Semisynthesis of miltirone, 1,2-dehydromiltirone, saligerone from carnosic acid and cytotoxities of their derivatives

Article abstract of DOI:10.1016/j.tetlet.2018.05.073

Miltirone, 1,2-dehydromiltirone, saligerone have been synthesized form carnosic acid. Among them, one step transformation of carnosic acid into miltirone was realized when decarboxylation/aromatization was promoted by Lewis acid. Moreover, 4 derivatives e

Full text of DOI:10.1016/j.tetlet.2018.05.073

Accepted Manuscript
Semisynthesis of miltirone, 1,2-dehydromiltirone, saligerone from Carnosic
acid and cytotoxities of their derivatives
Cheng-Ji Li, Fan Xia, Wen Zhang, Kou Wang, Gang Xu, Hong-Bo Qin
PII:
S0040-4039(18)30698-1
https://doi.org/10.1016/j.tetlet.2018.05.073
TETL 50020
DOI:
Reference:
Tetrahedron Letters
To appear in:
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Revised Date:
Accepted Date:
16 April 2018
21 May 2018
25 May 2018
Please cite this article as: Li, C-J., Xia, F., Zhang, W., Wang, K., Xu, G., Qin, H-B., Semisynthesis of miltirone,
1,2-dehydromiltirone, saligerone from Carnosic acid and cytotoxities of their derivatives, Tetrahedron Letters
(2018), doi: https://doi.org/10.1016/j.tetlet.2018.05.073
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1
Semisynthesis of miltirone, 1,2-dehydromiltirone, saligerone from Carnosic acid and
cytotoxities of their derivatives
†
†
Cheng-Ji Lia,b, , Fan Xiaa,b, , Wen Zhangc, Kou Wangc, Gang Xua,, and Hong-Bo Qina,
a State 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
b University of Chinese Academy of Sciences, Beijing 100049, P. R. China
c Kunming Medical University, Yunnan 650000, P. R. China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Miltirone, 1,2-dehydromiltirone, saligerone have been synthesized form carnosic acid. Among
them, one step transformation of carnosic acid into miltirone was realized when
decarboxylation/aromatization was promoted by Lewis acid .Moreover, 4 derivatives exhibited
potent cytotoxic activities against three human cancer lines in vitro.
Available online
Keywords: Miltirone, Carnosic acid, Cytotoxicity
Carnosic acid (CA), easily available from widely distributed
rosemary, has antioxidant and anti-inflammatory activities. Its
resource abundance ensures the use as additive and preservative
in food industry. It also exhibits antitumor activity, although gen-
erally inactive due to the carboxylic group.1 Our research interest
focuses on the discovery of antitumor molecules, especially the
2
tricyclic terpenoids derivatives . So we chose carnosic acid as
starting material to access c-20 norditerpenoid derivatives and
thereafter, the synthesized compounds were tested against three
human cancer cell lines (i.e., HepG2: human hepatoblastoma
(HepG2) cells; MCF7: human breast adenocarcinoma cells; A549:
human alveolar basal epithelial cells) by MTT method.
Scheme 1. Synthetic route to miltirone.
At first, we synthesized carnosol from carnosic acid by Ag2O
oxidation. Considering the structure similarity between carnosol
with miltirone, we envisioned that decarboxylation could lead to
the formation of miltirone directly because methyl groups were
just temporary protecting groups. When BBr3 was used, miltirone
was isolated as minor product (20%). Encouraged by this result,
we then applied the same strategy directly from carnosic acid. To
our delight, the yield was improved to 35% (Entry, 1, Table 1).
Hence, several Lewis acids were investigated and the results
were collected in Table 1.
In 1996, Luis group reported an easy hemisynthesis of
miltirone, which displayed significant activity in the central ben-
zodiazepine receptor binding assay,3 from carnosol through an
interesting rearrangement triggered by potassium tert-butoxide
and subsequent decarboxylation.4 (Scheme 1) Based on the fact
that carnosol could be obtained from carnosic acid by air-
oxidation,5 the biomimetic transform from carnosic acid to
miltirone in short steps, would be attractive..
In above process, spontaneous decarboxylation at C-7 and
aromatization occurred when treated with BBr3. We recognized
that if decarboxylation at C-10 also occur in a Lewis acid condi-
tion, the process could be largely simplified. Inspired by a BiCl3
promoted decarboxylation,6 we began the screening for an ideal
Lewis acid to operate in one-step decarboxylation/aromatization
sequence.
Reaction was carried out at rt in CH2Cl2. Boron tribromide
promoted the reaction in similar results under Ar or O2, a little
surprise to our speculation that oxidation of catechol to ortho
quinone would be easier with O2. When the amount of BBr3 was
reduced to 2.5 equiv., no desired product was isolated. Weaker
Corresponding authors. Tel.: +86-871-65217971; e-mails: qinhongbo@mail.kib.ac.cn; xugang008@mail.kib.ac.cn.
†Both authors contributed equally to this work.

