Synthesis, α-glucosidase inhibitory and molecular docking studies of prenylated and geranylated flavones, isoflavones and chalcones

Article abstract of DOI:10.1016/j.bmcl.2015.08.059

Three series of prenylated and/or geranylated flavonoids were synthesized and evaluated for their α-glucosidase inhibitory activity. The 3′,5′-digeranylated chalcone (16) was identified as a new α-glucosidase inhibitor whose activity (IC₅₀ = 0.

Full text of DOI:10.1016/j.bmcl.2015.08.059

Accepted Manuscript
Synthesis, α-glucosidase inhibitory and molecular docking studies of prenylated
and geranylated flavones, isoflavones and chalcones
Hua Sun, Yashan Li, Xiaoting Zhang, Yanan Lei, Weina Ding, Xue Zhao,
Haomeng Wang, Xiaotong Song, Qingwei Yao, Yongmin Zhang, Ying Ma,
Runling Wang, Tao Zhu, Peng Yu
PII:
S0960-894X(15)30011-1
DOI:
http://dx.doi.org/10.1016/j.bmcl.2015.08.059
BMCL 23053
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
29 June 2015
10 August 2015
21 August 2015
Please cite this article as: Sun, H., Li, Y., Zhang, X., Lei, Y., Ding, W., Zhao, X., Wang, H., Song, X., Yao, Q.,
Zhang, Y., Ma, Y., Wang, R., Zhu, T., Yu, P., Synthesis, α-glucosidase inhibitory and molecular docking studies
of prenylated and geranylated flavones, isoflavones and chalcones, Bioorganic & Medicinal Chemistry Letters
(2015), doi: http://dx.doi.org/10.1016/j.bmcl.2015.08.059
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Graphical Abstract
Synthesis, α-glucosidase inhibitory and
molecular docking studies of prenylated and
geranylated flavones, isoflavones and
chalcones
Leave this area blank for abstract info.
Hua Sun, Yashan Li, Xiaoting Zhang, Yanan Lei, Weina Ding, Xue Zhao, Haomeng Wang, Xiaotong Song,
Qingwei Yao, Yongmin Zhang, Ying Ma, Runling Wang, Tao Zhu and Peng Yu
Compound 16 (IC50 = 0.90 M)

Bioorganic & Medicinal Chemistry Letters
jo u r n a l h o m e p a g e : w w w . e ls e v i e r . c o m
Synthesis, α-glucosidase inhibitory and molecular docking studies of prenylated
and geranylated flavones, isoflavones and chalcones
Hua Suna, b , Yashan Lia, b , Xiaoting Zhang a, b , Yanan Leia, b , Weina Dinga, b , Xue Zhaoa, b , Haomeng
Wanga, b , Xiaotong Song a, b , Qingwei Yaoe , Yongmin Zhangb, d , Ying Mac , Runling Wangc , Tao Zhua, b,
and Peng Yua, b,
*
a
Key Laboratory of Industrial Fermentation Microbiology (Tianjin University of Science and Technology), Ministry of Education, Tianjin, 300457, China
b Tianjin Key Laboratory of Industry Microbiology, and Sino-French Joint Lab of Food Nutrition/Safety and Medicinal Chemistry, College of Biotechnology,
Tianjin University of Science and Technology, Tianjin 300457, China
c School of Pharmacy, Tianjin Medical University, Tianjin 300070, China
dUniversité Pierre et Marie Curie-Paris 6, Institut Parisien de Chimie Moléculaire UMR CNRS 8232, 4 place Jussieu, 75005, Paris, France
e 382 Longvalley Lane, DeKalb, USA
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Accepted
Available online
Three series of prenylated and/or geranylated flavonoids were synthesized and evaluated for
their α-glucosidase inhibitory activity. The 3’, 5’-digeranylated chalcone (16) was identified as a
new α-glucosidase inhibitor whose activity (IC50 = 0.90 μM) was 50-fold more than that of
acarbose (IC50 = 51.32 μM). Molecular docking studies revealed the existence of strong
hydrophobic interaction and H-bonding between compound 16 and α-glucosidase’s active site.
The inhibitory mode analysis showed that 16 exhibited a competitive inhibitory mode.
Keywords:
α-Glucosidase inhibitors
Prenyl
2009 Elsevier Ltd. All rights reserved.
Geranly
Flavonoid
Docking study
———
 Corresponding author. Tel.: +86-22-6091-2592; fax: +86-22-6060-2298; e-mail: yupeng@tust.edu.cn; tao.zhu@cansinotech.com

