New maplexins F-I and phenolic glycosides from red maple (Acer rubrum) bark

Article abstract of DOI:10.1016/j.tet.2011.11.062

Four new gallotannins, maplexins F-I (1-4), two new phenolic glycosides, rubrumosides A-B (5,6), and eleven known compounds were isolated from red maple (Acer rubrum) bark. Their structures were elucidated based on spectroscopic analysis. The maplexins contained three galloylated derivatives attached to different positions of 1,5-anhydro-glucitol and were 10-20 fold more potent α-glucosidase inhibitors than the clinical drug, Acarbose (IC ₅₀=7-16 vs 161 μM), in vitro. These results support previous data suggesting that gallotannins are the main contributors to the α-glucosidase inhibitory activities of maple plant part extracts and that three substituents on the 1,5-anhydro-glucitol moiety are important for activity.

Full text of DOI:10.1016/j.tet.2011.11.062

Tetrahedron 68 (2012) 959e964
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
New maplexins FeI and phenolic glycosides from red maple (Acer rubrum) bark
Tao Yuan , Chunpeng Wan , Ke Liu , Navindra P. Seeram a
a
a
,
b
c
,
*
a
Bioactive Botanical Research Laboratory, Department of Biomedical and Pharmaceutical Sciences, College of Pharmacy, 41 Lower College Road, Fogarty Hall, University of Rhode
Island, Kingston, RI 02881, United States
State Key Lab of Food Science and Technology, Nanchang University, Nanchang 330047, Jiangxi, China
b
c
Department of Medicinal Chemistry, University of Kansas, Lawrence, KS 66045, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 October 2011
Received in revised form 18 November 2011
Accepted 21 November 2011
Available online 27 November 2011
Four new gallotannins, maplexins FeI (1e4), two new phenolic glycosides, rubrumosides AeB (5,6), and
eleven known compounds were isolated from red maple (Acer rubrum) bark. Their structures were
elucidated based on spectroscopic analysis. The maplexins contained three galloylated derivatives at-
tached to different positions of 1,5-anhydro-glucitol and were 10e20 fold more potent
inhibitors than the clinical drug, Acarbose (IC50¼7e16 vs 161 M), in vitro. These results support previous
data suggesting that gallotannins are the main contributors to the -glucosidase inhibitory activities of
a-glucosidase
m
a
Keywords:
Acer rubrum
Red maple
Maplexins
maple plant part extracts and that three substituents on the 1,5-anhydro-glucitol moiety are important
for activity.
Ó 2011 Elsevier Ltd. All rights reserved.
a
-Glucosidase inhibition
1
. Introduction
anhydro-glucitol, was 20 times more potent in inhibiting
a
-gluco-
sidase activity than the clinical drug, Acarbose, in vitro. Thus, we
were very interested in identifying additional -glucosidase in-
5
The red maple species, Acer rubrum L. (Aceraceae), is indigenous
a
to eastern North America and widely known for its sap, which is
used to produce maple syrup, a premium natural sweetener. In-
hibitors from different plant parts of these Acer species.
The prevalence of diabetes is increasing rapidly worldwide
(World Health Organization, 2010) and the search for natural
therapies for diabetes management is of great scientific interest.
The inhibition of carbohydrate hydrolyzing enzymes, such as
glucosidase, has been shown to be a promising and effective ap-
proach to type 2 diabetes management in clinical studies. Thus,
we focused our current attention on the bark of the red maple
1
terestingly, the red maple species was widely regarded by the Na-
tive Americans as a medicinal plant and its bark infusion was used
1
4
2
,3
as a blood purifier to treat various diseases. Despite these tradi-
tional and anecdotal medicinal claims, there is limited knowledge
on the chemical constituents of the red maple species. To date,
previous studies on the red maple species have yielded gallic acid
a-
15
4
,5
derivatives, ellagic acid, procyanidins, and flavonoids.
species with the objective of identifying new
inhibitors.
a-glucosidase
Our group has recently initiated a program of phytochemical
and biological studies of maple syrup and various plant parts of the
Here we report the isolation and characterization of four new
gallotannins, maplexins FeI (1e4) (Fig. 1), two new phenolic gly-
cosides, rubrumosides A and B (5,6), and eleven known compounds
from the red maple bark. All of the new compounds were evaluated
5
e10
red maple and sugar maple (Acer saccharum) species.
enthused by our identification of new gallotannins, maplexins AeE,
with -glucosidase inhibitory activities relevant to type 2 diabetes
We were
a
5
management. These maplexins varied in number and location of
galloyl groups linked to different positions of a 1,5-anhydro-glucitol
core. To date, gallotannins with a 1,5-anhydro-glucitol moiety
for in vitro a-glucosidase inhibitory activities.
5
11e13
have only been isolated from members of the genus Acer.
