(figure) $\xrightarrow[{(2)\,Zn}]{{(1)\,{O_3}}}\mathop {\begin{array}{*{20}{c}}
  {\,\,\,\,O\,\,\,\,\,\,\,O\,\,} \\ 
  {\,||\,\,\,\,\,\,\,\,\,||} \\ 
  {H - C - C - H} 
\end{array}}\limits_{Glyoxal}  + $ $\mathop {\begin{array}{*{20}{c}}
  {\,\,\,O\,\,\,\,\,O\,\,} \\ 
  {\,||\,\,\,\,\,\,\,\,||} \\ 
  {C{H_3} - C - C - C{H_3}} 
\end{array}}\limits_{2,3 - Bu\tan edione}  + \mathop {\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,O\,\,\,\,\,O} \\ 
  {\,\,\,\,\,\,||\,\,\,\,\,\,||} \\ 
  {C{H_3} - C - C - H} 
\end{array}}\limits_{Pyrualdehyde} $