α-Glucosidase, which catalyzes the breakage of the 1,4-α
glycosidic bonds found in starch and certain disaccharides and
convert them into glucose, is likely to be involved in many
diseases, such as diabetes,1 cancer,2 viral infection3 and pompe
disease.4 Therefore, α-glucosidase is an attractive target for
pharmaceutical studies and several α-glucosidase inhibitors are
already in the market or in clinical trial.5, 6
possess any improved α-glucosidase inhibition. Therefore,
prenylated and geranylated chalcones, isoflavones, such as
lupiwighteone (2a) and lespedeza E1 (2b), and flavones, such as
8-prenyl quercetin 3a and 8-geranyl quercetin (3b) (Fig.1), were
synthesized. In addition, evaluation of their α-glucosidase
inhibition as well as a molecular docking analysis and studies on
the inhibitory mode of these three series of prenylated and/or
geranylated flavones, isoflavones and chalcones were explored.
Flavonoids, a class of natural drugs with high biological
activity, are abundant in various plants. Flavones, isoflavones
and chalcones are three types of representative flavonoids, some
of which have been identified as good candidates as α-
glucosidase inhibitors to reduce postprandial hyperglycemia.7, 8
In 2010, Park’s group reported that the natural 5’-prenylated
broussochalcon (1, Fig.1) exhibited α-glucosidase inhibitory
activity (IC50 = 5.3 μM)9. Our previous studies also suggested
that the 5'-prenylated and geranylated chalcones possess
significantly enhanced cytotoxic activity.10 The above findings
encourage us to investigate whether the flavonoid and
isoflavonoid skeletons which possess different ring-B connection
pattern with chalcone, and longer geranyl side chain would
Fig.1 Structures of natural prenylated and geranylated flavonoids 1, 2a, 2b,
3a and 3b.
As shown in scheme 1, regiospecific bromination of the fully
MOM protected 5, readily prepared from the commercial
material genistein (4), with NBS provided the key intermediate 6;
the latter was converted to the 8-prenylated and geranylated
isoflavones 7a and 7b with prenyl/geranyl boronic esters11 via a
microwave assisted Suzuki coupling reaction. Removal of the
MOM groups accomplished the improved total synthesis of
lupiwighteone (2a)12 and lespedeza E1 (2b)13. In order to illustrate
the influence of the free phenolic hydroxyls on the inhibition, 2a
and 2b were further converted to the trimethylated and
dimethylated counterparts 8 and 9 as shown.14
Scheme 1. Synthesis of 2a and 2b and their methyl derivatives. Conditions
and reagents: a, MOMCl, 60% NaH, DMF, 0 °C-r.t., 2h, 87%; b, NBS,
DCM, r.t., 2h, 93%; c, Pd(dppf) Cl2, Cs2CO3, DMF, 110 °C, microwave, 2
h, 71-75 %; d, 2M HCl, MeOH/THF (5:1), 0-60 °C, 75-81%; e, MeI,
K2CO3, DMF, 40 °C, 4 h, 93-95%; f, BCl3, DCM, 0 °C, 30 min, 85-88 %.
The syntheses of 8-prenyl quercetin (3a) and 8-geranyl
quercetin (3b) were achieved through
a
sigmatropic
rearrangement of the key 5-O-prenylated and geranylated
precursors 12a and 12b to give 13a and 13b followed by MOM-
deprotection with an overall yield of 9-15% after 4 steps (Scheme
2).15
OH
OMOM
OMOM
OMOM
OMOM
OH
HO
O
MOMO
O
O
MOMO
O
O
b
a
c
OH
OMOM
OMOM
OH
O
10
OH
HO
OR
11
12
OMOM
OH
OMOM
OH
R
R
MOMO
O
O
O
d
12a, 13a, 3a R = prenyl
12b, 13b, 3b
R = geranyl
OMOM
OH
OH
OH
O
3
13
Scheme 2. Synthesis of 3a and 3b. Condition and reagents: a, i) MOMCl,
60% NaH, DMF, 0 °C-r.t., 2h, 85%; ii) MeOH, HCl, 0 °C, 3h, 78%; b, RBr,
Bu4NOH, DCM, toluene, r.t., 3h, 45-48 %; c, N,N-Diethylaniline, 220 °C, 2
h, 40-55 %; d, 2M HCl, MeOH, 70 °C, 2 h, 78-82 %.
We have recently reported the synthesis and anticancer activity

