Glycoside synthesis anomeric selectivity

This procedure has been demonstrated to provide moderate yields and anomeric selectivity in oligosaccharide synthesis. For instance, the disaccharide 110 was obtained in 50% yield as a 1 2 a p ratio. The reaction side products were mainly the self-condensed donor (10-25%) and unreacted hemiacetal (5-10% or higher). Alternatively, the a-linked glycosides were favored with diethyl ether solvent. In this way, trisaccharide 111 was prepared from the disaccharide hemiacetal donor in 49% yield, favoring the a-anomer by 4 1. 

Glycomimetics, 468 Glycoproteins, biosynthesis, 486, 487 Glycosidases, 487, 491, 492 Glycoside synthesis, 381 anomeric selectivity, 382 challenges, 382 via electrochemistry, 385 via electron transfer, 385 inductive effects, 452 miscellaneous methods, 432 new glycosyl donors, 386 orthogonal, 452 overview, 381... 

For the preparation of trichloroacetimidates used in glycoside synthesis (see Section 10.3.2), the anomeric position must be selectively deacetylated. This is possible starting from the j3-anomer by treatment with hydrazine acetate (Excoffier et al. 1975). Thus, compound 5,3 can be prepared from 5.2 by this method. 

Biocatalytic synthetic pathways are very attractive as they have many advantages such as mild reaction conditions, high enantio-, regio-, chemoselectivity and are nontoxic natural catalysts. In enzymatic methods for glycoside and saccharide synthesis no selective protection/deprotection steps are necessary and control of configuration at newly formed anomeric centers is absolute. 

F. W. Lichtenthaler, U. Klaeres, M. Lergenmueller, and S. Schwidetzky, Enantiopure building blocks from sugars. 14. Various glycosyl donors with a ketone or oxime function next to the anomeric center facile preparation and evaluation of their selectivities in glycosidations, Synthesis, (1992) 179-184. 

Similar to glycosyl fluorides, thioglycosides - first introduced in glycosidation reactions by Ferrier [104,105] - offer efficient temporary protections of the anomeric center. They can be introduced at an early stage of the synthesis and selectively activated to form active glycosyl donors [106] (Scheme 8). 

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