A compound with molecular formula $\mathrm{C}_{4} \mathrm{H}_{6} \mathrm{O}_{2}$ shows band at $1770 \mathrm{cm}^{-1}$ in IR spectrum and peaks at $178,68,28$ and 22 ppm in ${ }^{13} \mathrm{C} \mathrm{NMR}$ spectrum. The correct structure of the compound is