The discovery of Kahne glycosylation forwards a big step in the carbohydrate chemistry. Despite an extensive series of studies, the precise mechanism of this powerful glycosylation is still not fully understood. To address these 30-year puzzles, the far side of Kahne glycosylation is explored in this study. After a series of control and tracking experiments, a number of important intermediates including glycosyl oxo-sulfonium ion and sulfenic anhydride (Crich's intermediate) are suggested to be responsible for the complex reaction pathway. It also reveals that in addition to the conventional ionic glycosylation pathway, a novel free radical pathway is very likely to contribute to the various side-and co-products. This study provides further understanding of Kahne glycosylation, and it also sheds light on how to improve the efficiency of sulfoxide-based glycosylation reactions.
Carbohydrates, along with proteins and nucleic acids are known as basic life substances, which not only are the energy source and structure material, but also play an extremely important role in many biochemical processes, such as molecules recognition, information transformation in cells, interactions in immune response, differentiation and apoptosis of cells, etc. Compared to proteins and nucleic acids, the synthesis of oligosaccharides in chemical or enzymatic ways is more difficult, due to their diversified and complicated structures. Recently photo especially visible light promoted organic synthesis has become one of the fastest growing fields in organic chemistry attributed to its environmental friendliness, easy availability and low cost. This chemistry has also been applied to the photo-mediated glycosylation reactions by using various light sources (ultraviolet, visible light), photosensitizers (or photocatalysts), and additives (oxidants, reductants etc.), which provides milder and more effective ways for oligosaccharide assembly. To help chemists understand this field, we briefly reviewed recent advances and potential applications of photo-mediated glycosylation reactions according to their types (e.g. light sources, photosensitizers). In this review, we also detailly described the mechanisms and highlighted the advantages and limitations of these reactions. In addition, the further prospects of this area are proposed.
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