Gérard Excoffier

Effect of steam explosion treatment on the physico-chemical characteristics and enzymic hydrolysis of poplar cell wall components

Abstract The purpose of this study was to determine the effect of steam explosion pretreatment on pore size distribution and enzymic hydrolysis of poplar wood chips under various conditions (duration and temperature of treatment, pH impregnation). The measurements showed an increase in the available surface area with the severity of treatment and a good correlation with the initial rate of enzymic hydrolysis. This increase in enzymic hydrolysis can be explained by the removal of the hemicelluloses, but also by the melting and agglomeration of the depolymerized lignin, as shown on electron micrographs of ultrathin cross-sections.

Le group trichloroacétyle comme substituant temporaire; synthése du gentiotétraose

Abstract Glycosylation of 1,2,3,4-tetra- O -acetyl-β- d -glucopyranose with 2,3,4,6-tetra- O -acetyl-α- D -glucopyranosyl bromide and 2,3,4-tri- O -acetyl-6- O -trichloroacetyl-α- d -glucopyranosyl bromide, in the presence of mercuric cyanide, is 96–98% stereospecific. The trichloroacetyl group has been used as a temporary protecting group in a sequential synthesis of gentiotriose and gentiotetraose derivatives, first in homogeneous phase, and subsequently on a polymeric support: in the latter case the yield is about 70%.

Stéréospecificité des réactions de glycosylation; synthèse de l'isomaltose

Abstract Condensation of 1,2,3,4-tetra-O-acetyl-β- D -glucopyranose with 2-O-benzyl-3,4,6-tri-O-acetyl-α- D -glucopyranosyl bromide (6) gave 1,2,3,4-tetra-O-acetyl-6-O-(3,4,6-tri-O-acetyl-2-O-benzyl-α- (11) and β- D -glucopyranosyl)-β- D -glucopyranose (16). In the presence of molecular sieves and tetraethylammonium bromide, the ratio (1:24) of 11 to 16 is independent of the solvent used. In the presence of mercuric cyanide, however, 16 is preponderant ≈85%) in benzene or dichloroethane, whereas 75% of 11 is obtained in acetonitrile. On a solid support, condensation of 6 with 2,3,4-tri-O-acetyl-1-O-[(4-oxo-4 polystyryl)butyryl]- D -glucopyranose (22) in dichloroethane yielded a ratio of the anomers α to β identical to that obtained from the analogous reaction in a homogeneous phase.

Étude par r.m.n. de quatre disaccharides peracétylés sélectivement enrichis en 13C

Abstract Four peracetylated disaccharides 13 C-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- 13 C]glucose. They were studied on the basis of their 3 J 13 CH 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 J 13 C-1′H given, particularly the 2 J coupling (in the same cycle and with sign determination) and the 3 J 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.

Synthesis of reducing disaccharides of d-xylopyranose

Abstract Twenty-one derivatives of reducing disaccharides, d -xylopyranosyl-(1→2)-, -(1→3)-, and -(1→4)-α- or -β- d -xylose were synthesized according to the Helferich and Zirner method. Disaccharides having a pyranose reducing unit were prepared by condensation of 2,3,4-tri-O-acetyl-, 2,4-di-O-acetyl-3-O-chloroacetyl-, and 2,3-di-O-acetyl-4-O-chloroacetyl-α- d -xylopyranosyl bromide with tri-O-acetyl-β- d -xylopyranose derivatives; among these, 1,2,4-tri-O-acetyl-3-O-(2,4-di-O-acetyl-β- d -xylopyranosyl)-β- d -xylopyranose and 1,2,3-tri-O-acetyl-4-O-(2,3-di-O-acetyl-β- d -xylopyranosyl)-β- d -xylopyranose having a free hydroxyl group in the nonreducing sugar group were prepared for the synthesis of higher-mol.wt. linear oligomers. Glycosylation of the readily available 5-O-chloroacetyl- or 5-O-acetyl-1,2-O-isopropylidene-α- d -xylofuranose gave 1,2-O-isopropylidene-3-O-β- d -xylopyranosyl- d -xylofuranose derivatives, further transformed into hexa-O-acetyl-β-(1→3)-xylobiose. Some analogs of α- d -linked disaccharides were formed during glycosylation. The location of the acetyl groups in the reducing and nonreducing residues were identified by comparison of the methyl-proton resonances for solutions in (2H6)benzene of 2,3,4-tri-O-acetyl-β- d -xylopyranosyl-(1→3)-1,2,4-tri-O-acetyl-β- d -xylopyranose and of a hexa-O-acetyl disaccharide having deuterated acetyl groups in the reducing residue, obtained by acetylation of 2,3,4-tri-O-acetyl-β- d -xylopyranosyl-(1→3)- d -xylopyranose with (2H6)acetic anhydride.