Synthesis of 3- <i>C</i> -Methyl- <scp>d</scp> -Mannopyranoside Derivatives Functionalized at the 3-Position

3-<i>C</i>-Methyl-d-mannopyranose is a rare branched sugar whose derivatives also serve as precursors to the more common d-evalose (6-deoxy-3-<i>C</i>-methyl-d-mannose) and, after esterification at O3, as probes for the study of the mechanism of the stereodirecting effect of esters at the 3-position in mannopyranosylation reactions. We describe alternative syntheses of 4,6-<i>O</i>-benzylidene-protected 3-<i>C</i>-methylmannopyranosyl donors and their 3-<i>O</i>-benzoates and an interesting, reversible migration of a 2-<i>O</i>-silyl derivative on the attempted esterification at O3. We also describe the reversal of stereoselectivity on application of the Corey-Chaykovsky reaction to a 3-keto sugar based on a 4,6-<i>O</i>-benzylidene-protected α-mannopyranoside: the trimethylsulfonium iodide-derived ylid favored axial attack leading to the desired mannoisomer, whereas the more bulky trimethylsulfoxonium iodide-derived ylid gave the altro isomer of the resulting spiroepoxide as a consequence of preferential equatorial attack.