MOLECULAR ORBITAL APPROACH
The approach of the dienophile (1) and diene (2) are shown below; notice the Highest Occupied Molecular Orbitals (HOMO) to the bottom right of the diene which are for the butadiene portion of the diene that undergoes the bond formation with the Lowest Unoccupied Molecular Orbitals (LUMO) of the dienophile above. The LUMO of the dienophile are at the front of the picture. The correct 'polarity' of the molecular orbital coefficient is shown by the colours red or blue and these must be of the same colour to interect. The left hand side of the diene represents orbital overlap with the oxygen atoms in the six-membered ring and the C-O-C bond linking it to the diene. At the front of the diene the three methyl groups bonded to the silicon atom are clearly seen. On the quinol-based dienophile, the two hydrogen atoms protrude towards the viewer in front of the C=C group and the carbonyl group sticks out to the left and right. In fact, because the D-glucose auxiliary presents a more complicated molecular orbital picture due to the presence of the acetate groups (which don't intefere with the diene), these were removed in this model for simplicity.
Legend: Carbon atom: grey ball; hydrogen atom: turquoise; oxygen: red and silicon purple.
The energy level diagram is shown below and the smallest energy difference is clearly between the HOMO (2) and LUMO (1) (8.98 eV1):
Compare this to Danishefsky's Diene (3):
The energy level diagram is shown below and the smallest energy difference is clearly between the LUMO (1) and HOMO (3) (3.51 eV).
1. The energies of the MO's were calculated from H�ckel surfaces using CambridgeSoft's Chem3D Pro software.