2. Results and discussion
Moreover, maplexin E, the first gallotannin identified with three
galloyl substituents linked to three different positions of 1,5-
Compound 1, a colorless amorphous solid, exhibited a molecular
formula of C27
H
24
O
17 as determined by HRESIMS at m/z 619.0949
17, 619.0935). Its IR absorptions sug-
ꢀ
[
MꢀH] (calcd for C27
23
H O
ꢀ1
gested the presence of ester carbonyl (1699 cm ) and aromatic
ꢀ
1
1
13
ring (1610, 1498 cm ). Initial analysis of the H and C NMR
spectra (Table 1) showed signals indicative of three sets of galloyl
*
Corresponding author. Tel.: þ1 401 874 9367; fax: þ1 401 874 5787; e-mail
address: nseeram@uri.edu (N.P. Seeram).
0
040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.11.062

960
T. Yuan et al. / Tetrahedron 68 (2012) 959e964
OH
OH
OH
OH
OH
OH
1
7
O
'''
'''
1'''
O
O
H CO
3
7
O
'''
O
R2
6
1''
6
HO
HO
5
5
O 1
O
7''
O
HO
O4
O
HO4
7
''
2
1
3
3
2
HO
O
1
''
O
7
'
O
7'
O
R1
1'
1'
OH
OH
HO
OH
HO
OH
1
2
4
1
2
3
R = R = OH
1
2
R = OH, R = OCH
3
1
2
R = OCH , R = H
3
OH
HO
6
9
1
8
2''
5
3''
O
7
OCH3
7
OH
1''
7''
O
4
5'
HO 4''
1
2
O
4'
OH
6
'
6
O
O6'
3
5'' 6''
2
8'
5
''
3
'
1'
HO
5
6
''
''
OCH3
OH
4'
1
''
5'
3
HO
O
3''
H CO
3
2'
O
O
1'
7'
9'
HO
4
4
2''
HO
2
'
HO
3'
OH
OH
5
7,8-erythro
6
Fig. 1. Structures of compounds 1e6.
Table 1
1
1
3
a
H
N
M
R
a
n
d
C
N
M
R
d
a
t
a
o
f
c
o
m
p
o
u
n
d
s
1
e
4
No.
1
2
3
4
d
C
d
H
(mult., J in Hz)
d
C
d
H
(mult., J in Hz)
d
C
d
H
(mult., J in Hz)
d
C
H
d (mult., J in Hz)
1
1
2
3
4
5
6
ax
eq
66.5
3.52 (dd, 11.0, 10.3)
4.22 (dd, 11.0, 5.2)
5.13 (ddd, 10.3, 9.4, 5.2)
5.45 (dd, 9.4, 9.2)
3.81 (dd, 9.5, 9.2)
3.69 (m)
66.6
3.53 (dd, 11.1, 10.7)
4.21 (dd, 11.1, 5.4)
5.15 (ddd, 10.7, 9.6, 5.4)
5.44 (dd, 9.6, 9.4)
3.83 (dd, 9.4, 8.8)
3.70 (m)
66.6
3.55 (dd, 10.9, 10.7)
4.22 (dd, 10.9, 5.3)
5.18 (ddd, 10.7, 9.5, 5.3)
5.47 (dd, 9.6, 9.5)
3.86 (dd, 9.6, 9.3)
3.71 (m)
66.6
3.47 (dd, 10.9, 10.7)
4.20 (dd, 10.9, 5.2)
5.04 (ddd, 10.7, 9.3, 5.2)
4.06 (t, 9.3)
5.24 (t, 9.5)
3.90 (m)
69.9
76.3
68.6
78.8
63.2
69.8
76.7
68.6
78.8
63.3
69.8
76.7
68.5
78.8
63.3
71.7
73.4
71.4
76.7
62.8
4.58 (br d, 11.9)
4.58 (dd, 12.0, 1.0)
4.43 (dd, 12.0, 5.2)
4.59 (dd, 11.9, 1.9)
4.44 (dd, 11.9, 5.2)
4.43 (dd, 12.2, 2.3)
4.23 (dd, 12.2, 5.1)
4.44 (dd, 11.9, 5.0)
0
0
1
2
3
4
7
1
2
3
4
5
6
7
1
2
3
4
7
119.1
108.9
145.0
138.7
166.0
119.8
109.0
145.0
138.5
145.0
109.0
166.7
119.9
108.8
145.1
138.5
166.8
119.1
108.9
145.0
138.7
166.0
119.8
105.0
147.6
139.3
144.7
110.7
166.8
119.9
108.8
145.1
138.5
166.9
55.3
119.1
108.9
145.0
138.7
166.0
120.9
112.4
147.2
151.4
114.4
123.8
166.6
119.9
108.8
145.1
138.5
166.9
55.0
119.5
108.9
145.0
138.6
166.2
119.6
105.1
147.7
139.5
144.8
110.8
166.0
119.8
108.8
145.1
138.5
166.7
55.3
0
0
, 6
6.97 (2H, d, 2.2)
6.96 (2H, d, 0.9)
7.15 (d, 1.9)
6.97 (2H, d, 1.6)
7.52 (d, 1.7)
7.10 (2H, s)
7.22 (d, 2.0)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
, 5
0
0
0
0
0
0
0
7.06 (d, 1.6)
6.80 (d, 8.3)
7.55 (dd, 8.3, 1.7)
7.06 (d, 1.6)
7.16 (d, 1.9)
7.24 (d, 2.0)
7.08 (2H, s)
00
00
00
00
00
000
000
, 6
, 5
7.11 (2H, d, 1.7)
7.11 (2H, d, 1.0)
7.12 (2H, d, 1.4)
3.87 (3H, s)
OCH
3
3.83 (3H, s)
3.88 (3H, s)
a
Data were measured in CD
3
OD at 500 MHz ( H) and 125 MHz (13C).
