Traceless Leaving-Group Directing Group for SN2 Glycosylation
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Solution Overview
Problem
The synthesis of oligosaccharides with defined configurations and connectivities is hindered by the challenge of forming 1,2-cis glycosidic bonds, which is not efficiently addressed by existing glycosylation strategies due to limitations in controlling anomeric configuration and the need for specific protecting groups, limiting their generality and synthetic flexibility.
Innovation Solution
A traceless directing group strategy is employed in the SN2 glycosylation process, where the directing group is attached to the anomeric leaving group, allowing for stereoselective formation of 1,2-cis glycosidic bonds applicable to all sugar types, independent of specific glycosyl donors, and enabling high yields and selectivity in forming 1,2-cis and trans glycosidic linkages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If neighboring group assistance or remote group assistance strategies are used to control anomeric configuration, then stereoselectivity in forming 1,2-trans glycosidic bonds is improved, but the need for specific O-protecting groups limits generality and increases device complexity
Solution Approach 1:
The patent extracts the directing group from the sugar ring and places it on the leaving group instead. This separates the directing function from the sugar core, allowing the same leaving group to work with different sugar types without requiring sugar-specific protecting groups. The directing group is taken out of the traditional position and relocated to the leaving group, enabling universal application across all sugar types.
Solution Approach 2:
The leaving group is designed to serve multiple functions simultaneously: it acts as the anomeric leaving group, carries the directing group for stereoselectivity control, and can be attached to any sugar type. This multi-functional design eliminates the need for different protecting groups for different sugars, achieving universality across all glycosyl donors.
2Manufacturing precision
If specific O-protecting groups are installed to enable controlling facial selectivity, then stereoselectivity is improved, but the synthesis complexity increases due to additional protecting group installation and manipulation steps
Solution Approach 1:
The patent extracts the facial selectivity control function from the protecting groups and relocates it to the leaving group. The directing group on the leaving group provides the necessary steric control for facial selectivity without requiring special protecting groups on the sugar. This extraction eliminates the complexity of installing and manipulating multiple protecting groups.
Solution Approach 2:
The leaving group acts as an intermediary that carries the directing group and mediates the glycosylation reaction. Instead of using protecting groups on the sugar to control selectivity, the directing group on the leaving group serves as the intermediary that provides steric control during the reaction, simplifying the overall synthesis.
3Adaptability or versatility
If traceless directing group strategy is used in SN2 glycosylation, then generality across all sugar types is improved, but the reaction mechanism becomes more complex requiring SN2 pathway
Solution Approach 1:
Instead of using the conventional SN1 mechanism with oxocarbenium intermediates, the patent inverts the approach to use an SN2 mechanism. The directing group on the leaving group directs the nucleophile to attack from the backend, inverting the traditional attack direction and mechanism. This inversion enables universality across all sugar types while providing a clear, controlled reaction pathway.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach achieves high levels of stereoselectivity and yield in forming 1,2-cis glycosides, including challenging 1,2-cis-glucosides, galactosides, and xylopyranosides, and allows for the synthesis of oligosaccharides with branched or linear chains, enhancing the flexibility and efficiency of carbohydrate synthesis.
Implementation Method 1
the basic group on the leaving group forming a hydrogen bond with the acceptor hydroxyl group
Implementation Method 2
activating the leaving group by an electrophile in a presence of an HO group-bearing acceptor so as to form an activated leaving group
Data Source
AI summary
Broadly applicable and stereoselective formation of glycosidic linkage remains challenging yet of critical importance in giycoscience. By developing an SN2 glycosylation, this work advances a general solution to this challenge via stereoinversion at the anomeric position of glycosyl ester donors. This SN2 process is enabled by a basic directing-group in the leaving-group, which is activated by a cationic gold catalyst or any other electrophilic reagent. Unlike all the reported directing group approaches, this strategy is applicable to any glycosyl donors—a long sought-after yet unmet goal in carbohydrate chemistry; moreover, the basic directing-group upon glycosylation is lost as part of the leaving-group and hence traceless in the glycoside products, therefore avoiding potential complications in downstream transformations. Highly selective construction of glycosidic bonds including challenging 1,2-cis glycosidic bonds is achieved in excellent yields. The strategy is applied iteratively to access oligosaccharides and can distinguish alcohols with different steric hindrance.