Regioselective synthesis of 3-(1H-indol-3-yl)-5-(1H-indole-3-carbonyl)-4- hydroxyfuroic acids: Route to hydroxyfuroic acid-based insulin receptor activators

Article abstract of DOI:10.1002/ddr.20391

A new class of insulin receptor activator with a hydroxyfuroic acid in place of a hydroxyquinone moiety is reported. The synthesis of 3-(1H-indol-3-yl)-5-(1H-indole-3-carbonyl)-4-hydroxyfuroic acids (26-30) requires seven major steps. Key elements in the syntheses include (1) sequential preparation of two 4-(N-protected indole)-3-methoxy-furoic 2,5-dicarboxylic esters (4 and 6); (2) regioselective conversion of the furoic diacid 8 into its C-5 methyl ester 10 with methyl chloroformate; and (3) acylation of 10 by a 7-substituted indole under a mild condition. This study demonstrates a feasible route of synthesizing insulin receptor activators with a hydroxyfuroic acid scaffold. Among those hydroxyfuroic acid compounds, compound 28 demonstrates insulin receptor activation potential comparable to Merck's compound 2 with a dihydroxybenzoquinone scaffold.

Full text of DOI:10.1002/ddr.20391

DRUG DEVELOPMENT RESEARCH 72 : 247–258 (2011)
Research Article
Regioselective Synthesis of 3-(1H-indol-3-yl)-5-
(1H-indole-3-carbonyl)-4-hydroxyfuroic Acids: Route
to Hydroxyfuroic Acid-Based Insulin Receptor Activators
Ã
Ã
Shan-Yen Chou1,2 and Henry J. Tsai1,3,4
1Pharmaceutical R&D Program, Development Center for Biotechnology,
Hsi-Chih City 221, Taiwan
2Taigen Biotechnology Co., Taipei 114, Taiwan
3Department of Biological Science and Technology, China Medical University,
Taichung City 404, Taiwan
4Department of Health and Nutrition Biotechnology, Asia University,
Taichung County 413, Taiwan
Strategy, Management and Health Policy
Enabling
Preclinical Development Clinical Development
Toxicology, Formulation Phases I-III
Technology,
Genomics,
Proteomics
Preclinical
Research
Postmarketing
Phase IV
Drug Delivery,
Regulatory, Quality,
Manufacturing
Pharmacokinetics
ABSTRACT
A new class of insulin receptor activator with a hydroxyfuroic acid in place of a hydroxyquinone
moiety is reported. The synthesis of 3-(1H-indol-3-yl)-5-(1H-indole-3-carbonyl)-4-hydroxyfuroic acids
(26–30) requires seven major steps. Key elements in the syntheses include (1) sequential preparation of two
4-(N-protected indole)-3-methoxy-furoic 2,5-dicarboxylic esters (4 and 6); (2) regioselective conversion of
the furoic diacid 8 into its C-5 methyl ester 10 with methyl chloroformate; and (3) acylation of 10 by a
7-substituted indole under a mild condition. This study demonstrates a feasible route of synthesizing insulin
receptor activators with a hydroxyfuroic acid scaffold. Among those hydroxyfuroic acid compounds,
compound 28 demonstrates insulin receptor activation potential comparable to Merck’s compound 2 with
a dihydroxybenzoquinone scaffold. Drug Dev Res 72:247–258, 2011.
r 2010 Wiley-Liss, Inc.
Key words: insulin receptor activator; regioselective synthesis; hydroxyfuroic acid
INTRODUCTION
agonists [Lee et al., 2003]; and (5) dipeptidyl peptidase
IV inhibitors [Conarello et al., 2003; Tsai et al., 2008].
Diabetes mellitus is a chronic disease character-
ized by poor glucose homeostasis resulting from either
lack of insulin secretion by pancreatic islets or the
irresponsiveness to insulin stimulation at the peripheral
organs/tissues [Ross et al., 2004; Gale, 2001]. To bring
the hyperglycemic conditions under control, several oral
hypoglycemic agents are available on the market and can
be categorized into five major classes based on their
target receptors or enzymes. They include (1) sulfony-
lurea, an insulin secretagogue [Bryan et al., 2005];
(2) biguanide, an AMP kinase agonist [Zhou et al.,
2001]; (3) a-glucosidase inhibitors [Hanefeld, 2007];
(4) peroxisome proliferator activated receptor (PPAR)
Grant sponsor: Ministry of Economic Affairs, Taiwan.
Ã
Correspondence to [for chemistry]: Shan-Yen Chou,
TaiGen Biotech Co., 138 Shin-Ming Rd., Neihu Dist., Taipei
114, Taiwan. E-mail: sychou@taigenbiotech.com.tw
Ã
Correspondence to [for biology]: Henry J. Tsai, Depart-
ment of Health and Nutrition Biotechnology, Asia University, 500
Lioufong Rd., Wufong Dist., Taichung County 41354, Taiwan.
E-mail: henrytsai@asia.edu.tw
Received 29 July 2010; Accepted 19 August 2010
Published online in Wiley Online Library (wileyonlinelibrary.
com). DOI: 10.1002/ddr.20391
ꢀc
2010 Wiley-Liss, Inc.

248
CHOU AND TSAI
Insulin receptor itself is an interesting target
EXPERIMENTAL
because an insulin receptor activator may potentially
replace insulin at treating insulin-dependent diabetic
patients, yet it is small enough to be orally active.
Disclosed by Merck [Zhang et al., 1999], demethylas-
terriquinone B1 (DAQ B1), extracted from tropical
fungus, Pseudomassaria sp., and its structurally simpli-
fied derivative ‘‘Compound 2,’’ are capable of activating
insulin receptor in the absence of insulin [Liu et al.,
2000; Strowski et al., 2004] (Fig. 1). Interestingly,
bioconverted products of DAQ B1, 4-hydroxyfuroic
acids, have been demonstrated to be insulin receptor
activators and display a hypoglycemic effect on db/db
mice [Chen et al., 2003]. Additionally, kojic acid
compounds, also derivatives of DAQ B1, have been
shown to be capable of promoting insulin receptor
phosphorylation [Xiong and Pirrung, 2008; Pirrung
et al., 2008]. Our recent report has presented an
approach for preparing simplified analogues of
4-hydroxyfuroic acids, designated series 1 compounds,
in which the C-3 reverse prentyl- and C-5 prentyl
indole rings were replaced with a phenyl and a
7-substituted indole, respectively [Chou et al., 2006].
However, series 1 compounds showed no appreciable
potential at activating insulin receptor. Therefore, we
describe the synthesis of structurally less modified
analogues, designated series 2 compounds, in which
the C-3 and C-5 indole rings of DAQ B1 are replaced
by an unsubstituted C-3 indole and a C-5 (hydroxyalkyl)-
indole or C-5 (carboxyalkyl)indole (Fig. 1).
General Remarks
Melting points were measured in open capillary
tubes using an Bu¨chi immersion apparatus, and are
uncorrected. H and 13C NMR spectra were recorded
1
on a Bruker-AC 500 (500 MHz) spectrometer (chemical
shifts in ppm). Mass spectra (electron spray-ES) were
recorded on a JEOL-JMSD-D-100 instrument. Exact
mass spectra were measured by a JEOL-JMSD-HX100
high-resolution mass spectrometer.
3-Hydroxy-4-[1-(4-methoxy-benzyl)-1H-indol-3-yl]-
furan-2,5-dicarboxylic acid dimethyl ester (3)
A suspension of potassium t-butoxide was pre-
pared by reacting potassium (37.7 g, 0.96 mol,
ꢁ300 mol% vs 1) and t-butanol (94 ml, 0.99 mol) in
dry benzene (1,200 ml). The suspension was then kept
under reflux. A solution of dimethyl diglycolic acid
ester 1 (52.1 g, 0.32 mol) and [1-(4-methoxy-benzyl)-
1H- indol-3-yl]-oxo-acetic acid methyl ester (2)
(136.6 g, 0.42 mol, ꢁ130 mol% vs 1) in benzene
(1,000 ml) was added dropwise to the above refluxing
suspension. After being refluxed under nitrogen for
2.5 h, the mixture was cooled, diluted with ice water,
and acidified with 3 N HCl. The resultant mixture was
diluted with ethyl acetate (500 ml); the separated
organic layer was washed with water, dried, and
concentrated to yield 3 (194.0 g) as a crude product.
An analytical sample was prepared by chromatographic
H
N
NH
O
HOOC
3
HO
4
O
Biotransformation
OH
OH
5
O
R1 = CH3, R2 = CH3
HN
HN
O
R1 = CH2OH, R2 = CH3
R1 = CH3, R2 = CH2OH
R1
R2
Demethylasterriquinone B1
Hydroxyfuroic Acids
structural
modification
structural
modifiction
O
HOOC
O
3
Ar
4
OH
HO
OH
5
O
N
HN
O
H3C
Compound 2
series 1: Ar = phenyl, R = hydroxyalkyl or
R
carboxyalkyl
series 2: Ar = 1H-indol-3-yl, R = hydroxyalkyl
or carboxyalkyl
Fig. 1. Rational design of 3-phenyl- or 3-(1H-indol-3-yl)-4-hydroxy furoic acid derivatives as potential insulin receptor activators (series 1 and
series 2, respectively).
Drug Dev. Res.

