Iterative Multifunctionalization of Unactivated C−H Bonds in Piperidines by Way of Intramolecular Rh(II)-Catalyzed Aminations

Synthesis and bio-evaluation of newer dihydropyridines and tetrahydropyridines based glycomimetic azasugars

This study presents the synthesis and bio-evaluation of new triazolylated dihydropyridine and tetrahydropyridine azasugar scaffolds (F1-14). Azasugar glycomimetics are the synthetic substances that mimic the structural and functional characteristics of natural carbohydrates showcasing promising potential as therapeutic agents for diabetes. The α-glucosidase inhibitory activity of synthesized final compounds were evaluated against the commercially available α-glucosidase enzyme. Majority of the screened compounds displayed excellent inhibition with IC50 values ranging from 2.12 to 75.11 μM, when compared to the standard drug Acarbose. Particularly, compound F5 with IC50 value of 2.12 μM was found to be the most active compound among the series. Further molecular docking studies of selected ligands were performed to investigate the binding interactions with enzyme active sites. Their specific binding patterns have been analysed with the binding sites of Saccharomyces cerevisiae α-glucosidase. These findings suggest these candidates as the potential leads for the anti-diabetic activity.

Photoinduced α-C-H Amination of Cyclic Amine Scaffolds Enabled by Polar-Radical Relay

Herein, we report a polar-radical relay strategy for α-C-H amination of cyclic amines with N-chloro-N-sodio-carbamates. The relay is initiated by in situ generation of cyclic iminium intermediate using N-iodosuccinimide (NIS) oxidant as an initiator, which then operates through a series of polar (addition and elimination) and radical (homolysis, hydrogen- and halogen atom transfer) reactions to enable the challenging C-N bond formation in a controlled manner. A broad range of α-amino cyclic amines were readily accessed with excellent regioselectivity, and the superb applicability was further demonstrated by functionalization of biologically relevant compounds.

Late-stage C-H amination of abietane diterpenoids

This study aims at highlighting the synthetic versatility of the rhodium-catalyzed C-H amination reactions using iodine(iii) oxidants for the late-stage functionalization of natural products. Inter- and intramolecular nitrene insertions have been performed from various abietane diterpenoids, leading to the amination of the C-3, C-6, C-7, C-11 and C-15 positions. Ca. 20 aminated compounds have been isolated with yields of up to 86% and high levels of regio-, chemo- and stereoselectivities.

Iterative direct C(sp3)–H functionalization of amines: diastereoselective divergent syntheses of α,α′-disubstituted alicyclic amines

Facile access to the syn-α,α′-disubstituted N-heterocycles via iterative C–H oxygenation and C–C bond formation.

Catalytic C–H amination at its limits: challenges and solutions

Pushing C–H amination to its limits fosters innovative synthetic solutions and offers a deeper understanding of the reaction mechanism and scope.

Pushing the limits of catalytic C-H amination in polyoxygenated cyclobutanes

A synthetic route to a new class of conformationally constrained iminosugars based on a 5-azaspiro[3.4]octane skeleton has been developed by way of Rh(ii)-catalyzed C(sp(3))-H amination. The pivotal stereocontrolled formation of the quaternary C-N bond by insertion into the C-H bonds of the cyclobutane ring was explored with a series of polyoxygenated substrates. In addition to anticipated regioselective issues induced by the high density of activated α-ethereal C-H bonds, this systematic study showed that cyclobutane C-H bonds were, in general, poorly reactive towards catalytic C-H amination. This was demonstrated inter alia by the unexpected formation of a oxathiazonane derivative, which constitutes a very rare example of the formation of a 9-membered ring by way of catalyzed C(sp(3))-H amination. A complete stereocontrol could be however achieved by activating the key insertion position as an allylic C-H bond in combination with reducing the electron density at the undesired C-H insertion sites by using electron-withdrawing protecting groups. Preliminary biological evaluations of the synthesized spiro-iminosugars were performed, which led to the identification of a new class of correctors of the defective F508del-CFTR gating involved in cystic fibrosis.

The synthesis of functionalized bridged polycycles via C-H bond insertion

This review presents examples from the chemical literature of syntheses of bridged-polycyclic products via C–H bond insertion by carbenes and nitrenes. Applications to natural product synthesis, a description of the essential elements in substrate-controlled reactions, and mechanistic details of transformations are presented. Overall, these transformations allow the construction of important ring systems rapidly and efficiently, though additional catalyst development is needed.

Synthesis of a new class of iminosugars based on constrained azaspirocyclic scaffolds by way of catalytic C-H amination

The synthesis of the first examples of a new class of iminosugars based on constrained spirocyclic scaffolds has been achieved via Rh-catalyzed C(sp(3))-H amination. In this process, the needed electronic control in securing high regioselectivity from substrates with a high density of activated C-H bonds was achieved by using a combination of activating and electron-withdrawing groups.

