Amino acid-derived cross-conjugated trienes were used as a starting point for the synthesis of a discovery library of over 200 polycyclic 5-iminooxazolidin-2-ones, hydantoins, and acylureas. The main feature of this library synthesis is a triple branching strategy which provides efficient access to five skeletally diverse scaffolds. In addition, four sets of building blocks were applied in both a front end and a back end diversification strategy. Multiple fused rings were obtained by cyclization of diamides with phosgene and stereoselective Diels-Alder reactions with maleimides. The 5-iminooxazolidin-2-one scaffold was rearranged into the isomeric hydantoin scaffold through a sequence of ring opening and ring closing reactions.