Metal-Catalyzed Annulations through Activation and Cleavage of C−H Bonds

Rhodium(III)-Catalyzed Annulation of Pyridinones with Alkynes via Double C-H Activation: A Route to Functionalized Quinolizinones

A Rh(III)-catalyzed oxidative annulation of pyridin-2(1H)-ones with alkynes via double C-H activation to produce highly functionalized 4H-quinolizin-4-ones is disclosed. This reaction features easily available starting materials, simple manipulation, a relatively wide substrate scope, and good functional group tolerance. The application of this protocol is demonstrated by the synthesis of a known fluorescent quinolizino[3,4,5,6-ija]quinolinium salt.

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.

Dehydrogenative Aromatic Ring Fusion for Carbazole Synthesis via C-C/C-N Bond Formation and Alkyl Migration

An intermolecular dehydrogenative annulation (IDA) for carbazole synthesis via sequential C-C/C-N bond formation with a selective alkyl group migration is reported. Using the hypervalent iodine(III) reagent PhI(OAc)₂ (PIDA), in a one-pot operation, up to five C(sp2)-H bonds, one N(sp3)-H bond functionalization, and one alkyl (Me, Et) group migration could all be achieved from non-prefunctionalized 1,3,5-trialkylbenzenes and anilides under ambient laboratory conditions. Mechanistically, it is shown that PIDA reacts with anilides to generate a nitrenium ion or an equivalent carbenium ion which influences the second aromatic ring to be activated for C-C/C-N bond formation. Strategically, regioselective fusion of arenes to anilides is described.

Synthesis of bioactive and stabilized cyclic peptides by macrocyclization using C(sp3)-H activation

Synthesis of cyclic peptides with novel Cβ–Ar crosslinks has been achieved by C(sp³)–H activation, and their biological properties have been evaluated for the first time.

Cyclic peptides have attracted increasing attention in recent years due to their ability to inhibit protein–protein interactions. Current strategies to prepare cyclic peptides often rely on functional amino acid side chains or the incorporation of unnatural amino acids, thus limiting their structural diversity. Here, we describe the development of a highly versatile peptide macrocyclization strategy through a palladium-catalyzed C(sp³)–H activation and the synthesis of cyclic peptides featuring unique hydrocarbon linkages between the β-carbon of amino acids and the aromatic side chains of Phe and Trp. We demonstrate that such peptides exhibit improved biological properties compared to their acyclic counterparts. Finally, we applied this method in the synthesis of the natural product celogentin C.

Rhodium-Catalyzed [3 + 2] Annulation of Cyclic N-Acyl Ketimines with Activated Olefins: Anticancer Activity of Spiroisoindolinones

The rhodium(III)-catalyzed redox-neutral coupling reaction of N-acyl ketimines generated in situ from 3-hydroxyisoindolinones with various activated olefins is described. This approach leads to the synthesis of bioactive spiroisoindolinone derivatives in moderate to high yields. In the case of internal olefins such as maleimides, maleates, fumarates, and cinnamates, spiroindanes were obtained by the [3 + 2] annulations reaction. In sharp contrast, acrylates and quinones displayed the β-H elimination followed by Prins-type cyclization furnishing spiroindenes. The synthetic compounds were evaluated for in vitro anticancer activity against androgen-sensitive human prostate adenocarcinoma cells (LNCaP), human prostate adenocarcinoma cells (DU145), human endometrial adenocarcinoma cells (Ishikawa), human breast cancer cell (MCF-7), and triple negative human breast cancer cells (MDA-MB-231). Notably, quinone-containing spiroindenes displayed potent anticancer activity about 2- to 3-fold stronger than that of anticancer agent doxorubicin.

Tertiary amine self-catalyzed intramolecular Csp3–H functionalization with in situ generated allenes for the formation of 3-alkenyl indolines

Tandem Pd(0) catalyzed allenylation/tertiary amine self-catalyzed C_sp3–H functionalization reaction for the synthesis of 3-alkenyl indolines.

Rhodium(iii)-catalyzed synthesis of indanones via C–H activation of phenacyl phosphoniums and coupling with olefins

Phosphonium ylide acts as an efficient directing group in the oxidative carbocyclization between arenes and activated olefins for indanone synthesis.

Mechanism and origins of selectivity in rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes

DFT calculations were performed to investigate the rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes.

Silver-mediated oxidative C–H difluoromethylation of phenanthridines and 1,10-phenanthrolines

An efficient silver-mediated oxidative C–H difluoromethylation of phenanthridines and 1,10-phenanthrolines with TMSCF₂H was developed.

Palladium(ii)-catalyzed ortho-C–H olefination of phenylalanine and phenylethylamine derivatives directed by removable picolinamide group

Palladium-catalyzed ortho-C–H olefination of phenylalanine and phenylethylamine derivatives assisted by a removable picolinamide group has been achieved.

Facile one-pot synthesis of 2,3-dihydro-1H-indolizinium derivatives by rhodium(iii)-catalyzed intramolecular oxidative annulation via C–H activation: application to ficuseptine synthesis

Various substituted indolizidinium, quinolizinium and pyrido[1,2-a]azepinium salts were synthesized from benzaldehydes (α,β-unsaturated aldehydes) and alkyne–amines via Rh-catalyzed C–H activation.

Visible-light-mediated radical insertion/cyclization cascade reaction: synthesis of phenanthridines and isoquinolines from isocyanides

The development of a visible-light promoted radical insertion/cyclization cascade reaction to generate phenanthridines and isoquinolines from isocyanides under mild reaction conditions is reported.

Rhodium(III)-Catalyzed C-H Functionalization of 1-(2H)-Phthalazinones at C8

The rhodium(III) catalyst tris(acetonitrile)pentamethylcyclopentadienylrhodium(III) hexafluoroantimonate ([Cp*Rh(MeCN)3](SbF6)2) reacts with 1-(2H)-phthalazinones to promote a C-H functionalization event at C8. Preparation of a set of compounds arising from oxidative alkenylation with olefins, hydroarylation with alkynes, and iodination with N-iodosuccinimide is reported here. Oxidative alkenylation proceeds in very good yield, and the scope and limitations of the hydroarylation and halogenation reactions are discussed. Notably, this strategy enables rapid preparation of C8-substituted phthalazinones without requiring phthalazinone ring synthesis starting from a prefunctionalized arene.

Ru-Catalyzed One-Pot Diannulation of Heteroaryls: Direct Access to π-Conjugated Polycyclic Amides

A novel Ru-catalyzed oxidative double annulation of heteroarenes with symmetrical and unsymmetrical alkynes is reported. A general method for the unsymmetrical annulation of heteroarenes with two distinct alkynes is showcased for the first time. Methylphenyl sulfoximine (MPS) plays an important role in the annulations of heteroarenes and allows the construction of structurally complex π-conjugated heteroarene-fused polycyclic amide skeletons via the formation of multiple C-C and C-N bonds in a single operation. The reaction exhibits excellent substrate scope and tolerates a wide range of functional groups.