- Highly diastereoselective α-mannopyranosylation in the absence of participating protecting groups
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S-Phenyl 2,6-di-O-benzyl-3,4-O-(2',3'-dimethoxybutane-2',3'-diyl)- 1- thia-α-D-mannopyranoside and its sulfoxide, following activation at -78 °C with benzenesulfenyl triflate or trifiic anhydride, respectively, provide the corresponding α-mannosyl triflate as demonstrated by NMR spectroscopy. On addition of an acceptor alcohol α-mannosides are then formed. Similarly, S- phenyl 2,3-O-carbonyl-4,6-O-benzylidene-1-thia-α-D-mannopyranoside and ethyl 3-O-benzoyl-4,6-O-benzylidene-2-O-(tert-butyldimethylsilyl)-1-thia-α-D- mannopyranoside both provide α-mannosides on activation with benzenesulfenyl triflate followed by addition of an alcohol. These results stand in direct contrast to the highly β-selective couplings of comparable glycosylations with 2,3-di-O-benzyl-4,6-O-benzylidene protected mannosyl donors and draw attention to the subtle interplay of reactivity and structure in carbohydrate chemistry.
- Crich, David,Cai, Weiling,Dai, Zongmin
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p. 1291 - 1297
(2007/10/03)
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- Are glycosyl triflates intermediates in the sulfoxide glycosylation method? A chemical and 1H, 13C, and 19F NMR spectroscopic investigation
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The title question is addressed by low-temperature 1H, 13C, and 19F NMR spectroscopies in CD2CL2 as well as by the preparation of authentic samples from glycopyranosyl bromides and AgOTf. At -78°C glycosyl triflates are cleanly generated with either nonparticipating or particpating protecting groups at O-2. The glycosyl triflates identified in this manner were allowed to react with methanol, resulting in the formation of methyl glycosides. Glycosyl triflates were generated at -78°C in CD2Cl2 and allowed to warm gradually until decomposition was detected by 1H and 19F NMR spectroscopy. The decomposition temperature and products are functions of the protecting groups employed.
- Crich, David,Sun, Sanxing
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p. 11217 - 11223
(2007/10/03)
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- Synthesis and Reactions of Sulfenic Trifluoromethanesulfonic Anhydrides
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Sulfenyl halogenides 1 and 4 react with silver trifluoromethanesulfonate (2) to give aryl 3 and alkylsulfenic trifluoromethanesulfonic anhydrides 5, resp., in good yields; 3 and 5 could not be isolated because of their instability. 1H NMR spectroscopic data of 5a, b, each in dichloromethane and nitromethane, resp., are indicative of a dissoziation to adducts of alkylsulfenylium ions and the solvent.Whereas 3 do not react with aromatic compounds, addition products with diphenylacetylene, arylsulfenylvinyl trifluoromethanesulfonates 11, are formed smoothly, which under the reaction conditions immediately cyclize to give benzothiophenes 10 in excellent yields.
- Effenberger, Franz,Russ, Werner
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p. 3719 - 3736
(2007/10/02)
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