Synthesis of 3‑C‑Methyl‑d‑Mannopyranoside Derivatives Functionalized at the 3‑Position

3-C-Methyl-d-mannopyranose is a rare branched sugar whose derivatives also serve as precursors to the more common d-evalose (6-deoxy-3-C-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-O-benzylidene-protected 3-C-methylmannopyranosyl donors and their 3-O-benzoates and an interesting, reversible migration of a 2-O-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-O-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.