T. Suishu et al., SINGLET AND TRIPLET PHOTOCYCLOADDITION REACTIONS OF 2-PYRIDONES WITH PROPENOATE AND 2,4-PENTADIENOTES, AND THE FRONTIER MOLECULAR-ORBITAL ANALYSIS, Journal of heterocyclic chemistry, 34(3), 1997, pp. 1005-1011
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.