The first total synthesis of (±)-methyl salvianolate A using a convergent strategy

Article abstract of DOI:10.3390/molecules24050999

Herein, a convergent, practicable and first total synthesis of the natural product, (±)-methyl salvianolate A, is reported. The key features of the approach are the use of a Horner–Wadsworth–Emmons reaction and the protection of multiple hydroxyls using s

Full text of DOI:10.3390/molecules24050999

molecules
Article
The First Total Synthesis of (±)-Methyl Salvianolate
A Using a Convergent Strategy
Bo Wang 1,†, Liping Wang 2,†, Ying Peng 2, Yiying Pang 2, Hesheng Xiao 2, Xiaoji Wang 2,* and
Shuangping Huang 3,4,
*
1
School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University,
Nanchang 330013, China; wbo0728@163.com
School of Life Science, Jiangxi Science and Technology Normal University, Nanchang 330013, China;
wlping92@163.com (L.W.); dapengpeng@163.com (Y.P.); 13667080401@163.com (Y.P.);
xiaohes93@163.com (H.X.)
School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang 330013, China
College of Biomedical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
Correspondence: 2012207455@tju.edu.cn (X.W.); hsping02@gmail.com (S.H.); Tel.: +86-791-8380-5358 (S.H.)
These two authors contributed equally to this work.
2
3
4
*
†
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: David Díez and María Ángeles Castro
Received: 15 January 2019; Accepted: 7 March 2019; Published: 12 March 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Herein, a convergent, practicable and first total synthesis of the natural product,
)-methyl salvianolate A, is reported. The key features of the approach are the use of a
(
±
Horner–Wadsworth–Emmons reaction and the protection of multiple hydroxyls using silyl protecting
groups. The employment of the readily removable silyl protecting groups allows the synthesis of
(±)-methyl salvianolate A and its derivatives on a reasonably large scale.
Keywords: methyl salvianolate A; Danshen; total synthesis; Horner–Wadsworth–Emmons reaction;
silyl protecting group
1. Introduction
Traditional Chinese medicines, which are rich in bioactive natural products, and are widely
used in China and Asian countries, play a significant role in the treatment of several diseases.
The Chinese herb Danshen from the Chinese Pharmacopoeia has been widely used, for example, to
treat cerebro-cardiovascular, gynecologic, and digestive diseases [1]. The significant pharmacological
activity may be attributed to the novel biological active components present in Danshen. Phenolic
acids are one of the most important types of water-soluble bioactive components. In 1984, Li
and co-workers first isolated the polyphenol acid, (+)-salvianolate A (Figure 1), from the root of
Danshen [
2
]. Salvianolate A showed considerable biological properties, including antioxidant [
3–6],
cardio- protective [
7
,
8
], antiplatelet, and antithrombotic activities [ ], as well as reversing the paclitaxel
9
resistance and inhibiting tumor migration and invasion of human breast cancer [10]. These multiple
pharmacological effects attracted numerous pharmacological groups to devote their research efforts
toward investigating salvianolate A. Twenty three years later, its methyl derivative, ( )-methyl
±
salvianolate A, was first isolated by the Chen’s group [11] from the aqueous extracts of the roots of
another species of the Salvia genus, Salvia yunnanensis, and was found to possess pharmacological
effects similar to those of salvianolate A, including anti-HIV-1 activity. Due to its similar structure to
salvianolate A, (
those displayed by salvianolate A. However, no attention has been directed toward the synthesis of
)-methyl salvianolate A. Despite the numerous biological studies conducted on salvianolate A, only
four total syntheses of salvianolate A [12 15] and one for salvianolate F [16], which possesses a similar
±
)-methyl salvianolate A may also possess the other pharmacological effects than
(
±
–
Molecules 2019, 24, 999; doi:10.3390/molecules24050999
www.mdpi.com/journal/molecules

Molecules 2019, 24, 999
2 of 11
structure to (
and potential pharmacological applications of (
Salvia spp. tissues, we became interested in developing an efficient approach towards the synthesis of
)-methyl salvianolate A. Herein, we describe the first total synthesis of ( )-methyl salvianolate A,
using a convergent approach [17].
±
)-methyl salvianolate A, have been reported to date. Considering the excellent activity
±
)-methyl salvianolate A, but its low concentration in
(
±
±
Figure 1. The structures of salvianolate F and A, and (±)-methyl salvianolate A.
2. Results
To the best of our knowledge, most of the published formal syntheses of polyphenol compounds
that are structurally similar to ( )-methyl salvianolate A involve the use of methyl ether protecting
±
groups, which effectively avoids the influence of the phenol groups during the synthesis. However,
the use of the methyl ether protecting group results in significant side reactions and low yields during
its deprotection, in which a strong Lewis acid, such as BBr3, is usually used. Although an allylic
protective group which is more easily removed was employed by Zhang’s group, only a moderate
yield (41%) was achieved under the deprotection conditions (Pd(PPh3)4, morpholine) [15]. As an
alternative to using an alkyl group, we looked to install the readily removable tert-butyldimethylsilyl
(TBS) group as the protecting group in our total synthesis. Our retrosynthetic analysis of methyl
salvianolate A is shown in Scheme 1. (
via a Horner–Wadsworth–Emmons (HWE) reaction to give two Fragments
fragment could be obtained from fragments and via a Wittig reaction using Cotelle’s protocol [16],
and that fragment 3 could be prepared from caffeic acid (fragment 6).
±
)-Methyl salvianolate A was disconnected at the C7-C8 bond
2
and . We envisaged that
3
2
4
5
Scheme 1. Retrosynthetic analysis of (±)-methyl salvianolate A.
