Can preferential β-mannopyranoside formation with 4,6-O-benzylidene protected mannopyranosyl sulfoxides be reached with trichloroacetimidates?

Studies with 3-O-allyl-2-O-benzyl-4,6-O-benzylidene-α-d-mannopyranosyl sulfoxide (3d) and the corresponding trichloroacetimidate (4) as glycosyl donors and various acceptors (A–F) led under similar reaction conditions to similar glycosylation results, i.e. mainly or exclusively the β-anomers of glycosides 5dA–5dF could be obtained. Thus, the versatile building block 5dA for N-glycan synthesis is readily available. For the activation of the sulfoxide leaving group, one equivalent of Tf2O and two equivalents of DTBMP are required, whereas for trichloroacetimidate activation catalytic amounts of TMSOTf are sufficient; hence, the trichloroacetimidate based procedure is very convenient. Various parameters were modified in the reaction of 4 with A (catalyst concentration, configuration of 4, size of the 2-O-protective group, solvent), thus, a proposal for the reaction course was derived.