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

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)-alpha-D-glucopyranoside gave alpha/beta mixtures with standard NIS/TfOH mediated activation, whereas the corresponding fluoride was found to be highly beta-selective, when using SnCl2/AgE(C6F5)(4) as the promoter system. Mannosyl donors were highly alpha-selective despite the altered conformation. Galactosylations using NIS/TfOH were generally alpha-selective, but more beta-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)-alpha-D-glucopyranoside was found to be alpha-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 alpha-thioglycosides were significantly more reactive than the beta; 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.