Synthesis of furofuran lignans as antidiabetic agents simultaneously achieved by inhibiting α-glucosidase and free radical

Article abstract of DOI:10.1007/s12272-016-0778-9

Furofuran lignans such as sesamin have been recognized as promising antidiabetic agents as they possess curative as well as preventive effects toward diabetes complications. However, to date the structure–activity relationship has not been investigated due to the lack of a practical synthetic route capable of producing diverse furofuran lignans. Herein, we first introduced a single-step synthesis of these compounds starting from samin (4). Reaction of samin with a variety of electron-rich phenolics under acidic conditions afforded a total of 23 diverse furofuran lignans. On examination their inhibitions against α-glucosidase and free radicals, lignans having a free hydroxy group showed considerably enhanced inhibition, compared with their corresponding starter 4 and related lignans sesamin (1) and sesamolin (3). In addition, the mechanism underlying the α-glucosidase inhibition of a particular active lignan (epi-6) was verified to be mixed manner between competitive and noncompetitive inhibition.

Full text of DOI:10.1007/s12272-016-0778-9

Arch. Pharm. Res.
DOI 10.1007/s12272-016-0778-9
RESEARCH ARTICLE
Synthesis of furofuran lignans as antidiabetic agents
simultaneously achieved by inhibiting a-glucosidase and free
radical
Wisuttaya Worawalai1 Phonpimon Khongchai1 Nantaporn Surachaitanawat1
Preecha Phuwapraisirisan1
•
•
•
Received: 24 February 2016 / Accepted: 16 June 2016
Ó The Pharmaceutical Society of Korea 2016
Abstract Furofuran lignans such as sesamin have been
recognized as promising antidiabetic agents as they possess
curative as well as preventive effects toward diabetes
complications. However, to date the structure–activity
relationship has not been investigated due to the lack of a
practical synthetic route capable of producing diverse
furofuran lignans. Herein, we first introduced a single-step
synthesis of these compounds starting from samin (4).
Reaction of samin with a variety of electron-rich phenolics
under acidic conditions afforded a total of 23 diverse
furofuran lignans. On examination their inhibitions against
a-glucosidase and free radicals, lignans having a free
hydroxy group showed considerably enhanced inhibition,
compared with their corresponding starter 4 and related
lignans sesamin (1) and sesamolin (3). In addition, the
mechanism underlying the a-glucosidase inhibition of a
particular active lignan (epi-6) was verified to be mixed
manner between competitive and noncompetitive
inhibition.
Introduction
Hyperglycemia in diabetes is the most pivotal initiating
cause of impairment of various vulnerable tissues such as
capillary endothelial cells in the retina and Schwann cells
in peripheral nerves (Brownlee 2005). Prolonged tissue
impairment thus results in the rapid incidence of diabetes
complications such as cardiomyopathy, nephropathy, neu-
ropathy and retinopathy. To relieve the onset of diabetes
and prevent its complications, an alternative approach that
concomitantly attenuates the release of glucose from
oligosaccharides by blocking the a-glucosidase function
and scavenging the causative radicals generated during
glycolysis was introduced (Maritim et al. 2003).
This approach prompted us to search for potent antidiabetic
agents having dual functions from natural sources (Ramadhan
and Phuwapraisirisan 2015a, b) and through semisynthesis
(Rattanangkool et al. 2013; Worawalai et al. 2015). Noticeably,
introducing polyphenol or phenolic moiety to glucose mimic
core structure considerably improved a-glucosidase inhibition
together with antioxidation.
Keywords Sesamum indicum Á Diabetes Á Glucosidase Á
Antioxidant Á Friedel-Crafts Á Oxocarbenium
In the present communication, we are interested in modi-
fying sesamin (1, Fig. 1) structure by replacing methylene-
dioxybenzene moiety (A) with a variety of phenolics (ArOH).
This idea was inspired by the considerable evidence con-
cerning its antidiabetic activity as well as other beneficial
effects that would alleviate diabetic complications (Hong
et al. 2013; Baluchnejadmojarad et al. 2013; Guo et al. 2012;
Roghani et al. 2011). Other related structures such as (?)-
pinoresinol (2) also displayed an antidiabetic effect through
inhibiting a-glucosidase (Wikul et al. 2012). Therefore, syn-
thesis of furofuran lignans, the sesamin-like structures, vary-
ing with different phenolics could lead to the discovery of
new antidiabetic agents that have dual beneficial effects that
reduced the administrated dose.
Electronic supplementary material The online version of this
article (doi:10.1007/s12272-016-0778-9) contains supplementary
material, which is available to authorized users.
& Preecha Phuwapraisirisan
preecha.p@chula.ac.th
1
Natural Products Research Unit, Department of Chemistry,
Faculty of Science, Chulalongkorn University, Bangkok,
Thailand
123

W. Worawalai et al.
To date, there have been two distinct methods employed
to synthesize furofuran lignans. Total synthesis starting
from small phenolics enabled the construction of the
desired lignans (Hull and Knight 1997; Brown et al. 2001;
Pohmakotr et al. 2006). However, this methodology could
not practically target a wide variety of furofuran lignans
due to the low overall yield and limit of phenolics applied.
Alternatively, a particular furofuran lignans could be
facilely synthesized from naturally abundant precursors
such as sesamolin (3) in a single step under acidic condi-
tions (Huang et al. 2012; Marchand et al. 1997a). The
presence of acetal in sesamolin (3) makes it more reactive
toward nucleophilic substitution. However, to date, appli-
cation of 3 as a starter in the synthesis of various furofuran
lignans has been limited.
spectrometer using tetramethylsilane (TMS) as an internal
standard. Chemical shifts are reported in ppm downfield
from TMS. Analytical thin layer chromatography (TLC)
was performed on pre-coated Merck silica gel 60 F254
plates (0.25 mm thick layer) and visualized by p-
anisaldehyde reagent. Column chromatography was per-
formed using Merck silica gel 60 (70–230 mesh) and
Sephadex LH-20. Sucrose, maltose and rat intestinal ace-
tone powder were purchased from Sigma-Aldrich (St.
Louis, MO, USA). Glucose assay kit was purchased from
Human Gesellschaft fu¨r Biochemica und Diagnostica mbH
(Germany). Acarbose was obtained from Bayer (Germany).
Chemistry
To circumvent the restricted application of 3 in
semisynthesis, we introduced the idea of employing samin
(4), instead of 3, as the starting material. This approach is
based on the fact that hemiacetal moiety in 4 is relatively
more reactive, than the acetal residue in 3 toward nucle-
ophilic substitution. In addition, the release of H2O gen-
erated upon protonation of hemiacetal could be readily
trapped by a molecular sieve, preventing the regeneration
of samin (4). Therefore, this proposed methodology would
generate the desired furofuran lignans, by varying electron-
rich phenolics (ArOH) as nucleophiles (Scheme 1), in
sufficient amounts that can be further investigated for
structure–activity relationship (SAR).
Hydrolysis of sesamolin
To a solution of sesamolin (3, 0.27 mmol) in a mixture of
acetonitrile/H2O (9:1, 10 mL) was treated with acidic resin
Amberlyst-15 (1 mg/0.005 mmol of 3). After stirring at
70 °C for 8 h, the reaction mixture was evaporated to
dryness and purified by flash chromatography
(50 %EtOAc-hexane) to give samin 4 (60 mg, 90 %) as a
colorless oil.
25
D
Samin (4); colorless solid; ½aŠ ¼ þ198 (c 0.1, CHCl3);
1H NMR (CDCl3, 400 MHz) d 6.85 (s, 1H, H-20),
6.80–6.75 (m, 2H, H-50 and H-60), 5.94 (s, 2H, OCH2O),
5.36 (s, 1H, H-2), 4.35 (d, J = 8.4 Hz, 2H, H-6 and H-8),
4.16 (dd, J = 9.2, 6.0 Hz, 1H, H-4), 3.89 (d, J = 9.2 Hz,
1H, H-4), 3.56 (dd, J = 8.8, 7.2 Hz, 1H, H-8), 3.25 (brs,
1H, –OH), 3.05 (m, 1H, H-1), 2.86 (m, 1H, H-5); 13C NMR
(CDCl3, 100 MHz) d 148.1, 147.4, 134.7, 119.7, 108.3,
Materials and methods
General
O
O
ArOH
*
OH
All moisture-sensitive reactions were carried out under a
nitrogen atmosphere. All solvents were distilled prior to
use. High resolution electrospray ionisation mass spectra
(HRESIMS) was recorded with a Bruker microTof spec-
H
H
H
H
+
ArOH
(a-k)
O
O
O
O
1
trometer. H and 13C NMR spectra were recorded (CDCl3
O
5
4
O
as solvent) at 400 and 100 MHz, respectively, on a Varian
Mercury? 400 NMR and a Bruker (Avance) 400 NMR
Scheme 1 Synthetic plan for furofuran lignans (5) in this work
Fig. 1 Structures of selected furofuran lignans
123

