Total synthesis of apigenin-4′-yl 2-O-(p-coumaroyl)-β-d- glucopyranoside

Article abstract of DOI:10.1016/j.carres.2010.10.008

The first total synthesis of apigenin-4′-yl 2-O-(p-coumaroyl)-β- d-glucopyranoside, which exhibits good inhibitory activity against xanthine oxidase (XO), was accomplished in seven steps from a 1,2-blocked sugar unit and natural apigenin. A unique allyl p

Full text of DOI:10.1016/j.carres.2010.10.008

Carbohydrate Research 345 (2010) 2714–2717
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Carbohydrate Research
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Note
Total synthesis of apigenin-40-yl 2-O-(p-coumaroyl)-b-D-glucopyranoside
Lei Zhang a,b, Guohua Wei a, Yuguo Du a,b,
⇑
a State key laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences (CAS), Beijing 100085, China
b College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
The first total synthesis of apigenin-40-yl 2-O-(p-coumaroyl)-b-
D-glucopyranoside, which exhibits good
Article history:
Received 24 September 2010
Received in revised form 11 October 2010
Accepted 12 October 2010
Available online 15 October 2010
inhibitory activity against xanthine oxidase (XO), was accomplished in seven steps from a 1,2-blocked
sugar unit and natural apigenin. A unique allyl protecting group, a phase-transfer-catalyzed (PTC) regio-
selective coupling reaction, and robustness in large-scale preparation are the merits of this synthetic
strategy.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Glycosylation
Carbohydrates
Natural products
Apigenin
Apigenin-40-yl 2-O-(p-coumaroyl)-b-
D-glucopyranoside (1,
O
OH
Fig. 1) is one of the major active components of club moss, Palhin-
haea cernua, a traditional Chinese medicine to treat rheumatism,
whooping cough, hepatitis, and nephrolithiasis.1,2 The same folk
medicine has also been used in attenuating gout-generated joint
pains,3 a typical late-stage symptom of hypercuricaemia resulting
from the overproduction and/or insufficient excretion of uric acid.4
It was reported that compound 1 showed good inhibitory activity
OH
C
O
HO
HO
O
OH
O
O
O
1
OH
against xanthine oxidase (IC50 = 23.95 0.43 lM), a key enzyme
involved in producing uric acid as a metabolite from purine-con-
taining foodstuffs.5 The difficulties in obtaining compound 1
(31.8 mg from 2 kg of dry plant materials) have hindered further
pharmacological studies. In a collaborative project, we were
requested to develop a practical synthesis of apigenin-40-yl 2-O-
Figure 1. Structure of apigenin-40-yl 2-O-(p-coumaroyl)-b-
D-glucopyranoside (1).
OHs. However, the final debenzylation under various reported con-
ditions failed.12 We thus chose the allyl group13 to block both the
alcoholic and the phenolic hydroxyl groups, because allyl can be
readily installed and removed under uniquely mild conditions.
The commercially available apigenin 2 was treated with allyl
bromide under PTC conditions, namely, in the presence of tetrabu-
tylammonium bromide (TBAB) and K2CO3 in 1:1 DMF–H2O at rt,
giving 7-O-allyl-apigenin (3) in 75% yield (Scheme 1). Convergent-
(p-coumaroyl)-b-D-glucopyranoside (1). We report herein the first
total synthesis of this natural product.
The 7-hydroxy group of apigenin is para to the electron-with-
drawing pyrone carbonyl function. It possesses the highest acidity
in the molecule, and therefore generates most easily the
corresponding phenolate, such as 7-hexanoyl phenolate.6–10 The
subsequent glycosylation of this phenolate (flavonol 40,5-diol) with
ly, 1,2-O-ethylidene-
allyl bromide in DMF using NaH as the base, to afford 3,4,6-tri-O-
allyl-1,2-O-ethylidene- -glucopyranose (5) in an excellent yield.