HYDROXYFUROIC ACID AS AN INSULIN RECEPTOR ACTIVATOR
249
purification using 1/2 ethyl acetate-hexane as an eluent 3-Methoxy-4[1-(toluene-4-sulfonyl)-1H-indol-3-yl]-
to give 3a as yellow oil. 1H-NMR (500 MHz, CDCl3) d: furan-2,5-dicarboxylic acid dimethyl ester (6)
Method A.
Intermediate 5 (15.1 g, 0.046 mol) was
3.73 (s, 6H), 3.97 (s, 3H), 5.26 (s, 2H), 6.81 (d,
J 5 8.6 Hz, 2H), 7.11 (d, J 5 8.6 Hz, 2H), 7.13–7.15
(m, 1H), 7.32 (d, J 5 7.9 Hz, 1H), 7.51 (s, 1H), 7.62 (d,
J 5 7.9 Hz, 1H). MS (ESI): 436.0 (M11H); HREIMS:
Calculated for C24H21NO7 (M1) 435.1318, found
435.1321.
mixed with TsCl (17.5 g, 0.092 mol) and K2CO3 (19.0 g,
0.14 mol) in 2-butanone (460 ml). After the mixture was
refluxed under nitrogen for 2 h, additional amounts of
TsCl (8.7 g, 0.046 mol) and K2CO3 (9.5 g, 0.069 mol)
were added. The resultant mixture was then refluxed
overnight. The reaction mixture was filtered from a
sintered glass funnel. The filtrate was concentrated
under vacuum at o401C, and the residue was treated
with ice-cooled t-BuOMe (100 ml) to form a suspen-
sion. The suspension was again filtered from a sintered
glass funnel. The precipitate was collected and dried to
afford 6 (20.4 g, 91.9%) as a white powder, mp
3-Methoxy-4-[1-(4-methoxy-benzyl)-1H-indol-3-yl]-
furan-2,5-dicarboxylic acid dimethyl ester (4)
Potassium carbonate (238.8 g, 1.73 mol) and MeI
(183.2 g, 0.572 mol) were added sequentially to a
solution of the crude 3 (194.0 g, obtained from the
previous step) in DMF (3,750 ml). After being stirred
at room temperature for 24 h, the reaction mixture was
filtered from a sintered glass funnel. The filtrate was
concentrated under vacuum at o501C. The residue
thus obtained was diluted with dichloromethane
(1,000 ml) and washed with brine and water. The
separated organic layer was concentrated, and the
residue was purified by silica gel column chromato-
graphy using 1/4 ethyl acetate-hexane as an eluent to
produce 4 (93.0 g, yield in two steps: 64.4%) as a white
powder, mp: 116–1181C (1/4 ethyl acetate-hexane).
1H-NMR (500 MHz, CDCl3) d: 3.78 (s, 3H), 3.79
(s, 3H), 3.80 (s, 3H), 3.97 (s, 3H), 5.32 (s, 1H), 6.85
(d, J 5 8.6 Hz, 1H), 7.15 (t, J 5 8.6 Hz, 3H), 7.22
(t, J 5 7.6 Hz, 1H), 7.34 (d, J 5 8.1 Hz, 1H), 7.48
(s, 1H), 7.54 (d, J 5 7.9 Hz, 1H). MS (ESI): 450.0
(M11H); HREIMS: Calculated for C25H23NO7 (M1)
449.1475, found 449.1477.
1
107–1081C (1/2 (v/v) ethyl acetate-hexane). H-NMR
(500 MHz, CDCl3) d: 2.37 (s, 3H), 3.78 (s, 3H), 3.81
(s, 3H), 4.00 (s, 3H), 7.27 (d, J 5 8.2 Hz, 2H), 7.28–7.29
(m, 1H), 7.34–7.36 (m, 1H), 7.40–7.42 (m, 1H), 7.87
(d, J 5 8.2 Hz, 2H), 7.89 (s, 1H), 8.03 (d, J 5 8.3 Hz,
1H). 13C-NMR (125 MHz, CDCl3) d: 22.0, 52.7
(overlapping peaks), 62.6, 110.1; 114.0, 120.8, 121.4,
123.9, 125.3, 127.4, 127.7, 129.7, 130.4, 133.4, 135.0,
135.4, 140.7, 145.6, 153.6, 158.7 (overlapping peaks).
MS (ESI): 484.0 (M11H); HREIMS: Calculated for
C24H21NO8S (M1) 483.0988, found 483.0975. Anal.
Calcd for C24H21NO8S: C, 59.62; H, 4.38; N, 2.90.
Found: C, 59.77; H, 4.36; N, 2.85.
Method B.
A solution of t-butyllithium in pentane
(24.8 ml of 1.7 M solution; 42.0 mmol) was added to a
stirred solution of 1-tosyl-3-iodoindole (7.94 g,
20.0 mmol) in anhydrous THF (100 ml) at ꢂ781C in a
nitrogen atmosphere. The solution was stirred at this
temperature for 0.5 h; trimethyl borate (22.8 ml, 20.9 g,
200.0 mmol) was dissolved in THF (400 ml) and pre-
cooled to ꢂ781C and then added via a cannula. The
reaction mixture was stirred at ꢂ781C for 1 h; 8 ml of
50% MeOH was then added, and the solution was left
to reach room temperature. The reaction mixture was
poured into water, phases were separated, and the
aqueous phase was extracted with ether. The organic
phases were combined, dried (MgSO4), and evapo-
rated. The crude oily boronic acid was triturated with
hexane-acetone to yield the boronic acid 8 as an
amorphous white powder (2.8 g, 44.4%). 1H-NMR
(500 MHz, CDCl3) d: 2.30 (s, 3H), 7.06 (d, J 5 7.7 Hz,
1H), 7.41–7.47 (m, 2H), 7.79 (d, J 5 7.7 Hz, 2H), 7.97
(d, J 5 7.5 Hz, 1H), 8.04 (d, J 5 7.5 Hz, 1H), 8.35 (d,
J 5 7.0 Hz, 1H), 8.62 (s, 1H). 13C-NMR (125 MHz,
CDCl3) d: 145.7, 138.7, 136.2, 135.2, 135.1, 133.8,
130.5, 127.4, 125.2, 124.5, 123.5, 113.9, 21.9. MS
(ESI): 316 (M11H); HREIMS: Calculated for
C15H14BNO4S (M1) 315.0737, found 315.0748. Anal.
3-(1H-indol-3-yl)-4-methoxy-furan-2,5-dicarboxylic
acid dimethyl ester (5)
DDQ (25.9 g, 0.11 mol) was added to a solution of
4 (25.6 g, 0.057 mol) in CH2Cl2 (770 ml) and H2O
(43 ml). After the reaction mixture was stirred at room
temperature for 21 h, it was placed on an ice-water
bath; 5% Na2CO3 (500 ml) was then added with
stirring. The resultant mixture was diluted with CH2Cl2
(300 ml) and washed with brine and water. The
separated organic layer was dried and concentrated.
The crude product was purified by silica gel column
chromatography using 1/2 ethyl acetate-hexane as an
eluent to afford 5 (11.3 g, 60.2%) as a white powder,
1
mp: 150–1511C (1/2 ethyl acetate-hexane). H-NMR
(500 MHz, CDCl3) d: 3.77 (t, J 5 2.9 Hz, 3H), 3.82 (s,
3H), 3.97 (d, J 5 4.3 Hz, 3H), 7.16 (t, J 5 7.6 Hz, 1H),
7.24 (t, J 5 7.6 Hz, 1H), 7.41 (d, J 5 8.1 Hz, 1H),
7.53–7.55 (m, 2H), 8.52 (s, 1H). MS (ESI): 330.3
(M11H); HREIMS: Calculated for C17H15NO6 (M1)
329.0899, found 329.0897.
Drug Dev. Res.