As depicted in Scheme 2, our synthesis of (
preparation of intermediate from commercially available 4-methylcatechol (
subjected to TBS protection under conventional conditions (TBSCl/imidazole in DMF) to give
±
)-methyl salvianolate A commenced with the
). 4-Methylcatechol was
in an
4
7
8
almost quantitative yield. Compound
8 was then subjected to bromination at the benzylic position via
a radical process using N-bromosuccinimide (NBS) in the presence of 2,20-azobis- (isobutyronictrile)
◦
(AIBN) in CCl4 at 80 C. Without further purification, the resultant bromide was reacted with triphenyl
phosphine in anhydrous MeCN at 80 ◦C, followed by washing with toluene to give phosphonium
salt
available o-vanillin (
reacted with acetic anhydride to afford 10, which was then converted to monobromide 11 in the
4
in an overall 76% yield from
8. Meanwhile, intermediate
5
was prepared from commercially
9). To avoid the influence of the free phenol hydroxyl group, aldehyde
9
first

Molecules 2019, 24, 999
3 of 11
presence of potassium bromide and bromine in water (64% over two steps). The acetyl group in 11 was
removed under mild reaction conditions (sodium hydrogen carbonate in MeOH) followed by removal
of the methyl group using BBr3 at 0 ◦C and a second protection step using TBSCl in the presence of
triethylamine and 4-dimethylaminopyridine (DMAP) to achieve intermediate
three steps.
5 in 69% yield over
Scheme 2. Synthesis of intermediates 4 and 5.
With intermediates
n-BuLi gave the corresponding ylide that was then reacted with aldehyde
13 in 78% yield with a E/Z ratio of 84:16. On exposure to n-BuLi at
4
and
5
in hand, we then conducted the Wittig reaction. Treating salt
4
with
◦
5
at
−
78 C to afford alkene
◦
−
78 C, the resulting anion prepared
in situ from the lithium-halogen exchange of alkene 13 was reacted with N,N-dimethylformamide at
−20 ◦C to give aryl aldehyde 2 in 76% yield (Scheme 3).
Scheme 3. Synthesis of fragment 2.
For fragment
18 and alcohol 17. The latter was synthesized following an established procedure [18] starting from
caffeic acid , as shown in Scheme 4, which included esterification, TBS protection, dihydroxylation
and selective dehydroxylation steps. We obtained intermediate 17 in an overall 72% yield from
. Subsequently, when treated with N,N0-dicyclohexylcarbodiimide (DCC) and catalytic DMAP in
CH2Cl2 at room temperature, 17 and 18 were assembled into fragment 3 in 90% yield.
3, we envisaged an esterification reaction between the commercially available acid
6
6

Molecules 2019, 24, 999
4 of 11
Scheme 4. Synthesis of fragment 3.
At this stage, we needed to connect fragments
2
and
3
to construct the
α
,β-unsaturated ester
moiety via a HWE reaction. After conducting some experiments in different conditions (n-BuLi,
◦
◦ ◦
78 C; NaH, 0 C), we successfully obtained intermediate 19 in 65% yield with an E/Z
−
78 C; NaH,
−
ratio of ~4:1 when performing the HWE reaction at 0 ◦C for 3 h using NaH as base (Scheme 5).
Scheme 5. Assembly of intermediate 19 via HWE reaction.
Finally, with 19 in hand, we turned to conduct the global deprotection step according to our
retrosynthetic analysis. However, the deprotection proved surprisingly difficult. It was disappointing
to find that the desired product was not obtained when treating 19 with the conventional reagent,
tetrabutylammonium fluoride (TBAF). A great deal of TBAF and silicide accompanied
purification step, which hindered the isolation of the target product (Table 1, Entry 1).
1
during the
A similar phenomenon was observed when conducting the experiment under basic [19
acidic conditions, it gave either no reaction (Entry 2–4,6) or trace amounts of the isolated purified
product mixed with a large amount of silicide (Entry 5,7,8). Fortunately, we finally obtained ( )-methyl
–
21] or
±
salvianolate A (1) in excellent yield (92%) without any silicide after purification using triethylamine
trihydrofluoride in anhydrous pyridine (Entry 9) [18,22].
Considering the low efficiency of the demethylation step in the endgame of the formally published
total syntheses of the salvianolates, the deprotection of 19 employing the readily removable silyl ether
protection group demonstrated that ( )-methyl salvianolate A and its derivatives can be prepared on
±
a large scale using our strategy. The physical properties and 1H- and 13C-NMR spectra of compound 1
obtained using our synthetic route agreed with those reported in the literature [11].

Molecules 2019, 24, 999
5 of 11
Table 1. Completion of the total synthesis of (±)-methyl salvianolate A.
Entry
Reagent
Solvent
Yield (%) a
b
1
2
3
4
5
6
7
8
9
TBAF
CSA
TsOH
0.1M HCl
2.0 M HCl
Cs2CO3
LiOH
THF
MeOH
MeOH
H2O
–
No reaction
No reaction
No reaction
b
H2O
DMF-H2O (1:1)
DMF
–
No reaction
b
–
–
b
NaOH
Et3N·3HF
DMF
pyridine
92%
a
b
Isolated yield after silica gel column chromatography. Starting material was completely consumed, but only a
trace amount of the target product was isolated using silica gel column chromatography with a large amount of 1
not separable from the crude mixture containing silicide.
3. Experimental Section
3.1. General Methods
All air and water sensitive reactions were carried out under a nitrogen atmosphere with dry
solvents under anhydrous conditions, unless otherwise noted. Reactions were monitored by thin-layer
chromatography (TLC) carried out on 0.25 mm silica gel plates (60F-254, Qingdao Ocean Chemical
Factory, Qingdao, China) that were analyzed by fluorescence upon 254 nm irradiation or by staining
with anisaldehyde (450 mL of 95% EtOH, 25 mL of con. H2SO4 (15 mL) of acetic acid, and 25 mL
of anisaldehyde, phosphomolybdic acid (100 mL of 95% EtOH, 10 g phosphomolybdic acid) and
dinitrophenylhydrazine (80 mL H2O, 200 mL of 95% EtOH, 12 g dinitrophenylhydrazine and
60 mL concentrated sulfuric acid). Silica gel (60, particle size 0.040–0.063 mm) was used for flash
column chromatography. All the chemicals were purchased commercially and used without further
purification. Anhydrous THF was distilled from sodium-benzophenone. DMF, CH3CN and CH2Cl2
were distilled from calcium hydride. Yields refer to chromatographically, unless otherwise stated.