Synthesis of furofuran lignans as antidiabetic agents simultaneously achieved by inhibiting…
106.7, 102.4, 101.2, 87.0, 71.4, 69.5, 53.7, 52.9. These data
coincided well with those in previous report (J. Agric.
Food Chem., 2011, 59, 3214).
6.81–6.76 (m, 2H), 6.50 (dd, J = 8.4, 2.4 Hz, 1H), 6.44 (s,
1H), 5.94 (s, 2H), 4.91 (d, J = 6.0 Hz, 1H), 4.36 (d,
J = 8.0 Hz, 1H), 4.09 (d, J = 9.2 Hz, 1H), 3.81 (s, 3H, –
OCH3), 3.80 (s, 3H, –OCH3), 3.78–3.74 (m, 2H), 3.47 (m,
1H), 3.22 (dd, J = 8.8, 8.8 Hz, 1H), 2.84 (m, 1H); 13C
NMR (CDCl3, 100 MHz) d 160.2, 156.6, 148.1, 147.3,
135.6, 127.3, 127.3, 119.7, 108.3, 106.8, 103.9, 101.1,
98.3, 87.6, 78.6, 70.5, 69.9, 55.5, 55.4, 54.9, 48.7; HRMS
m/z 393.1310 [M ? Na]? (calcd for C21H22NaO6,
393.1314).
General procedure for synthesis of furofuran lignans 5
To a solution of samin 4 (1 equiv) in acetonitrile (1.0 mL/
0.1 mmol of 4) was treated with phenolics (1.5–2 equiv),
˚
Amberlyst-15 (1 mg/0.005 mmol of 4) and a 4 A molec-
ular sieve. After stirring at 70 °C for 8 h, the reaction
mixture was evaporated to dryness and purified by flash
chromatography.
5c Yield: 20 %, white powder; 1H NMR (CDCl3,
400 MHz) d 6.94–6.76 (m, 3H), 6.38 (s, 2H), 5.95 (s, 2H),
5.47 (d, J = 5.9 Hz, 1H), 4.70 (d, J = 6.2 Hz, 1H), 4.27
(dd, J = 8.5, 6.6 Hz, 1H), 4.16 (dd, J = 8.9, 7.2 Hz, 1H),
3.89–3.81 (m, 2H), 3.80 (s, 6H, –OCH3 (92)), 3.55–3.44
(m, 1H), 3.16–3.01 (m, 1H), 2.33 (s, 3H, –CH3); 13C NMR
(CDCl3, 100 MHz) d 158.9, 148.0, 147.1, 139.9, 136.0,
119.6, 114.1, 108.3, 106.7, 105.4, 101.1, 85.6, 78.3, 73.2,
72.3, 56.4, 55.9, 51.2, 22.2; HRMS m/z 407.1469
[M ? Na]? (calcd for C22H24NaO6, 407.1471).
5a Yield: 30 %, white powder; 1H NMR (CDCl3,
400 MHz) d 7.89 (brs, 1H, –OH), 6.92 (d, J = 7.6 Hz, 1H,
H-600), 6.83–6.78 (m, 3H, H-20, H-50, and H-60), 6.71 (s, 1H,
H-300), 6.67 (d, J = 7.6 Hz, 1H, H-500), 5.95 (s, 2H, H-70),
4.87 (d, J = 4.0 Hz, 1H, H-2), 4.78 (d, J = 4.0 Hz, 1H,
H-6), 4.34 (dd, J = 9.2, 7.6 Hz, 1H, H-4), 4.15 (dd,
J = 9.2, 6.8 Hz, 1H, H-8), 3.92–3.85 (m, 2H, H-4 and
H-8), 3.21 (m, 1H, H-1), 3.14 (m, 1H, H-5), 2.29 (s, 3H, –
CH3); 13C NMR (CDCl3, 100 MHz) d 155.5, 148.2, 147.4,
139.8, 134.8, 126.8, 120.9, 120.9, 119.5, 117.9, 108.4,
106.7, 101.3, 86.7, 85.6, 72.5, 70.9, 53.6, 53.1, 21.2;
HRMS m/z 363.1212 [M ? Na]? (calcd for C20H20NaO5,
363.1208).
1
epi-5c Yield: 21 %, white powder; H NMR (CDCl3,
400 MHz) d 6.91–6.72 (m, 3H), 6.33 (brs, 1H), 6.32 (brs,
1H), 5.96 (s, 2H), 5.22 (d, J = 8.0 Hz, 1H), 4.81 (d,
J = 8.0 Hz, 1H), 4.38 (t, J = 8.0 Hz, 1H), 4.10–3.97 (m,
1H), 3.95–3.85 (m, 1H), 3.79 (s, 7H), 3.39–3.27 (m, 1H),
3.24–3.08 (m, 1H), 2.41 (s, 3H, –CH3); 13C NMR (CDCl3,
100 MHz) d 160.0, 159.3, 148.1, 147.1, 139.5, 135.8,
135.4, 125.2, 119.5, 108.3, 108.0, 106.7, 101.2, 97.0, 85.1,
81.5, 72.7, 71.9, 55.8, 55.4, 55.4, 52.1, 21.0; HRMS m/z
407.1470 [M ? Na]? (calcd for C22H24NaO6, 407.1471).
1
epi-5a Yield: 15 %, white powder; H NMR (CDCl3,
400 MHz) d 7.85 (brs, 1H, –OH), 6.92 (d, J = 7.6 Hz, 1H,
H-600), 6.86–6.80 (m, 3H, H-20, H-50, and H-60), 6.71 (s, 1H,
H-300), 6.67 (d, J = 7.6 Hz, 1H, H-500), 5.97 (s, 2H, H-70),
4.85 (d, J = 5.6 Hz, 1H, H-2), 4.55 (d, J = 8.0 Hz, 1H,
H-6), 4.11 (d, J = 9.6 Hz, 1H, H-4), 3.90 (dd, J = 8.4,
7.6 Hz, 1H, H-8), 3.82 (dd, J = 9.6, 6.0 Hz, 1H, H-4),
3.38–3.28 (m, 2H, H-1 and H-8), 3.04 (m, 1H, H-5), 2.29
(s, 3H, –CH3); 13C NMR (CDCl3, 100 MHz) d 155.5,
147.9, 146.9, 139.8, 132.0, 126.9, 121.2, 120.8, 118.8,
118.0, 108.4, 106.5, 101.2, 88.6, 82.0, 70.7, 70.2, 53.4,
49.9, 21.3; HRMS m/z 363.1213 [M ? Na]? (calcd for
C20H20NaO5, 363.1208).
5b Yield: 55 %, colorless oil; 1H NMR (CDCl3,
400 MHz) d 7.25 (dd, J = 8.0, 2.8 Hz, 1H), 6.84 (s, 1H),
6.81–6.74 (m, 2H), 6.45 (d, J = 7.2 Hz, 1H), 6.44 (d,
J = 2.4 Hz, 1H), 5.92 (s, 2H), 5.03 (d, J = 4.8 Hz, 1H),
4.64 (d, J = 5.6 Hz, 1H), 4.30 (dd, J = 9.2, 7.6 Hz, 1H),
4.19 (dd, J = 8.8, 6.4 Hz, 1H), 3.98 (dd, J = 9.2, 5.2 Hz,
1H), 3.90 (dd, J = 9.2, 4.0 Hz, 1H), 3.80 (s, 3H, –OCH3),
3.79 (s, 3H, –OCH3), 3.01 (m, 1H), 2.91 (m, 1H); 13C
NMR (CDCl3, 100 MHz) d 160.2, 157.4, 147.9, 147.1,
135.4, 126.1, 122.8, 119.5, 108.1, 106.6, 103.8, 101.0,
98.6, 85.5, 82.0, 73.3, 71.2, 55.4, 55.3, 54.7, 53.7; HRMS
m/z 393.1310 [M ? Na]? (calcd for C21H22NaO6,
393.1314).
1
epi-5c0 Yield: 23 %, white powder; H NMR (CDCl3,
400 MHz) d 6.92–6.76 (m, 3H), 6.33 (brs, 1H), 6.32 (brs,
1H), 5.96 (s, 2H), 5.02 (d, J = 7.7 Hz, 1H), 4.82 (d,
J = 5.6 Hz, 1H), 4.07 (d, J = 9.3 Hz, 1H), 3.85–3.70 (m,
8H), 3.37 (m,1H), 3.20 (m, 2H), 2.41 (s, 3H, –CH3); 13C
NMR (CDCl3, 100 MHz) d 159.9, 159.3, 147.7, 146.7,
146.6, 139.7, 132.7, 118.8, 108.2, 108.1, 106.6, 101.1,
96.8, 82.0, 81.9, 71.7, 69.6, 55.9, 55.4, 51.8, 51.1, 20.9;
HRMS m/z 407.1469 [M ? Na]? (calcd for C22H24NaO6,
407.1471).
5d Yield: 42 %, yellow oil; 1H NMR (CDCl3, 400 MHz)
d 7.16 (d, J = 8.5 Hz, 1H), 6.90–6.72 (m, 3H), 6.63 (d,
J = 8.5 Hz, 1H), 5.95 (s, 2H), 5.06 (d, J = 4.0 Hz, 1H), 4.68
(d, J = 4.0 Hz, 1H), 4.30 (t, J = 8.2 Hz, 1H), 4.26–4.13 (m,
1H), 3.98 (dd, J = 9.0, 4.8 Hz, 1H), 3.90 (dd, J = 9.1,
3.9 Hz, 1H), 3.82 (s, 3H, –OCH3), 3.75 (s, 3H, –OCH3), 3.10
(m, 1H), 3.00 (m, 1H), 2.16 (s, 3H); 13C NMR (CDCl3,
100 MHz) d 158.6, 157.0, 148.1, 147.2, 135.5, 127.0, 123.8,
120.1, 119.6, 108.3, 106.7, 106.0, 101.2, 85.7, 82.3, 73.1,
71.5, 60.9, 55.8, 54.8, 54.1, 9.2; HRMS m/z 407.1469
[M ? Na]? (calcd for C22H24NaO6, 407.1471).
epi-5b Yield: 45 %, colorless oil; 1H NMR (CDCl3,
400 MHz) d 7.44 (d, J = 8.4 Hz, 1H), 6.87 (s, 1H),
epi-5d Yield: 31 %, yellow oil; 1H NMR (CDCl3,
400 MHz) d 7.36 (d, J = 8.6 Hz, 1H), 6.90–6.74 (m, 3H),
123