2
Tetrahedron Letters
Lewis acid like BF3·OEt2 did not afford the desired product.
TMSOTf gave the best result of 70% isolated yield, while TMSI
was not effective. Metal containing Lewis acid like TiCl4 de-
stroyed the reaction and strong brØnsted acid like CF3COOH and
MeSO3H did not lead to miltirone.(Scheme 2).
Further transformations of Miltirone were investigated. Since
the modification of Tanshinones were mainly concentrated to the
C-ring.7 and the studies on A-ring are rare. Considering the pro-
cess from 1b to 1c, 1,2-dehydromiltirone could be obtained in a
similar manner. Finally, treated with p-TsOH in Toluene and
with the aid of dehydrating agent of triethyl ortho acetate or MS
4Å, dehydromiltirone was isolated in 78%.8 It is noteworthy that
dehydrogenation proceeded very slowly without dehydrator. It
was further oxidized into allylic alcohol 4 or enone 5 (Scheme 4).
The enone 5, as a natural product, was isolated in 2003. 9 Its first
synthesis was accomplished. Another type of enone of A-ring
was synthesized from 1-oxomiltirone with DDQ as oxidative
reagent.
Scheme 2. Synthesis of miltirone from carnosic acid..
Table 1. Optimization of decarboxylation/aromadization.
Entry
Reagent
BBr3 (6.0eq)
BBr3 (6.0eq)
BF3.OEt2
Conditions
Results
35%
1
2
3
4
5
6
7
8
9
Ar
O2
Ar
Ar
Ar
Ar
Ar
Ar
Ar
40%
NR
TMSOTf (2.5eq)
TMSOTf (5.0 eq)
TMSI
40%
70%
messy
messy
NR
TiCl4
CF3COOH
CH3SO3H
Messy
Scheme 4. Synthesis of miltirone derivatives.
a Reaction was carried out for 12h. b NR for no reaction, CA recov-
ered.
In vitro inhibitory activities against HepG2, MCF7 and A549
human tumor cell lines of synthesized compounds were tested
using the MTT method. The results indicated that 4 tanshinone
derivatives exhibited some cytotoxic activities against three hu-
man cancer lines in vitro (Table 2).
In order to investigate the substrate scope, we conducted this
transformation using royleanonic acid as starting material and no
desired product was isolated. It implies that the
orthobenzoquinone moiety is important for decarboxyla-
tion/aromatization. Therefore, the proposed mechanism is depict-
ed below.
Table 2. Cytotoxicities of compounds against three cancer cell lines
(IC50 μM)
Compoundsa
HepG2
8.31
14.69
MCF-7
6.33
5.08
A549
12.06
12.99
2.24
2
4
5
2.40
0.83
2.79
0.04
1.51
0.10
2.43
0.0055
7
STSb
a Other derivatives with IC50 > 40 μM for all the cell lines are not
listed. b Positive control.
Our researches revealed that, for the first time, carnsonic
could be directly transformed into miltirone with Lewis acid and
air. In this process, decarboxylation/aromatization occurred
concurrently. Our effective transformation provide larger
amount of miltirone, comparing with reported total synthesis
10~13
and it would largely boost the bioactivity research of
miltirone. From miltirone, p-TsOH mediated isomerization af-
forded 1,2-dehydromiltirone. Subsequent Oxidation completed
the first synthesis of saligerone . The synthesized compounds
were subjected to antitumor assay towards three human tumor
cell line and saligenone showed potent activity and it demon-
strated the importance of Michael acceptor.
Scheme 3. Proposed Mechanism for decarboxylation/aromatization.
Acknowledgments
Decarboxylation occurred firstly, followed by oxidation of
catechol 1a into ortho benzoquinone 1b (during workup, because
the reaction proceeds effectively even under Ar) and enol isomer-
ization gave 1c, a second time oxidation of catechol afforded
miltirone.
The work was financially supported by the foundations from
NSFC (81373291) to Dr. G. Xu, NSFC (21372229) to Dr. H. B.
Qin, and from Foundation of Kunming Institute of Botany
(KIB2017007) to Dr. G. Xu.

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Supplementary data
The details of chemical transformation, and biological exper-
imental procedures, MS and NMR spectra of the synthesized
compounds associated with this article can be found in the online
version at http://dx.doi.org/XXX.
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


One step synthesis of miltirone was accomplished
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