β‑Selective Mannosylation with a 4,6-Silylene-Tethered
Thiomannosyl Donor
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Abstract
Mannosylations
using the new conformationally restricted donor
phenyl 2,3-di-<i>O</i>-benzyl-4,6-<i>O</i>-(di-<i>tert</i>-butylsilylene)-1-thio-α-d-mannopyranoside
(<b>6</b>) have been found to be β-selective with a variety
of activation conditions. The simplest activation conditions were
NIS/TfOH, in which case it is proposed that the β-mannoside
is formed from β-selective glycosylation of the oxocarbenium
ion <b>25</b> in a <i>B</i><sub>2,5</sub> conformation