Metal-catalyzed nitrogen-atom transfer methods for the oxidation of aliphatic C-H bonds

For more than a century, chemists have endeavored to discover and develop reaction processes that enable the selective oxidation of hydrocarbons. In the 1970s, Abramovitch and Yamada described the synthesis and electrophilic reactivity of sulfonyliminoiodinanes (RSO(2)N═IPh), demonstrating the utility of this new class of reagents to function as nitrene equivalents. Subsequent investigations by Breslow, Mansuy, and Müller would show such oxidants to be competent for alkene and saturated hydrocarbon functionalization when combined with transition metal salts or metal complexes, namely those of Mn, Fe, and Rh. Here, we trace our own studies to develop N-atom transfer technologies for C-H and π-bond oxidation. This Account discusses advances in both intra- and intermolecular amination processes mediated by dirhodium and diruthenium complexes, as well as the mechanistic foundations of catalyst reactivity and arrest. Explicit reference is given to questions that remain unanswered and to problem areas that are rich for discovery. A fundamental advance in amination technology has been the recognition that iminoiodinane oxidants can be generated in situ in the presence of a metal catalyst that elicits subsequent N-atom transfer. Under these conditions, both dirhodium and diruthenium lantern complexes function as competent catalysts for C-H bond oxidation with a range of nitrogen sources (e.g., carbamates, sulfamates, sulfamides, etc.), many of which will not form isolable iminoiodinane equivalents. Practical synthetic methods and applications thereof have evolved in parallel with inquiries into the operative reaction mechanism(s). For the intramolecular dirhodium-catalyzed process, the body of experimental and computational data is consistent with a concerted asynchronous C-H insertion pathway, analogous to the consensus mechanism for Rh-carbene transfer. Other studies reveal that the bridging tetracarboxylate ligand groups, which shroud the dirhodium core, are labile to exchange under standard reaction conditions. This information has led to the generation of chelating dicarboxylate dinuclear rhodium complexes, exemplified by Rh(2)(esp)(2). The performance of this catalyst system is unmatched by other dirhodium complexes in both intra- and intermolecular C-H amination reactions. Tetra-bridged, mixed-valent diruthenium complexes function as effective promoters of sulfamate ester oxidative cyclization. These catalysts can be crafted with ligand sets other than carboxylates and are more resistant to oxidation than their dirhodium counterparts. A range of experimental and computational mechanistic data amassed with the tetra-2-oxypyridinate diruthenium chloride complex, [Ru(2)(hp)(4)Cl], has established the insertion event as a stepwise pathway involving a discrete radical intermediate. These data contrast dirhodium-catalyzed C-H amination and offer a cogent model for understanding the divergent chemoselectivity trends observed between the two catalyst types. This work constitutes an important step toward the ultimate goal of achieving predictable, reagent-level control over product selectivity.

Highly diastereoselective synthesis of tetrahydropyridines by a C-H activation-cyclization-reduction cascade

A versatile reaction cascade leading to highly substituted 1,2,3,6-tetrahydropyridines has been developed. It comprises rhodium(I)-catalyzed C-H activation-alkyne coupling followed by electrocyclization and subsequent acid/borohydride-promoted reduction. This one-pot procedure affords the target compounds in up to 95% yield with >95% diastereomeric purity.

Rhodium-Catalyzed C-H Amination - An Enabling Method for Chemical Synthesis

Reaction methods for selective C-H amination are finding ever-increasing utility for the preparation of nitrogen-derived fine chemicals. This brief account highlights the remarkable versatility of dirhodium-based catalysts for promoting oxidation of aliphatic C-H centers in both intra- and intermolecular reaction processes.

Diastereoselective synthesis of 2,3,6-trisubstituted piperidines

We report the diastereoselective and chromatography-free syntheses of four 2-phenyl-6-alkyl-3-aminopiperidines. Ring construction was accomplished through a nitro-Mannich reaction linking a nitroketone and phenylmethanimine, followed by a ring-closure condensation. Relative stereocontrol was achieved between C-2 and C-3 by kinetic protonation of a nitronate or by equilibration of the nitro group under thermodynamic control. Stereocontrol at C-6 was accomplished by utilizing a variety of imine reduction methods. The C-2/C-6-cis stereochemistry was established via triacetoxyborohydride iminium ion reduction, whereas the trans relationship was set either by triethylsilane/TFA acyliminium ion reduction or by Lewis acid catalyzed imine reduction with lithium aluminum hydride.

Catalytic C-H amination: recent progress and future directions

Recent developments in catalytic C-H amination are discussed in this feature article. The careful design of reagents and catalysts now provides efficient conditions for exquisitely selective intramolecular as well as intermolecular nitrene C-H insertion. The parallel emergence of C-H activation/amination reactions opens new opportunities complementary to those offered by nitrenes.

Metal-free one-pot oxidative amidation of aldoses with functionalized amines

Metal-free one-pot oxidative amidation of aldoses with functionalized amines using iodine provides a rapid access to functionalized aldonamides. The main advantage of this approach relies on the fact that aldehyde oxidation and C-N bond formation are performed in a single synthetic operation.