A highly efficient transformation of cis- to trans-cinnamic acid derivatives by iodine

Article abstract of DOI:10.1016/j.tetlet.2015.11.050

Cinnamic acid derivatives (CAs), which have proven to be versatile components, are present in abundance in biologically active natural products, and are widely used as intermediates in the manufacture of pharmaceuticals, and chemicals. The presence of cis- and trans-CAs created difficulties for natural product and organic synthetic studies. A highly efficient method that utilized iodine to entirely convert cis- CAs into their trans-forms was developed to solve this problem. The mechanism of study revealed this conversion occurred via an anti-diiodo intermediate.

Full text of DOI:10.1016/j.tetlet.2015.11.050

Tetrahedron Letters 56 (2015) 7197–7200
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A highly efficient transformation of cis- to trans-cinnamic acid
derivatives by iodine
Zhen-Jie Li 1, Le Cai 1, Rui-Feng Mei, Jian-Wei Dong, Shu-Quan Li, Xue-Qiong Yang, Hao Zhou,
⇑
Tian-Peng Yin, Zhong-Tao Ding
Key Laboratory of Medicinal Chemistry for Nature Resource, Ministry of Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cinnamic acid derivatives (CAs), which have proven to be versatile components, are present in abundance
in biologically active natural products, and are widely used as intermediates in the manufacture of phar-
maceuticals, and chemicals. The presence of cis- and trans-CAs created difficulties for natural product and
organic synthetic studies. A highly efficient method that utilized iodine to entirely convert cis- CAs into
their trans-forms was developed to solve this problem. The mechanism of study revealed this conversion
occurred via an anti-diiodo intermediate.
Received 23 September 2015
Revised 8 November 2015
Accepted 16 November 2015
Available online 17 November 2015
Keywords:
Ó 2015 Elsevier Ltd. All rights reserved.
Cinnamic acid derivative
Transformation
Iodine
Anti-diiodo intermediate
Cinnamic acid derivatives (CAs) have attracted a great deal of
interest over the years due to their widespread occurrence in
plants1–7 and are widely used as intermediates for the manufacture
of pharmaceuticals, and chemicals.8–10 From a chemical point of
view, CAs possess an enormous degree of structural diversity,
although their molecular backbone consists only of one phenyl-
propane (C6–C3) unit. Previous studies showed CAs exist as both
cis- and trans-forms in nature.1,11,12 The trans-CAs have been
shown to be the predominant form in nature (>99%), because they
are much more stable than the cis-isomers.13,14 The trans-CAs were
shown to photoisomerize to cis-forms under sunlight15 or ultravi-
olet light16,17, which led to the isolation of numerous cis-CA deriva-
tives, including triterpenoids,1,18–21 alkaloids,18 flavonoids,22–25
steroids,26 cyclic peptides,27 and phenolic compounds.28–33 The
highly selective synthesis of trans-CAs is a hot topic and it is diffi-
cult to obtain the pure trans- or cis-isomer for chemists. The mix-
tures of cis- and trans-CAs refer to natural products18,19,34,35 and
organic synthetic products36–42 were reported. It was indicated
that it was difficult to separate these isomers to obtain pure CAs.
During our phytochemical investigations on the plant material,
twelve pairs of cis- and trans-CAs (1–12) were isolated
(Fig. 1).21,29,33,43 The overlapped and complex NMR signals of the
cis- and trans-isomers created difficulties in assigning signals for
these CAs (see Supplementary information). It took a great amount
of time and energy to separate these isomers by silica gel column
chromatography and semi-preparative HPLC. Undoubtedly, com-
pleted transformations to cis- or trans-isomers would facilitate to
obtain pure CAs. Studies of the transformation of trans-CAs to their
cis-forms have been reported. However, a photostationary equilib-
rium was maintained after the transformation.12,44,17 Cis-ionic liq-
uid ammonium cinnamates were reported to be converted partly
to trans-forms.44 Two ionic liquids with photoisomerizable p-
hydroxycinnamic acid moieties were synthesized and their photo-
chemistry transformations were studied in solution and neat con-
ditions.45 Nevertheless, these methods were helpless to obtain
pure cis- or trans-CAs (Scheme 1). Hence, developing a straightfor-
ward and efficient approach to converting the mixture to pure cis-
or trans-form is intensely required.
Iodine is a friendly catalyst and a remarkable radical initiator in
synthetic chemistry.46–48 Rando and Chang reported the I2-cat-
alyzed isomerization of the retinal isomers with a rate constant
of 1.9 Â 10À4 sÀ1 49
.
Puri et al. reported the iodo-Meyer–Schuster
-iodo- ,b-unsat-
rearrangement of 3-alkoxypropargyl alcohols to
a
a
urated esters using iodine.46 Herein we report for the first time a
highly efficient and green method using iodine to completely
transform cis-CAs to their corresponding trans-forms (Scheme 1).
Cis-p-hydroxy-cinnamic acid (1a), prepared from commercially
available trans-p-hydroxy-cinnamic acid (1b) using a previous
described method12,50 was selected for the optimization of the
reaction conditions. Reactions of cis-p-hydroxy-cinnamic acid
(1a) with 0.1, 0.2, 0.3, 0.4 and 0.5 equiv of iodine were screened
at 50 °C in NMR tubes. In the end, the optimal reaction conditions
⇑
Corresponding author. Tel.: +86 871 65033910.
E-mail address: ztding@ynu.edu.cn (Z.-T. Ding).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.tetlet.2015.11.050
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

