Boron-Catalyzed Site-Selective Reduction of Carbohydrate Derivatives with Catecholborane

Catalytic sp<sup>3</sup> C–O bond cleavage using B­(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>/HBcat is reported. This method is first demonstrated on simple ethers and silyl protected alcohols, which exhibit reactivity parallel to the known B­(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>/HSiR<sub>3</sub> system. In more complex carbohydrate derivatives, however, unique selectivities that are not possible with hydrosilane reductants have been achieved, including regioselective cyclizations. Preliminary computational studies suggest that diboryl oxonium ions are disfavored and that four-coordinate boronium ions may contribute to selectivity.