Total syntheses and structure-activity relationship study of parthenolide analogues

Article abstract of DOI:10.1016/j.tetlet.2016.01.039

Two new parthenolide analogues were obtained by total synthesis and assayed for their in vitro anticancer activities to study their structure-activity relationship. Based on the structure and anticancer activity results, two new SAR can be drawn: (1) replacement of the lactone moiety with lactam moiety greatly decreased the anticancer activity; (2) the C-14 methyl group of parthenolide might be important for its high anticancer activity.

Full text of DOI:10.1016/j.tetlet.2016.01.039

Accepted Manuscript
Total Syntheses and Structure-Acitvity Relaltionship Study of Parthenolide An-
alogues
Jing Long, Ya-Hui Ding, Pan-Pan Wang, Quan Zhang, Yue Chen
PII:
S0040-4039(16)30039-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.01.039
TETL 47203
Reference:
Tetrahedron Letters
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Revised Date:
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7 November 2015
8 January 2016
12 January 2016
Please cite this article as: Long, J., Ding, Y-H., Wang, P-P., Zhang, Q., Chen, Y., Total Syntheses and Structure-
Acitvity Relaltionship Study of Parthenolide Analogues, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/
j.tetlet.2016.01.039
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Total Syntheses and Structure-Acitvity Relaltionship
Study of Parthenolide Analogues
Jing Long, Ya-Hui Ding, Pan-Pan Wang, Quan Zhang* and Yue Chen*
College of Pharmacy, The State Key Laboratory of Elemento-Organic Chemistry, and Tianjin Key
Laboratory of Molecular Drug Research, Nankai University, Tianjin 300071, People's Republic of
China
*Correspondence author:
Tel./Fax.: + 86 22 23508090; E-mails: zhangquan612@163.com (Q.Z.), yuechen@nankai.edu.cn
(Y.C.)
1

Abstract: Two new parthenolide analogues were obtained by total synthesis and assayed for their
in vitro anticancer activities to study their structure–activity relationship. Based on the structure
and anticancer activity results, two new SAR can be drawn: (1) replacement of the lactone moiety
with lactam moiety greatly decreased the anticancer activity; (2) the C-14 methyl group of
parthenolide might be important for its high anticancer activity.
Keywords: Parthenolide; Total synthesis; Anticancer activity; Analogue
2

Accumulating evidence has demonstrated that cancer stem cells (CSCs) are responsible for
tumor initiation and progression. CSCs contribute to therapeutic resistance and are considered to
be the most likely cause of cancer relapse.1−3 Consequently, much effort has been made to design
molecules that can target CSCs specifically and sensitize them to therapy.4
Sesquiterpene lactones (SLs), which usually contain a γ-lactone ring, are a large group of
natural products with a wide range of biological activities such as anticancer, anti-inflammatory,
antiviral, antibacterial and antifungal activities. SLs are primarily classified on the basis of their
carbocyclic skeletons into pseudoguainolides, guaianalides, germanocranolides, eudesmanolides,
heliangolides and hyptocretenolides etc.5,6 Parthenolide (PTL, 1, Figure 1), a prominent member
of germanocranolides,7,8 is the major SL responsible for bioactivity of feverfew (Tanacetum
parthenium), an herbal medicine that has been used to treat migraine and rheumatoid arthritis for
centuries.9 Recent studies have demonstrated that PTL can induce death of leukemia stem cells
(LSC)10,11 and breast cancer stem cells (BCSC)12 while sparing normal hematopoietic cells.
However, PTL was unstable and demonstrated low water solubility13,14, which led to poor oral
bioavailability. A PTL analogue, 11,13-dehydro-13-dimethylamino-parthenolide (DMAPT),
presented improved oral bioavailability and retained comparable anticancer activity to PTL.11
Notably, DMAPT has entered clinical trial for the treatment of acute myelocytic leukemia (AML),
acute lymphoblastic leukemia (ALL), and chronic lymphocytic leukemia (CLL) in the United
Kingdom.15
3