a-D
-glucopyranose (4)14 was allylated with
an
a-glycosyl bromide under phase-transfer-catalyzed (PTC) con-
ditions was suggested to afford the 40-O-b-glycoside in a highly
regioselective manner.10,11 Accordingly, we designed a practical
route for the facile synthesis of compound 1, as shown in Scheme
1.
a-D
Hydrolysis of 5 with 90% trifluoroacetic acid (TFA) at rt gave an
anomeric mixture of 1,2-diol 6 in 94% yield. Condensation of 6
and 4-O-allyl-coumaric acid15 in the presence of N,N0-dicyclohexyl-
carbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) in
CH2Cl2 at ꢀ20 °C gave 3,4,6-tri-O-allyl- 1,2-di-O-[p-(O-allyl)cou-
During the exploration of a promising strategy towards total
synthesis of 1, we initially applied benzyl groups to block all active
maroyl]-D a,b mixture with
-glucopyranose (7)16 in 90% yield as an
⇑
the b anomer as the major component. Treatment of 7 with 33%
HBr in HOAc readily provided the desired glycosyl bromide 8,
Corresponding author. Tel./fax: +86 10 62849126.
E-mail address: duyuguo@rcees.ac.cn (Y. Du).
0008-6215/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2010.10.008

L. Zhang et al. / Carbohydrate Research 345 (2010) 2714–2717
2715
OH
OH
HO
O
O
AllO
O
O
a
OH
OH
Apigenin, 2
3
O
RO
RO
RO
AllO
AllO
AllO
AllO
AllO
AllO
OAll
O
O
O
O
C
c
d
OH
O
O
HO
OAll
O
C
4
5
R = H
R = All
Me
6
b
O
7
O
AllO
AllO
AllO
AllO
OAll
C
O
AllO
AllO
O
O
OAll
e
f
O
1
O
g
O
O
Br
AllO
C
O
9
8
OH
Scheme 1. Reagents and conditions: (a) TBAB, AllBr, K2CO3, 1:1 DMF–H2O, 45 °C, 75%; (b) AllBr, NaH, DMF, rt, 95%; (c) 90% TFA, rt, 94%; (d) 4-O-allylcoumaric acid, DCC,
DMAP, CH2Cl2, ꢀ20 °C, 90%; (e) HBr–HOAc, CH2Cl2, rt; (f) 3, TBAB, K2CO3, 1:1 CHCl3–H2O, 40 °C, 76%; (g) Pd/C, p-TsOH, 10:1 MeOH–H2O, 60–80 °C, 79%.
which was subjected to the glycosylation with 3 under the above-
mentioned PTC conditions at 40 °C in chloroform–water generating
1.2. 7-O-Allyl-apigenin (3)
7-O-allylapigenin-40-yl
royl]-b-
3,4,6-tri-O-allyl-2-O-[p-(O-allyl)couma-
To a solution of apigenin 2 (5.00 g, 18.5 mmol) in DMF (180 mL)
and H2O (180 mL) were added K2CO3 (3.83 g, 27.7 mmol), TBAB
(596 mg, 1.85 mmol), and AllBr (2.4 mL, 27.7 mmol). The mixture
was stirred vigorously at 45 °C for 5 h, and the resulting mixture
was extracted with CH2Cl2 (3 ꢂ 180 mL). The organic extracts were
combined, then washed with brine, dried over Na2SO4, and concen-
trated. The residue was purified by column chromatography (1:3
acetone–petroleum ether) to obtain 3 (4.31 g, 75%) as a yellowish
D-glucopyranoside (9) in a yield of 76% with complete b
selectivity. Global removal of all five allyl groups from 9 was car-
ried out smoothly using Pd/C and p-TsOH in 10:1 MeOH–H2O at
60–80 °C,17 producing the target compound 1 in a 79% isolated
yield. The physical and spectral data of our synthetic sample 1
were in excellent agreement with the literature report5 {½a D20
ꢁ
+0.2 (c 1, MeOH); lit.,5
½
a 2D0
+0.17 (c 0.16, MeOH)}.
