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-dimethylphthalic anhydride. 

(a) Deduce the structure of compound A. Draw all non-bonding electrons (as applicable) 

(b) Give a curved-arrow mechanism for the conversion of A into 3,6-dimethylphthalic anhydride. The catalyst H2SO4 is abbreviated as "H-B" below and its conjugate base is abbreviated as B Follow the directions above each drawing box. 

First step: Delete/Add bonds to form A,  Add 2 curved arrows to give next product.

Second step: Draw missing bond andcurved arrow.

Answer 1

Given above is the diels-alder reaction and the further ring opening rearrangement reaction

(a) Drawn below is the structure of A (diels-alder product)

(b) mechanism for A to 3,6-dimethyl phthalic anhydride product formation

From A, first step is protonation of oxygen followed by C-C bond cleavage to generate a tertiary carbocation species. Loss of H+ gives a diene structure. Reprotonation of -OH to form H2O which leaves and gives another tertiary carbocation. Loss of another H+ gives the desired product.