The methanolysis of some derivatives of 2,3,4-tri-O-benzyl-α-D-glucopyranosyl bromide in the presence and absence of silver salts

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-pull$\text{\$}\text{?}$mechanism is postulated in the presence of silver tetrafluoroborate or hexafluorophosphate. Glucosyl triflate intermediates are proposed for the silver triflate-assisted methanolyses.     

Synthesis of methyl 4,6-O-methylene-D-glycopyranosides

Methyl 4,6-O-methylene-D-glycopyranosides having the α-D-altro, α- and β-D-gluco, α-D-manno, and α-D-galacto configurations were prepared in 3.4 to 27.4% yields by condensing formaldehyde from 1,3,5-trioxane with the methyl glucosides in anhydrous 1,4-dioxane at 95° with boron trifluoride as the catalyst. A crystalline methyl 2,3:4,6-di-O-methylene-α-D-mannopyranoside was also isolated. Crystalline methyl 4,6-O-methylene 2,3-di-O-p-tolylsulfonyl-α-D-galacto- and α-D-glucopyranosides were prepared in 78 and 54.4% yields. N.m.r. coupling constants of the 2,3-di-O-acetyl derivatives of the 4,6-O-methylene glycosides were used to establish the Cl(D) conformation for each derivative.     

The use of 1-O-sulfonyl-d-mannopyranose derivatives in β-d-mannopyranoside synthesis

Several 1-O-sulfonyl derivatives of d-mannopyranose having a nonparticipating benzyl ether group at C-2 and ester functions at C-6 and C-4 were synthesized from the corresponding d-mannopyranosyl chloride derivatives with silver sulfonates in acetonitrile. The reaction of 1-O-sulfonyl-d-mannopyranose compounds with methanol in various solvents at room temperature gave high yields of glycosides with low degrees of stercoselectivity. On the other hand, 1-O-suffonyl-d-mannopyranose derivatives having an acyl participating-group at O-2 and benzyl ethers at C-3, C-4, and C-6 gave high yields and high stereoselectivity of α-d-mannopyranosides with primary and secondary alcohols in several solvents. Model studies were carried out to determine the best combination of 2-O-acyl group, solvent, time, temperature, and 1-O-sufonyl group to give high yields with high stereoselectivity. The method has been used to prepare in good yields more complex glycosides, including perbenzylated methy 2-O-(α-d-mannopyranosyl)-α-d-mannopyranoside.     

Binding of p-substituted-phenyl glycosides to the lectins from the pea and the lentil

The effect of p-substitution on the behaviour of phenyl α-D-mannopyranosides as specific ligands for the lectins from Pisum sativum (pea) and Lens culinaris (lentil) is reported. The binding affinities are related to the electronic properties of the substituents, expressed as the Hammett substituent constant (p = ?0.4). Inductive and mesomeric contributions are best considered separately. Neither steric hindrance nor hydrophobic binding of the substituents is observed. For the binding of the corresponding α-D-glucopyranosides to the Pisum sativum lectin, the same effect is maintained (p = ?0.4). In a series of p-substituted-phenyl β-D-glucopyranosides, the nature of the substituent has almost no effect on the binding to the Pisum sativum lectin.     

Synthése,conformation et méthanolyse des chlorures de 2,3,4-tri-O-chlorosulfonyl-β- et α-L-fucopyranosyle

Treatment of α-L-fucose with sulfuryl chloride at low temperature gave mainly 2,3,4-tri-O-chlorosulfonyl-β-L-fucopyranosyl chloride (1) and a small proportion of the α-anomer (2). Both compounds adopt a ¹C₄ chair conformation. Methanolysis of 1 in the presence of silver carbonate and anhydrous calcium sulfate gave methyl 2,3,4-tri-O-chlorosulfonyl-α-L-fucopyranoside (the β-anomer being only present in small proportion), further converted into methyl α-L-fucopyranoside by treatment with a basic resin and a catalytic amount of sodium iodide. Methanolysis of 1 in the presence of sodium iodide gave directly methyl α-L-fucopyranoside, in a more rapid but less stereoselective way. Methanolysis of 2 in the presence of silver carbonate is very slow and gave, after removal of the chlorosulfonyl groups, methyl β-L-fucopyranoside with a rather poor stereoselectivity.     

Stereochemistry of the β-cyanoalanine synthetase and S-alkylcysteine lyase reactions

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.     

