[Winga]

According to this case, which rule can be applied to assign the priority of A & B substituents?

By CIP rule or by counting the molecular weight of the substituents(heavier one has higher priority)?

[Albert]

I'm not sure to have understood your question. Diels-Alder reaction yields endo-products only.

[Winga]

This reaction is stereospecific.
If the dienophile is a trans-isomer, the substituents of it will also give trans relationship after the reaction.

Moreover, endo-products of the reaction are kinetic control. If the reaction undergoes at high temperature, it will become thermodynamic control. Exo-products are more stable thermodynamically.

[Albert]

So, what did I miss? I mean, what do you want to know in particular?

[Winga]

If A & B align in the same direction, no matter they are pointing upward or downward, it is easy to find out that the product is endo or exo.

This case shows that they are align in different directions, so, which one has a higher priority in order to govern the product types?
My question is that the priority used in this case is as same as CIP rule or there is another rule?