A compound of formula (I), or stereoisomers or pharmaceutically acceptable salts thereof, formula (1), or stereoisomers or pharmaceutically acceptable salts thereof, wherein A, U1, U2, R1a, R1b, R2 and R3 have the meanings as indicated in the specification, are useful for treating conditions mediated by activation of the adenosine A2A receptor, especially inflammatory or obstructive airways diseases. Pharmaceutical compositions that contain the compounds and a process for preparing the compounds are also described.