Stereoselective synthesis of 1,2-cis- and 2-deoxyglycofuranosyl azides from glycosyl halides

Protected 1,2-cis glycofuranosyl azides with alpha-D-ribo, beta-D-arabino and 2-deoxy-2-fluoro-beta-D-arabino configurations were efficiently prepared from the appropriate 1,2-trans glycosyl halides bearing non-participating 0-2 substituent by inversion with sodium azide under phase transfer catalytic conditions (80-85% yields, 90-96% de). The same method failed to result in sufficiently good beta-selectivity starting from 2-deoxy-3,5-di-O-(p-toluoyl)-alpha-D-ery-thro-pentofuranosyl chloride (5alpha) (40% de). The selectivity in favour of the protected 2-deoxy-beta-D-erythro-pentofura-nosyl azides was greatly improved (74-80% de) by treating 5alpha and its p-chlorobenzoyl analog 6alpha with cesium or potassium azide in dimethylsulfoxide at room temperature (83-85% yields).     

Synthesis and characterization of N-aryl chitosan derivatives

Selective N-arylation of chitosan was performed via a Schiff bases formed by the reaction between the 2-amino group of glucosamine residue of chitosan with an aromatic aldehyde under acidic condition followed by reduction of the Schiff base intermediate with sodium cyanoborohydride (Borch reduction). Aromatic aldehydes bearing either an electron donating or electron withdrawing substituent were used. The chemical structures and thermal properties of the N-aryl chitosans were characterized by FT-IR, (1)H NMR, (13)C NMR, TGA, and DSC. The extent of N-substitution (ES) was influenced by the molar ratio of the aldehyde to the glucosamine residue of chitosan, the reaction time and the substituent on the aromatic ring. Lower ESs resulted from N-arylation using an aldehyde with an electron donating substituent. A linear relationship between the targeted ES and the ES obtained was observed when aldehydes bearing electron withdrawing substituents were employed.     

Syntheses of amphiphilic glycosylamides from glycosyl azides without transient reduction to glycosylamines

Protected glycosyl azides react with acyl chlorides in the presence of triphenylphosphine to afford glycosylamides in high yields, at room temperature. Starting from the beta-glycosyl azides, the reaction is highly stereoselective and occurs with retention of configuration, whereas the alpha-azido anomers display a lower stereoselectivity giving rise to alpha/beta mixtures of glycosylamides. The reaction was applied to several monosaccharidic azides and to lactosyl azide with various acyl chlorides; it was shown to be of general use for preparing 1,2-trans beta-glycosylamides.     

1,3-Dipolar cycloaddition of sugar azides with benzyne: a novel synthesis of 1,2,3-benzotriazolyl glycoconjugates

Glycosyl azides undergo smooth 1,3-dipolar cycloaddition with benzyne generated in situ from 2-(trimethylsilyl)phenyltrifluoromethanesulfonate and cesium fluoride under mild conditions to furnish 1,2,3-benzotriazole-linked glycoconjugates in excellent yields and with high stereoselectivity. This method provides a novel class of benzotriazole linked glycoconjugates in a single-step reaction. This is the first example of a fluoride- triggered 1,3-dipolar cycloaddition of benzyne with glycosyl azides.     

Salophen Copper(II) Complex‐Assisted Click Reactions for Fast Synthesis of 1,2,3‐Triazoles Based on Naphthalene‐1,4‐dione Scaffold,Antibacterial Evaluation,and Molecular Docking Studies

The salophen copper(II) complex was successfully used for the efficient synthesis of new 1,2,3‐triazoles based on the naphthalene‐1,4‐dione scaffold. The reaction of 2‐chloro‐3‐(prop‐2‐yn‐1‐yloxy)naphthalene‐1,4‐dione or 2,3‐bis(prop‐2‐yn‐1‐yloxy)naphthalene‐1,4‐dione with aromatic azides in the presence of a low copper catalyst (loading 1 mol‐%) afforded 2‐chloro‐3‐[(1‐phenyl‐1H‐1,2,3‐triazol‐4‐yl)methoxy]naphthalene‐1,4‐dione or 2,3‐bis[(1‐phenyl‐1H‐1,2,3‐triazol‐4‐yl)methoxy]naphthalene‐1,4‐dione, respectively. The advantages of these reactions are short reaction times, high‐to‐excellent reaction yields, operational simplicity, and mild experimental conditions. The new 1,2,3‐triazoles obtained were screened for their in vitro antibacterial activities and were subjected to molecular docking studies.     

