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1.
The synthesis of several 2-azido-2-deoxy-D-gluco- and -D-galactopyranosyl halides is described. With tetraethylammonium chloride, α-pyranosyl bromides react to give β-pyranosyl chlorides. This provides a facile method for obtaining selectively blocked halides for the synthesis of α-linked, amino sugar of oligosaccharides. The inversion reaction at the anomeric centre is shown to be of second order, corresponding to an SN2 mechanism. The rates of the inversion reactions were correlated to the 13C-n.m.r. data of C-1 of α-bromides. Within the gluco series, the 13C-n.m.r. shift of C-1 proves to be proportional to the natural logarithm of the rate constant. An analogous correlation in the galacto series could not be observed. 相似文献
2.
The mechanism of, and the solvent effects in, the Koenigs—Knorr reaction of D-glucosyl chlorides having a non-participating group at C-2, using silver perchlorate as principal catalyst, were investigated. When a large excess of methanol was used, methyl D-glucopyranosides with inversion of the configuration at C-1 were predominantly obtained, except in one case. When 1 molar equivalent of nucleophile, such as methanol, methyl trityl ether, or 2-propanol, was used, the ratio of α- and β-D-glucopyranosides obtained varied with the solvent used. It is proposed that the reactions proceed via a common intermediate such as a D-glucosyl-perchlorate. The following conclusions are made for the preparation of α-D-glucopyranosides: anhydrous ether is a preferable solvent, silver perchlorate and sym-collidine are superior to a mixture of silver perchlorate and silver carbonate in the presence of Drierite, β-D-glucosyl chloride is preferred to the α-D anomer, and the solvent and reagents should be as dry as possible. 相似文献
3.
In contrast to reactions with high concentration, reactions of several derivatives of 2,3,4-tri-0-benzyl-α-D-glucopyranosyl bromide with low concentrations of methanol gave mainly the α-D-glucosides regardless of the structure of the C-6 substituent. Methanolysis of the same α-D-glucosyl bromides or the corresponding chlorides in the presence of silver tetrafluoroborate or hexafluorophosphate at —78° gave mainly the β-D-glucosides. The use of these silver salts led to side reactions, particularly when the glucosyl halide had an acyl blocking group at C-6. The side reactions were minimized when silver trifluoromethanesulfonate (triflate) was used. The relative amounts of α- and β-D-glucosides produced in the presence of silver triflate depended on the structure of the C-6 substituent and the solvent polarity. A rapid methanolysis of 2,3,4-tri-0-benzyl-6-0-(N-phenylcarbamoyl)-α-D-glucopyranosyl bromide with silver triflate in ether at —78° gave a high proportion of the methyl α-D-glucoside.The results of direct methanolysis seem to be due to competitive methanolysis of the anomeric bromides and a p?ush-pullmechanism is postulated in the presence of silver tetrafluoroborate or hexafluorophosphate. Glucosyl triflate intermediates are proposed for the silver triflate-assisted methanolyses. 相似文献
4.
Maria I. Bilan Alexander S. Shashkov Craig J. Sanderson Nikolay E. Nifantiev 《Carbohydrate research》2010,345(14):2038-2047
The polysaccharide composition of a fucoidan preparation isolated from the brown alga Saccharina latissima (formerly Laminaria saccharina) was reinvestigated. The preparation was fractionated by anion-exchange chromatography, and the fractions obtained were analyzed by chemical methods combined with NMR spectroscopy. Several 2D procedures, including HSQC, HMQC-TOCSY, and HMQC-NOESY, were used to obtain reliable structural information from the complex spectra, and the signal assignments were additionally confirmed by comparison with the literature spectra of the related polysaccharides and synthetic oligosaccharides. In accordance with the previous data, the main polysaccharide component was shown to be a fucan sulfate containing a backbone of 3-linked α-l-fucopyranose residues sulfated at C-4 and/or at C-2 and branched at C-2 by single sulfated α-l-fucopyranose residues. In addition, three other types of sulfated polysaccharide molecules were detected in the total fucoidan preparation: (i) a fucogalactan having a backbone of 6-linked β-d-galactopyranose residues branched mainly at C-4 and containing both terminal galactose and fucose residues; (ii) a fucoglucuronomannan having a backbone of alternating 4-linked β-d-glucopyranosyluronic acid and 2-linked α-d-mannopyranose residues with α-l-fucopyranose residues as single branches at C-3 of α-d-Manp; and (iii) a fucoglucuronan having a backbone of 3-linked β-d-glucopyranosyluronic acid residues with α-l-fucopyranose residues as single branches at C-4. Hence, even a single algal species may contain, at least in minor amounts, several sulfated polysaccharides differing in molecular structure. Partial resolution of these polysaccharides has been accomplished, but unambiguous evidence on their presence as separate entities was not obtained. 相似文献
5.
