Synthesis of the tetrasaccharide repeating unit of the O-specific polysaccharide of Azospirillum doebereinerae type strain GSF71T using linear and one-pot iterative glycosylations

Abstract

A straightforward synthetic strategy was developed for the synthesis of the tetrasaccharide repeating unit corresponding to the O-specific polysaccharide of Azospirillum doebereinerae type strain GSF71^T in a very good yield adopting sequential glycosylation followed by removal of the p-methoxybenzyl (PMB) group in the same pot. Further, the synthetic strategy was modified by carrying out three stereoselective iterative glycosylations followed by in situ removal of the PMB group in one pot. The stereochemical outcome of the newly formed glycosidic linkages was excellent using thioglycoside derivatives as glycosyl donors and a combination of N-iodosuccinimide (NIS) and perchloric acid supported on silica (HClO₄-SiO₂) as the glycosyl activator.

Keywords: Azospirillum doebereinerae; HClO4-SiO2; O-polysaccharide; glycosylation; tetrasaccharide.

Figure 1

Structure of the synthesized tetrasaccharide and its synthetic intermediates.

Scheme 1

Stepwise stereoselective synthesis of tetrasaccharide 1. Reagents and conditions: (a) NIS, HClO₄-SiO₂, CH₂Cl₂, MS 4 Å, −15 °C, 30 min, then 20 °C, 30 min, 73% for compound 5, 70% for compound 6, and 74% for compound 7; (b) CH₃COSH, pyridine, rt, 16 h; (c) HClO₄-SiO₂, CH₃CN, rt, 15 min; (d) acetic anhydride, pyridine, CH₂Cl₂, 0 °C, 5 h; (e) Et₃SiH, 20% Pd(OH)₂/C, CH₃OH, rt, 24 h, 50% overall yield for three steps.

Scheme 2

Iterative stereoselective three-step one-pot glycosylation. Reaction conditions: (a) NIS, HClO₄-SiO₂, CH₂Cl₂, MS 4 Å, –15 °C, 30 min, then 20 °C, 30 min.