Silicates Silver

Mannosides are difficult to obtain since here a 2-O-acyl group blocks the -position. 2-O-Benzyl-a-mannosyl bromides give, however, high yields of pure -glycosides with a heterogeneous silver silicate catalyst preventing anomerization and SnI reaction of the bromide H. Paulsen, 1981 B, Q. 

Carbohydrate-coupling or glycosylation, is a major synthesis method in carbohydrate preparation. Silver silicates and Ag(I)-exchanged zeolite A - so-called insoluble Ag(I) - have been advocated as promoting agents, applied in more than stoichiometric amount (Fig. 9). All hydroxyl groups except the attacking one are suitably protected. 

The Koenigs-Knorr method in the presence of an insoluble silver salt proceeds mainly with inversion of configuration. Silver silicate and silver-silicate-aluminate have often been used as the heterogeneous catalyst. This procedure has been traditionally used for the synthesis of p-mannosidcs and has been recently reviewed.35 However, it only works well with very reactive halides and sufficiently reactive alcohol components. 

Ammonium chloride or ammonium carbonate Silver nitrate Gelatinous precipitate Yellow precipitate of silver silicate soluble in dilute acids as well as ammonia solution... 

Fig. 5. Synthesis of 1,2-trans glycosides using a participating group at C-2 for example, synthesis of glycosylated hydroxylysine derivative (p-D-Gal-( 1 >0)-IIyl) using (a) insoluble promoter-silver silicate (56) (b) soluble promoter-silver trifluo-romethanesulfonate (57).

At ordinary temperature and pressure quartz is the stable modification of silica. Quartz does not noticeably react with water but is readily attacked by HF. A less dense metastable modification of Si02 called silica W [after Weiss and Weiss (35)] reacts readily with water and forms a yellow silver silicate in contact with an aqueous solution of AgNOs. Another newly discovered but dense modification of Si02, called silica C [after Coes (6)], which is also metastable under ordinary conditions, is so unreactive that even the smallest crystals are not noticeably attacked by HF. The rates of the reactions of Si02 are determined by the polarizability of the O-2 ions. The O-2 ions are most polarizable in the low density form (silica W) and least polarizable in the high density form (silica C). The reactions of silica with water are initiated by the penetration of protons into the electron clouds of the O-2 ions and the rate of proton penetration increases with increasing polarizability of the anions. The phenomena described are strictly rate phenomena, because neither quartz nor the two metastable forms of silica are in equilibrium with HF. 

A different approach to control the stereoselectivity of glycosylations is through the use of heterogeneous catalysis. Catalysts such as silver silicate were developed for this purpose [46]. Reactions of glycosyl halides on the surface of silver silicate are thought to proceed by a concerted mechanism providing, for example, (3-D-mannopyranosides 44 from a-D-mannosyl bromide 43 (Scheme 4.7). 

More widely used promoters for this purpose are silver zeolite and silver silicate, which were developed by Paulsen [18] and Garegg [19], respectively. Although not being commercially available, these reagents can be conveniently prepared from silver nitrate and proved to be quite powerful. More recently, van Boeckel developed the use of silver silica-alumina, which was reported to be more reactive than other insoluble salts and to have an enhanced cation capacity [20]. Representative examples of successful y0-mannosylation by using insoluble salts are depicted inO Scheme 8 [19,21,22]. 

An even more impressive result was reported by Takeda and coworkers [33] (O Scheme 11). In their synthetic studies on the spermatozoa-derived glycolipid, the disaccharide donor 20 was coupled with 21 by the action of silver silicate. The reaction proceeded in a remarkably selective manner a P = l) to afford the desired disaccharide in 70% yield. This is one of a very few examples of the direct formation of a p-manno-g yco idQ using an oligosaccharide... 

Reduction with tin butyl hydride then gives the a-D-glycosides. 2-De-oxy— D-glycopyranosides are formed on the surface of silver silicate or silver zeolite. 

Heidebrecht, K., and Jansen, M. (1991) Agig(Si04)2(Si40i3), the first silver silicate with mixed anions, Z Anorg. Allg. Chem., 597,79-86, (1991) 606,242. 

The influences of -allyl, benzyl, acetyl, trichloroacetyl and levullnoyl substituents at 0-3, 4 or 6 on the a,S ratio in the products of silver silicate or silver zeolite activated coupling of 2- -benzyl-a-D-gluco- and manno-pyranosyl bromide derivatives with the anhydrides (24) and (25) have been examined. Highest 3 a-... 

The introduction of the more reactive silver silicate promoter by Paulsen and Lockoff overcame the problem of coupling less reactive secondary carbohydrate acceptors [30]. Activation of the mannosyl bromide donor 8 with silver silicate in the presence of the anhydro acceptor 9 exclusively afforded P-mannoside 10 in an excellent 81% yield (Scheme 8.3). 

This methodology was further enhanced by the realisation that an acyl protecting group at the C-2 position effectively hinders the formation of a carbonium ion intermediate [31]. This phenomenon was exploited by Paulsen and co-workers in the coupling of mannoside donor 11 with the GlcNAc precursor acceptor 12 by activation with silver silicate, to afford the P-disaccharide 13 in a 65% yield (Method 1) [32]. 

Silver silicate promoted P-mannosylation of bromo mannopyranosyl donors [32]. 

Notes and discussion. This method allows efficient entry to 4-0-(4-0-acetyl-3,6-di-(9-allyl-2-0-benzyl-P-D-mannopyranosyl)-l,6-anhydro-2-azido-3-0-benzyl-2-deoxy-P-D-glucopranose by use of the silver silicate catalyst procedure. The target is of use for entry to a branched p-mannose containing pentasaccharide, present within A-gly coproteins. 

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