7198
Z.-J. Li et al. / Tetrahedron Letters 56 (2015) 7197–7200
OH OH
OH
OH
OH
O
HO
HO
HO
O
O
OMe
OH
OH
COOH
1a (cis); 1b (trans )
COOCH3
3a (cis); 3b (trans )
COOH
4a (cis); 4b (trans)
2a (cis); 2b (trans )
O
O
HO
O
OH
O
O
OH
O
HO
HO
HO
HO
HO
O
O
O
O
OMe
OH
OH
OH
OH
OH
6a (cis); 6b (trans)
5a (cis); 5b (trans )
O
OH
OH
O
OH
O
O
HO
HO
HO
HO
O
O
OH
OH
O
O
OH
OH
8a (cis); 8b (trans)
7a (cis); 7b (trans)
OH
O
O
HO
O
O
O
HO
HO
O
OH
O
HO
HO
OH
HO
OH
OH
O
O
9a (cis); 9b (trans)
OH
O
OH
10a (cis); 10b (trans)
HO
HO
O
O
COOH
HO
O
COOH
O
HO
HO
HO
11a (cis); 11b (trans)
12a (cis); 12b (trans)
Figure 1. Structures of compounds 1–12.
Table 1
Previous work:
R
The reaction of cis-p-hydroxy-cinnamic acid (1a) and iodine in NMR tubers at 50 °C
O
UV or sunlight
3~80%
Entry
I2 (equiv)
Time (min)
Percent conversiona (%)
R
OH
O
OH
1
2
3
4
5
6
7
8
0.1
0.1
0.1
0.1
0.2
0.2
0.2
0.2
0.3
0.3
0.3
0.3
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
2
4
8
10
2
4
8
10
2
4
8
10
2
4
8
10
2
4
8
10
8
11
14
51
35
57
76
92
51
73
88
100
84
86
84
90
74
72
72
80
R = H, OH, OCH3
This work:
O
′
R
I2 0.3 equiv, Solvent, 50 ºC
1~10 min, 100%
R
′
R
O
R
O
O
9
R = H, OH; R' = H, CH3, sugar, triterpenoid, flavone units
Solvent: chloroform, methanol, acetone and pyridine
10
11
12
13
14
15
16
17
18
19
20
Scheme 1. Transformation between cis- and trans-cinnamic acid derivatives.
to obtain 100% conversion of trans-p-hydroxy-cinnamic acid (1b)
were I2 (0.3 equiv) and 10 min at 50 °C (Table 1, entry 12). The
reaction could occur in several solvents, including chloroform,
methanol, acetone, and pyridine. However, when the reaction
was conducted in dimethyl sulfoxide (DMSO), percent conversion
was very low. The unsatisfactory solvent was related with
iodine–DMSO complex51–53 and dimethyl sulfoxide should be
avoided in order to obtain excellent conversion.
a
Percent conversion was determined by 1H NMR analysis.
corresponding trans-forms and the original complex NMR spectra
became simple and unambiguous (see Supporting information).
For example, Fig. S1 showed the 1H and 13C NMR spectra of the mix-
ture of cis- and trans-chlorogenic acid methyl ester (4a and 4b).
To investigate the scope of the above transformation, an addi-
tional eleven pairs of natural CAs (2a–12a) were treated with
0.3 equiv of iodine for a few minutes in NMR tubes. The result
indicated that all the cis-CAs were completely transformed to the

