Development of a concise synthetic approach to access oroxin A

Article abstract of DOI:10.1039/c4ra08573f

A novel environment-friendly method to access bioactive oroxin A through a one-pot/two-step process from naturally abundant and inexpensive baicalin is described. The procedure presented here has several advantages including clean, one-pot, synthetic ease, and large-scale feasibility. This work also provides a model strategy for rapid and diverse access to natural molecules sharing the common skeleton of this family.

Full text of DOI:10.1039/c4ra08573f

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Development of a concise synthetic approach to
access oroxin A†
ab
Cite this: RSC Adv., 2014, 4, 45151
Guihua He,a Cailong Li,a Longrong Dong,a Xiaobo Xie,a Jianlei Wu,a
*
Haijun Chen,
a
b
*
Yu Gao and Jia Zhou
Received 12th August 2014
Accepted 10th September 2014
DOI: 10.1039/c4ra08573f
www.rsc.org/advances
A novel environment-friendly method to access bioactive oroxin A similar bioactivity, stability and improved solubility, are more
through a one-pot/two-step process from naturally abundant and efficacious than their aglycones in pharmaceutical studies.11
inexpensive baicalin is described. The procedure presented here has However, in contrast to an extensive investigation on avo-
several advantages including clean, one-pot, synthetic ease, and noids, avonoid glycosides have not yet been thoroughly
large-scale feasibility. This work also provides a model strategy for explored, owing to the limited preparative accessibility and the
rapid and diverse access to natural molecules sharing the common lack of an efficient and convenient synthetic methodology.
skeleton of this family.
Among them, oroxin A (1, baicalein-7-O-glucoside, Fig. 1) was
identied from a traditional herbal medicine of Asian coun-
tries, Oroxylum indicum.12,13 Accumulating studies have
demonstrated the benecial biological effects of oroxin A
including antioxidant, anticancer, antibacterial and anti-viru-
lence properties.14–17 Despite the promising biological evidence,
extensive investigation including in vivo toxicological study and
efficacy evaluation of oroxin A is limited because of scarce
availability.
In order to obtain sufficient oroxin A for pharmacological
evaluation, several groups have made substantial efforts in
recent years. Generally, oroxin A was previously produced either
by natural product purication or through biological engi-
neering. For instance, oroxin A can be isolated as one of the
major constituents in the seeds of Oroxylum indicum by high-
speed counter-current chromatography (HSCCC).18–20 However,
the presence of strong polar hydroxyl groups in oroxin A results
in a low solubility in organic solvents. Hence, the separation
and purication of oroxin A by HSCCC using conventional
solvents is very difficult. To overcome this limitation, Liu et al.
established a preparative HSCCC by using ionic liquids as the
modier of the two-phase solvent system.21 Despite application
of ionic liquids in separation procedure makes it possible to
produce oroxin A in a relatively large scale; however, the cost of
Plant-derived natural products have the potential to treat
various human diseases. More than 65% people in the world
rely primarily on traditional herbal medicines for the health. It
is estimated that above 50% of current clinical drugs are derived
from natural products.1–3 Notably, there is increasing interest
on the development of low toxic natural antioxidants as prom-
ising leads especially those compounds that are widely
distributed in the plant kingdom.4 Polyphenolic avonoids as a
large group in dietary plants exhibit a diverse range of phar-
macological and biological properties including anticancer,
antioxidant, antithrombotic, antiplatelet, and antibacterial
effects.5 Till now, more than 5000 polyphenolic avonoids have
been isolated and characterized, which are classied into over
10 subgroups.6 The multifunctional properties of these prom-
ising natural products are due to the presence of multiple
oxygenated moieties.7,8 Accumulating evidence has demon-
strated that avonoids exhibit potential health protective effects
in vivo, resulting in several lead compounds in preclinical and
clinical trials.9,10 Besides the avonoids having favorable phar-
macological effects, avonoid glucosides as another subgroup
of avonoids also gained much of interest in the research
recently. It was found that glycosylated avonoids possessing
aCollege of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China. E-mail:
chenhaij@gmail.com; Fax: +86-591-22866227; Tel: +86-591-22866234
bChemical Biology Program, Department of Pharmacology and Toxicology, University
of Texas Medical Branch, Galveston, Texas 77555, USA. E-mail: jizhou@utmb.edu;
Fax: +1-409-772-9648; Tel: +1-409-772-9748
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra08573f
Fig. 1 Chemical structures of oroxin A (1) and baicalein (2).