NMR spectra were recorded on an Avance 400 MHz instrument (1H: 400 MHz, 13C: 100 MHz, Bruker
Corporation, Karlsruhe, Germany). The following abbreviations were used to explain themultiplicities:
s = singlet, d = doublet, m = multiplet. High-resolution mass spectra were obtained from a MALDI-TOF
Mass Spectrometer (Applied Biosystems, Carlsbad, CA, USA). IR spectra were obtained using FT-IR
Spectrometer (Bruker Vertex 70, Bruker Corporation, Karlsruhe, Germany). NMR and HRMS of
compounds 1–5, 8, 10–17, 19 can be found in Supplementary Materials.
3.2. 3,4-Bis(tert-butyldimethylsilyloxy)toluene (8)
Imidazole (13.7 g, 201.39 mmol) and tert-butyldimethylchlorosilane (18.2 g, 120.83 mmol) were
added to a solution of
7
(5.0 g, 40.28 mmol), which was dissolved in dry DMF (200 mL), at 0 ◦C under
a nitrogen atmosphere. The resulting mixture was warmed to room temperature and stirred overnight.
The reaction was quenched with brine, extracted with ethyl acetate, and the combined organic layers
were washed with brine, dried over anhydrous sodium sulfate, filtered and concentrated in vacuo.
The resulting residue was purified by silica gel chromatography to afford compound 8 (14.20 g, 99%)
as a colorless liquid. 1H-NMR (CDCl3):
δ 6.72 (d, J = 8.0 Hz, 1H), 6.65 (s, 1H), 6.61 (d, J = 8.0 Hz,

Molecules 2019, 24, 999
6 of 11
1H), 2.24 (s, 3H), 1.01 (s, 9H), 1.00 (s, 9H), 0.21 (s, 6H), 0.20 (s, 6H).; HRMS (TOF-MS): m/z calcd for
C19H37O2Si2 [M + H]+ 353.2327. Found 353.2327.
3.3. (3,4-Bis((tert-butyldimethylsilyl)oxy)benzyl)triphenylphosphonium Bromide (4)
Under a nitrogen atmosphere, N-bromosuccinimide (7.28 g, 40.83 mmol) and AIBN (1.1 g,
6.81 mmol) were added to a solution of compound
8 (12.0 g, 34.03 mmol) in CCl4 (70 mL) and
the resulting mixture heated at reflux for 1 h. The reaction mixture was filtered and the residue was
washed with petroleum ether and concentrated in vacuo to afford the crude product, which was
immediately used in the next step without any purification. Triphenylphosphine (13.38 g, 51.04 mmol)
was then added to the crude product solution in dry MeCN (70 mL) and the resulting mixture was
heated under reflux for 3 h. The reaction mixture was cooled to room temperature and concentrated
in vacuo. Toluene was added to the crude product and the resulting suspension filtered to give
1
phosphonium salt
4
(17.94 g, 76% over two steps) as a pale yellow solid. H-NMR (CDCl3):
δ
7.79–7.62
0.05
(m, 15H), 6.61 (s, 2H), 6.47 (s, 1H), 5.16 (d, J = 13.6 Hz, 2H), 0.93 (s, 9H), 0.84 (s, 9H), 0.15 (s, 6H),
(s, 6H); HRMS (TOF-MS): m/z calcd for C37H50O2Si2P [M-Br-]+ 613.3081. Found 613.3082.
−
3.4. 2-Formyl-6-methoxyphenyl Acetate (10)
A catalytic quantity of 4-dimethylaminopyridine (0.96 g, 7.89 mmol) and acetic anhydride
(8.07 mL, 85.44 mmol) were added to an o-vanillin (9, 10.01 g, 65.72 mmol) solution in
N,N-diisopropylethylamine (33 mL) at 0 ◦C under a nitrogen atmosphere. The solution was allowed
to warm to room temperature and stirred overnight. After compound 9 was completely consumed,
hydrochloric acid (2 M, 100 mL) was added. The resulting mixture was stirred for another 10 min and
then extracted with dichloromethane (100 mL
×
3). The combined organic layers were concentrated
in vacuo to afford a crude yellow solid, which was recrystallized from ethanol to give compound 10
1
(10.85 g, 85%) as a yellow solid. H-NMR (CDCl3):
δ 10.12 (s, 1H), 7.43 (d, J = 7.6 Hz, 1H), 7.31 (t,
J = 8.4 Hz, 1H), 7.20 (d, J = 8.4 Hz, 1H), 3.85 (s, 3H), 2.39 (s, 3H).; 13C-NMR (100 MHz, CDCl3):
δ 188.7,
168.7, 151.8, 141.5, 129.2, 126.8, 121.2, 117.9, 56.3, 20.4.
