β-l-Rhamnosylation and β-d-Mannosylation Mediated by 4- O -Ester Groups in a Weakly Nucleophilic Environment.

eq -4- O -Acyl group directed β-rhamnosylation and β-mannosylation are achieved in a carborane or BARF anion formed weakly nucleophilic environment with the assistance of a 2,3-orthocarbonate group. The 4- O -acyl group plays a critical role in directing the β-selectivity, and the weakly coordinating anion is essential to amplify this direction. The orthocarbonate group could be readily removed with 1,3-propanediol in the presence of BF 3 ·Et 2 O.