Figure 1. Design of compounds 3 and 4.
In view of the intensive biological activities of PTL, and in particular its ability to selectively
kill cancer stem cells, we reported the first total synthesis of PTL and a series of PTL analogues
and established their preliminary SAR.16–18 Recently, we reported a protecting group-free
semisynthesis of parthenolide and its cyclopropyl analogue (2) from the abundant natural product
costunolide.18 Cyclopropyl analogue was significantly more stable than parthenolide under acidic
condition and in plasma, and it maintained comparable activities against acute myeloid leukemia
(AML) cell lines and AML stem cells. Our ongoing efforts are to elaborate this natural product
scaffold for further study of structure activity relationship (SAR) and obtain PTL analogues with
enhanced potency and improved drug-like properties. Herein, we report the asymmetric total
synthesis and further SAR study of parthenolide analogues (Figure 1).
It was reported that the C-14 methyl group is liable to undergo hydroxylation reaction by
cytochrome P450 enzymes, and the P450-catalyzed oxidation led to a complete loss of activity.19
Therefore, we designed the compound 3 with removal of the C-14 methyl group. Based on
strategy of the total synthesis of PTL and its analogues,17 we envisioned that compound 3 could be
generated from 5 through Barbier reaction (Scheme 1). The C1-C10 double bond was supposed to
be furnished by cross metathesis reaction between aldehyde 6 and known compound 7.
4

Scheme 1. Retrosynthesis of compound 3.
Our investigation started with synthesis of aldehyde 6, which was obtained from commerical
available geraniol (8) in three steps (Scheme 2). Asymmetric cyclopropanation of geraniol20
provided alcohol 9. The formed primary alcohol 9 was subjected to ozonation and wittig reaction
in one pot to afford compound 10 in yield of 62% for two steps, which was treated with
pyridinium chlorochromate (PCC) to furnish the corresponding aldehyde 6 in yield of 86%.
Scheme 2. Synthesis of aldehyde 6.
Primary alcohol 11 underwent oxidation with PCC, followed by Baylis-Hillman reaction with
methylacrylate to yield ester 12 in yield of 30% for two steps (Scheme 3). Bromization of 12 with
CBr4 and PPh3 furnished compound 7.
Scheme 3. Synthesis of compound 7.
5

With aldehyde 6 and compound 7 in hand, we implemented the cross metathesis reaction
between 6 and 7. Fortunately, acceptable selectivity (1,10 double bond E/Z up to 9:1) and
moderate yield (68%) were achieved by using the Zhan Catylyst 1B as the catalyst in refluxing
CH2Cl2 (Scheme 4). The cyclization precursor (E)-5 was subjected to intramolecular Barbier
reaction to obtained intermediate 13, without further purification, which was transformed to
lactone 6,7-cis-14 in 65% yield without the desired 6,7-trans-3. The structure of lactone 14 was
further confirmed by X-ray assay.
Scheme 4. Synthesis of compound 14.
There is a lactone in the structure of PTL. Normally, lactone moiety is prone to be hydrolyzed
in base condition, acid condition or hydrolase. PTL is highly lipophilic and the lactone moiety is
prone to metabolic cleavage in vivo.21 Therefore, we designed compound 4 with replacement of
the lactone with a lactam moiety to improve the stability and hydrophilic character.6,21,22 Lin et al.
have developed an efficient and facile one-pot approach for the asymmetric synthesis of highly
substituted γ-lactam containing a α-methylene moiety through Zn-promoted intermolecular
aza-Barbier reaction.23 According to the strategy, we envisioned that the Zn-promoted
intramolecular aza-Barbier reaction of 15 could constructed the ten-membered ring of lactam 4.
Cyclization precursor 15 could be obtained from intermediate 5 by condensation with tert-butyl
6

sulfonamide (Scheme 5). Unfortunately, treatment of 15 with the common Zn-promoted
aza-Barbier reaction condition did not provide any of the desired lactam 4 (Scheme 6).
Scheme 5. First retrosynthesis of compound 4.
Scheme 6. Attempt for synthesis of compound 4.
Therefore, we changed our synthesis route. We expected to obtain compound 4 from cyclization
precursor 16 by ring cross metathesis (RCM) reaction. Cyclization precursor 16 was supposed to
be synthesized by coupling of 17 and 7 according to Lin et al developed strategy. Compound 17
could be obtained from 6 (Scheme 7).
7

