Ni-to-Ni + 3-Ethylene-Bridged Partially Modified Retro-Inverso Tetrapeptide β-Turn Mimetic: Design, Synthesis, and Structural Characterization†

A 10-membered heterocyclic ring system 1,3,8-trisubstituted 2,5,7-trioxo-1,4,8-triazadecane that represents a N i -to-N i + 3 -ethylene-bridged partially modified retro-inverso tetrapeptide β-turn mimetic (EBRIT-BTM) has been designed, synthesized, and structurally analyzed. These compounds utilize an ethylene bridge to replace the CO i ...HN i + 3 10-membered hydrogen bond of standard β-turns. The N,N'-ethylene-bridged dimer was obtained in 90% yield by reductive alkylation of phenylalanylamide with a tert-butyl N-(9-fluorenylmethyloxycarbonyl),N-(2-formylmethyl)-glycinate. An orthogonal protection strategy and HATU-mediated couplings allowed efficient stepwise additions of monomeric building blocks leading to a N i -to-N i+3 -ethylene-bridged linear precursor: H-Asp-(OcHex)-NGly-OBu t HO 2 CCH 2 CO-NPhe-NH 2 . Further elaboration of the linear precursor generated the ethylene-bridged model compounds H 2 N-rPheN-mGly-Asp-NGly-OH (16) and Ac-gPheN-mGly-Asp-NGly-OH (18) (g, gem-diaminoalkyl; m, malonyl; and r, direction-reversed amino acid residue) in 44 and 39% yields, respectively. The structural features of the two EBRIT-BTM compounds were determined using 1 H NMR and extensive computer simulations. The results indicate that the 10-membered rings are conformationally constrained with well-defined structural features and that the three amide bonds in the ring are in the trans orientation. The topological arrangement of the residues in the ring system closely resembles a type II'β-turn. Transformation of CONH 2 in the N-terminal amino acid residue of 16 into NHCOCH3 in 18 resulted in the formation of a hydrogen bond between the NH of gPhe-COCH 3 and the C-terminal carboxyl of Gly, initiating an antiparallel β-sheet. The formulation of the concept applying a minimalistic structural elaboration approach and the synthetic exploration, together with the conformational analysis, offer a new molecular scaffolding system and a true tetrapeptide secondary structure mimetic that can be used to generate peptidomimetics of biological interest.