Temperature-induced formation of Pd nanoparticles in heterogeneous nanobiohybrids: application in C-H activation catalysis.

The effect of the temperature in the synthesis of Pd nanoparticles in the metal-enzyme biohybrids is evaluated. The effect on the formation, size, and morphology of nanoparticles was evaluated using C. antarctica B lipase as the protein scaffold. XRD analyses confirmed the formation of crystalline Pd(0) as the metal species in all cases. TEM analyses revealed spherical crystalline nanoparticles with average diameter size from 2 nm at 4 °C synthesis to 10 nm obtained at 50 °C synthesis. The thermal phenomenon was also critical in the final hybrid formation using more complex enzymes, where the relation of the protein structure and temperature and the influence of the latter has been demonstrated to be critical in the reducing efficiency of the enzyme in the final Pd nanoparticle formation, in the metal species, or even in the final size of the nanoparticles. Different Pd biohybrids were evaluated as catalysts in the C-H activation of protected l-tryptophan under mild conditions. Pd@CALB4 showed the best results, with >99% conversion for C-2 arylation in methanol at room temperature with a TOF value of 64 min-1, being 2 or 4 times higher than that of the other synthesized hybrids. This catalyst showed a very high stability and recyclability, maintaining >95% activity after three cycles of use.

Practical synthesis and biological screening of sulfonyl hydrazides.

A methodology for the synthesis of sulfonyl hydrazides mediated by hypervalent iodine is described. Taking advantage of the umpolung properties of hypervalent iodine reagents, the polarity of sodium sulfinate salts is reversed, and a key intermediate is generated and reacted with mono- and disubstituted hydrazines. To highlight the practical utility of this protocol, a diverse range of sulfonyl hydrazides were synthesized in yields up to 62%. Furthermore, a gram-scale reaction was performed, showing the robustness of the procedure. Mechanistic studies, including DFT calculations, were performed and the bioactivity of the generated compounds was evaluated.

Synthesis of Bis(3-indolyl)methanes Mediated by Potassium tert-Butoxide.

The indole moiety is an important N-heterocycle found in natural products, and a key structural component of many value-added chemicals including pharmaceuticals. In particular, bis(3-indolyl)methanes (BIMs) are an important subgroup of indoles, composed of two indole units. Herein, we report the development of a simple method to access BIMs derivatives in yields of up to 77 % by exploiting a tBuOK-mediated coupling reaction of indoles and benzyl alcohols.

Hypervalent Iodine(III) Reagents with Transferable Primary Amines: Structure and Reactivity on the Electrophilic α-Amination of Stabilized Enolates.

A new family of hypervalent iodine reagents containing transferable primary amine groups is described. Benziodoxolone-based reagents were synthesized on the gram-scale through operationally simple reactions in up to quantitative yields. These bench-stable solids were characterized by X-ray analysis and successfully employed in the α-amination of indanone-based ß-ketoesters in up to 83% yield. Mechanistic studies indicate a substitution mechanism involving an electrophilic amine.

Assembly of Peptidoglycan Fragments-A Synthetic Challenge.

Peptidoglycan (PGN) is a major constituent of most bacterial cell walls that is recognized as a primary target of the innate immune system. The availability of pure PGN molecules has become key to different biological studies. This review aims to (1) provide an overview of PGN biosynthesis, focusing on the main biosynthetic intermediates; (2) focus on the challenges for chemical synthesis posed by the unique and complex structure of PGN; and (3) cover the synthetic routes of PGN fragments developed to date. The key difficulties in the synthesis of PGN molecules mainly involve stereoselective glycosylation involving NAG derivatives. The complex synthesis of the carbohydrate backbone commonly involves multistep sequences of chemical reactions to install the lactyl moiety at the O-3 position of NAG derivatives and to control enantioselective glycosylation. Recent advances are presented and synthetic routes are described according to the main strategy used: (i) based on the availability of starting materials such as glucosamine derivatives; (ii) based on a particular orthogonal synthesis; and (iii) based on the use of other natural biopolymers as raw materials.

A top-down chemo-enzymatic approach towards N-acetylglucosamine-N-acetylmuramic oligosaccharides: Chitosan as a reliable template.

