The presence of water improves reductive openings of benzylidene acetals with trimethylaminoborane and aluminium chloride

The acidic reagent formed in situ from anhydrous AlCl(3) and H(2)O in 3:1 ratio is much more efficient for the reductive openings of the cyclic benzylidene acetals with Me(3)N x BH(3) in tetrahydrofurane than the AlCl(3) alone. Under proposed conditions, the dioxane-type 4,6-O-bezylidene acetals of hexopyranosides give regioselectively the corresponding 4-hydroxy,6-O-benzyl derivatives in excellent yields. Reductive openings of the dioxolane-type 3,4-O-benzylidene acetals of galactopyranoside are also very efficient and regioselective and give either 3-O-benzyl derivative (from 3,4-O-exo-benzylidene acetal) or 4-O-benzyl derivative (from 3,4-O-endo-benzylidene acetal) depending on the configuration of the acetal carbon atom.     

Regioselective ring opening of exo- and endo-3,4-benzylidene acetals of arabinopyranoside derivatives with Lewis acids and reducing agents

Dioxolane type 3,4-benzylidene acetals of benzyl β-l-arabinose either as a mixture or pure exo- and endo-isomers cleavaged with BF₃·OEt₂/Et₃SiH in dichloromethane or acetonitrile regioselectively, provided the 4-O-benzyl-3-hydroxy derivative. The reaction with TiCl₄/Et₃SiH or Cu(OTf)₂/Et₃SiH provided a mixture of 3- and 4-O-benzyl derivatives whereas with Cu(OTf)₂/BH₃·THF gave only hydrolyzed product. The regioselectivity of the reaction was proved to be directed by the acetyl substitution at C-2. Benzyl substitution provided a mixture of 3- and 4-O-benzyl derivatives in 1:1 ratio whereas non-substitution yielded the same mixture in 2:1 ratio.     

Reductive openings of benzylidene acetals. Kinetic studies of borane and alane activation by Lewis acids

The reaction kinetics for a number of reductive openings of methyl 2,3-di-O-benzyl-4,6-O-benzylidene-α-d-glucopyranoside have been investigated. Openings to give free HO-6 (using BH₃·THF-AlCl₃-THF or LiAlH₄-AlCl₃-Et₂O) follow first order kinetics, while reactions yielding free HO-4 (using BH₃·NMe₃-AlCl₃-THF or BH₃·NMe₃-BF₃·OEt₂-THF) follow higher order kinetics. The addition of water to the BH₃·NMe₃-AlCl₃-THF results in faster reactions. The BH₃·SMe₂-AlCl₃-THF system constitutes a borderline case, yielding both free HO-6 (by a first order reaction) and free HO-4 (by a higher order reaction). These results correlate well with the concept of regioselectivity by activation of borane complexes.     

Allyl deprotection of galacturonic acid derivatives: mechanistic aspects of mercuric-catalyzed prop-1-enyl acetal cleavage

Different deallylation methods were assayed for selective deprotection of allyl galactopyranosiduronic acid derivatives. A two-step procedure using DABCO and (Ph(3)P)(3)RhCl followed by mercuric-assisted cleavage gave quantitative yields. Reaction in the presence of [(18)O]water allowed us to obtain evidence about the mechanism of prop-1-enyl cleavage.     

Selective oxygen transfer catalysed by heme peroxidases: synthetic and mechanistic aspects

The synthetic and mechanistic aspects of the use of heme peroxidases as functional mimics of the cytochrome P450 monooxygenases in oxygen-transfer reactions have been described. The chloroperoxidase from Caldariomyces fumago (CPO) is the catalyst of choice in sulfoxidation, hydroxylation and epoxidation on account of its high activity and enantioselectivity. Other heme peroxidases were less active by orders of magnitude; protein engineering has resulted in impressive improvements but even the most active mutant was still at least an order of magnitude less active than CPO. The 'oxygen-rebound' mechanisms of oxygen transfer mediated by heme enzymes - as originally conceived - have proved to be untenable. Dual pathway mechanisms, via oxoferryl species that insert oxygen as well as iron hydroperoxide species that insert OH(+), have been proposed that accommodate all of the known experimental data.     

Thermodynamic stabilities and conformational analyses of 4,6-O-acetalized 1,5-anhydro-5-thio-dl-threo-2-enitols

4,6-O-Methylidene and 4,6-O-neopentylidene derivatives of 1,5-anhydro-2,3-dideoxy-5-thio-dl-thero-hex-2-enitol having the C-inside form were found to be thermodynamically more stable than the corresponding O-inside conformers. Thermodynamic stabilities, as well as the conformation of sulfoxide and sulfone derivatives of the 4,6-O-neopentylidene compound were examined by experiment and ab initio MO and DFT calculations. These thermodynamic stabilities, and the most stable conformations determined by NMR data, were corroborated by calculations.     

