Showing papers on "Pyranose published in 1993"

Enantiospecific synthesis by transformations of chiral pool-derived metal .pi.-complexes. A strategy for the introduction of substituents on a pyranose-derived lateral .pi.-ligand either syn or anti to the coordinating metal

Abstract: In order to ectend the use of stoichiometric transition metal π-complexes to the stereocontrolled synthesis of organic target structures bearing substituents either cis or trans to each other on a cyclic unszaturated π-ligand, new strategies for the functionalieation of π-complexes are required. A first-generation appriach to the stereocontrolled introfuction of substituents syn or anti relative to an η 5 -CpMp-CO) 2 unit coordinated to a lateral π-ligand is reported.

Catalytic mechanism of 3-deoxy-D-manno-2-octulosonate-8-phosphate synthase. The use of synthetic analogues to probe the structure of the putative reaction intermediate.

Abstract: The proposed mechanistic pathway for the reaction catalyzed by 3-deoxy-d-manno-2-octuloson-ate-8-phosphate (Kdo8P) synthase was examined in terms of the structure of the putative bisphosphate intermediate. Two 2-deoxy analogues of the product Kdo8P, having been structurally prohibited from undergoing the ring-opening and possessing the stereochemistry of either the α-pyronase (compound 1) or the β-pyranose form (compound 2) of the product, were synthesized and probed as inhibitors for the synthase. It was found that both analogues bind to the enzyme and are competitive inhibitors with respect to phosphoenolpyruvate binding, having Ki values of 470 μM and 303 μM, respectively. Comparison of this data to the Ki value of the tautomeric mixture of the product Kdo8P (Ki= 590 μM) suggests that both the α- and the β-pyranose anomers (65.8% and 3.1%, respectively at neutral pH) bind to the enzyme with a slight (1.13 kJ/mol) preference for the β-anomer, and that the C2 hydroxyl does not contribute to the binding. This uncertain stereochemical preference exhibited by the enzyme for the stereoisomers at the anomeric carbon suggests that the carboxylate binding site of the product is indistinct, while the hydroxyl and carboxylate binding sites may be interchangeable. More importantly, however, the isosteric phosphonate analogue 2,6-anhydro-3-deoxy-2β-phosphonylmethyl-8-phosphate-d-glycero-d-talo-octonate (3), which mimics the topological and electrostatic properties of the proposed cyclic intermediate, was found to be the most potent inhibitor of the enzyme with a Ki, value of 5 μM. Two hitherto unrecognized aspects of the mechanism of the synthase were identified. First, the results showing that the cyclic analogues 1, 2 and 3 are inhibitors of the enzyme whereas the previously reported acyclic analogue, which contains no carbonyl group at C2 and may thus resemble the open-chain form of Kdo8P, is not an inhibitor, suggest that the pyranose form and not the open-chain acyclic form of the putative bisphosphate intermediate is handled by the enzyme. Second, since the overall stereochemical course of the transformation mediated by the synthase has been shown to involve si face addition of phosphoenol-pyruvate to the re face of the carbonyl of arabinose 5-phosphate, the present observation involving analogue 3 suggests that the bisphosphate intermediate formed during the initial steps of synthesis may have the pyranose structure with the anomeric phosphate located in the β-configuration.

Carbohydrates to carbocycles: synthesis of the densely functionalized carbocyclic core of tetrodotoxin by radical cyclization of an anhydro sugar precursor

Abstract: The core of tetrodotoxin is a densely functionalized carbocycle for which an annulated pyranose can be envisaged as a retron. The carbocyclic ring is constructed upon a rigid 1,6-anhydro template which permits the early introduction of the key angular nitrogen, and concomitantly positions a trap for reaction with a carbon-centered radical generated chemospecifically at C6. The carbocyclic entity is elaborated in this process leading to a caged system with the capability for generating all of the stereogenic centers. A novel procedure using tert-butylhyponitrite is described for C6 dehydrogenation pursuant to the intramolecular radical cyclization. © 1993, American Chemical Society. All rights reserved.

