English

• The functionalized nine-membered ring plays an important role in the bioactivities of natural products, and has been found in meroterpenoids [1], sesquiterpenoids [2], diterpenoids [3,4], and alkaloids from plants [5], fungi, and marine organisms. Neocarzinostatin (Fig. S1 in Supporting information), a polypeptide containing an enediynyl nine-membered core, has been used to treat various human cancers in the clinic [6]. α-Viniferin and protoxenicin A (Fig. S1) bearing a nine-membered ring exhibit their potential as lead compounds for anti-inflammatory and anticancer drugs, respectively [7]. The unique architectural features and potent medicinal properties of such natural products have attracted more and more attention from synthetic and medicinal chemists.

  Ericaceae plants are widely used as traditional Chinese medicines and folk medicines, and many structurally diverse meroterpenoids [8], diterpenoids [9,10], triterpenoids [11], and flavonoids [11], have been reported. In a continuing to search for structurally novel natural products from Ericaceae plants, two pairs of novel 6/6/6/9 tetracyclic merosesquiterpenoid enantiomers featuring an unprecedented 9,15-dioxatetracyclo[8.5.3.0^4,17.0^14,18]octadecane skeleton (Fig. 1), namely dauroxonanols A (1) and B (2), were isolated from the leaves of Rhododendron dauricum, a famous traditional Chinese medicine Man-Shan-Hong. The nuclear magnetic resonance (NMR) spectra of 1 showed very broad resonances, and ¹³C NMR spectrum of 1 exhibited only 13 instead of 22 carbon resonances. These coalescent NMR phenomena made a great challenge to elucidate the structure of 1 using NMR data analysis.

  Figure 1

  

  Figure 1.  Chemical structures of (±)-dauroxonanols A (1) and B (2) and the nomenclature of the unprecedented ring system.

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  The ¹H NMR spectrum of dauroxonanol A (1) acquired in chloroform-d at room temperature (Fig. 2A and Table S1 in Supporting information) displayed two extremely broad resonances for an aromatic proton (δ_H 6.48, br. s, H-6) and an oxymethine (δ_H 4.04, br. s, H-15α), along with normal resonances for four methyl groups (δ_H 1.26, s, H₃-17; 1.25, s, H₃-18; 1.34, s, H₃-20; 2.21, s, H₃-21), an aromatic proton (δ_H 6.30, br. s, H-8), and an olefinic proton (δ_H 4.73, s, H-19a). ¹³C NMR spectrum of 1 only revealed 13 recognizable carbon resonances (Fig. 2B and Table S2 in Supporting information) instead of 22 carbon resonances assigned by its HRESIMS ion at m/z 365.2079 (calcd. for C₂₂H₃₀O₃Na, 365.2093). To obtain high-resolution data, the NMR spectra of 1 were acquired in pyridine-d₅ and methanol-d₄, respectively, and increasing the number of scans (ns) from 1000 (1 h) to 2000 (2 h). However, none of them showed sharp NMR resonances (Figs. 2A and B, Fig. S2 in Supporting information).

  Figure 2

  

  Figure 2.  ¹H (400 MHz, A) and ¹³C (100 MHz, B) NMR spectra of 1 at 298 K with broad signals. Key ¹H–¹H COSY and HMBC correlations of 1 (C). ORTEP drawing of the crystal structure of 1 with the ellipsoid contour at 15% probability level (D). ¹H–¹H COSY, HMBC, and NOESY correlations of 2 (E).

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  Heteronuclear single quantum correlation (HSQC) data assigned ten protonated carbons and two methylenes with broad carbon resonances at δ_C 37.3 (C-3) and 23.5 (C-10). ¹H–¹H correlation spectroscopy (COSY) spectrum only showed one spin system for H₂-9/H₂-10 (Fig. 2C). The connections of C-3/C-9/C-20 to the oxygenated tertiary carbon C-2, C-15/C-17/C-18 to the oxygenated tertiary carbon C-16, and C-21/C-6/C-8 to C-7 were defined by HMBC correlations (Fig. 2C). Except for these, there were no more useful NMR data to determine the structure of 1.

