The arabinofuranosyl oxacarbenium ion could be locked in the E3 conformation by employing a 3,5-O-di-tert-butylsilane protecting group, which places C-5 and O-3 in a pseudoequatorial orientation, resulting in a perfect chair conformation of the protecting group. The new glycosyldonor gave excellent β selectivities in a range of glycosylations with glycosyl acceptors hav...