3.5. 3-Bromo-2-formyl-6-methoxyphenyl Acetate (11)
Bromine (3.73 mL, 72.64 mmol) was added to a potassium bromide (21.47 g, 180.48 mmol) solution
in H2O (133 mL). Aldehyde 10 (10.85 g, 55.88 mmol) was added to the resulting dark red solution and
stirred overnight. The reaction was monitored by TLC until 10 was completely consumed. The reaction
mixture was extracted with ethyl acetate, the layers separated and the organic layer concentrated
in vacuo. The resulting residue was recrystallized from ethyl acetate–hexane to give compound 11
1
(11.44 g, 75%) as yellow crystals. H-NMR (CDCl3):
δ 10.35 (s, 1H), 7.59 (d, J = 8.8 Hz, 1H), 7.14 (d,
J = 8.8 Hz, 1H), 3.94 (s, 3H), 2.47 (s, 3H); 13C-NMR (CDCl3):
δ 190.3, 168.7, 151.9, 140.4, 131.5, 126.6,
117.8, 116.3, 56.4, 20.5.
3.6. 6-Bromo-2-hydroxy-3-methoxybenzaldehyde (12)
Sodium bicarbonate (4.93 g, 58.64 mmol) was added to a solution of aldehyde 11 (11.44 g,
41.89 mmol) solution in a mixture of methanol and water (v/v = 2:1, 84 mL). The resulting turbid yellow
solution was stirred for 2 h at room temperature. Hydrochloric acid (1 M) was then added to adjust
the pH to 6–7 and the mixture extracted with ethyl acetate (100 mL
×
3). The combined organic layers
were concentrated in vacuo and the crude yellow solid recrystallized from ethyl acetate/hexane to
1
give compound 12 (8.23 g, 85%) as yellow acicular crystals. H-NMR (CDCl3):
δ 12.26 (s, 1H), 10.27
(s, 1H), 7.07 (d, J = 8.4 Hz, 1H), 6.90 (d, J = 8.4 Hz, 1H), 3.88 (s, 3H); 13C-NMR (CDCl3):
δ 198.3, 154.4,
148.3, 123.3, 118.2, 117.1, 116.3, 56.3.

Molecules 2019, 24, 999
7 of 11
3.7. 6-Bromo-2,3-bis((tert-butyldimethylsilyl)oxy)benzaldehyde (5)
Boron tribromide (13.47 mL, 142.48 mmol) in CH2Cl2 (60 mL) was added dropwise to a solution
of compound 12 (8.23 g, 35.62 mmol) in dry CH2Cl2 (120 mL) at 0 ◦C under a nitrogen atmosphere.
The resulting mixture was then stirred for 2 h. Ice–water (100 mL) was added slowly to quench the
reaction, the layers separated and the organic layer concentrated in vacuo. The residue was washed
with water, filtered and the resulting yellow filter cake dried at 40 ◦C to give the crude product as
a yellow solid, which was used in the next step without any further purification. Et3N (16.50 mL,
118.37 mmol), DMAP (0.826 g, 6.76 mmol) and tert-butyldimethylchlorosilane (15.29 g, 101.46 mmol)
◦
were added to the above crude solid (7.34 g, 33.82 mmol) in dry DMF (307 mL) at 0 C under a nitrogen
atmosphere. The resulting solution was stirred for 1 h at 0 ◦C. The reaction was quenched with brine,
extracted with ethyl acetate (200 mL
×
3) and the combined organic layers concentrated in vacuo.
The resulting residue was purified by silica gel chromatography to give intermediate 5 (12.85 g, 81%
1
over two steps). H-NMR (CDCl3):
δ
10.29 (s, 1H), 7.11 (d, J = 8.4 Hz, 1H), 6.87 (d, J = 8.8 Hz, 1H),
189.4, 149.3, 146.9, 127.0, 126.0,
4.8; HRMS (TOF-MS): m/z calcd for C19H34BrO3Si2 [M + H]+
1.00 (s, 9H), 0.96 (s, 9H), 0.23 (s, 6H), 0.12 (s, 6H); 13C-NMR (CDCl3):
δ
124.6, 112.8, 25.1, 25.0, 17.7, 17.5,
445.1224. Found 445.1230.
−
4.7,
−
3.8. (E)-6-Bromo-2,3,3’,4’-tetrakis(tert-butyldimethylsilyloxy)stilbene (13)
n-BuLi (10.3 mL, 2.5 M in hexane, 25.81 mmol) was slowly added to a suspension of phosphonium
◦
salt
4
(19.47 g, 28.06 mmol) in dry THF (100 mL) at –78 C under a nitrogen atmosphere and the resulting
mixture was stirred for 30 min at –78 ◦C. Aldehyde
5
(5.0 g, 11.22 mmol) dissolved in dry THF (40 mL)
was added dropwise to the reaction mixture and the resulting solution stirred for another 1 h at –78 ◦C.
The reaction was quenched with a saturated aqueous solution of ammonium chloride. The mixture was
extracted with ethyl acetate (100 mL
over anhydrous sodium sulfate, filtered and concentrated in vacuo. The resulting residue was purified
by silica gel chromatography to give compound 13 (17.08 g, 78%, E:Z = 84:16) as a colorless oil. H-NMR
×
3) and the combined organic layers washed with brine, dried
1
(CDCl3):
δ 7.11 (d, J = 8.4 Hz, 1H), 7.02 (d, J = 8.8 Hz, 1H), 7.00 (d, J = 17.2 Hz, 1H), 6.94 (s, 1H), 6.85 (d,
J = 16.8 Hz, 1H), 6.84 (d, J = 8.4 Hz, 1H), 6.66 (d, J = 8.4 Hz, 1H), 1.04 (s, 18H), 1.02 (s, 9H), 0.95 (s, 9H),
0.28 (s, 6H), 0.26 (s, 12H), 0.09 (s, 6H).; 13C-NMR (CDCl3): δ 147.3, 146.9, 146.8, 145.6, 135.2, 131.8, 131.2,
125.4, 123.5, 121.0, 120.0, 119.1, 115.5, 26.2, 26.2, 26.0, 25.9, 18.8, 18.5, 18.5, 18.3,
−
3.5,
−
3.6,
−
4.0
−
4.1;
HRMS (TOF-MS): m/z calcd for C36H68BrO4Si4 [M + H]+ 779.3373. Found 779.3378.