W. Worawalai et al.
6.66 (d, J = 8.5 Hz, 1H), 5.94 (s, 2H), 4.96 (d, J = 6.2 Hz,
1H), 4.38 (d, J = 7.4 Hz, 1H), 4.10 (d, J = 9.3 Hz, 1H),
3.83 (m, 4H), 3.74 (m, 4H), 3.46 (m, 1H), 3.24 (t,
J = 8.6 Hz, 1H), 2.87 (m, 1H), 2.16 (s, 3H); 13C NMR
(CDCl3, 100 MHz) d 158.3, 155.7, 148.1, 147.3, 135.6,
124.5, 124.2, 123.6, 119.8, 108.3, 106.78, 105.9, 101.2,
87.7, 78.8, 70.6, 69.9, 60.6, 55.8, 55.0, 49.3, 9.3; HRMS m/
z 407.1470 [M ? Na]? (calcd for C22H24NaO6, 407.1471).
5e Yield: 27 %, yellow oil; 1H NMR (CDCl3, 400 MHz)
d 6.89–6.73 (m, 4H), 6.62 (d, J = 8.6 Hz, 1H), 5.94 (s,
2H), 5.05 (d, J = 4.0 Hz, 1H), 4.68 (d, J = 4.0 Hz, 1H),
4.31 (dd, J = 9.1, 7.3 Hz, 1H), 4.22 (dd, J = 9.1, 6.6 Hz,
1H), 4.01 (dd, J = 9.2, 4.7 Hz, 1H), 3.92 (d, J = 4.3 Hz,
4H), 3.89 (d, J = 7.1 Hz, 4H), 3.10–3.02 (m, 1H),
3.01–2.93 (m, 1H); 13C NMR (CDCl3, 100 MHz) d 148.1,
147.4, 144.6, 138.7, 135.4, 128.3, 119.6, 115.9, 108.3,
106.7, 105.9, 101.2, 85.6, 82.4, 73.1, 71.6, 60.6, 56.4, 54.8,
54.2; HRMS m/z 409.1260 [M ? Na]? (calcd for C21H22-
NaO7, 409.1263).
epi-5e Yield: 37 %, yellow oil; 1H NMR (CDCl3,
400 MHz) d 7.02 (d, J = 8.5 Hz, 1H), 6.89–6.73 (m, 3H),
6.65 (d, J = 8.4 Hz, 1H), 5.95 (s, 2H), 4.95 (d, J = 5.9 Hz,
1H), 4.36 (d, J = 8.0 Hz, 1H), 4.09 (d, J = 9.4 Hz, 1H),
3.97–3.84 (m, 7H), 3.86–3.74 (m, 2H), 3.51–3.40 (m, 1H),
3.24 (t, J = 8.6 Hz, 1H), 2.86 (dd, J = 15.4, 7.2 Hz, 1H);
13C NMR (CDCl3, 100 MHz) d 148.1, 147.3, 147.2, 138.2,
135.5, 129.9, 124.6, 119.7, 116.8, 108.3, 106.8, 105.8, 101.2,
87.7, 78.7, 70.6, 69.9, 60.3, 56.4, 54.9, 49.2; HRMS m/z
409.1261 [M ? Na]? (calcd for C21H22NaO7, 409.1263).
5f Yield: 31 %, brown oil; 1H NMR (CDCl3, 400 MHz)
d 7.03 (d, J = 8.6 Hz, 1H), 6.90–6.73 (m, 3H), 6.64 (d,
J = 8.6 Hz, 1H), 5.95 (s, 2H), 5.03 (d, J = 4.0 Hz, 1H),
4.67 (d, J = 4.0 Hz, 1H), 4.33 (dd, J = 8.9, 7.5 Hz, 1H),
4.21 (dd, J = 9.0, 6.6 Hz, 1H), 3.99 (dd, J = 8.0, 4.0 Hz,
1H), 3.92 (s, 3H, –OCH3), 3.89 (m, 1H), 3.87 (s, 3H, –
OCH3), 3.85 (s, 3H, –OCH3), 3.05 (m, 1H), 2.98 (m, 1H).
13C NMR (CDCl3, 100 MHz) d 153.4, 151.2, 148.1, 147.3,
142.4, 135.4, 128.0, 120.3, 119.6, 108.3, 107.1, 106.7,
101.2, 85.6, 82.4, 73.2, 71.5, 60.9, 60.9, 56.2, 54.8, 54.2;
HRMS m/z 423.1431 [M ? Na]? (calcd for C22H24NaO7,
423.1420).
5g Yield: 68 %, white powder; 1H NMR (CDCl3,
400 MHz) d 7.71 (brs, 1H, –OH), 6.84–6.79 (m, 3H), 6.54
(s, 1H), 6.49 (s, 1H), 5.96 (s, 2H), 4.82 (d, J = 8.0 Hz,
1H), 4.78 (d, J = 8.0 Hz, 1H), 4.36 (dd, J = 8.8, 7.2 Hz,
1H), 4.16 (dd, J = 9.6, 6.4 Hz, 1H), 3.92–3.86 (m, 2H),
3.84 (s, 3H, –OCH3), 3.82 (s, 3H, –OCH3), 3.21–3.14 (m,
2H); 13C NMR (CDCl3, 100 MHz) d 150.3, 150.1, 148.2,
147.4, 142.6, 134.8, 125.2, 119.5, 111.2, 108.4, 106.7,
102.1, 101.3, 86.7, 85.6, 72.6, 70.8, 57.2, 56.1, 53.6, 53.2;
HRMS m/z 409.1286 [M ? Na]? (calcd for C21H22NaO7,
409.1263).
1
epi-5g Yield: 17 %, white powder; H NMR (CDCl3,
400 MHz) d 8.05 (brs, 1H, –OH), 6.87–6.77 (m, 3H), 6.46
(s, 1H), 6.42 (s, 1H), 5.95 (s, 2H), 5.01 (d, J = 8.0 Hz,
1H), 4.44 (d, J = 6.8 Hz, 1H), 4.19 (d, J = 9.6 Hz, 1H),
3.98 (t, J = 8.8 Hz, 1H), 3.88 (m, 1H), 3.85 (s, 3H, –
OCH3), 3.80 (s, 3H, –OCH3), 3.49 (dd, J = 8.4, 9.2 Hz,
1H), 3.40 (m, 1H), 2.91 (m, 1H); 13C NMR (CDCl3,
100 MHz) d 150.1, 149.8, 148.2, 147.5, 142.7, 134.8,
125.2, 119.8, 110.5, 108.4, 106.7, 101.9, 101.2, 87.7, 84.6,
71.9, 70.1, 57.0, 56.0, 53.7, 50.8; HRMS m/z 409.1275
[M ? Na]? (calcd for C21H22NaO7, 409.1263).
5h Yield: 50 %, colorless oil; 1H NMR (CDCl3,
400 MHz) d 6.89 (s, 1H), 6.84 (d, J = 8.0 Hz, 1H), 6.78
(d, J = 8.0 Hz, 1H), 6.13 (s, 2H), 5.95 (s, 2H), 5.42 (d,
J = 6.0 Hz, 1H), 4.70 (d, J = 6.0 Hz, 1H), 4.25 (dd,
J = 8.8, 6.8 Hz, 1H), 4.16 (dd, J = 8.8, 7.2 Hz, 1H),
3.85–3.82 (m, 2H), 3.81 (s, 3H, –OCH3), 3.80 (s, 6H, –
OCH3 (92)), 3.49 (m, 1H), 3.07 (m, 1H); 13C NMR
(CDCl3, 100 MHz) d 161.3, 159.9, 159.9, 148.0, 147.1,
136.0, 119.6, 109.6, 108.3, 106.7, 101.1, 91.2, 91.2, 85.6,
78.3, 73.2, 72.2, 56.3, 56.0, 56.0, 55.5, 51.1; HRMS m/z
423.1417 [M ? Na]? (calcd for C22H24NaO7, 423.1420).
5i Yield: 47 %, white powder; 1H NMR (CDCl3,
400 MHz) d 8.96 (brs, 1H, –OH), 6.82–6.77 (m, 3H), 6.06
(d, J = 2.4 Hz, 1H), 6.01 (d, J = 2.4 Hz, 1H), 5.95 (s,
2H), 5.21 (d, J = 4.8 Hz, 1H), 4.81 (d, J = 4.0 Hz, 1H),
4.47 (dd, J = 9.2, 8.4 Hz, 1H), 4.13 (dd, J = 9.2, 2.8 Hz,
1H), 4.03 (dd, J = 9.2, 6.8 Hz, 1H), 3.79 (m, 1H), 3.76 (s,
6H, –OCH3 (x2)), 3.19 (m, 1H), 3.01 (m, 1H); 13C NMR
(CDCl3, 100 MHz) d 161.0, 158.0, 157.6, 148.2, 147.3,
134.9, 119.5, 108.3, 106.7, 105.0, 101.2, 94.6, 91.0, 84.2,
84.2, 72.7, 71.0, 55.5, 55.5, 54.8, 53.7; HRMS m/z
387.1449 [M ? H]? (calcd for C21H23O7, 387.1444).
epi-5f Yield: 22 %, brown oil; 1H NMR (CDCl3,
400 MHz) d 7.21 (d, J = 8.6 Hz, 1H), 6.87–6.76 (m, 3H,
H-20, H-50, and H-60), 6.67 (d, J = 8.6 Hz, 1H), 5.94 (s,
2H, H-70), 4.93 (d, J = 5.6 Hz, 1H, H-2), 4.37 (d,
J = 8.0 Hz, 1H, H-6), 4.09 (d, J = 9.2 Hz, 1H, H-4), 3.92
(s, 3H, –OCH3), 3.86 (s, 6H, –OCH3 (x2)), 3.83–3.77 (m,
2H), 3.43 (m, 1H), 3.24 (m, 1H), 2.86 (m, 1H); 13C NMR
(CDCl3, 100 MHz) d 153.2, 150.0, 148.1, 147.3, 141.8,
135.5, 124.4, 121.2, 119.7, 108.3, 107.0, 106.8, 101.2,
87.7, 78.6, 70.6, 69.9, 60.9, 60.8, 56.1, 55.0, 49.2; HRMS
m/z 423.1431 [M ? Na]? (calcd for C22H24NaO7,
423.1420).
1
epi-5i Yield: 51 %, white powder; H NMR (CDCl3,
400 MHz) d 9.15 (brs, 1H, –OH), 6.87–6.77 (m, 3H), 6.07
(d, J = 2.0 Hz, 1H), 6.00 (d, J = 2.4 Hz, 1H), 5.95 (s,
2H), 5.17 (d, J = 8.0 Hz, 1H), 4.40 (d, J = 6.8 Hz, 1H),
4.17 (d, J = 10.0 Hz, 1H), 3.91 (dd, J = 8.0, 8.0 Hz, 1H),
3.81 (dd, J = 9.6, 6.4 Hz, 1H), 3.77 (s, 3H, –OCH3), 3.76
(s, 3H, –OCH3), 3.51–3.42 (m, 2H), 2.87 (m, 1H); 13C
NMR (CDCl3, 100 MHz) d 160.9, 158.1, 157.4, 148.2,
147.5, 134.9, 119.8, 108.3, 106.8, 101.7, 101.2, 94.3, 90.8,
123

