It is known that hydroxyl groups in equatorial positions are more reactive than axially oriented ones. In our synthesis of lupane-type saponins bearing OSW-1 disaccharide unit we observed that glycosylation had preferentially taken place at 4-OH group of arabinopyranoside ring. Such reversed reactivity of some L-arabinopyranoside derivatives was reported and explained by an influence of bulky substituent at the O-2 position and/or inversion of the pyranoside ring which adopts unusual 1C4 conformation .
Here we propose that the formation of the hydrogen bond between anomeric oxygen atom and the 3-OH group is responsible for observed ring inversion. The presence of hydrogen bond was confirmed by the NMR studies. In this communication we will present the influence of hydrogen bond on the regioselectivity of glycosylation reaction. Plausible mechanism explaining such unusual regioselectivity will also be presented .
The support from the National Science Centre (NCN), Poland (grant No: 2016/21/B/ST5/02141) is acknowledged.
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