Stereoseletive glycosylation is of primary importance for glycochemistry and glycobiology. Many factors can influence the stereo outcome of a glycosylation event like the leaving group at the glycosyl donor, the activating system, the reaction conditions, the nature of the protecting group/template, etc. Many innovative methods have been recently developed. However, stereoselective synthesis of 1,2-cis glycosidic linkages is still challenging. Light has been employed to induce the glycosylation reaction. However, to the best of our knowledge, light-controlled tool to influence the glycosylation efficiency and stereoselectivity has never been developed.
Photochromic molecules are bistable molecules featuring different structural and/or electronic properties which can be reversibly isomerized by light. They offer numerous opportunities for reversibly photomodulating chemical, biological or pharmacological activities or properties. There is increasing use of the photoisomerization to control the conformation as well as the activities of biomolecules. We envisioned using photochromic molecule as linker or template to study the influence of the photoisomerization of photochromic linker on the stereoselectivity of the glycosylation, through intramolecular aglycone delivery (IAD) strategy (Figure). As photochromic molecules, azobenzenes are simple and readily accessible. Besides their remarkable photostability, azobenzenes undergo reversible E to Z photoisomerization upon UV or visible light illumination, resulting in important changes in molecular size, conformation, and dipole moment between the thermodynamically favored planar E isomer and the energetically higher, bent Z isomer. Since protecting group induced conformational change of glycosyl donors can influence dramatically the reactivity and the stereoselectivity of the glycosylation, it’s expected that photoisomerization of azobenzene linker will change the disposition between the glycosyl donor and acceptor and consequently influence the glycosylation reaction. We will present the intramolecular glycosylation results with E- and Z-azobenzene as linker, as well as the photochromic properties of generated photoswitchable glycomacrocycles.
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