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Aromatic stacking in Glycochemistry: can glycosidations be tamed?

Periodic Reporting for period 1 - SWEET-PI (Aromatic stacking in Glycochemistry: can glycosidations be tamed?)

Reporting period: 2020-01-01 to 2021-12-31

The biological relevance of the human glycome, together with the increasing importance of carbohydrates in medicinal chemistry, provides significant opportunities and challenges for chemists in the field of synthetic oligosaccharide and glycoconjugate research. Regarding oligosaccharide synthesis, the formation of glycosidic bonds usually represents the key step. In this context, supra-molecular interactions can offer significant opportunities to modulate the properties of glycosidic donors and acceptors, and in fact, this bio-mimetic strategy presents the potential to ultimately dictate the yield and stereochemical course of the reaction. In particular, carbohydrate/aromatic stacking represents a frequent structural motif for the molecular recognition of glycosides, either by protein binding domains, enzymes, or synthetic receptors. Interestingly, it has also been proposed that aromatic residues can assist in the formation/cleavage of glycosidic bonds by stabilizing positively charged intermediates through cation/π interactions. Despite the potential benefits derived from understanding aromatic catalysis in glycosylations, this had not been explored yet. In this project, we tackled the first experimental study on this relevant topic, based on the design, synthesis, and reactivity evaluation of a large number of carbohydrate systems, some of them equipped with varying aromatic platforms. Different geometries and dynamic features, anomeric leaving groups, sugar configurations, and reaction conditions have been explicitly considered. The use of variable temperature NMR techniques has been of crucial importance not only for the monitorization of reaction progress, but also for the identification of highly unstable reaction intermediates.
Recently, we determined the structural bases required for CH/π catalysis to happen under normal lab glycosylation conditions. Two families of glycosides were synthesized, one exhibiting a modified benzylidene group equipped with an aromatic appendage, and the other composed of a series of more rigid disaccharides skeletons. Interestingly, the monitorization by NMR of these species during glycosylations revealed a diverging reactivity trend: while the first family of compounds did experience a noticeable rate enhancement of in the presence of the aromatic ring, the second family of compounds underwent a significant rate deceleration accompanied by an improvement in the stereoselectivity of the process. These experimental results underline the opposing influence exerted by van der Waals and Coulombic forces, and how they can be affected by dynamic aromatic/glycosyl cation contacts; the conclusions of our study suggest there is an open window for aromatic assistance to glycosylation processes, especially for glycosyl donors participating in dissociative mechanisms, with far reaching implications for enzyme engineering and organocatalysis.
During the course of this study, the monitorization of activated reaction mixtures devoid of glycosyl acceptor allowed the identification of several intermediates, where the alpha-glycosyl triflate was the predominant species. Unfortunately, no signals could be directly ascribed to an oxocarbenium-like species, despite the sugar donor laid on a stabilizing aromatic platform, overall indicating that no such intermediates were accumulating and instead, if any, they were very minor or had a transition-state character. However, these findings sparked a more fundamental study on glycosylation mechanisms. With this aim in mind, we set out to gauge the scope of our methodology in isolating activated reaction species acting as oxocarbenium reservoirs. In particular, we focused on α-selective glycosylations involving poor acceptors, for which an SN1 has been traditionally invoked. A similar methodology was applied, relying on the synthesis of 13C-labeled sugar donors and their analysis by low temperature NMR. For glucose, these experiments revealed the presence of an apparently exclusive α-triflate species (>99%), in agreement with previous reports. In fact, despite the extra sensitivity provided by the isotopically labeled samples, significant accumulation times were required before an anomeric cross-peak could be confidently assigned to a β-triflate intermediate. Further experiments conducted on a more favorable allose model showed a much higher ratio of β-triflate, which allowed the first-ever structural and kinetic characterization of such intermediate. To our delight, additional 13C kinetic isotopic effects and in silico calculations pointed toward an oxocarbenium cationic species, in the form of a contact ion pair, as the key intermediate in the production of the major glycoside. Thus, the detected anomeric β-triflate acts as a high-energy reservoir of even more unstable species. Ultimately, the obtained results showed that the analyzed glycosylations do not satisfy the Curtin−Hammett boundary requirements for which triflate anomerization should be much faster than the alcohol substitution. This conclusion is illustrated by the preferential consumption of the β-allosyl triflate over its α-counterpart.
This work is described in: Andrés G. Santana, Laura Montalvillo-Jiménez, Laura Díaz-Casado, Francisco Corzana, Pedro Merino, Francisco J. Cañada, Gonzalo Jiménez-Osés, Jesús Jiménez-Barbero, Ana M. Gómez, Juan Luis Asensio: Dissecting the Essential Role of Anomeric β‑Triflates in Glycosylation Reactions. J. Am. Chem. Soc. 2020, 142, 12501-12514.
Although the COVID-19 pandemic negatively affected the planned activities of this proposal, especially those concerning public outreach and dissemination of the results, we were still able to successfully carry out this research project, for which we have been able to achieve most of the objectives. These mechanistic studies provide a deeper understanding on the dynamics involved in glycosylations, which in turn are necessary to rationally control the reaction outcome. Additionally, we have presented a series of supramolecular tools that allow tuning of the reaction rate and/or stereoselectivity.
The training received during the implementation of Sweet-Pi project has positively impacted my career prospects by increasing my skills in fundamental scientific areas, such as technical and scientific training in state-of-the art techniques, lab management, dissemination of results in high impact journals, as well as in specialized scientific meetings. Of note, the MSCA fellowship has qualified me to officially appear as a PhD co-supervisor, a valuable new merit in my CV.