Transition-Metal-Free Access to Pyridocarbazoles from 2‑Alkynylindole-3-carbaldehydes via Azomethine Ylide

An efficient approach for the synthesis
of functionalized tetrahydro-pyrido/quinolinocarbazoles
from 2-alkynylindole-3-carbaldehydes and l-proline utilizing
a metal-free decarboxylative cyclization, ring expansion, and ring
contraction strategy via the generation of azomethine ylide was developed.
The reaction of 2-alkynylindole-3-carbaldehydes with l-thioproline
leads to the formation of γ-carbolines. By virtue of this expedient
method, a diverse range of biologically active heteroannulated carbazoles
can be synthesized efficiently.