$($ii$)$ Shown below is a redox activated prodrug $1$ for gene$-$directed enzyme

prodrug therapy $($GDEPT$)$ where E$.$ coli nitro reductase gene has been

introduced to the cancer cells. Show both the mechanism of conversion

of the prodrug and the products that will be produced when the prodrug

is acted on by $E.$ coli nitro$-$reductase. $[15$ marks$]$

$($iii$)$ Compound $2$ below was also prepared as a potential redox activated

prodrug for GDEPT. However, the biological activity of $2$ was very poor

compared to $1.$ Explain the difference in the observed biological activity

between $1$ and $2.[10$ marks$]$