Dérivés C-glycosyliques : Partie XII. Élaboration de cycles furoxannes,isoxazoles et 2-isoxazoline par l'intermédiaire d'oxydes de nitrile dérivés de sucres

Treatment with a base of hydroxymoyl chlorides derived from sugars led to the corresponding unstable nitrile oxides which, in the absence of nucleophilic or dipolarophilic reagents, dimerize into furoxans. When nitrile oxides are formed in the presence of an excess of acetylenic (or olefinic) dipolarophilic reagents, they give 1,3-dipolar cycloaddition reactions with the formation of the corresponding 3-glycosylisoxazoles (or -2-isoxazolines). The action of nucleophilic reagents such as the alkaline cyanides or ethylmagnesium bromide allows the elongation of the sugar chain by one or two carbon atoms, respectively. The action of ethynylmagnesium bromide on these nitrile oxides gave a mixture of two compounds: an α-ethynyloxime and a 3-glycosylisoxazole; in alkaline medium the former compound can undergo cyclization and transformation into the latter, thus allowing deuterium labeling of that compound.     

Pyrrolidide formation as a side reaction during activation of carboxylic acids by phosphonium salt coupling reagents

Pyrrolidide derivatives are observed as unwanted by-products from slow reactions of activated carboxylates with nucleophilic amines, as mediated by phosphonium salt coupling reagents (PyAOP, PyBOP, PyBroP). This side reaction is attributed to the presence of small amounts (e.g., 0.5%, w/w) of pyrrolidine as a contaminant to commercial phosphonium salts, and does not occur when the reagents are crystallized before their use in coupling reactions.     

Pyrrolidide formation as a side reaction during activation of carboxylic acids by phosphonium salt coupling reagents

Summary Pyrrolidide derivatives are observed as unwanted by-products from slow reactions of activated carboxylates with nucleophilic amines, as mediated by phosphonium salt coupling reagents (PyAOP, PyBOP, PyBroP). This side reaction is attributed to the presence of small amounts (e.g., 0.5%, w/w) of pyrrolidine as a contaminant to commerical phosphonium salts, and does not occur when the reagents are crystallized before their use in coupling reactions.     

Effect of high concentrations of sucrose on the enzymatic activity of alpha-chymotrypsin

The effect of the low-molecular-mass natural reagents in high concentrations is important for investigating enzymatic reactions in near "in vivo" conditions and for optimisation of biotechnology processes. A model system, including p-nitrophenyl acetate as substrate and alpha-chymotrypsin as proteolytic enzyme, has been used to study the effect of high concentrations of sucrose, both influencing the viscosity of the reaction medium and acting as a nucleophilic effector (activator) on the enzymatic reaction. A kinetic scheme at high concentrations of nucleophilic effectors (sucrose) has been proposed.     

Modification of end phosphate gruops in mono- and oligonucleotides.

A method is described for selective activation of phosphomonoester end groups of oligonucleotides and nucleosidedi-(tri) phosphates via mixed anhydrides with mesitoic acid. Mixed anhydrides are synthesized in high yield and isolated by paper or DEAE-cellulose column chromatography. The ability of such anhydrides to phosphorylate different nucleophilic agents was used for synthesis of amidates, imidazolidates, esters, thioesters and pyrophosphates of mono- and oligonucleotides. Mixed anhydrides mono-, oligonucleotides and nucleosidedi-(tri)phosphates and mesitoic acid were also applied to achieve immobilization of the mono- and oligonucleotides via their end groups on hexamethylenediamine - Sepharose support. Mixed anhydrides studied may be efficiently used for affinity labeling of proteins and nucleic acids and also as material for preparating reagents for template reactions.     

Metal ions in biological processes. Volume 15. Zinc and its role in biology and nutrition. : Edited by Helmut Sigel. (Institute of Inorganic Chemistry,University of Basel.) Marcel Dekker,Inc., New York,N.Y. and Basle,Switzerland, 1983. 520 pp., bound,illustrated. SFr. 198.

Seven new complexes where the chelate ring was nucleophilically substituted were synthesized by the reactions of asymmetrical tetradentate Schiff base complexes of nickel(II) and copper(II) with benzenethiols and thiolycollic ehtyl ester. They were characterized by means of elemental analyses, electronic and ¹H-NMR spectra. Kinetics of the nucleophilic ring-substitution reactions were spectrophotometrically studied in a benzene solution. A four-centered intermediate is proposed for the reactions.     