Prostaglandin H synthase: Resolved and unresolved mechanistic issues

The cyclooxygenase and peroxidase activities of prostaglandin H synthase (PGHS)-1 and -2 have complex kinetics, with the cyclooxygenase exhibiting feedback activation by product peroxide and irreversible self-inactivation, and the peroxidase undergoing an independent self-inactivation process. The mechanistic bases for these complex, non-linear steady-state kinetics have been gradually elucidated by a combination of structure/function, spectroscopic and transient kinetic analyses. It is now apparent that most aspects of PGHS-1 and -2 catalysis can be accounted for by a branched chain radical mechanism involving a classic heme-based peroxidase cycle and a radical-based cyclooxygenase cycle. The two cycles are linked by the Tyr385 radical, which originates from an oxidized peroxidase intermediate and begins the cyclooxygenase cycle by abstracting a hydrogen atom from the fatty acid substrate. Peroxidase cycle intermediates have been well characterized, and peroxidase self-inactivation has been kinetically linked to a damaging side reaction involving the oxyferryl heme oxidant in an intermediate that also contains the Tyr385 radical. The cyclooxygenase cycle intermediates are poorly characterized, with the exception of the Tyr385 radical and the initial arachidonate radical, which has a pentadiene structure involving C11-C15 of the fatty acid. Oxygen isotope effect studies suggest that formation of the arachidonate radical is reversible, a conclusion consistent with electron paramagnetic resonance spectroscopic observations, radical trapping by NO, and thermodynamic calculations, although moderate isotope selectivity was found for the H-abstraction step as well. Reaction with peroxide also produces an alternate radical at Tyr504 that is linked to cyclooxygenase activation efficiency and may serve as a reservoir of oxidizing equivalent. The interconversions among radicals on Tyr385, on Tyr504, and on arachidonate, and their relationships to regulation and inactivation of the cyclooxygenase, are still under active investigation for both PGHS isozymes.     

Regiospecific Reduction of 4,6-Dinitrobenzimidazoles: Synthesis,Characterization, and Biological Evaluation

The regiospecific reduction of 4,6-dinitrobenzimidazole derivatives leading to the corresponding 4-amino-6-nitrobenzimidazoles was studied. The identification of the formed product structures was accomplished by spectroscopic and X-ray diffraction data. The anticancer and antiparasitic activities of the synthesized compounds were examined, and promising activities against Toxoplasma gondii and Leishmania major parasites were discovered for certain 4,6-dinitrobenzimidazoles in addition to moderate anticancer activities of the 4-amino-6-nitrobenzimidazole derivatives against T. gondii cells. However, the tumor cell experiments revealed a promising sensitivity of p53-negative colon cancer cells to these compounds.     

Design,synthesis, spectral analysis and in vitro microbiological evaluation of 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones

A series of novel 2-phenyl-3-(4,6-diarylpyrimidin-2-yl)thiazolidin-4-ones 23-33 were synthesized, and studied for their in vitro antibacterial and antifungal activities against clinically isolated strains. Generally compounds possessing electron donating groups showed good antibacterial activity. Compound 31, which contain both electron withdrawing chloro and electron donating methyl groups showed potent activity against all the tested Gram positive and Gram negative bacterial strains whereas compounds 32 and 33 which contain electron donating methoxy functional group at the para position of the phenyl ring attached to pyrimidine ring showed promising activity against S.aureus, S.typhii and E.coli. Compounds 32 and 33, both containing electron withdrawing groups (-Cl, -F) showed excellent activities against all the tested A. flavus, Mucor, Rhizopus and M.gypsuem fungal strains. while against Mucor, compound 27 which contains an electron donating methyl group at the para position of the phenyl ring attached to pyrimidine ring showed promising activity. Also compound 31, which contains both electron withdrawing chloro and electron donating methyl groups showed potent activity against A. flavus and Rhizopus.     