Synthesis of furofuran lignans as antidiabetic agents simultaneously achieved by inhibiting…
O
O
OH
O
O
O
Amberlyst-15, CH3CN-H2O (9:1)
70 oC, 90%
H
H
H
H
O
O
O
O
3
4
O
O
Scheme 2 Synthesis of samin (4)
OH
=
-OH or -OCH3
OCH3
CH3
monooxygenated
dioxygenated
a
OCH3
OCH3
CH3
OCH3
H3C
OCH3
OCH3
b
c
d
OCH3
OCH3
OCH3
OH
1,2,3-
1,3,4-
OCH3
OCH3
e
OH
f
OCH3
OCH3
g
OCH3
OH
CH3O
O
1,3,5-
H3CO
H3CO
OCH3
H3CO
OCH3
H3CO
OCH3
trioxygenated
h
i
j
OH
OCH3
OCH3
k
tetraoxygenated
Fig. 2 Phenolics (a–k) used in synthesis of furofuran lignans. Bold arrow indicates where the bonding between phenolic and furan moiety was
formed whereas dash arrow suggests another possible regioselective site
87.5, 81.9, 71.4, 70.3, 55.7, 55.4, 53.7, 49.6; HRMS m/z
409.1273 [M ? Na]? (calcd for C21H22NaO7, 409.1263).
5j Yield: 15 %, white powder; 1H NMR (CDCl3,
400 MHz) d 6.88 (m, 3H), 6.27 (s, 1H), 5.95 (s, 2H), 5.23
(d, J = 6.0 Hz, 1H), 4.74 (d, J = 5.6 Hz, 1H), 4.31 (dd,
J = 9.2, 7.2 Hz, 1H), 4.13 (dd, J = 8.8, 7.6 Hz, 1H),
3.92–3.90 (m, 2H), 3.85 (s, 3H, –OCH3), 3.83 (s, 3H, –
OCH3), 3.77 (s, 3H, –OCH3), 3.44 (m, 1H) 3.12 (m, 1H),
2.49 (s, 3H, –CH3); 13C NMR (CDCl3, 100 MHz) d 202.0,
160.6, 158.1, 157.8, 148.1, 147.2, 135.7, 119.7, 119.5,
123