Silacyclobutenes - Synthesis and reactivity

Silacyclobutenes can be prepared using different synthetic approaches such as: nucleophilic substitution reactions of organometallic reagents with silahalides; reaction of intermediary silenes or silylenes generated under FVP-conditions or by complex reagents, subsequently the silenes reacting in inter- or intramolecular [2+2] cycloaddition with CC or CC bonds and the silylenes commonly via intramolecular insertion reaction into C-H bonds to give silacyclobutenes; use of transition metal complexes as catalysts or reagents. The reactivity of silacyclobutenes is depending on the substituents at the silicon center and at the carbon ring atoms with their reactions leading to a wide range of materials.     

Crystal and molecular structure of a 4-oximino-5-imino-pyrazoline active ester.

The crystal structure of an active 4-oximino-5-imino-pyrazoline ester has been determined by single-crystal X-ray diffraction methods. The possible reasons for its high reactivity towards nucleophilic reagents are briefly discussed.     

Affinity labeling of catechol O-methyltransferase by N-haloacetyl derivatives of 3,5-dimethoxy-4-hydroxyphenylethylamine and 3,4-dimethoxy-5-hydroxyphenylethylamine. Kinetics of inactivation.

In an attempt to elucidate the relationship between the chemical structure and the catalytic function of catechol O-methyltransferase (COMT), several classes of affinity labeling reagents have been synthesized and their interaction with COMT has been studied. Earlier studies have shown that various N-haloacetyl derivatives of 3,5-dimethoxy-4-hydroxyphenylethylamine were effective affinity labeling reagents for this enzyme. In this report we have shown that N-haloacetyl derivatives of the isomeric 3,4-dimethoxy-5-hydroxyphenylethylamine also rapidly and irreversibly inactivate COMT ant they satisfy many of the criteria established for affinity labeling reagents. This latter group of agents appear to modify a nucleophilic residue at the active site of COMT different from that modified by the 3,5-dimethoxy-4-hydroxyphenylethylamine series. Evidence to support this conclusion has been obtained by comparing the kinetics of COMT inactivation and the substrate protection profiles for these two classes of affinity labeling reagents.     

Polynucleotides and their components in the process of aromatic nucleophilic substitution. I. Chemistry and dynamics of nucleotide arylation with pentafluoropyridine. Intermediate product ofr molecular design of nucleic acid analogs

Pentafluoropyridine reacts with thymidine, adenosine, and uridine hydroxy groups to give quantitative yields of the corresponding nucleoside di- and triaryl ethers. The nucleophilic substitution reactions proceed successively and in parallel, with the slowest step being the nucleophilic substitution of the nucleoside secondary hydroxyls. The resulting ethers contain tetrafluoropyridyl moieties, which could be smoothly modified by nucleophilic substitution of fluorine atoms. The ethers are useful intermediate synthons (both isolated and in situ) for molecular design of oligonucleotide analogues. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2004, vol. 30, no. 1; see also http://www.maik.ru.     

Synthesis and some reactions of 1,3:4,6-di-O-benzylidene-d-threo-2,5-hexodiulose

Oxidation of 1,3:4,6-di-O-benzylidene-d-mannitol in ethyl acetate with ruthenium tetraoxide gave the corresponding diketone as its hydrate, which was dehydrated to the free diketone. These compounds were treated with a variety of reagents containing hydride, oxygen, nitrogen, and carbon nucleophiles. Only in reactions with methylmagnesium iodide and with diazomethane was the symmetrical bis-adduct obtained. In all other cases, the reactivity of the two carbonyl groups was different. Hydride addition occurred with opposite stereoselectivity at each carbonyl group. With oxygen and nitrogen nucleophiles, cyclic adducts were obtained, arising from bridging across the two carbonyl groups by one molecule of nucleophilic reagent. The configurations of the ring fusions were deduced by p.m.r spectroscopy, and the cis,cis-structure was preferred in compounds containing one five- and two six-membered rings. In the reactions with other carbon nucleophiles, complex mixtures were obtained due, at least in the case of the reaction with phenylmagnesium bromide, to elimination reactions.     

Stable, nonreducible cross-links of mature collagen.