3.9. (E)-2-(3,4-Bis((tert-butyldimethylsilyl)oxy)styryl)-3,4-bis((tert-butyldimethyl-silyl)oxy)benzaldehyde (2)
n-BuLi (6.7 mL, 2.5 M in hexane,◦16.82 mmol) was added to a solution of compound 13 (6.56 g,
8.41 mmol) in dry THF (40 mL) at
−
78 C under a nitrogen atmosphere. After the reaction mixture was
stirred for 30 min at
was slowly added to the mixture and stirred for another 30 min at
−
78 ◦C, dry N,N-dimethylformamide (6.50 mL, 84.08 mmol) in dry THF (10 mL)
−
78 ◦C until 13 was completely
consumed. The reaction was quenched with a saturated aqueous solution of ammonium chloride and
hydrochloric acid (1 M) added to adjust the pH to 1. The mixture was extracted with ethyl acetate
(
50 mL × 3) and the combined organic layers dried over anhydrous Na2SO4, filtered and concentrated
in vacuo. The resulting residue was purified by silica gel chromatography to give compound
76%) as a yellow oil. H-NMR (CDCl3):
2 (4.66 g,
1
δ 10.08 (s, 1H), 7.56 (d, J = 8.4 Hz, 1H), 7.18 (d, J = 16.4 Hz, 1H),
7.02 (s, 1H), 6.99 (d, J = 8.4 Hz, 1H), 6.91 (d, J = 8.4 Hz, 1H), 6.85 (d, J = 8.0 Hz, 1H), 6.46 (d, J = 16.4 Hz,
1H), 1.03 (s, 9H), 1.03 (s, 9H), 1.02 (s, 9H), 0.99 (s, 9H), 0.31 (s, 6H), 0.26 (s, 6H), 0.25 (s, 6H), 0.13 (s, 6H).;
13C-NMR (CDCl3):
δ
192.6, 152.9, 148.4, 147.9, 145.5, 139.4, 137.3, 131.4, 130.2, 124.0, 122.0, 121.4, 120.5,
120.4, 120.2, 27.1, 27.0, 26.9, 19.8, 19.4, 19.4, 19.3, 2.4, 2.6, 3.1, 3.1; HRMS (TOF-MS): m/z calcd
for C39H69O5Si4 [M + H]+ 729.4217. Found 729.4218.
−
−
−
−

Molecules 2019, 24, 999
8 of 11
3.10. (E)-methyl 3-(3,4-dihydroxyphenyl)acrylate (14)
Concentrated sulfuric acid (4 mL) was slowly added to a solution of caffeic acid (
6, 2.0 g,
11.10 mmol) in methanol (140 mL). The resulting mixture was heated at reflux for 1 h. Aqueous
NaOH (1 M) was added to quench the reaction and adjust the pH to 6–7. The aqueous phase was
extracted with CH2Cl2 (100 mL
anhydrous sodium sulfate, filtered and concentrated in vacuo. The resulting crude product (2.15 g)
was used in the next step without any further purification. H-NMR (CD3OD):
×
3). The combined organic layers were washed with brine, dried over
1
δ
7.51 (d, J = 16.0 Hz,
1H), 7.03 (s, 1H), 6.90 (d, J = 8.0 Hz, 1H), 6.77 (d, J = 8.4 Hz, 1H), 6.22 (d, J = 16.0 Hz, 1H), 4.98 (br s,
2H), 3.71 (s, 3H).; 13C-NMR (CD3OD): δ 167.5, 147.1, 144.6, 144.4, 125.5, 120.7, 114.2, 113.0, 112.6, 49.8;
HRMS (TOF-MS): m/z calcd for C10H11O4 [M + H]+ 195.0652. Found 195.0658.
3.11. (E)-methyl 3-(3,4-bis((tert-butyldimethylsilyl)oxy)phenyl)acrylate (15)
Imidazole (5.28 g, 77.51 mmol) and tert-butyldimethylchlorosilane (5.0 g, 33.22 mmol) were added
to the crude compound 14 (2.15 g, 11.07 mmol) dissolved in dry DMF (60 mL) at 0 ◦C. The reaction
mixture was allowed to warm up to room temperature and stirred overnight. The reaction was
quenched with brine, extracted with ethyl acetate (100 mL
dried over anhydrous sodium sulfate, filtered and concentrated in vacuo. The resulting residue was
purified by silica gel chromatography to afford compound 15 (4.74 g, 98%) as a white solid. H-NMR
×
3) and the combined organic layers were
1
(CDCl3):
J = 16.0 Hz, 1H), 3.77 (s, 3H), 0.98 (s, 9H), 0.97 (s, 9H), 0.20 (s, 6H), 0.20 (s, 6H).; 13C-NMR (CDCl3):
167.7, 149.4, 147.2, 144.8, 128.0, 122.2, 121.1, 120.4, 115.4, 51.5, 25.9, 25.9, 18.5, 18.4, 4.1, 4.1; HRMS
(TOF-MS): m/z calcd for C22H39O4Si2 [M + H]+ 423.2381. Found 423.2376.
δ 7.56 (d, J = 16.0 Hz, 1H), 7.01 (s, 1H), 6.99 (d, J = 5.6 Hz, 1H), 6.81 (d, J = 8.4 Hz, 1H), 6.23 (d,
δ
−
−
3.12. Methyl 3-(3,4-bis((tert-butyldimethylsilyl)oxy)phenyl)-2,3-dihydroxypropanoate (16)
Methyl caffeate (15, 2.30 g, 5.43 mmol) dissolved in tert-butyl alcohol (20 mL) was added to a
suspension of K2OsO4 (2.80 g, 7.60 mmol), K3[Fe(CN)6] (2.50 g, 7.60 mmol), K2CO3 (1.05 g, 7.60 mmol)
◦
and methanesulfonamide (10 mmol) in tert-butyl alcohol (40 mL) under a nitrogen atmosphere at 0 C.
