We report here the discovery of a novel series of selective mTOR kinase inhibitors. A series of imidazo [4, 5-b] pyrazin-2-ones, represented by screening hit 1, was developed into lead compounds with excellent mTOR potency and exquisite kinase selectivity. Potent compounds from this series show> 1000-fold selectivity over the related PI3Kα lipid kinase. Further, compounds such as 2 achieve mTOR pathway inhibition, blocking both mTORC1 ...