| Abstract
| - A series of 4,6-O-benzylidene-protected 2-O-benzyl-3-deoxy-3-fluoro- and 3-O-benzyl-2-deoxy-2-fluorogluco- and mannopyranosyl thioglycosides were synthesized and their coupling reactions with aseries of alcohols, on preactivation with 1-benzenesulfinylpiperidine and trifluoromethanesulfonicanhydride, investigated. In all cases, the selectivities were lower than those observed with the correspondingsimple 4,6-O-benzylidene 2,3-di-O-benzylgluco- and mannopyranosyl thioglycosides. This leads to theconclusion that the high β-selectivity observed with 4,6-O-benzylidene 2,3-di-O-benzylmannopyranosyldonors under the same conditions is in large part derived from the compression of the O2−C2−C3−O3torsion angle on going from the intermediate covalent glycosyl triflate to the oxacarbenium ion, ascompared to the relaxation of this torsion angle in the gluco series.
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