Synthesis and conformational analysis of N-glycopeptides. II. CD, molecular dynamics, and nmr spectroscopic studies on linear N-glycopeptides†

Abstract: The comprehensive structural analysis reported herein of eight N-glycopeptides, in three different solvents, is based on quantitative CD experiments, homonuclear nuclear Overhauser effect measurements, and molecular dynamics (MD) calculations. Although several orientations of the two amide planes attached to the carbohydrate pyranose ring are possible, according to NOE, CD data, and MD simulations, of all of the glycopeptide models, regardless of the type of the carrier peptide, only one dominant conformer population was found. This conformer is characterized by a nearly trans orientation of the CH and NH hydrogens of both acetamido groups. This finding is in perfect agreement with x-ray crystallographic data on the solid state conformation of the 1-N-acetyl- and 1-N-(β-aspartoyl)-2-acetamido-2-deoxy-β-D-glucopyranosylamine. The precise identification of this dominant conformer of N-glycopeptides in solution was the major question addressed herein by the structural analyses. A “CD additivity” experiment was carried out using an equimolar solution of Boc-Pro-Asp-NHCH3 and l-N-acetyl-3,4,6-tri-O-acetyl-2-acetamido-2-deoxy-β-D-gluco-pyranosylamine at ambient temperature in acetonitrile. The CD spectrum obtained from the equimolar solution of the above two molecules (the “spectroscopic sum”) was identical with the CD curve obtained from the algebraic summation of the individually recorded CD spectra of the peptide and the carbohydrate moiety (“mathematical sum”). The global picture of the CD spectral analyses of the eight parent peptides with the eight N-glycopeptides revealed that in trifluoroethanol and acetonitrile, the side-chain modification of the Asn models (natural N-glycopeptide analogues) by N-glycosylation has a significant effect on the conformation of the carrier peptide, resulting in a decrease in the original type I β-turn content. Simultaneously, the type II β-turn conformational percentage increased to ≈ 20%. Such a conformational ratio change seems to be larger than the expected errors arising from the CD analyses, and agrees with the results of MD calculations. N-glycosylation of Asn residues causes perturbations, not only through the covalent bond, but also through specific hydrogen bonds between the backbone and side chain atoms. CD spectroscopy, augmented by efficient CD curve deconvolution techniques, has proved to be a useful tool for studying multicomponent conformer mixtures of small linear peptides in solution and changes of conformational equilibria caused by N-glycosylation. © 1993 John Wiley & Sons, Inc.

Trifluoromethylated pyranose derivatives as the chiral dopant for ferroelectric liquid crystals

Abstract: The novel pyranose derivatives with a trifluoromethyl group were found to be excellent chiral dopants for ferroelectric liquid crystals, inducing large spontaneous polarization by their addition even in a small quantity. These materials showed some characteristic physical properties depending on their stereochemical relationship : thus, the corresponding cis isomers exhibited high ability in inducing large spontaneous polarization and quick response but lowered the upper limit of the temperature of the Sc phase. However, the same trend in the spontaneous polarization was observed for the compounds with trans stereochemistry, without lowering the transition temperature of SA to SC, of the host liquid crystal. MoFover, it was also proved that the adjustment of the helical pitch of the N* phase was readily carried out. Based on these results, we have succeeded in the development of the practical ferroelectric liquid crystalline compositions with good alignment and quick response.

Unprecedented influence of azides and the effect of bulky groups on zinc-induced reductions of deoxy halogeno sugars

Abstract: Zinc/silver–graphite induced ring-opening reactions of deoxy halogeno sugars by dealkoxyhalogenation may be suppressed by bulky protecting groups in the substrates, giving rise to simple dehalogenation products. Furanose derivatives are more sensitive to such steric effects than pyranose systems, although an X-ray analysis of 1,2-O-isopropylidene-3-O-benzyl-5-deoxy-5-iodo-α-D-xylofuranose 3 shows no evidence of steric hindrance by the C-3 substituent. Moreover, dehalogenation instead of reductive ring-opening reactions are observed in all cases in which organic azides are present in the reaction medium. This unprecedented effect of azides on zinc-induced reductions is equally effective both intra-and intermolecularly. This behaviour is best rationalized by assuming radical intermediates for dehalogenations, in contrast to transient organometallic species closely bound to the zinc surface when reductive ring-opening is observed. This mechanistic interpretation is supported by a labelling experiment.

