[mor]

According to the "endo rule", aldehyde substituent on the dienophile directs into the endo Diels-Alder product.

As you can see, when the dienophile is subsituted with OPMB the preference for the "endo product" is 6:1. However, when the dienophile is subsituted with TMS-C(sp) the preference for the "endo product" rises to 20:1. This fact is not reasonable because the endo rule is influenced by steric effect, and the OPMB group is much bulky than TMS-C(sp).
Are there any other effects that can explain the outcome, i.e. the preferable "endo rule"?

thanks!

[movies]

The endo rule can also be explained by electronics.  This explanation usually involves "secondary orbital overlap" between the diene and the dienophile.  This effect is disputed, somewhat, but I haven't seen a good alternative explaination.