A substrate-unspecified glycosylation reaction promoted by copper(II) trifluoromethanesulfonate in benzotrifluoride
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Introduction
Glycosylation reactions that have been reported so far in the literature have been achieved by the appropriate combination of a leaving group on the anomeric carbon and suitable activating reagent(s). However, reagents that activate a variety of glycosyl donors are limited. Trimethylsilyl trifluoromethanesulfonate (triflate, hereafter) and boron trifluoride diethyl etherate have been reported as reagents that activate several kinds of glycosyl donors.1., 1.(a), 1.(b), 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15., 16., 17., 18., 19., 20. Recently, Mukaiyama and co-workers reported that TrB(C6F5)4 activates thioglycosides, glycosyl fluorides, glycosyl phenylcarbonates, and lactols.21 For a one-pot strategy of oligosaccharide synthesis, the search for reagents that activate several kinds of glycosyl donors is also important. Here we report a copper(II) triflate-mediated glycosylation reaction.22 Using benzotrifluoride (BTF) as the reaction solvent,23., 23.(a), 23.(b) copper(II) triflate was shown to activate five kinds of glycosyl donors, 2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl chloride (1a), 2,3,4,6-tetra-O-benzyl-α,β-d-glucopyranosyl fluoride (1b), 2,3,4,6-tetra-O-benzyl-α,β-d-glucopyranosyl trichloroacetimidate (1c), 1-O-acetyl-2,3,4,6-tetra-O-benzyl-α,β-d-glucopyranose (1d), and 2,3,4,6-tetra-O-benzyl-α,β-d-glucopyranose (1e) (Scheme 1).
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Results and discussion
Glycosylation reactions of 1a–1e are summarized in Table 1. Cyclohexylmethanol was used as a glycosyl acceptor. A stoichiometric amount of copper(II) triflate was used as a common activating reagent of the glycosyl donors, 1a–1e. Diethyl ether, acetonitrile, 1,2-dichloroethane, and BTF were employed as the reaction solvents. BTF is a solvent with a relatively low toxicity that was introduced as an alternative of dichloromethane by Ogawa and Curran.23., 23.(a), 23.(b) With the glucosyl chloride
General procedure for the glycosylations
The reactions were run under a positive pressure of argon. Acetonitrile and 1,2-dichloroethane used for the solvent of the reactions were distilled from calcium hydride. Diethyl ether was distilled from sodium benzophenone ketyl before use. Benzotrifluoride was distilled from phosphorus pentoxide.
Copper(II) triflate was dried before use by heating to 180–200 °C with a heat gun under reduced pressure (ca. 1 mmHg). A benzene solution of each glycosyl donor (1.0 equiv) and an acceptor alcohol (1.1
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scientific Research (Grant no. 13874078) from the Japan Society for the Promotion of Science, The Naito Foundation, and by a Sunbor Grant from Suntory Institute for Bioorganic Research.
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