  Finally, the structure of 1 was established as a 6/6/6/9 tetracyclic meroterpenoid with an unprecedented 9,15-dioxatetracyclo[8.5.3.0^4,17.0^14,18]octadecane scaffold by single crystal X-ray diffraction analysis (Fig. 2D and Fig. S3 in Supporting information).

  Since Rhododendron meroterpenoids have been reported as scalemic mixtures [8,12], 1 having a large specific rotation ([α]_D²⁵ −41) was subjected to chiral high performance liquid chromatography (HPLC) analysis (Fig. S4A and Table S3 in Supporting information), revealing that 1 is a scalemic mixture in a ratio of 1:4, differing from the crystallographic data. Subsequent chiral HPLC separation yielded a pair of enantiomers, (+)-1 ([α]_D²⁵ +67) and (−)-1 ([α]_D²⁵ −67), and their absolute configurations were established as 2S, 4S, 11R, 15R and 2R, 4R, 11S, 15S, respectively, by ECD calculations (Fig. S4B) [[13], [14]–15].

  The crystal structure (Fig. S5 in Supporting information) and the P2₁ space group of (−)-1 from a mixed solvent of MeOH/H₂O (v/v, 20:1) were found to be identical to those of 1 from methanol (Fig. 2D). Thus, the crystal of 1 previously obtained from methanol should be the major enantiomer (−)-1. The absolute configuration of (−)-1 was further confirmed by the Flack parameter of 0.00(9) [16]. While, crystals of (+)-1 for X-ray diffraction were not obtained.

  The structure of dauroxonanol B (2) was established as a 15-epimer of 1 by the comprehensive spectroscopic data analyses (Fig. 2E). Detailed structural elucidation and chiral separation of 2 were described in Figs. S6–S8 and Tables S4–S6 (Supporting information).

  Dauroxonanols A (1) and B (2) represent the first 6/6/6/9 tetracyclic merosesquiterpene skeleton bearing an unprecedented 9,15-dioxatetracyclo[8.5.3.0^4,17.0^14,18] octadecane core, and their biosynthetic pathways are proposed in Scheme S1 (Supporting information).

  NMR spectra are usually the average behavior of all conformations of molecules in solvents under certain conditions. To interpret the coalescent NMR phenomenon of 1, a conformational analysis was carried out [[17], [18]–19], revealing the presence of 20 low-energy conformers in 1 (Fig. S9 and Tables S7 and S8 in Supporting information). These conformers mainly differ in the conformation of the oxonane ring moiety and could be classified into three distinct groups: skewed chair-boat, twist chair-boat, and twist chair-chair.

  To investigate the conformational exchange of the oxonane ring, the relaxed potential energy surface scan along the torsion angle ω_{C11–C12–C13–C14} was conducted at B3LYP/6-31G(d, p) level using Gaussian 09 program [13,20], starting from conformer SCB1. Results (Fig. S10 in Supporting information) revealed the inversion involves two steps: SCB1 migrates to TCC1 through TS1, and then, TCC1 transitions to TCB1 through TS2. Subsequent optimization and Gibbs free energies calculations at the M06-2X/def2-TZVP level in chloroform (Fig. 3 and Table S9 in Supporting information) revealed a lower energy barrier $\left(\Delta G^{\ddagger}\right)$ of 15.86 kcal/mol from SCB1 to TCB1 [21]. Thus, SCB1 and TCB1 readily interconvert in chloroform at 298 K.

  Figure 3

  

  Figure 3.  DFT calculated free energy profiles for the pathways of the conformational changes in 1 and 2 at the M06-2X/def2-TZVP level in chloroform at 298 K. The key states along the pathways were obtained by full optimization based on the potential energy surface scan in Fig. S10.

