Bausteine von Oligosacchariden, XXX. Neue effektive β‐Glycosidsynthese für Mannose‐Glycoside Synthesen von Mannose‐haltigen Oligosacchariden

Abstract

Es wird eine neue Methode zur selektiven Synthese von Oligosacchariden mit P-verknupfter Mannose-Einheit entwickelt, fur deren direkte Gewinnung es bisher kein Verfahren gab. Als Katalysator dient ein auf A1203 niedergeschlagenes Silbersilicat. Es werden die Trisaccharide S-D- Building Units for Oligosaccharides, X X X *) New Efficient 0-Glycoside Synthesis of Mannose Glycosides Syntheses of Mannose Containing Oligosaccharides A new method was developed for the stereo selective synthesis of oligosaccharides containing a P-linked mannose-unit which were hitherto not available by direct synthesis. The glycoside synthesis is promoted by a silversilicate-catalyst which was precipitated on aluminium oxide. The trisaccharides 20 and 31 (see german summary) and the tetrasaccharide 33 were synthesized, demonstrating the advantages offered by block synthesis.In der repeating-unit der 0-antigenen Kette des Lipopolysaccharides aus Escherichia coli 0 75, dessen Synthese wir geplant haben, und von der schon einige von uns synthetisierte Teilstrukturen vorliegen'), ist eine Mannose-Einheit in fbglycosidischer Bindung enthalten. Die repeating-unit besitzt die folgende Formel*):Es ist zu ersehen, daB die Mannose-Einheit B(1 + 4)-glycosidisch an die D-Galactose am Verzweigungspunkt der Oligosaccharidkette gebunden ist. Die Herstellung derarti-

Direct and stereoselective synthesis of β-d-mannosides using 4,6-O-benzylidene-protected mannosyl diethyl phosphite as a donor

“…2 Despite these efforts, the stereoselective synthesis of β-mannosides is recognized as one of the most challenging problems in carbohydrate chemistry, because both the anomeric effect and the steric repulsion between a nonparticipating group disposed axially at C-2 and an incoming alcohol uniformly favor the formation of α-mannosidic linkages. Departing from the seminal work of Paulsen and Lockhoff on the direct construction of this linkage via an S N 2-type displacement in the presence of insoluble silver silicate, 3 a number of indirect methods involving the epimerization of accessible β-glucosides at C-2, 4 the hexo-2-ulosyl bromide approach, 5 the reductive cleavage of mannosyl anomeric orthoesters, 6 intramolecular aglycon delivery (IAD) 7 using a temporary mixed acetal [8][9][10] or silyl ether connectors, 11 and intramolecular mannosylation via prearranged glycosides, 12 have been reported.…”

General Aspects of the Glycosidic Bond Formation

“…Although occasionally substantial amounts of 1,2-cis glycosides were obtained even with 2-acylated glycosyl donors, the purposeful 1,2-cis glycosylations were best achieved with a nonparticipating ether group at C-2, such as methyl or benzyl. Further search for suitable promoters for the activation of glycosyl halides led to the discovery of Ag-silicate that proved to be very efficient for direct b-mannosylation, as these reactions often proceed via a concerted S N 2 mechanism [13,14].…”

“…In addition, it is further disfavored by the repulsive interactions that would have occurred between the axial C-2 substituent and the nucleophile approaching from the top face of the ring. For many years the only direct procedure applicable to b-mannosylationAg-silicate promoted glycosidation of a-halides -was assumed to follow bimolecular S N 2 mechanism [13,14]. The difficulty of the direct b-mannosylation was addressed by developing a variety of indirect approaches such as C-2 oxidationreduction, C-2 inversion, anomeric alkylation and intramolecular aglycone delivery (Scheme 1.7) [151][152][153][154][155].…”

Study of the Glycosidation Properties of 1‐Thiomannosazidopyranosides and 1‐Thiomannosaziduronic Acid Esters

“…[6] Other procedures involve silver silicate mediated S N 2 displacement of α-bromides [7] or intramolecular aglycon delivery [8] as the key step. The finding by Crich and co-workers that activation of 4,6-O-benzylidene-protected mannopyranosyl sulfoxide donors with triflic anhydride leading to the formation of β-mannoside linkages proved to be a breakthrough in this field of research.…”