The mixture was stirred for 28 h until 15 was completely consumed. The reaction was quenched with
a saturated solution of sodium sulfite. The mixture was allowed to warm up to room temperature
and stirred for 1 h. The reaction mixture was extracted with CH2Cl2 (100 mL
organic layers washed with brine, dried over anhydrous sodium sulfate, filtered and concentrated
×
3) and the combined
in vacuo. The resulting residue was purified by silica gel chromatography to give dihydroxyl ester
1
16 (2.10 g, 85%) as a white solid. H-NMR (CDCl3):
δ 6.91 (s, 1H), 6.81 (s, 2H), 4.85 (s, 1H), 4.30 (s,
1H), 3.77 (s, 3H), 3.12 (s, 1H), 2.70 (s, 1H), 0.99 (s, 9H), 0.98 (s, 9H), 0.19 (s, 6H), 0.19 (s, 6H); 13C-NMR
(CDCl3): δ 173.2, 146.8, 146.8, 133.0, 120.8, 119.5, 119.4, 74.9, 74.3, 52.6, 25.9, 18.4, 18.4, −4.1, −4.1.
3.13. Methyl 3-(3,4-bis((tert-butyldimethylsilyl)oxy)phenyl)-2-hydroxypropanoate (17)
Et3SiH (7.78 mL, 48.83 mmol) and trifluoroacetic acid (7.26 mL, 97.7 mmol) were added to a
◦
solution of dihydroxyl ester 16 (4.46 g, 9.77 mmol) dissolved in dry CH2Cl2 (40 mL) at 0 C. The reaction
mixture was allowed to warm up to room temperature and stirred for 10 h. The reaction was quenched
with a saturated aqueous solution of sodium bicarbonate and extracted with CH2Cl2 (80 mL
The combined organic layers were washed with brine, dried over anhydrous sodium sulfate, filtered
×
3).
and concentrated in vacuo. The resulting residue was purified by silica gel chromatography to give the
1
monohydroxy product 17 (3.70 g, 86%) as a white semi-solid. H-NMR (CDCl3):
δ 6.76 (d, J = 8.0 Hz,
1H), 6.75 (s, 1H), 6.66 (d, J = 8.0 Hz, 1H), 4.41 (br s, 1H), 3.74 (s, 3H), 3.00 (dd, J = 4.4 and 14.0 Hz, 1H),
2.89 (br s, 1H), 2.88 (dd, J = 6.4 and 14.0 Hz, 1H), 1.01 (s, 18H), 0.21 (s, 12H).; 13C-NMR (CDCl3):
174.5, 146.6, 145.8, 129.3, 122.5, 122.4, 120.9, 71.4, 52.2, 39.9, 26.0, 18.4, −4.1.
δ

Molecules 2019, 24, 999
9 of 11
3.14. Methyl 3-(3,4-bis((tert-butyldimethylsilyl)oxy)phenyl)-2-(2-(dimethoxyphosphoryl)acetoxy)-propanoate (3)
Dicyclohexylcarbodiimide (DCC, 2.58 g, 12.49 mmol) and a catalytic amount of
4-dimethylaminopyridine (0.19 g, 1.56 mmol) were added to a solution of the monohydroxy
product 17 (3.44 g, 7.81 mmol) and 2-(dimethoxyphosphoryl)acetic acid 18 (2.10 g, 12.49 mmol)
◦
in dry CH2Cl2 (40 mL) at 0 C under a nitrogen atmosphere, and the resulting mixture was then
stirred for 1 h. The reaction was quenched with brine, extracted with CH2Cl2 (50 mL
combined organic layers dried over anhydrous sodium sulfate, filtered and concentrated in vacuo.
×
3) and the
The resulting residue was purified by silica gel chromatography to give compound
3 (4.15 g, 90%) as a
1
white semi-solid. H-NMR (CDCl3):
δ 6.69 (d, J = 8.0 Hz, 1H), 6.64 (s, 1H), 6.60 (d, J = 8.4 Hz, 1H),
5.15 (dd, J = 4.8 and 7.2 Hz, 1H), 3.72 (d, J = 11.2 Hz, 3H), 3.70 (d, J = 11.2 Hz, 3H), 3.64 (s, 3H), 2.97
(d, J = 21.6 Hz, 2H), 2.94–3.00 (m, 2H), 0.93 (s, 9H), 0.92 (s, 9H), 0.14 (s, 6H), 0.13 (s, 6H).; 13C-NMR
(CDCl3) :
(d, J = 135.0 Hz), 25.9, 18.4, 18.4,
613.2388. Found 613.2386.
δ 169.4, 165.1 (d, J = 4.7 Hz), 146.7, 146.0, 128.4, 122.2, 122.2, 120.9, 74.1, 53.2, 52.2, 36.6, 32.9
−
4.1; HRMS (TOF-MS): m/z calcd for C26H47O9PSi2Na [M + Na]+
3.15. 2-(3,4-Bis(tert-butyldimethylsilyloxy)phenyl-1-methoxycarbonyl)ethyl(E)-3-((E)-2,3,30,40-tetrakis(tert
-butyldimethylsilyloxy)stylben-6-yl)acrylate (19)
NaH (30.9 mg, 0.77 mmol, 60% in mineral oil) was added to a solution of
in dry THF (4 mL) at 0 ◦C under a nitrogen atmosphere and the reaction mixture stirred for 30 min.