An unprecedented approach towards oligosaccharides containing N-acetylglucosamine-N-acetylmuramic (NAG-NAM) units was developed. These novel bacterial cell wall surrogates were obtained from chitosan via a top down approach involving both chemical and enzymatic reactions. The chemical modification of chitosan using a molecular clamp based strategy, allowed obtaining N-acetylglucosamine-N-acetylmuramic (NAG-NAM) containing oligomers. Intercalation of NAM residues was confirmed through the analysis of oligosaccharide fragments from enzymatic digestion and it was found that this route affords NAG-NAM containing oligosaccharides in 33% yield. These oligosaccharides mimic the carbohydrate basic skeleton of most bacterial cell surfaces. The oligosaccharides prepared are biologically relevant and will serve as a platform for further molecular recognition studies with different receptors and enzymes of both bacterial cell wall and innate immune system. This strategy combining both chemical modification and enzymatic digestion provides a novel and simple route for an easy access to bacterial cell wall fragments - biologically important targets.

Metal-Catalyzed Cross-Coupling Reactions on Azaindole Synthesis and Functionalization.

Azaindoles are rare in nature but extremely attractive for drug discovery programs. Azaindoles can be obtained by diverse methods, including those involving metal-catalyzed reactions. This important core has been fascinating the scientific community due to their challenging synthesis and relevant bioactivity. This paper highlights the diverse synthetic methodologies developed to date involving metal-catalyzed reaction to attain azaindoles and its functionalization.

One-Pot Synthesis of 1,2-Disubstituted 4-, 5-, 6-, and 7-Azaindoles from Amino-o-halopyridines via N-Arylation/Sonogashira/Cyclization Reaction.

A direct synthesis of several 1,2-disubstituted 4-, 5-, 6-, and 7-azaindoles from available amino-o-halopyridines is described. This procedure involves a palladium-catalyzed N-arylation followed by a Sonogashira reaction and subsequent cyclization in a one-pot manner, exhibiting a wide scope and compatibility with electron-withdrawing and electron-donating groups. The strategy represents an advancement in azaindole chemistry with a straightforward approach toward 1,2-disubstituted azaindoles, while avoiding complex N-arylations of hindered 2-substituted azaindoles and difficult purification steps of intermediates.

Selective Modification of Chitin and Chitosan: En Route to Tailored Oligosaccharides.

Chitin and chitosan are attractive biopolymers with enormous structural possibilities for chemical modification, creating platforms for new chemical entities with a broad scope of applications, ranging from material science to medicine. During the last few years, incredible efforts have been dedicated to the regioselective modification of these biopolymers paving the way for improved properties and tailored activities. Herein, the most recent advances in chitin/chitosan regioselective modification, reaction conditions, selectivity, and the impact on its applications are highlighted. Moreover, the recent focus on chitooligosaccharides, their regioselective and chemoselective functionalization, as well as their role in biological studies, including molecular recognition with several biological targets are also covered.

Synthesis of Substituted 4-, 5-, 6-, and 7-Azaindoles from Aminopyridines via a Cascade C-N Cross-Coupling/Heck Reaction.

A practical palladium-catalyzed cascade C-N cross-coupling/Heck reaction of alkenyl bromides with amino-o-bromopyridines is described for a straightforward synthesis of substituted 4-, 5-, 6-, and 7-azaindoles using a Pd2(dba)3/XPhos/t-BuONa system. This procedure consists of the first cascade C-N cross-coupling/Heck approach toward all four azaindole isomers from available aminopyridines. The scope of the reaction was investigated and several alkenyl bromides were used, allowing access to different substituted azaindoles. This protocol was further explored for N-substituted amino-o-bromopyridines.

Inhibition of LOX by flavonoids: a structure-activity relationship study.

The lipoxygenase (LOX) products have been identified as mediators of a series of inflammatory diseases, namely rheumatoid arthritis, inflammatory bowel disease, psoriasis, allergic rhinitis, atherosclerosis and certain types of cancer. Hence, LOX inhibitors are of interest for the modulation of these phenomena and resolution of the inflammatory processes. During LOX activity, peroxyl radical complexes are part of the reaction and may function as sources of free radicals. Thus antioxidants, such as flavonoids, capable of inhibiting lipid peroxidation and scavenging free radicals, may act as LOX inhibitors. The aim of this work was to assess the structure-activity relationship among a series of flavonoids concerning 5-LOX inhibition, through a systematic study of the inhibition of the formation of LTB4 in human neutrophils. The type of inhibition of the flavonoids was further studied using soybean LOX, type I, and Saturation Transfer Difference (1)H NMR (STD-(1)H NMR) was used to characterize the binding epitopes of the compounds to LOX-1. The obtained results reinforce flavonoids as effective inhibitors of LTB4 production in human neutrophils. It was also possible to establish a structure/activity relationship for the inhibitory activity and the type of inhibition.

