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(+)-Meyeniins A–C, Novel Hexahydroimidazo[1,5-c]thiazole Derivatives from the Tubers of Lepidium meyenii: Complete Structural Elucidation by Biomimetic Synthesis and Racemic Crystallization

Zhou, Min, Ma, Hang-Ying, Liu, Zhi-Hua, Yang, Guang-Yu, Du, Gang, Ye, Yan-Qing, Li, Gan-Peng, Hu, Qiu-Fen
Journal of agricultural and food chemistry 2017 v.65 no.9 pp. 1887-1892
Lepidium meyenii, X-ray diffraction, biomimetic synthesis, circular dichroism spectroscopy, condensation reactions, crystallization, crystals, cytotoxicity, dietary supplements, enantiomers, functional foods, human cell lines, inhibitory concentration 50, neoplasms, thiazoles, tubers, China
(+)-Meyeniins A–C (1–3), a novel class of sulfur-containing hexahydroimidazo[1,5-c]thiazole derivatives, were isolated from the tubers of Lepidium meyenii (maca) cultivated in Lijiang, Yunnan province, China. Guided by their biosynthetic hypothesis, a stereocontrolled biomimetic synthesis of meyeniins A–C and their individual enantiomers was efficiently accomplished by a combination of a condensation reaction and Edman degradation. The formation of high-quality crystals for X-ray crystallography occurred much more readily from a racemic mixture of (±)-meyeniin A than with the single enantiomer alone in this case. These extensive strategies, combined with circular dichroism (CD) spectra, allowed the complete structural assignments of (+)-meyeniins A–C. Among them, (+)-meyeniin A showed moderate selective cytotoxicities against the HL-60, A549 and MCF-7 human cell lines with IC₅₀ values of 14.41, 32.22, and 33.14 μM, respectively. To some extent, these findings support traditional applications of maca as healthy nutritional supplements or functional foods for cancer prevention.