W. Worawalai et al.
Table 1 Synthesis of furofuran lignans
Entry
Isolated product (% yield)
5
epi-5
1
2
3
5a (30 %)
5b (50 %)
5c (19 %)
5c0 (NI)
epi-5a (15 %)
epi-5b (45 %)
epi-5c (21 %)
epi-5c0 (24 %)a
epi-5d (42 %)
epi-5e (37 %)
epi-5f (22 %)
epi-5g (7 %)
epi-5h (NI)
4
5d (31 %)
5e (27 %)
5f (31 %)
5g (68 %)
5h (50 %)
5i (51 %)
5j (14 %)
5k (30 %)
5
6
7
8
9
epi-5i (47 %)
epi-5j (NI)
10
11
epi-5k (40 %)
NI not isolated. Due to trace amount, structure characterization and bioactivity evaluation could not be accomplished
a
The connectivity between furan moiety and 1,3-dimethoxy-5-methylbenzene (c) in product epi-5c0 is indicated by a dotted arrow
Synthesis of sesaminol (6) and epi-sesaminol (epi-6)
108.3, 106.7, 101.2, 101.2, 92.0, 85.3, 79.1, 72.8, 72.6,
64.4, 56.0, 56.0, 55.9, 51.5, 29.8; HRMS m/z 465.1538
[M ? Na]? (calcd for C24H26NaO8, 465.1525).
To a solution of sasemolin 3 (82 mg, 0.221 mmol) in
acetonitrile (1.5 mL) was treated with acidic resin amber-
lyst-15 (40 mg). After stirring at 70 °C for 5 h, the reaction
mixture was evaporated to dryness and purified by flash
chromatography yielding sesaminol (6, 66 mg, 80 %) and
epi-sesaminol (epi-6, 11 mg, 13 %) as a white powder.
Sesaminol (6) 1H NMR (CDCl3, 400 MHz) d 7.76 (brs,
1H, –OH), 6.83–6.78 (m, 3H), 6.51 (s, 1H), 6.45 (s, 1H),
5.96 (s, 2H), 5.89 (s, 2H), 4.77 (d, J = 4.0 Hz, 2H), 4.35
(dd, J = 8.8, 7.6 Hz, 1H), 4.14 (dd, J = 9.2, 6.0 Hz, 1H),
3.89–3.83 (m, 2H), 3.18–3.11 (m, 2H); 13C NMR (CDCl3,
100 MHz) d 150.9, 148.3, 148.2, 147.4, 141.1, 134.7,
125.2, 119.5, 115.2, 108.4, 106.7, 106.3, 101.3, 99.6, 86.7,
85.5, 72.6, 70.7, 53.5, 53.1; HRMS m/z 393.0962
[M ? Na]? (calcd for C20H18NaO7, 393.0950).
5k Yield: 40 %, colorless oil; 1H NMR (CDCl3,
400 MHz) d 8.58 (brs, 1H, –OH), 6.82–6.77 (m, 3H), 6.22
(s, 1H), 5.95 (s, 2H), 5.12 (d, J = 8.0 Hz, 1H), 4.83 (d,
J = 8.0 Hz, 1H), 4.49 (dd, J = 8.4, 8.4 Hz, 1H), 4.13 (dd,
J = 9.6, 2.8 Hz, 1H), 4.04 (dd, J = 9.2, 6.8 Hz, 1H), 3.90
(s, 3H, –OCH3), 3.81 (s, 3H, –OCH3), 3.80 (m, 1H), 3.79
(s, 3H, –OCH3), 3.22 (m, 1H), 3.03 (m, 1H); 13C NMR
(CDCl3, 100 MHz) d 153.9, 152.1, 150.9, 148.2, 147.3,
135.2, 134.7, 119.5, 109.1, 108.4, 106.8, 101.2, 97.0, 84.4,
84.2, 72.9, 70.8, 61.1, 60.9, 56.0, 54.7, 53.7; HRMS m/z
439.1366 [M ? Na]? (calcd for C22H24NaO8, 439.1369).
epi-5k Yield: 30 %, colorless oil; 1H NMR (CDCl3,
400 MHz) d 8.87 (brs, 1H, –OH), 6.87 (s, 1H), 6.83–6.77
(m, 2H), 6.21 (s, 1H), 5.95 (s, 2H), 5.15 (d, J = 8.0 Hz,
1H), 4.40 (d, J = 4.0 Hz, 1H), 4.18 (d, J = 10.0 Hz, 1H),
3.92 (m, 1H), 3.90 (s, 3H, –OCH3), 3.82 (s, 3H, –OCH3),
3.79 (m, 1H), 3.78 (s, 3H, –OCH3), 3.48–3.43 (m, 2H),
2.90 (m, 1H); 13C NMR (CDCl3, 100 MHz) d 153.8, 152.6,
150.2, 148.2, 147.5, 135.0, 134.8, 119.8, 108.3, 106.7,
105.7, 101.2, 96.8, 87.5, 82.0, 71.4, 70.3, 61.1, 60.9, 55.9,
53.8, 50.2; HRMS m/z 439.1366 [M ? Na]? (calcd for
C22H24NaO8, 439.1369).
1
epi-Sesaminol (epi-6) H NMR (CDCl3, 400 MHz) d
8.17 (brs, 1H, –OH), 6.87–6.77 (m, 3H), 6.42 (s, 1H), 6.40
(s, 1H), 5.96 (s, 2H), 5.90 (s, 2H), 4.97 (d, J = 5.9 Hz,
1H), 4.41 (d, J = 7.0 Hz, 1H), 4.17 (d, J = 10.0 Hz, 1H),
4.00 (dd, J = 9.2, 8.8 Hz, 1H), 3.84 (dd, J = 9.6, 6.4 Hz,
1H), 3.49 (dd, J = 9.2, 8.4 Hz, 1H), 3.37 (m, 1H), 2.89 (m,
1H); 13C NMR (CDCl3, 100 MHz) d 150.8, 148.2, 147.9,
147.5, 141.2, 134.7, 119.8, 112.1, 108.3, 106.7, 105.8,
123

