4-aza-2,7-dimethyl-[1-H]cyclopropa[b]-naphthalene

Synthesis and reactions of a stable o-quinoid 10-pi-electron system, Furo

Methyl 4,6-dichloro-3-(diethylamino)furo[3, 4-c]pyridine-1-carboxylate (6), an intermediate in the Hamaguchi-Ibata reaction involving the Rh(II)-catalyzed intramolecular reaction of a diazo group with the carbonyl of an adjacent amido group, has been isolated and characterized. PM3 calculations reveal the heat of formation (DeltaH(f)) of this remarkably stable molecule to be -77.7 kcal/mol. Compound 6 undergoes a facile Diels-Alder cycloaddition with a variety of dienophiles to give polysubstituted isoquinoline derivatives via ring opening of initially formed cycloadducts. In each case the cycloaddition proceeds with high regioselectivity, with the electron-withdrawing group located ortho to the amino group. The most favorable FMO interaction is between the HOMO of the azaisobenzofuran 6 and the LUMO of the dienophile. The atomic coefficient at the ester carbon of the azaisobenzofuran 6 is larger than the amino center, and this nicely accommodates the observed regioselectivity.