Stereoselective THE DIELS-ALDER REACTION: Synthesis of cis-4-Cyclohexene-1,2-dicarboxylic Anhydride SO2
The Correct Answer and Explanation is:
Correct Answer
The reaction illustrates a two-step synthesis of cis-4-cyclohexene-1,2-dicarboxylic anhydride. The first step involves the thermal decomposition of 3-sulfolene in a retro-cheletropic reaction to generate 1,3-butadiene and sulfur dioxide gas. In the second step, the in situ generated 1,3-butadiene acts as a diene and undergoes a [4+2] Diels-Alder cycloaddition reaction with maleic anhydride, the dienophile, to yield the final bicyclic product with cis stereochemistry.
Explanation
The chemical transformation shown is a classic and elegant example of the Diels-Alder reaction, a powerful method in organic chemistry for constructing six-membered rings. The overall synthesis is ingeniously designed in two distinct stages.
The first stage addresses the practical challenge of handling 1,3-butadiene, which is a flammable gas at room temperature. Instead of using it directly, the reaction starts with 3-sulfolene, a stable and easy-to-handle solid. Upon heating, 3-sulfolene undergoes a retro-cheletropic reaction. This is a type of pericyclic reaction where two sigma bonds break, releasing a small, stable molecule. In this case, gaseous sulfur dioxide (SO2) is extruded, a process that is thermodynamically favorable and drives the formation of the reactive diene, 1,3-butadiene, directly within the reaction mixture, or in situ.
The second stage is the core Diels-Alder reaction. The freshly generated 1,3-butadiene (the conjugated diene) immediately encounters maleic anhydride (the dienophile). The electron-rich diene and the electron-poor dienophile, activated by its two electron-withdrawing carbonyl groups, readily engage in a [4+2] cycloaddition. This concerted mechanism involves a cyclic flow of six pi electrons (four from the diene, two from the dienophile) to simultaneously form two new carbon-carbon sigma bonds and a new pi bond, creating the cyclohexene ring system.
The reaction is stereoselective, which is a key feature of the Diels-Alder reaction. The geometry of the dienophile is preserved in the product. Since maleic anhydride is a cis-alkene, the resulting anhydride ring fuses to the newly formed cyclohexene ring in a cis fashion. The two hydrogen atoms on the bridgehead carbons are therefore on the same face of the ring, as depicted in the final product, cis-4-cyclohexene-1,2-dicarboxylic anhydride.thumb_upthumb_down
