The syntheses of potent small molecule inhibitors of the CDK2/cyclinA recruitment site are described. Structure-activity trends of nanomolar octapeptides were examined through amino-acid substitution and truncation of the sequence resulting in the identification of a smaller, albeit significantly less potent, tetrapeptide lead. These losses in affinity were recovered by side-chain optimization and by rigidification of the peptide backbone using a combination of solid-phase parallel synthesis and structure-based design. Finally, two guanidine functionalities were replaced to improve drug-like properties, resulting in neutral small molecules equal in activity to that of the peptide lead.