Isolation, structural elucidation, and synthetic study of salviyunnanone A, an abietane derived diterpenoid with a 7/5/6/3 ring system from Salvia yunnanensis

Fan Xiaab, Da-Wei Zhangab, Chun-Yan Wua, Hui-Chun Genga, Wen-Dan Xua, Yu Zhanga, Xing-Wei Yanga, Hong-Bo Qin*a and Gang Xu*a
aState Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, and Yunnan Key Laboratory of Natural Medicinal Chemistry, Kunming 650201, P. R. China. E-mail:;
bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China

Received 21st December 2017 , Accepted 19th February 2018

First published on 19th February 2018

Salviyunnanone A (1), a cytotoxic diterpenoid possessing an unprecedented 7/5/6/3 ring system derived from a normal abietane skeleton, was characterized from Salvia yunnanensis. Its structure was established by extensive MS and NMR spectroscopic analyses and its absolute configuration was defined by the comparison of experimental and calculated ECDs. The synthetic study of 1 was carried out and its 6-epi-isomer (12) was achieved in 9 steps involving the conjugated addition of an aryl boronic acid as a key step.


The name of the genus, Salvia, is derived from the Latin salvere, in reference to the curative properties of the plants which were used as medicinal herbs throughout the world.1,2 The investigation of Salvia plants has delivered an array of secondary metabolites, mainly abietane and clerodane types of diterpenoids and polyphenols.1 More than 400 abietane type diterpenoids with 5/7/6, 5/6/6, 6/6/6, 6/7/6, 6/7/5, 6/5/6, 7/6/6, 6/6/7 and 6/6/5/5 ring systems have been identified from Salvia plants since 1976.1 Meanwhile, many abietanes showed significant biological activities, such as tanshinone IIA (treating cardiovascular diseases), salvicine (a significant antitumor agent), neo-tanshinlactone (significant and selective activity toward human breast cancer cells), etc.1–3

We have focused on the diterpenoid constituents from the Salvia plants growing in southwest China for more than 10 years and discovered many diterpenoids (such as przewalskins A and B, castanolide, and przewalskone) with intriguing chemical structures and extensive biological activities.4 As part of our ongoing search for secondary metabolites from Salvia genus, a novel rearranged abietane diterpenoid, salviyunnanone A (1), was isolated from a traditional Chinese herb, S. yunnanensis.5 The structure was elucidated by NMR spectroscopy, CD data, and computational approaches. Compound 1 possesses an unprecedented 7/5/6/3 fused ring system comprising an unusual 9,11-epoxy moiety. Compound 1 can be seen as the first abietane derived diterpenoid featured with a 7/5/6 carbon skeleton from plants. In addition, compound 1 exhibited significant cytotoxic activities against five human cancer cell lines in vitro (IC50 1.18–4.84 μM). Subsequently, the synthetic study of 1 was also carried out and its 6-epi-isomer (12) was obtained in 9 steps. The key steps involved conjugated addition of an aryl boronic acid to cycloheptenone, and aldol condensation and epoxidation. Herein, we describe the isolation, structural elucidation, plausible biosynthetic pathway, and bioactivities of compound 1, as well as the synthesis of (±)-6-epi-salviyunnanone A (12).

Results and discussion

Salviyunnanone A (1) was obtained as a yellow powder. Its molecular formula C20H26O3 was established from positive HRESIMS ([M + Na]+ m/z 337.1786, calcd 337.1780), indicating 8 degrees of unsaturation. The IR spectrum showed the absorptions for carbonyl and olefinic groups at 1691, 1662, and 1639 cm−1. The 13C NMR and DEPT spectra (Table 1) showed 20 carbon resonances due to seven quaternary carbons (including two ketones, an oxygenated one, and two olefinic ones), five methines (including an oxygenated one and two olefinic ones), three methylenes, and five methyls. A detailed analysis of these data indicated that the characteristic signals for an abietane type diterpenoid at δC 43.7, 28.8, 21.3, and 24.6 ascribable to C-10, Me-18, Me-19, and Me-20 can all be found, respectively. What's more, three typical methylenes for C-1 (δC 27.3), C-2 (δC 18.8), and C-3 (δC 34.6), as well as the isopropyl group for C-15 (δC 27.0, δH 2.92, sept, J = 6.8 Hz), C-16 (δC 21.8, δH 1.01, d, J = 6.8 Hz), and C-17 (δC 21.1, δH 1.09, d, J = 6.8 Hz), can be distinguished, suggesting that compound 1 should be an abietane derivative.6
Table 1 1H (400 MHz) and 13C (100 MHz) NMR data of compound 1 in CDCl3
No. δC, type δH (J in Hz)
1 27.3, CH2 1.15, m
1.36, m
2 18.8, CH2 1.76, m
3 34.6, CH2 1.58, m
2.04, m
4 46.2, C  
5 215.2, C  
6 63.1, CH 3.93, d (2.9)
7 136.0, CH 6.24, d (2.9)
8 135.9, C  
9 75.8, C  
10 43.7, C  
11 57.5, CH 3.60, s
12 194.8, C  
13 144.1, C  
14 129.3, CH 6.90, s
15 27.0, CH 2.92, sept (6.8)
16 21.8, CH3 1.01, d (6.8)
17 21.1, CH3 1.09, d (6.8)
18 28.8, CH3 1.12, s
19 21.3, CH3 1.12, s
20 24.6, CH3 1.28, s