RSC Adv., 2014, 4, 45151–45154 | 45151
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natural purication limits it further application. To address this product (only $60 per kg), which is abundantly available as the
issue, Sohng and coworkers developed the biotransformation of major ingredient extracted from traditional Chinese medicine
baicalein (2, Fig. 1) into oroxin A by applying engineered Scutellaria baicalensis.23 Our efficient method follows the semi-
Escherichia coli (E. coli).22 Glucosylation of baicalein in engi- synthetic route using baicalin as the starting material.
neered E. coli might be benecial for the large scale industrial
As shown in Scheme 2, key intermediate (4) was prepared in
production of oroxin A; however, various uncertain factors high yield by treating baicalin with catalytic amount of
including time-consuming, complex of products, low yield and concentrated H2SO4 in the solution of methanol without puri-
high cost in biological engineering still make it far from prac- cation. Sufficient amount of methanol was crucial for the
tical application.
outcome of this reaction due to the low solubility of baicalin.
Chemical synthesis remains to be an ideal option to yield The formation of gel/colloidal material becomes feasible when
pure desired natural products and plenty of key intermediates the concentration of key intermediate is high. In this regard, the
for further investigation of structure–activity relationships and concentration of the starting material of baicalin in methanol
potential applications in drug discovery. To this end, we report should not be less than 25 mg mLꢀ1. This phenomenon indi-
the chemical synthesis of oxorin A by a facile and efficient cates that baicalin and this key intermediate may be a good
synthetic strategy.
natural material for cosmetics and food industry.
According to the chemical structure of oroxin A, baicalin (3)
With the key intermediate 4 in hand, we set out to nd a
has the similar structure which contains a glucuronide moiety reliable condition for the simple conversion of 4 to the desired
at 7-O-position of baicalein, while, oroxin A is the glucoside of product oroxin A. Sodium borohydride (NaBH4) is the tradi-
baicalein (Scheme 1). Notably, baicalin is a cheap natural tional reagent for the reduction of esters. We utilized this
reducing agent in order to get a feasible and scalable process.
The key intermediate 4 is suspended in methanol and cooled to
0 ꢁC, and then 1.0 equiv. of NaBH4 is added portionwise (Table
1, entry 1). By reaction of compound 4 with NaBH4, we observed
that the reduction of glucuronic acid methyl ester was capable
ofꢁaffording the desired product aer 1 h of reaction time at
0 C. Despite the yield is only 14%, it was noteworthy that the
desired product was smoothly generated and about 85% of the
starting material was recovered. Since this key reduction step
appeared to be applicable for cost-effective and fast synthesis,
Scheme 1 Synthesis of oroxin A (1) from baicalin (3).
the reduction of 4 by using NaBH4 was investigated under
different reaction conditions in terms of time and temperature
as well as a variety of solvents (Table 1).
Based upon our preliminary result mentioned above, we
extended the reaction time from 1 h to 24 h. However, this
reduction reaction was still incomplete and the reductive
product was a complex reaction mixture of several components
(entry 2). It seems that the desired product might decompose
under this condition for a long time. In order to improve the
yield and to avoid the undesired reaction, we added 5.0 equiv. of
NaBH4 and the reaction was stirred at 0 ꢁC for 1 h or 4 h,
Scheme 2 Reagents and conditions: (a) cat. H2SO4, MeOH, reflux; (b) respectively. In the case of 1 h, no undesired byproduct was
NaBH4, different solvents, 0 ꢁC to 25 ꢁC.
Table 1 Optimization of the NaBH4 reduction condition
Entry
NaBH4 (equiv.)
Solvent
T (ꢁC)
Time (h)
Conversiona (%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
1.0
1.0
5.0
5.0
1.0
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
0
0
0
1
24
1
4
1
1
1
1
1
15%
51%
40%
60%
36%
76%
82%
69%
2%
14%
19% (decomposed)
38%
49%
34%
69%
75%
64%
Trace
0
25
25
25
25
25
25
5.0
10.0
10.0
10.0
10.0
THF
MeOH
24
91%
15% (decomposed)
a
The conversion is based on the amount of isolated starting material. b Isolated yield.
