Double Palladium Catalyzed Reductive Cyclizations. Synthesis of 2,2′‑, 2,3′‑, and 3,3′-Bi‑1<i>H</i>‑indoles, Indolo[3,2‑<i>b</i>]indoles, and Indolo[2,3‑<i>b</i>]indoles

A palladium catalyzed, carbon monoxide mediated, double reductive cyclization of 1,4-, 1,3-, and 2,3-bis­(2-nitroaryl)-1,3-buta­dienes to afford 2,2′-, 2,3′-, and 3,3′-biindoles, respectively, was developed. In contrast, reductive cyclizations of 1,2-bis­(2-nitro­aryl)­ethenes were nonselective, affording mixtures of monocyclized indoles, indolo­[3,2-<i>b</i>]­indole, indolo­[1,2-<i>c</i>]­quinazolin-6­(5<i>H</i>)-ones, and 5,11-dihydro-6<i>H</i>-indolo­[3,2-<i>c</i>]­quinolin-6-ones. Nonselective product formation was also observed from reductive cyclization of 1,1-bis­(2-nitro­aryl)­ethenes, producing indolo­[2,3-<i>b</i>]­indoles and indolo­[2,3-<i>c</i>]­quinolin-6-ones. Carbon monoxide insertion to give the carbonyl containing products was the major or sole reaction path starting from 1,1- or 1,2-bis­(2-nitro­aryl)­ethenes

Total Synthesis of the Tetracyclic Indole Alkaloid Ht-13‑B

An expedient synthesis corroborating the proposed structure of the tetracyclic indole alkaloid ht-13-B is presented. Key synthetic steps include acyliminium ion allylation, a Mitsunobu reaction, a palladium-catalyzed Stille–Kelly cross coupling reaction, and a carbon monoxide-mediated palladium-catalyzed reductive <i>N</i>-heterocyclization. The chiral centers are ultimately derived from commercially available <i>trans</i>-4-hydroxy-l-proline