Bifunctional Chiral Dehydroalanines for Peptide Coupling and Stereoselective S-Michael Addition

Chemical synthesis and functional evaluation of glycopeptides and glycoproteins containing rare glycosyl amino acid linkages

Covering: 1987 to 2023Naturally existing glycoproteins through post-translational protein glycosylation are highly heterogeneous, which not only impedes the structure-function studies, but also hinders the development of their potential medical usage. Chemical synthesis represents one of the most powerful tools to provide the structurally well-defined glycoforms. Being the key step of glycoprotein synthesis, glycosylation usually takes place at serine, threonine, and asparagine residues, leading to the predominant formation of the O- and N-glycans, respectively. However, other amino acid residues containing oxygen, nitrogen, sulfur, and nucleophilic carbon atoms have also been found to be glycosylated. These diverse glycoprotein linkages, occurring from microorganisms to plants and animals, play also pivotal biological roles, such as in cell-cell recognition and communication. The availability of these homogenous rare glycopeptides and glycoproteins can help decipher the glyco-code for developing therapeutic agents. This review highlights the chemical approaches for assembly of the functional glycopeptides and glycoproteins bearing these "rare" carbohydrate-amino acid linkages between saccharide and canonical amino acid residues and their derivatives.

Electrochemical nickel-catalyzed cross-coupling of glycosyl thiols with preactivated phenols and ketones

An efficient electrochemical nickel-catalyzed cross-coupling reaction has been reported here for the synthesis of S-glycosides from preactivated phenols and ketones under mild conditions. Various glycosyl thiols, including unprotected sugar, and a diverse range of aryl/alkenyl triflates, including some complex biorelevant phenols and ketones, were well tolerated in this method.

Photocatalyzed Oxidative Decarboxylation Forming Aminovinylcysteine Containing Peptides

The formation of (2S,3S)-S-[(Z)-aminovinyl]-3-methyl-D-cysteine (AviMeCys) substructures was developed based on the photocatalyzed-oxidative decarboxylation of lanthionine-bearing peptides. The decarboxylative selenoetherification of the N-hydroxyphthalimide ester, generated in situ, proceeded under mild conditions at −40 °C in the presence of 1 mol% of eosin Y-Na2 as a photocatalyst and the Hantzsch ester. The following β-elimination of the corresponding N,Se-acetal was operated in a one-pot operation, led to AviMeCys substructures found in natural products in moderate to good yields. The sulfide-bridged motif, and also the carbamate-type protecting groups, such as Cbz, Teoc, Boc and Fmoc groups, were tolerant under the reaction conditions.

Towards Enantiomerically Pure Unnatural α-Amino Acids via Photoredox Catalytic 1,4-Additions to a Chiral Dehydroalanine

Chemo- and diastereoselective 1,4-conjugate additions of anionic and radical C-nucleophiles to a chiral bicyclic dehydroalanine (Dha) are described. Of particular importance, radical carbon photolysis by a catalytic photoredox process using a simple method with a metal-free photocatalyst provides exceptional yields and selectivities at room temperature. Moreover, these 1,4-conjugate additions offer an excellent starting point for synthesizing enantiomerically pure carbon-β-substituted unnatural α-amino acids (UAAs), which could have a high potential for applications in chemical biology.

Stereoselective α-Deuteration of Serine, Cysteine, Selenocysteine, and 2,3-Diaminopropanoic Acid Derivatives

Efficient methodologies for synthesizing enantiopure α-deuterated derivatives of serine, cysteine, selenocysteine, and 2,3-diaminopropanoic acid have been developed. H/D exchange was achieved by deprotonation of a chiral bicyclic serine equivalent followed by selective deuteration. Additionally, diastereoselective additions of thiols, selenols, and amines to a chiral bicyclic dehydroalanine in deuterated alcohols allowed site-selective deuteration at the Cα atom of cysteine, selenocysteine, and 2,3-diaminopropanoic acid derivatives. A deuterated analogue of carbocysteine, a drug for the treatment of bronchiectasis, was synthesized.

Recent Advances in the Synthesis and Biological Applications of Peptidoglycan Fragments

The biosynthesis, breakdown, and modification of peptidoglycan (PG) play vital roles in both bacterial viability and in the response of human physiology to bacterial infection. Studies on PG biochemistry are hampered by the fact that PG is an inhomogeneous insoluble macromolecule. Chemical synthesis is therefore an important means to obtain PG fragments that may serve as enzyme substrates and elicitors of the human immune response. This review outlines the recent advances in the synthesis and biochemical studies of PG fragments, PG biosynthetic intermediates (such as Park's nucleotides and PG lipids), and PG breakdown products (such as muramyl dipeptides and anhydro-muramic acid-containing fragments). A rich variety of synthetic approaches has been applied to preparing such compounds since carbohydrate, peptide, and phospholipid chemical methodologies must all be applied.

