Glycosyl radical functionalization is one of the central topics in synthetic carbohydrate chemistry. Recent advances in metal-catalyzed cross-coupling chemistry and metallaphotoredox catalysis provided powerful platforms for glycosyl radical diversification. In particular, the discovery of new glycosyl radical precursors in conjunction with these advanced reaction technologies have significantly expanded the space for glycosyl compound synthesis. In this Minireview, we highlight the most recent progress in this area starting from 2021, and the reports included will be categorized based on different reaction types for better clarity.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/anie.202305138

Diversification of Glycosyl Compounds via Glycosyl Radicals.

While the amide bond is one of the most widespread backbones in natural peptides, pharmaceuticals, and agrochemicals, the atom-economic synthesis of amides remains a major challenge in organic chemistry. In this context, transition-metal-catalyzed hydroaminocarbonylation of cost-effective alkenes and alkynes has emerged as an ideal approach to access amides. However, it is only in recent times that more chemo- and regioselective hydroaminocarbonylation transformations, covering large substrate scope and under milder conditions, have been reported. In this review, we highlight some recent contributions to this field and elucidate the impact of ligands on regioselectivity, as well as discuss the potential applications of abundantly available nitrogen sources for hydroaminocarbonylation.

Transition-Metal-Catalyzed Hydroaminocarbonylations of Alkenes and Alkynes

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines.

Stereoselective C-glycosylation reactions are increasingly gaining attention in carbohydrate chemistry because they enable glycosyl precursors, easily accessible as anomeric mixtures, to converge to a single diastereomeric product. However, controlling the stereochemical outcome via transition metal catalysis remains challenging, and methods that leverage bench-stable heteroaryl glycosyl sulfone donors to facilitate glycosylation are rare. Here, we show two complementary nonprecious metal catalytic systems, based on iron or nickel, which are capable of promoting efficient C-C coupling between heteroaryl glycosyl sulfones and aromatic nucleophiles or electrophiles via distinct mechanisms and modes of activation. Diverse C-aryl glycosides were secured with excellent selectivity, scope, and functional group compatibility, and reliable access to both α and β isomers could be achieved for key sugar residues.

Stereoselective C-Aryl Glycosylation by Catalytic Cross-Coupling of Heteroaryl Glycosyl Sulfones.

We report a silver-catalyzed hydroalkoxylation of C2-alkynyl quinazolinones to prepare a series of novel quinazolinone-fused eight-membered N,O-heterocycles in good-to-excellent yields through a selective 8-endo-dig cyclization Mechanistic studies revealed that the silver catalyst might aid bidentate coordination of an imine group and alkyne to facilitate 8-endo-dig cyclization to afford eight-membered N,O-heterocycles Also, the proposed bimetal silver intermediates might promote hydroalkoxylation rapidly for quinazolinones bearing terminal alkynes at the C2-position Biological evaluations revealed that most of the designed quinazolinone-fused eight-membered N,O-heterocycles inhibited nitric-oxide generation significantly in lipopolysaccharide-stimulated RAW2647 cells and displayed their bioactivity as potentially good anti-inflammatory agents

Silver(I)-Catalyzed Selective Hydroalkoxylation of C2-Alkynyl Quinazolinones to Synthesize Quinazolinone-Fused Eight-Membered N,O-Heterocycles

As one of the effective strategies for carbohydrate synthesis, chemical synthesis can precisely control the structure of sugar chains, thus establishing the structure-activity relationship of complex sugars. The precise construction of glycosidic bonds is a long-standing challenge of stereo-controlled synthesis of carbohydrates. Therefore, research on developing stereoselective glycosylation to access carbohydrate mimics has received considerable attention from chemists, and this has led to remarkable progress and achievement in this field. In recent years, transition-metal-catalyzed glycosylation reactions involving anomeric C(sp3)-metal bond formation have emerged as a powerful tool for the synthesis of various glycosides and glycoconjugates due to their versatility, efficiency, and stereoselectivity. In this review, we highlight and discuss remarkable advances (from January 2017 to July 2022) in this rapidly expanding area. This review aims to provide timely illumination and beneficial guidance for the application of transition-metal catalysis in glycosyl cross-coupling reactions and endeavors to inspire further progress in this emerging area. 

Recent advances in transition-metal-catalyzed glycosyl cross-coupling reactions

C-Glycosyl peptides possess excellent metabolic stability and therapeutic properties and thus play critical roles in biological studies as well as drug discoveries. However, the limited accessibility of C-glycosyl amino acids has significantly hindered the broader research of their structural features and mode of action. Herein, for the first time we disclose a novel visible-light-driven radical conjugate addition of 1,4-dihydropyridine (DHP)-derived glycosyl esters with dehydroalanine derivatives, generating C-glycosyl amino acids and C-glycosyl peptides in good yields with excellent stereoselectivities. Redox-active glycosyl esters, as readily accessible and bench-stable radical precursors, could be easily converted to glycosyl radicals via anomeric C(sp3)–O bond homolysis under mild conditions. Importantly, the generality and practicality of this transformation were fully demonstrated in >40 examples including 2-dexosugars, oligosaccharides, oligopeptides, and complex drug molecules. Given its mild reaction conditions, robust sugar scope, and high anomeric control and diastereoselectivity, the method presented herein could find widespread utility in the preparation of C(sp3)-linked sugar-based peptidomimetics.

/pdf/stereoselective-alkyl-c-glycosylation-of-glycosyl-esters-via-14mt01zn.pdf

Stereoselective alkyl C-glycosylation of glycosyl esters via anomeric C–O bond homolysis: efficient access to C-glycosyl amino acids and C-glycosyl peptides

Palladium-Catalyzed Hydrocarbonylative Cyclization Enabled by Formal Insertion of Aromatic C═N Bonds into Pd-Acyl Bonds.

Brønsted Acid Catalyzed Cyclization of Aminodiazoesters with Aldehydes to 3-Carboxylate-N-Heterocycles.

A general strategy was developed for synthesizing of 2-C-glycals through a nickel-catalyzed cross-electrophile coupling reaction of 2-iodoglycals with (hetero)aryl iodides. Key to the success of this methodology is the use of an electron-deficient bipyridyl ligand. This innovative approach facilitates the efficient construction of 2-C-glycals, thereby broadening the synthetic repertoire available for glycochemistry.

Cross-Electrophile Coupling of 2-Iodoglycals Enables Efficient Access to 2-C-Glycals

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