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. 2014 Jan 17;12(1):317–351. doi: 10.3390/md12010317

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© 2014 by the authors; licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

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Scheme 12 — Synthesis of a and b series central core by Hashimoto and co-workers. Reagents and conditions: (a) Et₃N, THF, −25 °C, 71%; (b) TFA, EtOH; (c) NaBH₃CN, AcOH, EtOH, 52% (2 steps); (d) Boc₂O, DMAP, Et₃N, THF, 0 °C, 84%; (e) (S)-Boc-Ala-OH, CIP, HOAt, DIPEA, CH₂Cl₂, 93%; (f) tBuOCl, THF, −78 °C, then cat. DMAP, Et₃N, 95%. CIP = 2-chloro-1,3-dimethylimidazolidium hexafluorophosphates.