ꢁ
In conclusion, the first total synthesis of apigenin-40-yl 2-O-(p-
coumaroyl)-b- -glucopyranoside, a powerful inhibitor of xanthine
amorphous solid: ½a D20
ꢁ
+16 (c 1, CH3COCH3); 1H NMR (400 MHz,
D
acetone-d6): d 4.70 (d, 2H, J 5.0 Hz, OCH2), 5.29 (d, 1H, J 10.6 Hz,
@CH2), 5.45 (d, 1H, J 17.2 Hz, @CH2), 6.07–6.11 (m, 1H, @CH–),
6.33 (s, 1H, H-6), 6.65 (s, 1H, H-8), 6.68 (s, 1H, H-3), 7.01 (d, 2H, J
8.2 Hz, H-30, H-50), 7.93 (d, 2H, J 8.2 Hz, H-20, H-60), 12.90 (s, 1H,
OH); 13C NMR (125 MHz, acetone-d6): d 69.1, 93.1, 98.3, 103.2,
103.6, 115.9, 117.3, 123.5, 128.4, 132.9, 157.7, 161.1, 161.8,
162.2, 164.4, 164.5, 182.2. Anal. Calcd for C18H14O5: C, 69.67; H,
4.55. Found C, 70.05; H, 4.59.
oxidase (XO), was accomplished in seven steps from a 1,2-blocked
sugar unit and natural apigenin. The synthesis takes advantages of
regioselective protection and a stereoselective coupling reaction,
and is suitable for large-scale preparation. This method may pro-
vide a practical way towards the availability of acylated flavone
glycosides.
1. Experimental
1.3. 3,4,6-Tri-O-allyl-1,2-O-ethyliene-a-D-glucopyranose (5)
1.1. General methods
To a solution of compound 4 (5.16 g, 25.0 mmol) in dry DMF
(25 mL) was added NaH (3.61 g, 60% in oil, 90.2 mmol). After
15 min, AllBr (6.87 mL, 79.6 mmol) was added dropwise at 0 °C.
The reaction mixture was stirred overnight at rt, and TLC (1:6
EtOAc–petroleum ether) indicated that the reaction was complete.
Water was then slowly added into the reaction mixture until no
bubbles were observed. The mixture was diluted with EtOAc
(200 mL) and washed with water (3 ꢂ 150 mL). The organic layer
was dried and concentrated. Purification by column chromatogra-
phy (1:6 EtOAc–petroleum ether) gave 5 (7.75 g, 95%) as a syrup.
Optical rotations were determined at 20 °C with a Perkin–Elmer
model 241-Mc automatic polarimeter and are reported in units of
10ꢀ1 deg cm2 gꢀ1 1H NMR and 13C NMR spectra were recorded
.
with a Bruker ARX 400 (or 500) spectrometer for solutions in CDCl3
or acetone-d6. The chemical shifts are given in ppm downfield from
internal Me4Si. Mass spectra were measured using a MALDI-TOF
mass spectrometer with
a-cyano-4-hydroxycinnamic acid (CCA)
as the matrix. TLC was performed on Silica Gel HF254 with detec-
tion by charring with 30% (v/v) H2SO4 in MeOH or in some cases
by a UV lamp. Column chromatography was conducted by elution
of a column of silica gel (100–200 mesh) with EtOAc–petroleum
ether (bp 60–90 °C) or acetone–petroleum ether as the eluent.
The solutions were concentrated at <50 °C under reduced pressure.
