Selective methylation of kaempferol via benzylation and deacetylation of kaempferol acetates

Article abstract of DOI:10.3762/bjoc.11.33

A strategy for selective mono-, di- and tri-O-methylation of kaempferol, predominantly on the basis of selective benzylation and controllable deacetylation of kaempferol acetates, was developed. From the selective deacetylation and benzylation of kaempferol tetraacetate (1), 3,4′,5,-tri-O-acetylkaempferol (2) and 7-O-benzyl-3,4′5,-tri-O-acetylkaempferol (8) were obtained, respectively. By controllable deacetylation and followed selective or direct methylation of these two intermediates, eight O-methylated kaempferols were prepared with 51-77% total yields from kaempferol.

Full text of DOI:10.3762/bjoc.11.33

Selective methylation of kaempferol via benzylation and
deacetylation of kaempferol acetates
Qinggang Mei1,2,3, Chun Wang2, Weicheng Yuan1 and Guolin Zhang*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 288–293.
1Chengdu Institute of Organic Chemistry, Chinese Academy of
Sciences, Chengdu 610041, China, 2Chengdu Institute of Biology,
Chinese Academy of Sciences, Chengdu 610041, China and
3University of Chinese Academy of Sciences, Beijing 100049, China
doi:10.3762/bjoc.11.33
Received: 20 December 2014
Accepted: 05 February 2015
Published: 25 February 2015
Email:
Guolin Zhang* - zhanggl@cib.ac.cn
Associate Editor: S. Bräse
* Corresponding author
© 2015 Mei et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
benzylation; deacetylation; kaempferol; methylation; regioselectivity
Abstract
A strategy for selective mono-, di- and tri-O-methylation of kaempferol, predominantly on the basis of selective benzylation and
controllable deacetylation of kaempferol acetates, was developed. From the selective deacetylation and benzylation of kaempferol
tetraacetate (1), 3,4′,5,-tri-O-acetylkaempferol (2) and 7-O-benzyl-3,4′5,-tri-O-acetylkaempferol (8) were obtained, respectively. By
controllable deacetylation and followed selective or direct methylation of these two intermediates, eight O-methylated kaempferols
were prepared with 51–77% total yields from kaempferol.
Introduction
Kaempferol [2-(4-hydroxyphenyl)-3,5,7-trihydroxychromen-4- and cytotoxic [10] activities. 7-O-Methylkaempferol (rhamno-
one] (Figure 1) and its derivatives are widely distributed in citrin) shows a very strong inhibition against platelet aggrega-
plants such as beans, broccoli, strawberries, teas, and propolis tion induced by sodium arachidonate [11], and antibacterial
[1,2]. They are well known to have a wide range of pharmaco- [12], anti-atherogenic [13], anti-inflammatory and antispas-
logical activities [3]. Recent studies have indicated that diets modic activities [14]. 4′-O-Methylkaempferol (kaempferide) is
abundant in kaempferol-rich fruits and vegetables may have reported to reduce hypoxia-inducible factor (HIF)-1-dependent
prophylactic effects against the development of coronary heart luciferase expression [15], and possess potent antiproliferative
disease and certain cancers [4,5]. Among them, methylated effects on the human colon cancer cell line Caco-2 [16]. 3,7-Di-
derivatives of kaempferol have attracted attention for a long O-methylkaempferol (kumatakenin) demonstrates high
time due to their broad spectrum of biological activities, inhibitory activity against porcine pancreas α-amylase [7]. 4′,7-
including induction of apoptosis in adipocytes, anti-inflamma- Di-O-methylkaempferol exhibits modest cytotoxicity [17].
tion and anti-atherogenic properties [6,7]. 3-O-Methyl- These methylated derivatives of flavonols are found as natural
kaempferol evidently possesses antiviral [8], antimicrobial [9] products in relative low quantity, whereas kaempferol is rela-
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Beilstein J. Org. Chem. 2015, 11, 288–293.
tively rich in plants. Therefore, it is quite worthwhile to develop ation of flavonoids [21,22], we report here a convenient and
procedures for the preparation of methylated derivatives of efficient method for mono-, di- and tri-O-methylation of
kaempferol.
kaempferol, by taking the advantage of the different reactive
potential (hydrolysis, benzylation and methylation) of
O-acetoxy groups in multi-acetylated kaempferol. The major
strategy depends on the selective benzylation and controllable
deacetylation of kaempferol acetates.
