Pavol Kováč

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

Abstract 13 C-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. 13 C-N.m.r. spectroscopy is shown to be a rapid and convenient method for the structural analysis of xylose-rich polysaccharides.

Sequential synthesis and 13C-N.m.r. spectra of methyl β-glycosides of (1→4)-β-d-xylo-oligosaccharides

Abstract The reaction of 2,3-di- O -acetyl-4- O -benzyl-α,β- d -xylopyranosyl bromide ( 2 ) with methyl 2,3-di- O -acetyl-β- d -xylopyranoside gave methyl O -(2,3-di- O -acetyl-4- O -benzyl-β- d -xylopyranosyl)-(1→4)-2,3-di- O -acetyl-β- d -xylopyranoside ( 22 ). Catalytic hydrogenolysis of 22 exposed HO-4′ which was then condensed with 2 . This sequence of reactions was repeated three more times to afford, after complete removal of protecting groups, a homologous series of methyl β-glycosides of (1→4)-β- d -xylo-oligosaccharides. 13 C-N.m.r. spectra of the synthetic methyl β-glycosides (di- to hexa-saccharide) are presented together with data for six other, variously substituted, homologous series of (1→4)- d -xylo-oligosaccharides.

Synthesis and reactions of uronic acid derivatives : Part I. Unambiguous synthesis of methyl (Methyl α-d-glucopyranosid)uronate 2-, 3-, 4-, 2,3-dI-, 2,4-dI-, and 3,4-dI-Methyl ethers

Abstract Oxidation of O -benzyl- O -methyl derivatives of methyl α- d -glucopyranoside having only HO-6 unsubstituted, with chromium trioxide and dilute sulphuric acid in acetone, followed by esterification, gave high yields of the corresponding methyl (methyl α- d -glucopyranosid)uronates. Removal of the benzyl groups by hydrogenolysis then gave the partially methylated derivatives of methyl (methyl α- d -glucopyranosid)uronate, which were characterized as the amides. In this way, the complete series of partially methylated derivatives was obtained.

An approach to the systematic synthesis of (1→4)-β-d-xylo-oligosaccharides☆

Abstract The first approach to a general synthesis of (1→4)-β- d -xylo-oligosaccharides is described. Thus, 1,2,3-tri- O -acetyl-4- O -benzyl-β- d -xylopyranose was treated with hydrogen bromide in dichloromethane to give the corresponding glycosyl bromide 3 , which was condensed with 1,2,3-tri- O -acetyl-β- d -xylopyranose. The β-linked disaccharide formed, bearing at O-4 of its non-reducing end the selectively removable benzyl group, was submitted to hydrogenolysis to afford a disaccharide nucleophile 21 , having HO-4′ free, which was again treated with 3 . This sequence of reactions was repeated two more times and the isolated products 17–20 were deprotected to give a homologous series of lower xylo-oligosaccharides (from the disaccharide to the pentasaccharide), demonstrating thus the feasibility of stepwise construction of xylo-oligosaccharide molecules. The corresponding α-linked oligosaccharides 5–8 , present in the mixtures from the condensation reactions, were also isolated and characterized. 13 C-N.m.r. spectral characteristics for six homologous series of variously substituted (1→4)- d -xylo-oligosaccharides are presented.

The 13C-N.M.R. spectra of methyl (methyl O-methyl-α-d-glucopyranosid)uronates

Abstract 13 C-N.m.r. spectra of all of the methyl ethers of methyl (methyl α- d -glucopyranosid)uronate have been interpreted. The data can be used as an aid in the analysis of 13 C-n.m.r. spectra of α- d -glucopyranosyluronic acid-containing polysaccharides.

Hydrogenolysis of 3,5-0-benzylidene acetals with the LiAlH4-AlCl3 reagent in methyl d-xylofuranosides

Abstract The hydrogenolysis of methyl 3,5-0-benzylidene-α- and -β-D-xylofuranoside derivatives with the LiAlH4-AlCl3 reagent gave 5-benzyl ethers as main products. In some cases the attack of the reagent occured at the ring oxygen of the furenoside skeleton to yield 5-0-benzyl-1-0-methylxylitol derivatives. The structure of the synthesized compounds was proved by 13C-NMR spectroscopy. Unambiguous assignment of lines in the 13C-NMR spectra of numerous partially methylated methyl α-and β- d -xylofuranoside derivatives has been made.

