Linear and cyclic β3-oligopeptides with functionalized side-chains (-CH2OBn, -CO2Bn, -CH2CH2CO2Bn) derived from serine and from aspartic and glutamic acid
The natural beta-amino acid deriv. Boc-Asp-OBn (Boc = Me3CO2C; Bn = CH2Ph), as well as I (R = CH2CH2CO2Bn, CH2OBn; TCE = CH2CCl3) (prepd. from appropriately protected glutamic acid and serine, resp., by Arndt-Eistert homologation), were employed as building blocks for the synthesis of linear and cyclic beta-oligopeptides consisting of 2-6 beta-amino acids (using TCE ester groups for C-terminal protection and pentafluorophenyl ester activation for macrocyclization). While the linear derivs. are sol. enough for reactions and structural investigations in soln., the cyclo-beta-tri- and -hexapeptides are not (according to FT-IR measurements they form networks of hydrogen bonds, perhaps consisting of so-called nanotubes). The CD spectra of protected and unprotected beta-hexapeptides II (R1 = Boc, R2 = TCE; R1 = R2 = H) differ drastically; only the unprotected form shows the familiar pattern of a neg. Cotton effect between 210 and 220 nm (indicative of a 314 helix). An NMR anal. in methanol of the beta-hexapeptide II (R1 = R2 = H) with free termini reveals the presence of a single, central, left-handed helix turn (14-membered hydrogen-bonded ring). The results are discussed and compared with those obtained previously for analogous beta-peptides carrying non-functionalized side chains.