Catalytic Asymmetric Allylic Substitution/Isomerization with Central Chirality Transposition

Asymmetric [3 + 3] cycloaddition of cinnamaldehyde-derived N-aryl nitrones with 2-indolemethanols enabled by chiral phosphoric acid

We described a chiral phosphoric acid (CPA) catalyzed asymmetric [3 + 3] cycloaddition of cinnamaldehyde-derived N-aryl nitrones with 2-indolylmethanols to prepare various indole-fused 1,2-oxazines in high yields (up to 96%) with excellent enantioselectivity (>99% ee). Control experiments indicate that hydrogen bonding plays important roles in controlling the enantioselectivity of products. This strategy provides an efficient pathway to construct enantioenriched indole-fused 1,2-oxazines from N-aryl nitrones with 2-indolylmethanols.

Stereospecific 3-Aza-Cope Rearrangement Interrupted Asymmetric Allylic Substitution-Isomerization

Transition-metal catalyzed asymmetric allylic substitution with alkyl and heteroaryl carbon nucleophiles has been well-established. However, the asymmetric allylic arylation of acyclic internal alkenes with aryl nucleophiles remains challenging and underdeveloped. Herein we report a stereospecific 3-aza-Cope rearrangement interrupted asymmetric allylic substitution-isomerization (Int-AASI) that enables asymmetric allylic arylation. By means of this stepwise strategy, both enantioenriched allylic arylation products and axially chiral alkenes could be readily obtained in high enantioselectivities. Experimental studies support a mechanism involving a cascade of asymmetric allylic amination, stereospecific 3-aza-Cope rearrangement and alkene isomerization. Density functional theory studies detailed the reasons of achieving the high chemoselectivity, regioselectivity, stereoselectivity and stereospecificity, respectively.

Synthesis of Alkene Atropisomers with Multiple Stereogenic Elements via Catalytic Asymmetric Rearrangement of 3-Indolylmethanols

Catalytic enantioselective preparation of alkene atropisomers with multiple stereogenic elements and discovery of their applications have become significant but challenging issues in the scientific community due to the unique structures of this class of atropisomers. We herein report the first catalytic atroposelective preparation of cyclopentenyl[b]indoles, a new kind of alkene atropisomers, with stereogenic point and axial chirality via an unusual rearrangement reaction of 3-indolylmethanols under asymmetric organocatalysis. Notably, this novel type of alkene atropisomers have promising applications in developing chiral ligands or organocatalysts, discovering antitumor drug candidates and fluorescence imaging materials. Moreover, the theoretical calculations have elucidated the possible reaction mechanism and the non-covalent interactions to control the enantioselectivity. This approach offers a new synthetic strategy for alkene atropisomers with multiple stereogenic elements, and represents the first catalytic enantioselective rearrangement reaction of 3-indolylmethanols, which will advance the chemistry of atropisomers and chiral indole chemistry.

Catalytic, Enantioselective Cycloaddition of Pyrrole-2-methides with Aldehydes toward a Synthesis of 2,3-Dihydro-1H-pyrrolizin-3-ols

An organocatalytic, highly enantioselective [6 + 2]-cycloaddition of 2-methide-2H-pyrroles with aryl acetaldehydes represents a novel and straightforward route toward densely substituted 2,3-dihydro-1H-pyrrolizin-3-ols, which were generated with good yields and high enantio- and diastereoselectivity. This one-step process involves a BINOL-phosphoric acid catalyzed reaction of 1H-pyrrole-2-carbinols with aryl acetaldehydes via the corresponding hydrogen-bonded, chiral 2-methide-2H-pyrroles.

Cooperative Catalysis-Enabled (4 + 3) Cycloaddition of 2-Indolylmethanols with In Situ-Generated ortho-Naphthoquinone Methides

A cooperative catalysis-enabled (4 + 3) cycloaddition of 2-indolylmethanols with ortho-naphthoquinone methides (o-NQMs), which were in situ-generated from enynones, has been established in the presence of silver/Brønsted acid cocatalysts. In the reaction pathway, the key o-NQM intermediates were formed through Ag(I)-catalyzed cyclization of enynones, while the indole-based carbocation intermediates were generated via Brønsted acid-catalyzed dehydration of 2-indolylmethanols. By this approach, a wide range of seven-membered cyclohepta[b]indoles were synthesized in good yields with high efficiency under mild reaction conditions, which serves as a useful strategy toward constructing indole-fused seven-membered rings. Moreover, the catalytic asymmetric version of this (4 + 3) cycloaddition has been realized under the cooperative catalysis of Ag(I) with chiral phosphoric acid, which offered chiral cyclohepta[b]indole with a good enantioselectivity (75% ee). This work not only represents the first cooperative catalysis-enabled (4 + 3) cycloaddition of 2-indolylmethanols but also provides a good example for o-NQM-involved cycloadditions, which will contribute to the chemistry of 2-indolylmethanols and enrich the research contents of cooperative catalysis.

The rapid construction and biological evaluation of densely substituted pyrrolo[1,2-a]indoles via a BF3·OEt2-assisted cascade approach

Lewis-acid cascade reactions promoted by BF3·OEt2 are reported for the synthesis of highly substituted pyrrolo[1,2-a]indoles and congeners of benzofuro[2,3-b]indoles. These reactions are highly regio- and diastereoselective towards generating up to five contiguous stereogenic centers, including two vicinal quaternary centers. Furthermore, an established cascade approach and the mechanism proposed herein are well supported by quantum chemistry calculations. In addition, a self-dimerization intermediate was trapped and isolated to establish a strategy for potential access to both pyrrolo and benzo indole derivatives, leaving sufficient freedom for broadening. Furthermore, in-silico molecular docking and all atomistic molecular dynamic (MD) simulation analysis suggests that the synthesized pyrrolo[1,2-a]indole derivatives stably bind at the active site of the mycobacterial secreted tyrosine phosphatase B (MptpB) enzyme, an emerging anti-mycobacterial drug target. Deep learning-based affinity predictions and MMPBGBSA-based energy calculations of the docked poses are presented herein.