Aldehyde (0.427 g, 0.59 mmol) dissolved in dry THF (1.0 mL) was added slowly to the mixture
and stirred for another 3 h at 0 ◦C. The reaction was quenched with a saturated aqueous solution of
ammonium chloride, extracted with ethyl acetate (10 mL 3) and the combined organic layers dried
3 (0.453 g, 0.77 mmol)
2
×
over anhydrous sodium sulfate, filtered and concentrated in vacuo. The resulting residue purified by
1
silica gel chromatography to give compound 19 (0.60 g, 65%, E:Z = 77:23) as a yellow oil. H- NMR
(CDCl3):
δ 8.04 (d, J = 15.6 Hz, 1H), 7.18 (d, J = 8.0 Hz, 1H), 7.01 (d, J = 16.0 Hz, 1H), 6.99 (s, 1H), 6.96 (d,
J = 8.8 Hz, 1H), 6.81 (d, J = 8.4 Hz, 1H), 6.73 (s, 1H), 6.72 (d, J = 8.8 Hz, 1H), 6.63 (d, J = 7.2 Hz, 1H), 6.45
(d, J = 16.8 Hz, 1H), 6.32 (d, J = 15.6 Hz, 1H), 5.25 (br s, 1H), 3.69 (s, 3H), 2.97–3.08 (m, 2H), 1.06 (s, 18H),
0.98 (s, 18H), 0.96 (s, 18H), 0.28 (s, 6H), 0.23 (s, 12H), 0.19 (s, 12H), 0.11 (s, 6H).; 13C-NMR (CDCl3):
δ
169.9, 165.9, 148.3, 146.6, 146.4, 146.1, 145.5, 145.3, 144.2, 136.7, 133.0, 130.7, 128.6, 126.5, 121.9, 121.7,
121.1, 120.6, 120.4, 119.9, 119.1, 118.7, 115.0, 72.6, 51.6, 36.4, 25.7, 25.5, 25.5, 18.3, 18.0, 17.9, 3.8, 4.0,
4.0,
4.5; HRMS (TOF-MS): m/z calcd for C63H108O10Si6Na [M + Na]+ 1215.6450. Found 1215.6457.
−
−
−
−
3.16. 3-(3,4-Dihydroxyphenyl)-1-methoxy-1-oxopropan-2-y(E)-3-(2-((E)-3,4-dihydroxystyryl)-3,4-
dihydroxyphenyl)acrylate (1)
Et3N
•
3HF (0.81 g, 5.02 mmol) was added to a solution of 19 (0.50 g, 0.42 mmol) in dry pyridine
◦
(4 mL) at 0 C under a nitrogen atmosphere. The reaction mixture was stirred for 1 h and then diluted
with methanol. The mixture was extracted with CH2Cl2 (10 mL
layers washed with brine, dried over anhydrous sodium sulfate, filtered and concentrated in vacuo.
×
3) and the combined organic
The resulting residue was purified by silica gel chromatography to give (
±
)-methyl salvianolate A (
1
,
1
0.196 g, 92%) as a yellow solid. H-NMR (CD3OD):
δ 8.00 (d, J = 16.0 Hz, 1H), 7.07 (d, J = 16.4 Hz, 1H),
7.05 (d, J = 8.0 Hz, 1H), 6.98( s, 1H), 6.81 (d, J = 7.2 Hz, 1H), 6.69 (d, J = 8.0 Hz, 1H), 6.68 (d, J = 8.4 Hz
1H), 6.58 (s, 1H), 6.57 (d, J = 7.6 Hz, 1H), 6.56 (d, J = 16.0 Hz, 1H), 6.44 (d, J = 8.0 Hz, 1H), 6.21 (d,
J = 16.0 Hz, 1H), 5.11 (dd, J = 5.2 and 7.2 Hz, 1H), 3.61 (s, 3H), 2.89-2.94 (m, 1H).; 13C-NMR (CD3OD):
,
δ
170.8, 167.1, 146.9, 146.2, 145.3, 145.1, 144.7, 143.9, 143.0, 136.5, 130.0, 127.4, 127.1, 124.7, 120.6, 119.3,
119.0, 118.7, 116.0, 115.1, 115.0, 113.9, 113.5, 112.7, 73.3, 51.3, 36.5; HRMS (TOF-MS): m/z calcd for
C27H24O10Na [M + Na]+ 531.1262. Found 531.1262.
4. Conclusions
We have accomplished the first total synthesis of the polyphenol natural product, (
salvianolate A, using a convergent approach. The key features of the approach are the
±
)-methyl

Molecules 2019, 24, 999
10 of 11
Horner–Wadsworth–Emmons reaction and the protection of multiple hydroxyl groups using silyl
protecting groups, which avoid the side reactions often observed during the deprotection step.
Employment of the readily removable silyl protecting group in the present approach is a practicable
route to methyl salvianolate A and its derivatives. Further studies on the application of this approach
to natural products containing polyphenol units are in progress and will be disclosed in due course.
Supplementary Materials: NMR spectra for all synthetic compounds are available online.
Author Contributions: S.H. and X.W. conceived and designed the experiments; B.W. and L.W. performed the
experiments; Y.P. (Ying Peng), Y.P. (Yiying Pang) and H.X. analyzed the data and contributed reagents; S.H. and
X.W. wrote the manuscript.
Funding: This research was funded by the National Science Foundation of China, grant number 21062088
and 21562020; Science and Technology Plan Project of Jiangxi Province, grant number 20151BBG70028
and 20142BBE50006.
Acknowledgments: We thank the National Science Foundation of China (21062088, 21562020) and Science and
Technology Plan Project of Jiangxi Province (No. 20151BBG70028, 20142BBE50006) for funding support.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
Chinese Pharmacopoeia Commission. Pharmacopoeia of the People’s Republic of China; China Medical Science
Press: Beijing, China, 2015; Volume 1, p. 76.
Li, L.N.; Tan, R.; Chen, W.M. Salvianolic acid A: A new depside from roots of Salvia miltiorrhiza. Planta Med.
1984, 50, 227–228.