1
1
moieties, eight proton signals at
sp carbon signals at d
C
d
H
3.52e5.45, and six oxygenated
78.8, 76.3, 69.9, 68.6, 66.5, and 63.2.
The aforementioned data suggested that compound 1 is a gal-
was established by the 1He H COSY spectrum. The relative ste-
reochemistry of the 1-deoxysugar moiety was determined by the
NOESY data (Fig. 2b), in which the H-2/H-4 correlation revealed
that they were co-facial leading to their assignment as being in the
3
1
1
lotannin. Further combined analysis of the He H COSY, HSQC, and
HMBC spectra allowed the establishment of the structure of 1. A 1-
deoxysugar moiety (C-1 to C-6), drawn with bold bonds in Fig. 2a,
b
-configuration. Similarly, H-3 and H-5 were assigned as being in
the -configuration based on the NOESY correlations of H-1 /H-3
a
a

T. Yuan et al. / Tetrahedron 68 (2012) 959e964
961
OH
supported the above structure. Therefore, compound 4 was eluci-
OH
OH
dated as 2,6-di-O-galloyl-4-O-(3-methoxygalloyl)-1,5-anhydro-D-
glucitol.
1
7
O
'''
'''
O
Compound 5, obtained as a colorless amorphous solid, displayed
H
4
O
6
6
a molecular formula of C26
H
36
O
11 as determined by HRESIMS at m/z
HO
HO
H
5
5
þ
O
O
O
547.2129 [MþNa] (calcd for C26
36
H O11Na, 547.2155). The IR data
HO
O4
HO
H
1
7
''
2
O
showed the absorption bands corresponding to hydroxy
HO
3
O
3
2
1
1
''
H
ꢀ1
ꢀ1
O
7'
(3406 cm ), and aromatic (1591 and 1501 cm ) functionalities.
O
H
H
1
1'
The H NMR (Table 2) spectrum of 5 displayed signals for two sets
OH
HO
OH
Table 2
1
1
3
a
H
N
M
R
a
n
d
C
N
M
R
d
a
t
a
o
f
c
o
m
p
o
u
n
d
s
5
a
n
d
6
(a)
(b)
No.
5
6
Fig. 2. (a) Key 1He H COSY (▬) and selected HMBC correlations (H/C) of 1. (b) Se-
1
lected NOESY correlations (H4H) of 1.
d
C
d
H
(mult., J in Hz)
d
C
H
d (mult., J in Hz)
1
2
3
4
5
6
7
8
9
136.7
111.4
150.3
144.8
117.7
119.3
72.5
112.0
157.0
117.8
125.9
149.8
117.2
169.9
7.06 (d, 1.4)
6.81 (d, 9.0)
7.26 (dd, 9.0, 2.0)
and H-3/H-5. Thus, the 1-deoxysugar moiety was determined as
1,5-anhydro-glucitol. The three galloyl groups were assigned to the
7.03 (d, 8.3)
6.92 (br d, 8.3)
4.81 (d, 5.0)
4.29 (m)
C-2, C-3, and C-6 positions of the 1,5-anhydro-glucitol by the HMBC
7.49 (d, 2.0)
0
00
correlations from H-2 to C-7 , from H-3 to C-7 , and from H
7
2
-6 to C-
D
0
00
85.0
60.8
, respectively. Acid hydrolysis of 1 afforded 1,5-anhydro- -glu-
3.80 (m)
citol, which was identified by direct co-TLC and optical rotation
comparisons with an authentic sample. Thus, the structure of
3.78 (m)
0
0
0
0
0
0
1
2
3
4
5
6
136.7
112.6
150.3
145.8
118.1
120.4
102.1
73.4
76.4
70.4
74.3
63.5
4.77 (d, 7.2)
3.45 (m)
3.47 (m)
3.42 (m)
3.70 (m)
compound 1 was determined as 2,3,6-tri-O-galloyl-1,5-anhydro-
D-
6.77 (br s)
glucitol assigned the trivial name of maplexin F.
