1040752-03-1Relevant academic research and scientific papers
Conformationally armed 3,6-tethered glycosyl donors: Synthesis, conformation, reactivity, and selectivity
Heuckendorff, Mads,Pedersen, Christian Marcus,Bols, Mikael
, p. 7234 - 7248 (2013/08/23)
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)-α-d- glucopyranoside gave α/β mixtures with standard NIS/TfOH mediated activation, whereas the corresponding fluoride was found to be highly β-selective, when using SnCl2/AgB(C6F 5)4 as the promoter system. Mannosyl donors were highly α-selective despite the altered conformation. Galactosylations using NIS/TfOH were generally α-selective, but more β-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)-α-d- glucopyranoside was found to be α-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 α-thioglycosides were significantly more reactive than the β; 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.
