Pd-Catalyzed Directed ortho-C–H Alkenylation of Phenylalanine Derivatives

Research progress on chemical modifications of tyrosine residues in peptides and proteins

The chemical modifications (CMs) of protein is an important technique in chemical biology, protein-based therapy, and material science. In recent years, there has been rapid advances in the development of CMs of peptides and proteins, providing new approaches for peptide and protein functionalization, as well as drug discovery. In this review, we highlight the methods for chemically modifying tyrosine (Tyr) residues in different regions, offering a comprehensive exposition of the research content related to Tyr modification. This review summarizes and provides an outlook on Tyr residue modification, aiming to offer readers assistance in the site-selective modification of macromolecules and to facilitate application research in this field.

Picolinamide-assisted ortho-C-H functionalization of pyrenylglycine derivatives using aryl iodides

Chemical transformations involving the pyrenylglycine motif (an unnatural amino acid) and practical methods toward it are seldom known. This work aimed at developing a method for synthesizing novel pyrenylglycine (pyrene-based glycine) unnatural amino acid derivatives. To realize this, initially, a new pyrenylglycine substrate possessing the picolinamide moiety was assembled via the Ugi multicomponent reaction. The picolinamide moiety linked to amine substrates is a well-known bidentate directing group for accomplishing the site-selective γ-C-H functionalization of amines. Subsequently, it was aimed at using a Pd(II)-catalyzed bidentate directing group-aided γ-C-H arylation strategy for generating a wide range of unprecedented examples of C(2)-H arylated pyrenylglycines. Accordingly, pyrenylglycine possessing the picolinamide moiety was subjected to Pd(II)-catalyzed C(2)-H arylation in the non-K-region to afford a library of C(2)-arylated pyrenylglycines (π-extended pyrenes). Additionally, pyrenylglycine-based small peptides were assembled using C(2)-arylated pyrenylglycines. The X-ray structure of a representative compound was obtained, which corroborated the structure of pyrenylglycine and the regioselectivity of C(2)-H arylation of the pyrene in the non-K-region.

Pd(II)-Catalyzed Fujiwara-Moritani Reactions for the Synthesis and Functionalization of Substituted Coumarins

Highly substituted coumarins, privileged and versatile scaffolds for bioactive natural products and fluorescence imaging, are obtained via a Pd(II)-catalyzed direct C-H alkenylation reaction (Fujiwara-Moritani reaction), which has emerged as a powerful tool for the construction and functionalization of heterocyclic compounds because of its chemical versatility and its environmental advantages. Thus, a selective 6-endo cyclization led to 4-substituted coumarins in moderate yields. Selected examples have been further functionalized in C3 through a second intermolecular C-H alkenylation reaction to give coumarin-acrylate hybrids, whose fluorescence spectra have been measured.

Access to benzene-modified 2nd generation strigolactams and GR24 by merging C-H olefination with decarboxylative Giese cyclization

Herein, we reported an efficient and general synthetic route to assemble benzene-modified 2^nd generation strigolactams and GR24. The key features of this synthesis include a palladium-catalyzed ortho-selective olefination of the commercially available substituted N-Boc phenylalanine and a decarboxylative Giese radical cyclization. The bioactivities of these compounds to stimulate the seed germination of Orobanche aegyptiaca parasitic weed were also analysed. 2^nd generation strigolactam 15f derived from para-OMe phenylalanine showed superior bioactivity to the original unsubstituted 15b.

Two Decades of Progress in the Asymmetric Intramolecular aza-Michael Reaction

The asymmetric intramolecular aza-Michael reaction (IMAMR) is a very convenient strategy for the generation of heterocycles bearing nitrogen-substituted stereocenters. Due to the ubiquitous presence of these skeletons in natural products, the IMAMR has found widespread applications in the total synthesis of alkaloids and biologically relevant compounds. The development of asymmetric versions of the IMAMR are quite recent, most of them reported in this century. The fundamental advances in this field involve the use of organocatalysts. Chiral imidazolidinones, diaryl prolinol derivatives, Cinchone-derived primary amines and quaternary ammonium salts, and BINOL-derived phosphoric acids account for the success of those methodologies. Moreover, the use of N-sulfinyl imines with a dual role, as nitrogen nucleophiles and as chiral auxiliaries, appeared as a versatile mode of performing the asymmetric IMAMR.

Palladium-catalysed Heck-type alkenylation reactions in the synthesis of quinolines. Mechanistic insights and recent applications

Recent developments in Pd(0)- and Pd(ii)-catalysed alkenylation reactions for the synthesis of quinolines focusing on mechanistic understanding.

Ligand Promoted, Palladium-Catalyzed C(sp2)-H Arylation of Free Primary 2-Phenylethylamines

A mono- N-protected amino acid (MPAA) ligand promoted, Pd(II)-catalyzed C(sp²)-H arylation of free primary 2-phenylethylamines using the native NH₂ as the directing group has been achieved. This method is compatible with challenging simple primary 2-phenylethylamines bearing α-hydrogen atoms. Application of this protocol in the direct structure modification of the drug molecule amphetamine is also demonstrated.

Synthesis of unnatural α-amino acid derivatives via selective o-C–H functionalization

Selective o-C–H functionalization of aryl based amino acids including arylation, alkylation, alkynylation, halogenation, alkoxylation, and acyloxylation were developed.

Recent advances in positional-selective alkenylations: removable guidance for twofold C–H activation

Recent advances in transition-metal catalyzed positional-selective alkenylations via twofold C–H activation directed by removable or traceless directing groups are reviewed.

Palladium-catalyzed C–H olefination of uracils and caffeines using molecular oxygen as the sole oxidant

The palladium-catalyzed oxidative C–H olefination of uracils or caffeines with alkenes using an atmospheric pressure of molecular oxygen as the sole oxidant has been disclosed.

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.

Catalytic Multisite-Selective Acetoxylation Reactions at sp2 vs sp3 C-H Bonds in Cyclic Olefins

The first Pd-catalyzed multisite-selective acetoxylation reactions are disclosed at an unactivated alkene sp² C-H bond versus secondary allylic sp³ C-H bond in cyclic olefins via the modulation of directing groups. The different directing groups overcome the key challenge in differentiating C-H bonds and provide a new controlling approach for site-specific C-H activation. A wide variety of substrates are readily acetoxylated under operationally simple conditions. Mechanistic studies suggest that different Pd (IV) intermediates were involved in the multisite-selective acetoxylation reactions.

H-D exchange in deuterated trifluoroacetic acid via ligand-directed NHC-palladium catalysis: a powerful method for deuteration of aromatic ketones, amides, and amino acids

A method has been developed for one-step ortho-selective ligand-directed H-D exchange, accompanied in some cases by concurrent acid-catalyzed electrophilic deuteration. This method is effective for deuteration of aromatic substrates ranging from ketones to amides and amino acids, including compounds of biological and pharmaceutical interest such as acetaminophen and edaravone. Use of a palladium catalyst featuring an NHC ligand is critical for the observed reactivity. Experimental evidence strongly suggests that palladium facilitates C-H activation of the aromatic substrates, a mechanism seldom observed under strongly acidic conditions. 2015 Elsevier Ltd. All rights reserved.

Palladium-catalyzed ortho-C–H alkenylation of 2-benzyl-1,2,3-triazoles

A mild and efficient method for the direct alkenylation of 2-benzyl-1,2,3-triazoles via Pd-catalyzed C–H bond activation was developed.