Liu, G.T.; Zhang, T.M.; Wang, B.E.; Wang, Y.W. Protective action of seven natural phenolic compounds
against peroxidative damage to biomembranes. Biochem. Pharmacol. 1992, 43, 147–152. [CrossRef]
Lin, T.J.; Zhang, K.J.; Liu, G.T. Effects of salvianolic acid a on oxygen radicals released by rat neutrophils and
on neutrophil function. Biochem. Pharmacol. 1996, 51, 1237–1241. [CrossRef]
Wang, X.J.; Wang, Z.B.; Xu, J.X. Effect of salvianic acid A on lipid peroxidation and membrane permeability
in mitochondria. J. Ethnopharmacol. 2005, 97, 441–445. [CrossRef] [PubMed]
Tsai, M.K.; Lin, Y.T.; Huang, Y.L. Effects of salvianolic acids on oxidative stress and hepatic fibrosis in rats.
Toxicol. Appl. Pharmacol. 2010, 242, 155–164. [CrossRef] [PubMed]
Wang, S.B.; Tian, S.; Yang, F.; Du, G.H. Cardioprotective effect of salvianolic acid A on isoproterenol-induced
myocardial infarction in rats. Eur. J. Pharmacol. 2009, 615, 125–132. [CrossRef] [PubMed]
Jiang, B.H.; Li, D.F.; Deng, Y.P.; Teng, F.K.; Chen, J.; Xue, S.; Kong, X.Q.; Luo, C.; Shen, X.; Jiang, H.L.;
et al. Salvianolic acid A, a novel matrix metalloproteinase-9 inhibitor, prevents cardiac remodeling in
spontaneously hypertensive rats. PLoS ONE 2013, 8, e59621. [CrossRef] [PubMed]
Fan, H.-Y.; Fu, F.-H.; Yang, M.-Y.; Xu, H.; Zhang, A.-H.; Liu, K. Antiplatelet and antithrombotic activities of
salvianolic acid A. Thromb. Res. 2010, 126, e17–e22. [CrossRef] [PubMed]
9.
10. Zheng, X.W.; Chen, S.Y.; Yang, Q.T.; Cai, J.X.; Zhang, W.P.; You, H.S.; Xing, J.F.; Dong, Y.L. Salvianolic acid A
reverses the paclitaxel resistance and inhibits the migration and invasion abilities of human breast cancer
cells by inactivating transgelin 2. Cancer Biol. Ther. 2015, 16, 1407–1414. [CrossRef] [PubMed]
11. Zhang, Z.-F.; Peng, Z.-G.; Gao, L.; Dong, B.; Li, J.-R.; Li, Z.-Y.; Chen, H.-S. Three new derivatives of anti-HIV-1
polyphenols isolated from Salvia yunnanensis. J. Asian Nat. Prod. Res. 2008, 10, 391–396. [CrossRef] [PubMed]
12. Kan, S.D.; Chen, Z.Z.; Shao, L.; Li, J.-A. Transformation of salvianolic acid b to salvianolic acid A in aqueous
solution and the in vitro liver protective effect of the main products. J. Food Sci. 2014, 79, 499–504. [CrossRef]
[PubMed]
13. Chen, W.Q.; Zhou, H.G.; Li, J.N.; Sun, B. Synthetic Method for Salvianolic Acid A. CN Patent 105085267A, 2015.
14. Zheng, Y.; Song, W.-B.; Xuan, L.-J. The asymmetric total synthesis of (t)-salvianolic acid A. Tetrahedron 2016
72, 5047–5050. [CrossRef]
,
15. Xu, K.; Liu, H.; Liu, D.L.; Sheng, C.; Shen, J.F.; Zhang, W.B. Synthesis of (+)-salvianolic acid A from sodium
Danshensu. Tetrahedron 2018, 74, 5996–6002. [CrossRef]
16. Dalla, V.; Cotelle, P. The total synthesis of salvianolic acid F. Tetrahedron 1999, 55, 6923–6930. [CrossRef]

Molecules 2019, 24, 999
11 of 11
17. Huang, S.P.; Wang, J.W.; Wang, L.P.; Xu, T.X.; Li, X.T.; Len, X.; Wang, X.J. Method for Synthesizing Natural
Product Salvianolic Acid A Methyl Ester. CN Patent 106905154A, 2017.
18. Hwu, J.R.; Varadaraju, T.G.; Abd-Elazem, I.S.; Huang, R.C.-C. First total syntheses of oresbiusins A and B,
their antipodes, and racemates: Configuration revision and anti-HIV activity. Eur. J. Org. Chem. 2012, 25,
4684–4688. [CrossRef]
19. Jiang, Z.-Y.; Wang, Y.-G. A mild, efficient and selective deprotection of t-butyldimethylsilyl-protected phenols
using cesium carbonate. Tetrahedron Lett. 2003, 44, 3859–3861. [CrossRef]
20. Ankala, S.V.; Fenteany, G. Selective deprotection of either alkyl or aryl silyl ethers from aryl, alkyl bis-silyl
ethers. Tetrahedron Lett. 2002, 43, 4729–4732. [CrossRef]
21. Crouch, R.D.; Stieff, M.; Frie, J.L.; Cadwallader, A.B.; Bevis, D.C. Selective Deprotection of Silyl-Protected
Phenols Using Solid NaOH and a Phase Transfer Catalyst. Tetrahedron Lett. 1999, 40, 3133–3136. [CrossRef]
22. Duynstee, H.I.; de Koning, M.C.; Ovaa, H.; van der Marel, G.A.; van Boom, J.H. Synthesis of verbascoside: A
dihydroxyphenylethyl glycoside with diverse bioactivity. Eur. J. Org. Chem. 1999, 10, 2623–2632. [CrossRef]
Sample Availability: Samples are not available from the authors.
©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).