The adenosine 5'-triphosphate (ATP) competitive cyclin-dependent kinase inhibitor O(6)-cyclohexylmethylguanine (NU2058, 1) has been employed as the lead in a structure-based drug discovery program resulting in the discovery of the potent CDK1 and -2 inhibitor NU6102 (3, IC(50) = 9.5 nM and 5.4 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively). The SAR for this series have been explored further by the synthesis and evaluation of 45 N(2)-substituted analogues of NU2058. These studies have confirmed the requirement for the hydrogen bonding N(2)-NH group and the requirement for an aromatic N(2)-substituent to confer potency in the series. Additional potency is conferred by the presence of a group capable of donating a hydrogen bond at the 4'-position, for example, the 4'-hydroxy derivative (25, IC(50) = 94 nM and 69 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively), 4'-monomethylsulfonamide derivative (28, IC(50) = 9 nM and 7.0 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively), and 4'-carboxamide derivative (34, IC(50) = 67 nM and 64 nM vs CDK1/cyclinB and CDK2/cyclinA3, respectively). X-ray crystal structures have been obtained for key compounds and have been used to explain the observed trends in activity.