Stereocontrolled 1,2-trans-arabinofuranosylation in the absence of 2-O-acyl group in glycosyl donor

Stereocontrolled 1,2-trans-α-arabinofuranosylation using polysilylated mono- and disaccharide glycosyl donors was investigated. A complete α-stereoselectivity of 1,2-trans-arabinofuranosylation was found for Ara-β-(1 → 2)-Ara disaccharide glycosyl donors containing five triisopropylsilyl (TIPS) groups with arylthiol (1) (as shown in our previous publications) or N-phenyltrifluoroacetimidoyl (2) (this work) leaving groups. Conversely, in case of monosaccharide thioglycosides polysilylated with acyclic silyl groups (TIPS, TBDPS), stereoselectivity of glycosylation was lower (α:β = 7-8:1), although the desired α-isomer still dominated. Disaccharide glycosyl donor 2 was successfully used in the synthesis of linear α-(1 → 5)-, β-(1 → 2)-linked hexaarabinofuranoside useful for further preparation of conjugates thereof as antigens valuable for the diagnosis of mycobacterioses.