Oxidative transformation of aminodinitrotoluene isomers by multicomponent dioxygenases.

The electron-withdrawing nitro substituents of 2,4,6-trinitrotoluene (TNT) make the aromatic ring highly resistant to oxidative transformation. The typical biological transformation of TNT involves reduction of one or more of the nitro groups of the ring to produce the corresponding amine. Reduction of a single nitro substituent of TNT to an amino substituent increases the electron density of the aromatic nucleus considerably. The comparatively electron-dense nuclei of the aminodinitrotoluene (ADNT) isomers would be expected to be more susceptible to oxygenase attack than TNT. The hypothesis was tested by evaluating three nitroarene dioxygenases for the ability to hydroxylate the ADNT isomers. The predominant reaction was dioxygenation of the ring to yield nitrite and the corresponding aminomethylnitrocatechol. A secondary reaction was benzylic monooxygenation to form aminodinitrobenzyl alcohol. The substrate preferences and catalytic specificities of the three enzymes differed considerably. The discovery that the ADNT isomers are substrates for the nitroarene dioxygenases reveals the potential for extensive bacterial transformation of TNT under aerobic conditions.     

Oxidative Transformation of Aminodinitrotoluene Isomers by Multicomponent Dioxygenases

The electron-withdrawing nitro substituents of 2,4,6-trinitrotoluene (TNT) make the aromatic ring highly resistant to oxidative transformation. The typical biological transformation of TNT involves reduction of one or more of the nitro groups of the ring to produce the corresponding amine. Reduction of a single nitro substituent of TNT to an amino substituent increases the electron density of the aromatic nucleus considerably. The comparatively electron-dense nuclei of the aminodinitrotoluene (ADNT) isomers would be expected to be more susceptible to oxygenase attack than TNT. The hypothesis was tested by evaluating three nitroarene dioxygenases for the ability to hydroxylate the ADNT isomers. The predominant reaction was dioxygenation of the ring to yield nitrite and the corresponding aminomethylnitrocatechol. A secondary reaction was benzylic monooxygenation to form aminodinitrobenzyl alcohol. The substrate preferences and catalytic specificities of the three enzymes differed considerably. The discovery that the ADNT isomers are substrates for the nitroarene dioxygenases reveals the potential for extensive bacterial transformation of TNT under aerobic conditions.     

Electronic effects on the regio- and enantioselectivity of the asymmetric aminohydroxylation of O-substituted 4-hydroxy-2-butenoates

Regio- and enantioselectivity in the asymmetric aminohydroxylation (AA) reaction of O-substituted 4-hydroxy-2-butenoates as well as the mechanism of the reaction were studied. When the electronic properties of the phenyl group in a substrate were altered by using different substituents, two conflicting trends were observed: The O-benzoyl substrates showed greater regio- and enantioselectivity when an electron-donating substituent was attached at the C-4 position of the phenyl group, while the O-benzyl substrates exhibited better regio- and enantioselectivity with an electron-withdrawing substituent at the C-4 position of the phenyl moiety. Thus, these results have disclosed hitherto unknown remarkable electronic effects in the AA reaction. Detailed analysis of possible electronic interactions in the chiral catalyst-substrate complex has revealed the importance of dipolar aromatic-aromatic interactions between the aromatic substituent of the substrate and the nitrogen heteroaromatic moiety of the chiral ligand for effective regiocontrol as well as enantioface selection in the AA reaction. A plausible model of the key intermediate in the AA reaction of O-substituted 4-hydroxy-2-butenoates is proposed.     