The stepwise synthesis of an aldopentaouronic acid derivative related to branched (4-O-methylglucurono)xylans

Abstract Benzylation of methyl 2,3-anhydro-4- O -[2- O -benzyl-3,4-di- O -(β- D -xylop yranosyl]-β- D -xylopyranosyl]-β- D -ribopyranoside ( 1 ) afforded the crystalline. fully benzylated tetrasaccharide derivative 2 . The octa- O -benzyl derivative 3 , having only HO-2 unsubstituted, obtained by treatment of 2 with benzyl alcoholate anion in benzyl alcohol, was allowed to react in dichloromethane with methyl 2,3-di- O -benzyl- 1-chloro-1-deoxy-4- O -methy]-α,β-glucopyranuronate in the presence of silver perchlorate and triethylamine to give the branched, 4- O -methyl-α- D -glucuronic acid-containing pentasaccharide derivative 4a as the major product. Subsequent debenzylation afforded the aldopentaouronic acid derivative 5a , which contains all the basic structural features of branched, hardwood (4- O -methylglucurono)xylans. The structure of 5a was confirmed by analysis of its 13 C-n.m.r. spectrum and the mass-spectral fragmentation pattern of the corresponding fully methylated derivative 6a .

1H NMR Study of Methyl O-Acetyl-α-and-β-D-Xylopyranosides Conformational Studies and Non Additivity of 1H-Shift Increments

Abstract 1H NMR spectra of the complete series of fully and partially acetylated methyl α-and β-D-xylopyranosides have been studied. The α-anomers occur exclusively in 4C1 chairs but the 1C4 chair becomes increasingly populated in the β-forms especially when the OH-3 is not acetylated. Increments used for the prediction of the chemical shifts of ring protons are discussed and compared with the literature data. The predictability for changes in shifts upon acetylation is poor.

Structural analysis by mass spectrometry of oligosaccharides related to xylans

Abstract Fully methylated, xylan-type oligosaccharides, namely, positionally and inter-glycosidically isomeric O - D -xylopyranosyl-β- D -xylopyranoses (xylobioses, 2–7 ), a complete series of O -β- D -xylopyranosyl(l →4)- O -β- D -xylopyranosyl-β- D -xylopyranoses (xylotrioses, 8 , 9 , 11 , 12 , and 14 ), a branched β- D -xylotetraose ( 15 ), and α- D -xylopyranosyl β- D -xylopyranoside ( 1 ), have been studied by 70- and 12-eV mass spectrometry. By using high-resolution techniques metastable-transition measurements, ion species formed through hitherto unknown fragmentation-processes have been found for the (1→1)- and (1→3)-linked disaccharides 1 , 4 , and 5 . Based on the qualitative and quantitative differences in the fragmentations, criteria for unambiguous determination of the positional mode of linkage in xylobioses are proposed. Similarly, by studying the fragmentation of xylotriose ( 10 ) labelled in the side-chain with tri-deuteriomethyl groups, the fragmentation-pathways for xylotrioses have been clarified. A new fragmentation series has been discovered in the fission of trioses 13 and 14 having a D -xylopyranosyl group attached to O-3 of the nonreducing end of the basic (1→4)-linked skeleton. Within the series of di- and tri-saccharides studied, the criteria proposed permit reliable determination of any of the theoretically possible branching points. The basic possibilities for structural analysis by mass spectrometry of related pentose tetraoses are also shown.

Identification of methyl O-methyl-D-xylofuranosides by mass spectrometry

Abstract The fragmentation of methyl O -methyl- D -xylofuranosides under conditions of electron-impact mass spectrometry has been studied and compared with that of fully methylated methyl pentofuranosides. Characteristic differences in fragmentation of the differently substituted methyl O -methyl- D -xylofuranosides have been found which permit unambiguous assignment of both the number and the location of the methyl groups. Thus, compounds in this series can be identified without derivatization prior to the analysis. The g.l.c.-m.s. technique can be conveniently used to identify the products of methanolysis of methylated polysaccharides.