Compound 2 (maplexin G), a colorless amorphous solid, had
6.78 (d, 8.3)
6.65 (dd, 8.3, 1.6)
a molecular formula of C28
H
26
O
17 as determined by HRESIMS at m/z
4.61 (br d, 11.8)
4.36 (dd, 11.8, 7.0)
ꢀ
6
33.1076 [MꢀH] (calcd for C28
25
H O17, 633.1092). The UV and IR
1
13
0
0
0
00
00
0
00
0
0
spectra, as well as the H and C NMR data (Table 1), were similar
to compound 1 and it was apparent that their only difference was
likely the presence of an additional methyl group in 2. Further
analysis of the 2D-NMR data allowed the establishment of the
7
8
9
1
2
3
4
5
6
31.2
34.1
60.8
100.0
70.6
70.8
72.4
69.3
16.6
2.59 (2H, t, 7.8)
1.79 (2H, quint, 7.1)
3.54 (2H, t, 6.4)
5.31 (d, 1.4)
4.05 (dd, 3.0, 1.4)
3.86 (m)
3.44 (t, 9.6)
3.79 (m)
1.21 (d, 6.1)
120.0
108.8
145.1
138.5
145.1
108.8
7.05 (d, 1.9)
0
structure of 2. The HMBC correlation from the methoxy protons (d
H
0
0
00
3
.83) to C-3 revealed that a methoxy group was linked to C-3 .
Thus the structure of compound 2 was elucidated as 2,6-di-O-gal-
loyl-3-O-(3-methoxygalloyl)-1,5-anhydro- -glucitol.
Compound 3 (maplexin H), a colorless amorphous solid, pos-
0
0
00
(7 )
7.05 (d, 1.9)
3.77 (3H, s)
D
166.8
OCH3
3eOCH3:
51.4
sessed a molecular formula of C28
H
26
O
16 from the HRESIMS data at
d
C
0
55.0,
d
H
3.78 (3H, s)
3.77 (3H, s)
ꢀ
3 -OCH :
d
3
m/z 617.1154 [MꢀH] (calcd for C28
25
H O16, 617.1143), which was
C
55.0,
d
H
sixteen mass units less than that of 2. The IR and UV spectra, as well
1
13
a
OD at 500 MHz ( H) and 125 MHz (13C).
1
Data were measured in CD
3
as the H and C NMR data (Table 1), were similar to 2. Extensive
analysis of the 2D-NMR data revealed that the difference between
compounds 3 and 2 is one less hydroxyl group in 3. The HMBC
0
0
00
00
00
00
00
correlations from H-5 to C-1 and C-3 , from H-6 to C-2 and C-
4
of a 1,3,4-trisubstituted aromatic ring [
d
H
7.06 (1H, d, J¼1.4 Hz, H-2),
6.77
0
0
00
00
, and from H-2 and H-6 to C-7 , indicated the presence of
7.03 (1H, d, J¼8.3 Hz, H-5), 6.92 (1H, br d, J¼8.3 Hz, H-6);
d
H
0
0
0
0
a vanilloyl group. The HMBC correlation between H-3 and C-7
(1H, br s, H-2 ), 6.78 (1H, d, J¼8.3 Hz, H-5 ), 6.65 (1H, dd, J¼8.3,
0
implied that the vanilloyl group was linked to C-3 of the glucitol
moiety via an oxygen. The two additional galloyl groups were
connected to C-2 and C-6 of the glucitol moiety based on the HMBC
1.6 Hz, H-6 )], a series of oxygenated methylene and methine pro-
ton signals at
(each 3H, s) and a methyl signal at
d
H
3.44e5.31, two methoxy signals at
H
d 3.78, 3.77
13
d
H
1.21 (3H, d, J¼6.1 Hz). The
C
0
000
correlations from H-2 to C-7 , and from H
Thus, the structure of compound 3 was determined as 2,6-di-O-
galloyl-3-O-vanilloyl-1,5-anhydro- -glucitol.
Compound 4 (maplexin I), an amorphous solid, had the same
2
-6 and C-7 , respectively.
NMR spectrum (Table 2) of 5 displayed 24 signals (two of them
coincidental but distinguishable by the HMBC spectrum), of which
six carbon signals were assigned to a sugar moiety. The afore-
mentioned data suggested that compound 5 is likely a lignan
glycoside.
Detailed analysis of the 1D and 2D-NMR data (including He H
COSY, HSQC, HMBC) allowed the establishment of the structure of 5.
The He H COSY spectra suggested three substructures: a rham-
D
molecular formula of C28
H
26
O
17 as 2 based on the HRESIMS data at
ꢀ
1
1
m/z 633.1101 [MꢀH] (calcd for C28
H
25
O17, 633.1092) data. Analysis
1
13
of the H and C NMR data (Table 1) showed the presence of two
sets of galloyls, a 3-methoxygalloyl, and a 1,5-anhydro-glucitol
moiety. This suggested that compounds 4 and 2 possessed similar
substituents and the only difference was likely in the location of
these groups on the 1,5-anhydro-glucitol. The HMBC correlations
1
1
0
0
00
nose (C-1 to C-6 ), a trioxygenated propyl function (C-7 to C-9),
0
0
and a hydroxypropyl group (C-7 to C-9 ) (drawn with bold bond in
Fig. 3a). Analysis of the HMBC spectrum enabled the connectivity of
the three spin coupling fragments and the other functional groups.
