Stereoselective Total Synthesis of α‐Galacto‐Oligosaccharides Using Boron‐Mediated Aglycon Delivery and Evaluation of their Immune‐Enhancing Activities

α-Galacto-oligosaccharides (α-GOSs) are natural oligomeric saccharides found in plants that have garnered significant interest due to their indigestible nature and various biological activities. However, they often exist as inseparable mixtures, particularly long-chain substances. This study successfully achieves the total synthesis of α-GOSs 1-6, featuring different lengths of galactose chains, using the boron-mediated aglycon delivery method developed in our laboratory. Additionally, this study investigates the immune-enhancing activity and structure-activity relationships of the chemically synthesized α-GOSs. It reveals that the length of the galactose chain in the α-GOSs significantly influences their immune-enhancing properties. Specifically, α-GOS 2, which contains two galactose units, is found to markedly promote macrophage phagocytosis and increase the production of cytokines such as IL-6, TNF-α, and IL-1β. These results suggest that α-GOS 2 could serve as a natural immunomodulatory agent in functional foods.