Chemoselective intramolecular annulation of 3-alkylindolines into dihydro or tetrahydrofuro[2,3-b]indoles

Abstract

3-Alkyl-2-hydroxyindolines, conveniently prepared from 2-hydroxyindolenines and a Grignard reagent, cyclize in the aprotic solvent tetrahydrofuran to afford tetrahydro-3-cyano-2-oxofuro[2,3-b]indoles, while in the protic solvent methanol the chemoselectivity changed to give dihydro-2-amino-3-carbomethoxyfuro[2,3-b]indoles. The steric effect of the alkyl group on the reactivity of 3-alkyl-2-hydroxyindolines is discussed for both processes. The ring transformation of tetrahydro-3-cyano-2-oxofuro[2,3-b]indoles into dihydro-2-amino-3-carbomethoxyfuro[2,3-b]indoles via γ-lactone imines is also discussed. Keywords: furo[2,3-b]indoles, α-cyano-γ-lactones, chemoselectivity, ring transformation, β-enamino esters.