One of the two isomers gave the following physical data: ½a D20
ꢁ
+26 (c 1, CHCl3); 1H NMR (400 MHz, CDCl3): d 1.45 (d, 3H, J
4.7 Hz, CHCH3), 3.54 (dd, 1H, J 9.5, 3.7 Hz, H-4), 3.64 (br s, 2H, H-
6), 3.75 (d, 1H, J 3.4 Hz, H-5), 3.83 (dd, 1H, J 9.5, 3.8 Hz, H-3),

2716
L. Zhang et al. / Carbohydrate Research 345 (2010) 2714–2717
4.00–4.15 (m, 6H, 3CH2), 4.19 (dd, 1H, J 5.5, 12.6 Hz, H-2), 5.04 (q,
1H, J 4.7 Hz, CH), 5.13–5.30 (m, 6H, @CH2), 5.51 (d, 1H, J 5.0 Hz, H-
1), 5.85–5.91 (m, 3H, CH2@CH–); 13C NMR (125 MHz, CDCl3): d
19.9, 69.2, 69.8, 70.9, 71.7, 72.4, 74.9, 75.8, 76.9, 77.1, 77.3, 78.3,
97.2, 100.5, 117.1, 117.4, 117.5, 134.4, 134.6, 134.7. Anal. Calcd
for C17H26O6: C, 62.56; H, 8.03. Found C, 62.84; H, 8.14.
40 °C for 1 h, then glycosyl bromide 8 (1.162 g, 2.11 mmol) was
added. The reaction mixture was stirred at 40 °C for 5.5 h, then di-
luted with CH2Cl2, and washed sequentially with water and brine.
The organic layer was dried over Na2SO4 and concentrated. The
residue was purified by silica gel column chromatography (1:2
EtOAc–petroleum ether) to give 9 (1.25 g, 76%) as a yellowish
amorphous solid: ½a D20
ꢁ
ꢀ38 (c 1, CHCl3); 1H NMR (400 MHz, CDCl3):
1.4. 3,4,6-Tri-O-allyl-D-glucopyranose (6)
d 3.58–3.70 (m, 4H), 3.80 (d, 1H, J 9.3 Hz), 4.00–4.10 (m, 2H), 4.16–
4.20 (m, 2H), 4.26 (dd, 1H, J 12.7, 5.7 Hz), 4.35 (dd, 1H, J 12.2,
5.6 Hz), 4.55 (br s, 2H), 4.55–4.61 (m, 2H), 5.12 (d, 2H, J 9.48 Hz),
5.19 (d, 2H, J 11.3 Hz), 5.26 (d, 2H, J 7.4 Hz), 5.32 (br s, 3H), 5.37
(d, 1H, J 7.9 Hz), 5.42 (d, 1H, J 5.1 Hz), 5.81–5.94 (m, 3H), 5.98–
6.07 (m, 2H), 6.07 (s, 1H), 6.34 (d, 1H, J 9.4 Hz), 6.45 (d, 1H, J
7.5 Hz), 6.53 (d, 2H, J 1.1 Hz), 6.90 (d, 2H, J 7.9 Hz), 6.99 (d, 1H, J
8.2 Hz), 7.09 (d, 1H, J 8.1 Hz), 7.46 (d, 2H, J 7.9 Hz), 7.70 (d, 1H, J
15.9 Hz), 7.77 (d, 2H, J 7.6 Hz), 12.74 (d, 1H, J 9.3 Hz); 13C NMR
(125 MHz, CDCl3): d 68.9, 69.2, 72.6, 73.9, 75.6, 76.8, 77.1, 77.3,
82.3, 93.3, 98.7, 98.9, 104.8, 114.6, 115.1, 117.1, 117.3, 118.1,
118.3, 118.5, 125.4, 127.0, 127.9, 129.9, 132.1, 132.7, 134.5,
134.6, 145.6, 157.6, 160.1, 160.6, 162.1, 163.7, 164.4, 165.7,
182.4. MALDI-TOFMS: calcd for C45H46O12: 778.3 [M]+; found
801.3. [M+Na]+. Anal. Calcd for C45H46O12: C, 69.40; H, 5.95. Found:
C, 69.28; H, 5.87.
To a solution of compound 5 (3.26 g, 10 mmol) in CH2Cl2 (4 mL)
was added 90% aq trifluoroacetic acid (18 mL) at rt. The mixture
was stirred under these conditions for 1.5 h, and then co-evapo-
rated with toluene (2 ꢂ 15 mL) under reduced pressure. Purifica-
tion of the residue on a short silica gel column (1:1 EtOAc–
petroleum ether) gave diol 6 (2.83 g, 94%) as a syrupy a,b mixture.
