A Complementary Brønsted–Lewis Acid Sequence for the Synthesis of 1‐Alkenylindoles From 2‐Alkynylanilines
Sukit Chonradeenitchakul, Kunlayanee Punjajom, Jumreang Tummatorn, Somsak Ruchirawat, Charnsak ThongsornkleebABSTRACT
We report a complementary one‐pot protocol for the synthesis of 1‐alkenylindoles from 2‐alkynylanilines and ethyl acetoacetate through an N ‐vinylogous carbamate intermediate. The sequence involves CSA‐promoted condensation followed by PtCl 2 ‐mediated 5‐ endo ‐dig cyclization in DCM/HFIP. The method provides access to a range of N ‐alkenylindoles in moderate yields under mild conditions, with products generally obtained as mixtures of ( E )/( Z ) isomers. Competitive formation of the corresponding NH‐indoles was observed for several substrates, indicating that direct nitrogen–alkyne cyclization competes with the desired condensation–cyclization pathway. Compared with previously reported InBr 3 ‐catalyzed tandem cyclization, this study provides a complementary Brønsted–Lewis acid manifold and defines the substrate‐dependent limitations of this approach.