Anomerization carbonium ion

With tln(IV) chloride, benzylated glycofuranosyl acetates give anomeric carbonium ions which cause Friedel Crafts alkylation of 0 -2 benzyl substituents. Scheme 19 illustrates the reaction in the D-rlbose series the corresponding L-arabinose-based product was produced in 60 yield. 

This methodology was optimised further by protecting the C-4 and C-6 hydroxyl groups with a cyclohexylidene acetal [66]. It was envisaged that the rigid chair conformation of the cyclohexylidene protected thiomethyl mannoside donor 83 would favour an Sn2 type pathway during the lAD reaction. This in turn would suppress any side reactions occurring from the formation of an anomeric carbonium ion. 

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