$C{H_2}CN\xrightarrow{{Na/{C_2}{H_5}OH}}\,A\,\xrightarrow{{HN{O_2}}}\,B\,\xrightarrow{{C{r_2}O_{_7}^{ - 2}}}\,C$
$Hex - 3 - ynal\xrightarrow[\begin{subarray}{l}
(ii)\,PB{r_3} \\
(i)\,Mg/ether \\
(i)\,C{O_2}/{H_3}{O^ + }
\end{subarray} ]{{(i)\,NaB{H_4}}}\,?$