The Modes of Binding Methyl—α (and β)-D-glucopyranosides and Some of their Derivatives to Concanavalin A—A Theoretical Approach

Abstract

The probable modes of binding of Methyl—α (and β)-D-glucopyranosides and some of their derivatives to concanavalin A have been proposed from theoretical studies. Theory predicts that βMeGlcP can bind to ConA in three different modes whereas α-MeGlcP can bind only in one mode. βMeGlcP in its most favourable mode of binding differs from α-MeGlcP in its alignment in the active-site of the lectin where it binds in a flipped or inverted orientation. Methyl substitution at the C-2 atom of the α-MeGlcP does not significantly affect the possible orientations of the sugar in the active-site of the lectin. Methyl substitution at C-3 or C-4, however, affects the allowed orientations drastically leading to the poor inhibiting power of Methyl-3-O-methyl-α-D-glucopyranoside and the inactivity of Methyl-4-O-methyl-α-D-glucopyranoside. These studies suggest that the increased activity of the α-MeGlcP over β-MeGlcP may be due to the possibility of formation of better hydrogen bonds and to hydrophobic interactions rather than to steric factors as suggested by earlier workers. These models explain the available NMR and other binding studies.     

Stereoselective synthesis of some methyl-substituted steroid hormones and their in vitro cytotoxic activity against human gastric cancer cell line MGC-803

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.     

The crystal structure of 6-epi-desacetyllaurenobiolide,a germacra-1(10),4-diene-12,8α-olide from Montanoa grandiflora

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.     

Differential mutagenicity of streptozotocin analogs of the carbohydrate moiety

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.     

The use of 1-O-tosyl-d-glucopyranose derivatives in α-d-glucoside synthesis

1-O-Tosyl-d-glucopyranose derivatives having a nonparticipating benzyl group at O-2 have been shown to react rapidly in various solvents with low concentrations of alcohols, either methanol or methyl 2,3,4-tri-O-benzyl-α-d-glucopyranoside. The stereospecificity of the glucoside-forming reaction could be varied from 80% of β to 100% of α anomer by changing the solvent or modifying the substituents on the 1-O-tosyl-d-glucopyranose derivative. 2,3,4-Tri-O-benzyl-6-O-(N-phenylcarbamoyl)-1-O-tosyl-α-d-glucopyranose in diethyl ether gave a high yield of α-d-glucoside. Kinetic measurements of reaction with various alcohols (methanol, 2-propanol, and cyclohexanol) show a high rate even at low concentrations of alcohol, and give some insight into the reaction mechanism. The high rate and stereoselectivity of their reaction suggest that the 1-O-tosyl-d-glucopyranose derivatives may be used as reagents for oligosaccharide synthesis.     

Changes in serum lipoproteins during metamorphosis of the bullfrog,Rana catesbeiana

Serum lipoproteins of the bullfrog, Rana catesbeiana, were studied during metamorphosis. Adult bullfrog has essentially one lipoprotein, designated β-lipoprotein. This β-lipoprotein migrates during electrophoresis to β-globulin region and it has a low hydrated density such that it exhibits floatation in a solvent of density 1.063. On the other hand, tadpole serum has one more lipoprotein, designated as α-lipoprotein, in addition to the β-lipoprotein. The α-lipoprotein migrates to the α-globulin region in zone electrophoresis and corresponds to the so called high density lipoprotein judging from ultracentrifugal behavior. Serum α-lipoprotein disappears and β-lipoprotein content decreases during metamorphosis.     

Further studies on the composition and structure of a fucoidan preparation from the brown alga Saccharina latissima

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.     

Synthesis of a series of fully methylated aldobiouronic acids,and β-elimination reactions of their synthetic precursors

Treatment of methyl 2,3,4-tri-O-acetyl-l-bromo-l-deoxy-α-d-glucopyranuronate severally with 2,4,6-, 2,3,6-, and 2,3,4-tri-O-methyl derivatives of methyl α-d-glucopyranoside and with methyl 4,6-O-benzylidene-3-O-methyl-α-d-glucopyranoside, in the presence of silver carbonate, afforded crystalline aldobiouronic acid derivatives in high yield. Deacetylation followed by methylation gave a series of fully methylated derivatives of laminaribiouronic, cellobiouronic, and gentiobiouronic acids, and the (1 → 2)-linked analogue. Methylation with methyl iodide and silver oxide in N,N-dimethylformamide was invariably accompanied by a small amount ofβ-elimination, with the formation of olefinic disaccharides which were also obtained by β-elimination reactions of the precursor acetates followed by methylation. Methyl 4,5-unsaturated 4-deoxyhexopyranosyluronate derivatives were the main products of the reaction, but these underwent further degradation with cleavage of the interglycosidic linkage and formation of 6-methoxycarbonyl-4-pyrone.     