1H NMR (400 MHz, CDCl3): d 3.30 (dd, 1H, J 9.2, 3.6 Hz, H-4), 3.70
(dd, 1H, J 10.3, 6.9 Hz, H-6a), 3.54–3.60 (m, 2H, H-5, H-6b), 3.65
(dd, 1H, J 10.4, 3.8 Hz, H-3), 3.93–4.09 (m, 4H, 2CH2), 4.25–4.33
(m, 3H, H-2, CH2), 5.13 (d, 1H, J 5.4 Hz, H-1), 5.16–5.29 (m, 6H,
3CH2), 5.84–6.00 (m, 3H, @CH); 13C NMR (125 MHz, CDCl3): d
68.8, 70.1, 72.4, 73.6, 74.1, 74.6, 75.0, 76.9, 77.1, 77.3, 77.4, 77.6,
81.9, 83.9, 92.2, 96.7, 116.9, 117.6, 134.4, 134.7, 135.2. Anal. Calcd
for C15H24O6: C, 59.98; H, 8.05. Found C, 60.12; H, 8.11.
1.5. 3,4,6-Tri-O-allyl-1,2-di-O-[p-(O-allyl)coumaroyl]-
D
-
1.7. Apigenin-40-yl 2-O-(p-coumaroyl)-b-
D-glucopyranoside (1)
glucopyranose (7)
A mixture of compound 9 (1.06 g, 1.36 mmol), Pd/C (605 mg,
10% content), p-TsOH (22 mg), CH3OH (40 mL), and H2O (4 mL)
was stirred at 60–80 °C for 40 h, at the end of which time TLC
(6:1 EtOAc–acetone) indicated the completion of the reaction.
The mixture was filtered and the filtrate was co-evaporated to dry-
ness with toluene (3 ꢂ 10 mL). The residue was purified by silica
gel column chromatography (10:1 EtOAc–acetone) to furnish 1
To a solution of DMAP (319.7 mg, 2.62 mmol), 4-O-allyl-couma-
ric acid (4.097 g, 20.06 mmol), and compound (2.62 g,
6
8.72 mmol) in CH2Cl2 (54 mL) was added N,N-dicyclohexylcarbodi-
imide (4.49 g, 21.8 mmol) in about 5 portions over 15 min at
ꢀ20 °C. The resulting mixture was allowed to warm up to rt, and
additional amounts of DMAP (3 ꢂ 15 mg) were added after 15,
24, and 34 h, respectively. TLC analysis (1:4 EtOAc–petroleum
ether) was used to monitor the process and determine when the
reaction was complete. The organic solvent was removed in vacuo,
the residue was diluted with EtOAc (50 mL), and the N,N-dicyclo-
hexylurea was filtered off. The filtrate was washed with water,
dried over Na2SO4, and concentrated to dryness. Purification of
the residue by silica gel column chromatography (1:4 EtOAc–
(624 mg, 79%) as a yellowish amorphous powder: ½a D20
ꢁ
+0.2 (c 1,
MeOH); 1H NMR (400 MHz, acetone-d6): d 3.61 (t, 1H, J 9.4 Hz,
H-400), 3.70–3.72 (m, 1H, H-500), 3.77 (dd, 1H, J 11.6, 5.2 Hz, H-
6a00), 3.82 (t, 1H, J 9.4 Hz, H-300), 3.97 (dd, 1H, J 11.6, 1.2 Hz, H-
6b00), 5.17 (dd, 1H, J 9.4, 8.1 Hz, H-200), 5.39 (d, 1H, J 8.0 Hz, H-100),
6.26 (d, 1H, J 2.0 Hz, H-8), 6.37 (br d, 1H, J 15.9 Hz, H-8000), 6.55
(d, 1H, J 2.0, H-6), 6.67 (s, 1H, H-3), 6.88 (d, 2H, J 8.5 Hz, H-3000
