Stereospecific Furanosylations Catalyzed by Bis-thiourea Hydrogen-Bond Donors

We report a new method for stereoselective O-furanosylation
reactions promoted by a precisely tailored bis-thiourea hydrogen-bond-donor
catalyst. Furanosyl donors outfitted with an anomeric dialkylphosphate
leaving group undergo substitution with high anomeric selectivity,
providing access to the challenging 1,2-cis substitution
pattern with a range of alcohol acceptors. A variety of stereochemically
distinct, benzyl-protected glycosyl donors were engaged successfully
as substrates. Mechanistic studies support a stereospecific mechanism
in which rate-determining substitution occurs from a catalyst–donor
resting-state complex.