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  Generally, when a molecule possesses multiple conformers and their interconversion rates are fast, the observed NMR signals represent an average from each conformer and manifest as sharp peaks [17]. Thus, the broadening and missing NMR signals of 1 indicated that its conformers interconvert at a rate slower than but still at a rate relatively close to the NMR timescale [17,22,23]. To promote the interconversion rates of conformers of 1, the temperature-rising NMR experiments of 1 in pyridine-d₅ and chloroform-d were performed. In pyridine-d₅ (Fig. 4A), the peak pattern of H-15 was changed from a broad singlet at 298, 303, and 313 K to be a triplet-like at 323 K, and a normal sharp triplet at 333 K. At 343 K, H-6 showed a very broad peak at δ_H 7.09. Similar dynamic effects occurred in chloroform-d (Figs. S11 and S12 in Supporting information), in which H-4β, H-6, H-15α, H-19, C-3, C-13–C-15, and C-19 showed broad signals at 326 K. With increasing temperature, the NMR spectra of 1 in pyridine-d₅ and chloroform-d showed sharper and higher resolution compared to those at 298 K. Therefore, higher temperatures may accelerate the interconversion between different conformers of 1, thereby improving the resolution of NMR spectrum. However, not all peaks of 1 still could be observed even at 373 K, due to the limitations of the interconversion of conformers.

  Figure 4

  

  Figure 4.  Temperature-dependent ¹H NMR (600 MHz) spectra of 1 in pyridine-d₅ (298–373 K, A) and in chloroform-d (213–293 K, B), respectively. Broadening or missing NMR signals in 1 in 298 K (C). The experimental (213 K) and calculated ¹H (D) and ¹³C (E) NMR chemical shifts differences between conformers SCB1 and TCB1.

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  When conformational equilibrium is slow, more than one conformer may be observed in the NMR spectrum [24–26]. To observe the coexisting conformers, the temperature-decreasing NMR experiments of 1 in chloroform-d (Fig. 4B) were carried out. At 283 K, the ¹H NMR spectrum of 1 still showed broadening and missing signals as those at 293 and 298 K. From 273 K, the ¹H NMR spectrum started to exhibit distinct resonances for two conformers. As temperature decreases, the ¹H NMR spectrum shows sharper and higher resolution signals. At 213 K, the ¹H NMR spectrum of 1 displayed well-resolved signals for two independent conformers in a ratio of 1:0.88 (Fig. S13 in Supporting information), and their NMR data (Tables S1 and S2 in Supporting information) were completely assigned by HSQC and HMBC data analysis. For H-6 (Fig. 4B), two distinct signals (Δν = 143.8 Hz) were observed below 253 K, and they were broadening from 263 K to 283 K, and finally coalesced to be a very broad peak at 293 K. The free energy of activation $\Delta G_{\mathrm{c}}{ }^{\ddagger}$ and the rate constant k_c of the process of transformation between two conformers at the coalescence temperature T_c = 293 K were calculated to be 13.8 kcal/mol and 319.5 s^‒1, respectively, using the Eyring equation [27]. The free energies of activation $\Delta G_{\mathrm{c}}{ }^{\ddagger}$ of H-4β, H-6, H-8, H-15, H-19a, H-19b, H-20, and H-21, ranging from 13.3 kcal/mol to 14.0 kcal/mol (Table 1), were found to be very close to the calculated energy barrier $\Delta G^{\ddagger}$ of 15.86 kcal/mol from SCB1 to TCB1 (Fig. 3). Their rate constants k, ranging from 12.1 to 723.6 s^‒1, fall within the intermediate-exchange regime of 1 < k < 10⁴ s^‒1, and some hydrogens coalesced around 293 K. This is why some peaks are broadening and missing in the ¹H NMR spectrum at 298 K.

  Table 1

  

  Table 1.  Representative activation parameters of 1.

