Harnessing C−H Activation of Benzhydroxamates as a Macrocyclization Strategy: Synthesis of Structurally Diverse Macrocyclic Isoquinolones

Rhodium-Catalyzed Asymmetric Macrocyclization towards Crown Ethers Using Hydroamination of Bis(allenes)

Enantiomerically enriched crown ethers (CE) exhibit strong asymmetric induction in phase transfer catalysis, supramolecular catalysis and molecular recognition processes. Traditional methods have often been used to access these valuable compounds, which limit their diversity and consequently their applicability. Herein, a practical catalytic method is described for the gram scale synthesis of a class of chiral CEs (aza-crown ethers; ACEs) using Rh-catalyzed hydroamination of bis(allenes) with diamines. Using this approach, a wide range of chiral vinyl functionalized CEs with ring sizes ranging from 12 to 36 have been successfully prepared in high yields of up to 92 %, dr of up to >20 : 1 and er of up to >99 : 1. These vinyl substituted CEs allow for further diversification giving facile access to various CE derivatives as well as to their three-dimensional analogues using ring-closing metathesis. Some of these chiral CEs themselves display high potential for use in asymmetric catalysis.

Transition metal catalysed direct construction of 2-pyridone scaffolds through C-H bond functionalizations

Substituted 2-pyridone is one of the most frequent scaffolds among nitrogen-containing bioactive natural products, pharmaceuticals and organic materials. Besides the classical syntheses to construct this class of molecules, retrosynthetically more straightforward approaches based on transition metal catalysed C-H bond functionalizations have been explored recently. In this review, we have summarized the recent progress in the direct transition metal catalysed construction of substituted 2-pyridone scaffolds via site-selective C-H bond functionalizations.

Rh(III)-Catalyzed mild straightforward synthesis of quinoline-braced cyclophane macrocycles via migratory insertion

An efficient Rh(III)-catalyzed straightforward strategy is developed for the synthesis of quinoline braced cyclophane macrocycles via methyl (sp³) C-H functionalization. The method is mild, simple and regioselective with various ring sizes and has good functional group tolerance. The method proceeds via C8-methyl metalation, metal-carbene formation and a subsequent migratory insertion. High dilution is not necessary for this macrocyclization and the only byproduct is nitrogen. A preliminary investigation shows that the C-H metalation step is the rate-determining step.

Pd(II)-Catalyzed Annulation Reactions of Epoxides with Benzamides to Synthesize Isoquinolones

Epoxides as alkylating reagents are unprecedentedly applied in Pd(II)-catalyzed C-H alkylation and oxidative annulation of substituted benzamides to synthesize isoquinolones rather than isochromans, which is accomplished through alerting the previously reported reaction mechanism by the addition of oxidant and TEA. Under these conditions, various isoquinolones have been prepared with yields up to 92%. In addition, this methodology has been successfully employed in the total syntheses of rupreschstyril, siamine, and cassiarin A in an expedient fashion.

Peptide macrocyclization by transition metal catalysis

Peptide macrocyclization has traditionally relied on lactam, lactone and disulfide bond-forming reactions that aim at introducing conformational constraints into small peptide sequences. With the advent of ruthenium-catalyzed ring-closing metathesis and copper-catalyzed alkyne-azide cycloaddition, peptide chemists embraced transition metal catalysis as a powerful macrocyclization tool with relevant applications in chemical biological and peptide drug discovery. This article provides a comprehensive overview of the reactivity and methodological diversification of metal-catalyzed peptide macrocyclization as a special class of late-stage peptide derivatization method. We report the evolution from classic palladium-catalyzed cross-coupling approaches to more modern oxidative versions based on C-H activation, heteroatom alkylation/arylation and annulation processes, in which aspects such as chemoselectivity and diversity generation at the ring-closing moiety became dominant over the last years. The transit from early cycloadditions and alkyne couplings as ring-closing steps to very recent 3d metal-catalyzed macrocyclization methods is highlighted. Similarly, the new trends in decarboxylative radical macrocyclizations and the interplay between photoredox and transition metal catalysis are included. This review charts future perspectives in the field hoping to encourage further progress and applications, while bringing attention to the countless possibilities available by diversifying not only the metal, but also the reactivity modes and tactics to bring peptide functional groups together and produce structurally diverse macrocycles.

Rhodium-catalyzed synthesis of substituted isoquinolones via a selective decarbonylation/alkyne insertion cascade of phthalimides

A rhodium-catalyzed decarbonylation/alkyne insertion cascade of phthalimides has been established. The reaction can be carried out in an operationally simple manner and provides expedient access to a series of isoquinolones in moderate to good yields. This reaction proceeded through a sequential decarbonylation/alkyne insertion/intramolecular annulation procedure and featured good functional group tolerance, ample substrate scope, and the construction of C-C and C-N bonds in one pot.

Macrocyclization via C-H functionalization: a new paradigm in macrocycle synthesis

The growing emphasis on macrocycles in engaging difficult therapeutic targets such as protein-protein interactions and GPCRs via preferential adaptation of bioactive and cell penetrating conformations has provided impetus to the search for de novo macrocyclization strategies that are efficient, chemically robust and amenable to diversity creation. An emerging macrocyclization paradigm based on the C-H activation logic, of particular promise in the macrocyclization of complex peptides, has added a new dimension to this pursuit, enabling efficacious access to macrocycles of various sizes and topologies with high atom and step economy. Significant achievements in macrocyclization methodologies and their applications in the synthesis of bioactive natural products and drug-like molecules, employing strategic variations of C-H activation are captured in this review. It is expected that this timely account will foster interest in newer ways of macrocycle construction among practitioners of organic synthesis and chemical biology to advance the field.

Palladium-catalyzed annulation of N-alkoxy benzsulfonamides with arynes by C–H functionalization: access to dibenzosultams

Palladium-catalyzed C–H/N–H functionalization of N-alkoxy benzsulfonamides with arynes provides dibenzosultams via C–C/C–N bond formation, accompanied by an unexpected N–O bond cleavage.

Rhodium(III)-Catalyzed C-H Activation/Heterocyclization as a Macrocyclization Strategy. Synthesis of Macrocyclic Pyridones

Structurally diverse macrocyclic pyridones can be efficiently synthesized by a rhodium(III)-catalyzed C-H activation/heterocyclization of ω-alkynyl α-substituted acrylic hydroxamates. The use of a O-pivaloyl hydroxamate as directing group was crucial to achieve efficient catalyst turnover in a redox-neutral process.

Ring-Expansion Approach to Medium-Sized Lactams and Analysis of Their Medicinal Lead-Like Properties

Medium-sized rings are widely considered to be under-represented in biological screening libraries for lead identification in medicinal chemistry. To help address this, a library of medium-sized lactams has been generated by using a simple, scalable and versatile ring-expansion protocol. Analysis of the library by using open-access computational tool LLAMA suggested that these lactams and their derivatives have highly promising physicochemical and 3D spatial properties and thus have much potential in drug discovery.