Synthesis of β2,2-Amino Acids by Stereoselective Alkylation of Isoserine Derivatives Followed by Nucleophilic Ring Opening of Quaternary Sulfamidates

Chiral bicyclic N,O-acetal isoserine derivatives have been synthesized by an acid-catalyzed tandem N,O-acetalization/intramolecular transcarbamoylation reaction between conveniently protected l-isoserine and 2,2,3,3-tetramethoxybutane. The delicate balance of the steric interactions between the different functional groups on each possible diastereoisomer controls their thermodynamic stability and hence the experimental product distribution. These chiral isoserine derivatives undergo diastereoselective alkylation at the α position, proceeding with either retention or inversion of the configuration depending on the relative configuration of the stereocenters. Quantum mechanical calculations revealed that a concave-face alkylation is favored due to smaller torsional and steric interactions at the bicyclic scaffold. This synthetic methodology gives access to chiral β^2,2-amino acids, attractive compounds bearing a quaternary stereocenter at the α position with applications in peptidomimetic and medicinal chemistry. Thus, enantiopure α-alkylisoserine derivatives were produced upon acidic hydrolysis of these alkylated scaffolds. In addition, α-benzylisoserine was readily transformed into a five-membered ring cyclic sulfamidate, which was ring opened regioselectively with representative nucleophiles to yield other types of enantiopure β^2,2-amino acids such as α-benzyl-α-heterofunctionalized-β-alanines and α-benzylnorlanthionine derivatives.

Recent Development of Bis-Cyclometalated Chiral-at-Iridium and Rhodium Complexes for Asymmetric Catalysis

The field of asymmetric catalysis has been developing to access synthetically efficacious chiral molecules from the last century. Although there are many sustainable ways to produce nonracemic molecules, simplified and unique methodologies are always appreciated. In the recent developments of asymmetric catalysis, chiral-at-metal Lewis acid catalysis has been recognized as an attractive strategy. The catalysts coordinatively activate a substrate while serving the sole source of chirality by virtue of its helical environment. These configurationally stable complexes were utilized in a large number of asymmetric transformations, ranging from asymmetric Lewis acid catalysis to photoredox and electrocatalysis. Here we provide a comprehensive review of the current advancements in asymmetric catalysis utilizing iridium and rhodium-based chiral-at-metal complexes as catalysts. First, the asymmetric transformations via LUMO and HOMO activation assisted by a chiral Lewis acid catalyst are reviewed. In the second part, visible-light-induced asymmetric catalysis is summarized. The asymmetric transformation via the electricity-driven method is discussed in the final section.

Combining flavin photocatalysis with parallel synthesis: a general platform to optimize peptides with non-proteinogenic amino acids

Most peptide drugs contain non-proteinogenic amino acids (NPAAs), born out through extensive structure-activity relationship (SAR) studies using solid-phase peptide synthesis (SPPS). Synthetically laborious and expensive to manufacture, NPAAs also can have poor coupling efficiencies allowing only a small fraction to be sampled by conventional SPPS. To gain general access to NPAA-containing peptides, we developed a first-generation platform that merges contemporary flavin photocatalysis with parallel synthesis to simultaneously make, purify, quantify, and even test up to 96 single-NPAA peptide variants via the unique combination of boronic acids and a dehydroalanine residue in a peptide. We showcase the power of our newly minted platform to introduce NPAAs of diverse chemotypes-aliphatic, aromatic, heteroaromatic-directly into peptides, including 15 entirely new residues, and to evolve a simple proteinogenic peptide into an unnatural inhibitor of thrombin by non-classical peptide SAR.

Synthesis of C-linked α-Gal and α-GalNAc-1'-hydroxyalkanes by way of C2 functionality transfer

Inspired by reports of water sculpted Tn antigen (α-GalNAc-O-Ser/Thr) epitopes and our interest in producing metabolically more stable C-linked analogs of Tn, we explored the utility of C2 functionality on α-Gal-C-alkenes to deliver hydroxyl to the pendant alkenyl chain. Toward this end, a cyclic carbonate approach gave rise to a single C-linked α-Gal-1'(S)-hydroxyethane in 3 steps, and use of a 2-(hydroxyimino)galactoside cyclization transferred an oxygen to a pendant cis-substituted C-linked alkene affording the R-configuration at the newly formed stereocenter (7:1 dr). Reduction and acetylation of the resultant isoxazoline demonstrated this approach as a viable route to C-linked α-GalNAc-1'-hydroxyalkanes.

Toward Enantiomerically Pure β-Seleno-α-amino Acids via Stereoselective Se-Michael Additions to Chiral Dehydroalanines

The first totally chemo- and diastereoselective 1,4-conjugate additions of Se-nucleophiles to a chiral bicyclic dehydroalanine (Dha) are described. The methodology is simple and does not require any catalyst, providing exceptional yields at room temperature, and involves the treatment of the corresponding diselenide compound with NaBH₄ in the presence of the Dha. These Se-Michael additions provide an excellent channel for the synthesis of enantiomerically pure selenocysteine (Sec) derivatives, which pose high potential for chemical biology applications.

