Charbon de bois | EPFL Graph Search

Le charbon de bois est un combustible obtenu en carbonisant du bois en atmosphère contrôlée par pyrolyse (en l'absence d'oxygène). La charbonnette est un bois, de faible dimension, destiné à cette carbonisation. Le procédé permet d'éliminer, par élévation de la température, les fractions liquéfiables (acide pyroligneux) et gazéifiables du bois : son humidité et toute matière végétale ou organique volatile, afin de ne laisser que le carbone et les minéraux. Le charbon de bois a été historiquement utilisé comme combustible pour le chauffage et la cuisson des aliments. Il a été largement utilisé en métallurgie avant d’être remplacé par le charbon. Il entre également dans la composition de la poudre à canon. Intérêt Le charbon de bois a un pouvoir calorifique plus élevé que le bois et il se consume plus lentement. La densité calorifique du charbon de bois est de l’ordre de 28 à 33 MJ/kg contre 18 MJ/kg pour le bois sec. Le charbon de bois se consume par « incandescence

Synthesis of C-glycosides and C-linked disaccharides through carbonylative Stille coupling reactions

Peter Steunenberg

In the first part, we described the synthesis of C-(1'→1) and C-(1'→4) disaccharide precursors, which are readily obtained via the carbonylative Stille coupling of glucal and isolevoglucosenone derivatives. Tin glycals and triflates, derived from isolevoglucosenone, undergo the carbonylative Stille condensation under special conditions requiring AsPh3, LiCl and powdered charcoal as co-catalysts to give the corresponding cross-conjugated dienones, in which the bicyclic alkene moiety is more reactive than the glucal alkene moiety. The bicyclic part of the formed disaccharides precursors undergo selective nucleophilic additions with hydrogen, methoxybenzylamine and thiophenol. The other alkene moiety can undergo a selective epoxidation. The reactivity of the bicyclic part permitted a regio- and stereoselective hydrogenation of the bicyclic enone, that was then reduced with K-Selectride with high diastereoselectivity. The allylic alcohols so-obtained can undergo hydroboration giving a diol protected as an acetonide. The anhydro-moiety is then opened by a thioglycosidation reaction to form a partially protected glycoside-donor which can be used, in principle, for O-glycosidation. The C(1'→4) linked disaccharide so-obtained consists of a protected form of β-D-glucopyranose, attached at position C(4) of a α-D-3-deoxy-lyxo-hexopyranoside derivative via a (R)-hydroxymethanol linker. Another methodology based on 1,4-addition of thiophenol, allow one to form of several C-(1'→4) linked disaccharide precursors. Simple C-(1'→1)-disaccharides could also be prepared in similar fashion. In a second part the palladium-catalyzed cross-coupling reaction of tinglucal and tingalactal derivatives with arenesulfonyl chlorides provide aryl C-glycosides precursors. Desulfitative carbonylative Stille cross-coupling between 1-naphthalenesulfonyl chloride and a tinglucal derivative gives, after stereoselective reduction of the keto moiety and stereoselective oxidative hydroboration, C-glycosides consisting of a β-C-glucopyranoside and (R)-(naphtha-1-yl)methanol. Benzyl C-glycoside precursors are obtained by desulfitative Stille coupling of phenylmethanesulfonyl chloride with the corresponding protected tinglycals. In a third part we discussed the synthesis of carbohydrate based scaffolds for the deprotection of allylic alcohols by polysulfones. In this part we successfully applied the cleavage of allyl-protected carbohydrate derivatives by polysulfones, solid catalysts for the chemoselective cleavage of methyl substituted allyl ethers, such as prenyl, methylallyl and methylprenyl, developed by Markovic in our group. Allyl ethers are not cleaved under these conditions. This establishes a new method of selective polyol semi-protection in which a neutral, solid catalyst induces cleavage of ethers with the following reactivity sequence: 2-methylprenyl > prenyl > methallyl >> allyl.

EPFL2005

Past uppermost tree limit in the Central European Alps (Switzerland) based on soil and soil charcoal

The uppermost limits of past treelines in the Alps are established using soil type and soil charcoal mass. In all the studied sites, a sharp decrease of soil charcoal mass is correlated with the upper altitudinal limit of podzols. On the basis of this evidence, the uppermost tree limit reached 2500 +/- 100 m a.s.l. in the Valaisan Alps during the Holocene, i.e., it was 250 +/- 100 in higher than today's potential treeline. Consequently, the timberline would have reached 2400 +/- 100 m a.s.l.. From the strong decline of charcoals concentration in soils above 2500m a.s.l., we infer that conifer species were rare or very rare above this altitude during the Holocene. Joint interpretation of charcoal, pollen, soil and macrofossil data suggest that alpine meadows with at most scattered conifers were present throughout the Holocene in the today's middle and upper alpine belt.

2004

Synthesis of C-glycosides and C-linked disaccharides through carbonylative Stille coupling reactions

Peter Steunenberg

EPFL2005