Study on the synthesis of the cyclopenta[f]indole core of raputindole A

Gold(I)-Catalyzed Synthesis of Benzofurans from Tetrahydropyranyl (THP) Ethers of o-Alkynylphenols

The activation of π-systems in alkenes and alkynes by homogeneous gold catalysts is a powerful tool for the synthesis of benzo-fused heterocycles. Decades of development in the area of gold catalysis have brought an array of new catalysts and with them the discovery of new modes of carbophilic activation of π-systems in alkenes and alkynes, and investigations of previously unknown mechanistic pathways. In alignment with these efforts, we report a cyclization of tetrahydropyranyl ethers of 2-alkynylphenols to form benzofurans catalyzed by a gold(I)-NHC complex. The reaction proceeds efficiently at very low catalyst loadings, and the products form in good yields.

Total Synthesis of (+)-Raputindole A: An Iridium-Catalyzed Cyclization Approach

This work describes the total synthesis of raputindole A (1) through a convergent approach that features (1) an iridium-catalyzed cyclization to assemble the tricyclic core of the northern part, (2) enzymatic resolution to secure the preparation of an enantiomerically pure benzylic alcohol intermediate, and (3) the installation of the isobutenyl side chain via methallylation of the corresponding benzylic carbocation and coupling of the northern and southern parts via the Heck reaction. (+)-Raputindole A (1) was prepared in 10 steps (longest linear sequence) in 3.3% overall yield.

Total Synthesis and Absolute Configuration of Raputindole A

The first total synthesis of the bisindole alkaloid raputindole A from the rutaceous plant Raputia simulans is reported. The key step is a Au(I)-catalyzed cyclization that assembles the cyclopenta[f]indole tricycle from a 6-alkynylated indoline precursor. The isobutenyl side chain was installed by Suzuki-Miyaura cross-coupling, followed by a regioselective reduction employing LiDBB. Starting from 6-iodoindole, the sequence needs nine steps and provided (±)-raputindole A in 6.6% overall yield. The absolute configuration of the natural product (+)-raputindole A was determined by quantum chemical calculation of the ECD spectrum.