Stereoselective Ring-Opening of Acetylated Pyranose-1, 2-(ethyl orthoacetates)

Abstract: When acetylated pyranose-1, 2-(ethyl orthyl orthoacetates) were hydrolyzed in acidic solvents, the ring-opening of the orthoacetate rings was influenced by the axial or equatorial OAc group at C-4 on the pyranoses; on acid-catalyzed hydrolysis, 3, 4, 6-tri-O-acetyl-α-D-glactopyranose- (8) and methyl 3, 4-di-O-acetyl-α-D-galacturonatopyranose-1, 2-(ethyl orthoacetate) (16) having an axial OAc group at C-4 on the pyranose rings gave 1, 3, 4, 6-tetra-O-acetyl-α-D-galactopyranose (9) and methyl 1, 3, 4-tri-O-acetyl-α-D-galacturonatopyranose (23), respectively, whereas 3, 4, 6-tri-O-acetyl-α-D-glucopyranose- (10) and methyl 3, 4-di-O-acetyl-α-D-glucuronatopyranose-1, 2-(ethyl orthoacetate) (22)having an equatorial OAc group at C-4 on the pyranose rings gave 2, 3, 4, 6-tetra-O-acetyl-D-glucopyranose (11) and methyl 2, 3, 4-tri-O-acetyl-D-glucuronatopyranose (24), respectively. On the acid-catalyzed hydrolysis, 3, 4-di-O-acetyl-β-L-arabinopyranose-1, 2-(ethyl orthoacetate) (34) having an axial OAc group at C-4 on the pyranose ring gave a mixture of 1, 3, 4-tri-O-acetyl-β-L- (35) and 2, 3, 4-tri-O-acetyl-L-arabinopyranose (36). These selectivities of ring-opening of the 1, 2-(orthoacetates) were considered to have resulted from the differences of the conformers fo the 1, 2-(orthoacids)intermediates derived from the 1, 2-(othoacetates) and the orientation of the acetyl groups at C-4 on the pyranose rings.

New cellulose cuprammonium solution and production thereof

Abstract: PURPOSE:To obtain a new cellulose cuprammonium solution having low content of copper and ammonia by preparing cellulose having adsorbed copper and then dissolving the cellulose in an aqueous solution containing ammonia CONSTITUTION:For example, cellulose is immersed in a cuprammonium solution containing an alkali metal hydroxide to prepare cellulose having adsorbed copper in an amount of 03-07 molar ratio to pyranose ring of cellulose Then the cellulose is dissolved in an aqueous solution containing ammonia to give the objective cuprammonium solution containing >=3% cellulose, 05-5% ammonia and 025-1% alkali metal hydroxide

Two Polymorphs of Structures of $\alpha,\alpha$ -Trehalose Octaacetate Monohydrate

Abstract: Structures of two polymorphs of -trehalose octaacetate monohydrate, , have been studied by X-ray diffraction method. -trehalose (-D-glucopyranosyl -D-glucopyranoside) is a nonreducing disaccharide. The polymorph I belongs to the monoclinic , and has unit cell parameters of a=10.725(l), b=15.110(4), c=11.199(5) , and Z=2. The polymorph II is orthorhombic , with a=13.684(4), b=15.802(4), c=17.990(9) and Z=4. The final R and R values for monoclinic polymorph I are 0.043 and 0.048 and for orthorhombic polymorph II are 0.116 and 0.118, respectively. Those R values of polymorph II are high because the large thermal motions of acetyl groups and the poor quality of the crystal. The molecular conformations in the two polymorphs are similar. Both D-glucopyranosyl rings have chair conformations and atoms of glycosidic chain linkage are coplanar. The primary acetate groups of the pyranose residues assume both gauche-trans conformations. The molecules of two polymorphs have pseudo-C symmetry at glycosidic O(1) atom. The bond lengths and angles are normal compared with those in other acetylated sugar compounds. The molecules in the monoclinic crystal are held by the hydrogen bonds with the water molecules and by van der Waals forces.