evaluation of the 5’-prenylated/geranylated and 3’, 5’-di-
prenylated/geranylated chalcones.10 The diprenyl/digeranyl
derivatives 15-18 were prepared from 2’-O-prenyl/geranyl ethers
by 1, 3’-rearrangement catalyzed by magnesium silicate at 140
°C. Here, the prenylated/geranylated chalcones 14-19 (Fig. 2)
were evaluated for the α-glucosidase inhibition.
8-prenyl quercetin (3a) was 2.5-fold more active than 10,
suggesting that the flavone core of 3b should have a better fit to
the enzyme’s active site.
Table 2. α-Glucosidase inhibition of prenyl and geranyl flavones.
OH
OH
R
HO
O
14 R1=geranyl, R2=R3=R4=R5=H
15 R1=R2=prenyl, R3=R4=R5=H
16 R1=R2=geranyl, R3=R4=R5=H
17 R1=prenyl, R2=geranyl, R3=R4=R5=H
18 R1=geranyl, R2=prenyl, R3=R4=R5=H
19 R1=prenyl, R2=H, R3=R4=R5=CH3
R1
5'
R3O
R2
OR5
OH
OH
O
No.
R
H
IC50(μΜ)a
10.98
4.38
1.15
51.32
3'
OR4
O
10
3a
3b
prenyl
geranyl
-
Fig. 2 Synthesis of mono- and di-prenylated/geranylalted chalcones 14-19.
Acarboseb
The α-glucosidase inhibition of all the newly synthesized three
series of prenyl and geranyl flavonoids, along with unsubstituted
genistein (4), quercetin (10) and chalcone (20), was evaluated
according to the literature protocol.16 Acarbose, an α-glucosidase
inhibitor used to treat type 2 diabetes mellitus (T2DM), was
chosen as a reference compound for activity comparison.
a Results are the average of three independent experiments, each performed in
duplicate. Standard deviations were below ±10%. b Reference compound.
Similiar to the flavone series, both 5’-prenylated chalcone 1
and 5’-geranylated chalcone 14 exhibited better activity than the
unsubstituted chalcone 20 and were slightly more active than
acarbose (Table 3). Notably, the 3’, 5’-diprenylated and
digeranylated chalcones 15-18 exhibited significantly enhanced
activity. Among them, the digeranylated chalcone 16 showed the
strongest α-glucosidase inhibitory activity (IC50 = 0.90 μM),
which is about 100-fold and 50-fold more active than the parent
compound 20 and the marketed drug Acarbose, respectively.
Conversion of the free phenolic hydroxyls into methoxyl groups,
as in the case of compound 19, brought about a diminishment in
activity. This finding is consistent with the SAR analysis of the
isoflavone series (2b vs. 8b, 9b).
The results of our activity study were compiled in Table 1-3.
As can be seen from the data in Table 1, lespedeza E1 (2b)
bearing a geranyl side chain was 8-fold more active than the
unsubstituted genistein (4) and acarbose, pointing to a favorable
effect of the geranyl side chain in improving the α-glucosidase
inhibitory activity. On the other hand, tri- and di-methylation of
phenolic hydroxyls led to significantly decrease in the inhibitory
activity (2b vs. 8b, 9b), suggesting that the free phenolic
hydroxyls are crucial for the desired activity. Unexpectively,
compound 2a with C-8 prenyl substitution did not show an
increased activity. One possible reason maybe that the C-8
prenylation or geranylation would increase the hydrophobic
interaction between the drug molecule and α-glucosidase, but a
bigger substitution group may also decrease the H-bond tendancy
between the C-7 hydroxyl group and the amino residues of the
enzyme. The overall effect will come from the interplay of these
