Highly Selective β-Mannosylations and β-Rhamnosylations Catalyzed by Bis-thiourea

We report highly beta-selective bis-thioureas-catalyzed 1,2-cis-O-pyranosylations employing easily accessible acetonide-protected donors. A wide variety of alcohol nucleophiles, including complex natural products, glycosides, and amino acids were beta-mannosylated and beta-rhamnosylated successfully using an operationally simple protocol under mild and neutral conditions. Less nucleophilic acceptors such as phenols were also glycosylated efficiently in excellent yields and with high beta-selectivities.