Results and Discussion
Selectivity of direct methylation of kaempferol
Kaempferol is both significantly similar to and different from
quercetin. Rao et al. found that the methylation of the hydroxy
groups in quercetin occurs gradually following a specific
Figure 1: Kaempferol.
On the other hand, O-methylation is an effective approach to sequential position order: 4′ > 7 > 3 > 3′ > 5 [23]. Recently,
protect the reactive hydroxy groups of flavonoids and a strategy however, Roland ascertained that the relative reactivity order of
to improve the physicochemical properties such as solubility OH in quercetin presents an invert sequence: 7 > 3 ≈ 4′ [19].
and intracellular compartmentation [18]. Some of the O-alky- And this sequence varies with different O-substituted functions
lated flavonoids possess biological activities, and some have [24].
been used for the preparation of natural and therapeutic
products of biological significance. To date, however, the We first investigated the direct methylation of kaempferol in
regioselective direct O-alkylation of polyhydroxy favonoids is order to get an insight into the reactivity of the hydroxy groups.
still of challenge. Until recently, selective O-methylation of Dimethyl sulfate was chosen as methylating reagent. As illus-
quercetin was carried out by the protection of phenolic func- trated in Table 1, with 1 equiv of dimethyl sulfate and 1.2 equiv
tions with benzyl or diphenylmethylene ketal [19]. Quercetin of potassium carbonate at room temperature for 12 h or more,
was converted to pentabenzylquercetin and thereby four mono- three mono-O-methylkaempferols (3-, 4′- and 7-O-methyl-
methylated isomers were synthesized through selective depro- kaempferol) were obtained in only 34% isolated yield. Using
tection [20]. The regioselective O-methylation of kaempferol, to 2 equiv of dimethyl sulfate at the same conditions, mono-, di-
the best of our knowledge, has not been reported up to now.
and a trace of tri-O-methylated kaempferols were isolated using
silica gel column chromatography followed by reversed phase
We tried to directly methylate kaempferol using dimethyl HPLC. Tetra-O-methylkaempferol was obtained quantitatively
sulfate as methylating reagent, but the methylation was uncon- when dimethyl sulfate was increased to 6 equiv at 30 °C for
trollable and resulted in a mixture of multiple methylated 24 h (Table 1).
products. The direct methylation of the hydroxy groups in
kaempferol is non-selective. We turned to the selective or These results indicated a reactivity order of each hydroxy group
controllable hydrolysis of kaempferol tetraacetate (1) intending of kaempferol: 7 > 4′ > 3 >> 5, different from that of quercetin:
to get the specific hydroxy group free for methylation. The 3 ≈ 4′ [19]. The 7-OH of kaempferol should be the most highly
solubility of acetylated kaempferols is improved, and the condi- reactive hydroxy group in nucleophilic alkylation, followed by
tions for the methylation possess more possibilities. As a part of the 4'-OH. The 5-OH resists to alkylation because of the strong
our ongoing work on the synthesis and biological activity evalu- intramolecular H-bond with the 4-carbonyl group.
Table 1: Direct methylation of kaempferol in acetone (yield, %).
1 equiva, rt
2 equiva, rt
6 equiva, 30 °C
kaempferol
62
4
0
3-, 4′-, 7-mono-O-methylkaempferol
3,7-, 3,4′-, 4′,7-di-O-methylkaempferol
3,4′,7-tri-O-methylkaempferol
3,4′,5,7-tetra-O-methylkaempferol
6, 11, 17
0, 12, 19
0
0
0
0
16, 10, 29
0
4
0
0
96
a1 equiv is defined as the stoichiometric amount of Me2SO4 in the presence of 1.2 equiv of K2CO3.
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Beilstein J. Org. Chem. 2015, 11, 288–293.
Synthesis of 7-O-methylkaempferol (4), 4′,7-
di-O-methylkaempferol (5) and 3,7-di-O-
methylkaempferol (7)
and 3-OH based on NMR as well as HMBC experiments (see
Supporting Information File 1 for experimental and spectra
data), different from the tentative assigment [27,28].