Synthesis of the NAG-NAM disaccharide via a versatile intermediate.

A simple strategy for the synthesis of a ß-GlcNAc-(1→4)-MurNAc (NAG-NAM) moiety, crucial for the preparation of synthetic components of a bacterial peptidoglycan, was achieved. This strategy relies on the use of three O-protecting groups, 4,6-O-benzilidene acetal, benzyl, and acetyl group, which allows further regioselective manipulation at O-3, O-4 positions, and on the insertion of the peptide chain at the lactate moiety in an advanced and versatile intermediate. Overall, a simple route to achieve the biological relevant NAG-NAM is presented, which may serve as a conceptual framework in the designing of synthetic strategies of different natural and non-natural polysaccharides.

Indole based cyclooxygenase inhibitors: synthesis, biological evaluation, docking and NMR screening.

The close structural similarity between the two cyclooxygenase (COXs) isoforms and the absence of selective inhibitors without side effects continues to stimulate the development of novel approaches towards selective anti-inflammatory drugs. In the present study a small library of new indolic compounds involving two different substitutions patterns at the indole scaffold was synthesized. In order to establish a relation between the spatial distribution of known functional groups related with inhibitory activity, two substitution patterns were explored: one with substituents at N-1, C-3, C-5 positions and another at C-2, C-3 and C5 positions. Accordingly, indole positions C-5, C-3 and N-1 were substituted with: sulfonamide or methylsulfone at C-5, p-halo-benzyl group at C-3, and an alkyl chain with a trifluoromethyl group at N-1. Alternatively, a p-halo-benzyl group was introduced at C-2, leaving the indolic nitrogen free. Inhibitory studies were performed and the activity results obtained against both COXs isoforms were rationalized based on docking and NMR studies. Docking studies show that dialkyation at C-2 and C-3 favors a binding with an orientation similar to that of the known selective inhibitor SC-558. From the tested compounds, this substitution pattern is correlated with the highest inhibitory activity and selectivity: 70% COX-2 inhibition at 50 µM, and low COX-1 inhibition (18 ± 9%). Additionally, Saturation Transfer Difference NMR experiments reveal different interaction patterns with both COXs isoforms that may be related with different orientations of the sulfonamide group in the binding pocket. Despite the moderated inhibitory activities found, this study represents an innovative approach towards COXs inhibitory activity rationalization and to the design of anti-inflammatory drugs.

Stereoselective glycosylation of glucosamine: the role of the N-protecting group.

Oligosaccharides and glycoconjugates play an important role in biological processes. The use of these complex polymers as biocompatible materials for medicinal applications as well as therapeutic agents for the treatment of several diseases has attracted considerable interest. However, these investigations require large and pure amounts of glycostructures. Glucosamine is one of the major building blocks of these highly important glycoconjugates. Recently, considerable synthetic efforts have been devoted to improving stereoselective glycosylation. In this Focus review, the role of the amine protecting group in the outcome of the glucosamine glycosylation reaction is highlighted.

Binding of ibuprofen, ketorolac, and diclofenac to COX-1 and COX-2 studied by saturation transfer difference NMR.

Saturation transfer difference NMR (STD-NMR) spectroscopy has emerged as a powerful screening tool and a straightforward way to study the binding epitopes of active compounds in early stage lead discovery in pharmaceutical research. Here we report the application of STD-NMR to characterize the binding of the anti-inflammatory drugs ibuprofen, diclofenac, and ketorolac to COX-1 and COX-2. Using well-studied COX inhibitors and by comparing STD signals with crystallographic structures, we show that there is a relation between the orientations of ibuprofen and diclofenac in the COX-2 active site and the relative STD responses detected in the NMR experiments. On the basis of this analysis, we propose that ketorolac should bind to the COX-2 active site in an orientation similar to that of diclofenac. We also show that the combination of STD-NMR with competition experiments constitutes a valuable tool to address the recently proposed behavior of COX-2 as functional heterodimers and complements enzyme activity studies in the effort to rationalize COX inhibition mechanisms.

Developments towards regioselective synthesis of 1,2-disubstituted benzimidazoles.

1,2-Disubstituted benzimidazoles play an important role in several areas and particularly as drug discovery targets. Herein, several methods to assemble these structures are reviewed, from the classical approaches to the more recently developed metal-catalyzed intramolecular amination process, the cascade arylamination/condensation reaction and polymer-supported benzimidazole assembly under microwave conditions.

