Diels–Alder reactions of pyrano[3,4-b]indol-3-ones and a 2-benzopyran-3-one with hetero substituted olefins: generation of carbazole and naphthalene derivatives by elimination instead of dehydrogenation

Abstract

Pyrano[3,4-b]indol-3-ones are shown to undergo cycloaddition with electron-rich olefins. However, after loss of CO₂ from the adducts, the intermediate indole-2,3-quindodimethanes are not dehydrogenated—after a 1,5-sigmatropic hydrogen shift—to yield the expected heterosubstituted carbazoles. Rather, aromatisation arising from elimination of the hetero substituent takes place and other carbazoles are formed. In the same way, a hydroxyethylated naphthalenic compound was obtained from the adduct of 6,7-dimethoxy-1-methyl-2-benzopyran-3-one with 3,4-dihydrofuran. The hetero substituent is only conserved in cycloadditions of enamides with pyrano[3,4-b]indol-3-ones