two factors. The prenyl group in 2a is five carbons shorter than
the geranyl group in 2b and might not be able to provide enough
additional hydrophobic affinity and this would offset its gain in a
more favorable H-bond interaction. This hypothesis was
supported by the docking analysis shown in Fig. 3. To compare
the impact of the prenylation and geranylation of the flavonoid
and chalcone skeletons which possess different ring-B
connection pattern, and hence, may adopt different orientations
inside the enzyme’s active center, the 8-prenylated and
geranylated quercetins as well as the 5’-prenylated and
geranylated and 3’, 5’-di-prenylated and digeranylated chalcone
derivatives were prepared and evaluated.
Table 3. α-Glucosidase inhibition of prenyl/geranyl chalcones.
R1
R3O
OR5
R2
OR4
O
No.
1
14
15
16
17
18
R1
prenyl
geranyl
prenyl
R2
H
H
R3
H
H
H
H
H
H
R4
H
H
H
H
H
H
R5
H
H
H
H
H
H
IC50(μΜ)a
31.31
16.31
6.90
0.90
11.51
9.00
prenyl
geranyl geranyl
prenyl
geranyl
prenyl
H
geranyl
prenyl
H
CH3 CH3 CH3
H
>100
>100
51.32
19
H
H
H
20
Acarboseb
-
a Results are the average of three independent experiments, each performed in
duplicate. Standard deviations were below ±10%. b Reference compound.
In order to gain some structural insight into the inhibitory
mechanisms for the α-glucosidase inhibitors, the binding modes
in the active site were investigated using Glide 5.5. Figure 3
illustrates the molecular interactions of 2a, 2b, 16 and acarbose
with α-glucosidase.
Table 1. α-Glucosidase inhibition of prenyl and geranyl isoflavones.
R1
R2O
O
O
prenyl=
The calculated binding modes of 2a and 2b in the active site
of α-glucosidase were illustrated in Figure 3 (B and C), which
support the difference in the inhibition of 5-prenyl isoflavone 2a
(IC50 >100 μΜ) and 5-geranyl isoflavone 2b (IC50 = 6.26 μΜ)
(Table 1). Although H-bond formation by acarbose and 2b with
the key residues Asp 568 and Asp 469 were observed in the
docking study (Fig. 3, A and B), a stronger hydrophobic
interaction (blue area) between 2b and enzyme’s active site was
formed than that of acarbose. The hydrophobic nature of the long
geranyl side chain of 2b draws a strong interaction with the deep
hydrophobic pocket, which provides further stabilization for 2b
in the active site and makes it fit very well in the enzyme’s active
pocket. In contrast, a shorter 5-prenyl side chain in 2a could not
provide enough additional hydrophobic affinity and 2a cannot fit
well in the enzyme’s active pocket (Fig. 3, C). The calculated
docking score of 2a (-10.79) was significantly less than that of 2b
(-13.67), consistent with the observation that 2a is a less efficient
OR3
geranyl=
OR4
R2
H
No.
4
2a
2b
8a
R1
H
prenyl
geranyl
prenyl
R3
H
H
H
R4
H
H
H
IC50(μΜ)a
50.05
>100
6.26
>100
H
H
CH3 CH3 CH3
geranyl CH3 CH3 CH3
>100
8b
prenyl
geranyl CH3
CH3
H
H
CH3
CH3
>100
>100
51.32
9a
9b
Acarboseb
-
a Results are the average of three independent experiments, each performed in
duplicate. Standard deviations were below ±10%. b Reference compound.
The α-glucosidase inhibitory activity of 8-geranyl quercetin
(3b) was almost 10-fold higher than that of quercetin (10) (Table
2), which is consistent with the result of the isoflavone series in
Table 1. It is interesting to note that the inhibitory activity of the

