Synthesis of icariin from kaempferol through regioselective methylation and para-Claisen - Cope rearrangement

Article abstract of DOI:10.3762/bjoc.11.135

The hemisynthesis of the naturally occurring bioactive flavonoid glycoside icariin (1) has been accomplished in eleven steps with 7% overall yield from kaempferol. The 4?-OH methylation of kaempferol, the 8-prenylation of 3-O-methoxymethyl-4?-O-methyl-5- O-prenyl-7-O-benzylkaempferol (8) via para-Claisen-Cope rearrangement catalyzed by Eu(fod)₃ in the presence of NaHCO₃ , and the glycosylation of icaritin (3) are the key steps.

Full text of DOI:10.3762/bjoc.11.135

Synthesis of icariin from kaempferol through regioselective
methylation and para-Claisen–Cope rearrangement
Qinggang Mei1,2, Chun Wang1, Zhigang Zhao3, Weicheng Yuan2 and Guolin Zhang*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1220–1225.
1Chengdu Institute of Biology, Chinese Academy of Sciences,
Chengdu 610041, China, 2Chengdu Institute of Organic Chemistry,
Chinese Academy of Sciences, Chengdu 610041, China and 3College
of Chemistry and Environmental Protection Engineering, Southwest
University for Nationalities, Chengdu 610041, China
doi:10.3762/bjoc.11.135
Received: 12 May 2015
Accepted: 26 June 2015
Published: 20 July 2015
Email:
Associate Editor: J. S. Dickschat
Guolin Zhang* - zhanggl@cib.ac.cn
© 2015 Mei et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
Claisen–Cope rearrangement; flavonol; icariin; prenylation;
regioselectivity
Abstract
The hemisynthesis of the naturally occurring bioactive flavonoid glycoside icariin (1) has been accomplished in eleven steps with
7% overall yield from kaempferol. The 4′-OH methylation of kaempferol, the 8-prenylation of 3-O-methoxymethyl-4′-O-methyl-5-
O-prenyl-7-O-benzylkaempferol (8) via para-Claisen–Cope rearrangement catalyzed by Eu(fod)3 in the presence of NaHCO3, and
the glycosylation of icaritin (3) are the key steps.
Introduction
The plants of the Genus Epimedium, also known as “Yin-Yang- anti-oxidative [6], anti-inflammatory [7], anti-osteoporotic [8],
Huo”, were used in traditional Chinese medicine and are anticancer [9], neuroprotective [10], angiogenesis stimulating
believed to invigorate the kidney and enhance the “Yang”. For [11], testosterone mimetic [12], antidepressant-like [13] and
more than one thousand years, some plants of Epimedium have multidrug resistance reversal [14] activities.
been widely used in China to treat cardiovascular diseases,
amnesia, arthritis, asthenia, impotence, infertility, lumbago and Although icariin exerts a variety of important bioactivities,
other chronic illnesses [1,2]. Icariin (1), a 3,7-diglyco- reports concerning its synthesis are very scarce. One Chinese
sylflavone (Figure 1), is recognized as the major pharmacologi- patent reported a 15-step total synthesis of icariin from benzyl
cally active ingredient of these plants [3,4], and has been used alcohol with 0.6% overall yield [15]. The 11-step synthesis of
as the index for quality control of the herbs and relative drug icaritin, starting from 2,4,6-trihydroxyacetophenone via
preparations [5]. Icariin, icariside I (2), and their aglycone, icar- microwave-assisted Claisen rearrangement reaction as the key
itin (3) (Figure 1), possess multiple biological activities, such as step, was succeeded with an overall yield of 23% [16]. In view
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Beilstein J. Org. Chem. 2015, 11, 1220–1225.
a new approach to icaritin and then icariin through semi-syn-
thesis from the commercially available natural product
kaempferol. Our previously developed regioselective methyl-
ation of kaempferol [19], Europium(III)-catalyzed para-
Claisen–Cope rearrangement and the bis-glycosylation are the
key features of this linear synthesis. Previously, we succeeded
in the selective methylation of 4′-OH in kaempferol. In this
work, we focus on developing an efficient procedure for the
selective prenylation of flavonols for facile access to icariin (1).
Results and Discussion
Figure 1: Structures of icariin (1), icariside I (2) and icaritin (3).
Our synthetic approach to 1 commenced with the preparation of
aglycone 3, as illustrated in Scheme 1. First, 7-O-benzylka-
empferide (6) was easily obtained from kaempferol through
tetraacetylation, followed by benzylation at C-7 and selectively
methylation, according to our previously developed procedure
[19]. In order to methylate exclusively the 4′-OH in kaempferol,
we initially attempted to use methoxymethyl (MOM) as 7-OH
of the long synthetic routes, tedious work-up and harsh reaction
conditions, an alternative access to icariin (1) and related com-
pounds is demanded for large-scale production and practical
application.
