Glycals, electrophilic addition

Glycals can be glycosylated not only directly via electrophilic addition, cyclo addition, nucleophilic addition and rearrangement reactions but also indirectly by conversion into a range of other glycosyl donors. One of the most important classes of these glycal-derived donors, the 1,2-anhydro sugars, is discussed in detail in Section 1.3. 

The determination of the lifetime of the ionic intermediates using the azide-clock method has been however useful in showing that electrophilic addition of Br2 can occur, even through a fully concerted mechanism, definable as SN2-like. Bromination of cyclic enol ethers (glycals) 8-10 in methanol in the presence of... 

The concept of armed/disarmed glycosyl donors was subsequently extended by other groups to thioglycosides21 and selenoglycosides.64 A similar strategy has been used by Friesen and Danishefsky to achieve chemoselectivity in electrophilic addition to glycal double bonds.65... 

As the project progressed, a major study was made of the electrophilic addition of various fluorinated species to the double bonds of peracylated carbohydrate glycals,... 

Unlike the carbohydrates with double bonds at positions other than between C-1 and C-2 ( isolated alkenes ), which exhibit normal alkene chemistry, glycals are vinyl ethers and therefore undergo a number of highly selective addition reactions due to the strongly polarized double bonds and the presence of bulky substituents at the C-3 allylic centers. Straightforward addition reaction includes initial electrophilic addition at the double bond, followed by the addition of a nucleophile at C-1 to give the 1,2-trans adduct (O Scheme 19). 

AIBN. The ease of the allylic bromination of 18 has been attributed to the presence of the endoperoxide, which can initiate the radical reaction [44], Similar results were obtained with Br2 without initiator, whereas Br2 reacts with glycal 10 through an electrophilic addition on the double bond [44, 67],... 

It was at one time thought that l-like reactions were anti and FlcB-like reactions were more likely to be syn but electrophilic additions to glycals are rarely trans and they go through transition states in the SW corner of the More O Ferrall-Jencks diagram. 

Figure 6.68 Some electrophilic additions of glycals. For details of relative proportions of products, see text.

The electrophilic addition of arylbis(arylthio)sulfonium salts to glycals in the presence of alcohols, directly provide 2-thio-(5-glycosides with good stereoselectivity [162]. Stereoselective syntheses of [5-glycosides were also obtained from the reaction of 1,2-trans acetoxysele-nides, prepared by treatment of glycals with phenylselenyl chloride and silver acetate, in the presence of trimethylsilyl triflate [163]. Combined application of this method and the trichlo-roacetimidate method (see section 3.3) was suggested for the synthesis of functionalized precursors of the olivomycin C-D-E trisaccharide [164] (Scheme 34). 

Figure 6. Possible mechanism of addition of fluorine to glycals with Xep2. (a) Electrophilic addition of F from XeF2-BF3 complex, (b) addition of F on carboxonium ion,...

Various pathways for the electrophilic addition to a glycal followed by nucleophilic opening of the resulting halonium ion can account for the different product distributions obtained from these reactions [74]. Irreversible formation of the halonium ion and subsequent nucleophilic displacement leads to the (B-g/wco compound, while reversible formation of the halonium ion followed by slow nucleophilic trans diaxial opening leads to the a-manno compound. Common T sources used to synthesise a-manno species include V-iodosuccinimide (NIS), and iodonium di-iym-collidine perchlorate (IDCP). An iodonium source is particularly favoured over a bromonium source, as this more readily allows further subsequent functionalisation of the product often via radical chemistry. The yields of... 

Electrophilic Additions to Glycals Mechanistic Aspects and Applications to the Synthesis of 2-Deoxyglycosides... 

Introduction of a fluorine atom at C-2 by addition to glycals is effected by electrophilic reagents, (Q is a strongly electron-withdrawing... 

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