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  Nuclei	Δν (Hz)	Tc (K)	kc (s‒1)	$\Delta G_{\mathrm{c}}{ }^{\ddagger}$ (kcal/mol)
  H-4β	325.7	293	723.6	13.3
  H-6	143.8	293	319.5	13.8
  H-8	5.46	263	12.1	14.0
  H-15	164.8	293	366.1	13.7
  H-19a	179.6	293	399.1	13.7
  H-19b	208.0	293	462.1	13.5
  H-20	16.6	273	36.8	14.0
  H-21	27.1	273	60.3	13.7

  Larger NMR chemical shift differences between two major conformers SCB1 and TCB1 would hinder the observation of the NMR signals of 1 (Fig. 4C). To further prove this deduction, the NMR data of SCB1 and TCB1 were calculated at the mPW1PW91/6–311+G(2d, p) level using the gauge independent atomic orbital (GIAO) method [10,28,29]. The calculated NMR data of conformers SCB1 and TCB1 well matched the experimental ones at 213 K (Tables S10 and S11 in Supporting information) with much smaller root mean square deviation (RMSD) values (0.07–0.14 ppm for ¹H and 2.00–2.07 ppm for ¹³C) than the expected precision limits (0.16 ppm for ¹H and 2.45 ppm for ¹³C) [30]. The calculated ¹H NMR chemical shift differences of H₂–3, H-4, H-6, H₂–10, H-11, H₂–13, H₂–14, and H₂–19 exceeded 0.1 ppm (Fig. 4D and Table S12 in Supporting information), and the calculated ¹³C NMR chemical shift differences of C-3, C-4, C-10–C-15, and C-19 between two conformers SCB1 and TCB1 exceeded 1.8 ppm (Fig. 4E and Table S13 in Supporting information). These notable chemical shift differences indicated an NMR distinction between two conformers of 1 (Fig. 4C). In contrast, the chemical shift differences of C-2, C-4a, C-8a, C-5, C-6–C-9, C-17, C-18, C-20, and C-21 were less than 1.0 ppm, and exhibited sharp and normal carbon resonances. Thus, the dynamic conformation inversion of the oxonane ring and the large NMR differences between conformers are responsible for the coalescent NMR characteristics of 1. Compared to 1, dauroxonanol B (2) showed relatively normal NMR resonances except for C-13, which was too broad and too weak to be observed (Fig. S14 in Supporting information). Conformational analysis and density functional theory (DFT) calculations elucidated the transformation of the oxonane ring conformation in 2 from a twisted chair-boat to a skewed chair-boat, with relatively low energy barriers at 12.84 kcal/mol (Fig. 3 and Table S14 in Supporting information). Furthermore, ¹³C NMR calculations revealed a larger chemical shift difference (∆δ_C 8.8 ppm) of C-13 between SBC1 and TBB1 (Table S15 in Supporting information), potentially accounting for the broadening and weak C-13 resonance in the ¹³C NMR spectrum of 2. Interestingly, the energy barrier of 2 ($\Delta G^{\ddagger}$ = 12.84 kcal/mol) was found to be lower than 1 ($\Delta G^{\ddagger}$ = 15.86 kcal/mol), indicating faster interconversion rates between conformers of 2. This observation could explain the higher resolution of the NMR signals in 2 compared to 1.

  Nine-membered rings are important structural blocks of bioactive natural products [24–26]. Due to the flexible nine-membered ring, the intermediate exchange rate often manifests itself in broadened NMR linewidths. Thus, it is difficult to determine their structures based on NMR data analysis. This study provided a new example to elucidate the structures of natural products with a novel 9,15-dioxatetracyclo[8.5.3.0^4,17.0^14,18]octadecane skeleton.

  (+)/(−)-Dauroxonanols A (1) and B (2) displayed significant α-glucosidase inhibitory activities with half maximal inhibitory concentration (IC₅₀) values ranging from 90.68 µmol/L to 363.98 µmol/L (Table S16 and Fig. S16 in Supporting information), which were 2–8 times more potent than the positive control acarbose (IC₅₀ = 714.75 µmol/L). Molecular docking revealed the binding modes of (+)/(−)-1 and 2 with α-glucosidase. Detailed structure-activity relationship analysis and molecular docking studies (Fig. S17 in Supporting information) were described in Supporting Information.