Ten C-glycosyl β2- and β/β2-peptides with three to eight amino acid residues have been prepared. Solution and solid-phase peptide syntheses were employed to assemble β2-amino acids in which C-glycosylic substituents are attached to the C-2 position of β-amino acids. Conformational analysis of the C-glycosyl β2-peptides using NMR and CD spectra indicates that the tripeptide can form a helical secondary structure. Besides, helix directions of the C-glycosyl β/β2-peptides are governed by the configuration at the α-carbon of the peptide backbone that originates from the stereocenter of the C-glycosyl β2-amino acids. 相似文献
6.
The mutagenicity of streptozotocin (SZN), 8 of its analogs and N-msthyl-N-nitrosourea (MNU) were compared in Salmonella typhimurium. SZN and its analogs carry MNU attached to the carbohydrate moiety at the C-2 position. The C-1 analogs tested were α- and β-methyl-SZN, α-ethyl-SZN, β-propyl-SZN, α- and β-butyl-SZN; in 2 analogs glucose was replaced by α- or β-inositol. When the ability of these compounds to revert the hisG46 auxotroph was compared, they fell into 4 groups which differed by about 10-fold in mutagenicity from one another. The most mutagenic was (i) SZN, followed by (ii) β-methyl-SZN; (iii) α-methyl-SZN, α-ethyl-SZN, β-propyl-SZN, α- and β-butyl-SZN; (iv) α and β-inositol-MNU. These results suggest that the presence of the glucose moiety is conducive to a high level of mutagenicity of SZN. Alterations of the glucose moiety by addition of larger alkyl groups, especially in the α position lead to decreased mutagenicity. The least mutagenic analogs are those in which the glucose moiety is replaced by inositols.The mutagenicity of SZN, β-methyl-SZN and of β-butyl-SZN was also compared in a mouse tissue-mediated assay. SZN was about 500-fold more mutagenic than its β-methyl analog, while the β-butyl analog was not mutagenic.Depletion of SZN and 4 of its analogs from the medium in presence of bacteria was determined spectrophotometrically. The more mutagenic compounds were depleted more rapidly but the quantitative differences in mutagenicity between these compounds could not be accounted for by depletion alone. 相似文献
7.
Fe(II)/α-ketoglutarate (αKG)-dependent hydroxylases catalyze an amazing diversity of reactions that result in protein side-chain modifications, repair of alkylated DNA/RNA, biosynthesis of antibiotics and plant products, metabolism related to lipids, and biodegradation of a variety of compounds. These enzymes possess a β-strand “jellyroll” structural fold that contains three metal-binding ligands found in a His1-X-Asp/Glu-Xn-His2 motif. The cosubstrate, αKG, chelates Fe(II) using its C-2 keto group (binding opposite the Asp/Glu residue) and C-1 carboxylate (coordinating opposite either His1 or His2). Oxidative decomposition of αKG forms CO2 plus succinate and leads to the generation of an Fe(IV)-oxo or other activated oxygen species that hydroxylate the primary substrate. The reactive oxygen species displays alternate reactivity in related enzymes that catalyze desaturations, ring expansions, or ring closures. Other enzymes resemble the Fe(II)/αKG-dependent hydroxylases in terms of protein structure or chemical mechanism but do not utilize αKG as a substrate. This review describes the reactions catalyzed by this superfamily of enzymes, highlights key active site features revealed by structural studies, and summarizes results from spectroscopic and other approaches that provide insights into the chemical mechanisms. 相似文献
8.
