Synthesis and Structural Revision of a Natural Tetrasaccharide from Starfish Asterias rollestoni Bell

Abstract

AbstractStarfish provide important saponins with diverse bioactivities as the secondary metabolites, among which 2‐O‐glycosylated glycosides are commonly found. Preparation of those 1,2‐trans 2‐O‐glycosylated glycosides usually relies on 2‐O‐acyl participation requiring the selective installation and cleavage of 2‐O‐acyl groups. A convergent synthesis using 2‐O‐glycosylated oligosaccharide donors would be more straightforward but also pose greater challenges. Herein, we report a convergent synthesis of a distinctive tetrasaccharide isolated from starfish Asterias rollestoni Bell. Dual 2‐(diphenylphosphinoyl)acetyl (DPPA) groups at O3 and O4 on galactose moiety led to high β‐selectivities (β/α=12/1 or β only) in the challenging [2+2] glycosylation, giving the desired tetrasaccharides in >90 % yields from the 2‐O‐glycosylated disaccharide donors. These synthetic studies have also unambiguously revised the structure of these natural tetrasaccharides. This work would facilitate further studies on new inhibitors of α‐glucosidase as hypoglycemic drugs.