Novel chemistry of indole in the synthesis of heterocycles
Indoles that are substituted at the 2- or 3-position with electron-withdrawing groups (nitro, phenylsulfonyl) undergo nucleophilic addition, 1,3-dipolar cycloaddition, and Diels–Alder reactions to give a variety of indoles, pyrroloindoles, and carbazoles. New methods for the synthesis of furo[3,4-b]indoles and the novel ring system furo[3,4-b]pyrrole are described for the first time. Diels–Alder reactions of furo[3,4-b]pyrroles afford indoles after dehydration of the primary cycloadducts. Efficient syntheses of both 2- and 3-nitroindoles from indole are reported, and the first generation and successful electrophilic trapping of a 2,3-dilithioindole has been achieved.