Leaves of Spinacea oleracea and Medicago sativa were incubated with (2-14C, (4R)-43H1 mevalonic acid and the sterols isolated. Cycloartenol had a 3H: 14C atomic ratio of 6:6 whilst oxidation to cycloartenone resulted in a ratio of 5:6 showing that tritium was present in the 3α-position and that the cycloartenol was symmetrically labelled. Separation of the 4-demethyl sterols gave α-spinasterol and a mixture of stigmast-7-enol and 24-methylcholest-7-enol, which had 3H: 14C atomic ratios of 3:5. Ozonolysis of α-spinastery] acetate gave the terminal side chain fragment as 2-ethyl-3-methyl butanoic acid. The acid contained 14C but no tritium thus showing that the C-24 hydrogen of cycloartenol is lost during the alkylation reactions leading to the C-24 ethyl group of α-spinasterol. 相似文献
9.
Leovigildo Quijano Jose S. Calderon G.Federico Gomez P.Jesus Lopez Tirso Rios Frank R. Fronczek 《Phytochemistry》1984,23(9):1971-1974
A chloroform extract of Montanoa grandiflora afforded a novel 6β-hydroxy-germacradien-8,12-olide. Its structure was shown to be 6-epi-desacetyllaurenobiolide by spectral studies, chemical transformations and single crystal X-ray diffraction. The X-ray data demonstrate that the ten-membered ring exists in the crystal in the highly unusual [15D5,1D14] conformation, in which the methyl group at C-4 is α-oriented and the methyl group at C-10 is β-oriented. The two double bonds are approximately parallel rather than crossed. 相似文献
10.
Carbon-13 spin-lattice relaxation times, T1, have been measured for aqueous solutions of L-aspartic acid, L-alanine, O-phospho-L-serine, and 2-mercapto-L-succinic acid in the presence of the paramagnetic metal ions, Cu2+ and Mn2+, and Mg2+ as a diamagnetic control, at ambient temperature and neutral pH. Nitrogen-15, oxygen-17 and proton relaxation times were also obtained for L-aspartic acid and phosphorus-31 relaxation times for O-phospho-L-serine under similar conditions. The structures of these complexes in solution were determined from the various metal ion-nuclei distances calculated from the paramagaetically-induced relaxation. These results indicate that the Cu2+ interaction with L-aspartic acid is through α-amino and β-carboxyl groups while Mn2+ coordinates most strongly through α-and β-carboxyl groups, with the possibility of a weak interaction through the amino group.An examination of the coordination of these divalent metal ions to an analog of L-aspartic acid in which the β-carboxyl group is replaced by a phosphate group (O-phospho-L-serine) indicated that Cu2+ coordination is now probably through the α-amino and phosphate groups, while this analog is a monodentate ligand for Mn2+ coordinating through the phosphate group. Removal of the β-carboxyl group (L-alanine) also results in Cu2+ coordination through the α-carboxyl and α-amino groups, and the same ligand interactions are observed with Mn2+. Replacement of the α-amino group of L-aspartic acid with an - SH group (2-mercapto-L-succinate) is sufficient to eliminate any specific coordination with either Cu2+ or Mn2+. 相似文献
11.
Parimi Atchuta Ramaiah Parimi Uma Devi Felix Frolow David Lavie 《Phytochemistry》1984,23(10):2251-2255
3-Oxo-friedelan-20α-oic acid, named as maytenonic (polpunonic) acid, has been isolated along with β-amyrin and sitosterol from Gymnosporia emarginata. 1H and 13C NMR signals have been assigned for the structure. X-ray analysis of the single crystal confirmed that the carboxylic acid is α-oriented at C-20 and the D/E rings of this D:A-friedo-oleanane are in chair-chair conformation. 13C NMR data of the present compound also enabled the assignment of the C-20α- and β-methyl carbons in friedelin. 相似文献
12.
