Cycloaddition Reactions - Multi-Step Synthesis
The diene shown on the left can be converted into the product shown by performing a simple [4+2] cycloaddition reaction, followed by a reaction of the cyclic alkene. For each problem, draw the structure of the diene and dienophile which would be required for the synthesis, the structure of the intermediate product, and the reaction conditions necessary to convert the intermediate into the final product.Click the mouse on the buttons to view the synthetic logic, the structure of the intermediate and the solution to the problem.
The final product is an alkane and two bromines have been introduced into the bicyclic system. The formation of trans dihalides is a classic reaction of alkenes and most likely resulted from addition of Br2 to the alkene. It is most likely that the the intermediate alkene is a simple hydrocarbon.
This intermediate was formed from the starting diene and a dienophile. To identify the reactants in a [4+2] cycloaddition reaction, identify the carbons of the dienophile (they will be the two carbons in a six-member ring which are opposite to the double bond in the product) and mentally split the bonds, separating the carbon skeletons of the diene and the dienophile.
1,3-cyclopentadiene undergoes [4+2] cycloaddition with ethene (ethylene) to give the bicyclic addition product. Addition of Br2 gives the trans-dibromo adduct.