Kaempferol was acetylated to furnish the peracetylated deriva-
tive 1. Jurd found that with excess of methyl iodide and potas-
sium carbonate in dry acetone, only the 7-O-acetyl group of
quercetin pentaacetate is replaced [25]. For the synthesis of
7-O-methylkaempferol (4), we first adopted Jurd’s method.
Under similar conditions, however, a mixture of 7-O-, 4′-O- and
4′,7-di-O-methylkaempferol was obtained whether methyl
iodide or dimethyl sulfate was used as methylating reagent.
Separation of 7- and 4′-mono isomers from the crude product
involved tedious and laborious work-up. We turned to employ
thiophenol with imidazole in N-methylpyrrolidinone (NMP) at
0 °C [26], for selective removal of the most electrophilic 7-O-
acetyl group from the fully acetylated kaempferol. 3,4′,5-Tri-O-
acetylkaempferol (2) was obtained in excellent yield (91%).
Methylation of 2 with 1.3 equiv of dimethyl sulfate at room
temperature generated 3 in a yield of 87%, which was further
treated with methanolic ammonia (7.0 M) for deacetylation to
afford compound 4 (Scheme 1).
Treatment of 2 with an excess of dimethyl sulfate (3 equiv) and
K2CO3 (3.6 equiv) in acetone/MeOH (3:1) under reflux, unex-
pectedly, leads to the formation of 4′,7-di-O-methylkaempferol
(5) in 90% yield (Scheme 1). We speculated that this was due to
the OH reactivity (7 > 4′ > 3 >> 5), and the hydrolysis rate of
OAc in a reversed order (5 > 4′ > 3) in kaempferol. Deacetyl-
ation of 4′-OAc in 2 and methylation of thus derived 4′-OH
were accomplished simultaneously in one pot. Caution should
be taken to the ratio of acetone to MeOH and the amount of
K2CO3, since high polarity of solvent or excess of base result in
heterocycle cleavage of the flavonoid skeleton.
To obtain 3,7-di-O-methylkaempferol (7), we envisioned the
possibility of methylation from the 4′-O-acetylkaempferol (6),
which can be derived from 2. We were delighted that the
combination of reflux with aluminum chloride in polar solvent
CH3CN and then hydrolysis in 1 M HCl (aq) at room tempera-
ture was an excellent method for selective removal of the 3- and
The location of the methoxy group in 4 was unambiguously 5-O-acetyl groups from triacetylated kaempferol (2). No side
determined by the NOESY correlations of OCH3 at δH 3.86 to products were detected by TLC analysis. With other Lewis
8-H (δH 6.75) and 6-H (δH 6.35). Three characteristic singlets at acids such as ZnCl2, the result was very different. The desired
δH 12.47, 10.14 and 9.51 were thus assigned separately to 5-, 4′- product 6 was obtained with reproducible yield (89%) after
Scheme 1: Reagents and conditions: (a) PhSH, imidazole, NMP (N-methylpyrrolidinone), 0 °C, 91%; (b) Me2SO4 (1.3 equiv), K2CO3, acetone, rt,
87%; (c) NH3 (g), MeOH, 97%; (d) Me2SO4 (3 equiv), K2CO3, acetone, MeOH, reflux, 90%; (e) i: AlCl3, CH2Cl2, CH3CN, reflux, ii: 1 M HCl (aq), 89%;
(f) i: Me2SO4 (2.6 equiv), K2CO3, acetone, rt, ii: NH3 (g), MeOH, 84%.
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Beilstein J. Org. Chem. 2015, 11, 288–293.
chromatographic separation. Compound 7 was thus prepared by tection of 10 involving deacetylation with 7.0 M methanolic
methylation of 6 and subsequent hydrolysis at appropriate ammonia and debenzylation by hydrogenolysis with 10% palla-
conditions in good yield (Scheme 1).
dium on carbon in EtOAc/MeOH (1:3) at room temperature
afforded 12 (Scheme 2).