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detected on silica gel TLC (entry 3), while the reaction was still 12.57 (s, 1H), 8.59 (s, 1H), 8.08 (d, 2H, J ¼ 8.0 Hz), 7.57–7.63 (m,
incomplete aer a reaction time of 4 h and some small amounts 3H), 7.06 (s, 1H), 7.02 (s, 1H), 5.42 (d, 1H, J ¼ 4.0 Hz), 5.16 (d,
of byproducts can be found in TLC (thin-layer chromatography) 1H, J ¼ 4.0 Hz), 5.11 (d, 1H, J ¼ 4.0 Hz), 5.02 (d, 1H, J ¼ 8.0 Hz),
(entry 4). We decided to carry out the reduction at 25 ꢁC with 1.0 4.68 (t, 1H, J ¼ 4.0 Hz), 3.74–3.78 (m, 1H), 3.48–3.52 (m, 2H),
equiv. of NaBH4. As expected, this reaction proceeded faster at 3.18–3.24 (m, 1H). 13C NMR (100 MHz, DMSO-d6): 183.1, 164.0,
elevated temperature (entry 5). Increasing the amount of NaBH4 152.1, 149.7, 147.0, 132.5, 131.3, 131.1, 129.6, 126.9, 106.6,
resulted in signicant improving the synthetic effect, especially 105.2, 101.5, 94.8, 77.8, 76.4, 73.7, 70.2, 61.2. HRMS (ESI) calcd
when we used 10.0 equiv. of NaBH4, the isolated yield was up to for C21H19O10 431.0984 (M ꢀ H)ꢀ, found 431.0973.
75% (entry 6 and entry 7). Among different solvents including
EtOH and THF, the results showed that MeOH was superior for
this reaction (entries 7–9). In this regard, the evaporation of
One-pot large-scale synthesis of oroxin A (1)
To the solution of baicalin (10.0 g, 22.4 mmol) in methanol (400
mL) was added catalytic amount of sulfuric acid (0.05 mL), and
the mixture was heated at reux temperature for 4 h. The
mixture was cooled to 0 ꢁC and then NaBH4 (8.51 g, 224 mmol)
was added portionwise in 1 h. Aer the addition was complete,
the mixture was stirred for an additional 2 h at 25 ꢁC and then
quenched with 200 mL of 10% AcOH/H2O. The solution was
evaporated to obtain the crude product. The residue was sus-
pended in H2O/MeOH (500 mL/100 mL, v/v 5/1), and the reac-
tion mixture was acidied to pH 4 by dropwise addition of 1 N
HCl (aq). The desired product was precipitated and ltered. The
solid was washed with cold water and dried in vacuo to yield 6.97
g (72%) of oroxin A (1). The structural characterization data are
same as those described above.
methanol in the rst step seems unnecessary. Therefore, a more
straightforward procedure was possible. Aer reaction of bai-
calin with cat. H2SO4 in MeOH, followed by reduction with
NaBH4 and subsequent workup procedure by adding appro-
priate amount of 10% AcOH/H2O, the mixture was concentrated
to afford the desired product in an excellent yield.
Conclusions
In summary, a facile and efficient synthetic approach to access
oroxin A from inexpensive baicalin has been developed. Opti-
mized NaBH4 reduction as the key step aer methyl esterica-
tion of baicalin, resulted in a satisfactory yield of oroxin A.
Development of this method will greatly facilitate the biological
studies of oroxin A that has demonstrated great potentials in
food and pharmaceutical applications. The reported synthetic
protocol is concise and may be readily extended to the synthesis
of other avonoid glycosides.
Conflicts of interest
The authors declare no competing nancial interest.
Experimental section
Synthesis of baicalin methylate (4)24
Acknowledgements
This work was supported by the Technology Development
Foundation of Fuzhou University (Project Numbers 2013-XQ-8
and 2013-XQ-9), grants P30 DA028821, R21 MH093844 from the
National Institutes of Health, R. A. Welch Foundation Chem-
istry and Biology Collaborative Grant from the Gulf Coast
Consortia (GCC), John Sealy Memorial Endowment Fund,
Institute for Translational Sciences (ITS), and the Center for
Addiction Research (CAR) at UTMB.
To the solution of baicalin (2.0 g, 4.48 mmol) in methanol (80
mL) was added catalytic amount of sulfuric acid (0.01 mL), and
the solution was heated at reux temperature for 3 h. The
mixture was cooled at room temperature, and the solvent was
concentrated under reduced pressure to give the key interme-
diate as a yellow solid for direct use in the next step without
further purication (2.05 g, 99%).
Synthesis of oroxin A (1)
Notes and references
The key intermediate baicalin methylate (4) (0.916 g, 2.0 mmol)
was suspended in methanol (40 mL) and cooled to 0 C. Then
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