The HMBC correlations from H-7 to C-1, C-2 and C-6 supported the
attachment between C-7 of the trioxygenated propyl and C-1 of the
aromatic ring. Similar HMBC correlations allowed the attachment
0
00
000
from H-2 to C-7 , from H-4 to C-7 , and from H -6 and C-7 , in-
2
dicated that the two galloyls and the 3-methoxygalloyl groups were
attached at C-2, C-6, and C-4 of the glucitol moiety, respectively.
The distinct changes in chemical shifts of C-3 and C-4, and the
carbons in the vicinity of C-3 and C-4 as compared to those of 2,
0
0
0
between C-7 and C-1 . The HMBC correlation from H-8 to C-4 , and

962
T. Yuan et al. / Tetrahedron 68 (2012) 959e964
OH
HO
7
9
O
7
OCH3
1
8
O
OH
HO
7
''
O
1
4
OH
O
O
''
2
3
4'
1'
8'
HO
6'
5
1
OCH3
OH
O
1'
O
3
'
HO
HO
HO
O
H CO
3
7'
9'
OH
HO
OH
(a)
(b)
Fig. 3. (a) Key 1He H COSY (▬) and selected HMBC correlations (H/C) of 5. (b) Key He H COSY (▬) and selected HMBC correlations (H/C) of 6.
1
1
1
0
the correlations between two methoxy protons and C-3, C-3 , de-
As shown in Table 3, while the phenolic glycosides (5,6)
were not active, the gallotannins (1e4) inhibited -glucosidase
termined the aglycone of 5 as 1-(4-hydroxy-3-methoxyphenyl)-2-
a
[
4-(3-hydroxypropyl)-2-methoxyphenoxy]-1,3-propanediol. The
activity in a concentration-dependent manner (Fig. 4). Further-
00
HMBC correlation between H-1 and C-4 readily placed the sugar
moiety at C-4. Thus, the planar structure of 5 was established. The
stereochemistry for C-7 and C-8 was assigned to be erythro from the
J coupling constant value (5.0 Hz) of H-7. Acid hydrolysis of 5
afforded -rhamnose, which was identified by direct comparison
with an authentic sample (see Experimental section). The structure
of compound 5 was thus elucidated as 1-[4-O-( -rhamnopyr-
anosyl)-3-methoxyphenyl]-2-[4-(3-hydroxypropyl)-2-methoxyp-
henoxy]-1,3-propanediol, assigned the common name of
rubrumoside A.
Table 3
a
-Glucosidase inhibitory activities of compounds 1e6a
16
No.
IC50 (mM)
L
1
2
3
4
11.45ꢂ0.45
16.14ꢂ0.05
7.91ꢂ0.03
11.46ꢂ0.85
a-L
5
n.d.
6
n.d.
161.38ꢂ5.50
Acarboseb
Compound 6, a white amorphous solid, had a molecular formula
n.d.¼not detected.
a
ꢀ
of C21
calcd for C21
exhibited an ABX spin system at
1H, dd, J¼9.0, 2.0 Hz, H-4), 6.81 (1H, d, J¼9.0 Hz, H-3), two aro-
H
22
O
13 as determined by HRESIMS at m/z 481.0983 [MꢀH]
IC50 values are shown as meanꢂSD from three independent experiments.
b
1
Positive control.
(
21
H O
13, 481.0982). Its H NMR spectrum (Table 2)
7.49 (1H, d, J¼2.0 Hz, H-6), 7.26
d
H
(
0
0
00
matic proton signals at
a methoxy signal at 3.77 (3H, s), as well as sugar moiety reso-
nances at 3.42e4.77. Apart from carbon signals typical of a gal-
loyl moiety at
166.8, 145.1 (2ꢁ C), 138.5, 120.0, 108.8 (2ꢁ C), the
C NMR spectrum of 6 showed six aromatic carbons signals, one
ester carbonyl signal at 169.9, a methoxy group at 51.4, and six
d
H
7.05 (2H, d, J¼1.9 Hz, H-2 , H-6 ),
d
H
d
H
d
C
1
3
d
C
d
C
1
1
sugar-derived carbons. The He H COSY spectrum (Fig. 3b)
revealed the presence of a glucopyranosyl unit in 6, and the reso-
0
nance of the anomeric proton
dicated that it was
Fig. 3b) from H-3 to C-1 and C-5, from H-4 to C-2, from H-6 to C-2,
d
H
4.77 (1H, d, J¼7.2 Hz, H-1 ) in-
b
-glycosidic linkage. The HMBC correlations
(
1
2
3
4
acarbose
C-4, and C-7, and from the methoxy protons to C-7, indicated that
the aglycone of 6 was methyl gentisate. The glucopyranosyl unit
was attached to C-5 of methyl gentisate and the galloyl group was
Fig. 4. Concentration-dependent
bose (concentration from high to low, 1: 18.0, 9.0, 4.5, 2.3
4.0, 7.0, 3.5, 1.8 M; 4: 15.0, 7.5, 3.8, 1.9 M; Acarbose: 774.0, 387.0, 193.5, 96.8 m
a
-glucosidase inhibition of compounds 1e4 and Acar-
M; 2: 22.0, 11.0, 5.5, 2.8 M; 3:
M).
