Synthesis of the Branched Tetrasaccharide Repeating Unit of Lactobacillus fermentum TDS030603 and Its Regioisomer

The branched tetrasaccharide repeating unit beta-Glc-(1 -> 3)-[alpha-Glc-(1 -> 6)-alpha-Gal-(1 -> 2)]-Glc of Lactobacillus fermentum TDS030603 was synthesized. In addition, its regioisomer beta-Glc(1 -> 2)-[alpha-Glc-(1 -> 6)-alpha-Gal-(1 -> 3)]-Glc was synthesized for a future assay study. In these syntheses, we examined di-n-butyl phosphate (DBP) and phenylthio (SPh) groups as leaving groups for glycosidation and examined p-methoxyphenyl (MP) and 2-(trimethylsilyl) ethyl (TMSEt) groups as acceptor-reducing end anomeric protecting groups. In the selective synthesis of the target tetrasaccharide repeating unit, the combination of MP acceptors and DBP donors was effective, achieving regio- and stereo-selective glycosidation. However, purification after removal of the MP group in the final step proved difficult. Nevertheless, we achieved simultaneous synthesis of both the branched tetrasaccharide repeating unit and its regioisomer in high yield using a combination of TMSEt acceptors and SPh donors.