Answer:
Hydroxyzine is converted into Cetirizine in humans. The latter can be synthesized as described at: (or see the "related link" below)

http://books.google.se/books?id=fsKKI82uqK8C&pg=PA48&lpg=PA48&dq=hydroxyzine+synthesis&source=bl&ots=IOZsQ5FRpP&sig=BIUdoupvg3zHieI0yqrtUUsHc-o&hl=sv&sa=X&oi=book_result&resnum=1&ct=result#PPA48,M1

(page 48 and 49)

This is the closest I could find on a short notice, I hope it helps. (if you have access to scientifc journals, the references 22-25 in the text could have more specific information)

First answer by HurryKain. Last edit by HurryKain. Contributor trust: 3 [recommend contributor recommended]. Question popularity: 1 [recommend question].