This invention provides compounds of formula I: wherein R1, R2, R4, R5, R6, R7, R8a, R8b, W, a, b, c and m are as defined in the specification, or a pharmaceutically acceptable salt or solvate or stereoisomer thereof. The compounds of this invention possess both β2 adrenergic receptor agonist and muscarinic receptor antagonist activity. Accordingly, such compounds are expected to be useful as therapeutic agents for treating pulmonary disorders, such as chronic obstructive pulmonary disease and asthma.