Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

In (A), 4,5-dimethylcyclopent-4-en-1,3-dione, enolisation of the central $\mathrm{C}{\mathrm{H}}_2$ would give a fully conjugated cyclopentadienone system with 4 pi electrons - anti-aromatic and very unstable, so the enol content is negligible (essentially 100% keto). (B) cyclopentane-1,3-dione, (C) pentane-2,4-dione and (D) 2-acetylcyclopentanone all give ordinarily stabilised enols. Hence (A).