Glycosyl fluorides, glycosidation

A combination of Cp nCX —A CXC) (where = cyclopentadienyl) effectively promotes the Friedel-Crafts coupling of glycosyl fluorides with aromatic compounds, such as trimethoxyben2ene or methoxynaphthalenes. The derived C-aryl glycosides are potent antitumor agents (39). 

Now with the two requisite coupling partners available, the next step is the elaboration of the AE ring system (see Scheme 11). Through the application of previously developed conditions for activation of glycosyl fluorides,24 the convergent union of compounds 19 and 20 can be achieved, giving a 4.5 1 mixture of axial and equatorial isomers in favor of the desired axial glycoside 60 (ca. 70 % yield). 

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)... 

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. " ... 

Scheme 8.3. Glycoside formation from glycosyl fluorides with a metallocene activator.

The characteristic feature of the divalent tin compounds is that they have both a vacant orbital and a lone pair of electrons (Figure 2.5). In this glycosylation reaction, it is assumed that SnCl2 behaves as a Lewis acid, where the vacant orbital accepts one of the three lone pairs in the fluorine atom of the glycosyl fluoride. As a result of this interaction, the C—F bond cleaves to give the oxocarbenium ion intermediate that is then attacked by an alcohol to give the glycoside. 

Table 2.2 Synthesis of 1,2-c/s-glycosides by using glycosyl fluorides as glycosyl donors promoted by SnCI2-AgCI04 [6] or SnCI2-TrCI04 [7],...

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