Manipulation of Spiroindolenine Intermediates for Enantioselective Synthesis of 3‐(Indol‐3‐yl)‐Pyrrolidines

Organocatalytic enantioselective reactions involving prochiral carbocationic intermediates

Since the discovery of carbocations in 1901, the past 120 years have witnessed many marvelous advances in the chemistry of carbocations. The state-of-the-art research in this field is to overcome the intrinsic instability and high reactivity of the prochiral carbocationic intermediates to develop catalytic asymmetric reactions. Such transformations enable the facile synthesis of structurally diverse value-added products from readily available starting materials such as alkenes, alcohols, and carbonyl derivatives, and enjoy high and even perfect atom-economy in most cases. Notably, such allows catalytic stereoconvergent synthesis from racemic substrates and can realize regioselectivity in olefin functionalization reactions complementary to radical processes. With the rapid developments in modern asymmetric organocatalysis, a variety of highly enantioselective protocols evolving prochiral carbocationic intermediates have been achieved by employing three strategies, namely chiral ion-pairing, chiral nucleophile, or chiral carbenium ion strategy. This feature article aims to summarize the exciting advances in this emerging area and briefly showcase the possible mechanisms. The advantages and limitations of each strategy are presented as well as their synthetic applications in the synthesis of natural products or bioactive compounds.

Rhodium-Catalyzed Stereoselective Cyclization of 3-Allenylindoles and N-Allenyltryptamines to Functionalized Vinylic Spiroindolenines

Herein, we report a highly enantio- and diastereoselective rhodium-catalyzed cyclization of N-allenyltryptamines and 3-allenylindoles to 6-membered spirocyclic indolenines. This allylic addition methodology offers the advantage of using a comparably cheap commercially available ligand with low loadings of an affordable rhodium precursor. The products can be converted into functionalized spirooxindoles and spiroindolines, which are regarded as important building blocks for the synthesis of a lot of natural products with biological activities.

Exploring the Chemistry of Spiroindolenines by Mechanistically-Driven Reaction Development: Asymmetric Pictet-Spengler-type Reactions and Beyond

The Pictet-Spengler reaction is a fundamental named reaction in organic chemistry, and it is the most straightforward method for the synthesis of tetrahydro-β-carbolines, a core structure embedded in numerous alkaloids. Spiroindolenines are often proposed as possible intermediates in Pictet-Spengler reactions. However, whether the spiroindolenine species is an intermediate in the mechanism of the asymmetric Pictet-Spengler reaction remains unclear. Questions about the role of the spiroindolenine species regarding the mechanism include the following: Can the spiroindolenine species be formed effectively under Pictet-Spengler conditions? If so, what is its fate? Is the delivery of the enantioenriched tetrahydro-β-carboline product related to the spiroindolenine intermediate? Previous studies regarding these questions have not reached a consensus. Therefore, elucidating these questions will advance the field of synthetic organic chemistry.The first highly enantioselective synthesis of spiroindolenines that have the same molecular scaffold as the proposed key intermediate of the Pictet-Spengler reaction was accomplished by an Ir-catalyzed intramolecular asymmetric allylic substitution reaction of an indol-3-yl allylic carbonate. In this reaction, a piperidine, pyrrolidine, or cyclopentane ring can be introduced in conjunction with the indolenine structure.Spiroindolenines were found to undergo ring-expansive migration reactions when treated with a catalytic amount of an acid, leading to tetrahydro-β-carbolines or related tetrahydrocarbazoles. Comprehensive DFT calculations and Born-Oppenheimer molecular dynamics simulations have provided insight into the mechanism of the migration process. It has been found that the stereochemistry is strongly correlated with the electronic properties of the migratory group along with the acidity of the catalyst. Close interactions between the positively charged migratory group and the electron-rich indole ring favor the stereospecificity of the migration. Furthermore, a continuous mechanistic spectrum of the Pictet-Spengler reactions can be obtained on the basis of two readily accessible energetic parameters that are derived from computed energies for competing transition states relative to a key intermediate species. This theoretical model provides a unified mechanistic understanding of the asymmetric Pictet-Spengler reaction, which has been further supported by rationally designed prototype reactions. Chemically and stereochemically controllable migration can be achieved when multiple potential migratory groups are available.The reactivity of spiroindolenines has also been explored beyond Pictet-Spengler reactions. A one-pot Ir-catalyzed asymmetric allylic dearomatization/stereoconvergent migration allows the facile synthesis of enantioenriched tetrahydro-β-carbolines from racemic starting materials. An unprecedented six- to seven-membered ring-expansive migration can be achieved when a vinyliminium moiety is involved as a highly reactive migratory group. This reaction facilitates the stereoselective synthesis of thermodynamically challenging indole-annulated seven-membered rings. It has also been found that the migration process can be interrupted. The electrophilic migratory group released from the retro-Mannich reaction of a spiroindolenine can be captured by an inter- or intramolecular nucleophile, thus providing new entries into structurally diverse polycyclic indole derivatives.Therefore, the mechanism of the Pictet-Spengler reaction can be probed by manipulating the reactivity of the spiroindolenine species. In turn, the mechanistic insights gained herein will aid in chemical transformations toward various target molecules. This study serves as a vivid example of the positive interplay between experimental and theoretical investigations in synthetic organic chemistry.

Catalyst-controlled formal [4 + 1] annulation of N-vinyl fluorenone nitrones and allenoates to prepare spirofluorenylpyrrolines

A simple gimeracil-controlled formal [4 + 1] annulation was developed to prepare various spirofluorenylpyrrolines in good yields with high diastereoselectivity from N-vinyl fluorenone nitrones and allenoates.

Chiral phosphoric acid-catalyzed asymmetric dearomatization reactions

We summarize in this review the recent development of chiral phosphoric acid (CPA)-catalyzed asymmetric dearomatization reactions.

Copper-catalyzed nitrene transfer/cyclization cascade to synthesize 3a-nitrogenous furoindolines and pyrroloindolines

Copper-catalyzed nitrene transfer for amination/cyclization of tryptophols and tryptamines to generate the corresponding indole alkaloids in good to excellent yields was successfully developed.