250
CHOU AND TSAI
Calcd for C15H14BNO4S: C, 57.17; H, 4.48; N, 4.44. chromatography using 1/2 (v/v) ethyl acetate/hexane
Found: C, 57.20; H, 4.43; N, 4.35. A mixture of the through ethyl acetate to 1/20 (v/v) methanol/ethyl
triflate 7 (0.36 g, 1.0 mmol), bis-(triphenylphosphine) acetate as an eluent to yield successively the diester 6
palladium (II) chloride (0.07 g, 0.1 mmol), the boric (1.3 g, 15.3%), 100 (0.64 g, 7.8%), 10 (4.5 g, 54.6%),
acid 8 (0.38 g, 1.2 mmol), Na2CO3 (0.32 g, 3.0 mmol) and recovered 9 (1.7 g, 21.2%). 100, a white powder,
and DMF (10 ml) was stirred at 901C for 12 h. The mp 148–1501C (methanol). 1H-NMR (500 MHz,
solvent was removed in vacuo, and the residue was 6d-DMSO) d: 2.30 (s, 3H), 3.66 (s, 3H), 3.83 (s, 3H),
diluted with ethyl acetate and water. The separated 7.21–7.22 (m, J 5 7.5, 1H), 7.30–7.32 (m, 1H), 7.35
organic layer was washed with water, dried (MgSO4), (s, 1H), 7.37 (d, J 5 8.1 Hz, 2H), 7.88–7.91 (m, 4H).
6
and evaporated. The residue was subjected to flash 13C-NMR (125 MHz, d-DMSO) d: 21.9, 52.3, 62.5,
chromatography on silica gel using 1/2 (v/v) ethyl 113.1, 113.5, 113.8, 122.2, 124.1, 125.3, 127.4, 127.7,
acetate-hexane to afford 6 (0.32 g, 65.5%) as a white 129.9, 131.0, 131.1, 134.7, 134.9, 146.3, 150.8, 154.5,
powder. The product is identical to that prepared by 159.1, 162.4. MS (ESI): 470.0 (M11H); HREIMS:
condensation (Method A).
Calculated for C24H21NO8S (M1) 469.4639, found
469.4645. Anal. Calcd for C23H19NO8S: C, 58.84; H,
3-Methoxy-4-[1-(toluene-4-sulfonyl)-1H-indol-3-yl]- 4.08; N, 2.98. Found: C, 58.87; H, 4.33; N, 2.71. 10, a
1
furan-2,5-dicarboxylic acid (9)
white powder, mp 139–1401C (methanol). H-NMR
Lithium hydroxide (4.3 g, 177.7 mmol, 700 mol% (500 MHz, 6d-DMSO) d: 2.33 (s, 3H), 3.68 (s, 3H), 3.74
vs intermediate 6) was added at 251C to a solution of (s, 3H), 7.26–7.27 (m, 1H), 7.29–7.44 (m, 4H),
intermediate
6
(12.3 g, 25.4 mmol) in methanol 7.88–7.92 (m, 2H), 7.97–8.02 (m, 2H). 13C-NMR
(280 ml) and H2O (23 ml). The resultant solution was (125 MHz, 6d-DMSO) d: 21.9, 53.0, 62.9, 110.9,
stirred at 251C for 5 h, acidified with 10% sulfuric acid, 114.0, 120.6, 121.9, 124.5, 125.8, 127.7, 128.0, 130.1,
and concentrated. The residue was diluted with ethyl 131.2, 134.6, 134.7, 134.8, 140.4, 146.6, 153.1, 158.7,
acetate and was washed with water and brine, dried, 159.4. MS (ESI): 470.0 (M11H); HREIMS: Calcu-
and concentrated to give 9 (11.5 g, 99.3%) as a white lated for C24H21NO8S (M1) 469.4639, found 469.4643.
powder, mp 153–1551C (ethyl acetate). 1H-NMR Anal. Calcd for C23H19NO8S: C, 58.84; H, 4.08; N,
6
(500 MHz, d-acetone) d: 2.35 (s, 3H), 3.84 (s, 3H), 2.98. Found: C, 58.75; H, 4.37; N, 2.75.
7.26–7.30 (m, 1H), 7.37–7.50 (m, 3H), 7.50 (d,
J 5 7.9 Hz, 1H), 7.93 (d, J 5 8.3 Hz, 1H), 8.00 (s, 1H), General procedure for the synthesis
8.05 (d, J 5 8.3 Hz, 1H). 13C-NMR (125 MHz, 6d- of 4-methoxyfuroic acid esters 16–20
acetone) d: 158.7, 158.4, 153.7, 146.1, 141.2, 135.3,
Oxalyl chloride (10 ml/g 10) was added to 10
135.0, 133.8, 130.5, 130.4, 127.8, 127.4, 125.2, 123.8, (12.2 mmol) and stirred for 20 min. The mixture was
121.6, 120.4, 113.8, 111.1, 62.1, 21.0. MS (ESI): 456.0 condensed under vacuum to give the acyl chloride of
(M11H); HREIMS: Calculated for C22H17NO8S 10. In a separated flask, a 60-ml dichloromethane
(M1) 455.0675, found 455.0672. Anal. Calcd for solution containing 12.4 mmol of the 7-substituted
C22H17NO8S: C, 58.02; H, 3.76; N, 3.08. Found: C, indoles 11–15 was added with Et2AlCl (1 M in hexane,
58.05; H, 3.73; N, 3.11.
30 mmol) at 01C and stirred for 0.5 h before 50 ml
dichloromethane solution containing the aforemen-
3-Methoxy-4-[1-(toluene-4-sulfonyl)-1H-indol-3-yl]- tioned acyl chloride of 10 was added. After 1 h stirring
furan-2,5-dicarboxylic acid 5-methyl ester (10) at 01C and 14 h at room temperature, the reaction was
The typical procedure for the regioselective quenched with 3 N HCl. The separated organic layer
synthesis of the half ester 10 involved the following was condensed and the residue was purified by silica
steps. To a solution of the diacid 9 (8.0 g, 17.6 mmol) gel chromatography to yield 16–20 (51–66% yield).
and triethylamine (1.9 g, 18.8 mmol, 107 mole% vs 9) in
dichloromethane (106 ml) at 01C was added a solution 5-[7-(3-Acetoxy-propyl)-1H-indole-3-carbonyl]-4-
of methyl chloroformate (1.70 g, 18.0 mmol, 103 mole% methoxy-3[1-(toluene-4-sulfonyl)-1H-indol-3-yl]-
vs 9) in dichloromethane (2 ml). After stirring at 01C furan-2-carboxylic acid methyl ester (16)
for 60 min, DMAP (0.26 g) was added. The resulting
1H-NMR (500 MHz, 6d-acetone) d: 2.12–2.14 (m,
solution was stirred at 01C for 1 h and at room 2H), 2.36 (s, 3H), 3.06 (t, J 5 7.5 Hz, 2H), 3.76 (s, 3H),
temperature for a further 18 h, acidified with 10% 3.91 (s, 3H), 4.12 (t, J 5 6.6 Hz, 2H), 7.17–7.18 (m,
sulfuric acid, and concentrated. The residue was 1H), 7.22–7.25 (m, 1H), 7.29–7.30 (m, 1H), 7.39–7.40
diluted with ethyl acetate, washed with water and (m, 1H), 7.42 (d, J 5 8.3 Hz, 2H), 7.51 (d, J 5 8.0 Hz,
brine, dried, and concentrated. Half esters 10 and 100 1H), 7.95 (d, J 5 8.3 Hz, 2H), 7.99 (s, 1H), 8.06
were isolated from the mixture by silica gel column (d, J 5 8.4 Hz, 1H), 8.39 (d, J 5 8.0 Hz, 1H), 8.66
Drug Dev. Res.