Microwave-induced enzyme-catalyzed chemoselective reduction of organic azides

A lipase enzyme, suspended in organic media along with organic azides and irradiated under microwaves, enhances the reaction rate over thermal heating and affords the corresponding amines in high yields. The present biocatalytic method employing lipase is a significant development with remarkable regio- and chemoselectivity under microwave irradiation in organic media with excellent yields for the reduction of azide functionality.     

Oxidation of aromatic sulfides by lignin peroxidase from Phanerochaete chrysosporium.

The reaction of H2O2 with 4-substituted aryl alkyl sulfides (4-XC6H4SR), catalysed by lignin peroxidase (LiP) from Phanerochaete chrysosporium, leads to the formation of sulfoxides, accompanied by diaryl disulfides. The yields of sulfoxide are greater than 95% when X = OMe, but decrease significantly as the electron donating power of the substituent decreases. No reaction is observed for X = CN. The bulkiness of the R group has very little influence on the efficiency of the reaction, except for R = tBu. The reaction exhibits enantioselectivity (up to 62% enantiomeric excess with X = Br, with preferential formation of the sulfoxide with S configuration). Enantioselectivity decreases with increasing electron density of the sulfide. Experiments in H218O show partial or no incorporation of the labelled oxygen into the sulfoxide, with the extent of incorporation decreasing as the ring substituents become more electron-withdrawing. On the basis of these results, it is suggested that LiP compound I (formed by reaction between the native enzyme and H2O2), reacts with the sulfide to form a sulfide radical cation and LiP compound II. The radical cation is then converted to sulfoxide either by reaction with the medium or by a reaction with compound II, the competition between these two pathways depending on the stability of the radical cation.     

Synthesis of C-4 and C-7 triazole analogs of zanamivir as multivalent sialic acid containing scaffolds

The relative reactivities of C-4 and C-7 azides derived from zanamivir were compared in cycloaddition reactions with a panel of alkynes. All of the reactions proceeded efficiently with no observable differences between primary and secondary azides. Significant rate differences were observed between several members of the alkyne panel. Most notably, a trialkyne derived from a 1,3,5-triazine core underwent complete reaction within 4 h, whereas an analogous trialkyne with an all carbon aromatic core required 18 h. These results suggest that the triazine core serves as an internal catalyst.     

Comparison of the myotropic activity of position-2 modified analogues of proctolin on the hindgut of Periplaneta americana and the oviduct of Locusta migratoria

The largest series of position-2 modified proctolin analogues to have been examined to date were tested for their ability to mimic the basal contraction induced by proctolin on hindgut of the cockroach, Periplaneta americana, and oviduct of the locust, Locusta migratoria. Twelve analogues of proctolin (Arg-Tyr-Leu-Pro-Thr), differing in the substituent (H, OMe, OEt, OPr, F, Cl, Br, I, NO(2), NH(2), N(3), Me) located at the para-position of the aromatic amino acid, caused dose-dependent contractions of both tissues at concentrations quite similar to proctolin. Seven showed greater or equal potency on the hindgut but, with one exception, they were less active on the oviduct than proctolin. The rank order of potency of the analogues depends on the tissue, lending more support to the notion that insects have more than one type of proctolin receptor. No relationship was observed between myoactivity and lipophilic, steric, electron donating or electron withdrawing properties of the substituents at the para-position of the aromatic amino acid. This may be the result of more than one sub-type of proctolin receptor on the specific tissue with differing structural requirements for optimum activity.     

