The present invention provides novel compounds of Formulae (I′) and (II), and pharmaceutically acceptable salts, solvates, hydrates, polymorphs, co-crystals, tautomers, stereoisomers, isotopically labeled derivatives, prodrugs, and compositions thereof. Also provided are methods and kits involving the inventive compounds or compositions for treating and/or preventing proliferative diseases (e.g., cancers (e.g., leukemia, acute lymphoblastic leukemia, lymphoma, Burkitt's lymphoma, melanoma, multiple myeloma, breast cancer, Ewing's sarcoma, osteosarcoma, brain cancer, ovarian cancer, neuroblastoma, lung cancer, colorectal cancer), benign neoplasms, diseases associated with angiogenesis, inflammatory diseases, autoinflammatory diseases, and autoimmune diseases) in a subject. Treatment of a subject with a proliferative disease using a compound or composition of the invention may inhibit the aberrant activity of a kinase, such as a cyclin-dependent kinase (CDK) (e.g., CDK12), and therefore, induce cellular apoptosis and/or inhibit transcription in the subject.