Four peracetylated disaccharides 13C-labelled at the C-1′ position and having α-d-(1′→3), β-d-(1′→3), α-d-(1′→4), and β-d-(1′→4) linkages were prepared starting from the commercially available d-[1-13C]glucose. They were studied on the basis of their 3J13CH coupling constants in relation with the conformation in solution of oligosaccharides as models for the corresponding polymer. A method of analysis of the n.m.r. spectra is described and the coupling constants J13C-1′H given, particularly the 2J coupling (in the same cycle and with sign determination) and the 3J coupling (through the glycosidic bond). In that case, the values obtained give experimental information on the ψ angle values. They are compared with the known X-ray data for similar compounds. 相似文献
13.
Structure of hexasaccharide 1-phosphate polymer from Arthrobacter uratoxydans VKM Ac-1979T cell wall
N. V. Potekhina A. S. Shashkov S. N. Senchenkova L. V. Dorofeeva L. I. Evtushenko 《Biochemistry. Biokhimii?a》2012,77(11):1294-1302
A hexasaccharide 1-phosphate polymer of original structure and two teichoic acids (TA) belonging to different structural types were found in Arthrobacter uratoxydans VKM Ac-1979T cell wall. The poly(hexasaccharide 1-phosphate) combines features of teichuronic acids and glycosyl 1-phosphate polymers, and its structure has never been reported earlier. Its composition includes residues of α- and β-D-glucuronic acid as well as α-D-galacto-, β-D-gluco-, α-D-mannopyranose, and 6-O-acetylated 2-acetamido-2-deoxy-α-D-glucopyranose. The phosphodiester bond in the polymer joins the glycoside hydroxyl of α-D-glucuronic acid and O6 of α-D-galactopyranose. TA 1 is β-D-glucosylated 1,3-poly(glycerol phosphate), and TA 2 is 3,6-linked poly[α-D-glucosyl-(1→2)-glycerol phosphate]. The phosphate-containing polymers were studied by chemical methods and on the basis of one-dimensional 1H-, 13C-, and 31P-NMR spectra, homonuclear two-dimensional 1H/1H COSY, TOCSY, ROESY, and heteronuclear 1H/13C HSQC, HSQC-TOCSY, HMBC, and 1H/31P HMBC experiments. The set and structure of the polymers revealed as well as the cell wall sugars (galactose, glucose, mannose, glucosamine) and glycerol can be used in microbiological practice for taxonomic purposes. 相似文献
14.
A series of 3-, 7-, 15-, and 16-methyl-substituted steroid analogs were synthesized via a highly stereoselective 1,6-conjugate addition. Under the catalysis of CuBr, AlMe3 reacted with four steroid dienone precursors to afford either the corresponding α-epimer of C-3 and C-7 methyl-substituted steroids as the major products, and the ratio of α/β was up to 10/1. No β-epimer has been detected for methyl addition at C-16. However, under the same reaction conditions, enantioselective methyl addition at C-15 afforded the 15β-epimer as the major product. The preliminary SAR analysis showed that the methyl substituents at C-7α and C-15β positions lead to a dramatical increase in potency against human gastric cancer cell line MGC-803. 相似文献
15.
Ming-Daw Tsai Janet Weaver Heinz G. Floss Eric E. Conn Richard K. Creveling Mendel Mazelis 《Archives of biochemistry and biophysics》1978,190(2):553-559
The stereochemistry of the replacement of the SH-group of cysteine by CN catalyzed by β-cyanoalanine synthetase was studied using cysteine stereospecifically tritiated at C-3. Analysis of the resulting β-cyanoalanine by conversion into fumarate via aspartate and malate showed that the reaction had occurred with retention of configuration at C-3. Using cystine stereospecifically labeled at C-3 with tritium or with tritium and deuterium, it was found that the α,β-elimination reaction catalyzed by S-alkylcysteine lyase involves stereo-specific replacement of the β-substituent of the substrate by a hydrogen derived from the solvent, D2O or H2O, with retention of configuration to give pyruvate containing a chiral methyl group. The results are discussed, particularly in the light of mechanistic proposals by Braunstem and co-workers. 相似文献
16.
