We found 30d and 30e, which showed a potent in vitro activity (CDK12/cyclinK IC50 = 21 nM and 85 nM, respectively), and a potent growth-inhibitory activity against the both HER2+ breast-cancer cell lines (SK-Br3 GI50 = 46–52 nM, HCC1954 GI50 = 34–36 nM). Here, CDK12 is linked to breast cancer.