HYDROXYFUROIC ACID AS AN INSULIN RECEPTOR ACTIVATOR
251
(d, J 5 3.2 Hz, 1H). MS (ESI): 669.0 (M11H); (d, J 5 7.9 Hz, 1H), 7.83–7.85 (m, 3H), 8.03 (d,
HREIMS: Calculated for C36H32N2O9S (M1) J 5 8.3 Hz, 1H), 8.46 (d, J 5 8.0 Hz, 1H), 8.60 (d,
668.1828, found 668.1831.
J 5 3.0 Hz, 1H), 9.74 (br s, 1H). MS (ESI): 769.5 (M1
1H); HREIMS: Calculated for C41H40N2O11S (M1)
768.2353, found 768.3348.
5-[7-(3-Acetoxy-butyl)-1H-indole-3-carbonyl]-4-
methoxy-3[1-(toluene-4-sulfonyl)-1H-indol-3-yl]-
furan-2-carboxylic acid methyl ester (17)
General procedure for the synthesis
1H-NMR (500 MHz, 6d-acetone) d: 1.75–1.77 (m, of 4-hydroxyfuroic acid esters 21–25
2H), 1.84–1.86 (m, 2H), 1.99 (s, 3H), 2.38 (s, 3H), 3.04
A 50-ml dichloromethane solution containing
(t, J 5 7.5 Hz, 2H), 3.79 (s, 3H), 3.94 (s, 3H), 4.10 (t, 5.3 mmol of 16–20 was added with BCl3 (1 M in
J 5 6.6 Hz, 2H), 7.18–7.19 (m, 1H), 7.22–7.25 (m, 1H), hexane, 35 mmol) at 01C. The reaction mixture was
7.31–7.32 (m, 1H), 7.39–7.40 (m, 1H), 7.43 (d, stirred at 01C for 0.5 h and at room temperature for 3 h,
J 5 8.4 Hz, 2H), 7.53 (d, J 5 8.0 Hz, 1H), 7.98 (d, and then diluted with additional 50 ml dichloro-
J 5 8.4 Hz, 2H), 8.01 (s, 1H), 8.08 (d, J 5 8.4 Hz, 1H), methane. After another 0.5 h of stirring, the mixture
8.40 (d, J 5 7.8 Hz, 1H), 8.67 (d, J 5 3.2 Hz, 1H). MS was quenched with ice water and the separated organic
(ESI): 683.0 (M11H); HREIMS: Calculated for layer was condensed. The residue was purified by silica
C37H34N2O9S (M1) 682.1985, found 682.1983.
gel chromatography to yield 21–25 (80–85% yield).
5-[7-(3-Acetoxy-pentyl)-1H-indole-3-carbonyl]-4-
methoxy-3[1-(toluene-4-sulfonyl)-1H-indol-3-yl]-
furan-2-carboxylic acid methyl ester (18)
5-[7-(3-Acetoxy-propyl)-1H-indole-3-carbonyl]-4-
hydroxy-3[1-(toluene-4-sulfonyl)-1H-indol-3-yl]-
furan-2-carboxylic acid methyl ester (21)
1H-NMR (500 MHz, 6d-acetone) d: 1.48–1.50 (m,
1H-NMR (500 MHz, 6d-acetone) d: 2.10–2.12 (m,
2H), 1.67–1.69 (m, 2H), 1.75–1.85 (m, 2H), 1.98 (s, 2H), 2.39 (s, 3H), 3.10 (t, J 5 7.9 Hz, 2H), 3.86 (s, 3H),
3H), 2.37 (s, 3H), 3.00 (t, J 5 7.5 Hz, 2H), 3.77 (s, 3H), 4.15 (t, J 5 6.6 Hz, 2H), 7.23 (d, J 5 7.2 Hz, 1H),
3.92 (s, 3H), 4.05 (t, J 5 6.9 Hz, 2H), 7.16–7.17 (m, 7.28–7.30 (m, 1H), 7.31–7.33 (m, 1H), 7.42–7.44 (m,
1H), 7.22–7.25 (m, 1H), 7.30–7.31 (m, 1H), 7.39–7.39 1H), 7.45 (d, J 5 8.3 Hz, 2H), 7.65 (d, J 5 7.9 Hz, 1H),
(m, 1H), 7.40 (d, J 5 8.4 Hz, 2H), 7.52 (d, J 5 7.9 Hz, 8.00 (d, J 5 8.3 Hz, 2H), 8.08–8.10 (m, 1H), 8.11 (s,
1H), 7.97 (d, J 5 8.4 Hz, 2H), 8.00 (s, 1H), 8.07 (d, 1H), 8.37 (d, J 5 8.0 Hz, 1H), 8.80 (d, J 5 3.2 Hz, 1H).
J 5 8.4 Hz, 1H), 8.40 (d, J 5 7.8 Hz, 1H), 8.67 (d, MS (ESI): 655.0 (M11H); HREIMS: Calculated for
J 5 3.2 Hz, 1H). MS (ESI): 697.0 (M11H); HREIMS: C35H30N2O9S (M1) 654.1672, found 654.1668.
Calculated for C38H36N2O9S (M1) 696.2142, found
696.2138.
5-[7-(3-Acetoxy-butyl)-1H-indole-3-carbonyl]-4-
hydroxy-3[1-(toluene-4-sulfonyl)-1H-indol-3-yl]-
furan-2-carboxylic acid methyl ester (22)
4-Methoxy-5-[7-(4-methoxycarbonyl-butyl)-1H-
indole-3-carbonyl]-3-[1-(toluene-4-sulfonyl)-1H-
1H-NMR (500 MHz, 6d-acetone) d: 1.77–1.79 (m,
indol-3-yl]-furan-2-carboxylic acid methyl ester (19) 2H), 1.85–1.87 (m, 2H), 2.00 (s, 3H), 2.39 (s, 3H), 3.06
1H-NMR (500 MHz, 6d-acetone) d: 1.63–1.80 (t, J 5 7.8 Hz, 2H), 3.87 (s, 3H), 4.11 (t, J 5 6.6 Hz,
(m, 4H), 2.34–2.36 (m, 2H), 2.93 (s, 3H), 3.60 (t, 2H), 7.21–7.23 (m, 1H), 7.31–7.32 (m, 2H), 7.41–7.43
J 5 5.8 Hz, 2H), 3.76 (s, 3H), 3.88 (s, 3H), 3.94 (s, 3H), (m, 1H), 7.44 (d, J 5 8.4 Hz, 2H), 7.65 (d, J 5 7.9 Hz,
7.20–7.30 (m, 2H), 7.3–7.40 (m, 4H), 7.47 (d, 1H), 8.00 (d, J 5 8.4 Hz, 2H), 8.10 (d, J 5 8.4 Hz, 1H),
J 5 7.9 Hz, 1H), 7.90–7.95 (m, 3H), 8.00 (s, 1H), 8.11 (s, 1H), 8.37 (d, J 5 7.8 Hz, 1H), 8.79 (s,
8.05–8.10 (m, 1H), 8.40 (d, J 5 3.2 Hz, 1H). MS J 5 3.3 Hz, 1H). MS (ESI): 669.0 (M11H); HREIMS:
(ESI): 683.0 (M11H); HREIMS: Calculated for Calculated for C36H32N2O9S (M1) 668.1829, found
C37H34N2O9S (M1) 682.1985, found 682.1983.
668.1833.
5-[7-(5-Acetoxy-4-acetoxymethyl-pentyl)-1H-indol-
3-carbonyl]-4-methoxy-3-[(1-(toluene-4-sulfonyl)-
1H-indol-3-yl)-furan-2-carboxylic acid methyl
ester (20)
5-[7-(3-Acetoxy-pentyl)-1H-indole-3-carbonyl]-4-
hydoxy-3[1-(toluene-4-sulfonyl)-1H-indol-3-yl]-
furan-2-carboxylic acid methyl ester (23)
1H-NMR (500 MHz, 6d-acetone) d: 1.48–1.50 (m,
1H-NMR (500 MHz, CDCl3) d: 1.25–1.27 (m, 2H), 1.67–1.69 (m, 2H), 1.75–1.85 (m, 2H), 1.97 (s,
1H), 1.46–1.48 (m, 2H), 1.82–1.84 (m, 2H), 2.10 (s, 3H), 2.37 (s, 3H), 3.00 (t, J 5 7.5 Hz, 2H), 3.84 (s, 3H),
6H), 2.35 (s, 3H), 2.87 (t, J 5 8.1 Hz, 2H), 3.76 (s, 3H), 4.04 (t, J 5 6.9 Hz, 2H), 7.15–7.17 (m, 1H), 7.18–7.20
3.83 (s, 3H), 4.12–4.14 (m, 4H), 7.12 (d, J 5 7.2 Hz, (m, 1H), 7.25–7.30 (m, 2H), 7.30–7.40 (m, 3H), 7.65
1H), 7.24–7.28 (m, 4H), 7.34–7.36 (m, 1H), 7.46 (d, J 5 7.9 Hz, 1H), 7.98 (d, J 5 8.4 Hz, 2H), 8.07
Drug Dev. Res.