Z.-J. Li et al. / Tetrahedron Letters 56 (2015) 7197–7200
7199
I
COOH
COOH
I2 0.3 equiv
COOH
HO
HO
I
CD3OD, 40 ºC,1 min
HO
1a
1b
1c
Scheme 2. Addition reaction of iodine and cis-p-hydroxy-cinnamic acid (1a).
I
O
O
R
I2
COOH
′
O
OH
R
O
O
R′
O2N
I
HO
O
O
O
A
OCH3
13a (cis); 13b (trans)
14a (cis); 14b (trans)
O
I
O
′
R
′
R
O
I2
R
O
15a (cis); 15b (trans) 16a (cis); 16b (trans)
Figure 2. Structures of compounds 13–16.
R
I
B
Scheme 3. Reaction mechanism of cis- to trans-isomerization.
Table 2
Investigation on the effect of vicinal moieties
After reacting with 0.3 equiv weight of iodine for 2 min at 50 °C,
the 1H and 13C NMR spectra of the above mixture became simple
and pure (Fig. S2), indicating that all the cis-chlorogenic acid
methyl ester (4a) was converted to its trans-isomer (4b). Therefore,
this reaction could be a very convenient method that natural
product chemists could use to study CAs. This reaction could also
be utilized in organic synthesis researches.
Entry Starting
material
Reaction
time
Product Percent conversiona
(%)
1
2
3
4
5
Oleinic acid
5 h
1 h
2 h
4 min
2 h
—
—
13a
14a
15a
16a
13b
14b
15b
16b
—
30
100
100
To investigate the applicability of this reaction on gram scale,
1.05 g of a mixture of cis- and trans-caffeic acids (2a:2b = 43:57)
were transformed by iodine (41.3 mg, 0.16 mmol). Results indi-
cated that all the cis-caffeic acid was converted to its trans-isomer
within 5 min, which indicated that this reaction could be scaled up
commendably. Particularly, iodine in catalytic amounts could be
taken away along with solvent evaporation after the reaction
because of its sublimation property, which made the post-
treatment easy. Thus, this reaction is green, efficient, and has
potential industrial applications.
In view of the speed of the iodine transformation, the reaction
was first considered to proceed via a radical anion intermediate.
If this conversion was a radical reaction, other radical initiators
should also promote this conversion. Therefore, two additional
radical initiators, azobisisobutyronitrile and tri-n-butyltin hydride,
were used to react with cis-p-hydroxy-cinnamic acid (1a) and 4-
Percent conversion was determined by 1H NMR analysis.
a
To evaluate the effect of vicinal moieties on the transformation
of cis-double bonds to their trans-isomers, various compounds
(13–16) (Fig. 2) were screened in this reaction (Table 2). The phe-
nyl group was supposed to facilitate the formation of an iodonium
cation because of its electron donating effect. Cis-4-nitrocinnamic
acid (14a), whose electron donating ability was significantly
decreased, could not be completely transformed to its trans-form
(30%), indicating that the electron donating effect of the phenyl
group was very important to this reaction. Two cis-vinyl com-
pounds, cis-aconitic acid methyl ester (13) and oleinic acid, which
both contain no phenyl linking with the cis-double bond, could not
be transformed to their trans-isomers by iodine which further sug-
gested that the phenyl group was essential for the success of this
transformation. To examine the effect of the carboxyl group on this
transformation, cis-stilbene (16a), which has no carboxyl group
adjacent to the double bond, was used in this transformation.
The result showed that the transformation could be completed in
2 h, which was much longer than for CAs, indicating that carboxyl
was positive to this conversion. The reason might be that carboxyl
hydroxy-cis-cinnamomic acid 4-b-D-glucopyranosyloxybenzyl
ester (7a). Unexpectedly, the transformation did not occur. Addi-
tionally, an efficient radical-trapping reagent, 2,2,6,6-tetram-
ethylpiperidine-1-oxyl (TEMPO), was further involved in this
mechanism study.54,55 The result indicated that TEMPO could not
prevent the transformation from cis-p-hydroxy-cinnamic acid
(1a) to its trans-isomer (1b), which was consistent with results of
azobisisobutyronitrile and tri-n-butyltin hydride experiment.
The above two experiments incontrovertibly confirmed that this
conversion was not promoted by a free radical.
Therefore, the iodonium cation was suggested as an intermedi-
ate in this reaction. When the reaction temperature was deceased
to 40 °C, the signals of a key anti-7,8-diiodo intermediate (1c) were
detected in 1H NMR spectrum (dH 4.45, d, J = 10.5 Hz and dH 4.34, d,
J = 10.5 Hz, Fig. S3). Compound 1c was unstable and two labile
iodine groups were eliminated to form trans-p-hydroxy-cinnamic
acid (1b) (Scheme 2). The above experiments definitely confirmed
that the iodine-catalyzed transformation of cis-CAs occurred via an
iodonium cation (A), followed the production of the key
trans-diiodo intermediate (B) (Scheme 3).
unit increased the polarization of
p electron cloud in vinyl, which
facilitated the formation of the iodonium cation. Another com-
pound, 4-phenyl-cis-3-buten-2-one (15a), whose hydroxyl group
in the carboxyl moiety was replaced by a methyl, could also be
converted to its trans-isomer in 4 min, which was slightly longer
than common CAs, implying the hydroxyl in carboxyl was also
positive to this transformation. This might be caused by the
stabilization of the iodonium cation by the lone pair electrons in
the hydroxyl oxygen (Scheme 4). It can be concluded that two
substitutions adjacent to the cis-double bond had a significant
influence on the transformation capabilities, which decreased in
the following order: phenyl, carboxyl > phenyl, carbonyl > phenyl,
phenyl ꢀ carboxyl, carboxyl = alkyl, alkyl (no reaction).

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Z.-J. Li et al. / Tetrahedron Letters 56 (2015) 7197–7200
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