inhibitor than 2b. Therefore, 2b was capable of inhibiting -
glucosidase by binding in the active site through hydrophobic
interactions and multiple hydrogen bonds in a cooperative way.
This may also explain the reason why the free phenolic hydroxyls
were important for inhibitory activity because phenolic hydroxyls
serve as hydrogen bond donors with the residues of glucosidase.
D). This interaction stabilizes 16 in the active site and makes 16
fit very well in the enzyme’s active pocket. Additional H-bonds
are formed between 16 and the key Asp 232 residue to provide
extra affinity. The binding conformation of 16 differs from that
of 2b in that the hydrogen bonds between Asp232 and the
inhibitor is observed. Another feature that discriminates the
binding mode of 16 from that of 2b is that the former is stabilized
in the active site by more hydrophobic interactions with different
nonpolar residues than the latter. Although the number of
hydrogen bonds was less in the α-glucosidase-16 complex, the
weakening of the hydrophobic interactions should be responsible
for the diminished inhibitory activity of 2b than 16.
The most potent compound 16 formed hydrophobic
interactions by one of its geranyl groups with the deep
hydrophobic pocket. The ring-B and the α, β-unsaturated ketone
of chalcone mother core also have a strong hydrophobic
interaction as shown by the blue surface of α-glucosidase (Fig. 3,
(C)
(A)
(B)
(D)
Figure 3. Binding modes of compounds in the active site of α-glucosidase. acarbose (A), compound 2b (B), compounds 2a and 2b (C), and compound 16 (D)
dock on the α-glucosidase.
(C) Compound 16
(B) Compound 3b
(A) Compound 2b
Figure 4. Lineweaver-Burk plots for inhibition of compound 2b (*, 6.0 μM; ▲, 4.0 μM; ▼, 0 μM), 3b (▼, 2.0 μM; ▲, 1.5 μM; *, 0 μM) and 16 (▼,
2.0 μM; ▲, 1.0 μM; *, 0 μM) on α-glucosidase.
In conclusion, three series of prenylated and geranylated
isoflavonoids, flavonoids and chalcones were synthesized and
evaluated for their α-glucosidase inhibitory activity. The 3’, 5’-
digeranyl chalcone 16 was identified as a potentially new α-
glucosidase inhibitor with an IC50 value of 0.90 μM in a
competitive inhibitory mode and was found to fit well in the
active pocket of α-glucosidase. This compound should hold a
great potential as a leading compound for the treatment of T2DM
and other diseases. Moreover, 8-prenyl and geranyl quercetins 3a
and 3b showed a stonger inhibition than that of corresponding 8-
prenyl and geranyl isoflavone 2a and 2b and 5’-prenyl and
geranyl chalcone 1 and 14, respectively. The synthesis and

evaluation for α-glucosidase inhibition of 6, 8-di-prenyl and
geranyl quercetin are ongoing in our laboratory.
Acknowledgments
The authors sincerely thank the financial support from the
International Science & Technology Cooperation Program of
China (2013DFA31160) and the Scientific Research Foundation
of Tianjin University of Science & Technology (No. 20110115).
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Supplementary data associated with this article can be found
in the online version at http://dx.doi.org/.