1,3-Bis[N-sulfonyl-(1R,2S)-1,3-diphenyl-2-aminopropanol]benzene: an excellent ligand for titanium-catalyzed asymmetric AlPh3(THF) additions to aldehydes

Asymmetric AlPh(3) (THF) additions to a wide variety of aldehydes catalyzed by a titanium catalyst of 20 mol % 1,3-bis[N-sulfonyl-(1R,2S)-1,3-diphenyl-2-aminopropanol]benzene (1) are reported. The catalytic system works excellently for aromatic aldehydes bearing either an electron-donating or an electron-withdrawing substituent on the aromatic ring to afford secondary diaryl alcohols in excellent isolated yields of >or=95% and excellent enantioselectivities of >or=94% ee. The phenyl addition to cinnamaldehyde or 2-furylaldehyde gave corresponding secondary alcohols in 85% and 95% ee, respectively. For aliphatic aldehydes, increasing enantioselectivities of the addition products in terms of increasing steric sizes of aldehydes are observed, and this trend goes from the linear 1-pentanal (87% ee), the secondary cyclohexylaldehyde (95% ee) or the 2-methylpropanal (97% ee), to the tertiary 2,2-dimethylpropanal (99% ee).     

A novel domino-click approach for the synthesis of sugar based unsymmetrical bis-1,2,3-triazoles

Aryl or sugar azides were treated with allenylmagnesium bromide to generate 1,5-disubstituted-butynyl-N-aryl or N-glycosyl-1,2,3-bistriazoles in a domino fashion. Upon Cu(I) catalyzed 1,3-dipolar cycloaddition with sugar azides, these compounds afford novel unsymmetrical bis-1,2,3-triazoles in high yields.     

Electrochemical impedance spectroscopy sensor for ascorbic acid based on copper(I) catalyzed click chemistry

Copper(I) species can be acquired from the reduction of copper(II) by ascorbic acid (AA) in situ, and which in turn quantitative catalyze the azides and alkynes cycloaddition reaction. In this study, propargyl-functionalized ferrocene (propargyl-functionalized Fc) has been modified on the electrode through reacting with azide terminal modified Au electrode via copper(I) catalyzed azides and alkynes cycloaddition (CuAAC) reaction. The electrochemical impedance spectroscopy (EIS) measurement has been applied to test the electron transfer resistance of the Au electrode before and after click reaction. The changes of the fractional surface coverage (θ) with different AA concentrations are characterized. It is found that the θ value has a linear response to the logarithm of AA concentration in the range of 5.0 pmol/L to 1.0 nmol/L with the detection limits of 2.6 pmol/L. The sensor shows a good stability and selectivity. And it has been successfully applied to the AA detection in the real samples (urine) with satisfactory results.     

Mixed ligand complexes of iron with cyanide and phenanthroline as new probes of metalloprotein electron transfer reactivity. Analysis of reactions involving rusticyanin from Thiobacillus ferrooxidans

A family of 12 different mixed ligand complexes of iron with cyanide and substituted 1,10-phenanthroline was prepared. The electron transfer properties of each reagent were systematically manipulated by varying the substituent(s) on the aromatic ring system and the stoichiometry of the two types of ligands in the complex. Values for the standard reduction potentials of each member of this family of electron transfer reagents were determined and spanned from 500 to 900 mV. The one-electron transfer reactions between each of these substitution-inert reagents and the high potential blue copper protein, rusticyanin, from Thiobacillus ferrooxidans were studied by stopped flow spectrophotometry under acidic conditions. For comparison with the protein results, the kinetics of electron transfer between each of these reagents and sulfatoiron were also investigated. The Marcus theory of electron transfer was successfully applied to this set of kinetic data to demonstrate that 10 of the 12 reagents had equal kinetic access to the redox center of the rusticyanin and utilized the same reaction pathway for electron transfer. The utility of these synthetic electron transfer reagents in characterizing the electron transfer properties of very high potential, redox-active metalloproteins is illustrated.     

