Disclosed are compounds of formula (I), their tautomeric forms, stereoisomers, and pharmaceutically acceptable salts thereof, wherein R1-R4, and n are as defined in the specification, pharmaceutical compositions including a compound, tautomer, stereoisomer, or salt thereof, and methods of treating or preventing diseases or disorders, for example, cancer, that are amenable to treatment or prevention by inhibiting the PI3K enzyme of a subject.
