β‑Selective Mannosylation with a 4,6-Silylene-Tethered Thiomannosyl Donor

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>_2,5 conformation