Diazo compounds that are well-known to undergo [3 + 2]-cycloaddition provide the first examples of the previously unknown [4 + 2]-cycloaddition with dienes that occur thermally under mild conditions and in high yields. Reactions are initiated from reactants prepared from propargyl aryldiazoacetates that undergo gold(I)-catalyzed rearrangement to activated 1,3-dienyl aryldiazoacetates. These reactions proceed to mixtures of both [4 + 2]-cydoaddition and the 1,3-dienyl aryldiazoacetate after long reaction times. At short reaction times, however, both E- and Z-1,3-dienyl aryldiazoacetates are formed and, after isolation, thermal reactions with the E-isomers form the products from [4 + 2]-cycloaddition with Delta H-298(double dagger) = 15.6 kcal/mol and Delta S-298(double dagger) =-27.3 cal/(mol degrees C). The Z-isomer is inert to [4 + 2]-cydoaddition under these conditions. The Hammett relationships from aryl-substituted diazo esters (rho = +0.89) and aryl-substituted dienes (rho = -1.65) are consistent with the dipolar nature of this transformation.