Direct synthesis of β-mannosides. Synthesis of β-D-xyl-(1→2)- β-D-man-(1→4)- α-D-Glc-OMe: A trisaccharide component of the Hyriopsis schlegelii glycosphingolipid. Formation of an orthoester from a xylopyranosyl sulfoxide

Abstract

A concise synthesis of the title trisaccharide is described in which the key β-mannosylation reaction is achieved directly, with high yield and selectivity, by triflic anhydride mediated coupling of S-ethyl2- O-allyl-3- O-benzyl-4,6- O-benzylidene-1-deoxy-1-thia-α-D-mannopyranoside S-( R)-oxide with methyl 2,3,6-tri- O-benzyl-α-D-glucopyranoside. After deallylation of the so-formed disaccharide, β-xylopyranosylation is brought about by reaction with α-tri- O-benzoyl xylopyranosyl bromide activated with silver triflate. Attempted xylosylation of cyclohexanol with S-phenyl2,3,4-tri- O-benzoyl-1-deoxy-1-thio-β-D-xylopyranoside S-oxide, activated by triflic anhydride, and with the corresponding thioglycoside in the presence of benzenesulfenyl triflate both led to the formation of an orthoester rather than the anticipated β-xyloside. Treatment of this orthoester with tin tetrachloride promoted its rearrangement to the β-xyloside. The conformation and configurational assignment of tri- O-benzoyl-β-xylopyranosides is discussed.