Our interest in flavonoids with activities such as estrogen protecting group, but this method could not provide an ideal
biosynthesis regulation and PDE inhibition, prompted us to seek yield in the 4′-OH selective methylation and in subsequent
for a practically synthetic approach to icariin. Previously our Claisen–Cope rearrangement.
group prepared icaritin from phloroglucin through an
Algar–Flynn–Oyamada reaction and europium-promoted The resulting free 3-OH in 6 is protected with an orthogonal
prenylation [17]. However, a number of challengable problems protecting group to give 3-O-methoxymethyl-4′-O-methyl-7-O-
such as strict conditions, numerous byproducts and poor yields, benzylkaempferol (7) in 93% yield. Then introduction of a
were calling for our continuous efforts to overcome these requisite prenyl ether moiety at C-5 of 7, with 3,3-dimethylallyl
hitches [18]. As a continuation of this program, herein we report bromide in the presence of potassium carbonate and
Scheme 1: Reagents and conditions: (a) Ac2O, pyridine, 94%; (b) BnBr, KI, K2CO3, acetone, 85%; (c) Me2SO4, K2CO3, acetone, MeOH, 82%;
(d) MOMCl, N,N-diisopropylethylamine (DIPEA), CH2Cl2, 93%; (e) 3,3-dimethylallyl bromide, 18-crown-6, K2CO3, acetone, 86%; (f) Eu(fod)3,
NaHCO3, PhCl, 85 °C, 61%; (g) MeOH, 3 M HCl (aq), reflux, 95%; (h) Pd/C, 1,4-cyclohexadiene, MeOH, 84%.
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18-crown-6, formed 8, whose all hydroxy groups were the para/ortho-selectivity was still unsatisfactory, even when
protected for the rearrangement accompanied in the next step. It prolonging the reaction time to 36 h or raising the temperature
is noticeable that this compound is slightly less stable in solu- to 100 °C. By carefully investigating the two routes in
tion, easily decomposing into 7 and 11 (Scheme 2), especially Scheme 3, we assumed that an appropriate base is in favor of
under acidic conditions.
the [1,5]H σ migration in route 2, as well as lowering the acidity
of Eu(fod)3 to prevent 12b from cyclization in route 1, and
The Claisen rearrangement is commonly accepted as an effi- therefore enhances the proportion of the para-rearrangement
cient method for the preparation of C-isopentenyl. In the case of product 8. Obviously, strong alkali could lead to heterocycle
flavonoids, control of the regioselectivity of ortho (C6)/para cleavage of the flavonoid skeleton. Gratifyingly, when prenyl
(C8)-rearranged products is still remained a challenging issue ether 8 was treated with catalytic amounts of Eu(fod)3 and
[20]. Once the prenyl ether 8 was obtained, attention was NaHCO3 in PhCl at 85 °C for 24 h (Table 1), the Claisen para-
fastened on the para-Claisen–Cope rearrangement to acquire rearranged product was obtained selectively in 61% yield with
the 8-prenylated product. Two pathways have been developed the ratio 9 to 12 of 2.1:1. With other bases such as K2CO3 or
for the rearrangement: direct heating at about 217 °C [17,21] DIPEA, the results were very different, probably due to the
and heating at about 60 °C in the presence of Eu(fod)3 [22,23]. partial inactivity of Eu(fod)3 under these conditions. It is note-
When we adapted the former path, reflux of 8 in N,N-diethyl- worthy that NaHCO3 only affected the 9/12 ratio, but not the
aniline for 3 h led to complete consumption of 8 and gave an total amount of C-isoprenoid flavonols 9 and 12, which was
intractable product mixture, probably due to the prevalence of found to stay around 90%. This to some extent, implies that our
the 5-O-prenyl chain elimination with no rearrangement. Then hypothesis on using appropriate base to hinder the Route 1 and
we turned to try the latter method with 10 mol % Eu(fod)3 as to promote the Cope rearrangement step from C-6 to C-8 could
the catalyst at 60 °C in dry CHCl3 (Table 1), in which the be true. Therefore, the combination of Eu(fod)3 and NaHCO3 is
Claisen rearrangement of 8 indeed took place. Unlike reported preferable for the Claisen–Cope rearrangement from 5-O- to
results [24-26], we obtained the ortho-rearranged product, 8-C-prenyl products.
which was not the 6-(1,1-dimethylallyl) product 12a, but an
inseparable epimeric mixture of the dihydrofuro-flavonol 12, Exposure of compound 9 to dilute methanolic HCl at reflux
presumably resulted from the cyclization of the 6-(1,1-di- resulted in the cleavage of the methoxymethyl group, giving
methylallyl) chain with the 5-hydroxy group (Scheme 3). The rise to 7-O-benzylicaritin (10). Removal of the benzyl group in
ratio of the para-rearranged product 9 to 12 was 0.7:1. When 10 resulting in 3 without affecting the double bond of the prenyl
PhCl was used as solvent, the rearrangement of 8 at 85 ºC for residue was another challenge. The hydrogenation of 10 with
24 h allowed the isolation of 9 and 12 in a 0.9:1 ratio (Table 1). H2–Pd/C using EtOAc/MeOH (1:1) as solvent at 10 °C for 3 h,
Although the proportion of 9 and the total yield were improved, followed by column chromatography and recrystallization,
Scheme 2: Decomposition of 8.