m
m
1
m
m
0
linked to C-6 of the sugar moiety based on the HMBC correlations
0
0
00
from H-1 to C-5, and from H
2
-6 to C-7 , respectively. The D-con-
more, maplexins FeI (1e4) showed superior activities compared to
Acarbose, which were similar to that observed for maplexin E.
Among the gallotannins identified here, maplexin H (3) showed
5
figuration of the glucopyranosyl was determined by acid hydrolysis
as for the other compounds previously discussed (see Experimental
section). The structure of compound 6 was therefore elucidated as
the strongest
which was 20 fold more potent than Acarbose (IC50¼161.38
In conclusion, four new -glucosidase inhibitory gallotannins
a
-glucosidase inhibitory activity (IC50¼7.91
m
M).
M),
5
-O-(6-galloyl)-
b
-D
-glucopyranosyl methyl gentisate assigned the
m
common name of rubrumoside B.
a
Eleven known compounds were identified as nymphaeoside A
were identified from red maple bark along with two new phenolic
glycosides and eleven known compounds. This study supports the
growing body of in vitro and in vivo data suggesting that gallo-
7),16 gallic acid (8), ginnalins AeC (9e11),
17
18,19
maplexins AeE
12e16), and methyl vanillate (17) on the basis of their spectral
(
(
5
20
1
13
data ( H and C NMR, and ESIMS).
tannins are the main contributors to the a-glucosidase inhibitory
5
,12,21
Our group and others have reported that gallotannins exhibit
activities of maple plant part extracts.
5
,12,21
in vitro and in vivo
a
-glucosidase inhibitory activities.
Struc-
ture activity related studies have shown that both number and
location of substituents on a 1,5-anhydro-glucitol moiety were
3. Experimental section
5
,12
important for
a
-glucosidase inhibitory activity.
Furthermore,
3.1. General experimental procedures
maplexin E, which was the only gallotannin identified until now
with three substituents linked to 1,5-anhydro-glucitol, was 20
Optical rotations were measured on an Auto Pol III automatic
polarimeter (Rudolph Research, Flanders, NJ, USA) at room tem-
perature. The IR spectra were recorded on a Nicolet 380 FT-IR
spectrometer. The UV spectra were measured on a Shimadzu UV-
2550 UVevisible spectrophotometer. NMR data were recorded on
times more potent in inhibiting
clinical drug, Acarbose. Given that all of the new maplexins
a-glucosidase activity than the
5
identified here also fulfilled these structural criteria, we were very
interested in their a-glucosidase inhibitory activities.

T. Yuan et al. / Tetrahedron 68 (2012) 959e964
963
a Varian 500 MHz instrument with TMS as internal standard.
HRESIMS data were acquired using a Waters LCT Premier Mass
Spectrometer (Waters Corporation, MA, USA). Semi-preparative
HPLC separations were performed on a Hitachi Elite LaChrom sys-
tem consisting of an L2130 pump, L-2200 autosampler, L-2455 di-
eluted with MeOH to give four sub-fractions A3c3c1eA3c3c4. Pu-
rification of fraction A3c3c1 (384.0 mg) by semi-preparative HPLC
(MeOH/H
compounds 8 (102 mg) and 17 (13 mg). Purification of fraction
A3c3c3 (165.0 mg) by semi-preparative HPLC (MeOH/H O: 40/60 to
2
O: 35/65 to 57/43 v/v in 22 min, 3 mL/min) yielded
2
ode array detector, and
a
Phenomenex Luna C18 column
47/53 v/v in 30 min, 3 mL/min) provided compounds 2 (16 mg), 3
(5 mg), 4 (12 mg), and 16 (20 mg). Fraction A3c3c4 (76.0 mg) was
(
250ꢁ10 mm, S-5 m), all operated by EZChrom Elite software.
m
Medium-pressure liquid chromatography (MPLC) separations were
carried out on pre-packed C18 columns (4ꢁ37 cm) connected to
a DLC-10/11 isocratic liquid chromatography pump (D-Star In-
struments, Manassas, VA, USA) with a fixed-wavelength detector.
All solvents were of ACS or HPLC grade and were obtained from
SigmaeAldrich (St. Louis, MO) through Wilkem Scientific (Pawca-
tuck, RI, USA). Silica gel (230e400 mesh, Sorbent Technologies),
Sephadex LH-20 gel (Amersham Biosciences), and MCI gel (CHP20P,
2
purified by semi-preparative HPLC (MeOH/H O: 35/65 to 51/49 v/v
in 16 min, 3 mL/min) to give compounds 1 (14 mg) and 6 (12 mg).
