FTotal synthesis of the trisaccharide repeating unit of biologically active heteroglycan isolated from a hybrid mushroom of Pleurotus florida and Lentinula edodes

Singh, Tushar (2025) FTotal synthesis of the trisaccharide repeating unit of biologically active heteroglycan isolated from a hybrid mushroom of Pleurotus florida and Lentinula edodes. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Mushroom-derived polysaccharides have garnered significant attention in recent years due to their diverse biological activities, including immunomodulatory, antioxidant, and anticancer properties. Among them, a novel water-soluble heteroglycan (PS-II) isolated from a somatic hybrid of Pleurotus florida and Lentinula edodes has shown promising bioactivity by stimulating key immune cells such as macrophages, thymocytes, and splenocytes. Structural studies revealed that PS-II consists of a repeating trisaccharide unit composed of D-galactose, D-glucose, and D-mannose in a 1:1:1 molar ratio. Motivated by its biological relevance and the challenges in accessing such complex oligosaccharides from natural sources, this thesis focuses on the total chemical synthesis of the trisaccharide repeating unit of PS-II in the form of its 2-aminoethyl glycoside. A retrosynthetic strategy was employed to deconstruct the trisaccharide into three key monosaccharide building blocks: a galactosyl donor, a glucosyl acceptor, and a mannosyl donor. Each unit was synthesized via multi-step protocols involving selective protection-deprotection of hydroxyl groups, the use of thioglycoside and trichloroacetimidate chemistry, and stereoselective glycosylation reactions. Special attention was given to controlling the anomeric configurations using neighboring group participation, the anomeric effect, and optimized reaction conditions. The synthesis of the disaccharide intermediate was achieved by coupling the galactosyl donor with the glucosyl acceptor, followed by detailed NMR (¹H, ¹³C, COSY, HSQC) and HRMS characterization. Parallel efforts were undertaken to construct the fully protected mannose derivative, which will serve as the final glycosyl donor in subsequent coupling steps. This work demonstrates the feasibility of assembling structurally complex and biologically significant oligosaccharides using modern synthetic carbohydrate chemistry. The synthetic access to this trisaccharide unit opens the door to further immunological studies and potential therapeutic applications of PS-II and its analogues. Moreover, this research contributes to the broader field of glycochemistry by highlighting challenges and strategies in stereoselective glycosylation, as well as the role of protecting group design in achieving regio- and stereocontrol.

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