Glycosyl cations are universally accepted key ionic intermediates in the mechanism of glycosylation, the reaction that covalently links carbohydrates to other molecules. These ions have remained hypothetical species so far because of their extremely short life in organic media as a consequence of their very high reactivity. Here, we report the use of liquid hydrofluoric acid-antimony pentafluoride (HF/SbF5) superacid to generate and stabilize the glycosyl cations derived from peracetylated 2-deoxy and 2-bromoglucopyranose in a condensed phase. Their persistence in this superacid medium allows their three-dimensional structure to be studied by NMR, aided by complementary computations. Their deuteration further confirms the impact of the structure of the glycosyl cation on the stereochemical outcome of its trapping.
The broad application of well-defined
synthetic oligosaccharides
in glycobiology and glycobiotechnology is largely hampered by the
lack of sufficient amounts of synthetic carbohydrate specimens. Insufficient
knowledge of the glycosylation reaction mechanism thwarts the routine
assembly of these materials. Glycosyl cations are key reactive intermediates
in the glycosylation reaction, but their high reactivity and fleeting
nature have precluded the determination of clear structure–reactivity-stereoselectivity
principles for these species. We report a combined experimental and
computational method that connects the stereoselectivity of oxocarbenium
ions to the full ensemble of conformations these species can adopt,
mapped in conformational energy landscapes (CEL), in a quantitative
manner. The detailed description of stereoselective S
N
1-type
glycosylation reactions firmly establishes glycosyl cations as true
reaction intermediates and will enable the generation of new stereoselective
glycosylation methodology.
α-Fluoroenamides, potent rigid fluorinated bioisosters of ureas, have been synthesized by a highly regio- and stereo-selective hydrofluorination of ynamides in anhydrous HF. This reaction provides the first general entry to α-fluoroenamides and can easily be applied to a wide range of substrates.
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