Bicyclic Piperazine Mimetics of the Peptide β‑Turn Assembled via the Castagnoli–Cushman Reaction

5-Oxopiperazine-2-carboxamides and respective carboxylic acids (obtained by the Castagnoli–Cushman reaction of protected iminodiacetic anhydride) were converted into cis- and trans-configured bicyclic piperazines containing two stereogenic centers. The latter are not only well-established mimetics of peptide β-turn but also attractive, high-Fsp3 cores for drug design in general. The methodology was applied to the preparation of ring-expanded version of bicyclic piperazines not described in the literature.