Question

A Diels-Alder reaction of 2,5-dimethylfuran and maleic anhydride gives a compound A that undergoes acid-catalyzed dehydration to give 3,6-dimethyiplithalic anhydride.

(a) Deduce the structure of compound A. 

(b) Give a curved-arrow mechanism for the conversion of A into 3,6-dimettiylphthalic anhydride. Make sure you show ALL lone pairs of electrons and formal charges. Complete the structure of sulfuric acid or the hydrosulfate ion as necessary in each step. Likewise, complete cadi organic framework. Don't include water. 

Answer 1

The given reaction is a Diels-alder reaction which involves the addition of a conjugated diene to a dienophile. It involves a concerted mechanism. a) The diene that is 2,5-dimethylfuran adds to the dienophile (maleic anhydride) to give compound A as follows O: maleic anhydride dienophile 2,5-dimethylfuran (diene) Structure of compound A b) The conversion of compound A into 3,6-dimethyl anhydride is shown below: Step 1 The first step involves the abstraction of proton from sulphuric acid by compound A O: O: он O: sulphuricacid O: 0:

Step 2 The second step involves the detachment of a bond to form a carbocation intermediate as shown below: O: но O: OH Step 3 The third step involves the abstraction of a proton from the intermediate by sulphuric acid to form a conjugated double bond as shown below: O : O: Step 4 The fourth step involves the formation of a hydronium ion by gain of a proton as follows O: O: OH2 O : o: 0:

Step 5 The fifth step involves the removal of a water molecule as shown below : OH2 O: O: -H20 O: 0 Step 6: The final step involves the abstraction of a proton to form the final product that is 3,6 dimethylpthalic anhydride as shown below: O: O: Required product

Answer 2