Stereochemistry of Biradical Cycloaddition

Conventional wisdom has it that whenever stepwise cycloadditions are stereoselective, the thermodynamically more stable stereoisomer will be the one formed preferentially. For example, an olefin that has subsituents with different steric requirements would be expected whenever possible to avoid dimerization to an isomer of the product in which bulky substituents eclipse one another. In those cases when the preferred product turns out to be the more sterically strained one, the mechanistic argument - not always explicit - usually runs as follows  

Brief reflection on the stepwise process shows that (b) does not necessarily follow from (a), and since (c) may be true even when (b) is not, it does not imply (d) The observed stereochemistry is fully consistent with a step- 

1) The first biradical is formed in the trans conformation, as required by Fig. 6.4. 

2) Both bond-forming steps occur preferentially from the conformation of the reactant(s) that leads to the least congested transition states. 

The preferred stereochemical course for dimerization of an olefin with two large substituents and two small substituents in the E (or trans) configuration is traced in Fig. 6.5. 

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