER MOLECULAR-ORBITAL ANALYSIS

Regioselective photocycloadditions of a-pyridones (1) with 2,4-pentadi enoates (3) were analysed and compared with the reactions with propeno ate (2), and origins of the different regioselectivities were inferred from frontier molecular orbital properties by the use of PM3-CI metho d. Direct photoreactions of 1 with 3 being alpha,beta:gamma,delta-unsa turated carboxylates gave four types of regioselective [2+2]cycloadduc ts, 4-alpha-4,5,3-delta:4-gamma-6,7,5-beta:6-alpha-8,9 and 5-delta:6-g amma-[2+2]cycloadducts 10,11. Sensitized photoreactions of 1 with 3 di d not occur, and fluorescence of 1 was weakly quenched by 3. Otherwise , main cycloadducts in the sensitized reactions of 1 with 2 have been obtained as 5-alpha:6-beta-[2+2]cycloadducts, which were different fro m main 3-beta:4-alpha-[2+2]cycloadducts of the direct reactions, and t hese site-and regio-selectivities were inferred from the different fro ntier molecular orbital coefficients at the 3-and 6-positions of 1 for the triplet and singlet states. Formations of 4-7 were also interpret ed by both effects of frontier orbital HSOMO-LUMO interactions and of the electrostatic interaction between 1 and 3, The other site-and regi oselective adducts 8-11 were inferred to be formed by the electrostati c interactions similar to 2-pyridone photodimerization.