The normal oxidising agents such as acidified `KMnO_(4)`, acidified `K_(2)Cr_(2)O_(7)` or conc. `HNO_(3)` cannot be used in this case as they will cleave double also giving a mixture of ketone/acid. The most effective reagent is `I_(2)" and "NaOH` mixture which converts methyl ketonic group to COOH group without affecting the double bond.