and H-5000), 7.20 (d, 2H, J 8.8 Hz, H-30 and H-50), 7.55 (d, 2H, J
7.8 Hz, H-2000 and H-6000), 7.66 (br d, 1H, J 15.9 Hz, H-7000), 7.99 (d,
petroleum ether) gave 7 (5.279 g, 90%,
a
/b = 1:9) as a syrup. For
the b isomer: ½a 2D0
ꢁ
+7 (c 1, CHCl3); 1H NMR (400 MHz, CDCl3): d
3.60 (dd, 1H, J 9.1, 1.6 Hz), 3.66–3.68 (m, 2H), 3.72 (br s, 2H),
4.01 (dd, 1H, J 12.6, 5.4 Hz), 4.01 (dd, 1H, J 12.6, 5.4 Hz), 4.09 (dd,
1H, J 12.6, 5.2 Hz), 4.16–4.19 (m, 2H), 4.25 (dd, 1H, J 12.7,
5.6 Hz), 4.33 (dd, 1H, J 12.1, 5.4 Hz), 4.55 (d, 4H, J 4.8 Hz), 5.11
(d, 1H, J 10.3 Hz), 5.16–5.42 (m, 9H), 5.79 (d, 1H, J 8.1 Hz), 5.83–
6.07 (m, 5H), 6.26 (t, 2H, J 15 Hz), 6.88 (m, 4H), 7.43 (m, 4H),
7.65 (dd, 2H, J 15.9, 4.3 Hz); 13C NMR (125 MHz, CDCl3): d 68.1,
68.8, 72.1, 72.5, 73.8, 73.9, 75.8, 76.8, 77.1, 77.3, 82.3, 92.3,
114.3, 114.7, 115.0, 117.2, 117.3, 118.1, 126.9, 127.0, 129.9,
130.1, 132.7, 134.5, 134.6, 134.7, 145.5, 146.4, 160.5, 160.6,
165.5, 165.8. Anal. Calcd for C39H44O10: C, 69.63; H, 6.59; Found:
C, 69.48; H, 6.87.
2H, J
8.8 Hz, H-20 and H-60), 12.92 (s, 1H, OH); 13C NMR
(125 MHz, acetone-d6): d 61.5, 71.1, 73.3, 74.2, 78.1, 94.6, 98.9,
99.4, 104.1, 104.5, 115.8, 116.8 (2C), 117.6 (2C), 126.0, 126.8,
128.7, 128.9, 131.1 (2C), 145.1, 157.9, 160.3, 161.1, 163.5, 164.2
(2C), 166.5, 182.2. MALDI-TOFMS: calcd for C30H26O12: 578.1
[M]+; found 579.3 [M+H]+, 601.3. [M+Na]+. Anal. Calcd for
C30H26O12: C, 62.28; H, 4.53; Found: C, 62.39; H, 4.62.
Acknowledgements
This work was supported in partial by NNSF of China (Projects
20872172, 20732001, and 20621703) and National S&T Special
Project 2009ZX09501-011.
1.6. 7-O-Allyl-apigenin-40-yl 3,4,6-tri-O-allyl-2-O-[p-(O-
allyl)coumaroyl]-b-D-glucopyranoside (9)
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carres.2010.10.008.
To a cooled solution of compound 7 (1.42 g, 2.11 mmol) in an-
hyd CH2Cl2 (22 mL) at 0 °C was added a solution of 33% HBr in
HOAc (3.36 mL). After stirring for 3 h, the mixture was neutralized
with satd aq NaHCO3. The organic phase was washed with cooled
brine, dried over Na2SO4, and then concentrated in vacuo, provid-
ing the crude bromide 8 (1.162 g) as a syrup. The mixture of com-
pound 3 (547 mg, 1.76 mmol), TBAB (567 mg, 1.0 equiv), K2CO3
(608 mg, 4.4 mmol) in 1:1 CHCl3–H2O (80 mL) was stirred at
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