Compounds of formula (I) wherein R1, R2, R3, R4, R5 and R6 are as defined herein, and salts thereof that are useful as JAK kinse inhibitors are described herein. Also provided are pharmaceutical compositions that include such a JAK inhibitor and a pharmaceutically acceptable carrier, adjuvant or vehicle, and methods of treating or lessening the severity of a disease or condition responsive to the inhibition of a Janus kinase activity in a patient.