2,3-Dimethylbuta-1,3-diene reacts with propenal ($\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CHO}$) in a Diels-Alder reaction. The product is

The four diene carbons and the two dienophile carbons form a cyclohexene ring, the new C=C lying between the two methyl-bearing carbons. The CHO carbon is C-1 of the ring and the double bond must not involve C-1, so the lowest locants are 3,4 for both methyl groups and the double bond. The product is 3,4-dimethylcyclohex-3-ene-1-carbaldehyde.