Asn - Ser $+\,\underset{(excess)}{\mathop{{{(C{{H}_{3}}CO)}_{2}}O}}\,\xrightarrow{NE{{t}_{3}}}P$
$\begin{array}{*{20}{c}}
{CHO} \\
| \\
{HCOH} \\
| \\
{HOCH} \\
| \\
{HCOH} \\
| \\
{C{H_2}OH} \\
I
\end{array}\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$\begin{array}{*{20}{c}}
{CHO} \\
| \\
{HCOH} \\
| \\
{HCOH} \\
| \\
{HOCH} \\
| \\
{C{H_2}OH} \\
{II}
\end{array}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$\begin{array}{*{20}{c}}
{CHO} \\
| \\
{HCOH} \\
| \\
{HCOH} \\
| \\
{HCOH} \\
| \\
{C{H_2}OH} \\
{III}
\end{array}$