A facile synthesis of fully protected meso-diaminopimelic acid (DAP) and its application to the preparation of lipophilic N-acyl iE-DAP

Synthesis and applications of symmetric amino acid derivatives

Symmetric α-amino acid derivatives can be used for the synthesis of intermolecularly linked peptides such as dimer-type peptides, and modified peptides in which two amino acids are intramolecularly linked. They are also synthetic intermediates for the total synthesis of natural products and functional molecules. These symmetric amino acid derivatives must be prepared based on organic synthesis. It is necessary to develop an optimal synthetic strategy for constructing the target symmetric amino acid derivative. In this review, we will introduce strategies for synthesizing symmetric amino acid derivatives. Additionally, selected applications of these amino acids in the life sciences will be described.

Synthesis and validation of click-modified NOD1/2 agonists

NOD1 and NOD2 sense small bacterial peptidoglycan fragments, often called muropeptides, that access the cytosol. These muropeptides include iE-DAP and MDP, the minimal agonists for NOD1 and NOD2, respectively. Here, we synthesized and validated alkyne-modified muropeptides, iE-DAP-Alk and MDP-Alk, for use in click-chemistry reactions. While it has long been known that many cell types respond to extracellular exposure to muropeptides, it is unclear how these innate immune activators access their cytosolic innate immune receptors, NOD1 and NOD2. The subcellular trafficking and transport mechanisms by which muropeptides access these cytosolic innate immune receptors are a major gap in our understanding of these critical host responses. The click-chemistry-enabled agonists developed here will be particularly powerful to decipher the underlying cell biology and biochemistry of NOD1 and NOD2 innate immune sensing.

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 characteristic Mycobacterium peptidoglycan (PGN) fragments utilizing with chemoenzymatic preparation of meso-diaminopimelic acid (DAP), and their modulation of innate immune responses

Peptidoglycan (PGN) is a major component of bacterial cell wall and is recognized as a potent immunostimulant. The PGN in the cell envelope of Mycobacterium Tuberculosis has been shown to possess several unique characteristics including the presence of N-glycolyl groups (in addition to N-acetyl groups) in the muramic acid residues, and amidation of the free carboxylic acid of d-Glu or of meso-DAP in the peptide chains. Using a newly developed, highly stereoselective, chemoenzymatic approach for the synthesis of meso-DAP in peptide stems, we successfully synthesized for the first time, a series of Mycobacterium PGN fragments that include both mono- and disaccharides of MurNGlyc or 1,6-anhydro-MurNGlyc, as well as peptide-amidated variants. The ability of these PGN fragments to stimulate the immune system through activation of human Nod1 and Nod2 was examined. The PGN fragments were found to modulate immune stimulation, specifically, amidation at the d-Glu and meso-DAP in the peptide stem strongly reduced hNod1 activation. This effect was dependent on modification position. Additionally, N-glycolyl (instead of acetyl) of muramic acid was associated with slightly reduced human Nod1 and Nod2 stimulatory capabilities.

Expeditious synthesis of enantiopure, orthogonally protected bis-α-amino acids (OPBAAs) and their use in a study of Nod1 stimulation

A convenient approach towards the synthesis of orthogonally protected chiral bis-α-amino acids (OPBAAs) is described. The key transformations include: (1) a highly stereoselective conjugation (alkylation) of the Schöllkopf bis-lactim ethers and oxazolidinyl alkyl halides to build a backbone skeleton; and (2) our orthogonal protection strategy. A series of enantiopure OPBAAs bearing a variety of alkyl chain as a spacer; two stereogenic centers; and three protecting groups were prepared as examples. These versatile molecules were applied to the synthesis of biologically interesting di- or tri-peptide analogues, including chiral iE-meso-DAP and A-iE-meso-DAP, for the study of Nod1 activation in the innate immune response.

[Synthetic medicinal chemistry of the biomolecular components mimics]

We are studying the medicinal synthetic chemistry of biomolecular component mimics such as carbohydrates, nucleosides, amino acids, and peptides. In this review, the synthesis and biological activities of iminosugars as carbohydrate mimics are discussed. Glycosidases and glycosyltransferases are involved in a wide range of anabolic and catabolic process, including digestion, the lysosomal catabolism of glycoconjugates, glycoprotein biosynthesis. Hence, modifying or blocking these processes in vivo using inhibitors is a topic of great interest from the therapeutic point of view. Iminosugars are sugars in which the endocyclic oxygen is replaced by a basic nitrogen atom. They are regarded as transition state mimics in certain types of enzyme reactions. This makes the field of iminosugars as carbohydrate mimics an exciting area of research. We synthesized all of the stereoisomers of polyhydroxypiperidines such as fagomine, 1-deoxynojirimycine, and isofagomine. In addition, their both enantiomers, as substrates for a variety of glycosidases were evaluated. Secondly, the asymmetric synthesis of α-1-C-alkyl-arabinoiminofuranoses was achieved by asymmetric allylic alkylation, RCM, and Negishi cross coupling as key reactions. Surprisingly, the L-forms showed a quite potent inhibitory activity toward rat intestinal maltase, while the activities of the D-forms were much weaker. Some of the prepared L-forms showed potent inhibitory activities towards intestinal maltase, with IC₅₀ values comparable to those of commercial drugs such as acarbose, voglibose, and miglitol, which are used in the treatment of type 2 diabetes. Among them, the inhibitory activity towards intestinal sucrase of α-1-C-L-butylarabinoiminofuranose was quite strong towards intestinal sucrase compared to the above commercial drugs.

Synthesis of conformationally constrained γ-D-glutamyl-meso-diaminopimelic acid derivatives as ligands of nucleotide-binding oligomerization domain protein 1 (Nod1)

Nod1, an important member of the pattern recognition receptor family, remains a virtually unexploited target. Harnessing its innate immune stimulatory properties still remains an unfulfilled goal of medicinal chemistry. Nucleotide-binding oligomerization domain protein 1 (Nod1) agonists have been shown to boost the inflammatory responses against pathogenic microbes and could thus constitute a new class of broad spectrum antimicrobial agents. To gain additional insight into the structure/activity relationships of Nod1 agonistic compounds, a series of novel, conformationally constrained γ-D-glutamyl-meso-diaminopimelic acid (iE-DAP) analogs have been designed and synthesized. Ramos-Blue cells expressing Nod1 were used to screen and validate our compounds for their Nod1-agonist activity. Their immunomodulatory properties were subsequently determined in vitro, by evaluating their capacity to induce pro-inflammatory cytokine and chemokine production from human peripheral blood mononuclear cells (PBMC), by themselves and in synergy with lipopolysaccharide (LPS), a Toll-like receptor 4 (TLR4) ligand. The synthesized iE-DAP analogs were shown to possess immuno-enhancing properties as a result of their potent and specific Nod1-agonistic effect. The activity of the compound exhibiting the greatest capacity to induce pro-inflammatory cytokine release from PBMC surpassed that of lauroyl-γ-D-glutamyl-meso-diaminopimelic acid (C12-iE-DAP).