Table 1: Rearrangement of 5-O-prenylflavone 8.
Methoda
T (ºC)
t (h)
Products (%)b
Ratio 9:12
Total 9 + 12
CHCl3, Eu(fod)3
60
85
85
24
24
24
9 (33), 12 (46)
9 (42), 12 (47)
9 (61), 12 (29)
0.7:1
0.9:1
2.1:1
79%
89%
90%
PhCl, Eu(fod)3
PhCl, Eu(fod)3, NaHCO3
aEu(fod)3: 10 mol %; NaHCO3: 100 mol %. bIsolated yield.
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Beilstein J. Org. Chem. 2015, 11, 1220–1225.
Scheme 3: Claisen rearrangement of flavonol 8.
provided 3 in 39% yield along with a large amount of the initially attempted the 7-OH glycosylation with tetra-O-acetyl-
byproduct with reduced double bond. Then we tried a transfer glucopyranosyl bromide (15) [30] as the donor and SrCO3 or
hydrogenolysis method using ammonium formate/10% Pd/C AgNO3 as an activator in pyridine or quinoline [15,31], but it
[27], but the result was not improved. The desired product was unsuccessful. TLC monitoring revealed that most of icartin
icaritin (3) was finally obtained in 84% yield from 10 when we was still unreacted even prolonging the reaction time to 48 h.
used 1,4-cyclohexadiene/10% Pd/C in MeOH [28]. Hence, we To our delight, switching the solvent to DMF/CHCl3 and the
optimized a convenient eight-step sequence from kaempferol activator to Ag2CO3 made the reaction to proceed smoothly,
to 3 through regioselective methylation and a domino resulting in the synthesis of 13. The yellow powder 13 was
Claisen–Cope rearrangement that delivered 3 in 26% overall treated with tri-O-acetylrhamnopyranosyl bromide (16) [30] in
yield. The NMR spectra of synthetic 3 (see Supporting Informa- CH2Cl2 using Ag2O as activator, following a known procedure
tion File 1 for experimental and spectral data) were in accor- [32]. Compound 14 was easily separated by normal silica gel
dance with the reported ones [16,29].
column chromatography, with a yield of 31% from 3.
With icaritin (3) in hand, the selective glycosylation was The final procedure in completing the synthesis of icariin (1) is
investigated (Scheme 4). The alkylation of OH in kaempferol the removal of all acetyl groups in 14 (Scheme 4). In contrast to
followed a specific reactivity order: 7 > 4′ > 3 >> 5 [19]. We the traditional method for deacetylation under basic conditions
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Scheme 4: Reagents and conditions: (a) 15, DMF/CHCl3, Ag2CO3, molecular sieves (4 Å, powder); (b) 16, CH2Cl2, Ag2O, molecular sieves (4 Å
powder), 31% for 2 steps; (c) NH3 (g), MeOH, 94%; (d) NH3 (g), MeOH, 63% for 2 steps.
such as with CH3ONa [15] or K2CO3 [33] in CH3OH, employ-
Supporting Information
ing methanolic ammonia (7.0 M) for 3 h at room temperature
effectively suppressed the cleavage of the sugar moiety [34] and
Supporting Information File 1
gave the target compound 1 in high conversion. Icariside I (2)
was also obtained using the same procedure from compound 13.
Thus, icariin (1) and icariside I (2) were synthesized from
kaempferol with 7% and 16% overall yields, respectively.
Experimental section and copies of NMR, ESI-HRMS,
HMBC, HSQC, COSY and NOESY spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-135-S1.pdf]
The compounds 14, 2 and 1 were fully characterized by ESI-
HRMS, 1H NMR, 13C NMR, 1H,1H COSY, 1H,13C HMBC/
HSQC and IR (see Supporting Information File 1 for experi-
mental and spectral data). In icariin (1), the configurations of
the anomeric C-atoms of rhamnose and glocuse were α and β on
the basis of the anomeric proton signals at δ 5.27 and 5.00 with
J = 1.3 Hz and 7.4 Hz, respectively. The NMR data of icariin
(1) were in agreement with those reported [29,35] and with the
spectra of the authentic, naturally derived sample.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (No. 21172214) is gratefully acknowledged.
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