The stereoisomers of 2,4-hexadiene react as dienes in Diels-Alder reactions at very different rates because of their varying abilities to exist in the s-cis conformation.(For interactive models of each structure just click on them - requires the HyperChem WebViewer plug-in)

isomer

s-trans or transoid conformation (double bond carbons shown in green)

s-cis or cisoid conformation (double bond carbons shown in green)

Comments (energy differences cited are based on molecular mechanics calculations)
trans, trans The s-cis conformation is only slightly higher in energy than the s-trans (3 kcal/mol). Therefore, a fairly high proportion of the molecules exist as s-cis and Diels-Alder reaction is fast.
cis, trans The s-cis conformation is moderately higher in energy than the s-trans (6 kcal/mol). Therefore, a lower proportion of the molecules exist as s-cis and Diels-Alder reaction is slower than with trans, trans.
cis, cis The s-cis conformation is much higher in energy than the s-trans (11 kcal/mol) due to steric strain between the methyls. Therefore, a very low proportion of the molecules exist as s-cis and Diels-Alder reaction is very slow.