Glycosyl preparation

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). 

Halide derivatives may be fluorides, chlorides, or bromides. Fluorides are best prepared by the reaction of hydroxy groups with (diethylamino)sulfur trilluoride ( DAST M. Sharma, 1977) or of glycosyl thioethers with DAST/NBS (K.C. Nicolaou, 1990 B). The other halides are usually only introduced at the glycosidic position, where treatment with hydrogen chloride... 

This ether, developed for the protection of a pyranoside anomeric hydroxyl, is prepared via a Konigs-Knorr reaction from the glycosyl bromide and 2-(ben-zylthio)ethanol in the presence of DIPEA. It is cleaved, after oxidation with dimethyldioxirane, by treatment with LDA or MeONa. ... 

More interesting are the substitution reactions on the triazole ring where a characteristically different course can be expected from that of analogous reactions in the purine series. These reactions were studied in more detail only in connection with the preparation of the glycosyl derivatives, and the experimental material does not permit the drawing of general conclusions. 

In the Koenigs-Knorr method and in the Helferich or Zemplen modifications thereof, a glycosyl halide (bromide or chloride iodides can be produced in situ by the addition of tetraalkylammonium iodide) is allowed to react with a hydrox-ylic compound in the presence of a heavy-metal promoter such as silver oxide, carbonate, perchlorate, or mercuric bromide and/or oxide,19-21 or by silver triflu-oromethanesulfonate22 (AgOTf). Related to this is the use of glycosyl fluoride donors,23 which normally are prepared from thioglycosides.24... 

In this approach [92], the glycosyl /3-azides were prepared according to literature procedure [93] and the acetylenes were synthesized in excellent yields via previously reported Pd-catalyzed microwave-assisted Sonogashira crosscoupling protocol with trimethylsilylacetylene, followed by desilylation upon treatment with TEAR... 

Carbohydrate-coupling or glycosylation, is a major synthesis method in carbohydrate preparation. Silver silicates and Ag(I)-exchanged zeolite A - so-called insoluble Ag(I) - have been advocated as promoting agents, applied in more than stoichiometric amount (Fig. 9). All hydroxyl groups except the attacking one are suitably protected. 

Glycosylation of the 7-hydroxy compound, prepared from 367 by trityla-tion, benzylation, and detritylation with acid, afforded the protected validamycin D in good yield. 

Because of the high C - F bond energy, glycosyl fluorides are stable in comparison to the other glycosyl halides, and this character has attracted much attention. They have been prepared in many different ways. One of them, rather classical, is through addition of the elements of HF (for example, HF in benzene ), BrF, or IF to per-O-acylated glycals. ... 

Glycosyl fluorides may be prepared by displacement of per-O-acyl or suitably protected 2-0-acyl 1-halides (Cl or Br) with fluoride [AgF (Refs. 28 and 29), KHF2 (Ref 30) or AgBF4 (Ref 31)]. -o-Glucopyranosyl [ F]fluo-... 

Glycosyl fluorides have also been prepared by treatment of per-O-acyl or partially 0-acylated sugars with hydrogen fluoride [liquid HF (for example, see Refs. 38 and 39) or HF in acetic acid or dichloromethane], as exemplified by 2,3,4-tri-O-benzyl-a-D-xylopyranosyl (18), a-D-glucopyranosyl (19), tetra-O-pivaloyl-a-D-glucopyranosyl (20), and 2,3,5-tri-O-acetyl-D-xylofur-anosyl fluorides (21) (see Table 1). Frequently, HF treatment - leads to... 

The preparation of glycosyl fluorides is described next. Aiming to have a convenient glycosyl donor convertible into 1,2-CM-furanosides, Mukaiyama and coworkers prepared 2,3,5-tri-O-benzyl-yS-D-ribofuranosyl fluoride (36y3) by treatment of a protected D-ribofuranose (35) with 2-fluoro-1 -meth-ylpyridinium tosylate (FMPTs) the total yield was raised by anomerizing the simultaneously produced a-1-fluoride (36a, 7, p 66, 72.f 24 Hz) to 36 (7, F 63.5 Hz, 72,F very small ) by treatment with BF3-OEt2. 

Glycosylations utilizing the aforedescribed glycosyl fluorides are described next. In 1981, Mukaiyama and coworkers attempted to prepare 1,2-cw-glycosides by utilizing the relatively stable (as compared with other glycosyl halides) 2,3,4,6-tetra-0-benzyl-)8-D-glucopyranosyl fluoride (47)5)... 

Glycosylation utilizing glycosyl fluorides in the presence of BF3 OEt2 was reported concurrently by Ishido, Voznij, Nicolaou, and their respective coworkers, and Kunz and Sagar. Ishido and coworkers prepared 1 — 1 a,a and y ,/ )-disaccharides under BFj catalysis by condensation of... 

Glycosides may also be prepared by enzyme-catalyzed condensation reactions utilizing a glycosyl fluoride. Thus 6-0-a-maltosylcyclodextrins were prepared enzymically from a-maltosyl fluoride (obtained from the corresponding heptaacetate by Zemplen deacetylation) and cyclodex-trins. " ... 

C-Glycosyl derivatives may be prepared by utilizing glycosyl fluorides. Ishido and coworkers reported that the reaction of 2,3,5-tri-O-benzyl-a-(36a) or - -D-ribofuranosyl fluoride (36fi) with isopropenyl trimethylsilyl ether under BF3 catalysis (0.1 -0.05 mol. equiv. for the fluoride, in ether or acetonitrile) gave a mixture of 4,7-anhydro-5,6,8-tri-C)-benzyl-l,3-dideoxy-D-altro- (139, major) and -D-a//o-2-octulose (140) 139 was stated to isomerized to 140 (should be vice versa) under Lewis acid catalysis. Similar... 

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