Chan-Lam-Evans Coupling of Cbz-Protected Histidines

Design, synthesis, and applications of ring-functionalized histidines in peptide-based medicinal chemistry and drug discovery

Modified and synthetic α-amino acids are known to show diverse applications. Histidine, which possesses numerous applications when subjected to synthetic modifications, is one such amino acid. The utility of modified histidines varies widely from remarkable biological activities to catalysis, and from nanotechnology to polymer chemistry. This renders histidine residue an important place in scientific research. Histidine is a well-studied scaffold and constitutes the active site of various enzymes catalyzing important reactions in the biological systems. A rational modification in histidine structure with a distinctly developed protocol extensively changes its physical and chemical properties. The utilization of modified histidines in search of potent, target selective and proteostable scaffolds is vital in the development of bioactive peptides with enhanced drug-likeliness. This review is a compilation and analysis of reported side-chain ring modifications at histidine followed by applications of ring-modified histidines in the synthesis of various categories of bioactive peptides and peptidomimetics.

Copper-Promoted N-Arylation of the Imidazole Side Chain of Protected Histidine by Using Triarylbismuth Reagents

The N-arylation of the side chain of histidine by using triarylbismuthines is reported. The reaction is promoted by copper(II) acetate in dichloromethane at 40 °C under oxygen in the presence of diisopropylethylamine and 1,10-phenanthroline and allows the transfer of aryl groups with substituents at any position of the aromatic ring. The reaction shows excellent functional group tolerance and is applicable to dipeptides where the histidine is located at the N terminus. A histidine-guided backbone N-H arylation was observed in dipeptides where the histidine occupies the C terminus.

Recent developments in selective N-arylation of azoles

Transition-metal based carbon-heteroatom (C-X) bond formation has attracted the attention of synthetic chemists over the past few years because the resultant aryl/heteroaryl motifs are important substructures in many natural products, pharmaceuticals, etc. Several efficient protocols such as Buchwald-Hartwig amination, Ullmann coupling, Chan-Lam coupling and metal-free approaches have proved beneficial in C-X bond formation. Selective arylation of one hetero-centre over other centres without protection/deprotection thus allowing minimum synthetic manipulation has been achieved for several substrates using these protocols. Azoles are one such novel five-membered heterocyclic core with huge pharmaceutical applications. Though N-arylation on azole-bearing analogues has been extensively practised, selective N-arylation either on one N-centre or the exocyclic N-site of the azole ring in competition with other hetero-centres in the framework has been recently explored for azole-carrying systems. Thus, this review would focus on recent advances in chemo- and regio-selective N-arylation (either on one N-centre or the exocyclic N-site of the azole ring) on azole-containing frameworks.

Advances in Carbon–Element Bond Construction under Chan–Lam Cross-Coupling Conditions: A Second Decade

Copper-mediated carbon–heteroatom bond-forming reactions involving a wide range of substrates have been in the spotlight for many organic chemists. This review highlights developments between 2010 and 2019 in both stoichiometric and catalytic copper-mediated reactions, and also examples of nickel-mediated reactions, under modified Chan–Lam cross-coupling conditions using various nucleophiles; examples include chemo- and regioselective N-arylations or O-arylations. The utilization of various nucleophiles as coupling partners together with reaction optimization (including the choice of copper source, ligands, base, and other additives), limitations, scope, and mechanisms are examined; these have benefitted the development of efficient and milder methods. The synthesis of medicinally valuable or pharmaceutically important nitrogen heterocycles, including isotope-labeled compounds, is also included. Chan–Lam coupling reaction can now form twelve different C–element bonds, making it one of the most diverse and mild reactions known in organic chemistry.

1 Introduction

2 Construction of C–N and C–O Bonds

2.1 C–N Bond Formation

2.1.1 Original Discovery via Stoichiometric Copper-Mediated C–N Bond Formation

2.1.2 Copper-Catalyzed C–N Bond Formation

2.1.3 Coupling with Azides, Sulfoximines, and Sulfonediimines as Nitrogen­ Nucleophiles

2.1.4 Coupling with N,N-Dialkylhydroxylamines

2.1.5 Enolate Coupling with sp3-Carbon Nucleophiles

2.1.6 Nickel-Catalyzed Chan–Lam Coupling

2.1.7 Coupling with Amino Acids

2.1.8 Coupling with Alkylboron Reagents

2.1.9 Coupling with Electron-Deficient Heteroarylamines

2.1.10 Selective C–N Bond Formation for the Synthesis of Heterocycle-Containing Compounds

2.1.11 Using Sulfonato-imino Copper(II) Complexes

2.2 C–O Bond Formation

2.2.1 Coupling with (Hetero)arylboron Reagents

2.2.2 Coupling with Alkyl- and Alkenylboron Reagents

3 C–Element (Element = S, P, C, F, Cl, Br, I, Se, Te, At) Bond Forma tion under Modified Chan–Lam Conditions

4 Conclusions

Regioselective copper-catalyzed N(1)-(hetero)arylation of protected histidine

We report regioselective N(1)-arylation of protected histidine using copper(i) iodide as a catalyst, trans-N,N′-dimethylcyclohexane-1,2-diamine as a ligand and readily available aryl iodides as coupling partners under microwave irradiation at 130 °C for 40 min.

Copper-mediated oxidative trifluoromethylation of boronic acids

A copper-mediated oxidative cross-coupling of aryl- and alkenylboronic acids with (trifluoromethyl)trimethylsilane (Me(3)SiCF(3)) under mild conditions has been developed. This method allows a wide range of functional group tolerant trifluoromethylated arenes and alkenes to be easily prepared. This oxidative trifluoromethylation has the potential to introduce trifluoromethyl groups into advanced, highly functionalized organic molecules.