Mechanistic studies on PseB of pseudaminic acid biosynthesis: a UDP-N-acetylglucosamine 5-inverting 4,6-dehydratase

UDP-N-acetylglucosamine 5-inverting 4,6-dehydratase (PseB) is a unique sugar nucleotide dehydratase that inverts the C-5″ stereocentre during conversion of UDP-N-acetylglucosamine to UDP-2-acetamido-2,6-dideoxy-β-l-arabino-hexos-4-ulose. PseB catalyzes the first step in the biosynthesis of pseudaminic acid, which is found as a post-translational modification on the flagellin of Campylobacter jejuni and Helicobacter pylori. PseB is proposed to use its tightly bound NADP⁺ to oxidize UDP-GlcNAc at C-4″, enabling dehydration. The α,β unsaturated ketone intermediate is then reduced by delivery of the hydride to C-6″ and a proton to C-5″. Consistent with this, PseB from C. jejuni has been found to incorporate deuterium into the C-5″ position of product during catalysis in D₂O. Likewise, PseB catalyzes solvent isotope exchange into the H-5″ position of product, and eliminates HF from the alternate substrate, UDP-6-deoxy-6-fluoro-GlcNAc. Mutants of the putative catalytic residues aspartate 126, lysine 127 and tyrosine 135 have severely compromised dehydratase, solvent isotope exchange, and HF elimination activities.     

Glycosylamines of 4,6-O-butylidene-alpha-D-glucopyranose: synthesis and characterization of glycosylamines, and the crystal structure of 4,6-O-butylidene-N-(o-chlorophenyl)-beta-D-glucopyranosylamine

A total of nine glycosylamines of 4,6-O-butylidene-alpha-D-glucopyranose were synthesized using primary amines having various groups in their ortho- or para-positions. Among these, six are monoglycosylamines, including one primary glycosylamine, and three are bis-glycosylamines. All these compounds were characterized by 1H, 1H-1H COSY, 1H-13C COSY and 13C NMR spectroscopy and FTIR spectra. The FAB mass spectra provided the molecular weights of the products by exhibiting the corresponding molecular ion peaks. The crystal structure of 4,6-O-butylidene-N-(o-chlorophenyl)-beta-D-glucopyranosylamine revealed the C-1 glycosylation, the beta-anomeric nature, and the 4C1 chair conformation of the saccharide unit in the product. In the lattice two types of dimers exist. While one type of dimer is formed through O-H...O type of interactions, the other type is formed via C-H...O type of interactions. In the direction of these C-H...O type of interactions, the dimeric units are connected to form a chain.     

Free radical scavenging reactions of sulfasalazine, 5-aminosalicylic acid and sulfapyridine: mechanistic aspects and antioxidant activity

Reactions of sulfasalazine (SAZ) and its metabolites, 5-aminosalicylic acid (5-ASA) and sulfapyridine (SP), with various oxidizing and reducing free radicals (hydroxyl, haloperoxyl, one-electron oxidizing, lipid peroxyl, glutathiyl, superoxide, tryptophanyl, etc.) have been studied to understand the mechanistic aspects of its action against free radicals produced during inflammation. Nanosecond pulse radiolysis technique coupled with transient spectrophotometry has been used for in situ generation of free radicals and to follow their reaction pathways. The transients produced in these reactions have been assigned and radical scavenging rate constants have been measured. In addition to scavenging of various primary and secondary free radicals by SAZ, 5-ASA and SP, 5-ASA has also been observed to efficiently scavenge radicals of biomolecules. 5-ASA has been found to be the active moiety of SAZ involved in the scavenging of oxidizing free radicals whereas reduction of SAZ produced molecular radical anion. The study suggests that free radical scavenging activity of 5-ASA may be a major path of pharmacological action of SAZ against inflammatory bowel diseases (IBD).     

In vitro cytotoxicity of melleolide antibiotics: structural and mechanistic aspects

Melleolide sesquiterpene aryl esters are secondary products of the mushroom genus Armillaria. We compared the cytotoxicity of eleven melleolides—five thereof are new natural products—against four human cancer cell lines. Armillaridin, 4-O-methylarmillaridin, and dehydroarmillylorsellinate were most active, at IC₅₀ = 3.0, 4.1 and 5.0 μM, respectively, against Jurkat T cells for the former two compounds, and K-562 cells for the latter. Dehydroarmillylorsellinate did not inhibit respiration and RNA-synthesis of K-562 cells at 5 μM. However, replication of DNA dropped to 35% after 120 min at this concentration, and translational activity also decreased.     

Halogenation of aromatic compounds: thermodynamic, mechanistic and ecological aspects

Biological halogenation of aromatic compounds implies the generation of reducing equivalents in the form of e.g. NADH. Thermodynamic calculations show that coupling the halogenation step to a step in which the reducing equivalents are oxidized with a potent oxidant such as O₂ or N₂O makes the halogenation reaction thermodynamically feasible without the input of additional energy in the form of e.g. NADH. In a current model on the halogenation of tryptophan to 7-chloro-l-tryptophan NADH and O₂ are proposed as co-substrates in a reaction in which the aromatic compound is oxidized via an epoxide as intermediate. The thermodynamic calculations thus indicate that such a route hinges on mechanistic insights but has no thermodynamic necessity. Furthermore the calculations suggest that halogenation of tryptophan and other aromatic compounds should be possible with N₂O, and possibly even with nitrate replacing O₂ as the oxidant.     