252
CHOU AND TSAI
(d, J 5 8.4 Hz, 1H), 8.10 (s, 1H), 8.30 (d, J 5 7.8 Hz, d: 27.8, 35.2, 61.1, 103.4, 112.4, 114.3, 116.9, 119.7,
1H), 8.70 (s, 1H). MS (ESI): 683.0 (M11H); 120.2, 121.5, 121.7, 121.9, 123.2, 123.7, 126.8, 127.2,
HREIMS: Calculated for C37H34N2O9S (M1) 682.1985, 127.7, 135.2, 136.0, 136.3, 136.4, 136.7, 158.7, 161.6,
found 682.1983.
178.3. MS (ESI, negative mode): 443.2 (M1–H);
HREIMS: Calculated for C25H20N2O6 (M1) 444.1321,
found 444.1324. Anal. Calcd for C25H20N2O6: C, 67.56;
H, 4.54; N, 6.30. Found: C, 67.61; H, 4.52; N, 5.98.
4-Hydroxy-5-[7-(4-methoxycarbonyl-butyl)-1H-
indole-3-carbonyl]-3-[1-(toluene-4-sulfonyl)- 1H-
indol-3-yl]-furan-2-carboxylic acid methyl ester (24)
1H-NMR (500 MHz, 6d-acetone) d: 1.63–1.80 4-Hydroxy-5-[7-(3-hydroxy-butyl)-1H-indole-3-
(m, 4H), 2.34–2.36 (m, 2H), 2.97–2.99 (m, 3H), 3.59 carbonyl]-3-(1H-indol-3-yl)-furan-2-carboxylic acid
(s, 3H), 3.61 (s, 3H), 3.78 (t, J 5 5.8 Hz, 2H), 7.16 (d, (27)
J 5 7.3 Hz, 2H), 7.20–7.30 (m, 2H), 7.30–7.40 (m, 4H),
A yellow powder, mp 145–1471C (ethyl acetate).
7.80–7.95 (m, 2H), 8.00 (s, 1H), 8.10–8.15 (m, 1H), 1H-NMR (500 MHz, 6d-acetone) d: 1.63–1.65 (m, 2H),
8.27 (s, 1H), 8.28 (d, J 5 7.4 Hz, 1H). MS (ESI): 669.0 1.84–1.86 (m, 2H), 3.01 (t, J 5 7.5 Hz, 2H), 3.64 (t,
(M11H); HREIMS: Calculated for C36H32N2O9S J 5 6.5 Hz, 2H), 7.07–7.08 (m, 1H), 7.15–7.17 (m, 2H),
(M1) 668.1829, found 668.1833.
7.23–7.25 (m, 1H), 7.68 (d, J 5 8.0 Hz, 1H), 7.35–7.36
(m, 1H), 7.48 (d, J 5 8.0 Hz, 1H), 7.79–80 (m, 1H),
8.35 (d, J 5 8.0 Hz, 1H), 8.60 (br s, 1H), 8.90 (br s, 1H).
5-[7-(5-Acetoxy-4-acetoxymethyl-pentyl)-1H-indol-
3-carbonyl]-4-hydroxy-3-[(1-(toluene-4-sulfonyl)-
6
13C-NMR (125 MHz, d-DMSO) d: 27.2, 30.7, 33.1,
1H-indol-3-yl)-furan-2-carboxylic acid methyl ester 61.5, 102.8, 112.5, 114.3, 119.9, 120.0, 120.2, 121.4,
(25)
122.0, 123.4, 123.9, 127.0, 127.5, 127.9, 135.4, 136.0,
1H-NMR (500 MHz, CDCl3) d: 1.25–1.27 (m, 136.5, 136.8, 139.6, 143.7, 153.3, 160.7, 178.5. MS
1H), 1.47–1.49 (m, 2H), 1.83–1.85 (m, 2H), 2.12 (s, (ESI, negative mode): 457.2 (M1-H); HREIMS:
6H), 2.35 (s, 3H), 2.87 (t, J 5 8.2 Hz, 2H), 3.81 (s, 3H), Calculated for C26H22N2O6 (M1) 458.1478, found
4.10–4.12 (m, 2H), 4.18–4.20 (m, 2H), 7.14 (d, 458.1459. Anal. Calcd for C26H22N2O6: C, 68.11; H,
J 5 7.2 Hz, 1H), 7.25–7.28 (m, 4H), 7.34–7.36 (m, 4.84; N, 6.11. Found: C, 68.23; H, 4.78; N, 5.88.
1H), 7.53 (d, J 5 7.9 Hz, 1H), 7.85 (d, J 5 8.2 Hz, 2H),
7.94 (s, 1H), 8.02 (d, J 5 8.4 Hz, 1H), 8.39 (d, 4-Hydroxy-5[7-(3-hydroxy-pentyl)-1H-indole-3-
J 5 7.9 Hz, 1H), 8.73 (s, 1H), 9.96 (br s, 1H). MS carbonyl]-3-(1H-indol-3-yl)-furan-2-carboxylic acid
(ESI): 755.5 (M11H); HREIMS: Calculated for (28)
C40H38N2O11S (M1) 764.2196, found 754.2193.
A yellow powder, mp 158–1601C (ethyl acetate).
1H-NMR (500 MHz, 6d-acetone) d: 1.53–1.55 (m, 2H),
1.61 (t, J 5 7.2 Hz, 2H), 1.82 (t, J 5 7.4 Hz, 2H), 3.01 (t,
J 5 7.8 Hz, 2H), 3.58 (t, J 5 6.2 Hz, 2H), 7.10–7.11 (m,
General procedure for the synthesis
of 4-hydroxyfuroic acid derivatives 26–30
A methanol solution (10–15 ml/g) containing 1H), 7.18–7.20 (m, 2H), 7.26–7.28 (m, 1H), 7.51 (d,
ꢁ0.11 g of the furan bisindole (21–25) from the J 5 8.0 Hz, 1H), 7.69 (d, J 5 8.0 Hz, 1H), 7.79
previous step was added with 1.0 ml of 5% aqueous (d, J 5 2.5 Hz, 1H), 8.38 (d, J 5 7.9 Hz, 1H), 8.84
NaOH and refluxed for 0.5 h. The mixture was (d, J 5 3.2 Hz, 1H), 10.7 (br s, 1H), 11.4 (br s, 1H).
6
concentrated, acidified with 3 N HCl, and then 13C-NMR (125 MHz, d-acetone) d: 26.2, 30.5, 31.4,
extracted with ethyl acetate. The extract was washed 33.0, 62.0, 102.9, 111.9, 114.2, 119.7, 120.0, 120.3,
with brine, dried, and solvent evaporated. The residue 121.4, 121.9, 123.2, 123.9, 126.8, 127.1, 127.3, 127.5,
was purified by silica gel chromatography to give the 135.2, 135.9, 136.2, 136.8, 139.1, 155.2, 159.7, 179.6.
title compounds with 66–70% yield.
MS (ESI): 473.0 (M11H); HREIMS: Calculated for
C27H24N2O6 (M1) 472.1634, found 472.1639. Anal.
Calcd for C27H24N2O6: C, 68.63; H, 5.12; N, 5.93.
Found: C, 68.47; H, 4.98; N, 5.87.
4-Hydroxy-5[7-(3-hydroxy-propyl)-1H-indole-3-
carbonyl]-3-(1H-indol-3-yl)-furan-2-carboxylic
acid (26)
A yellow powder, mp 140–1421C (ethyl acetate). 5-[7-(4-Carboxy-butyl)-1H-indole-3-carbonyl]-4-
6
1H-NMR (500 MHz, d-DMSO) d: 1.84–1.88 (m, 2H), hydroxy-3-(1H-indol-3-yl)-furan-2-carboxylic acid
2.95 (t, J 5 7.4 Hz, 2H), 3.51 (t, J 5 6.5 Hz, 2H), (29)
7.02–7.05 (m, 1H), 7.11–7.15 (m, 2H), 7.18–7.21 (m,
An amorphous yellow powder. 1H-NMR
1H), 7.46 (d, J 5 8.0 Hz, 1H), 7.50 (d, J 5 8.0 Hz, 1H), (500 MHz, 6d-acetone) d: 1.74 (d, J 5 7.6 Hz, 2H),
7.62–7.63 (m, 1H), 8.27 (d, J 5 7.5 Hz, 1H), 8.66 1.92 (d, J 5 2.1 Hz, 2H), 2.38 (t, J 5 7.5 Hz, 2H),
6
(d, J 5 2.2 Hz, 1H). 13C-NMR (125 MHz, d-DMSO) 3.02 (t, J 5 7.5 Hz, 2H), 7.08–7.11 (m, 1H), 7.17–7.21
Drug Dev. Res.

