Glycosyl cations

Noyori and coworkers found that tetrafluorosilane or trimethylsilyl tri-flate catalyzes the condensation of appropriately protected glycopyranosyl fluorides with trimethylsilyl ethers or alcohols. The strong affinity of silicon for fluorine was considered to be the driving force for this reaction. In the case of Sip4, attack of a nucleophile on the glycosyl cation-SiFj ion-pair intermediate was anticipated. Thus, condensation of 2,3,4,6-tetra-O-benzyl-a- and - -D-glucopyranosyl fluorides (47a and 47fi) with methyl... 

An Sn 1 -like reaction, on the other hand, is much more favorable, because the glycosyl cation intermediate is stabilized by charge distribution between C-1 and the ring-oxygen atom. The unfavorable formation of an ion pair on bond cleavage can be avoided by protonation of the glycosidic oxygen atom and thus the requirement for acid catalysis (see Scheme 1). 

A second example has been addressed in which the synthesis and potential application of a series of neutral O-glycosylated porphyrin dimers and two original O-glycosyl cationic dimers were examined in PDT. In order to understand the influence of the number of glycosyl moieties, the spatial geometry and the ionic character on their photodynamic activity was evaluated.144 The in vitro results... 

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. 

H. Liu, X. Liang, H. Sohoel, A. Billow, and M. Bols, Noeuromycin, a glycosyl cation mimic that strongly inhibits glycosidases, J. Am. Chem. Soc., 123 (2001) 5116-5117. 

KawakamiS,YamashitaF,NishidaK,Nakamura J, Hashida M (2002) Glycosylated cationic liposomes for ceU-selective gene delivery. Grit Bev Ther Drug Carrier Syst 19 171-190... 

Potential leaving groups other than hydroxy can also be activated with acidic amine-HF complexes, if a sufficiently stabilized carbocation is formed as a reaction intermediate. Thus, glycosyl fluorides can be conveniently prepared from a variety of different glycosidic precursors, because of the stability of the intermediately formed glycosyl cation [134, 135] (Scheme 2.62). 

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