Metal-Free C(sp3)-H Bond Arylation of 3-Methylindole Derivatives via 3-Indole Imine Methides

Direct arylation of the benzylic C(sp3)-H bond is one of the most straightforward strategies for the construction of multi-aryl methanes, owing to the extraordinary step and atom economy. In this paper, we developed the first metal-free arylation of the C(sp3)-H bond in 3-methylindoles, thereby providing rapid access to a range of diaryl- and triarylmethanes with two indole rings. Mechanistically, 3-indole imine methide serves as the key intermediate. Water plays a crucial role in this process, likely serving as a proton shuttle to facilitate the key 1,3-proton transfer step in this reaction and, thus, enhance the reaction efficiency.

Highly Enantioselective Synthesis of 3a-Fluorofuro[3,2-b]indolines via Organocatalytic Aza-Friedel-Crafts Reaction/Selective C-F Bond Activation

Fluoroalkylated compounds are of high interest in drug discovery and have inspired the evolution of diverse C-F bond activation methodologies. However, the selective activation of polyfluorinated compounds remains challenging. Herein, we describe an unprecedented strategy for synthesizing enantioenriched fluorofuro[3,2-b]indolines through the organocatalytic aza-Friedel-Crafts reaction coupled with selective C-F bond activation. These reactions feature excellent enantioselectivities (≤96% ee) and yields (≤96%) as well as good functional group compatibility. Mechanistic investigations by means of 19F nuclear magnetic resonance experiments provided sufficient support for silica gel as the key medium in this transformation.

One-pot synthesis of thioethers from indoles and p-quinone methides using thiourea as a sulfur source

Thiourea is an inexpensive and user friendly sulfur reagent that acts as a sulfur source. A simple and efficient protocol has been developed to access thioethers by reacting indoles with p-quinone methides using thiourea as the sulfur source. In our experiments, the reaction apparently proceeded through an S-(3-indolyl)isothiuronium iodide intermediate and subsequent generation of indolethiol that attacked the 1,6 position of p-quinone methides to give desired thioethers in good to excellent yields.

Bronsted Acid-Catalyzed Regioselective Carboxamidation of 2-Indolylmethanols with Isonitriles

A regioselective direct carboxamidation reaction of 2-indolylmethanols with readily available isocyanoesters/isocyanides has been reported in this work. The reaction was catalyzed by Bronsted acid such as p-TsOH to deliver the benzylic regioselective amides in 67-86% yield under mild conditions. The developed methodology provides alternative access to traditional metal-free carboxamidation via C-C and C-O bond formation with high atom economy. Furthermore, the developed approach was diversified to synthesize chiral indole-2-carboxamide derivatives with a moderate enantiomeric excess (61-73% ee) using an (R)-chiral phosphoric acid.

Catalytic asymmetric de novo construction of 4-pyrrolin-2-ones via intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones

The chiral phosphoric acid-catalyzed asymmetric intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones has been established. This convergent protocol leads to a facile and enantioselective de novo construction of a wide range of fully substituted 4-pyrrolin-2-ones bearing a fully substituted carbon atom in good yields and with excellent enantioselectivities (26 examples, 72-95% yields and 87-99% ee).

Catalytic Enantioselective Synthesis of 2,3'-Bis(indolyl)methanes Bearing All-Carbon Quaternary Stereocenters via 2-Indole Imine Methides

An organocatalytic enantioselective formal hydroarylation of 2-vinyl indoles for the preparation of enantioenriched 2,3'-bis(indolyl)methanes bearing an all-carbon quaternary stereocenter is described. This reaction features mild conditions, low catalyst loading, excellent efficiency and enantioselectivity. The obtained products showed promising anticancer activity.

Chiral phosphoric acid-catalyzed enantioselective aza-Friedel-Crafts reaction of naphthols and electron-rich phenols with 2-aryl-3H-indol-3-ones

The enantioselective aza-Friedel-Crafts reaction is one of the most straightforward and efficient strategies for constructing a new carbon-carbon bond bearing quaternary stereocenter in organic synthesis, but the catalytic asymmetric aza-Friedel-Crafts reaction of naphthols/phenols with cyclic-ketimines attached to a neutral functional group remains still relatively unexplored. Herein, a highly enantioselective aza-Friedel-Crafts reaction of cyclic-ketimines and naphthols/phenols has been realized using a chiral phosphoric acid catalyst. A variety of chiral aminonaphthols (chiral indolin-3-ones) containing a quaternary stereocenter at the C2 position were obtained with excellent outcomes (up to 97% yield, 98% ee). Moreover, the synthetic utility of the enantiomerically enriched chiral aminonaphthols was demonstrated in some efficient transformations. According to the experimental results, a possible transition state model has been proposed to rationalize the origin of asymmetric induction.

Insights into the regioselectivity and diastereoselectivity of the Nazarov cyclization of 3-alkenyl-2-indolylmethanol with tryptophol

Density functional theory calculations of Nazarov cyclizations showed that 3-alkenyl-2-indolylmethanol and tryptophol formed binary and ternary complexes, respectively, in the presence and absence of H2O to produce C–N and C–C products.