Lewis acid promoted glycosylation

Representative Procedure for Bronsted Add Promoted Glycosylation with Cl-Hemiacetal Donors Using Methoxyacetic Add 128 Representative Procedure for Lewis Acid Promoted Glycosylation with Cl-Hemiacetal Donors Using Sn(OTf)2 and LiC104 129... 

In addition to Bronsted acid promoted Fischer-type glycosylations, Lewis acids have been investigated (Scheme 3.4). A variety of Lewis acids promote glycosylation under mild conditions, often in substoichiometric amounts. The earliest examples include ZnCl2 [18] and FeCl3 [19], although these readions were demonstrated only for preparation of trehalose-type disaccharides. Mukaiyama et al. have very recently developed metal triflate catalysts for the dehydrative glycosylation with... 

SCHEME 4.40 Lewis acid-promoted glycosylation of hemiacetals. 

Scheme 3.4 Lewis acid promoted dehyrative glycosylations.

Representative Procedure for Lewis Acid Promoted Clycosylation with Glycosyl Acetate Donors Using SnCI4 [198]... 

Suitably protected glycosyl halides or acetates, upon Lewis-acid promoted SN1 heterolysis, generate glycosyl cation intermediates that can react with electron-rich arenes, heteroarenes, Me3SiCN, enoxysilanes, enamines, allyl silanes and stannanes, acetylenyl silanes and stannanes affording C-glycosyl compounds. 

Glycosyl fluorides are excellent precursors for glycosylations. Lewis acids promote the defluorination leaving a stabilized carbenium species which is able to react with nucleophiles, e.g. N-nucleophiles. The configuration at the newly formed C-N bond is not only influenced by the sugar configuration but also by the utilized Lewis acid. This means that this is not a clear SNl-type reaction SN2-type pathways also have to be considered.15 18... 

Glycosyl phosphites were introduced by Schmidt [451-453], Wong [454-457] and Watanabe [458,459], A variety of dialkyl and cyclic phosphites has proved suitable for the reaction [459,460], In fact, diethyl and dibenzyl phosphites are the most frequently used. Glycosyl phosphites react with alcohols at low temperamres in the presence of catalytic amounts of Lewis acid promoters such as TMSOTf, BF3-Et20 and Sn(OTf)2, thus creating glycosides. Phosphites are also activated by other promoters, which are summarized in Table 4.9. 

In typical procedures for glycosyl fluoride glycosylation reactions, the glycosyl fluoride is mixed with an aglycon alcohol and a Lewis acid promoter. As noted above, Mukaiyama s original disclosure described the use of a combination of tin(II) chloride and silver perchlorate as the activator [20], but since then, many other... 

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