A new series of pyrazolo [3, 4-d] pyrimidine-3, 6-diamines was designed and synthesized as potent and selective inhibitors of the nonreceptor tyrosine kinase, ACK1. These compounds arose from efforts to rigidify an earlier series of N-aryl pyrimidine-5-carboxamides. The synthesis and structure–activity relationships of this new series of inhibitors are reported. The most promising compounds were also profiled for their pharmacokinetic properties.