Chapter 14, Problem 14.65P

In the presence of a small amount of acid, a solution of acetaldehyde $\left({\text{CH}}_{\text{3}}\text{CHO}\right)$ in methanol ${\text{CH}}_{\text{3}}\text{OH}$ was allowed to stand and a new compound L was formed. L has a molecular ion in its mass spectrum at $90$ and IR absorptions at $2992$ and $2941{\text{cm}}^{-1}$. L shows three signals in its ${}^{13}\text{C}\text{NMR}$ at $19$, $52$, and $101$ ppm. The ${}^1\text{H}\text{NMR}$ spectrum of L is given below. What is the structure of L?