For a normal abietane diterpenoid, the chemical shifts of C-4 and C-6 were usually presented between δC 30–35 and δC 45–52 in the 13C NMR spectrum.6 Whereas in 1, these two characteristic signals were replaced by two signals for a quaternary carbon at δC 46.2 and a methine at δC 63.1, respectively, in which both were located on a distinct downfield in the 13C NMR spectrum. In addition, two unusual signals for oxygenated carbons at δC 75.8 (C-9) and δC 57.5 (C-11), as well as two carbonyl groups at δC 215.2 (C-5) and δC 194.8 (C-12) were also presented simultaneously (Table 1). These observations indicated that the structure of 1 was quite different from that of the normal abietane with an ordinary 6/6/6 carbon ring system.

In the HMBC spectrum, the correlations of H-20 with C-1, C-6, and C-10, H-18 and H-19 with C-3, C-4, and C-5, and H-6 with C-5, C-10, and C-20 established the linkage of C-3/C-4(Me-18 and Me-19)/C-5/C-6/C-10(Me-20)/C-1 (Fig. 2). The connections of C-1/C-2/C-3 were accomplished by the proton spin system, H-1/H-2/H-3, found in the 1H–1H COSY spectrum (Fig. 2), which incontrovertibly established the unique seven-membered ring A. The five-membered ring B was deduced by the HMBC correlations of H-7 with C-9 and C-10, of H-6 with C-8, C-9 and C-10, of H-20 with C-6 and C-9, as well as the proton spin system H-6/H-7 deduced from the 1H–1H COSY spectrum (Fig. 2). Hence, the oxygenated quaternary carbon at δC 75.8 was ascribed to C-9, which is rare in natural abietane diterpenoids.7

The oxygenated methine at δC 57.5 was assigned to C-11 based on its HMBC correlations with C-9 and C-10. Furthermore, the HMBC correlations of H-11 with C-12 and C-13, H-14 with C-7, C-9, and C-15, and H-15 with C-12, C-13, and C-14 can all be found. The evidence mentioned above, along with the proton spin system of Me-16/H-15/Me-17, established the ring C and the isopropyl substitution at C-13 in 1. The chemical shifts of C-9 and C-11 permitted the existence of the unique 9,11-epoxy moiety, otherwise their chemical shifts should be present in an obviously lower field region. This deduction can also be confirmed by the degrees of unsaturation. Accordingly, the planar structure of 1 was determined as shown (Fig. 1).

image file: c7qo01140g-f1.tif
Fig. 1 Structure of salviyunnanone A (1).

image file: c7qo01140g-f2.tif
Fig. 2 Key HMBC and 1H–1H COSY correlations of 1.

The relative configuration of 1 was established by the analysis of its ROESY correlations in combination with molecular modeling studies. The conformational searching was performed based on MMFF94S force field calculations for energy minimization with CONFLEX 6.2 software overlaid with key correlations observed in the ROESY spectrum. The corresponding minimum geometries found were further optimized by DFT calculations at the B3LYP/6-31G (d,p) level, leading to one minimum stereochemistry as shown (Fig. 3). Then, the diagnostic NOE correlations of H-19/H-20, H-20/H-6, and H-20/H-11 observed in the ROESY spectrum indicated that H-6, H-11, and Me-20 were all β-oriented.

image file: c7qo01140g-f3.tif
Fig. 3 Key NOE correlations of 1.

In order to determine its absolute configuration, we turned to the comparison of experimental and Time-Dependent Density Functional Theory (TDDFT) calculated electronic circular dichroism (ECD) spectra, which has been demonstrated as a powerful tool for defining the structure and absolute configuration of natural products.8 The ECD calculation was performed on the Gaussian 03 program using the TD-DFT-B3LYP/6-31G (d,p) level of theory on a B3LYP/6-31G (d) optimized geometry through the IEFPCM model (in MeOH).9 The ECD spectrum of 1 matched well with the experimental spectrum (Fig. 4). Thus, the absolute configuration of 1 was confirmed as 6S, 9S, 10S, and 11S.

image file: c7qo01140g-f4.tif
Fig. 4 Calculated and experimental ECDs of 1.

Biogenetically, compound 1 might rationally be generated from an abietane precursor ((+)-taxodone),10 undergoing dehydration and epoxidation to form the 5,6-epoxide moiety i. A pinacol rearrangement occurs starting from a 5α,6α-diol under acidic conditions, leading to an A-homo-B-nor-abietane with a rearranged 7/5/6 ring system. Meanwhile, the 9,11-epoxide is formed by the attack of the C-11 hydroxyl over C-9 with the transfer of electrons (Scheme 1). Subsequently, the intermediate ii via the tautomerization of enol produces the structure of 1.

image file: c7qo01140g-s1.tif
Scheme 1 Putative biosynthetic pathway to 1.

