A Traceless, Solid-Supported Synthesis of β-Turn Mimetics Based on the Hexahydropyrazino[1,2-a]pyrazine-1,2-dione Scaffold

The solid-supported synthesis of a library of beta-turn mimetics based on the three-component Petasis condensation and 2,5-diketopiperazine formation is reported. The eight-step sequence starts from optically pure (S)-piperazine-2-carboxylic acid dihydrochloride, which is first converted into an orthogonally protected, resin-bound amino derivative. The subsequent transformations lead to compounds having the common hexahydropyrazino[1,2-a]pyrazine-1,2-dione core and diverse side chains, which mimic the beta-turn structure. This synthetic route includes protection of the initial amino acid with two different protecting groups, followed by attachment to the Wang resin using the Mitsunobu reaction, deprotection of the beta-nitrogen atom, then Petasis reaction, amidation, deprotection of the alpha-nitrogen atom, coupling with a Boc-protected alpha-amino acid, cleavage of the Boc group, and the cyclizative cleavage from the resin, resulting in the requested bicyclic products obtained in good yields and having good to moderate purities. Six different boronic acids, four amines, and nine alpha-amino acids were applied to this synthetic route, to explore the efficiency and limitations of the described method.