《Journal of Molecular Catalysis .B, Enzymatic》1999,6(1-2):89-94
Selective enzymatic hydrolysis of the peracetylated disaccharides, namely cellobiose, lactose, maltose and melibiose, with lipase from Asperilligus niger in aqueous buffer and organic solvent for 30 min afforded exclusively the corresponding heptaacetates with a free hydroxyl group at C-1 in high yield. Prolonged reaction of the β-1,4 linked cellobiose and lactose peracetates afforded selectively their hexaacetates with free hydroxyl groups at C-1,2, whereas the α-1,4 linked disaccharides maltose and melibiose peracetate gave a complex mixture of products. The reaction of 2-acetamido-2-deoxy-1,3,4,6-tetra-O-acetylglucopyranose (11) for 22 h afforded as the major product the diacetate 12 with free hydroxyl groups at C-1,4. 相似文献
17.
D.John Aberhart 《Bioorganic chemistry》1977,6(2):191-201
Ammonium[2-3H,1-14C]isobutyrate was converted by Pseudomonas putida ATCC 21244 into S(+)-β-hydroxyisobutyric acid (β-HIBA) with loss of the α-tritium atom. The recovered isobutyrate had the same as the starting material. Ammonium (2S)-[3-13C]isobutyrate was synthesized and converted by P. putida into β-HIBA. The 13C-nmr of the corresponding methyl ester benzoate showed 13C enrichment in the hydroxymethyl carbon atom. The results therefore indicate that isobutyrate metabolism in this organism proceeds via an unsaturated intermediate (probably methacrylyl-CoA) formed by dehydrogenation of the 2-pro-S-methyl group of the precursor (isobutyryl-CoA). Hydration of the intermediate proceeds with addition of a proton at C-2 from the same side as the hydrogen removed in the dehydrogenation. 相似文献
18.
Shigeo Ukai Seinosuke Matsuura Chihiro Hara Tadashi Kiho Kazuo Hirose 《Carbohydrate research》1982,101(1):109-116
A water-soluble galactomannan (C-3), [α]D20 +30°, isolated from the rod-like ascocarps of Cordyceps cicadae, was determined to be homogeneous, and the molecular weight was estimated by gel filtration to be 27,000. The polysaccharide is composed of d-mannose and d-galactose in the molar ratio of 4:3. The results of methylation analysis, Smith degradation, stepwise hydrolysis with acid, and 13C-n.m.r. spectroscopy indicated that the polysaccharide is of highly branched structure, and composed of α-d-(1→2)-linked and α-d-(1→6)-linked mannopyranosyl residues in the core; some of these residues are substituted at O-6 and O-2 with terminal β-d-galactofuranosyl and α-d-mannopyranosyl groups, and with short chains of β-d-(1→2)-linked d-galactofuranosyl units. 相似文献
19.
Nancy S. Rowan Carlyle B. Storm Robert Rowan III 《Journal of inorganic biochemistry》1981,14(1):59-65
The 1H and 13C nmr spectra of Co(NH3)5ImH3+ and the 1H nmr spectra of αCotrien(ImH)23+ and βCotrien(ImH)23+ are reported. The pKa values determined from the dependence of the chemical shift on pH are 10.0, 9.6, and 10.1, respectively. The range of the chemical shift between the acid and base forms is unusually small in the 1H nmr, 0.5–0.7 ppm for the C-2 H and about 0.25 ppm for the C-4 H and C-5 H. In the 13C nmr, C-2 and C-4 have large shifts to low field and C-5 a small shift to high field on deprotonation. The C-2 proton is not exchanged with solvent 2H under acidic or basic conditions, in marked contrast to the corresponding proton in both imidazole and 1-methylimidazole. These spectroscopic and chemical properties should be useful for the direct identification of metal-ion coordinated histidines in proteins. 相似文献
20.
《Journal of steroid biochemistry》1983,18(1):81-87
The α- and β-anomers of the 17β-d-glucuronide conjugate of ethynylestradiol were synthesized by the SnCl4-promoted reaction between β-acetoxy GAM and the t-17β-hydroxyl group of EE2-3-acetate. The conjugates were resolved by crystallization and HPLC. Positive identification was established by u.v. spectrophotometry, i.r. and mass spectrometry and 1H- and 134C-n.m.r. The structure of the β-anomer was confirmed by X-ray crystallographic analysis. In addition, the α-anomer was refractory to hydrolysis by bovine β-glucuronidase, establishing a biochemical difference between the conjugate pair. 相似文献