13C-n.m.r. spectra of xylo-oligosaccharides and their application to the elucidation of xylan structures

¹³C-N.m.r. spectra of thirteen xylo-oligosaccharides [a complete series of α- and β-d-xylopyranosyl derivatives of methyl α-d-xylopyranoside, β-d-xylopyranosyl derivatives of methyl 4-O-β-d-xylopyranosyl-d-xylopyranoside, methyl O-α-d-xylopyranosyl-(1→3)-O-β-d-xylopyranosyl-(1→4)-d-xylopyranoside, and a branched methyl β-xylotetraoside] have been interpreted. The data obtained have been used for the carbon signal assignment in the spectra of a number of red-algal xylans. ¹³C-N.m.r. spectroscopy is shown to be a rapid and convenient method for the structural analysis of xylose-rich polysaccharides.     

Enzymatic regioselective deprotection of peracetylated mono- and disaccharides

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.     

Debenzylation of carbohydrate benzyl ethers and benzyl glycosides via free-radical bromination

The dealkylation of benzylated carbohydrates by free-radical bromination and hydrolysis has been further examined. Free-radical bromination of methyl 2,3,4,6-tetra-O-benzyl-α-D-glucopyranoside (1) methyl 2,3-di-O-benzyl-α-D-glucopyranoside (2) 6-O-benzyl-3,5-O-benzylidene-1,2-O-isopropylidene-α-D-glucofuranose (4) and 6-O-benzyl-D-glucose (3) appears to be quantitative. Spectroscopic evidence of a CBr bond indicates that an α-bromobenzyl ether is the product. Alkaline hydrolysis yielded methyl α-D-glucopyranoside from 1 and 2 and D-glucose from 3 and 4. A benzyl group present as an aglycon could be removed in the same way. Reaction of benzyl α-D-glucopyranoside tetraacetate (6) with bromine and chlorine under free-radical conditions and examination of the products by t.l.c. and i.r. spectrophotometry indicated that the first product was an α-halobenzyl glycoside and that the aglycon could be displaced by Br^- or Cl^- to form the tetra-O-acetyl-D-glucopyranosyl halide, undoubtedly with anomerization. Treatment of the mixture of products with moist ether and silver carbonate yielded only 2,3,4,6-tetra-O-acetyl-D-glucopyranose.     

Reaction of acetyl hypofluorite with pyranoid and furanoid glycals

The regiospecific syn-addition of acetyl hypofluorite to glycals derived from pentopyranoses led to mixtures of stereoisomers. Stereospecific reactions occurred with furanoid glycals, the direction of addition being governed by the nature of the substituent at C-3. Whereas a benzyloxy group caused attack from the opposite, less-hindered face of the double bond, a hydroxyl group induced addition from the same side. From these reactions, 2-deoxy-2-fluoro derivatives of β-d-arabino-, α-d-ribo-, β-d-lyxo-, and α-d-xylo-pyranose as well as β-d-manno-, α-d-gluco-, α-d-ribo-, and β-d-arabino-furanose were obtainedl their ¹H-, ¹³C-, and ¹⁹F-n.m.r. data are given.     

Synthesis of methyl 3-O- and 2-O-carbamoyl-α-D-mannopyranosides and carbamoyl-group migration between them

Methyl 3-O- and 2-O-carbamoyl-α-D-mannopyranosides, (2 and 3), were synthesized from methyl α-D-mannopyranoside via ammonolysis of a cyclic carbonate or a p-nitrophenoxycarbonate, as shown in Charts 1 and 2. Carbamoyl-group migration between the C-2 and C-3 hydroxyl groups, in methyl α-D-mannopyranoside under alkaline conditions, was also studied.     

Synthèse des l-fucose et l-(3-2H)fucose à partir du d-mannose

Oxidation with the dimethyl sulfoxide-acetic anhydride reagent of methyl 2-O-acetyl-4,6-O-benzylidene-α-d-mannopyranoside, obtained in quantitative yield from the corresponding 4,6-benzylidene acetal by stereoselective opening of a 2,3-orthoester, led in good yield to methyl 2-O-acetyl-4,6-O-benzylidene-α-d-arabino-hexopyranosid-3-ulose, which was reduced with either sodium borohydride or sodium borodeuteride into a methyl 4,6-O-benzylidene-α-d-altropyranoside or its 3-²H derivative. A sequence involving a C-6 halogenation-dehydrohalogenation followed by catalytic hydrogenation of the resulting methyl 6-deoxy-α-d-arabino-hex-5-enopyranoside gave methyl 6-deoxy-β-l-galactopyranoside (methyl β-l-fucopyranoside) and then α-l-fucose, with an overall yield of 24% with respect to the starting methyl α-d-mannopyranoside.