The invention is concerned with the compounds of formula (I): and pharmaceutically acceptable salts thereof, wherein R1, R2, R3 and X are defined in the detailed description and claims. In addition, the present invention relates to methods of manufacturing and using the compounds of formula I as well as pharmaceutical compositions containing such compounds. The compounds of formula I are antagonists or partial agonists at the CRTH2 receptor and may be useful in treating diseases and disorders associated with that receptor such as asthma.