WFIGR ID=jo0700575n00001> |
Diene substituent effects on the regiochemical and stereochemical outcomes of uncatalyzed Diels-Alderreactions of
N-alkoxycarbonyl-1,2-dihydropyridines
with both styrene and methyl vinyl ketone (MVK)
were studied. Alkyl substitution on the diene in all cases examined resulted in a kinetic preference for7-endo isomers (7-phenyl 51-96% exo and 7-acetyl 54-96% exo). For both dienophiles, the higheststereoselectivities (
89% endo)
were observed
with 5-methyl or 6-methyl substituents in the dihydropyridine. Theoretical calculations of the energies of gas phase endo and exo transition states at the RHF/3-21G(*) predict that total entropy,
Stotal, considerations favor endo cycloadducts for both dienophiles
with DHP,
while total energy considerations,
Eo, favor endo cycloadducts for styrene and exocycloadducts for MVK. At this level, favored
endo-phenyl isomers are correctly predicted for styrenereactions, but the calculation of 7-acetyl exo or endo isomer dominance is diene-substituent-dependentfor MVK reactions. The preference for endo addition of MVK to the parent, 5-methyl, and 6-methyl-DHPs
was successfully predicted by calculations at the B3LYP/6-31G* theory level.