466
3
Chemical Glycosylation Reactions
⊡ Scheme 7
to very successful results. Mannopyranosyl donors with diol O-protecting groups leading to
ring annelation had already been investigated but again with limited success [85]. Therefore, it was a big surprise that 2,3-di-O-alkyl-4,6-O-benzylidene-protected mannopyranosyl
sulfoxides as glycosyl donors gave preferentially β-products with various acceptors at low
temperatures [86,87]. The same result is more conveniently obtained with the corresponding
trichloroacetimidates as shown in > Scheme 9 with glycosyl donor 9a furnishing with accep-