  In conclusion, dauroxonanols A (1) and B (2), two pairs of novel merosesquiterpenoid enantiomers possessing an unprecedented 9,15-dioxatetracyclo[8.5.3.0^4,17.0^14,18] octadecane skeleton, were isolated from R. dauricum. NMR spectra of 1 showed very broad and missing resonances, leading to a great challenge to elucidate its structure using NMR data analysis. DFT calculations, conformational analysis, and dynamic NMR study revealed that the coalescent NMR phenomena of 1 and 2 result from the conformational changes of the flexible oxonane ring in 1 and 2. These results enriched the structural diversity of merosesquiterpenoids with a flexible nine-membered ring.

  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.

  CRediT authorship contribution statement

  Hanqi Zhang: Software, Methodology, Investigation, Formal analysis, Data curation. Biao Gao: Methodology, Investigation, Data curation. Yuanyuan Feng: Investigation, Data curation. Guijuan Zheng: Writing – review & editing, Funding acquisition, Conceptualization. Zhijun Liu: Software, Data curation. Lichun Kong: Data curation. Junjun Liu: Methodology, Investigation. Haji Akber Aisa: Supervision, Resources, Conceptualization. Guangmin Yao: Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Formal analysis, Conceptualization.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22207036, 22277034, 22477034, and 22107033), Interdisciplinary Research Program of Huazhong University of Science and Technology (No. 2023JCYJ037), and International Cooperation Project of Hubei Provincial Key R&D Plan (No. 2023EHA040). We are grateful to the Analytical and Testing Center at Huazhong University of Science and Technology for IR, ECD, and single crystal X-ray diffraction data collection, Medical Subcenter at Huazhong University of Science and Technology for NMR data acquisition. The computation is completed in the HPC Platform of Huazhong University of Science and Technology.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111234.

  
  
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• 

  Figure 1  Chemical structures of (±)-dauroxonanols A (1) and B (2) and the nomenclature of the unprecedented ring system.

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  Figure 2  ¹H (400 MHz, A) and ¹³C (100 MHz, B) NMR spectra of 1 at 298 K with broad signals. Key ¹H–¹H COSY and HMBC correlations of 1 (C). ORTEP drawing of the crystal structure of 1 with the ellipsoid contour at 15% probability level (D). ¹H–¹H COSY, HMBC, and NOESY correlations of 2 (E).

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  Figure 3  DFT calculated free energy profiles for the pathways of the conformational changes in 1 and 2 at the M06-2X/def2-TZVP level in chloroform at 298 K. The key states along the pathways were obtained by full optimization based on the potential energy surface scan in Fig. S10.

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  Figure 4  Temperature-dependent ¹H NMR (600 MHz) spectra of 1 in pyridine-d₅ (298–373 K, A) and in chloroform-d (213–293 K, B), respectively. Broadening or missing NMR signals in 1 in 298 K (C). The experimental (213 K) and calculated ¹H (D) and ¹³C (E) NMR chemical shifts differences between conformers SCB1 and TCB1.

  下载: 全尺寸图片 幻灯片

  Table 1.  Representative activation parameters of 1.

  Nuclei	Δν (Hz)	Tc (K)	kc (s‒1)	$\Delta G_{\mathrm{c}}{ }^{\ddagger}$ (kcal/mol)
  H-4β	325.7	293	723.6	13.3
  H-6	143.8	293	319.5	13.8
  H-8	5.46	263	12.1	14.0
  H-15	164.8	293	366.1	13.7
  H-19a	179.6	293	399.1	13.7
  H-19b	208.0	293	462.1	13.5
  H-20	16.6	273	36.8	14.0
  H-21	27.1	273	60.3	13.7

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