A concise, stereoselective synthesis of meso-2,6-diaminopimelic acid (DAP)

Nucleophilic Addition of Amides to Haloalkynes: Synthesis of (Z)-β-Halovinyl Amides as Dual Precursors of Alkylidene Carbenes and Allyl Halides

Toward a regioselective method for the synthesis of β-halovinyl amides, we developed a transition-metal-free nucleophilic addition reaction of amides to haloalkynes. The regioselective nucleophilic addition was achieved under solvent-free conditions using phosphonates to protonate the intermediate alkylidene carbenoids, thus suppressing their decomposition. Furthermore, we demonstrate that β-halovinyl amides can serve as dual precursors of allyl halides and alkylidene carbenes to obtain functionalized indoles and pyrrolidones.

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.

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.

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).

Peptidoglycan as Nod1 ligand; fragment structures in the environment, chemical synthesis, and their innate immunostimulation

Covering: up to 2011. This review focuses on the recent revealing of the immunostimulatory bacterial cell wall peptidoglycan (PGN) fragments as Nod1 ligands, especially a newly developed chemical synthesis of the partial structures, fragment structures in the environment and bacterial supernatant, and the immunostimulatory activities of the Nod1 ligands.

Structures, synthesis, and human Nod1 stimulation of immunostimulatory bacterial peptidoglycan fragments in the environment

Bacteria release immunostimulatory compounds to the environment, and one of the stimulants is the ligand of nucleotide-binding oligomerization domain protein 1 (Nod1), an intracellular protein involved in the recognition of the bacterial component peptidoglycans having a diaminopimelic acid (DAP) structure. The polymorphisms of Nod1 have been linked to several inflammatory diseases and allergies that are strongly affected by environmental factors. The present paper summarizes recent results on the isolation and structural elucidation of natural human Nod1 (hNod1) ligands from the Escherichia coli (E. coli) K-12 culture supernatant, the first chemical synthesis of these natural ligands and related PGN fragments structures, and the hNod1 stimulatory activities of the chemically synthesized DAP-type PGN fragments. For structural characterization studies, the 7-(diethylamino)coumarin-3-carbonyl (DEAC) labeling method was also used to enhance the sensitivity in mass spectrometry studies, in order to observe PGN fragments in a comprehensive manner. The results suggest that DAP-containing bacteria release certain hNod1 ligands to the environment and that these ligands accumulate in the environment and regulate the immune system through Nod1.

Synthesis of diaminopimelic acid containing peptidoglycan fragments and tracheal cytotoxin (TCT) and investigation of their biological functions

Bacterial cell wall peptidoglycan (PGN) is a potent immunostimulator and immune adjuvant. The PGN of Gram-negative bacteria and some Gram-positive bacteria contain meso-diaminopimelic acid (meso-DAP), and we have recently shown that the intracellular protein Nod1 is a PGN receptor and recognizes DAP-containing peptides. In this study, we achieved the synthesis of DAP-containing PGN fragments, including the first chemical synthesis of tracheal cytotoxin (TCT), GlcNAc-(beta1-4)-(anhydro)MurNAc-L-Ala-gamma-D-Glu-meso-DAP-D-Ala, and a repeating-unit of DAP-type PGN, GlcNAc-(beta1-4)-MurNAc-L-Ala-gamma-D-Glu-meso-DAP-D-Ala. For the synthesis of PGN fragments, we first established a new synthetic method for an orthogonally protected meso-DAP derivative, and then we constructed the glycopeptide structures. The ability of these fragments to stimulate human Nod1, as well as differences in Nod1 recognition of the variety of synthesized ligand structures were examined. The results showed that the substitution of the N terminus of iE-DAP is necessary for stronger Nod1 recognition, but the structure of the substituent seems not to be strictly recognized. The importance of the carboxyl group at the 2-position of DAP for human Nod1 stimulation was also shown.

