Photocatalytic enantioselective Minisci reaction of β-carbolines and application to natural product synthesis

A highly efficient enantioselective direct C-H functionalization of β-carbolines via a Minisci-type radical process under a photo-redox and chiral phosphoric acid cooperative catalytic system has been disclosed. Through this protocol, a wide range of C1 aminoalkylated β-carbolines were constructed directly with high levels of enantioselectivities from readily available β-carbolines and alanine-derived redox-active esters. This transformation allows straightforward access to highly valuable enantioenriched β-carbolines, which are an intriguing structural motif in valuable natural products and synthetic bio-active compounds. This protocol has been utilized as a highly efficient synthetic strategy for the concise asymmetric total synthesis of marine alkaloids eudistomin X, (+)-eudistomidin B and (+)-eudistomidin I.

Synthesis of Natural Products by C-H Functionalization of Heterocycless

Total synthesis is considered by many as the finest combination of art and science. During the last decades, several concepts were proposed for achieving the perfect vision of total synthesis, such as atom economy, step economy, or redox economy. In this context, C-H functionalization represents the most powerful platform that has emerged in the last years, empowering rapid synthesis of complex natural products and enabling diversification of bioactive scaffolds based on natural product architectures. In this review, we present an overview of the recent strategies towards the total synthesis of heterocyclic natural products enabled by C-H functionalization. Heterocycles represent the most common motifs in drug discovery and marketed drugs. The implementation of C-H functionalization of heterocycles enables novel tactics in the construction of core architectures, but also changes the logic design of retrosynthetic strategies and permits access to natural product scaffolds with novel and enhanced biological activities.

The Rich Legacy and Bright Future of Transition-Metal Catalyzed Peroxide Based Radical Reactions

This personal account is mainly focused on the author's involvement in the field of transition metal-catalyzed peroxide based radical reactions. Over the past decades, radical chemistry has flourished and become crucial in contemporary synthetic organic chemistry. Owing to the presence of a single electron in one orbital, radicals are very unstable and react very fast. To carry out desired transformations and to control the side reactions the stabilizations of these radicals is essential. Fortunately, the implementation of a suitable transition metal and peroxide combination into the radical reactions have proved beneficial. Transition metals not only stabilizes the radicals but also protects them from being quenched by undesired homo-coupling or fragmentation. Transition metal-catalyzed radical-radical reactions provide an innovative way for the construction and derivatization of carbocycles and heterocycles. The objective of this review is to give an overview of the construction and derivatization of heterocycles through the lens of radical chemistry, mainly focusing on research work done by our group.

Metal-, Photocatalyst-, and Light-Free Minisci C-H Acetylation of N-Heteroarenes with Vinyl Ethers

Herein, we report a mild, operationally simple method for Minisci C-H acetylation of N-heteroarenes using vinyl ethers as robust, inexpensive acetyl sources. The reactions do not require a conventional photocatalysis, electrocatalysis, metal catalysis, light activation, or high temperature. This method is thus significantly more sustainable than previously reported methods in terms of cost, reagent toxicity, and waste generation. This protocol can be expected to obtain medically relevant molecules from abundant feedstock materials.

Cobalt-Catalyzed Aerobic Oxidative Cleavage of Alkyl Aldehydes: Synthesis of Ketones, Esters, Amides, and α-Ketoamides

A widely applicable approach was developed to synthesize ketones, esters, amides via the oxidative C-C bond cleavage of readily available alkyl aldehydes. Green and abundant molecular oxygen (O₂ ) was used as the oxidant, and base metals (cobalt and copper) were used as the catalysts. This strategy can be extended to the one-pot synthesis of ketones from primary alcohols and α-ketoamides from aldehydes.

Photo-redox catalyzed dehydrazinative acylation of N-heterocycles via Minisci reaction

Visible light-induced acylation of heteroaromatic compounds have been achieved using benzoyl hydrazides as an efficient acyl source under mild reaction conditions. The photo-redox catalyzed oxidative cleavage of hydrazides leads to in situ formation of acyl radicals, which subsequently couple with various N-heterocycles to produce acylated products. This synthetic strategy performs the classic Minisci reaction in an eco-friendly and greener way with functional group tolerance and regioselectivity. Control experiments confirm the radical pathway for this transformation.

Visible light-induced acylation of heteroaromatic compounds have been achieved using benzoyl hydrazides as an efficient acyl source under mild reaction conditions.

Minisci aroylation of N-heterocycles using choline persulfate in water under mild conditions

A modified Minisci aroylation of isoquinoline using arylglyoxylic acids in the presence of choline persulfate or pre-composition (choline acetate and K2S2O8) in water at ambient temperature affording aroylated isoquinolines is reported.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.