Synthesis of 3-O-methylkaempferol (12), 5,4′-
di-O-methylkaempferol (16) and 4′-O-methyl-
In order to prepare 4′,5-di-O-methylkaempferol (16), selective
deprotection of 4′- and 5-O-acetyl groups in compound 8 was
necessary. We first employed acetone/MeOH or THF/MeOH
system with K2CO3 or Na2CO3 as base, but failed. We found
that under acidic conditions the 4′- and 5-O-acetyl groups could
be removed selectively. The best result was obtained using a
mixture of 1 M HCl (aq)/acetone/MeOH (3:3:1) at reflux for
3.5 h. Partial deacetylated product 13 was thus acquired in 82%
isolated yield. Next, in order to methylate the less reactive
5-OH, we applied an intermittent injection method with a higher
concentration of dimethyl sulfate. After 24 h of total reaction
time at 30 °C, 14 was obtained in 92% yield. The following step
of this synthesis was the deprotection of the benzyl and acetyl
groups (Scheme 2).
kaempferol (18)
For the synthesis of 3-O-methylkaempferol (12), we decided to
selectively protect the 7-OH with benzyl, in order to make the
methylation of 3-OH proceed smoothly. Previous studies have
revealed that benzylation of quercetin pentacetate in anhydrous
acetone and subsequent alkaline hydrolysis gives a mixture of
7-O- and 4',7-di-O-benzylquercetin [29]. Using this strategy,
peracetylated derivative 1 was treated with an excess of benzyl
bromide and catalytic amounts of potassium iodide in the pres-
ence of K2CO3 under reflux for 24 h. Fortunately, only 8 was
obtained by crystallization from absolute ethanol in an 85%
yield (Scheme 2).
With 8 in hand, the synthesis of 12 is based on similar synthetic
strategy to that of 7 from 2. The key step was the selective Two different methods are available for the preparation of 4′-O-
release of the 3-OH and 5-OH from compound 8, using the methylkaempferol (18): methylation of 13 with 1.3 equiv of
combination of AlCl3 and HCl (aq) (Scheme 2).
dimethyl sulfate and subsequent hydrolysis at appropriate
conditions, or deacetylation of 8 followed methylation succes-
Methylation of 9 with 1.3 equiv of dimethyl sulfate at room sively as described for 5. Obviously, the latter route is more
temperature provided 10 with a yield of 85%. The final depro- convenient. As may be expected, treatment of 7-O-benzyl tri-
Scheme 2: Reagents and conditions: (a) BnBr (1.9 equiv), KI, K2CO3, acetone, 85%; (b) i: AlCl3, CH2Cl2, CH3CN, reflux, ii: 1 M HCl (aq), 86%; (c)
Me2SO4 (1.3 equiv), K2CO3, acetone, rt, 85%; (d) NH3 (g), MeOH, 96%; (e) Pd/C, H2, 91%; (f) MeOH, acetone, 1 M HCl (aq), reflux, 82%; (g)
Me2SO4 (3.3 equiv), K2CO3, acetone, 30 °C, 92%; (h) Pd/C, H2, 92%; (i) NH3 (g), MeOH, 94%; (j) Me2SO4 (1.5 equiv), K2CO3, acetone, MeOH,
reflux, 82%; (k) Pd/C, H2, 93%.
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Beilstein J. Org. Chem. 2015, 11, 288–293.
acetate 8 with excess of dimethyl sulfate and K2CO3 in acetone/
MeOH at reflux afforded 17 in high yield (82%). Subsequent
hydrogenolysis of 17 catalyzed by 10% palladium on carbon in
EtOAc/MeOH (1:3) at room temperature provided 18
(Scheme 2).
Supporting Information
Supporting Information File 1
Experimental section; NMR and ESI-HRMS spectra of
1–23; NOESY spectra of 4 and 13; HMBC spectra of 4, 5,
7, 9, 12, 13, 16, 18, 22, 23.
Synthesis of 3,4′-di-O-methylkaempferol (22)
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-33-S1.pdf]
and 3,4′,5,-tri-O-methylkaempferol (23)
3,4′-Di-O-methylkaempferol (22) and 3,4′,5-tri-O-methyl-
kaempferol (23) were prepared from 19 derived from 8.
Controlled di-O-methylation and tri-O-methylation of 19 were
achieved by the quantity of Me2SO4, resulting in 20 and 21, res-
pectively. Hydrogenolysis of 20 and 21 afforded 22 and 23
(Scheme 3).
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (No. 21172214).
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