The present invention provides for compounds of formula (I) wherein R1, R2, R3, R4, R5, and R6 have any of the values defined in the specification, and pharmaceutically acceptable salts thereof, that are useful as agents in the treatment of diseases and conditions mediated and modulated by CFTR, including cystic fibrosis, Sjögren's syndrome, pancreatic insufficiency, chronic obstructive lung disease, and chronic obstructive airway disease. Also provided are pharmaceutical compositions comprised of one or more compounds of formula (I).