in diels alder reaction , consider a case when the dinucleophile is not a cyclic struture(eg cis- 1,3 buta diene) and reacts with a diene(ethene) to form a non aromatic cyclic structure. Now how is this reaction feaseble ! in the product the C-C bond angle is decreased below that of the original dinucleophile ! there also no resonance to stablilize this ! so why does this reaction take place????????

um...according to me, this reaction DOES NOT take place...there is no electron withdrawing group attached to ethene...so what's the enophile???  

experts, correct me if i am wrong.