Aldehydes: magnificent acyl equivalents for direct acylation

From the viewpoint of meeting the current green chemistry challenges in chemical synthesis, there is a need to disseminate how the cocktail of acylation and activation can play a pivotal role in affording bioactive acylated products comprising substituted ketone motifs in fewer reaction steps, with higher atom-economy and improved selectivity. In recent years, a significant number of articles employing the title compounds "aldehydes" as magnificent acylation surrogates which are less toxic and widely applicable have been published. This review sheds light on the compounds use for selective acylation of arene, heteroarene and alkyl (sp3, sp2 and sp) C-H bonds by proficient utilization of the C-H activation strategy. Critical insights into selective acylation of diverse moieties for the synthesis of bioactive compounds are presented in this review that will enable academic and industrial researchers to understand the mechanistic aspects involved and fruitfully employ these strategies in designing novel molecules.

Mechanism and Origin of MAD-Induced Ni/N-Heterocyclic Carbene-Catalyzed Regio- and Enantioselective C-H Cyclization of Pyridines with Alkenes

This work presents a DFT-based computational study on the regio- and enantioselective C-H functionalization of pyridines with alkenes at the relatively unreactive C4-position, which was successfully achieved by Shi et al. [J. Am. Chem. Soc. 2019, 141, 5628-5634] using Ni⁰ /N-heterocyclic carbene (NHC) catalysis under the assistance of an aluminum-based Lewis acid additive (2,6-tBu₂ -4-Me-C₆ H₂ O)₂ AlMe (MAD). The calculations indicate that the selective functionalization involves a three-step mechanism in which a unique H-migration assisted oxidation metalation (HMAOM) step is identified as the rate- and enantioselectivity-determining step. The newly proposed mechanism can well rationalize the experimental observation that the preferred product is the endo-type (vs. exo-type), R-configuration (vs. S-configuration) product at the C4 (vs. C2) position, and also unveil the reasons that the NHC ligand and the MAD additive can facilitate the reaction.

Chemoselective Intramolecular Formal Insertion Reaction of Rh-Nitrenes into an Amide Bond Over C-H Insertion

The past few decades have witnessed extensive efforts to disclose the unique reactivity of metal-nitrenes, because they could be a powerful synthetic tool for introducing the amine functionality into unactivated chemical bonds. The reactivity of metal-nitrenes, however, is currently mainly confined to aziridination (an insertion into a C=C bond) and C-H amination (an insertion into a C-H bond). Nitrene insertion into an amide C-N bond, however, has not been reported so far. In this work we have developed a rhodium-catalyzed one-nitrogen insertion into amide C-N and sulfonamide S-N bonds. Experimental and theoretical analyses based on density functional theory indicate that the formal amide insertion proceeds via a rhodium-coordinated ammonium ylide formed between the nitrene and the amide nitrogen, followed by acyl group transfer concomitant with C-N bond cleavage. Mechanistic studies have allowed rationalization of the origin of the chemoselectivity observed between the C-H and amide insertion reactions. The methodology presented herein is the first example of an insertion of nitrene into amide bonds and provides facile access to unique diazacyclic systems with an N-N bond linkage.

Chloroacetate Promotes and Participates in the Oxidative Annulation of Pyridines/Isoquinoline by Using Oxygen as the Oxidant

The aerobic oxidative annulation of chalcones, pyridines/isoquinoline and ethyl chloroacetate to indolizines was achieved by cascade reaction. Various functional groups on chalcones were tolerated. And different pyridines derivatives could also be suitable substrates. Ethyl chloroacetate is an essential component in participating of the oxidative annulation process. Overall, this protocol is very practical and efficient by using molecular oxygen as oxidant with high selectivity for the annulation product.

[1+1+1] Cyclotrimerization for the Synthesis of Cyclopropanes

The synthesis of small rings by functionalization of C(sp³)−H bonds remains a great challenge. We report for the first time a copper‐catalyzed [1+1+1] cyclotrimerization of acetophenone derivatives under mild reaction conditions. The reaction has a broad scope for the stereoselective synthesis of cyclopropanes by trimerization of acetophenone. The developed transformation is based on an extraordinary copper‐catalyzed cascade process that allows saturated carbocycles to be obtained for the first time by cyclotrimerization through functionalization of C(sp³)−H bonds. The cascade of sixfold C(sp³)−H bond functionalization allows the synthesis of cyclopropanes in a highly stereoselective approach.