Ni-to-Ni + 3-ethylene-bridged partially modified retro-inverso tetrapeptide β-turn mimetic: Design, synthesis, and structural characterizationt

Article abstract of DOI:10.1021/jo011041w

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 HO₂CCH₂CO-NPhe-NH₂. Further elaboration of the linear precursor generated the ethylene-bridged model compounds H2N-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 ¹H 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 CONH₂ in the N-terminal amino acid residue of 16 into NHCOCH₃ in 18 resulted in the formation of a hydrogen bond between the NH ofgPhe-COCH₃ 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.