Complex assembly rules renders carbohydrates challenging target for chemical synthesis and multiple strategies have been designed to control regio- and stereoselectivity of the reaction. The most common approach to couple two carbohydrates either trans- or cis- selectively with respect to substituents at C1 and C2 employs protecting group participation. First, leading to formation of trans-glycosidic bond involves participating group at C2 carbon, whereas the second - cis mechanism involves remote group participation at C4 or C6. Yet, understanding the role of protecting groups in the glycosylation reaction remains limited because traditional condensed phased techniques, such as NMR, cannot resolve the short-lived and reactive glycosyl ion intermediate what in effect hinders rational design and non-empirical reaction optimizations.
Here, we characterize the intermediates of the glycosylation reaction, the glycosyl cation, by combination of gas-phase IR spectroscopy in helium droplets and first principle methods. The investigated structures, encoded in the mid-IR fingerprint region, include a series of 2-acetylated glycosides as well as galactose with acetyl participating group at C4, C6 or both C4 and C6. We show that the intermediate involved in the trans-glycosylation adopts dioxolenium type of structure with a covalent bond formed between anomeric carbon and acetyl oxygen. In effect, this 1,2-cis-fused bicyclic motif distorts the low-energy chair conformation of each glycosyl cation in unique fashion which impacts the reaction energy profile. The remote group participation at C4 as well as both C4 and C6 shows similar behavior, with a formation of the bicyclic dioxolenium type of ion. However, in case of C6 participation, we observe significant oxocarbenium ion population in which the anomeric carbon weakly interacts with other groups groups which explains reduced stereoselectivity of this protecting group.
The structural characterization of the ion provides fundamental understanding of the reaction required to further improve the reaction conditions and building blocks in the future. Future in-depth ab initio molecular dynamics investigations of the glycosylation reaction in solution are discussed.