A new insight on the hypochlorous acid scavenging mechanism of tryptamine and tryptophan derivatives.

The reaction mechanisms of hypochlorous acid (HOCl) with several tryptophan and tryptamine derivatives, previously reported to scavenge this powerful oxidant, was investigated to determine whether ionic or radical pathways were involved. For this purpose, the reaction of tryptamine and tryptophan derivatives with HOCl was optimized and some compounds were isolated by HPLC and their structures assigned. In order to prevent possible radical reaction pathway, experiments have been carried in the presence of the radical trap TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). The obtained results show that the reaction mechanisms are influenced by the type of structure and that a complex pathway is involved, in which both ionic and radical mechanisms can occur.

Antioxidant activity of unexplored indole derivatives: synthesis and screening.

The present study envisaged the development of novel antioxidant candidates using the indole scaffold. Several tryptophan and tryptamine derivatives were synthesized, in particular prenylated indole compounds, and their scavenging activity for reactive oxygen species (ROS) and reactive nitrogen species (RNS) was investigated. The library substitution pattern included several alkyl chains at positions N-1, C-2 of the indole nucleus, including prenyl and isopentyl chain, as well as different groups at the side chain (C-3) that allowed the investigation of a possible radical stabilization. The results obtained showed that tryptophan (8), tryptamine (9), N-phthaloyl tryptamine (5) and N-prenyl tryptophan (13) were the most active against peroxyl radical (ROO(•)) with activities higher than Trolox, which was used as control. The scavenging of hypochlorous acid (HOCl) was also evaluated and tryptophan (8) and tryptamine (9) showed IC(50) of 3.50 ± 0.4 and 6.00 ± 0.60 µM, respectively. Significant activity was also found for the N-prenyl tryptophan (13) with an IC(50) of 4.13 ± 0.17 µM and C-2 prenylated derivative (14), with 4.56 ± 0.48 µM. The studies were extended to RNS and best results were obtained against peroxynitrite anion (ONOO(-)) in the presence of NaHCO(3). N-alkylated tryptophan (18) showed a high activity with an IC(50) of 14.0 ± 6.8 µM. The results show that the tested compounds are effective scavengers of ROS and RNS, and suggest that the radical stabilization is strongly dependent on the type of substituents on the indolic moiety and on their relative positions. In addition, the radical dissipation inside the indolic system is mandatory for the observed antioxidant activity.

Application of HR-MAS NMR in the solid-phase synthesis of a glycopeptide using Sieber amide resin.

The solid-phase synthesis (SPS) of a structurally complex glycopeptide, using Sieber amide resin, was monitored by high resolution magic angle spinning NMR, demonstrating the further application of this technique. A synthetic peptidoglycan derivative, a precursor of a biologically active PGN, known to be involved in the cellular recognition, was prepared by SPS. The synthesis involved the preparation of an N-alloc glucosamine moiety and the synthesis of a simple amino acid sequence L-Ala-D-Glu-L-Lys-D-Ala-D-Ala. Last step consisted the coupling, on solid-phase, of the protected muramyl unit to the peptide chain. Proton spectra with good suppression of the polystyrene signals in swollen resin samples were obtained in DMF-d(7) as a solvent and by using a nonselective 1D TOCSY/DIPSI-2 scheme, thus allowing to follow the SPS without losses of compound and cleavage from the resin. The assignment of the proton spectra of the resin-bound amino acid sequence and of the bound glycopeptide was achieved through the combination of MAS COSY, TOCSY and NOESY.

Studies in sigmatropic rearrangements of N-prenylindole derivatives--a formal enantiomerically pure synthesis of tryprostatin B.

Rearrangement of N(a)-prenyl-N(b)-acetyltryptamine, induced by BF3.Et2O at low temperature, leads to a 2-prenyl derivative, and thence to the tricyclic tryptamine 7 and the indoline 8. Similarly, N(a)-prenyl-N(b)-phthaloyl-l-tryptophan methyl ester furnished the corresponding 2-prenyl derivative 16, a known advanced precursor of tryprostatin B. Density functional (B3LYP) calculations for the putative rearrangement transition state for N-prenylskatole show that prior coordination of BF3 to the indolic nitrogen changes the character of the subsequent sigmatropic pericyclic shifts from being entirely covalent to acquiring a significant degree of ionic character. The shifting prenyl group favours the endo over the exo mode of the transition state by 4.1 kcal mol(-1).