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 a diketone and a chlorine has been introduced into the ring. Aldehydes and ketones can be prepared from alkenes by ozonolysis, followed by reductive workup; the chlorine must have come from the dienophile. To get the structure of the alkene, simply connect the two carbonyl carbons with a carbon-carbon double bond.

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.

2,3-dimethyl-1,3-cyclopentadiene undergoes [4+2] cycloaddition with 1-chloroethene (vinyl chloride) to give the bicyclic addition product. Ozonolysis of this, followed by reductive workup with Zn/H⁺, H₂O gives the diketone. Note that the methyl ketones are cis to each other, since the carbonyl carbons used to be connected, and that the chlorine is also cis to the ketones; a result of the preferential formation of the endo cycloaddition product.