Chapter 14, Problem 14.58P

As we will learn in Chapter 20, reaction of ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{CO}$ with $\text{LiC=CH}$ followed by ${\text{H}}_{\text{2}}\text{O}$ affords compound D, which has a molecular ion in its mass spectrum at $84$ and prominent absorptions in its IR spectrum at ${\text{3600–3200, 3303, 2938, and 2120 cm}}^{\text{–1}}$. D shows the following $\text{H}\text{ NMR}$ spectral data: $\text{1}\text{.53 }\left(\text{singlet, 6 H}\right)\text{, 2}\text{.37 }\left(\text{singlet, 1 H}\right)$, and $\text{2}\text{.43 (singlet, 1H) ppm}$.

What is the structure of D?