CONFORMATIONS OF 24-MEMBERED RING PSEUDOPEPTIDES CONTAINING N,N'-ETHYLENE-BRIDGED DIPEPTIDES CONSTRUCTED FROM (S)-ALANINE (S)-LEUCINE, (S)-ISOLEUCINE, AND (S)-PHENYLALANINE

24-Membered ring pseudopeptides, cyclo[Gly-eXX-Gly]2 {eXX; N,N'-ethylene-bridged dipeptide, X=(S)-alanine(A), -leucine(L), -isoleucine(I), and -phenylalanine(F)} and cyclo[Sar-eXX-Gly]2 {X = (S)-A and -F} were prepared and their conformations were investigated by H-1 and C-13 NMR spectroscopy in dimethylsulfoxide-d6. It was found from NMR measurements that the major conformers (greater-than-or-equal-to 90% abundance) of the four cyclo[Gly-eXX-Gly]2 and major ones (congruent-to 70%) of the two cyclo[Sar-eXX-Gly]2 have C2-symmetric structures, in which all peptide bonds of cyclo[Gly-eXX-Gly]2 (X=A, L, and 1) and cyclo[Sar-eAA-Gly]2 are trans, while two Gly-eFF peptide bonds of cyclo[Gly-eFF-GlY]2 and two Sar-eFF ones of cyclo[Sar-eFF-Gly]2 are cis though the other peptide bonds are trans. C-13 spin-lattice relaxation times (T1) of all macrocycles showed small contribution of segmental mobility to T1. The structures of cyclo[Gly-eAA-Gly]2 and cyclo[Sar-eAA-Gly]2 were optimized by molecular mechanics on the basis of NMR data.