2
0
3.3.1. Maplexin F (1). Colorless amorphous solid; ½
MeOH); UV (MeOH) max (log ): 276 (4.32), 218 (4.01) nm; IR
3422, 2981, 1699, 1610, 1498 cm ; H NMR (CD
a
ꢃ
þ68 (c 0.1,
D
l
3
ꢀ
n
max
1 1
3
OD, 500 MHz) and
OD, 125 MHz) data, see Table 1; HRESIMS at m/z
13
C NMR (CD
619.0949 [MꢀH] (calcd for C27
3
ꢀ
23
H O17, 619.0935).
6
3e150 mM, M & M Industries Inc.) were used for column chro-
2
0
3
.3.2. Maplexin G (2). Colorless amorphous solid; ½
a
ꢃ
þ74 (c 0.2,
matography, and pre-coated silica gel GF254 plates (Whatman Ltd,
Maidstone, Kent, England) were used for TLC analyses. Standards of
D
MeOH); UV (MeOH)
lmax (log
3
): 277 (4.42), 217 (4.10) nm; IR n
max
406, 2987, 1701, 1598, 1496 cm ; H NMR (CD OD, 500 MHz) and
ꢀ
1 1
3
3
L-rhamnose,
D-glucose, and 1,5-anhydro-D-glucitol were purchased
1
3
C NMR (CD
33.1076 [MꢀH] (calcd for C28
3
OD, 125 MHz) data, see Table 1; HRESIMS at m/z
from SigmaeAldrich (St. Louis, MO, USA).
ꢀ
6
25
H O17, 633.1092).
3
.2. Plant material
2
0
3
.3.3. Maplexin H (3). Colorless amorphous solid; ½
a
ꢃ
þ90 (c 0.1,
D
MeOH); UV (MeOH)
l
max (log ): 269 (4.03), 212 (3.91) nm; IR n
max
398, 2968, 1703, 1605, 1500 cm ; H NMR (CD OD, 500 MHz) and
3
Red maple (A. rubrum) bark were collected in the summer of
ꢀ1 1
3
3
2
009 by the Federation of Maple Syrup Producers of Quebec
1
3
C NMR (CD
3
OD, 125 MHz) data, see Table 1; HRESIMS at m/z
(
Canada), shipped to our laboratory in August 2009, and identified
ꢀ
617.1154 [MꢀH] (calcd for C28
25
H O16, 617.1143).
by Mr. J. Peter Morgan. A voucher specimen (JPMCB2) has been
deposited in the Heber Youngken Herbarium and Greenhouse,
College of Pharmacy, University of Rhode Island (Kingston, RI, USA).
2
0
3
.3.4. Maplexin I (4). Colorless amorphous solid; ½
a
ꢃ
þ67 (c 0.1,
D
MeOH); UV (MeOH)
lmax (log
3
): 278 (4.38), 217 (4.09) nm; IR n
max
410, 2995, 1705, 1601, 1502 cm ; H NMR (CD OD, 500 MHz) and
ꢀ
1 1
3
3
3
.3. Extraction and isolation
1
3
C NMR (CD
33.1101 [MꢀH] (calcd for C28
3
OD, 125 MHz) data, see Table 1; HRESIMS at m/z
ꢀ
6
25
H O17, 633.1092).
The air-dried ground bark (2.9 kg) of A. rubrum was extracted by
maceration with methanol (10 Lꢁ3 times for 7 days per time pe-
riod) at room temperature to afford 350.4 g of crude extract. The
extract was suspended in distilled water (1 L) and then extracted
successively with ethyl acetate (1 Lꢁ3 times) followed by n-butanol
2
0
3
0
.3.5. Rubrumoside A (5). Colorless amorphous solid; ½
a
ꢃ
ꢀ10 (c
D
.1, MeOH); UV (MeOH) max (log ): 277 (3.42), 230 (sh), 216 (3.93)
ꢀ
max 3406, 2968, 1591, 1501, 1460 cm ; H NMR (CD OD,
3
l
3
1 1
nm; IR
5
n
13
00 MHz) and C NMR (CD
3
OD, 125 MHz) data, see Table 2; HRE-
(
1 Lꢁ3 times). The ethyl acetate fraction (106.0 g) was chromato-
þ
SIMS at m/z 547.2129 [MþNa] (calcd for C26
H
36
O
11Na, 547.2155).
