Conformationally Armed 3,6-Tethered Glycosyl Donors: Synthesis, Conformation, Reactivity, and Selectivity

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• 作者：Mads Heuckendorff ; Christian Marcus Pedersen ; Mikael Bols
• 刊名：The Journal of Organic Chemistry
• 出版年：2013
• 出版时间：July 19, 2013
• 年：2013
• 卷：78
• 期：14
• 页码：7234-7248
• 全文大小：551K
• 年卷期：v.78,no.14(July 19, 2013)
• ISSN：1520-6904

文摘

The reactivity and selectivity of 3,6-tethered glycosyl donors have been studied using acceptors with different steric and electronic characteristics. Eight (four anomeric pairs) 3,6-bridged-glycosyl donors were synthesized in high yields from their common parent sugars. The glycosylation properties were tested using at least three different acceptors and several promoter systems. Thiophenyl 2,4-di-O-benzyl-3,6-O-(di-tert-butylsilylene)-伪-d-glucopyranoside gave 伪/尾 mixtures with standard NIS/TfOH mediated activation, whereas the corresponding fluoride was found to be highly 尾-selective, when using SnCl₂/AgB(C₆F₅)₄ as the promoter system. Mannosyl donors were highly 伪-selective despite the altered conformation. Galactosylations using NIS/TfOH were generally 伪-selective, but more 尾-selective using the galactosyl fluoride and depending on the acceptor used. Thiophenyl 2-azido-2-deoxy-4-O-benzyl-3,6-O-(di-tert-butylsilylene)-伪-d-glucopyranoside was found to be 伪-selective . The reactivity of the donors was investigated using competition experiments, and some but not all were found to be highly reactive. Generally it was found that the 伪-thioglycosides were significantly more reactive than the 尾; this difference in reactivity was not found for 3,6-anhydro-, armed-(benzylated), or the classic super armed (silylated) donors. A mechanism supporting the unusual observations has been suggested.