How does a Diels Alder reaction work?

It’s pretty cool, actually. It is probably the first ring formation reaction you will have learned.
DIELS-ALDER MECHANISM
What you have in a Diels-Alder reaction is an s-cis conjugated diene and a dienophile.
A conjugated diene, has two double bonds that are separated by a single bond. A dienophile is literally a “lover of dienes”, and can be a regular alkene if you wish.
A basic Diels-Alder mechanism looks like this:
where the conjugated diene must be s-cis for a reaction to occur. Otherwise, carbons 1 and 4 on the diene are too far apart to react in a concerted fashion.
And they’re pretty much all like that (with heat), when the reactants are symmetrical or don’t have any electron-withdrawing/donating groups.
ASYMMETRICAL SUBSTITUENTS GIVE RISE TO REGIOSELECTIVITY
When you have an asymmetrically substituted conjugated diene and/or dienophile… that’s when things get more complicated, because you’ll get a major and a minor product.
The identity of the substituents ##Y## and ##Z## (and any substituents on the opposite end of each molecule) change the preferred orientation of each participant. You can have either an electron-withdrawing group (EWG) or an electron-donating group (EDG).
For instance, let’s say ##Y## was ##”OCH”_3## and ##Z## was ##(“C”=”O”)-“H”## (aldehyde).
RESONANCE STRUCTURES ALLOW PREDICTION OF PREFERRED ORIENTATIONS
When you draw out the resonance structures for each participant, you should notice that, ##”OCH”_3##, being an EDG, will be partially positive, while the other end of the diene will be more partially negative.
##(“C”=”O”)-“H”##, on the other hand, being an EWG, will be partially negative, while the other end of the dienophile will be more partially positive.
We can assert that the partially positive end of one molecule will preferentially line up with the partially negative end of the other molecule when one or both molecules are asymmetrical.
STEREOSELECTIVITY OF DIELS-ALDER REACTIONS
Lastly, if both substituents on the dienophile are related to each other in a cis or trans conformation, the product will retain that conformation.
DIELS-ALDER PRACTICE PROBLEMS
You may recognize that, aside from preferred orientations, this mechanism is very straightforward and patterned. That means you should expect some very visual variations on the Diels-Alder reactants. Here are some “fun” examples for you to figure out.
Note that some may have no reaction, if you have an s-trans conjugated diene.
Answers:
1) ##color(white)(“The orientation is correct, so just draw the basic mechanism.”)##
2) ##color(white)(“Start from 1,3-butadiene and cyclohexene.”##
3) ##color(white)(“No reaction, because of an s-trans conjugated”)##
##color(white)(“diene. The ring locks the double bonds in place.”)##
4) ##color(white)(“Start from cyclopentadiene and 4-​Cyclopentene-​1,​3-​dione.”)##
##color(white)(“We expect the exo product to be thermodynamically”)## ##color(white)(“favorable, but the extra”)## ##color(white)(pi)## ##color(white)(“density promotes kinetic”)##
##color(white)(“favorability of the endo product.”)##