Enantioselective Construction of Functionalized Tetrahydrocarbazoles Enabled by Asymmetric Relay Catalysis of Gold Complex and Chiral Brønsted Acid

Iodophor-catalyzed sulfenylation of indoles with sulfonyl hydrazides for the synthesis of 3-sulfenylindoles

An iodophor-catalyzed sulfenylation of indoles using sulfonyl hydrazides as sulfur source to synthesize 3-sulfenylindoles in aqueous phase has been achieved. Notably, iodophor as catalyst and solvent is inexpensive, commercially available and no innocuous to the environment. The method is also easy to operate. Moreover, the synthetic strategy features a wide range of substrates with excellent tolerance to diverse functional groups. A plausible mechanism for the iodophor-mediated 3-sulfenylation of indoles with sulfonyl hydrazides has been proposed. In addition, 3-(phenylthio)-1H-indole was obtained on a multi-gram scale.

Enantioconvergent and diastereoselective synthesis of atropisomeric hydrazides bearing a cyclic quaternary stereocenter through ternary catalysis

An efficient and highly enantioconvergent and diastereoselective ternary catalysis in a one-pot process is reported, which represents an integrated strategy for the synthesis of atropisomeric hydrazides with defined vicinal central and axial chirality from readily available racemic α-amino-ynones, azodicarboxylates, and Morita-Baylis-Hillman (MBH) carbonates. This method utilizes in situ-generated racemic pyrrolin-4-ones via hydroamination of racemic α-amino-ynones by AuCl catalysis as a novel and versatile C1 synthon, which engage commercially available azodicarboxylates to generate amination products in high yields and uniformly excellent enantioselectivities under the catalysis of a chiral phosphoric acid. Following amination, N-alkylation catalyzed by diastereoselective organocatalyst afforded axially chiral hydrazides with excellent diastereoselectivities (>98 : 2 dr). The synthetic utility of the amination products and axially chiral hydrazides was also demonstrated by their facile conversion to diverse molecules in high yields with excellent stereopurity. Density functional theory calculations were performed to understand the origin of diastereoselectivity.

Diastereo-divergent synthesis of chiral hindered ethers via a synergistic calcium(II)/gold(I) catalyzed cascade hydration/1,4-addition reaction

Hindered ethers are ubiquitous in natural products and bioactive molecules. However, developing an efficient method for the stereocontrolled synthesis of all stereoisomers of chiral hindered ethers is highly desirable but challenging. Here we show a strategy that utilizes in situ-generated water as a nucleophile in an asymmetric cascade reaction involving two highly reactive intermediates, 3-furyl methyl cations and ortho-quinone methides (o-QMs), to synthesize chiral hindered ethers. The Ca(II)/Au(I) synergistic catalytic system enables the control of diastereoselectivity and enantioselectivity by selecting suitable chiral phosphine ligands in this cascade hydration/1,4-addition reaction, affording all four stereoisomers of a diverse range of chiral tetra-aryl substituted ethers with high diastereoselectivities (up to >20/1) and enantioselectivities (up to 95% ee). This work provides an example of chiral Ca(II)/Au(I) bimetallic catalytic system controlling two stereogenic centers via a cascade reaction in a single operation.

Enantioselective merged gold/organocatalysis

Gold complexes, because of their unique carbophilic nature, have evolved as efficient catalysts for catalyzing various functionalization reactions of C-C multiple bonds. However, the realization of enantioselective transformations via gold catalysis remains challenging due to the geometrical constraints and coordination behaviors of gold complexes. In this context, merged gold/organocatalysis has emerged as one of the intriguing strategies to achieve enantioselective transformations which could not be possible by using a single catalytic system. Historically, in 2009, this field started with the merging of gold with axially chiral Brønsted acids and chiral amines to achieve enantioselective transformations. Since then, based on the unique reactivity profiles offered by each catalyst, several reports utilizing gold in conjunction with various chiral organocatalysts such as amines, Brønsted acids, N-heterocyclic carbenes, hydrogen-bonding and phosphine catalysts have been documented in the literature. This article demonstrates an up-to-date development in this field, especially focusing on the mechanistic interplay of gold catalysts with chiral organocatalysts.

