Leading Question

Provide an explanation for the observed regioselectivity in the reductive opening of the benzylidene group.

The second step of our transformation involves the reductive opening of a ring containing 2 oxygen atoms. The regioselectivity of the opening is determined by where the electrophilic BF3 reagent chooses to bond. In order for there to be a ring opening in the first place the oxygen that bonds with the BF3 must have the option of undergoing loan pair assisted ionization. There are only two rings in the molecule where this is the case. However one of these rings contains an electron withdrawing ester substituent. The other ring is bonded to the electron releasing benzene substituent. Therefore the oxygens in one of the rings are more nucleophilic than the others. Within this more nucleophilic ring there are two loan pair containing oxygens with which the BF3 can bond. When the BF3 bonds with either of these oxygens a positive charge will develop on that oxygen. On the oxygen closest to the LevO group this positive charge is more stabilized because of the greater degree of substitution of the adjacent carbon. The other oxygen is close to an already partially positive carbon and so there is no stabilization of the positive charge and possibly even a destabilization. For a visual explanation, see the attached drawing.