Rhodium-catalyzed transformations of diazo compounds via a carbene-based strategy: recent advances

Diazo compounds are known to be good coupling partners in the synthesis of heterocycles, carbocycles and functionalized molecules via a rhodium carbene-based strategy. Many heterocyclic and carbocyclic compounds, including isoquinolones and isocoumarins, quinoxalines, indoles, pyrrones, benzothazines, enaminones, benzenes and seven-membered rings, can be constructed using this rhodium-catalyzed system. The reaction mechanism involves C-H activation, carbene insertion and an annulation/functionalization sequence. This review describes the progress made in the last five years in rhodium-catalyzed transformations of diazo compounds as easily accessible precursors in organic chemistry.

Novel chiral matrine derivatives as potential antitumor agents: Design, synthesis and biological evaluation

Since the thalidomide incident, research on chiral drugs has escalated immensely. Differences in drug configuration can lead to significant variations in therapeutic efficacy. Matrine, a natural product esteemed for its low toxicity and high water solubility, has garnered significant attention in research endeavors. Nonetheless, its precise target has proven elusive. In this study, we designed and synthesized a novel chiral matrine derivative. Their cytotoxicity against three types of tumor cells was assessed. Comparing the newly synthesized derivatives to the parent matrine, most compounds exhibited significantly enhanced inhibitory effects on cancer cells. Among them, Q12 exhibited the highest activity, with IC50 values of 8.31 μM against rat glioma cells C6, 6.3 μM against human liver cancer cells HepG2 and 7.14 μM against human gastric cancer cells HGC-27, meanwhile showing low toxicity. Based on IC50 values, we constructed a preliminary structure-activity relationship (SAR). Compound Q12 significantly suppressed the cloning and migration of HepG2 cells. Further mechanistic studies indicated that Q12 inhibited Topo I in HepG2 cells, leading to DNA damage, induction of G0/G1 cell cycle arrest and ultimately causing apoptosis. The molecular docking experiments provided a rational binding mode of Q12 with the Topo I-DNA complex. In vivo, experiments demonstrated that Q12 exhibited a higher tumor growth inhibition rate (TGI) compared to the positive control drug Lenvatinib, while maintaining good safety. In summary, it suggests that Topo I might be a potential target for matrine and Q12 represents a promising candidate for cancer treatment.

Divergent Diazo Approach toward Alkyl 5/4-Hydroxy-3H-benzo[e]indole-4/5-carboxylates

A divergent diazo approach toward alkyl 5/4-hydroxy-3H-benzo[e]indole-4/5-carboxylates has been developed. The reaction of 1,3-diketones with alkyl 2-diazo-3-oxo-3-(2H-azirin-2-yl)propanoates catalyzed by Co(acac)3 or Ni(acac)2 gives various alkyl 3-(1H-pyrrol-2-yl)-2-diazo-3-oxopropanoate in good yields. The latter undergo Wolff rearrangement followed by the 6π-cyclization of transient ketene to form alkyl 5-hydroxy-3H-benzo[e]indole-4-carboxylates bearing various substituents in positions 1, 2, 7, and 8, as well as derivatives of methyl 4-hydroxy-6H-thieno[2,3-e]indole-5-carboxylates and methyl 5-hydroxy-7H-benzo[c]carbazole-6-carboxylate under thermolysis or Rh2(OAc)4 catalysis. Isomeric benzoindoles, alkyl 4-hydroxy-3H-benzo[e]indole-5-carboxylates, have been prepared by Boc-protection of the pyrrole nitrogen of alkyl 3-(1H-pyrrol-2-yl)-2-diazo-3-oxopropanoates followed by an intramolecular formal carbene insertion into the aromatic C-H bond catalyzed by Cu(OTf)2. The hydroxyl group of alkyl 5/4-hydroxy-3H-benzo[e]indole-4/5-carboxylates, through the formation of the corresponding triflates, allows the introduction of various substituents into the 5/4 position of benzo[e]indoles using the cross-coupling reaction and even form a new heterocyclic backbone, benzo[k]pyrrolo[2,3-i]phenanthridine, via a tandem Suzuki reaction/nucleophilic acyl substitution.

Imine Directed Cp*RhIII -Catalyzed N-H Functionalization and Annulation with Amino Amides, Aldehydes, and Diazo Compounds

A multicomponent annulation that proceeds by imine directed Cp*RhIII -catalyzed N-H functionalization is disclosed. The transformation affords piperazinones displaying a range of functionality and is the first example of transition metal-catalyzed multicomponent N-H functionalization. A broad range of readily available α-amino amides, including those derived from glycine, α-substituted, and α,α-disubstituted amino acids, were effective inputs and enabled the incorporation of a variety of amino acid side chains with minimal racemization. Branched and unbranched alkyl aldehydes and various stabilized diazo compounds were also efficient reactants. The piperazinone products were further modified through efficient transformations. Mechanistic studies, including X-ray crystallographic characterization of a catalytically competent five-membered rhodacycle with imine and amide nitrogen chelation, provide support for the proposed mechanism.

Substrate-Dependent Denitrogenative Rearrangements of Rhodium Azavinyl Carbenes Involving 1,2-Aryl Migration

Herein, we disclose substrate-dependent rearrangements of 4-substituted N-sulfonyl-1,2,3-triazoles under Rh(II)-catalysis via denitrogenation. The reaction pathways included key 1,2-aryl migration via the formation of intermediatory phenonium ion, which is elusive so far with Rh-azavinyl carbenes. Intriguingly, the transformations were completely dependent on the substituent present leading to different scaffolds like enaminones, pyrrol-3-ones, and azadienes. Hammett studies provided essential insights into the carbocationic intermediate formation. The developed methodology featured simple reaction conditions, excellent functional group compatibility, and broad substrate scope.

Annulation-Triggered Denitrogenative Transformations of 2-(5-Iodo-1,2,3-triazolyl)benzoic Acids

The ability of [1,2,3]triazolobenzoxazinones to act as a source of "hidden" diazo group was discovered. These diazo precursors can be easily prepared by the intramolecular cyclization of 2-(5-iodo-1,2,3-triazolyl)benzoic acids. The Cu-catalyzed capture of the hidden diazo group allows for further functionalization through the denitrogenative pathway. The transformations proceed via the formation of either diazoimine or diazoamide intermediates. Novel routes to various anthranilamides as well as thiolated benzoxazinones were developed using the one-pot cyclization/diazo capture procedure.