During in vivo maturation, and also during in vitro incubation with physiological buffers, native collagen fibers display a progressive increase in tensile strength and insolubility. Paralleling these physiologically important changes is a progressive loss of the reducible cross-links which initially join the triple-chained subunits of collagen fibers. Although there is evidence suggesting that the reducible cross-links are gradually transformed into more stable, nonreducible cross-links during maturation, the nature of the transformation process and the structure of the stable "mature" cross-links has remained a mystery. In order to test the possibility that cross-link transformation involves addition of a nucleophilic amino acid residue to the reducible cross-links, histidine, arginine, glutamate, aspartate, lysine, and hydroxylysine residues were chemically modified, and the effect of each modification procedure on the in vitro transformation of reducible cross-links was ascertained. The results of these experiments indicated that destruction of histidine, arginine, glutamate, and aspartate residues has no measurable effect on the rate and extent of reducible cross-link transformation in hard tissue collagens. In contrast, modification of lysine and hydrocylysine residues with a wide variety of specific reagents completely blocks the transformation of reducible cross-links. Removal of the reversible blocking groups from lysine and hydroxlylysine residues then allows the transformation to proceed normally. These results indicate that collagen maturation involves nucleophilic addition of lysine and/or hydroxylysine residues to the electrophilic double bond of the reducible cross-links, yielding derivatives which are not only more stable but also capable of cross-linking more collagen molecules than their reducible precursors.     

Polynucleotides and Their Components in the Processes of Aromatic Nucleophilic Substitution: II.1 Nucleophilic Modification of 3′,5′-Bis-O- (α,β,α′,β′-tetrafluoropyrid-γ-yl)thymidine

The interaction of 3,5-bis-O-(,,,-tetrafluoropyrid--yl)thymidine with various nucleophilic reagents was studied to evaluate the possibility of molecular design of new types of nucleic acid analogues using S _NAr reactions. The reactions with morpholine and sodium azide led to the introduction of one and two nucleophilic residues into each of the polyfluorinated pyridine rings. The nucleophilic polycondensation with bifunctional reagents ethylenediamine and hexamethylenediamine depended on the nature of nucleophile and reaction conditions and resulted in the formation of supramolecules containing about five or more than 20 pyrimidine bases.     

Using the general-purpose reactivity indicator: challenging examples

We elucidate the regioselectivity of nucleophilic attack on substituted benzenesulfonates, quinolines, and pyridines using a general-purpose reactivity indicator (GPRI) for electrophiles. We observe that the GPRI is most accurate when the incoming nucleophile resembles a point charge. We further observe that the GPRI often chooses reactive “dead ends” as the most reactive sites as well as sterically hindered reactive sites. This means that care must be taken to remove sites that are inherently unreactive. Generally, among sites where reactions actually occur, the GPRI identifies the sites in the molecule that lead to the kinetically favored product(s). Furthermore, the GPRI can discern which sites react with hard reagents and which sites react with soft reagents. Because it is currently impossible to use the mathematical framework of conceptual DFT to identify sterically inaccessible sites and reactive dead ends, the GPRI is primarily useful as an interpretative, not a predictive, tool.     

Polynucleotides and Their Components in the Processes of Aromatic Nucleophilic Substitution: I. Chemistry and Dynamics of Nucleotide Arylation with Pentafluoropyridine; Obtaining of Synthons for Molecular Design of Nucleic Acid Analogues

Pentafluoropyridine reacts with thymidine, adenosine, and uridine hydroxy groups to give quantitative yields of the corresponding nucleoside di- and triaryl ethers. The nucleophilic substitution reactions proceed successively and in parallel, with the slowest step being the nucleophilic substitution of the nucleoside secondary hydroxyls. The resulting ethers contain tetrafluoropyridyl moieties, which could be smoothly modified by nucleophilic substitution of fluorine atoms. The ethers are useful intermediate synthons (both isolated and in situ) for molecular design of oligonucleotide analogues.     

Cleavable linkers in chemical biology

Interest in cleavable linkers is growing due to the rapid development and expansion of chemical biology. The chemical constrains imposed by the biological conditions cause significant challenges for organic chemists. In this review we will present an overview of the cleavable linkers used in chemical biology classified according to their cleavage conditions by enzymes, nucleophilic/basic reagents, reducing agents, photo-irradiation, electrophilic/acidic reagents, organometallic and metal reagents, oxidizing reagents.     

Vinylstannylated alkylating agents as prosthetic groups for radioiodination of small molecules: design, synthesis, and application to iodoallyl analogues of spiperone and diprenorphine.

