Annulation of a Highly Functionalized Diazo Building Block with Indoles under Sc(OTf)₃/Rh₂(OAc)₄ Multicatalysis through Michael Addition/Cyclization Sequence

A highly functionalized and easily accessible six-carbon diazo building block has been developed and utilized as a 1,4-diacceptor for an efficient synthesis of functionalized tetrahydrocarbazoles, carbazoles, and tetrahydropyrido[1,2- a]indoles. The synthesis involves concurrent tandem catalysis by Sc(OTf)3 and Rh2(OAc)4. The role of Sc(OTf)3 is critical as it facilitates both the initial intermolecular Michael reaction of the indole and the subsequent Rh(II)-catalyzed intramolecular annulation. The products, tetrahydrocarbazoles and tetrahydropyridoindoles, are equipped with a β-ketoester and ester functionalities which can be utilized for further synthetic elaborations.