Activation of glycosyl trichloroacetimidates with perchloric acid on silica (HClO(4)-SiO(2)) provides enhanced alpha-selectivity

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Stereoselective ribosylation of amino acids

The glycosylation properties of ribofuranosyl N-phenyltrifluoroacetimidates toward carboxamide side chains of asparagine and glutamine were investigated. Conditions were found that promote nearly exclusive formation of the α-anomerically configured N-glycosides. The strategy allows for the synthesis of Fmoc-amino acids suitably modified for the preparation of ADP-ribosylated peptides. Furthermore, ribosylation of serine with these donors proved to be completely α-selective, and for the first time, α-ribosylated glutamic and aspartic acid, the naturally occurring sites for poly-ADP-ribosylation, were synthesized.

Convergent synthesis of the tetrasaccharide repeating unit of the O-antigen of Shigella boydii type 9

A convenient synthesis of the tetrasaccharide repeating unit of the O-antigen of Shigella boydii type 9 has been achieved in excellent yield using a [2 + 2] block glycosylation strategy. TEMPO-mediated selective oxidation of the primary alcohol of the tetrasaccharide derivative 8 to the carboxylic group followed by deprotection of the functional groups furnished target tetrasaccharide 1 as its 4-methoxyphenyl glycoside in high yield.

Study of the stereoselectivity of 2-azido-2-deoxygalactosyl donors: relationship to the steric factors of glycosyl acceptors

The stereoselectivity of a 2-azido-2-deoxygalactosyl (GalN(3)) donor is found to strongly depend on the nature of the acceptors in glycosylation reactions. The order of the acceptor, the stereochemistry, and the configuration of the monosaccharide all affect the stereochemistry outcome. More reactive acceptors are observed to favor β-products, while less reactive acceptors afford more α-products.

Study of the stereoselectivity of 2-azido-2-deoxygalactosyl donors: remote protecting group effects and temperature dependency

The stereoselectivity of glycosylation reactions is affected by many factors. Synthesis of 1,2-cis glycosidic linkages (such as α linkages in glucose and galactose like monosaccharides) is challenging due to lack of control of the stereoselectivity. Our systematic study of GalN(3) donors with different combination of protecting groups indicated that acetyl groups at the 3- and 4-positions are particularly important for high α-selectivity. Temperature is also recognized as a major factor in control of stereoselectivity. Mechanisms responsible for these experimental results are discussed and explored using computational methods. A remote participation model of the acetyl groups is proposed to explain the directing effects of the acetyl groups.