Studies on the endogenous L-selectin ligands: systematic and highly efficient total synthetic routes to lactamized-sialyl 6-O-sulfo Lewis X and other novel gangliosides containing lactamized neuraminic acid

Systematic syntheses of lactamized neuraminic acid-containing gangliosides GM4, sulfated sialylparagloboside, and sulfated/nonsulfated sialyl Lewis X are described. The highly efficient, one-step lactamization of neuraminic acid was accomplished by treatment of the N-deacetylated sialic acid (neuraminic acid)-containing gangliosides with HBTU and HOBt in DMF at 65 degrees C. Both the lactamized neuraminic acid residue and the sulfate group at O-6 of the GlcNAc residue were found to be involved in the antigenic determinant defined by G159 monoclonal antibody, while the fucose residue may not be critical for the recognition by G159 mAb.     

Tetrazoles as anticancer agents: A review on synthetic strategies,mechanism of action and SAR studies

Cancer is a leading cause of death worldwide. Even after the availability of numerous drugs and treatments in the market, scientists and researchers are focusing on new therapies because of their resistance and toxicity issues. The newly synthesized drug candidates are able to demonstrate in vitro activity but are unable to reach clinical trials due to their rapid metabolism and low bioavailability. Therefore there is an imperative requisite to expand novel anticancer negotiators with tremendous activity as well as in vivo efficacy. Tetrazole is a promising pharmacophore which is metabolically more stable and acts as a bioisosteric analogue for many functional groups. Tetrazole fragment is often castoff with other pharmacophores in the expansion of novel anticancer drugs. This is the first systematic review that emphasizes on contemporary strategies used for the inclusion of tetrazole moiety, mechanistic targets along with comprehensive structural activity relationship studies to provide perspective into the rational design of high-efficiency tetrazole-based anticancer drug candidates.     

In vitro bi-enzymatic synthesis of benzaldehyde from phenylalanine: practical and mechanistic studies

The oxidation of phenylalanine to benzaldehyde was performed in vitro by a bi-enzymatic system d-amino acid oxidase-peroxidase. We describe attempts to optimise experimental conditions and we propose a probable mechanism for the transformation.     

Evaluation of colorimetric assays for analyzing reductively methylated proteins: Biases and mechanistic insights

Colorimetric protein assays, such as the Coomassie blue G-250 dye-binding (Bradford) and bicinchoninic acid (BCA) assays, are commonly used to quantify protein concentration. The accuracy of these assays depends on the amino acid composition. Because of the extensive use of reductive methylation in the study of proteins and the importance of biological methylation, it is necessary to evaluate the impact of lysyl methylation on the Bradford and BCA assays. Unmodified and reductively methylated proteins were analyzed using the absorbance at 280 nm to standardize the concentrations. Using model compounds, we demonstrate that the dimethylation of lysyl ε-amines does not affect the proteins' molar extinction coefficients at 280 nm. For the Bradford assay, the responses (absorbance per unit concentration) of the unmodified and reductively methylated proteins were similar, with a slight decrease in the response upon methylation. For the BCA assay, the responses of the reductively methylated proteins were consistently higher, overestimating the concentrations of the methylated proteins. The enhanced color formation in the BCA assay may be due to the lower acid dissociation constants of the lysyl ε-dimethylamines compared with the unmodified ε-amine, favoring Cu(II) binding in biuret-like complexes. The implications for the analysis of biologically methylated samples are discussed.     

The mechanistic aspects in hydroxylation reactions catalyzed by fluorinated porphyrins of manganese and iron: Role of aqueous phosphate

The role of aqueous phosphate in the selective hydroxylation of cyclohexane to cyclohexanol (60-68% yields) and cyclohexene to 2-cyclohexene-1-ol (98% yield) has been discussed. Fluorinated porphyrins of manganese and iron were used as catalysts in acetonitrile with aqueous-K₂HPO₄ and t-BuOOH as co-catalyst and oxidant, respectively. The generation of the oxo-manganese reactive intermediate was identified by ¹H, ¹⁹F NMR, EPR, UV-Vis and mass spectral studies. The formation of phosphate adduct of the oxo-manganese reactive intermediates has been proposed on the basis of high binding constant of dipotassium hydrogen phosphate with the catalyst.