HYDROXYFUROIC ACID AS AN INSULIN RECEPTOR ACTIVATOR
253
(m, 2H), 7.25–7.27 (m, 1H), 7.50 (d, J 5 8.1 Hz, 1H), separated organic layer was washed successively with
7.67 (d, J 5 7.8 Hz, 1H), 7.77 (d, J 5 2.4 Hz, 1H), 8.37 saturated aqueous NaHCO3 (25 ml) and water (25 ml)
(d, J 5 7.8 Hz, 1H), 8.83 (d, J 5 3.0 Hz, 1H), 10.66 (br and then evaporated. The residue was purified by flash
s, 1H),11.44 (br s, 1H). 13C-NMR (125 MHz, 6d- chromatograph (silica, 3/1 (v/v) hexane-ethyl acetate) to
acetone) d: 25.1, 30.0, 31.0, 33.5, 102.9, 111.9, 114.3, yield 33 (6.5 g, 90% yield) as yellow oil. 1H-NMR
119.7, 120.0, 120.4, 121.4, 121.9, 123.3, 123.9, 126.7, (500 MHz,CDCl3) d: 2.12 (s, 3H), 4.80 (d, J 5 6.3 Hz,
127.0, 127.3, 127.5, 135.2, 135.8, 136.2, 136.8, 139.1, 2H), 6.29–6.40 (m, 1H), 6.57 (dd, J 5 3.0, 2.1 Hz, 1H),
155.1, 159.6, 174.3, 179.6. MS (ESI): 486.5 (M1); 6.93 (d, J 5 15.9 Hz, 1H), 7.09–7.20 (m, 2H), 7.26 (d,
HREIMS: Calculated for C27H22N2O7 (M1) 482.1427, J 5 7.8 Hz, 1H), 7.60 (d, J 5 7.8 Hz, 1H), 8.62 (br s,
found 482.1432. Anal. Calcd for C27H22N2O7: C, 66.66; 1H). MS (ESI): 216.0 (M11H); HREIMS: Calculated
H, 4.56; N, 5.76. Found: C, 66.57; H, 4.53; N, 5.68.
for C13H13NO2 (M1) 215.0946, found 215.0951.
4-Hydroxy-5-[7-(5-hydroxy-4-hydroxymethyl-
pentyl)-1H-indole-3-carbonyl]-3(1H-indol-3-yl)-
furan-2-carboxylic acid (30)
2-[3-(1H-Indol-7-yl)allyl]malonic acid diethyl ester
(34)
A solution of [(C6H5)3P]2PdCl2 (0.84 g, 1.2 mmol)
A yellow powder, mp 141–1431C (ethyl acetate). in THF (30 ml) was added the acetate 33 (6.5 g,
1H-NMR (500 MHz, 6d-acetone) d: 1.50–1.53 (m, 2H), 30.2 mmol) in THF (30 ml) by syringe. After a further
1.74–1.76 (m, 1H), 1.85–1.89 (m, 2H), 3.01 (t, 0.5 h of stirring, the solution was added to a stirred
J 5 7.9 Hz, 2H), 3.63–3.67 (m, 4H), 7.09–7.12 (m, suspension of sodium malonate (8.2 g, 45.0 mmol) in
1H), 7.17–7.21 (m, 2H), 7.25–7.28 (m, 1H), 7.51 (d, THF (30 ml). The reaction mixture was stirred at room
J 5 8.1 Hz, 1H), 7.68 (d, J 5 8.0 Hz, 1H), 7.79 temperature overnight and then diluted with ethyl
(d, J 5 2.6 Hz, 1H), 8.38 (d, J 5 7.8 Hz, 1H), 8.84 acetate (50 ml), and the organic phase washed with
(d, J 5 3.1 Hz, 1H), 10.68 (br s, 1H), 11.52 (br s, 1H). saturated aqueous NH4Cl (40 ml ꢃ 2) and water
6
13C-NMR (125 MHz, d-acetone) d: 28.4, 28.5, 31.7, (50 ml). The separated organic layer was condensed.
43.4, 64.0, 103.0, 111.8, 114.2, 119.7, 120.2, 121.4, The residue was purified by silica gel chromatography
1
121.9, 123.2, 123.9, 124.1, 126.9, 127.1, 127.3, 127.5, to yield 34 (4.8 g, 45% yield). H-NMR (500 MHz,
135.1, 135.3, 135.9, 136.1, 136.8, 137.8, 155.2, 179.6. CDCl3) d: 1.26 (t, J 5 7.2 Hz, 6H), 2.84 (t, J 5 7.2 Hz,
MS (ESI): 503.0 (M11H); HREIMS: Calculated for 2H), 3.56 (t, J 5 7.2 Hz, 1H), 4.21 (q, J 5 7.2 Hz, 4H),
C28H26N2O7 (M1) 502.1740, found 502.1746. Anal. 6.16–6.26 (m, 1H), 6.53–6.55 (m, 1H), 6.73 (d,
Calcd for C28H26N2O7: C, 66.92; H, 5.22; N, 5.57. J 5 15.9 Hz, 1H), 7.03–7.12 (m, 2H), 7.21–7.23 (m,
Found: C, 66.87; H, 5.52; N, 5.47.
1H), 7.52 (d, J 5 7.8 Hz, 1H), 8.65 (br s, 1H). MS
(ESI): 316.0 (M11H); HREIMS: Calculated for
C18H21NO4 (M1) 315.1471, found 315.1473.
3-(1H-Indol-7-yl)-pro-2-en-1-ol (32)
A solution of 3115 (8.0 g, 39.8 mmol) in toluene
(60 ml) was added, Dibal-H (1 M intoluene, 40 ml) at 2-[3-(1H-Indol-7-yl)-propyl]propane-1,3-
ꢂ301C. The reaction mixture was stirred at ꢂ301C for diacetoxymethyl (15)
0.5 h and then quenched with 20 ml of ice water (20 ml)
A solution of the diester 34 (4.8 g, 15.2 mmol) in
and 3 N HCl (15 ml). After another 0.5 h of stirring, the MeOH (100 ml) was hydrogenated (50 psi, Pd/C) using
separated organic layer was condensed. The residue 10% Pd-C (10 wt%) until completion of reduction
was purified by silica gel chromatography to yield 32 (ꢁ12 h). The resultant mixture was filtered from Celite
1
(5.8 g, 84% yield) as yellow oil. H-NMR (500 MHz, and was evaporated to give saturated diester in
CDCl3) d: 4.34 (d, J 5 5.1 Hz, 2H), 6.32–6.41 (m, 1H), quantitative yield. The intermediate was taken into
6.58 (dd, J 5 3.0, 2.1 Hz, 1H), 6.85 (d, J 5 15.9 Hz, 1H), ether (16 ml), and added to a suspension of LiAlH4
7.10–7.16 (m, 2H), 7.23 (d, J 5 6.9 Hz, 1H), 7.62 (d, (2.3 g, 60.6 mmol) in ether (120 ml) at 01C. After being
J 5 7.8 Hz, 1H), 8.79 (br s, 1H). MS (ESI): 174.1 (M1 stirred at 01C for 10 min and at room temperature for
1H); HREIMS: Calculated for C11H11NO (M1) 3 h, the reaction was quenched at 01C by careful
173.0841, found 173.0846.
addition of AcOEt (17 ml) followed by MeOH (2 ml)
and by 3 N HCl (81 ml). The mixture was extracted
with AcOEt, and the organic phase washed with brine
7-(3-Aceoxypropen-1-yl)-1H-indole (33)
A solution of 32 (5.8 g, 33.5 mmol) in chloroform and evaporated to dryness to give crude diol (2.0 g).
(100 ml) containing pyridine (10 ml) was added Ac2O The diol was acetylated (Ac2O, pyridine) in chloroform
(5 ml) at 0–101C. The reaction mixture was stirred at by a similar procedure as the acetylation of 32 to yield
0–101C for 5 h and then quenched with 20 ml of ice 15 (3.1 g, 64%). 1H-NMR (500 MHz, CDCl3) d:
water (20 ml) and 3 N HCl (20 ml) with stirring. The 1.40–1.50 (m, 2H), 1.80–2.00 (m, 3H), 2.10 (s, 6H),
Drug Dev. Res.