graphed over a column (5ꢁ50 cm) of MCI gel (MeOH/H
2
O, 50/50 to
9
0/10 v/v) to yield three fractions (AeC). Fraction A (73.3 g) was
20
D
3
0
.3.6. Rubrumoside B (6). Colorless amorphous solid; ½
a
ꢃ
þ52 (c
subjected to silica gel column chromatography (8ꢁ35 cm) eluted
.1, MeOH); UV (MeOH)
l
max (log ): 276 (3.45), 212 (3.91) nm; IR
max 3408, 2985, 1701, 1601, 1503 cm ; H NMR (CD OD, 500 MHz)
3
and C NMR (CD OD, 125 MHz) data, see Table 2; HRESIMS at m/z
3
3
with gradient solvent system of chloroform/methanol (10/1 to 5/
ꢀ1 1
n
1
v/v) to yield three fractions (A1eA3). Fraction A3 (28.1 g) was
chromatographed over MPLC eluted with MeOH/H O (10/90 to 70/
0 v/v, 3 mL/min) to give three sub-fractions (A3aeA3c). Fraction
13
2
81.0983 [MꢀH]ꢀ (calcd for C21
4
21
H O13, 481.0982).
3
A3a (3.7 g) was separated by
a Sephadex LH-20 column
3
.4. Acid hydrolysis of compounds 1e6 and sugar analysis
(
3.5ꢁ120 cm) eluted with MeOH to give three sub-fractions
A3a1eA3a3. Purification of A3a1 (87.0 mg) by semi-preparative
HPLC (MeOH/H O: 20/80 v/v, 3 mL/min) yielded compounds 12
11 mg) and 13 (10 mg). Fraction A3b (185.0 mg) was purified by
semi-preparative HPLC (MeOH/H O: 15/85 v/v, 3 mL/min) yielded
Each compound (1 mg) was added to a mixture of concentrated
2
HCl (0.5 mL), H
After completion of the reaction (monitored by TLC), H
added to the reaction mixture, which was then extracted with
CHCl
3
and then concentrated to dryness under reduced pressure and
purified by Sephadex LH-20 column chromatography to give
a sugar fraction. The sugar fractions of 1e4 were identified as 1,5-
anhydro-
authentic sample (R
optical rotation). The glycosides of 5 and 6 were identified as
O (1.5 mL), and dioxane (3 mL) and refluxed for 2 h.
2
(
2
O was
2
compounds 10 (22 mg) and 11 (25 mg). Fraction A3c (9.8 g) was
separated by a Sephadex LH-20 column (3.5ꢁ120 cm) eluted with
MeOH to give three sub-fractions A3c1eA3c3. Purification of frac-
3
(3ꢁ5 mL). The aqueous layer was neutralized with NaHCO
tion A3c1 (35.0 mg) by semi-preparative HPLC (MeOH/H
to 47/53 v/v in 24 min, 3 mL/min) yielded compounds 5 (3 mg) and
(4 mg). Fraction A3c3 (6.5 g) was subjected to MPLC eluted with
MeOH/H O (30/70 to 100/0 v/v, 3 mL/min) to give three sub-
2
O: 30/70
D
-glucitol by comparison co-TLC and specific rotation with
7
f
¼0.43, CHCl /MeOH, 10/1, positive value for
3
2
L-
fractions (A3c3aeA3c3c). Fraction A3c3b (1.8 g) and A3c3c (3.2 g)
precipitated compound 9 (2.8 g). The mother-liquor of fraction
A3c3b was further chromatographed over a Sephadex LH-20 col-
umn (3ꢁ80 cm) eluted with MeOH to give three sub-fractions
A3c3b1eA3c3b3. Purification of A3c3b2 (143.0 mg) by semi-
rhamnose (R
f
¼0.34, CHCl
3
/MeOH/H
2
O, 2/1/0.1 v/v/v) and D-glucose
(
R
f
¼0.45, CHCl
3
/MeOH/H
2
O, 1/1/0.1 v/v/v, positive value for optical
rotation) using similar methods as those of 1e4, respectively.
3.5.
a-Glucosidase inhibition assay
2
preparative HPLC (MeOH/H O: 28/72 v/v, 3 mL/min) yielded com-
pounds 14 (15 mg) and 15 (32 mg). The mother-liquor of fraction
A3c3c was separated by a Sephadex LH-20 column (3ꢁ80 cm)
A mixture of 50
mL of different concentrations of the compounds
and 100 L of 0.1 M phosphate buffer (pH 6.9) containing yeast
m

964
T. Yuan et al. / Tetrahedron 68 (2012) 959e964
a
-glucosidase solution (1.0 U/ml) was incubated in 96 well plates at
References and notes
ꢄ
2
5
C for 10 min. After pre-incubation, 50
mL of 5 mM p-nitro-
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inhibition and was calculated as follows:
mL buffer solutions in place of the
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a
8. Li, L.; Seeram, N. P. J. Funct. Foods 2011, 3, 125.
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control
Inhibition ¼
ꢁ 100
D
Abs
1902.
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Acknowledgements
1
1
This project was supported by Agriculture and Agri-Food Canada
from the Developing Innovative Agri-Products program. The Feder-
ation of Quebec Maple Syrup Producers participated in the collection
of plant materials from Quebec, Canada. The authors would like to
thank Mr. J. Peter Morgan for assistance with plant authentication.
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