Chemical constituents of Wikstroemia alternifolia and their neuroprotective activities

Abstract Phytochemical investigation on the plant of Wikstroemia alternifolia led to the isolation of 26 compounds including two new ones, wikstralternifols A and B (1 and 7). Their structures including the absolute configuration were elucidated by spectroscopic data together with analysis of experimental and calculated ECD data. All compounds were isolated from this plant for the first time, and their main structural types were lignans, sesquiterpenoids, and flavonoids. In the sodium nitroprusside-induced rat pheochromocytoma PC-12 cell model, the neuroprotective activities of the selected sesquiterpenoids (1 and 4) and lignans (7 − 14) were screened at the concentration of 10 µM, and 7 − 14 displayed better activities than the positive control edaravone. Graphical Abstract


Introduction
The genus Wikstroemia, belonging to the Thymelaeaceae family, contains about 70 species in the word, which mainly distributes in Eastern Asia, Malaysia, Australia, and Pacific islands (Wang et al. 2007).There are 49 species of Wikstroemia distributed in China (Wang et al. 2007), some of which such as W. indica, W. chamaedaphne, W. dolichantha, and W. nutans have important medicinal value in folk (Kim et al. 2019;Bailly 2021;Li et al. 2021;Liu et al. 2021;Li et al. 2022;Shi et al. 2022;Wu et al. 2022).Previous phytochemical investigation on these traditional Chinese medicines and other Wikstroemia species have revealed the genus Wikstroemia were rich in diterpenoids, lignans, flavonoids, sesquiterpenoids, and coumarins, and these chemical constituents possessed various biological activities such as cytotoxic, anti-inflammatory, anti-virus, anti-malaria, and so on (Kim et al. 2019;Liu et al. 2020;otsuki et al. 2020;Bailly 2021;Li et al. 2021;Liu et al. 2021;Li et al. 2022;Shi et al. 2022;Wu et al. 2022).
Wikstroemia alternifolia Batalin is an evergreen shrub, endemic to China, which distributes in Gansu and northern Sichuan (Wang et al. 2007).To the best of our knowledge, there is no report on the chemical constituents and bioactivity of this plant.In the course of our continuing research for active natural products from traditional Chinese medicines (Wang et al. 2020;Tang et al. 2021a;Su et al. 2022;Weng et al. 2022a;Yuan et al. 2023), a phytochemical investigation on W. alternifolia was carried out.Here are reported the isolation and structural elucidation of 26 compounds (Figure 1) including six sesquiterpenoids (1 − 6), ten lignans (7 − 15 and 23), four flavonoids (16 − 19), two diarylpentanoids (20 and 21), one triterpenoid (22), and three other compounds (24 − 26) from this plant, together with the neuroprotective effects of selected sesquiterpenoids and lignans.

Results and discussion
Wikstralternifol A (1) was obtained as a pale-yellow oil.142.2 (CH) and 134.7 (C)], three sp 3 methyls, three sp 3 methylenes, three sp 3 methines including an oxygenated one, and one sp 3 quaternary carbon.The collective information and the remaining two IHDs (three of the five IHDs were represented by two double bonds and a carbonyl group) indicated that 1 was a bicyclic sesquiterpenoid.
Compounds 1, 4, and 7 − 14 were selected to test their neuroprotective activities at an initial concentration of 10 µM in SNP-induced PC12 cells.As shown in Figure S1.4A (Supplemental Material), most of them at 10 µM exhibited significantly protective properties, being more potent than Eda (the positive control).Moreover, the protective effects of lignans (7 − 14) showed a good dose-dependent manner (Figure S1.4B in Supplemental Material).

Plant material
The branches and leaves of Wikstroemia alternifolia Batalin were purchased from Kunming City, Yunnan Province, P. R.China in october 2020.The voucher specimen (accession number: YWAT202008) was identified by Prof. G.-H. Tang and deposited at the School of Pharmaceutical Sciences, Sun Yat-sen university.

ECD calculation
For details of ECD calculation for compound 1, see S3.2 in Supplementary Material.

Evaluation of neuroprotective activity
In SNP-induced rat pheochromocytoma PC12 cells model, the neuroprotective activities of compounds 1, 4, and 7 − 14 were tested by the MTT assay, and edaravone (Eda) was the positive control (Weng et al. 2022b).

Conclusion
In the current study, a total of 26 compounds including two new ones were obtained from the plant of W. alternifolia for the first time.The main types of these compounds were involved sesquiterpenoids, lignans, and flavonoids.The bioassay results showed that the main chemical components lignans possessed significant neuroprotective activity in SNP-induced PC12 cells.All in all, the first phytochemical investigation on W. alternifolia not only confirmed that its main components were consistent with those found in this genus, but also found some lignans with neuroprotective effects.Additionally, the discovery of new compounds enriched the structural diversity of sesquiterpenoids and lignans, which could provide a diverse material basis for the study of biological activities.
Its molecular formula was determined as C 15 H 22 o 2 with five indices of hydrogen deficiency (IHDs) by the HRESIMS ion at m/z 257.1502 [M + Na] + (calcd 257.1512) and 13 C NMR data.The absorption bands at ν max 3434, 1711, 1650, and 1456 cm −1 in the IR spectrum of 1 indicated the presence of the characteristic groups of oH and conjugated carbonyl.