Formation of Pyrroloindolines via the Alkylation of Tryptamines with Trichloroacetimidates

Efficient construction of functionalized pyrroloindolines through cascade radical cyclization/intermolecular coupling

Pyrroloindolines are important structural units in nature and the pharmaceutical industry, however, most approaches to such structures involve transition-metal or photoredox catalysts. Herein, we describe the first tandem SET/radical cyclization/intermolecular coupling between 2-azaallyl anions and indole acetamides. This method enables the transition-metal-free synthesis of C3a-substituted pyrroloindolines under mild and convenient conditions. The synthetic utility of this transformation is demonstrated by the construction of an array of C3a-methylamine pyrroloindolines with good functional group tolerance and yields. Gram-scale sequential one-pot synthesis and hydrolysis reactions demonstrate the potential synthetic utility and scalability of this approach.

Polycyclic pyrroloindoline-containing natural products with a unique 3-heptyl-2a,4a-diazapentaleno[1,6-ab]indene core isolated from Alstonia scholaris

Alscholarine C (1), featuring an unprecedented pyrroloindoline-containing natural product (PiNP) with a 6/5/5/5 tetracyclic carbon skeleton, and four known PiNPs (2-5), namely demethylalstoscholarinine E (2), Nb-demethylechitamine (3), winphylline A (4), and echitamine (5), were isolated from Alstonia scholaris. Compound 1 was characterized by a hexahydropyrrolo[2,3-b] indole (HPI) core fused to a unique 4-heptylimidazolidine motif, forming an unparalleled 3-heptyl-2a,4a-diazapentaleno[1,6-ab]indene ring system. Their structures were established by spectroscopic analysis, quantum-chemical calculated 13C NMR data with DP4+ probability analyses, and ECD calculations and comparison. A plausible biosynthetic pathway of 1 was proposed. Compound 1 exhibited potential anti-inflammatory activity against LPS-stimulated NO production in RAW264.7 cells.

Photoredox-catalyzed diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indole derivatives

Prenylated and reverse-prenylated indolines are privileged scaffolds in numerous naturally occurring indole alkaloids with a broad spectrum of important biological properties. Development of straightforward and stereoselective methods to enable the synthesis of structurally diverse prenylated and reverse-prenylated indoline derivatives is highly desirable and challenging. In this context, the most direct approaches to achieve this goal generally rely on transition-metal-catalyzed dearomative allylic alkylation of electron-rich indoles. However, the electron-deficient indoles are much less explored, probably due to their diminished nucleophilicity. Herein, a photoredox-catalyzed tandem Giese radical addition/Ireland-Claisen rearrangement is disclosed. Diastereoselective dearomative prenylation and reverse-prenylation of electron-deficient indoles proceed smoothly under mild conditions. An array of tertiary α-silylamines as radical precursors is readily incorporated in 2,3-disubstituted indolines with high functional compatibility and excellent diastereoselectivity (>20:1 d.r.). The corresponding transformations of the secondary α-silylamines provide the biologically important lactam-fused indolines in one-pot synthesis. Subsequently, a plausible photoredox pathway is proposed based on control experiments. The preliminary bioactivity study reveals a potential anticancer property of these structurally appealing indolines.

Biosynthesis of pyrroloindoline-containing natural products

Covering: up to 2022Pyrroloindoline is a privileged tricyclic indoline motif widely present in many biologically active and medicinally valuable natural products. Thus, understanding the biosynthesis of this molecule is critical for developing convenient synthetic routes, which is highly challenging for its chemical synthesis due to the presence of rich chiral centers in this molecule, especially the fully substituted chiral carbon center at the C3-position of its rigid tricyclic structure. In recent years, progress has been made in elucidating the biosynthetic pathways and enzymatic mechanisms of pyrroloindoline-containing natural products (PiNPs). This article reviews the main advances in the past few decades based on the different substitutions on the C3 position of PiNPs, especially the various key enzymatic mechanisms involved in the biosynthesis of different types of PiNPs.