Effectiveness of the suzuki-miyaura cross-coupling reaction for solid-phase peptide modification

The Suzuki-Miyaura (SM) cross-coupling reaction has recently become one of the most efficient methods for C-C bond construction opening a wide range of opportunities in organic synthesis. This study focused on the evaluation of the use of the SM reaction to modify peptides using a solid-phase synthesis approach, an avenue that was still not investigated intensively. We used as a peptide model [Ala (1,2,3), Leu (8)]Enk linked to a polystyrene support on which it was previously assembled. The aromatic residues Tyr (4) and Phe (7) of [Ala (1,2,3), Leu (8)]Enk were respectively substituted with p-iodo-Phe, and an SM-related strategy was developed. Results indicated that the reaction conditions involving K 3PO 4 or Na 2CO 3 (base), DMF (solvent), Pd(PPh 3) 4 (catalyst), and temperatures ranging from 50 to 80 degrees C during 20 h were found as optimal. Finally applying those optimal conditions, a series of [Ala (1,2,3), Leu (8)]Enk analogs modified at Tyr (4) or Phe (7) positions was synthesized using diverse boronic acid derivatives.

A straightforward stereoselective synthesis of meso-, (S,S)- and (R,R)-2,6-diaminopimelic acids from cis-1,4-diacetoxycyclohept-2-ene

A straightforward synthesis of meso-2,6-diaminopimelic acid (DAP) meso-1 was developed from 1,4-diacetoxycyclohept-2-ene (2) via an oxidative ring cleavage. Subsequently, an enantio-divergent synthesis of (S,S)- and (R,R)-1 was performed using a homochiral monoacetate 7 available from 2 by enzymatic desymmetrization.

Organoboron Reagents in the Preparation of Functionalized ?-Amino Acids

Over the past decade, major advances in the preparation and utilization of organoboron reagents have been applied to virtually all areas of organic synthesis. The present review collates recent examples of the use of organoboron reagents in the synthesis of α-amino acids and their derivatives. Aryl- and alkenylboronic acids have been used in the asymmetric synthesis of α-amino acids through conjugate addition to unsaturated amino acids and the Petasis three-component coupling reaction. Additionally, α-amino acid derivatives with organoboron functionality on the side-chain have been prepared and used in metal-catalyzed cross-coupling reactions to prepare cross-linked amino acids and complex cyclic peptide natural products.

Synthesis and proinflammatory properties of muramyl tripeptides containing lysine and diaminopimelic acid moieties

The unusual amino acid diaminopimelic acid (DAP) was prepared by cross metathesis of appropriately protected vinyl glycine and allyl glycine derivatives. Catalytic hydrogenation of the cross-coupling product resulted in reduction of the double bond and the removal of protecting groups. The resulting compounds were appropriately protected for the polymer-supported and solution-phase synthesis of muramyl tripeptides 2 and 3, which differ in the amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties. Muramyl dipeptide (1, MDP), the DAP-containing muramyl tripeptide 3, and the lysine-containing muramyl tripeptides 4 and 5 induced TNF-alpha gene expression without TNF-alpha protein production in a human monocytic cell line. The observed block in translation could be removed by co-incubation with LPS, resulting in an apparent synergistic effect. Compound 2 did not induce TNF-alpha gene expression, neither did it exhibit a synergistic effect with LPS; this indicates that amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties results in a loss of biological activity. It is proposed that amidation of alpha-carboxylic acids is a strategy that may be used by pathogens to avoid detection by the innate immune system. Furthermore, the pattern recognition receptors Nod1 and Nod2 have been implicated in the possible induction of a synergistic effect of muropeptides with LPS.

Synthesis of dehydroalanine fragments as thiostrepton side chain mimetics

Syntheses of dehydroalanine derivatives via a solid-support route, starting from selenocystein, and via conventional solution phase chemistry are described along with initial biological testing. The target compounds were designed as mimetics of the dehydroalanine side chain of the macrocyclic antibiotic thiostrepton that acts on the bacterial ribosome.

Enantiomerically pure alpha-amino acid synthesis via hydroboration-suzuki cross-coupling

The Garner aldehyde-derived methylene alkene 5 and the corresponding benzyloxycarbonyl compound 25 undergo hydroboration with 9-BBN-H followed by palladium-catalyzed Suzuki coupling reactions with aryl and vinyl halides. After one-pot hydrolysis-oxidation, a range of known and novel nonproteinogenic amino acids were isolated as their N-protected derivatives. These novel organoborane homoalanine anion equivalents are generated and transformed under mild conditions and with wide functional group tolerance: electron-rich and -poor aromatic iodides and bromides (and a vinyl bromide) all undergo efficient Suzuki coupling. The extension of this methodology to prepare meso-DAP, R,R-DAP, and R,R-DAS is also described.