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Cyclopeptide Alkaloids: Stereochemistry and Synthesis of the Precursors of Discarines C and D and Myrianthine A

Mostardeiro, Marco A., Ilha, Vinicius, Dahmer, Janice, Caro, Miguel S. B., Dalcol, Ionara I., da Silva, Ubiratan F., Morel, Ademir F.
Journal of natural products 2013 v.76 no.7 pp. 1343-1350
Discaria, alkaloids, amino acids, crystallization, enantiomers, gas chromatography, nuclear magnetic resonance spectroscopy, stereochemistry, tripeptides
The stereochemistry of discarines C (1) and D (2) and myrianthine A (3), three cyclopeptide alkaloids isolated from Discaria febrifuga, was determined by a combination of NMR studies of 1–3, enantioselective gas chromatography, and comparison of NMR data with those of synthetic tripeptides. For the synthesis of peptides, the nonproteinogenic amino acid 3-phenylserine was also obtained in its four diastereoisomeric forms (l and d threo, obtained by recrystallization of the diastereoisomeric tripeptide, and l and d erythro, obtained by a Mitsunobu reaction with the threo-tripeptides). The general synthetic strategy described in this paper allows the tripeptide to be obtained with the free N-terminal extremity protected or dimethylated. This strategy also allows the synthesis of the corresponding peptide with an imidazolidinone ring.