Synthesis and conformational analysis of disaccharide analogues containing disulfide and selenosulfide functionalities in the interglycosidic linkages

The synthesis of novel disaccharides containing disulfide (methyl-4-S-(β-D-galactopyranosyl-1'-thio)-4-thio-α-D-glucopyranoside (1)) and selenosulfide (methyl-4-Se-(β-D-galactopyranosyl-1'-thio)-4-seleno-α-D-glucopyranoside (2)) functionalities in the interglycosidic linkages is described. The synthetic strategy relied on the reaction of a β-glycosylthiosulfonate with a carbohydrate thiol or selenol nucleophile. The resulting protected β-dihetero-linked disaccharides were deprotected to give the target compounds. The conformational preferences of these dihetero analogues were inferred from NOESY experiments and line-broadening effects in variable-temperature NMR spectra, and are rationalized in terms of molecular orbital theory. Low-energy conformations of these compounds can populate regions of conformational space not usually occupied by β-linked disaccharides, and offer the possibility for presentation of novel ligand topographies.Key words: disaccharides, disulfides, selenosulfides, interglycosidic linkages, conformations, MO explanation.

Siliciumverbindungen mit starken intramolekularen sterischen Wechselwirkungen, XIV [1] Versuche zur Synthese des Tetra-tert-butylsilans: Tri-tert-butylisopropylsilan und verwandte Verbindungen / Silicon Compounds with Strong Intramolecular Steric Interactions, XIV [1] Attempted Synthesis of Tetra-tert-butylsilane: Tri-tert-butylisopropylsilane and Related Compounds

Reaction of tri-tert-butyl(dihalomethyl)silanes (halo = chloro, bromo) with organolithium compounds, by lithium/halogen or lithium/hydrogen exchange, leads to the corresponding carbenoids which by lithium halide elimination could give the carbenes (t-C4H9)3SiCX. Further intermolecular or intramolecular reaction of these reactive intermediates provides a variety of crowded molecules including alkyltri-tert-butylsilanes, 1,2-bis(tri-tert-butylsilyl)- substituted ethanes and ethenes and ring substituted 1,2-di-tert-butyl-1-silacyclo-butanes. An X-ray structure determination of (E)-1,2-bis(tri-tert-butylsilyl)-1,2-dichloroethene reveals neither a twisting nor an elongation of the CC double bond, but the C = C-Si bond angles exhibit rather unusual values (up to 137.7°) and the Si-C (sp2) bonds are as long as 195 pm.Tri-tert-butylisopropylsilane was prepared by a two step procedure and characterized by its variable temperature NMR spectra. The attempted synthesis of tetra-tert-butylsilane by reactions of the compound with bromine followed by treatment with methyllithium afforded only tri-tert-butylisopropenylsilane.