Conjugated dienes react with alkenes to yield cyclohexene derivatives. The reaction is termed a 4+2 cycloaddition and is generally referred to as the Diels-Alder Reaction. The reactants in the cycloaddition are referred to, generically, as a diene and a dienophile. The reaction usually requires heat and pressure to give good yields and is promoted by electron withdrawing groups on the dienophile and electron donating groups on the diene.

The mechanism of the reaction is generally described as concerted involving an electrocyclic transition state in which the two new sigma bonds form simultaneously; this is usually represented by showing the electron movement with "curved arrows", as shown above. Since both bonds form at the same time, it is necessary for the diene to be in the proper conformation prior to the reaction, that is, the s-cis conformation is required, and dienes which cannot adopt this conformation will not react.

Examination of the animation shown above for the reaction of ethene with butadiene clearly shows that the initial product of the reaction is the boat cyclohexene. This can also be appreciated by examination of the sequence of images shown below for the reaction of butadiene with butenedinitrile. Lining up the reacting centers and allowing the cycloaddition to proceed generates the structure shown on the right. Rotating this along the X-axis (with the double bond remaining in the back) shows the compound in the "boat" conformation. Converting this to a "chair" by rotating one end down (and rotating the molecule slightly along the Z-axis) gives the middle image on the second row, which is an idealized cyclohexene "chair". In fact, the geometry of the double bond contorts the molecule as shown on the right, but the idealized chair is useful to establish stereochemical relationship between substituents on the diene and dienophile, as they appear in the cyclohexene product.

...rotate along the x-axis to give the intermediate "boat"...

Following a similar sequence of steps to those shown above, the product of the reaction of trans-trans-2,3-hexadiene with trans-2-butene can be shown to be the tetramethyl cyclohexene shown below. Using the numbering scheme shown (the corresponding numbers are also shown on the reactants), the methyl groups of the diene are 1,4-relative to each other and they are cis- (one is axial, one is equatorial). The methyl groups of the dienophile are 1,2-relative to each other (2,3- in the molecule) and they are trans- (both are equatorial).

A general rule can be established, as shown below, that the stereochemistry of the dienophile is retained and that a trans-trans-diene yields cis-substituents, while a cis-trans-diene yields trans-substituents (a cis-cis-diene will not react since it cannot achieve the s-cis conformation).

One further aspect of cycloaddition stereochemistry; when a bicycloheptane ring is formed in the cycloaddition (i.e., when cyclopentadiene or furan is the reacting diene), the preferred orientation of the substituents from the dienophile will be endo (facing into the cavity of the cyclohexene boat).