Single-Step Glycosylations with C-Labelled Sulfoxide Donors: A Low-Temperature NMR Cartography of the Distinguishing Mechanistic Intermediates.

Chemistry

Glycochemistry and Molecular Recognition group, Dpt. Bio-Organic Chemistry, Instituto de Química Orgánica General (IQOG-CSIC), Juan de la Cierva 3., 28006, Madrid, Spain.

Published: January 2021

Glycosyl sulfoxides have gained recognition in the total synthesis of complex oligosaccharides and as model substrates for dissecting the mechanisms involved. Reactions of these donors are usually performed under pre-activation conditions, but an experimentally more convenient single-step protocol has also been reported, whereby activation is performed in the presence of the acceptor alcohol; yet, the nature and prevalence of the reaction intermediates formed in this more complex scenario have comparatively received minimal attention. Herein, a systematic NMR-based study employing both C-labelled and unlabelled glycosyl sulfoxide donors for the detection and monitoring of marginally populated intermediates is reported. The results conclusively show that glycosyl triflates play a key role in these glycosylations despite the presence of the acceptor alcohol. Importantly, the formation of covalent donor/acceptor sulfonium adducts was identified as the main competing reaction, and thus a non-productive consumption of the acceptor that could limit the reaction yield was revealed.

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http://dx.doi.org/10.1002/chem.202003850DOI Listing

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