Solid-phase synthesis of oligosaccharides

A major contribution to the use of glycosyl trichloroacetimidates in the solid-phase synthesis of oligosaccharides came from the group of T. Ogawa (Scheme 4.7).5... 

Plante, O.J. Palmacci, E.R. Seebo-go-, P.H. (2001) Automated solid-phase synthesis of oligosaccharides. Science, 291, 1523-7. 

S. J. Danishefsky, K. F. McClure, J. T. Randolf, and R. B. Ruggeri, A strategy for the solid-phase synthesis of oligosaccharides. Science 260 1307 (1993) and references cited therein. 

Attachment of suitable linkers to the surface of silica can be achieved by transesterification with (3-aminopropyl)triethoxysilane, which leads to the support 2 (Figure 2.8) [198-200]. Alternatively, silica can be functionalized by reaction with alkyltri-chlorosilanes [201]. For the solid-phase synthesis of oligonucleotides, supports with a longer spacer, such as that in 3, have proven more convenient than 2 [202-206]. Supports 3, so-called LCAA-CPG (long chain alkylamine CPG [194,195]), are commercially available (typical loading 0.1 mmol/g) and are currently the most commonly used supports for the synthesis of oligonucleotides. For this purpose, protected nucleosides are converted into succinic acid monoesters, and then coupled to LCAA-CPG. CPG functionalized with a 3-mercaptopropyl linker has been used for the solid-phase synthesis of oligosaccharides [207]. 

Support-bound triacylmethanes (e.g. 2-acetyldimedone) readily react with primary aliphatic amines to yield enamines. These are stable towards weak acids and bases, and can be used as linkers for solid-phase peptide synthesis using either the Boc or Fmoc methodologies, as well as for the solid-phase synthesis of oligosaccharides [456]. Cleavage of these enamines can be achieved by treatment with primary amines or hydrazine (Entries 2 and 3, Table 3.23 see also Section 10.1.10.4). 

Silyl ethers of aliphatic alcohols are inert towards strong bases, oxidants (ozone [81], Dess-Martin periodinane [605], iodonium salts [610,611], sulfur trioxide-pyridine complex [398]), and weak acids (e.g., 1 mol/L HC02H in DCM [605]), but can be selectively cleaved by treatment with HF in pyridine or with TBAF (Table 3.32). Phenols can also be linked to insoluble supports as silyl ethers, but these are less stable than alkyl silyl ethers and can even be cleaved by treatment with acyl halides under basic reaction conditions [595], Silyl ether attachment has been successfully used for the solid-phase synthesis of oligosaccharides [600,601,612,613] and peptides [614]. 

The solid-phase synthesis of oligosaccharides is usually performed using acid-resistant linkers and protective groups, because of the slightly acidic reaction conditions required for glycosylations (Section 16.3). Hydroxyl group protection is conveniently achieved by conversion into carboxylic esters, such as acetates, benzoates, or nitro-benzoates. Support-bound esters of primary or secondary aliphatic alcohols can be cleaved by treatment with alcoholates [97-99] (Table 7.8), with DBU in methanol, with hydrazine in DMF [100] or dioxane [101], or with ethylenediamine [102], provided that a linker resistant towards nucleophiles has been chosen. 

Allyl carbonates can be cleaved by nucleophiles under palladium(O) catalysis. Allyl carbonates have been proposed for side-chain protection of serine and threonine, and their stability under conditions of /VT moc or /V-Boc deprotection has been demonstrated [107]. Prolonged treatment with nucleophiles (e.g., 20% piperidine in DMF, 24 h) can, however, lead to deprotection of Alloc-protected phenols [108,109]. Carbohydrates [110], tyrosine derivatives [107], and other phenols have been protected as allyl ethers, and deprotection could be achieved by palladium-mediated allylic substitution (Entry 9, Table 7.8). 9-Fluorenyl carbonates have been used as protected intermediates for the solid-phase synthesis of oligosaccharides [111]. Deprotection was achieved by treatment with NEt3/DCM (8 2) at room temperature. 

In early attempts directed towards the solid-phase synthesis of oligosaccharides, glycosidic bonds were formed by treating pyranosyl bromides with partially protected carbohydrates (Figure 16.16). These syntheses could be performed with either the pyranosyl bromide (glycosyl donor) [184,185] or the alcohol (glycosyl acceptor) [186— 188] linked to the support. 

Figure 16.16. Early solid-phase synthesis of oligosaccharides using glycosyl bromides [187],...

The use of glycals as monomers for the solid-phase synthesis of oligosaccharides has been explored by Danishefsky (Figure 16.20 [183,215]). Glycals can be converted into glycosyl donors by epoxidation or haloamination, which usually proceed with... 

Figure 16.20. Solid-phase synthesis of oligosaccharides from glycals [216,217],...

Scheme 19. Solid-phase synthesis of oligosaccharide-conjugated enediynes as putative DNA-binding and cleaving agents.

N. K. Kochetkov, Solid-phase synthesis of oligosaccharides and glycoconjugates, Russian Chem. Rev., 69 (2000) 795-820. 

P. H. Seeberger Automated solid-phase synthesis of oligosaccharides to address biomedical problems... 

Nicolau, K. C., Winssinger, N., Pastor, J., and DeRoose, F. (1997) A general and highly efficient solid phase synthesis of oligosaccharides. Total synthesis of a heptasaccharide phytoalexin elicitor (HPE)../. Am. Chem. Soc. 119,449—450. 

Some further examples of promising approaches towards the solid-phase synthesis of oligosaccharides are reported in [1],... 

Enzyme-Catalyzed Solid-Phase Synthesis of Oligosaccharides I 391... 

Fig. 8. Enzymatic glycosylation in the solid-phase synthesis of oligosaccharides elongation of the glycopeptide with /M, 4-galactosyltrans-ferase and a-2,3-sialyltransferase. The glycopeptide is finally released by cleavage with a-chymotrypsin.

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