Synthesis of δ- and α‑Carbolines via Nickel-Catalyzed [2 + 2 + 2] Cycloaddition of Functionalized Alkyne-Nitriles with Alkynes

A new method for the synthesis of
δ- and α-carbolines
through Ni-catalyzed [2 + 2 + 2] cycloaddition of ynamide-nitriles
or alkyne-cyanamides with alkynes has been developed. The catalytic
system of NiCl2(DME)/dppp/Zn with a low-cost Ni­(II)-precursor
was first utilized in Ni-catalyzed [2 + 2 + 2] cycloaddition reactions,
and the in situ generated Lewis acid may play an important role for
the successful transformation. Not only internal alkynes but also
terminal alkynes undergo the desired cycloaddition reactions efficiently
to furnish the carboline derivatives with wide diversity and functional
group tolerance.