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.