Stereoselective organocatalyzed glycosylations - thiouracil, thioureas and monothiophthalimide act as Bronsted acid catalysts at low loadings

Thiouracil catalyzes stereoselective glycosylations with galactals in loadings as low as 0.1 mol%. It is proposed that in these glycosylations thiouracil, monothiophthalimide, and the previously reported catalyst, Schreiner's thiourea, do not operate via a double H-bonding mechanism but rather by BrOnsted acid/base catalysis. In addition to the synthesis of 2-deoxyglycosides and glycoconjugates, we report the first organocatalytic synthesis of 1,1-linked trehalose-type sugars.