Piperic acid oxidation There are 2 double bonds $\ce{C=C}$ present within each piperic acid molecule,  thus when piperic acid reacts with hot excess $\ce{KMnO4}$, it oxidised into a carboxyllic acid. Why is $\ce{CO2}$ produced?

The carbon atom bonded directly to the benzene ring will get oxidised into a carboxylic acid and also that the carbon bonded to the $\ce{COOH}$ group will get oxidized also to a carboxylic acid to form a dicarboxylic acid. What changes will take place to the other 2 carbon atoms.
this paper stated that $\ce{CO2}$   could be formed as well, how?


Usually when we consider permanganate oxidations we think of


oxidation of alkyl benzenes to benzoic acid
cleavage and oxidation of a double bond to an acid, through the aldehyde, if there is one hydrogen on the double bond
cleavage and oxidation of a double bond to a ketone if there are no hydrogens on the double bond


However, there are a few special cases such as formic acid and oxalic acid, or their corresponding aldehydes.  These compounds are oxidized by permanganate to $\ce{CO2}$ (for an idea as to why these compounds react differently than, say, acetic acid, see Scheme 1 here).

With this information in hand we can re-examine the permanganate oxidation of piperic acid.



We see that oxalic acid is one of the primary products produced in this reaction.  As discussed above, oxalic acid can then undergo further reaction to produce carbon dioxide.