254
CHOU AND TSAI
2.82 (t, J 5 7.8 Hz, 2H), 4.00–4.20 (m, 4H), 6.53–6.55 coefficient. X represents compound concentration;
(m, 1H), 6.96 (d, J 5 6.9 Hz, 1H), 7.03 (t, J 5 7.2 Hz, Y represents observed response, while max and min
1H), 7.21–7.23 (m, 1H), 7.49 (d, J 5 8.1 Hz, 1H), 8.65 were approximated automatically by the program
(br s, 1H). MS (ESI): 318.0 (M11H); HREIMS: during the calculation.
Calculated for C18H23NO4 (M1) 317.1627, found
RESULTS AND DISCUSSION
317.1623.
Retroanalysis of Hydroxyfuroic Acid Derivatives
An interesting feature during the synthesis of
Insulin Receptor Activation Assay
Activation efficacies on cell-based insulin recep- series 1 compounds is the selective methylation of
tor tyrosine kinase were determined using a Chinese 3-methoxy-4-phenyl-furan-2,5-dicarboxylic acid to its
hamster ovary (CHO) cell line that overexpresses 5-methyl ester using methyl chloroformate and
recombinant human insulin receptor, followed by in 4-dimethylaminopyridine (DMAP), in a mixed anhy-
vitro determination of receptor tyrosine kinase activity dride method (see section below, Synthesis of the
[Zhang et al., 1999; Liu et al., 2000]. Approximately Furoic Acid 9) [Chou et al., 2006]. Since series 2
100,000 CHO cells were used for one assay in each well compounds share the same hydroxyl furan core, we
of a 96-well plate. Cells were incubated overnight and adapted the same strategy for series 2 compound
starved in serum free Ham’s F-12 medium for 2 h synthesis, and the retro synthetic disconnection is
before being stimulated with desired concentration of essentially the same as that of series 1 compound, as
insulin or test compound for 20 min. Test compounds illustrated in Figure 2. The desired half ester C could
were predissolved in DMSO as 100 ꢃ stocks and be prepared from the furoic diacid D by the mixed
serially diluted when needed. Stimulated CHO cells anhydride method. The series 2 derivatives A could be
were lysed in 60 ml of lysis buffer; 50 ml of the lysate was prepared by acylation with a 7-substituted indole B
transferred to a well on a Flexible Assay Plate with the half ester C.
precoated with Ab-3 antibody specific to the b-subunit
of insulin receptor. The well was then rinsed 3 times
Synthesis of the Furoic Acid 9
and dried before 10 ml of tyrosine kinase reaction
Scheme 1 presents the synthetic pathway for the
mixture (50 mM HEPES, pH 7.4, 5 mM MgCl2, 5 mM furoic diacid 9. The furan diester 4 was prepared by
MnCl2, 1 mg/ml polyGlu/Tyr and 250,000 cpm g-P33 adding a mixture of dimethyl diglycolate 1 and
ATP/well) was added. The reaction was carried out at p-methoxybenzyl (PMB)-protected indolylglyloxalate
251C for 40 min and terminated by adding 50 ml ice 2 (1.5:2 molar ratio) to a refluxing suspension of
cold 100 mM phosphoric acid. Next, 50 ml of the t-BuOK (2.7 molar eqs. vs dimethyl diglycolate) in
stopped reaction mixture was transferred to an inch- refluxing benzene; the resulting 3-hydroxyfuran inter-
square P81 paper. The P81 paper was rinsed in mediates (3a and 3b) were immediately protected by
Millipore water 5 times to remove leftover radioactive methylation to provide 4 [note: theoretically, one molar
ATP; 5 ml of scintillation cocktail was added to the equivalent of hydroxide ion would be generated by
paper before the count was determined in a liquid condensation, leading to partial hydrolysis of the
scintillation counter.
resulting furan diester, but in our setting, executing
All measurements were determined in triplicate methylation without purification is advantageous for a
with blank. Bovine insulin (10 nM) was used as a yield improvement about 15–20%]. Removal of PMB
positive control, to serve as the reference standard. from 4 would generate 5, which was achieved by
Fifty percentile effective concentration (EC50) was stirring 4 with 2,3-dichloro-5,6-dicyano-p-benzoqui-
estimated by the dose curve-fitting function in none (DDQ) in CH2Cl2/H2O 18:1 (v/v) at room
SigmaPlot 8, which models on Y 5 min1(maxꢂmin)/ temperature [Oikawa et al., 1984]. The deprotected
(11(X/EC50)^Hillslope), where Hillslope is Hill indole 5 was then tosylated to form diester 6.
H
N
HOOC
InP: N-proteted
indole group
2
HOOC
MeOOC
O
3
O
5
5
5
InP
HN
InP
4
4
OH
4
O
+
2
HN
2
3
OMe
3
OMe
O
7
X = OH or COOH
n = 0-3
( )n
HO
HO
X
O
O
7'
( )n
D
B
C
A
X
Fig. 2. Retroanalysis of 4-hydroxyfuroic acids.
Drug Dev. Res.

HYDROXYFUROIC ACID AS AN INSULIN RECEPTOR ACTIVATOR
255
O
O
MeO2C(O)C
MeO2C
CO2H
MeO2C
HO
CO2Me
i
+
+
MeO2C
O
CO2Me
ii
HO
N
PMB
N
PMB
N
PMB
1
PMB: p-methoxybenzyl
2
3a
3b
O
O
O
O
MeO2C
MeO
CO2Me
MeO2C
MeO
CO2Me
MeO2C
MeO
CO2Me
HO2C
MeO
CO2H
iv
v
iii
N
PMB
N
H
N
Ts
N
Ts
5
9
4
6
vi
OH
B
O
HO
MeO2C
CO2Me
OTf
+
N
Ts
MeO
8
7
Scheme 1. Reagents and conditions: (i) K, t-BuOH, benzene; (ii) MeI, DMF, 45.3% (two steps); (iii) DDQ, CH2Cl2/H2O (18/1, v/v); 45.5%; (iv)
TsCl, 2-butanone, K2CO3, reflux, 85.6%; (v) OHꢂ, MeOH, H2O, 95%; and (vi) PdCl2(PPh3) 2 (0.1 eq), Na2CO3 (3.0 eq), DMF, 901C, 12 h, 65.5%.
3.74 ppm
O
O
O
O
O
N
O
O
O
HNEt3
ii
O
O
O
OMe
HO
OMe
MeO
MeO
O
2
5
O
CO2H
N
Ts
O
H
3.68 ppm
i
4
Ts 10 (major)
3
O
3.83 ppm
Me
O
O
O
O
O
N
O
Ts
HNEt3
MeO
O
ii
MeO
OH
9
MeO
MeO
N
Ts
N
3.68 ppm
Ts 10' (minor)
Scheme 2. Reagents and conditions: (i) ClCO2Me (100 mol%), NEt3 (107 mol%), CH2Cl2 (6 ml/mmol 9) and (ii) DMAP (12 mol%), 65.5% (10/
100 5 7:1 molar ratio).
Alternatively, 6 can be prepared from the metal- treating it with methyl chloroformate and dichloro-
catalyzed cross-coupling of the triflate 7 [Chou et al., methane in the presence of triethylamine and DMAP
2006] with an indolyl boronic acid 8 [Witulski et al., (Scheme 2). Presumably, the C-2 carboxylic acid of 9 is
2000]. Hydrolysis of 6 yielded the diacid 9. The conjugated with an electron-donating 3-methoxyl group
purpose of tosyl replacement is to avoid undesired (3-OMe), and the acidic proton is locked intramole-
electrophilic acylation at the indole ring during the cularly by the 3-OMe group, i.e., intramolecular
subsequent acylation step. Without the tosyl replace- H-bonding effect. Therefore, the C-2 carboxylic acid
ment, the subsequent acylation would have yielded a should be less likely to be deprotonated and thus less
complicated mixture.
reactive than C-5 free carboxylic acid when treated with
methyl chloroformate and triethylamine. The mixed
anhydride intermediate was then decarboxylated to C-5
carboxylic acid methyl ester spontaneously in the
Regioselective Synthesis of the Half Ester 10 From
the Diacid 9: Mixed Anhydride Method
The diacid 9 was converted regioselectively into presence of a catalytic amount of DMAP [Chou et al.,
the half ester 10 (diastereomeric ratio: 10/100ꢁ7/1) by 2006]. As revealed by the 1H-NMR spectrum, the
Drug Dev. Res.

