Structure elucidation, biogenesis, and bioactivities of acylphloroglucinol-derived meroterpenoid enantiomers from Dryopteris crassirhizoma
Graphical abstract
Eighteen novel acylphloroglucinol meroterpenoid enantiomers were isolated from the rhizomes of Dryopteris crassirhizoma. Anti-inflammatory, antiviral, and antifungal activities were assessed. The anti-inflammatory mechanism of compounds 13 and 14 were investigated.
Introduction
Dryopteris crassirhizoma Nakai (Dryopteridaceae) is a perennial herb mainly distributed in the north region of Asia, such as northeast China, north Korea, and Japan. The dried rhizomes of D. crassirhizoma (“mian ma guan zhong” in Chinese) are one of the inevitable ingredients of two well known traditional Chinese medicines (TMCs), “kang gan ke li (KGKL)” and “lian hua qing wen (LHQW)”, which were widely used to treat cold and influenza [1], [2]. The latter, LHQW, a patented Chinese medicine product which was previously applied for the treatment of many public health emergencies in China, such as SARS, H1N1, and H3N2 influenza [3], [4], [5], has currently been approved by National Medical Products Administration in treating fever, cough, or fatigue caused by COVID-19 [1], [6], [7]. Most recently, the studies of “mian ma guan zhong” discovered that two major components, filixic acid ABA and dryocrassin ABBA, exhibited significant and stable NAs inhibitory activity against influenza viruses including H5N1 [8] and H7N9 [9]. Moreover, the extract of “mian ma guan zhong” was identified as potential anti-dengue agent, active against late stage of dengue virus replication cycle [10].
In a continuing search for structurally unique and pharmacologically active metabolites from this plant, we investigated the chemical constituents of the rhizomes of D. crassirhizoma. As a result, twenty-four unusual acylphloroglucinol based meroterpenoid enantiomers, (±)-dryoptols/(±)-3"-epi-dryoptols A-L (1 ∼ 24) and a known acylphloroglucinol, filixic acid ABA (25), were isolated and elucidated. The skeletons of the isolated meroterpenoids could be classified into two types (Fig. 1): mono and dimeric acylphloroglucinol-based adducts fusing at C-10"/C-11" of nerolidol (I, 1 ∼ 12) and C-6"/C-7" of nerolidol (Ⅱ, 13 ∼ 24), among which the dimeric (3 ∼ 10, 13, 14, and 19 ∼ 24) acylphloroglucinol derivatives possess unprecedented skeletons of mixed acylphloroglucinol and sesquiterpene biosynthetic origin. The mono-acylphloroglucinol meroterpenoids, 1/2, 11/12, and 15/16, were isolated recently from the same plant as three single compounds [11], and their stereochemistries were still confusing in the literature as the duplicate NMR signals caused by their C-3" epimerization in the side chains were hitherto unnoticed. Additionally, to elucidate these diastereomers with chiral centers separated by three or seven bonds in the side chains is very challenging. Moreover, the additional NMR signals produced by tautomerism [12] of the acylphloroglucinol-derived filicinic acid unit make the epimerization of the meroterpenoids more inconspicuous. In this paper, the structures of the six reported meroterpenoids were unambiguously reassigned by meticulous NMR spectroscopic analyses and single-crystal X-ray diffraction experiments. Herein, we report the isolation, structural elucidation, plausible biosynthetic pathway, and the bioactivity profiling of these acylphloroglucinol derivatives.
Section snippets
General experimental procedures
See Supporting Information.
Plant material
The scaly rhizomes of D. crassirhizoma were provided by Good Doctor Pharmaceutical Group. The sample was authenticated by Dr Yumei Zhang, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences. A voucher specimen (JIN2019016) has been deposited at the Key Laboratory of Phytochemistry, Kunming University of Science and Technology.
Extraction and isolation
The rhizomes of D. crassirhizoma were air-dried (25 Kg), ground, and extracted at room temperature with 95% ethanol. The
Structure elucidation of 1 ∼ 24
In this study, the NMR spectra of filicinic acid-containing meroterpenoids 3 ∼ 14 and 17 ∼ 24 exhibited anomalistic signals of filicinic acid ring in a 7:1 ratio, which was explained by keto-enol tautomerization of the filicinic acid moiety [11], [12]. In addition, the twenty-four meroterpenoids were initially isolated as 12 pairs of epimeric mixtures 1/2 ∼ 23/24, in which the signals of protons and carbons around the stereogenic centers in their sidechains appeared as duplicates in the NMR
Conclusions
A family of twenty-four racemic acylphloroglucinol meroterpenoids including eighteen novel ones (3 ∼ 10, 13, 14, and 17 ∼ 24) and six previously reported analogues (originally described as single compounds [11], now herein defined as six pairs of enantiomers 1, 2, 11, 12, 15, and 16), were isolated from the rhizomes of D. crassirhizoma, an inevitable ingredient of two traditional Chinese medicines (“kang gan ke li” and “lian hua qing wen” in Chinese). Among all isolates, the complicated dimeric
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (No. 22067012, 82073737, 82060637, and 21702181), the Yunnan Innovation Team (2019HC018), and the Higher Educational Key Laboratory for New drugs for Viral Respiratory Diseases (Chinese Traditional Medicine) of Yunnan Province.
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These authors made equal contributions to this work.