Michael glycosidation

Glycosidic thiol groups can be introduced into glycosyl bromides by successive reactions with thiourea and aqueous sodium disulfite (D. Horton, 1963 M. Cemy, 1961, 1963). Such thiols are excellent nucleophiles in weakly basic media and add to electrophilic double bonds, e.g., of maleic esters, to give Michael adducts in high yields. Several chiral amphiphiles have thus been prepared without any need for chromatography (J.-H. Fuhrhop, 1986 A). 

Michael addition/elimination 256 Mitsunobu glycosidation 543 ff. Mitsunobu reaction 259, 5291,... 

Michael, H.N., Saliband, J.Y., and Ishek, M.S., Acylated flavonol glycoside from Psidium guai-java, Pharmazie, 57, 859, 2002. 

The application of diazo coupling is somewhat limited by the availability of the p-aminophenyl glycosides, particularly of those of the oligosaccharides. Their precursors, the p-nitrophenyl glycosides, are usually obtained by the condensation of a per-O-acetylated glycosyl halide with p-nitrophenol in ethanol (Michael reaction)19,20 or by the reaction of a per-O-acetylated sugar with p-nitrophenol in the presence of a Lewis acid catalyst (Helferich reaction).21 p-Nitrobenzyl 1-thioglycosides have also been prepared by the condensation of the... 

J. Das and R. R. Schmidt, Convenient glycoside synthesis of amino sugars Michael-type addition to 2-nitro-D-galactal, Eur. J. Org. Chem., (1998) 1609-1613. 

The use of heteroaryl vinyl sulfides and vinyl dithiocarbamates (166) as hetero-Michael addition acceptors has been described. Combined chelating and electron-withdrawing effects were postulated to stabilize the transient anionic species and allow smooth Michael-induced ring closure to produce C-glycosides (167).191... 

By simulating evolution in vitro it has become possible to isolate artificial ribozymes from synthetic combinatorial RNA libraries [1, 2]. This approach has great potential for many reasons. First, this strategy enables generation of catalysts that accelerate a variety of chemical reactions, e.g. amide bond formation, N-glycosidic bond formation, or Michael reactions. This combinatorial approach is a powerful tool for catalysis research, because neither prior knowledge of structural prerequisites or reaction mechanisms nor laborious trial-and-error syntheses are necessary (also for non-enzymatic reactions, as discussed in Chapter 5.4). The iterative procedure of in-vitro selection enables handling of up to 1016 different compounds... 

C-Glycosides. Glycosyl fluorides react with a number of nucleophiles to provide C-glycosides. Lewis acid catalysts are not essential for reaction with A1(CH,), or A1(CH,),CN, but MgBr, is useful in reactions with Michael acceptors such as CH,=CHCN. The products are reduced by alane to tetrahydropyranes. They also react with S- and N-nucleophiles to afford the corresponding glycosides. 

Unlike base-mediated cyclizations, halocyclizations do not depend on the presence of Michael acceptors. As shown in Scheme 7.94, Armstrong and Teegarden [234] treated a hydroxyolefin (derived from a Wittig reaction applied to a protected arabinose) with A-bromosuccinimide and catalytic bromine to activate the illustrated cyclization. The reaction took place through an intermediate bromonium ion, providing a 52% yield of the (3 anomer. This observation is consistent with the preference for a anomers observed in base-mediated cyclizations. Moreover, the isolation of a bromo-substituted C-glycoside allows the preparation of new and novel structures. This chemistry was further adapted to the use of iodine instead of bromine [235]. 

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