Synthesis of furofuran lignans as antidiabetic agents simultaneously achieved by inhibiting…
101.3, 101.2, 99.4, 87.7, 84.5, 71.9, 70.1, 53.6, 50.7;
HRMS m/z 393.0946 [M ? Na]? (calcd for C20H18NaO7,
393.0950).
compounds (20 lL) were mixed with 80 lL of diluted
ABTSÁ? solution. After 2 h of incubation, the absorbance
was read at 750 nm. The percentage inhibition was cal-
culated by [(A0 - A1)/A0] 9 100, where A0 is the absor-
bance without the sample, and A1 is the absorbance with
the sample. The SC50 value was determined from a plot of
percentage inhibition versus sample concentration. Buty-
lated hydroxytoluene (BHT) was used as the standard
control and the experiment was performed in duplicate.
a-Glucosidase inhibitory activity
a-Glucosidase inhibitory activity against rat intestinal
maltase and sucrase was determined according to our
previous report. The crude enzyme solution prepared from
rat intestinal acetone powder was used as a source of
maltase and sucrase. Rat intestinal acetone powder (1 g)
was homogenized in 30 mL of 0.9 % NaCl solution. After
centrifugation (12,000g 9 30 min), the aliquot was sub-
jected to assay. The synthesized compounds (1 mg/mL in
DMSO, 10 lL) were added with 30 lL of the 0.1 M
phosphate buffer (pH 6.9), 20 lL of the substrate solution
(maltose: 10 mM; sucrose: 100 mM) in 0.1 M phosphate
buffer, 80 lL of glucose assay kit, and 20 lL of the crude
enzyme solution. The reaction mixture was then incubated
at 37 °C for 10 min (for maltose) and 40 min (for sucrose).
Enzymatic activity was quantified by measuring the
absorbance of quinoneimine formed (500 nm) using Bio-
Red microplate reader model 3550 UV. The percentage
inhibition was calculated by [(A0 - A1)/A0] 9 100, where
A0 is the absorbance without the sample, and A1 is the
absorbance with the sample. The IC50 value was deter-
mined from a plot of percentage inhibition versus sample
concentration. AcarboseÒ was used as the standard control
and the experiment was performed in duplicate.
DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging assay
Radical scavenging activity was validated using DPPH
colorimetric method. Briefly, the synthesized compounds
(20 lL) were added to 0.1 mM methanolic solution of
DPPH (100 lL). The mixture was kept dark at room
temperature in an incubator shaker for 15 min. The
absorbance of the resulting solution was measured at
517 nm with a 96-well microplate reader. The percentage
inhibition was calculated by [(A0 - A1)/A0] 9 100, where
A0 is the absorbance without the sample, and A1 is the
absorbance with the sample. The SC50 value was deduced
from a plot of percentage inhibition versus sample con-
centration. BHT was used as the standard control and the
experiment was performed in duplicate.
Results and discussion
Synthesis and characterisation
Kinetic study of a-glucosidase inhibition
In this experiment, naturally available sesamolin (3) was
obtained from sesame seed oil using the methodology
described elsewhere with minor modification (Reshma
et al. 2010). Briefly, sesame seed oil (150 g) dissolved in
MeOH (150 mL) was saponified with KOH (25 g) for 2 h.
After solvent removal, the resulting mixture was dissolved
in water and then extracted with EtOAc. The organic
extract mainly containing unsaponifiable lignans was sep-
arated on silica gel column using 10 % EtOAc-hexane to
obtain sesamin (1, 3 g, 2 %) and sesamolin (3, 1.5 g, 1 %)
(Hemalatha et al. 2004).
For kinetic analyses of maltase by the active compound,
enzyme and active compounds were incubated with
increasing concentrations of maltose (2–20 mM). The type
of inhibition was determined by the Lineweaver–Burk plot.
For calculation of Ki and Ki0 values, slope and intercept
from the Lineweaver–Burk plot were replotted vs. [I],
which gave the secondary plot.
Antioxidant activities
ABTS (2,20-azino-bis(3-ethylbenzothiazoline-6-sulfonate)
scavenging assay
Subsequently, we focused our attention on the prepara-
tion of samin (4) as starting material. Hydrolysis of 3 at
70 °C in MeCN-H2O (9:1) under acid-catalyzed conditions
using Amberlyst-15 (1 mg/0.005 mmol of samin) gave the
desired samin (4) in 90 % yield (Scheme 2) (Marchand
et al. 1997b).
The radical scavenging activity of synthesized compounds
against ABTSÁ? was carried out according to a procedure
described previously (Hirose et al. 2013). Briefly, ABTSÁ?
radical cation was produced by mixing 10 mL of 7.4 mM
ABTS with 0.5 mL of 2.6 mM potassium persulphate
(K2S2O8) for 16 h in the dark at room temperature. Before
use, the ABTSÁ? solution was diluted with ethanol to an
absorbance of 0.70 0.02 at 750 nm. The synthesized
Having the starter 4 in hand, diverse furofuran lignans 5
could be synthesized by nucleophilic substitution
(Scheme 1) with a variety of electron-rich phenolics.
Under the applied conditions, the reaction is expected to
proceed SN1 fashion via oxocarbenium ion (Huang et al.
123

W. Worawalai et al.
by X-ray analysis (Li et al. 2005). On the other hand, epi-
5a revealed key NOESY correlations of H-2/H-4ax and
H-6/H-8ax, which was indicative of endo-exo-2,6-diaryl-
furofuran (Fig. 4b). These observations correlated well
with the chair-boat conformation of epi-sesamin or asarinin
(Li et al. 2005).
In addition, the difference in the configuration at C-2 of
1
5a and epi-5a could be significantly observed in the H
NMR pattern and coupling constants of H-2 and neigh-
boring protons. H-2 and H-6 in 5a each showed doublet
signal with comparable coupling constants (J & 4.0 Hz)
whereas those in epi-5a displayed significantly distinct
values; 5.6 Hz for H-2 and 8.0 Hz for H-6. A more strik-
ingly significant observation is the splitting patterns of
diastereomeric H2-4, which appeared as expected as dou-
blet of doublet. However, only H-4eq of epi-5a showed an
exceptional doublet signal caused solely by germinal cou-
pling of H-4eq and H-4ax. This observation would account
for a nearly 90° dihedral angle between H-4eq and H-5 that
Fig. 3 Selected HMBC correlations of 5a and epi-5a
(a)
CH3
HO
O
2
H
Hax
Hax
1
2
H
H
H
1
8
HOAr
O
6
Heq
O
4
6
O
5
Heq
5a
H
O
Ar
3
O
gave rise to J4eq,5 & 0 Hz (Gu¨nter 1994) (Fig. 5). The
Hax
above evidence could be useful in readily distinguishing
the identity of other synthesized lignans 5 and their
epimers.
exo-exo
CH3
HO
(b)
HOAr
O
With the success in applying the above method, we
further synthesized other furofuran lignans by varying
electron-rich phenolics, namely dioxygenated (b–d), tri-
oxygenated (e–j) and tetraoxygenated (k) benzenes
(Fig. 2). All synthesized products (5b–5k and their epi-
O
2
Hax
1
8
H
H
1
H
6
2
6
H
Heq
4
5
O
O
H
Ar
H
epi-5a
O
Hax
1
mers) were securely characterized by H, 13C, HRMS as
O
Hax
well as 2D NMR where necessary (See also Supplementary
Information).
endo-exo
Fig. 4 Diagnostic NOESY correlations of (a) 5a and (b) epi-5a
As for regioselectivity, it could be generalized that the
product was formed through the carbon–carbon bond
between C-2 of furan moiety and phenolic at ortho-po-
sition to hydroxy or methoxy groups, as indicated by the
bold arrows in Fig. 2. This exclusive regioselectivity
could be accounted for ortho-directing effects of two or
more electron-donating groups that reinforce such a
preferential electrophilic site for oxocarbenium ion.
However, multiple products were also obtained if there
was more than one favored site. This observation was
demonstrated by the reaction between samin (4) and 1,3-
dimethoxy-5-methylbenzene (c) (Scheme 3). Once the
oxocarbenium ion was produced, electrophilic substitution
at C-2 of phenolic (pathway A) yielded lignan 5c and its
epimer (epi-5c) while the substitution at C-4 or C-6
(pathway B) afforded lignan 5c0 and its epimer (epi-5c0).
Alternatively, these reactions could be described as Frie-
del–Crafts-like reaction of the phenolic acceptor (Gut-
mann and Nidetzky 2013).
2012; Marchand et al. 1997a), generated upon protonation
with strong acid. In addition, the release of H2O after the
protonation of the hydroxyl group would lead to the
regeneration of 4 along with the occurrence of desired
products 5. To prevent unwanted results, removal of H2O
˚
by trapping with a molecular sieve 4A is required.
With the well-defined synthetic plan in hand, we first
applied it in the reaction with monooxygenated benzene.
Reaction of 4 and m-cresol (a, Fig. 2) yielded lignan 5a
(30 %) and its related isomer epi-5a (15 %) (Table 1).
Careful structure elucidation, particularly HMBC analysis,
indicated that m-cresol was connected to furan moiety
through C-2/C-100 bond formation (Fig. 3).
The relative configuration of 5a and epi-5a at C-2 was
further determined by NOESY data and coupling constant
analysis. The NOESY spectrum of 5a showed diagnostic
correlations of H-2/H-8ax and H-4ax/H-6, suggesting the
occupation of two aryls on the exo–exo face of the bicyclic
core (Fig. 4a). This interpretation was consistent with a
boat–boat conformation of sesamin (1), which was verified
Alternative, sesaminol (6, 80 %) and epi-sesaminol (epi-
6, 13 %) were also synthesized by refluxing sesamolin (3)
in Amberlyst-15/acetonitrile (Scheme 4).
123

