Mannosylation silver oxide activation

Silver cations on the surface of insoluble catalysts anchor mannosyl bromides from the a-side [52]. As a consequence, highly reactive alcohols can attack the anomeric center from the P-side in an Sn2 type reaction, thus leading to P-mannosidic linkages. The first application of this principle was reported by Perlin [53] in 1961 who condensed the cyclic carbonate-protected mannosyl bromide [54] 2 and the reactive aglycone 3 under silver oxide activation to afford the first synthetic disaccharide (4) that contains a P-mannopyranosidic linkage (Scheme 2). 

Silver oxide has also been successfully used for the formation of p-mannosyl-rhamnose linkages, which are present in the O-specific polysaccharide of Salmonella [17]. Kochetkov and co-workers effectively coupled identical donor 4 with the rhamnose acceptor 6 by activation with silver oxide to exclusively afford the P-Man-(1 — 4)-a-Rha disaccharide 7 in an impressive 91% yield (Scheme 8.2) [25]. 

High yields have been achieved with Paulsen s silver silicate activator precipitated on aluminum oxide that can also act as a water scavenger [64]. A threshold level of acceptor reactivity is necessary for satisfactory P-stereoselectivity—whereas coupling of mannosyl bromide 8 with the anhydro derivative 9 proceeds in an excellent yield and high P-selectivity [67] to afford 10, mannosylation of the less nucleophilic disaccharide 11 with the same donor affords the trisaccharide 12 as a 11 a/6 anomeric mixture (Scheme 4) [68]. ... 

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