4,6-O-Benzylidene-Directed β-Mannopyranosylation and α-Glucopyranosylation: The 2-Deoxy-2-fluoro and 3-Deoxy-3-fluoro Series of Donors and the Importance of the O2−C2−C3−O3 Interaction

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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 a series of alcohols, on preactivation with 1-benzenesulfinylpiperidine and trifluoromethanesulfonic anhydride, investigated. In all cases, the selectivities were lower than those observed with the corresponding simple 4,6-O-benzylidene 2,3-di-O-benzylgluco- and mannopyranosyl thioglycosides. This leads to the conclusion that the high β-selectivity observed with 4,6-O-benzylidene 2,3-di-O-benzylmannopyranosyl donors under the same conditions is in large part derived from the compression of the O2−C2−C3−O3 torsion angle on going from the intermediate covalent glycosyl triflate to the oxacarbenium ion, as compared to the relaxation of this torsion angle in the gluco series.

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