Promoters thioglycosides

Despite these restrictions, the acceptor-bound succinoyl MPEG28 was found to permit glycosylations with glycosyl trichloroacetimidates promoted by boron trifluoride, triflic anhydride, and trimethylsilyl or triethylsilyl triflates, with glycosyl halogenates promoted by silver triflate, and with thioglycosides promoted by the iodonium ion. It is compatible with long-term ester protection, with allylic,... 

Related to the above cyanoethylidene derivatives, thioorthoesters 231 also behave similarly, and can be activated with catalytic amounts of TrC104 to glycosylate trityl ethers [506,507]. By using the thioglycoside promoters NIS-TfOH [508] or I2 [254], glycosyl acceptors with free hydroxyls were glycosylated. 

Takeuchi, K, Tamura, T, Mukaiyama, T, Stereoselective glycosylation of thioglycosides promoted by respective combinations of A-iodo- or A-bromosuccinimide and trityl tetrakisfpentafluorophe-nyl)borate. Application to one-pot sequential synthesis of trisaccharide. Tetrahedron Lett., 124-125, 2000. 

Fukase, K, Kinoshita, I, Kanoh, T, Nakai, Y, Hasuoka, A, Kusumoto, S, A novel method for stereoselective glycosidation with thioglycosides promotion by hypervalent iodine reagents prepared from PhIO and various acids. Tetrahedron, 52, 3897-3904, 1996. 

Uchiro, H, Mukaiyama, T, An efficient method for catalytic and stereoselective glycosylation with thioglycosides promoted by trityl tetrakis(pentafluorophenyl)borate and sodium periodate, Chem. 

In the Koenigs-Knorr method and in the Helferich or Zemplen modifications thereof, a glycosyl halide (bromide or chloride iodides can be produced in situ by the addition of tetraalkylammonium iodide) is allowed to react with a hydrox-ylic compound in the presence of a heavy-metal promoter such as silver oxide, carbonate, perchlorate, or mercuric bromide and/or oxide,19-21 or by silver triflu-oromethanesulfonate22 (AgOTf). Related to this is the use of glycosyl fluoride donors,23 which normally are prepared from thioglycosides.24... 

Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA) differs from the other promoters in that its cation is a radical, and as such produces radical cationic sulfonium ions as glycosylating species from thioglycosides.85 The use of this promoter arose from earlier work on the electrochemical generation of 5-glycosyl radical cations as glycosylating species. 

B. Mukhopadhyay, B. Collet, and R. A. Field, Glycosylation reactions with disarmed thioglycoside donors promoted by /V-iodosuccinimide and HC104-silica, Tetrahedron Lett., 46 (2005) 5923-5925. 

Recently, a number of thiophilic activators that can activate thioglycosides of low reactivity at low temperature have been described. For example, thiophilic promoter systems, such as diphenylsulfoxide [87,99], S-(4-methoxyphenyl)benzenethiosulfinate... 

Thioglycosides can also be activated by a one-electron transfer reaction from sulfur to the activating reagent tris-(4-bromophenyl)ammoniumyl hexachloroanti-monate (TBPA+) [102,103]. The use of this promoter was inspired by an earlier report where activation was achieved under electrochemical conditions to give an intermediate S-glycosyl radical cation intermediate [104], and the reactivity and mechanism have also been explored [105,106]. 

Several other examples have been reported in which thioglycosides were used as glycosyl acceptors. Thus, thioglycosides containing free hydroxyls can be coupled chemoselectively with glycosyl bromides and chlorides in the presence of silver triflate or tin(II) chloride-silver perchlorate as the promoter system [12,53,56,162,218-221]. 

General Procedure for Synthesis of Thioglycosides from Peracetylated Hexapyranosides Promoted by BF3-Etherate [5-10]... 

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