Oxonium ions, stereoselective glycosylations with

Under optimized experimental conditions (CH2C12, MeOH (10 equivalents), HFTP (5 equivalents)) the reaction was complete at room temperature in less than 5h. Moreover, the reaction was still chemoselective (less than 5% of the elimination product 18), and completely stereoselective (the a stereoisomer was not detected) (see Scheme 6.9) [47], The nucleophile is delivered on the same P-face as the leaving group. Similar results from DHA glycosyl donor have been rationalized by the formation of the half-chair oxonium ion, and a stereoelectronically preferred axial addition with reactive nucleophiles [18c], Compared to the reaction of 17 with the stronger Ag+ electrophilic assistance (see Schemes 6.7 and 6.8), the low ratio of elimination product 16 and the high p-diastereoselectivity suggest a mechanism different from an addition on the oxonium ion 14. It is also clear... 

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