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

TL;DR

This paper presents new methods for synthesizing and studying 3-C-methyl-d-mannopyranoside derivatives, focusing on their structural variations and stereoselectivity.

Contribution

The paper introduces alternative syntheses and observations of stereoselectivity reversal using different reagents in sugar derivative reactions.

Findings

Alternative syntheses of 4,6-O-benzylidene-protected 3-C-methylmannopyranosyl donors and their 3-O-benzoates were developed.

A reversible migration of a 2-O-silyl derivative was observed during attempted esterification.

The Corey–Chaykovky reaction showed reversed stereoselectivity based on the reagent used, affecting the isomer produced.

Abstract

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.

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