Koenig JR, Liu H, Drizin I, et al. Rigidified 2-Aminopyrimidines as Histamine H4 Receptor Antagonists: Effects of Substitution about the Rigidifying Ring. Bioorganic & Medicinal Chemistry Letters. 2010. Available at: http://linkinghub.elsevier.com/retrieve/pii/S0960894X10001538