Abstract
Two cyclic dodecapeptides corresponding to newly found members of gramicidin S family, gramicidin S-2: cyclo(–Val–Orn–Leu–D-Phe–Pro–Abu–Orn–Leu–D-Phe–Pro–), and gramicidin S-3: cyclo(–Abu–Orn–Leu-D-Phe–Pro–Abu–Orn–Leu–D-Phe–Pro–), were synthesized. In the synthesis, the linear decapeptide precursors of the antibiotics prepared by a combination of the “hold-in-solution” method and the usual liquid phase method were converted to cyclic ones in excellent yields by one-pot reaction using 1-ethyl-3-(3-dimethylaminopropyl)carbo-diimide hydrochloride and 1-hydroxybenzotriazole. The synthesized antibiotics were identical with the natural products in HPLC analysis and assays of the antibiotic activities. The proposed structures for gramicidin S-2 and S-3 were confirmed. The CD spectra showed that the ring conformations of gramicidin S-2 and S-3 closely resemble each other and also that of gramicidin S-1, which has the already known structure.
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