Scheme 7. Second retrosynthesis of compound 4
Condensation of 6 with (S)-N-tert-butanesulfinylimine catalyzed by Ti(OEt)4 produced 17 in
91% yield (Scheme 8).24 Compound 17 went through the one pot coupling with 7 to get the
cyclization intermediate which was then treated with HCl-dioxane to afford 16 with removal of
the auxiliary. The final step was the ring close by cross metathesis reaction. However, there are
few reports of construction of ten-membered carbocycle by RCM reaction.25 Fortunately,
compound 4 (ratio of geometric isomers = 70:30) was smoothly obtained by using Zhan Catalyst
1B in CH2Cl2 in yield of 89%.
Scheme 8. Synthesis of compound 4.
With the compounds 4 and 14 in hand, we evaluated their anticancer activities against a series
of cancer cell lines, i.e. lung cancer cell line A549, AML progenitor cell line KG-1a, acute
8

myeloid leukemia (AML) cell line HL-60, doxorubicin-resistant cell line HL-60/A, chronic
myeloid leukemia (CML) cell line K562, and doxorubicin-resistant cell line K562/A. As shown in
Table 1, compound 2 showed comparable anticancer activity to that of parthenolide (1). The result
demonstrated that replacement of the epoxide moiety of parthenolide with the bioisosteric
cyclopropyl moiety maintained its anticancer activity.18 Compound 14 exhibited moderate potency
against the tested cancer cell lines with IC50 values ranging from 9.2 µM to 20.2 µM. For AML
cell line HL60, compound 14 (IC50 = 12.0 µM) was less potent than compound 18 (IC50 = 2.9 µM).
The result suggests that the C-14 methyl group of parthenolide might be important for its high
anticancer activity. However, Lactam 4 exhibited diminished cytotoxicity against the six cancer
cell lines (IC50 > 50 µM), which demonstrated that replacement of the lactone moiety with lactam
moiety greatly decreased the anticancer activity.
Table 1. Anticancer activity of compounds 1, 2, 4, 14 and 18 a
IC50 (µM) b
Compounds
A549 c KG-1a d HL-60 e HL-60/A f K562 g K562/A h
4.4±0.8 5.0±0.4
5.8±1.4 8.3±1.2 3.8±0.3 i
>50 >50 >50
2.1±0.5 i
3.6±0.9 i
6.2±0.6 i
>50
2.7±0.9
6.3±0.8
>50
4.6±0.3
5.9±1.4
>50
1
2
4
9.2±1.8 18.5±2.3 12.0±1.7 15.1±0.9 14.4±1.5 20.2±3.1
2.9 ± 0.8 j
14
18
-
-
-
-
-
9

a All values are the mean of three independent experiments, b IC50: 50% cytotoxic concentration,
c
e
f
A549: lung cancer cell line. d KG-1a: AML progenitor cell line, HL-60: AML cell line,
g
h
HL-60/A: doxorubicin-resistant cell line, K562: CML cell line, K562/A: doxorubicin-resistant
cell line, i Data from reference 18, j Data from reference 17.
In summary, two new parthenolide analogues were synthesized and evaluated for their in
vitro anticancer activities. Based on the above structure and anticancer activity results, the
following SAR can be drawn: (1) replacement of the lactone moiety with lactam moiety greatly
decreased the anticancer activity; (2) the C-14 methyl group might be important for high
anticancer activity. However, more investigations are necessary for extensive SAR study and
ultimate design and synthesis of parthenolide-derived anticancer agents.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (NSFC) (NOs.
81370086 and 81573308 to Q.Z. and NO. 21372129 to Y.C.), Program for New Century Excellent
Talents in University to Y.C., Hundred Young Academic Leaders Program of Nankai University
to Y.C..
10

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Total Syntheses and Structure-Acitvity Relaltionship
Study of Parthenolide Analogues
Jing Long, Ya-Hui Ding, Pan-Pan Wang, Quan Zhang* and Yue Chen*
Two new parthenolide analogues were obtained by total synthesis and assayed for their in vitro anticancer
activities to study their structure–activity relationship. Based on the structure and anticancer activity results, two
new SAR can be drawn: (1) replacement of the lactone moiety with lactam moiety greatly decreased the anticancer
activity; (2) the C-14 methyl group of parthenolide might be important for its high anticancer activity.
13