Preparation of exo-exo-5,6-bis(methoxycarbonyl)-7-oxabicyclo by Using a Diels-Alder Reaction.
The Diels-Alder reaction is essentially a cycloaddition reaction in which an alkene adds to a 1,3-diene to form a 6-membered ring. The substituted ethane in the reaction is normally referred to as the dienophile. This reaction takes place in one step, where new carbon-carbon single and double bonds form simultaneously. A reaction that takes place in one step, such as this reaction, is called a concerted reaction. Diels-Alder reactions are stereospecific with respect to the stereochemistry at the original double bond of the dienophile is retained in the product, and the orientation of the starting materials. It is a way to build six-membered rings from combining an electron-poor alkene with an electron-rich diene, which forms a sigma overlap of the pi-orbitals of two unsaturated systems. With some Diels-Alder reactions, a methylene bridge will form. If the substituents are on the same side as the bridge, cis, then it is called an exo-adduct. If the substituents are on the opposite side from the bridge, trans, then it is called an endo-adduct. This reaction is endo selective, meaning that the endo product is the kinetically favored product. Diels-Alder reactions do not need to occur only between two different products, but can also occur intramolecularly, meaning within the same molecule. Diels-Alder reactions are concerted, meaning they occur within one step, and prefer to react below room temperature which make the reaction fairly easy to perform.
Fischer esterification was also used in the two-step synthesis. The Fischer esterification reaction uses carboxylic acid, an alcohol, and an acid catalyst to form an ester. This reaction was discovered by Emil Hermann Fischer, another renowned German chemist who is also known for his contributions to carbohydrate chemistry and in his method of making molecular projections.