The preparation and synthetic utility of p-toluenesulfonate esters of (E)- and (Z)-3-(tri-n-butylstannyl)-prop-2-en-1-ol as bifunctional reagents for radioiodination are described. These vinylstannylated alkylating agents are prepared in two steps from propargyl alcohol, and readily couple with nucleophilic functionality (amide nitrogen, secondary amine, tertiary alcohol) in good yields (48-95%) to provide derivatives of the neuroreceptor ligands spiperone and diprenorphine. Regio- and stereospecific radioiododestannylation with retention of configuration occurs under mild, no-carrier-added conditions to give the corresponding radiolabeled N- or O-iodoallyl analogues in good radiochemical yields (55-95%) with high specific radioactivities. The methodology is versatile and well-suited to selective labeling of small molecules with radioisotopes of iodine such as 125I or 123I.     

The use of levoglucosenone and isolevoglucosenone as templates for the construction of C-linked disaccharides

Because of their functionalities (enone, ketone, and acetal) and their bicyclic structure (steric factors), levoglucosenone (1,6-anhydro-3,4-dideoxy-beta-D-glycero-hex-3-enopyran-2-ulose) and isolevoglucosenone (1,6-anhydro-2,3-dideoxy-beta-D-glycero-hex-3-enopyran-4-ulose) are useful templates for the convergent and combinatorial synthesis of (1-->2), (1-->3), and (1-->4)-linked C-disaccharides in reactions combining them with sugar-derived carbaldehydes. Synthetic methods relying on conjugate nucleophilic additions of these enones, their combination with aluminum reagents and aldehydes (Baylis-Hillman reaction) and modified Takai-Hiyama-Nozaki-Kishi couplings of enol triflates derived from them with sugar-derived aldehydes are reviewed. Highly stereoselective methods have thus been developed. These allow the generation of disaccharide mimetics with a high molecular diversity.     

Biochemistry of terminal deoxynucleotidyltransferase: identification, characterization, requirements, and active-site involvement in the catalysis of associated pyrophosphate exchange and pyrophosphorolytic activity

Terminal deoxynucleotidyltransferase (TdT) has been found to catalyze both pyrophosphate exchange and pyrophosphorolysis reactions. Both reactions are strongly inhibited by antiserum to TdT. The reactions require the presence of a divalent cation, a single- or double-stranded oligomeric or polymeric DNA or RNA, and deoxyribonucleoside triphosphates (for PPi exchange only). Of the three divalent cations tested, Mg2+ and Co2+ are equally effective, while Mn2+ neither is used for catalysis nor inhibits the Mg2+-catalyzed reactions. Ribonucleoside triphosphates have been found to support the PPi exchange reaction to a minor extent and have no inhibitory effect on the catalysis mediated by dNTPs. Inhibition studies, using SH group inhibitors, Zn chelator, and a substrate binding site specific reagent, revealed that PPi exchange and pyrophosphorolysis reactions may be distinguished by differences in their sensitivity to inhibition by various reagents. While the PPi exchange reaction is strongly inhibited by sulfhydryl reagents, o-phenanthroline, and pyridoxal phosphate, the pyrophosphorolysis reaction is insensitive to these reagents. In addition, the pyrophosphorolysis reaction is also found not to require a free 3'-OH terminus of a primer. This difference in the susceptibility of the two reactions indicates that discrete active-site structures exist in TdT which catalyze PPi exchange and pyrophosphorolysis reactions.     

Reactions of N-hydroxysulfosuccinimide active esters

N-Hydroxysulfosuccinimide esters are reactive functional groups employed in a variety of protein modification reagents, especially cross-linking reagents. For these compounds, hydrolysis is the most important reaction competing for reaction of the esters with nucleophilic groups in proteins. We have employed model compounds to investigate the rates of hydrolysis of N-hydroxysulfosuccinimide esters and their reactions with the alpha-amino group and the side chains of naturally occurring amino acids, under conditions comparable to those used for protein modification studies. The rats of hydrolysis observed were found to be very low, as compared with their rates of reaction with nitrogen nucleophiles found in proteins. Further, within the ranges investigated, the rate of aminolysis was observed to increase more rapidly than the rate of hydrolysis with increasing pH or with increasing temperature. Four amino acid side chains and the alpha-amino group were found to react measurably with N-hydroxysulfosuccinimide esters. At pH 7.4 and room temperature, the order of reactivity was found to be N alpha-Cbz-histidine greater than N alpha-Cbz-lysine approximately phenylalanine (alpha-amino group) much greater than N-acetylcysteine approximately N-acetyltyrosine; however, the acylimidazole adduct formed with the side chain of histidine was found to be a transient product, subject to hydrolysis or reaction with another nucleophile.