Conformationally Locked Pyramidality Explains the Diastereoselectivity in the Methylation of trans-Fused Butyrolactones

A stereoselectivity model inspired by the total synthesis of stemona alkaloids is developed to explain why enolate-derived 3,4-fused butyrolactones are methylated with a preference for syn alkylation. The model shows how conformational locking present in nonplanar enolate structures favors syn over anti methylation, due to less significant structural distortions in the syn pathway. The developed model was also successfully used to rationalize selectivities of previously documented methylation reactions.

Synthesis of Nβ-Substituted α,β-Diamino Acids via Stereoselective N-Michael Additions to a Chiral Bicyclic Dehydroalanine

The highly diastereoselective 1,4-conjugate additions of several nitrogen nucleophiles to chiral bicyclic dehydroalanines have been assessed effectively at room temperature in good to excellent yields without needing any catalyst or additional base. This methodology is general, simple, oxygen and moisture tolerant, high-yielding, totally chemo- and stereoselective. This procedure offers an efficient and practical approach for the synthesis of N_β-substituted α,β-diamino acids, such as 1-isohistidine, τ-histidinoalanine, β-benzylaminoalanine, β-(piperidin-1-yl)alanine, β-(azepan-1-yl)alanine, and fluorescent and ciprofloxacin-containing amino acid derivatives.

Recent advances in organocatalytic asymmetric oxa-Michael addition triggered cascade reactions

In this review, we summarized recent advances in organocatalytic asymmetric oxa-Michael addition triggered cascade reactions.

Facile triflic acid-catalyzed α-1,2-cis-thio glycosylations: scope and application to the synthesis of S-linked oligosaccharides, glycolipids, sublancin glycopeptides, and TN/TF antigens

Studies of S-linked glycoconjugates have attracted growing interest because of their enhanced chemical stability and enzymatic resistance over O-glycoside counterparts.

Studies of S-linked glycoconjugates have attracted growing interest because of their enhanced chemical stability and enzymatic resistance over O-glycoside counterparts. We here report a facile approach to access α-1,2-cis-S-linked glycosides using triflic acid as a catalyst to promote the glycosylation of a series of thiols with d-glucosamine, galactosamine, glucose, and galactose electrophiles. This method is broadly applicable for the stereoselective synthesis of S-linked glycopeptides, oligosaccharides and glycolipids in high yield and excellent α-selectivity. Many of the synthetic limitations associated with the preparation of these S-linked products are overcome by this catalytic method.

Access to α,γ-Diamino Diacid Derivatives via Organocatalytic Asymmetric 1,4-Addition of Azlactones and Dehydroalanines

A convenient and functional-group-tolerant organocatalytic asymmetric 1,4-addition of azlactones and dehydroalanine is disclosed. The reaction is used for the first synthesis of chiral α,γ-diamino diacid derivatives with nonadjacent stereogenic centers in moderate to high yields, with excellent diastereo- and enantioselectivities, under the catalysis of a chiral thiourea catalyst. In addition, the reaction could be conducted in gram-scale, and the products of the reaction could be readily converted to various α,γ-diamino diacid derivatives, α,γ-diamino dialcohols, and modified peptides with nonproteinogenic amino acid residues.

Progress in Lanthionine and Protected Lanthionine Synthesis

Lanthionine (Lan), a non-proteinogenic natural amino acid, is an essential component of peptidoglycan found in the cell wall of Fusobacterium species. Lan and β-methyllanthionine are also key constituents in lantibiotics, a prevalent class of peptide antibiotics. The development of those new antibacterial drugs with enhanced properties is the focus of recent research. Since multiple isomers of Lan are possible, a regio- and diastereoselective synthesis is challenging. This comprehensive review summarizes the known chemical syntheses of lanthionine from various precursors (e.g., β-chloroalanine, cystine, dehydroalanine, β-iodoalanine, aziridine, serine lactone, sulfamidate) since 1941. Methods for preparation of unprotected, protected, orthogonally protected, and mutually orthogonally protected lanthionine with relevant experimental details and perspectives on their usefulness are provided. The potential of these Lan derivatives is illustrated by one recent application.

Synthesis of modified proteins via functionalization of dehydroalanine

Dehydroalanine has emerged in recent years as a non-proteinogenic residue with strong chemical utility in proteins for the study of biology. In this review we cover the several methods now available for its flexible and site-selective incorporation via a variety of complementary chemical and biological techniques and examine its reactivity, allowing both creation of modified protein side-chains through a variety of bond-forming methods (C-S, C-N, C-Se, C-C) and as an activity-based probe in its own right. We illustrate its utility with selected examples of biological and technological discovery and application.

Tn Antigen Mimics by Ring-Opening of Chiral Cyclic Sulfamidates with Carbohydrate C1- S- and C1- O-Nucleophiles

Starting from commercially available ( S)-isoserine and effectively accessible ( S)-α-methylserine, enantiopure cyclic sulfamidates have been prepared as chiral building blocks for the synthesis of various S- and O-glycosylated amino acid derivatives, including unnatural variants of the Tn antigen, through highly chemo-, regio-, and stereoselective nucleophilic ring-opening reactions with carbohydrate C1- S- and C1- O-nucleophiles.