Co-oligopeptides of aromatic amino acids and glycine with variable distance between the aromatic residues. VII. Enzymatic synthesis of N-protected peptide amides

Several N-protected peptide amides, containing two aromatic residues spaced by one glycyl residue, have been enzymatically synthesized starting from P-Ar-OH and H-Gly-Ar-NH₂ (P is the protecting group and Ar is the aromatic residue) and using α-chymotrypsin as the catalyst for the coupling step. Reactions have been carried out in water solution, at room temperature, and afford yields ranging between 20 and 75% ca. This coupling reaction occurs in a much more restricted set of conditions than the hydrolysis reaction, e.g., only within a small pH range (ca. 6.5–7.5) and with particular buffering agents. The advantages and limitations of this type of reaction, compared with conventional coupling procedures, are discussed.     

Microbial energetics and stoichiometry for biodegradation of aromatic compounds involving oxygenation reactions

Oxygenation reactions significantly alter the energy and electron flows and, consequently, the overall stoichiometry for the microbial utilization of aromatic compounds. Oxygenation reactions do not yield a net release of electrons, but require an input of electrons to reduce oxygen molecules. The biodegradation pathway of phenanthrene as a model compound was analyzed to determine the impact of oxygenation reactions on overall stoichiometry using the half-reaction method. For individual oxygenation reactions, the half-reaction method for analyzing the electron and energy flows must be modified, because the reactions do not release electrons for synthesis or energy generation. Coupling the oxygenation reaction to subsequent reaction steps provides a net electron release for the coupled reactions. Modeling results indicate that oxygenation reactions increase the oxygen requirement and reduce the cell yield, compared to the conventional mineralization represented by hydroxylation reactions in place of oxygenations. The computed yields considering oxygenation reactions conform better to empirical yields reported in the literature than do yields computed by the hydroxylation single-step methods. The coupled-reaction model also is consistent with information about the ways in which micro-organisms that degrade aromatics accumulate intermediates, regulate degradation genes, and organize enzyme clusters.     

Biologically active aromatic retinoids bearing azido photoaffinity-labeling groups and their binding to cellular retinoic acid-binding protein

Retinoids bearing azido photoaffinity-labeling groups (azidoretinoids) have potential as probes for investigating the molecular mechanisms of action of all-trans-retinoic acid (RA) as mediated by its cellular retinoic acid-binding protein (CRABP) and nuclear receptor proteins. Two new azidoretinoids, 3-azido-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1E- propen-1-yl]-benzonic acid and 4-(4-azido-5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-anthracenyl)be nzoic acid were synthesized, and evaluated for their in vitro biological potency, and binding affinity for CRABP. Like RA, these aromatic azides had significant activity in modulating cell differentiation in retinoid-deficient hamster tracheal organ culture (ED500.02 nM and 0.03 nM, respectively) and in the inhibition of the induction of ornithine decarboxylase in mouse epidermis (ED50 7.0 nmol and 0.5 nmol, respectively). They also possessed high binding affinity for CRABP (ID50 0.9 microM and 0.85 microM, respectively). The tritiated aromatic azides were further evaluated for their ability to bind covalently to CRABP after photolysis. On photolysis at -78 degrees C, the two radiolabeled azidoretinoids formed stable adducts with CRABP. Treatment of the adducts with either RA or p-chloromercuriphenylsulfonic acid (CMPS) and subsequent dialysis did not cause any dissociation, indicating the formation of a covalent bond. In contrast, treatment of the unirradiated complexes with RA or CMPS led to dissociation of the complex. Synthesis of affinity labels and characterization of CRABP-retinoid complexes should provide useful information on the ligand-binding regions and insights into the mechanism of action of RA.     

Peptidomimetics Through Click Chemistry: Synthesis of Novel β-Keto Triazole Acids from <Emphasis Type="Italic">N</Emphasis>-Protected Amino Acids

An efficient procedure for the preparation of azidomethylketones from N-urethane protected amino acids and their application in Cu(I) catalyzed azide-alkyne cycloaddition reaction are described. The synthesis has been carried out under mild conditions with all the commonly used urethane protected (Fmoc, Boc and Z) amino acids and the desired azides/triazoles were obtained in good yields. Incorporation of these triazole amino acids into small peptides generating dipeptidomimetics containing β-keto triazole units has also been demonstrated.