Described herein are compounds that are useful as protein kinase inhibitors of formula I: or a pharmaceutically acceptable salt thereof, wherein Ring B, Z1, Z2, U, p, Q, R1, R2, R3, and R3 are as defined herein. These compounds, and pharmaceutically acceptable compositions thereof, are useful for treating or lessening the severity of a variety of disorders, including stroke, inflammatory disorders, autoimmune diseases such as SLE lupus and psoriasis, proliferative disorders such as cancer, and conditions associated with organ transplantation.