Water glycosyl cations

In the 1-phenylethyl system, 5 nI and 5 n2 pathways could occur in the same solution concerted reactions occurred when the carbenium ion did not have a meaningful lifetime in the presence of a nucleophile. This finding had important applications to the reactions of glycosyl derivatives in water, where the reality or otherwise of glycosyl cation intermediates is now known to depend on the leaving group. 

Most organic solvents are less polar than water, so that if intimate ion pairs of glycosyl cations and anions are too unstable to exist in water, a fortiori they have no real existence in organic solvents and mechanistic proposals which invoke them are simply in error. Even the reality of the ion-molecule complexes shown to exist in water must be questioned. Nonetheless, many literature sources draw intimate ion pairs and even solvent-equilibrated oxocarbenium ions, in the absence of any direct evidence. 

These reactions can be rationalized by envisaging a glycosyl cation intermediate that is subject to competitive interception either by the hydroxyl group of another sugar, leading to disaccharide formation, or by water, leading to overall hydrolysis (Figure 5.2). 

Certain cationic porphyrins containing glycosyl groups 454-461 were also reported by Krausz and coworkers (Scheme 93). " In these compounds the presence of a sugar moiety increased the water solubility and membrane interaction however, no PDT data are reported. 

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