Piperic acid oxidation
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.