Rhodium-Catalyzed Enantioselective Addition of Heteroarenium Salts Enabled by Nucleophilic Cyclization of 2-Alkynylanilines

Transition-metal-catalyzed cyclative coupling of 2-alkynylanilines provides a feasible routine for accessing functionalized indoles. Herein, a rhodium-catalyzed highly enantioselective addition of heteroarenium salts is presented, which is enabled by the nucleophilic cyclization of 2-alkynylanilines. It offers feasible protocols to access enantioenriched functionalized indoles tethered to 1,2-dihydropyridine and 1,2-dihydroquinoline motifs with excellent enantioselectivities.

Catalyst-Controlled Diastereoselective Synthesis of Bridged [3.3.1] Bis(Indolyl)-Oxanes and Oxepanes via Desymmetrization of Bis(Indolyl)-Cyclohexadienones

A catalyst-controlled divergent synthesis of bridged [3.3.1] bis(indolyl)-oxanes and cis-[6.7] fused bis(indolyl) oxepanes via diastereoselective desymmetrization of bis(indolyl)-cyclohexadienones is presented for the first time. The reaction is highly atom- and step-economic, furnishing sp³-rich functionalized bis(indolyl) derivatives in good to excellent yields with wide substrate scope. The reaction proceeds through Friedel-Crafts alkylation followed by catalyst-controlled selective C-C bond formation/rearrangement. Gram scale synthesis and synthetic utility to generate bis(indolyl) alkaloid-like molecular diversity were also illustrated.

Synthesis and Antimicrobial Activity of the Pathogenic E. coli Strains of p-Quinols: Additive Effects of Copper-Catalyzed Addition of Aryl Boronic Acid to Benzoquinones

A mild and efficient protocol for the synthesis of p-quinols under aqueous conditions was developed. The pivotal role of additives in the copper-catalyzed addition of aryl boronic and heteroaryl boronic acids to benzoquinones was observed. It was found that polyvinylpyrrolidone (PVP) was the most efficient additive used for the studied reaction. The noteworthy advantages of this procedure include its broad substrate scope, high yields up to 91%, atom economy, and usage of readily available starting materials. Another benefit of this method is the reusability of the catalytic system up to four times. Further, the obtained p-quinols were characterized on the basis of their antimicrobial activities against E. coli. Antimicrobial activity was further compared with the corresponding 4-benzoquinones and 4-hydroquinones. Among tested compounds, seven derivatives showed an antimicrobial activity profile similar to that observed for commonly used antibiotics such as ciprofloxacin, bleomycin, and cloxacillin. In addition, the obtained p-quinols constitute a suitable platform for further modifications, allowing for a convenient change in their biological activity profile.

Catalytic Friedel-Crafts Alkylative Desymmetrization of Cyclohexa-2,5-dienones: Access to Linear and Bridged Polycyclic Pyrroles and 3-Arylpyrroles

A catalytic [3 + 2]-cycloaddition/Friedel-Crafts alkylative desymmetrization strategy has been developed for the stereoselective construction of linear and bridged polycyclic pyrroles from alkynylcyclohexa-2,5-dienones. This strategy was further explored for the synthesis of 3-arylpyrroles under Brønsted acid catalysis. Reaction is highly chemo-, regio-, and stereoselective and is compatible with wide range of functionalized cyclohexa-2,5-dienones/pyrroles (>51 examples, ≤98% yields). Gram-scale synthesis and synthetic utility of the products have also been demonstrated to showcase the robustness of present method.

Copper-catalyzed multicomponent reactions for the efficient synthesis of diverse spirotetrahydrocarbazoles

In the presence of copper sulfate, three- or four-component reactions of 2-methylindole, aromatic aldehydes and various cyclic dienophiles in refluxing toluene afforded diverse spirotetrahydrocarbazoles. This reaction is an important development of the Levy reaction by using 2-methylindole to replace ethyl indole-2-acetate and successfully provides facile access to important polysubstituted spiro[carbazole-3,3'-indolines], spiro[carbazole-2,3'-indolines], spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cycloalkanes] in satisfactory yields and with high diastereoselectivity.

