help please.

Question 15: During a recent investigation into the chemistry of oligofurans (polymers of the furan heterocycle, which show potential to be used in materials), the investigators observed an interesting reaction with dienophiles. When trifuran is treated with maleimide, only a terminal furan ring participates in a [4+2] cycloaddition (as shown below). Provide a reasonable explanation for this observation. (1.5 points) Exclusive product Not observed