In: Chemistry
In a Diels-Alder reaction, why doesn't the carboxylic acid attack the ester to reform the acid anhydride? Why this Diels-Alder provides the cis-ring juncture instead of the trans?
Hello, carbonyl substitution reactions like any other substitution reactions are governed by the relative stabilities of the substrate and the product. The -OH group of the carbonyl is not a good nucleophile that can attach a stable carbonly group such as the ester and the resulting anhydride is very unstable. Even if such a reaction occurs the produced alcohol will attack on it to produce the ester and carboxylic acid.
look at the following reaction I got from wikipedia
the ring junction is always cis because the reaction follows a concerted mechanism. Both of the bonds are formed simultaneously and bond rotation that would lead to a trans product is impossible. You could get a trans ring junction if the reaction follows some radical mechanism but, that would obviously not be Diels-Alder reaction.