The bioassay of its cytotoxic activities against five human cancer cell lines HL-60, SMMC-7721, A-549, MCF-7, and SW480 using the MTT method has been reported previously.11 Compound 1 showed significant toxicity against five human cancer lines in vitro (Table 2).

Table 2 Cytotoxicity of compounds 1 and 12 (IC50 μM)
  MCF-7 SMMC-7721 HL-60 SW480 A-549
a Positive control.
1 1.18 2.52 2.63 3.23 4.84
12 16.66 10.03 3.45 14.87 12.54
Cisplatina 15.92 8.86 1.81 8.86 11.68

Considering its structural novelty and potential antitumor activities, we decided to carry out a total synthesis of 1. Our synthetic strategy is outlined in Scheme 2. The modification of C ring is the key to the total synthesis. It could be transformed from a benzene ring by dearomatization and subsequent epoxidation. The 7/5/6 tricyclic core could be accessed from aryl tertiary carbon ketone intermediate 4, while the tertiary aryl carbon could be formed by the coupling of heptenone and aryl boronic acid.12

image file: c7qo01140g-s2.tif
Scheme 2 Retrosynthetic analysis of 12.

Therefore, our synthesis commenced with the construction of an aryl tertiary carbon center by the coupling reaction of 3-methyl heptenone 2 and aryl boronic acid 3.13 Catalyzed by palladium diacetate, this reaction proceeded smoothly to produce the desired tertiary aryl carbon in 73% yield (Scheme 3). A little surprisingly, heptanone 5 was selectively double methylated in 75% yield when treated with iodomethane and potassium tertiary butoxide. This may attribute to the steric hindrance of the aryl group. To obtain the 7/5/6 tricyclic core, a one-carbon unit should be introduced. Accordingly, a formyl group was installed into the benzene ring in the para position of the methoxy group in 85% yield using dichloromethyl ether as a carbon source with the activation of TiCl4. An intramolecular aldol reaction in 6 proceeded smoothly when potassium hydride was used as the base to afford enone 7 in a yield of 95%. The hydrogenation of enone 7 yielded almost quantitatively; however, afforded two inseparable isomers in the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio. The deprotection of the methoxy group in 8 using boron tribromide proceeded well, but we isolated only the 7/5 trans-isomer because the cis-isomer seemed to be less stable by TLC analysis. We then turned our attention to the modification of the benzene ring. The dearomatization of 9 was performed on treating it with iodobenzene diacetate in a MeCN–water cosolvent. Selective epoxidation in 10 proceeded smoothly to afford epoxide 11 when Sharpless conditions14 were used. The epoxide was used directly and as predicted, dehydration with Martin sulfurane15 afforded a dienone epoxide. However, flip of C-6 hydrogen was unsuccessful after trials of different bases like NaH and DBU under thermal conditions, only resulted in the decomposition of the epoxy unit. As a result, (±)-6-epi-salviyunnanone A (12) was synthesized in 9 steps and 9% overall yield (Scheme 3). In the NOE spectrum of 12, the diagnostic correlations of H-20/H-19 and H-11, and H-18/H-6 further confirmed that 12 should be the 6-epi isomer of 1. Although the initial target was not realized, the synthesis of (±)-6-epi-salviyunnanone A (12) indirectly confirmed the correct assignment of the natural structure of 1. Additionally, compound 12 showed moderate toxicity against five human cancer lines in vitro (Table 2).

image file: c7qo01140g-s3.tif
Scheme 3 Synthesis of (±)-6-epi-salviyunnanone A (12).


In summary, salviyunnanone A (1), an unusual A-homo-B-nor-abietane with a 7/5/6/3 ring system, was characterized from S. yunnanensis. Although four diterpenoids with the 7/5/6 carbon ring system (pierisketolide A, pierisketones B and C, and euphomilone A16) have been reported in recent two years, compound 1 could be seen as the first abietane diterpenoid featured with this intriguing architecture. The 9,11-epoxy group of 1 is also rare in natural abietane derivatives. Subsequently, the synthetic study of 1 was carried out and its 6-epi-isomer (12) was achieved in 9 steps involving the conjugated addition of an aryl boronic acid as a key step. (±)-6-epi-Salviyunnanone A (12) represents the first example of the synthesis of a naturally occurring 7/5/6 carbocyclic ring diterpenoid and provides a certain reference value for the synthesis of other types of diterpenoids with a 7/5/6 ring system.

Conflicts of interest

There are no conflicts to declare.


This work was financially supported by the foundations from the National Natural Sciences Foundation of China (21372229 and 81373291) and from Kunming Institute of Botany, CAS (KIB2017007).

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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qo01140g
These authors contributed equally.

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