Journal
SCIENCE CHINA-CHEMISTRY
Volume 54, Issue 1, Pages 66-73Publisher
SCIENCE PRESS
DOI: 10.1007/s11426-010-4186-6
Keywords
stereoselectivity; pre-activation based glycosylation; additives; solvent effects
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Funding
- National Institutes of Health [R01-GM-72667]
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Stereochemical control is an important issue in carbohydrate synthesis. Glycosyl donors with participating acyl protective groups on 2-O have been shown to give 1,2-trans glycosides reliably under the pre-activation based reaction condition. In this work, the effects of additives and reaction solvents on stereoselectivity were examined using donors without participating protective groups on 2-O. While several triflate salt additives did not have major effects, the amount of AgOTf was found to significantly impact the reaction outcome. Excess AgOTf led to lower stereochemical control presumably due to its coordination with the glycosyl triflate intermediate and a more S(N)1 like reaction pathway. In contrast, the stereoselectivity could be directed by reaction solvents, with diethyl ether favoring the formation of alpha glycosides and dichloromethane leading to beta isomers. The trend of stereochemical dependence on reaction solvent was applicable to a variety of building blocks including the selective formation of beta-mannosides.
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