Synthesis of furofuran lignans as antidiabetic agents simultaneously achieved by inhibiting…
dd
H-2 H-6
H-4ax H-8ax
H-8eq
H-4eq
H-2
H-6
H-4eq H-8ax H-4ax
d
H-8eq
Fig. 5 1H NMR spectra of 5a (top) and epi-5a (bottom). Note that Newman projection of epi-5a demonstrates dihedral angle nearly 908 between
H-4eq and H-5, thus resulting in doublet (d) signal rather than expected doublet of doublet (dd). For clarity, particular atoms are omitted
a-Glucosidase inhibitory and antioxidant activities
(A)
All synthesized furofuran lignans were evaluated for a-
O
O
O
glucosidase inhibition toward rat intestinal maltase and
sucrase as well as antioxidation against DPPH and ABTS
radicals (Table 2). Of the products examined, furofuran
lignans having free hydroxy group on phenolic moiety,
namely 5a, 5e, 5g, 5i, 5k, and 6 along with their corre-
sponding epimers, revealed inhibition against both a-glu-
cosidase and free radicals in the range of 1.14–18.9 and
0.15–2.3 mM, respectively. These observations prelimi-
narily suggest that the presence of free hydroxy group
plays a critical role in exerting inhibition. For antioxida-
tion, phenolic hydroxyl could donate hydrogen radical (HÁ)
to terminate radical propagation (Yehye et al. 2015), thus
preventing the onset of cellular impairment.
OH
H
2
H+
-H2O
(B)
OCH3
H
H
H
O
1
O
O
5
3
O
O
H3C
OCH3
4
pathway (A)
H3CO CH3
pathway (B)
H3CO
OCH3
O
2
O
2
2
6
*
*
+
H
H
H
H
CH3
OCH3
O
O
5c
5c'
epi-5c'
Careful inspection on structures of the above active
compounds and a-glucosidase inhibition demonstrated
intriguing SAR data (Fig. 6). Generally, the epimeric
products (designed as epi-) revealed slightly more potent
inhibition (1–3 times) than their corresponding isomers in
O
O
epi-5c
O
O
Scheme 3 Mechanistic formation of lignans (5c, 5c0, epi-5c and epi-
5c0) synthesized from samin (4) and 1,3-dimethoxy-5-methylbenzene
(c)
123

W. Worawalai et al.
HO
O
HO
O
O
O
O
O
O
O
O
Amberlyst-15,CH3CN
70 oC
O
+
H
H
H
H
H
H
O
O
O
O
O
O
3
6
O
O
epi-6
(13%)
O
(80%)
Scheme 4 Synthesis of sesaminol (6) and epi-sesaminol (epi-6)
maltase
sucrase
20
Table 2 a-Glucosidase inhibitory effect and radical scavenging
activity of furofuran lignans
15
10
5
Compounds a-Glucosidase inhibitory
effect (IC50, mM)
Radical scavenging
(SC50, mM)a
Maltase
Sucrase
DPPH
ABTS
1
NAb
NA
NA
NAb
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
0.34
0.17
NA
NA
0.41
0.22
NA
NA
NA
NA
2.3
NA
3
NA
NA
4
NA
NA
NA
0
5a
8.23
7.01
NA
14.67
8.52
NA
0.34
0.20
NA
epi-5a
5b
epi-5b
5c
NA
NA
NA
NA
NA
NA
Fig. 6 Intestinal a-glucosidase inhibition of the synthesized lignans
containing free hydroxy group at introduced phenolic
epi-5c
epi-5c0
5d
NA
NA
NA
NA
NA
NA
NA
NA
NA
epi-5d
5e
NA
NA
NA
2.15
1.52
NA
3.83
3.13
NA
0.22
0.15
NA
0.15
epi-5e
5f
No Inhibitor
0.017 mM
0.085 mM
0.1
epi-5f
5g
NA
NA
NA
5.59
2.44
NA
8.21
3.30
NA
0.34
0.37
NA
0.425 mM
0.05
epi-5g
5h
5i
4.67
2.98
NA
18.89
11.1
NA
0.31
0.25
NA
0
epi-5i
5j
-3
-2
-1
0
1
2
1/[maltose] (mM)-1
5k
3.36
1.31
3.42
1.14
3.59
3.84
6.90
4.01
0.0023
NA
0.41
0.40
0.39
0.35
NA
epi-5k
6
1.45
0.65
0.42
NA
1.56
Fig. 7 Lineweaver–Burk plots for inhibitory activity of epi-6 against
rat intestinal maltase
epi-6
AcarboseÒ 0.0015
all bioassays examined; of which the most significant dif-
ference was discovered for 6 and epi-6 (3.42 vs 1.14 mM
against maltase). Noticeably, potent inhibition was likely to
depend largely on the number of hydroxyls along with
other electron donating groups (OMe and Me). These
observations were marked by the weaker inhibition of 5a
BHT
NA
0.14
a
SC50; 50 % radical scavenging concentration
b
Not active; %inhibition less than 30 % at highest concentration
examined against a-glucosidase (0.0625 mg/mL) and radical scav-
enging (1 mg/mL)
123

