Cycloaddition Reactions.

Background:

The key step in the synthesis of Xestoquinone is an intramolecular Diels-Alder addition (see full question here). A number of possible isomers can form. The easiest way to predict which is actually formed is to contruct quantitative modelular models, shown below with the calculated energies. These models are constructed using the semi-empirial PM3 method, although mechanics based methods give similar results.

To use these models, move the mouse cursor to the model and press the mouse button (rhs button for a windows machine). From the menu that appears, select File/Save molecule as, and save the model to your local hard disk. Select a modelling program (e.g. Weblab Viewer/Medchem Explorer, CAChe, Chem3D, MacMolPlt/GAMESS), open the saved file, and then proceed with the desired calculation.

Intermediate X and its isomers
X Energy = -67.1		Unformed isomer: Energy = -31.1
Zoom in out Display spacefill ball & stick stick wireframe _p2_s+_p4_s Cycloaddition [3,3] sigmatropic		Zoom in out Display spacefill ball & stick stick wireframe _p2_s+_p4_s Cycloaddition
Unformed isomer: Energy = -31.4		Unformed isomer: Energy = -54.0
Zoom in out Display spacefill ball & stick stick wireframe _p2_s+_p4_s Cycloaddition		Zoom in out Display spacefill ball & stick stick wireframe _p2_s+_p4_s Cycloaddition
Species Z is formed from X by a [3,3] sigmatropic shift
Z and its isomers
Z: Energy = -80.7		Unformed isomer: Energy = -60.4
Zoom in out Display spacefill ball & stick stick wireframe [3,3] sigmatropic		Zoom in out Display spacefill ball & stick stick wireframe
Unformed isomer: Energy = -75.9		Unformed isomer: Energy = -75.6
Zoom in out Display spacefill ball & stick stick wireframe		Zoom in out Display spacefill ball & stick stick wireframe

Electrocyclic Problem

The reaction sequence here is:
Squarate

Intermediate M and its HOMO as a 3D Model. To view the 3D orbital model, you must have the Quick3D plug-in installed.
Intermediate N and its isomer
N Energy = -149.3		Isomer: Energy = -153.3
Zoom in out Display spacefill ball & stick stick wireframe Aldol Reaction		Zoom in out Display spacefill ball & stick stick wireframe Aldol reaction
Product O and its isomer
Energy = -80.7		Unformed isomer: Energy = -60.4
Zoom in out Display spacefill ball & stick stick wireframe		Zoom in out Display spacefill ball & stick stick wireframe