Transition-Metal-Catalyzed Enantioselective Synthesis of Indoles from 2-Alkynylanilines

Optically active indole derivatives are ubiquitous in natural products and are widely recognized as privileged components in pharmacologically relevant compounds. Therefore, developing catalytic asymmetric approaches for constructing indole derivatives is highly desirable. In this short review, we summarize methods for the transition-metal-catalyzed enantioselective synthesis of indoles from 2-alkynyl­anilines.

1 Introduction

2 Aminometalation-Triggered Asymmetric Cross-Coupling Reactions/Insertion

2.1 Asymmetric Cross-Coupling Reactions

2.2 Asymmetric Insertion of C=O, C=C and C≡N Bonds

3 Asymmetric Relay Catalysis

4 Conclusion

Light-Mediated Polymerization Induced by Semiconducting Nanomaterials: State-of-the-Art and Future Perspectives

Direct capture of solar energy for chemical transformation via photocatalysis proves to be a cost-effective and energy-saving approach to construct organic compounds. With the recent growth in photosynthesis, photopolymerization has been established as a robust strategy for the production of specialty polymers with complex structures, precise molecular weight, and narrow dispersity. A key challenge in photopolymerization is the scarcity of effective photomediators (photoinitiators, photocatalysts, etc.) that can provide polymerization with high yield and well-defined polymer products. Current efforts on developing photomediators have mainly focused on organic dyes and metal complexes. On the other hand, nanomaterials (NMs), particularly semiconducting nanomaterials (SNMs), are suitable candidates for photochemical reactions due to their unique optical and electrical properties, such as high absorption coefficients, large charge diffusion lengths, and broad absorption spectra. This review provides a comprehensive insight into SNMs' photomediated polymerizations and highlights the roles SNMs play in photopolymerizations, types of polymerizations, applications in producing advanced materials, and the future directions.

Multicomponent Reaction for Diastereoselective Synthesis of Spiro[carbazole-3,4'-pyrazoles] and Spiro[carbazole-3,4'-thiazoles]

In the presence of copper sulfate, the three-component reaction of aromatic aldehydes, ethylindole-3-acetate and 4-arylidene-5-methyl-2-phenylpyrazol-3-ones, in refluxing toluene afforded spiro[carbazole-3,4'-pyrazoles] in good yields with high diastereoselectivity. More importantly, the similar CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes with ethylindole-3-acetate and 5-methyl-2-phenyl-pyrazol-3-one resulted in 2,4-diarylspiro[carbazole-3,4'-pyrazoles] in satisfactory yields. Additionally, CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes, ethylindole-3-acetate and 2-phenylthiazol-4-one, in refluxing toluene gave 2,4-diarylspiro[carbazole-3,4'-thiazoles] with diastereomeric ratios in the range of 3:1 to 20:1.

Asymmetric desymmetrization of alkene-, alkyne- and allene-tethered cyclohexadienones using transition metal catalysis

This review is covering the recent development of catalytic asymmetric domino reactions for the desymmetrization of alkene-, alkyne- and allene-tethered cyclohexadienones using transition metals and chiral ligands.

Recent advances in the asymmetric transformations of achiral cyclohexadienones

This review describes recent developments in the asymmetric transformations of achiral cyclohexadienones, including enantioselective desymmetrization of prochiral cyclohexadienones and kinetic resolution of racemic cyclohexadienones.

Development of a PET radioligand selective for cerebral amyloid angiopathy

INTRODUCTION

Positron emission tomography (PET) using radiolabeled amyloid-binding compounds has advanced the field of Alzheimer's disease (AD) by enabling detection and longitudinal tracking of fibrillar amyloid-β (Aβ) deposits in living people. However, this technique cannot distinguish between Aβ deposits in brain parenchyma (amyloid plaques) from those in blood vessels (cerebral amyloid angiopathy, CAA). Development of a PET radioligand capable of selectively detecting CAA would help clarify its contribution to global brain amyloidosis and clinical symptoms in AD and would help to characterize side-effects of anti-Aβ immunotherapies in AD patients, such as CAA.