Synthesis of furofuran lignans as antidiabetic agents simultaneously achieved by inhibiting…
of straight lines, all of which intersected within the second
quadrant (Fig. 7). The analysis displayed an increase in Km
values while Vmax decrease. This behavior indicated that
epi-6 inhibited maltase in a mixed-type manner comprising
two different pathways; namely, a competitively forming
enzyme-inhibitor (EI) complex and an interrupting
enzyme-substrate (ES) intermediate by forming an
enzyme-substrate-inhibitor (ESI) complex in a noncom-
petitive manner (Scheme 5).
Scheme 5 Putative inhibitory mechanism of epi-6 against rat
intestinal maltase. E, S, I and P represent enzyme, substrates
(maltose), inhibitors (epi-6) and glucose, respectively
We also further investigated the pathway in which epi-6
was preferentially preceded by deducing the dissociation
constants of EI (Ki) and ESI (Ki0) complexes. Apparently,
the secondary plots (Fig. 8) demonstrated Ki and Ki0 values
of 0.32 and 0.59 mM, respectively, thus indicating that epi-
6 was predominantly bound to maltase (EI) rather than the
formed ESI complex. The putative inhibitory mechanism is
summarized in Scheme 5.
0.045
(a)
0.03
0.015
0
0
0.1
0.2
0.3
0.4
0.5
Conclusion
[epi-6] mM
0.1
0.05
0
(b)
In conclusion, we first synthesized a series of new furo-
furan lignans by reaction of samin (4) and a variety of
phenolics in the presence of acidic resin. This approach
could be easily conducted in one step, and generated a
pair of diastereomeric products, which were useful for
investigating the relationship between bioactivity and
chemical structure together with stereochemistry. Of the
synthesized products, lignans comprising hydroxyl group
on the newly introduced phenolic residue demonstrated
much more improved inhibition than starter 4 and related
lignans 1 and 3 against a-glucosidase and free radicals.
The concomitant inhibition of the synthesized compounds
would provide benefits both in terms of the curative effect
by retarding the enzyme function and the preventive
effect by scavenging radicals generated from excess blood
glucose. Moreover, the mechanism underlying a-glucosi-
dase inhibition of epi-6, one of the most potent inhibitor,
was proven to inhibit in a mixed-type manner, therefore
suggesting that it would be applied as a single diabetic
agent or in a combination with antidiabetic drugs such as
acarbose. This investigation has provided fundamental
clues on the structural motifs required for synthesizing a
new series of improved inhibitors and a practical synthetic
approach.
0
0.06
0.12
0.18
0.24
0.3
0.36
0.42
0.48
[epi-6] mM
Fig. 8 Secondary replots of (a) slope vs. [I] and (b) intercept vs.
[I] from a primary Lineweaver–Burk plot for the determination of Ki
and K0i, respectively
and epi-5a (IC50 7.01–14.7 mM), whose phenolics
encompassed two electron donating groups (OH and Me).
This finding was reversed only for 5i and epi-5i, the lignans
containing three electron donating groups (OH and
2 9 OMe) on the introduced phenolics. To our knowledge,
this is the first report on the SAR of lignans.
Kinetic study
To gain insight into the mechanism underlying the inhibi-
tory effect of epi-6, the representative of synthesized
furofuran lignans in this experiment, toward rat intestinal
maltase, its kinetic study was performed. Lineweaver–Burk
plot of initial velocity versus maltose concentrations in the
presence of different concentrations of epi-6 gave a series
Acknowledgments This project was funded by the Thailand
Research Fund (RSA5880027) and the Ratchadapisek Sompoch
Endowment Fund, Chulalongkorn University (Sci-Super 2014–011).
PK and NS are recipients of the Science Achievement Scholarship of
Thailand (SAST). Chulalongkorn University Postdoctoral Fellowship
to WW is acknowledged. We would also like to express our thanks to
Dr. Rico Ramadhan for technical assistance in the kinetic study.
123

W. Worawalai et al.
Marchand PA, Zajicek J, Lewis NG (1997b) Oxygen insertion in
Sesamum indicum furofuran lignans. diastereoselective syntheses
of enzyme substrate analogues. Can J Chem 75:840–849
Maritim AC, Sanders RA, Watkins JB III (2003) Diabetes, oxidative stress,
and antioxidants: a review. J Biochem Mol Toxicol 17:24–38
Pohmakotr M, Pinsa A, Mophuang T, Tuchinda P, Prabpai S,
Kongsaeree P, Reutrakul V (2006) General strategy for stereos-
elective synthesis of 1-substituted exo, endo-2,6-diaryl-3,7-
References
Baluchnejadmojarad T, Roghani M, Nadoushan MRJ, Mahdavi MRV,
Moghaddam HK, Dehkordi FR, Dariani S, Raoufi S (2013) The
sesame lignan sesamin attenuates vascular dysfunction in
streptozotocin diabetic rats: involvement of nitric oxide and
oxidative stress. Eur J Pharmacol 698:316–321
Brown RCD, Bataille CJR, Hinks JD (2001) Total synthesis of ( )-
epimagnolin A. Tetrahedron Lett 42:473–475
Brownlee M (2005) The pathobiology of diabetic complications: a
unifying mechanism. Diabetes 54:1615–1625
Gu¨nter H (1994) NMR spectroscopy: basic principles, concepts, and
applications in chemistry, 2nd edn. John Wiley, New York,
pp 115–117
Guo LQ, Yang JR, Kong X (2012) Protective effect of sesamin on
endothelial function of type-2 diabetic rats. Chin Pharmacol Bull
28:392–396
Gutmann A, Nidetzky B (2013) Enzymatic C-glycosylation: insights
from the study of a complementary pair of plant O- and C-
glucosyltransferases. Pure Appl Chem 85:1865–1877
Hemalatha S, Ghafoorunissa (2004) Lignans and tocopherols in
Indian sesame cultivars. J Am Oil Chem Soc 81:467–470
dioxabicyclo[3.3.0]octanes: total synthesis of
J Org Chem 71:386–389
( )-gmelinol.
Ramadhan R, Phuwapraisirisan P (2015a) Arylalkanones from Hors-
fieldia macrobotrys are effective antidiabetic agents achieved by
a-glucosidase inhibition and radical scavenging. Nat Prod Com-
mun 10:325–328
Ramadhan R, Phuwapraisirisan P (2015b) New arylalkanones from
Horsfieldia macrobotrys, effective antidiabetic agents concomi-
tantly inhibiting a-glucosidase and free radicals. Bioorg Med
Chem Lett 25:4529–4533
Rattanangkool E, Kittikhunnatham P, Damsud T, Wacharasindhu S,
Phuwapraisirisan P (2013) Quercitylcinnamates, a new series of
antidiabetic bioconjugates possessing a-glucosidase inhibition
and antioxidant. Eur J Med Chem 66:296–304
Reshma MV, Balachandran C, Arumughan C, Sunderasan A,
Sukumaran D, Thomas S, Saritha SS (2010) Extraction, sepa-
ration and characterisation of sesame oil lignan for nutraceutical
applications. Food Chem 120:1041–1046
Roghani M, Baluchnejadmojarad T, Dehkordi FR (2011) The sesame
lignan sesamin attenuates vascular permeability in rats with
streptozotocin-induced diabetes: involvement of oxidative stress.
Int J Endocrinol Metab 9:248–252
Wikul A, Damsud T, Kataoka K, Phuwapraisirisan P (2012) (?)-
Pinoresinol is a putative hypoglycemic agent in defatted sesame
(Sesamum indicum) seeds though inhibiting a-glucosidase.
Bioorg Med Chem Lett 22:5215–5217
Worawalai W, Wacharasindhu S, Phuwapraisirisan P (2015) N-
arylmethylaminoquercitols, a new series of effective antidiabetic
agents having a-glucosidase inhibition and antioxidant activity.
Bioorg Med Chem Lett 25:2570–2573
Yehye WA, Rahman NA, Ariffin A, Hamid SBA, Alhadi AA, Kadir
FA, Yaeghoobi M (2015) Understanding the chemistry behind
the antioxidant activities of butylated hydroxytoluene (BHT): a
review. Eur J Med Chem 101:295–312
Hirose Y, Ohta E, Kawai Y, Ohta
S (2013) Dorsamin-A’s,
glycerolipids carrying a dehydrophenylalanine ester moiety from
the seed-eating larvae of the bruchid beetle Bruchidius dorsalis.
J Nat Prod 76:554–558
Hong L, Yi W, Liangliang C, Juncheng H, Qin W, Xiaoxiang Z
(2013) Hypoglycaemic and hypolipidaemic activities of sesamin
from sesame meal and its ability to ameliorate insulin resistance
in KK-Ay mice. J Sci Food Agric 93:1833–1838
Huang J, Song G, Zhang L, Sun Q, Lu X (2012) A novel conversion
of sesamolin to sesaminol by acidic cation exchange resin. Eur J
Lipid Sci Technol 114:842–848
Hull HM, Knight DW (1997) Diastereospecific approach to ( )-
samin and 2,6-diaryl-3,7dioxabicyclo[3.3.0]octane (furofuran)
lignans using the Ireland–Claisen rearrangement of unsaturated
oxamacrolides. J Chem Soc Perkin Trans 1(6):857–863
Li CY, Chow TJ, Wu TS (2005) The epimerization of sesamin and
asarinin. J Nat Prod 68:1622–1624
Marchand PA, Kato MJ, Lewis NG (1997a) (?)-Episesaminone, a
Sesamum indicum furofuran lignan.isolation and hemisynthesis.
J Nat Prod 60:1189–1192
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