3450
B. Baek et al. / Tetrahedron Letters 44 (2003) 3447–3450
Oxa,L formed folded structures, however, only L,(R)-
Nip,(S)-Nip,L promoted the stable folded conforma-
tions.
5. Since N-acetylated peptidomimetics possessing di-oxa-
nipecotic acids were sparingly soluble in nonpolar sol-
vents such as methylene chloride and chloroform, which
are ideal for examining hydrogen bond formations by
FT-IR and NMR, Boc-protected compounds were pre-
pared for structural studies.
Acknowledgements
6. According to FT-IR and NMR studies of four Boc-pro-
tected Nip-tetramers [L,(R)-Nip,(S)-Nip,L, L,(R)-
Nip,(R)-Nip,L, L,(S)-Nip,(S)-Nip,L, L,(S)-Nip,(R)-
Nip,L], only L,(R)-Nip,(S)-Nip,L (4b) exhibited folded
conformations. Accordingly, we selected 4b for these
studies. In fact, this result is a little different from previ-
ous results that heterochiral acetylated Nip-tetramers
(L,(R)-Nip,(S)-Nip,L and L,(S)-Nip,(R)-Nip,L) promote
turn formations.4c We assumed that carbonyl of Boc
group is a worse hydrogen bond acceptor to C-terminal
NH than that of acetyl group and thus only L,(R)-
Nip,(S)-Nip,L forms the folded conformation.
This work was supported by a grant of the Center for
Integrated Molecular Systems (KOSEF). We thank Dr.
Yong Jun Chung for several helpful discussions con-
cerning IR studies.
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