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

We
report highly β-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 β-mannosylated
and β-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 β-selectivities.