METHODS

A candidate CAA-selective compound (1) from a panel of analogues of the amyloid-binding dye Congo red was synthesized. The binding affinity to Aβ fibrils and lipophilicity of compound 1 were determined and selectivity for CAA versus parenchymal plaque deposits was assessed ex-vivo and in-vivo in transgenic APP/PS1 mice and in postmortem human brain affected with AD pathology.

RESULTS

Compound 1 displays characteristics of Aβ binding dyes, such as thioflavin-S, in that it labels both parenchymal Aβ plaques and CAA when applied to histological sections from both a transgenic APP/PS1 mouse model of Aβ amyloidosis and AD brain. Thus, compound 1 lacks molecular selectivity to distinguish Aβ deposits in CAA from those in plaques. However, when administered to living APP/PS1 mice intravenously, compound 1 preferentially labels CAA when assessed using in-vivo two-photon microscopy and ex-vivo histology and autoradiography.

CONCLUSION

We hypothesize that selectivity of compound 1 for CAA is attributable to its limited penetration of the blood-brain barrier due to the highly polar nature of the carboxylate moiety, thereby limiting access to parenchymal plaques and promoting selective in-vivo labeling of Aβ deposits in the vascular wall (i.e., "delivery selectivity").

Hydrogen-Bond-Guided Reaction of Cyclohexadienone-aldehydes with Amines: Synthesis of an Aminal Group Containing a Fused Tetracyclic Framework

A modular approach has been developed for an efficient synthesis of an aminal group containing a new tetracyclic framework. The strategy has been devised based on selective hydrogen-bond-guided aza-Michael addition of heteroaromatic amines to cyclohexadienone-aldehydes. The reaction is highly atom economic and practical and involves stereoselective construction of four new C-N bonds and four rings. The synthetic utility of the tetracyclic product was explored. The role of a H-bond was explained with the help of experimental and density functional theory (DFT) computation studies.

Cocatalyst-controlled divergent cascade cycloaddition reaction of arylalkynols and dioxopyrrolidienes: access to spiroketals and oxa-bridged eight-membered cyclic ethers

A cocatalyst-controlled divergent cascade cycloaddition reaction was developed for the synthesis of two different complex oxygen-containing heterocyclic compounds from arylalkynols and dioxopyrrolidienes in the presence of Au(i) catalyst.

Diastereoselective construction of carbazole-based spirooxindoles via the Levy three-component reaction

The CuSO₄ catalyzed three-component reaction of indole-2-acetate, aromatic aldehydes and 3-methyleneoxindoles in toluene at 130 °C afforded polysubstituted spiro[carbazole-3,3′-indolines] in good yields and with high diastereoselectivity.

Gold-catalyzed domino cyclization to diverse polyheterocyclic frameworks: mechanism, origin of the cooperative hydrogen bond, and role of π-stacking interactions

The detailed mechanism and origins of gold-catalyzed domino cyclization to diverse fused polyheterocyclic frameworks by cooperative catalysis and cascade catalysis were studied systematically.

Gold-catalyzed domino cyclization enabling construction of diverse fused azaspiro tetracyclic scaffolds: a cascade catalysis mechanism due to a substrate and counterion

The detailed mechanism and origins of gold-catalyzed domino cyclization to diverse fused azaspiro tetracyclic scaffolds by cooperative dual catalysis and cascade catalysis are systematically studied.

Catalytic asymmetric synthesis of tetrahydrocarbazoles

This feature article summarises recent advances in catalytic asymmetric synthesis of THCs, with emphases on reaction type and reaction mechanism.

Ir/PTC cooperatively catalyzed asymmetric umpolung allylation of α-imino ester enabled synthesis of α-quaternary amino acid derivatives bearing two vicinal stereocenters

A novel Ir/PTC (phase-transfer catalyst) cooperatively catalyzed asymmetric umpolung allylation of simple α-imino esters is developed and it provides facile access to α-quaternary amino acid derivatives bearing two vicinal stereocenters. Both the metal catalyst and the phase-transfer catalyst are crucial for this methodology. The reaction features good yields, high diastereo- and enantio-selectivities, ease of scale-up to gram scale and further transformation of the products (e.g. to quaternary proline analogues with multi stereocenters).