Discovery of Two Clinical Histamine H 3 Receptor Antagonists: trans - N -Ethyl-3-fluoro-3-3-fluoro-4-(pyrrolidinylmethyl)phenylcyclobutanecarboxamide (PF-03654746) and trans -3-Fluoro-3-3-fluoro-4-(pyrrolidin-1-ylmethyl)phenyl- N -(2-methylpropyl)cyclobutanecarboxamide (PF-03654764)