There is a lot of literature on the halodecarboxylation of alpha,beta-unsaturated carboxylic acids, usually via N-bromosuccinimide or some equivalent and various catalysts:

http://its.goofyti.me/u/http://pubs.acs.org/doi/abs/10.1021/jo025868h
http://its.goofyti.me/u/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6THS-3TKMC10-12&_user=10&_coverDate=04%2F08%2F1996&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=43e09fa17ba057fd610df4d1abb85a5f&searchtype=a
http://its.goofyti.me/u/http://www.4shared.com/dir/5526855/424dc959/Decarboxylation.html

Unique about these is that they can be carried out with catalysts like lithium acetate, triethylamine, etc, unlike typical Hunsdiecker reactions using silver, lead, and thallium.

With piperic acid, you get this:

http://its.goofyti.me/u/http://imgur.com/Xfc2F

which hydrolyses to this:

http://its.goofyti.me/u/http://imgur.com/NiNoR

which undergoes glycolisation/rearrangement to this:

http://its.goofyti.me/u/http://imgur.com/LIYfr

which upon oxidation gives a beta-keto acid that, like all beta-keto acids, spontaneously decarboxylates to P2P. A bit long, but hydrolysis and aldehyde oxidation are usually pretty easy.