Pyranose type nucleoside derivative

Abstract: PURPOSE: To obtain the subject novel compound which is useful for treatment of acquired immunodeficiency syndrome and cancers as an antiviral or an antitumor agent by binding a specific anhydrosugar with a nucleic acid base or acyl-protected nucleic acid base followed by deprotection. CONSTITUTION: A saccharide of formula I (R 4 is H, lower alkoxy; R 5 , R 6 are hydroxy-protecting groups) such as methyl 2,3-anhydro-4,6-benzylidene-α-D- allopyranoside is bonded to a nucleic acid base such as adenine or an acyl- protected nucleic acid base, when needed, the hydroxy-protecting groups are removed, further, if necessary, the product is led to a deoxy, double bonded or phosphorylated derivative to give the objective pyranose type nucleoside derivative of formula II [B is adenine, guanine, thymine, uracil, cytosine; R 1 , R 2 are OH, H, or may together form a bond; R 3 is formula III ((n) is 0, 1, 2); R 4 is H, lower alkoxyl], which is used as an antiviral or an antitumor agent. COPYRIGHT: (C)1994,JPO&Japio

The nitrile oxide/isoxazoline route to carbon-linked disaccharides

Abstract: A route to carbon-linked disaccharides (two monosaccharide units linked by a carbon bridge rather than a glycosidic oxygen) employing nitrile oxidefisoxazoline chemistry has been investigated. This convergent approach is based on cycloaddition of sugar-derived alkene and nitrile oxide fragments, followed by ring cleavage and functional group manipulation of the resulting 2-isoxazoline. Careful selection of the nitrile oxide and alkene fragments defines much of the stereochemistry of the products. Two 0)-unsaturated hexofuranoses were chosen for study: 3-O-benzyl-5,6dideoxy1 ,2-O-isopropylidene-(X-D-XYlo-hex-5-enOfUraflOSe (37) derived from Dglucose and methyl 5 ,6-dideoxy-2,3-O-isopropylidene-CL-D-lyXo-hex-5-eflofuranOside (137), which has the opposite configuration to (37) at C2, derived from D-mannose. Four nitrile oxides were employed in cycloadditions to these alkenes: ethoxycarbonylformoniirile oxide (36) was used as a model 1,3-dipole to probe the itfacial selectivity of cycloadditions to alkene (137), and three pyranose 1-nitrile oxides (122), (146) and (150), derived from D-xylose, D-arabinose and D-galactose respectively. Additions to alkene (37) proceeded with a high degree of it-facial selectivity (56-82% d.e.) in favour of the 2-isoxazoline with eryrhro configuration. Similar selectivity was noted in additions to alkene (137). This selectivity can be explained in terms of the "inside alkoxy effect" proposed by Houk and modified to include the "homoallylic effect" put forward by De Micheli. Reductive hydrolytic cleavage of the 2-isoxazolines to release the latent f3hydroxy ketone functionality was carried out by both PCl/C and Ra-Ni catalysed hydrogenolysis. In the Pd/C case significant loss of the 3-0-benzyl protecting group was noted, however, Ra-Ni hydrogenolysis afforded the required 3-hydroxy ketones ((1—+6)-carbonyl-linked C-disaccharides) in moderate yield (43—+56%) along with a minor product identified as an epimeric mixture of the corresponding ?-amino alcohols ((1 —*6)-aminomethylene-linked C-disaccharides). Finally, reduction of the -hythoxy ketone functionality and subsequent removal of the isopropylidene protecting groups afforded the dipyranose (1-46)hydroxymethylene-linked C-disaccharides.