Abstract
The stereoselective formation of 1,2‐cis‐glycosidic bonds is challenging. However, 1,2‐cis‐selectivity can be induced by remote participation of C4 or C6 ester groups. Reactions involving remote participation are believed to proceed via a key ionic intermediate, the glycosyl cation. Although mechanistic pathways were postulated many years ago, the structure of the reaction intermediates remained elusive owing to their short‐lived nature. Herein, we unravel the structure of glycosyl cations involved in remote participation reactions via cryogenic vibrational spectroscopy and first principles theory. Acetyl groups at C4 ensure α‐selective galactosylations by forming a covalent bond to the anomeric carbon in dioxolenium‐type ions. Unexpectedly, also benzyl ether protecting groups can engage in remote participation and promote the stereoselective formation of 1,2‐cis‐glycosidic bonds.
Highlights
Mateusz Marianski,* Eike Mucha, Kim Greis, Sooyeon Moon, Alonso Pardo, Carla Kirschbaum, Daniel A
We unravel the structure of glycosyl cations involved in remote participation reactions via cryogenic vibrational spectroscopy and first principles theory
Acetyl groups at C4 ensure a-selective galactosylations by forming a covalent bond to the anomeric carbon in dioxolenium-type ions
Summary
Mateusz Marianski,* Eike Mucha, Kim Greis, Sooyeon Moon, Alonso Pardo, Carla Kirschbaum, Daniel A. In the superacid medium, the protecting groups are protonated, such that results cannot be directly translated to classical reaction conditions.[10] In another approach, the isolation of side products provided strong, but indirect evidence for remote participation by acetyl esters at the C4-position of glycosyl donors.[8c,d] Alternatively, mass spectrometry-based techniques can be used to isolate and characterize the glycosylation reaction intermediates in the gas phase.[3a,b,11,12] Previously, we applied a combination of cryogenic ion spectroscopy and first-principles theory to determine the exact structures of glycosyl cations of custom-tailored model glycans carrying
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