256
CHOU AND TSAI
methyl ester protons of the desired isomer 10 showed a indicating that C-5 ester, without intramolecular
higher chemical shift than 100 because of the indole H-bonding, is consistently more reactive than C-2 ester
ring’s shielding effect, and the regiochemistry can also [note: hydrolysis of 6 with 150 mole% of LiOH in
be elucidated by an NOE spectrum. Conversely, the aqueous methanol (MeOH: 17.9 ml/mmol 6, H2O:
hydrolysis of the diester 6 using a 1.5 M equivalent of 0.9 ml/mmol 6) at room temperature for 12 h yielded
LiOH in aqueous methanol yielded the undesired 100 (undesired), 10 (desired) and intact 6 in 83:5:7
half acid 100 as the major product (10/100ꢁ1/16), molar ratio].
O
2
3
5
4
HOOC
COOMe
+
N
H
MeO
R
N
R =
R =
R =
(CH2)nOAc, n = 3, 11, n = 4, 12, n = 5, 13
(CH2)nCOOMe, n = 4, 14
(CH2)3CH(CH2OAc)2, 15
10
Ts
i, ii
O
O
COOMe
R
O
O
HN
iii
COOMe
R
MeO
HN
HO
N
Ts
N
R =
R =
R =
(CH2)nOAc, n = 3, 16, n = 4, 17, n = 5, 18
(CH2)nCOOMe, n = 4, 19
Ts
R =
(CH2)nOAc, n = 3, 21, n = 4, 22, n = 5, 23
(CH2)nCOOMe, n = 4, 24
(CH2)3CH(CH2OAc)2, 20
R =
R =
(CH2)3CH(CH2OAc)2, 25
iv
O
O
2
5
4
COOH
R
7'
HN
3
HO
R =
R =
R =
(CH2)nOH, n = 3, 26, n = 4, 27, n = 5, 28
(CH2)nCOOH, n = 4, 29
N
H
(CH2)3CH(CH2OH)2, 30
Scheme 3. Reagents and conditions: (i) oxalyl chloride, DMAP (cat.), room temperature; (ii) 7-substituted indole (11–15), Et2AlCl, CHCl3,
51–66%; (iii) BCl3, CH2Cl2, 01C to room temperature; 80–85%; and (iv) 5% NaOH in methanol, reflux, 66–70%.
iv
v
vi
i
iii
N
ii
N
H
N
N
H
H
H
CO2Me
EtO2C
RO
AcO
CO2Et
OAc
31
R = H, 32
R = Ac, 33
34
15
Scheme 4. Reagents and conditions: (i) DIBALH, toluene, 84%; (ii) Ac2O, pyridine, CHCl3, 86.4%; (iii) NaCH(COOEt)2, [(C6H5)3P]2PdCl2,
THF, 45%; (iv) H2, Pd-C; (v) LiAlH4, ether; and (vi) Ac2O, pyridine, CHCl3, 64% (three steps, 34–15).
Drug Dev. Res.

HYDROXYFUROIC ACID AS AN INSULIN RECEPTOR ACTIVATOR
257
Hydroxyfuroic Acid Derivatives
carbon chain and a protected hydrophilic end group
were prepared by chain extension reactions as described
by Chou et al. [2006]. The C-7 dihydroxyalkyl indole
intermediate 15, was prepared from 3-(1H-indol-7-
yl)acrylic acid methyl ester (31) by a six-step sequence,
in which the metal-catalyzed coupling (33-34) is the
key step (Scheme 4). The acyl intermediates 16–20 were
deprotected under mild reaction conditions using boron
trichloride to yield hydroxyfurans 21–25 (see Scheme 3).
Finally, simultaneous hydrolysis of the C-2 ester, the C-70
ester and the N-tosyl group under basic condition
resulted in the hydroxyfuroic acid derivatives (26–30)
[Tsai and Chou, 2009].
The synthetic routes of hydroxyfuroic acids 26–30
are presented in Scheme 3. The acylation of the half
ester 10 with 7-substituted indoles 11–15 was performed
under mild reaction conditions using diethylaluminum
chloride as the catalyst [Okauchi et al., 2000]. The
7-substituted indole intermediates (11–14) with a linear
TABLE 1. Activation of Insulin Receptor by 100 lM of Hydroxyfuroic
Acid Compounds
COOH
HN
O
NH
R
Biological Evaluations
OH
The 4-hydroxyfuroic acid compounds were sub-
jected to insulin receptor activation assay and the
results indicated that cells treated with 100 mM of
compounds 26–30 exhibited 12–67% of the receptor
tyrosine kinase activity as compared to that offered by
10 nM bovine insulin control (Table 1). Among these
compounds, hydroxyfuroic acid 28 is the most potent
one. When comparing 28 to Merck’s Compound 2, the
O
% Activationa
EC50
b
Compound
Side chain R
26
27
28
29
(CH2)2CH2OH
(CH2)3CH2OH
(CH2)4CH2OH
(CH2)4COOH
(CH2)3CH(CH2OH)2
NA
15.574.9
26.071.5
88.677.6
32.6715.1
26.3713.9
NA
32.972.9
30
Compound 2
(Merck)
38.874.6 EC50 are 32.972.9 mM for 28 and 38.874.6 mM for
Compound 2, suggesting they have similar potencies at
activating insulin receptor tyrosine kinase activity.
NA, not applicable.
aCompound activation is expressed as percentage of activation
provided by 10 nM bovine insulin. Values are average7standard
deviation (SD) of triplicate measurements.
bFifty percentile effective concentration (EC50) is expressed in mM.
EC50 of 28 was determined with compound concentrations ranging
Figure 3 exhibits dose responses of human insulin
receptor to increasing concentrations of compound A,
26 and 27. Compound A is a series 1 compound, which
has a 3-phenyl instead of 3-indole substitute, showed
from 23 to 300 mM, while Compound 2 of Merck ranging from 7.5 to no appreciable biological activity, whereas 26, 27 are
100 mM. Percentage activation of 28 maximums at 95 mM (48.7%
activation), while Compound 2 of Merck maximums at 75 mM (55.3%
activation). The EC50 parameters are determined by the dose curve
fitting function in SigmaPlot 8 with the regression curve of Y 5 min1
series 2 compounds and possess substantial insulin
receptor activation potential. The responses are
normalized against a concurrent insulin receptor
activation assay offered by 10 nM bovine insulin
stimulation.
(maxꢂmin)/[11(x/EC50)^Hillslope]. Compounds 26, 27, 29, and 30
have too little activation to be measured for EC50
.
35
30
25
20
15
10
5
COOH
R2
HN
O
R1
OH
O
27: R1=(CH2)3CH2OH, R2=1-H-indol-3-yl
26: R1=(CH2)2CH2OH, R2=1-H-indol-3-yl
A : R1=(CH2)3CH2OH, R2=phenyl
0
30
50
70
90
110
µM
A
26
27
Fig. 3. Dose responses of human insulin receptor to increasing concentrations of compound A of series 1, 26, and 27 of series 2. The responses
are normalized against a concurrent insulin receptor activation assay offered by 10 nM bovine insulin stimulation. [Color figure can be viewed in
the online issue which is available at wileyonlinelibrary.com]
Drug Dev. Res.

258
CHOU AND TSAI
protecting group by DDQ oxidation. Tetrahedron Lett 25:
CONCLUSION
5393–5396.
This work presents an approach of regioselective
Okauchi T, Itonaga M, Minami T, Owa T, Kitoh K, Yoshino H. 2000.
A general method for acylation of indoles at the 3-position with
acyl chlorides in the presence of dialkylaluminum chloride. Org
Lett 2:1485–1487.
synthesis for biologically active furandiindole deriva-
tives. The scaffold of these compounds is novel, and
this methodology may also be applied to introducing
various substituents into the furan ring with desired
regiochemistry.
Pirrung MC, Deng L, Lin B, Webster NJ. 2008. Quinone
replacements for small molecule insulin mimics. Chembio-
chemistry 9:360–362.
Ross SA, Gulve EA, Wang M. 2004. Chemistry and biochemistry of
type 2 diabetes. Chem Rev 104:1255–1282.
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