The present invention relates to compounds of formula I wherein R1, R2, R4, R5, Ra, Rb, n, W and Z are as defined in the application, their preparation and their use as P2Y12 receptor antagonists in the treatment and/or prevention of peripheral vascular, of visceral-, hepatic- and renal-vascular, of cardiovascular and of cerebrovascular diseases or conditions associated with platelet aggregation, including thrombosis in humans and other mammals.