First total synthesis of isoquinolinone alkaloid marinamide and its methyl ester

Article abstract of DOI:10.1016/j.cclet.2013.04.039

The first total synthesis of isoquinolinone alkaloid marinamide 1 and its methyl ester 2 was described. The key steps involved a regioselective Friedel-Crafts reaction of 1-benzyl-1H-pyrrole to form the intermediate 8.

Full text of DOI:10.1016/j.cclet.2013.04.039

Chinese Chemical Letters 24 (2013) 767–769
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Chinese Chemical Letters
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Original article
First total synthesis of isoquinolinone alkaloid marinamide and its methyl ester
*
Cheng-Liang Feng, Shu-Guang Zhang, Jun-Qing Chen, Jin Cai, Min Ji
Institute of Pharmaceutical Engineering, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The first total synthesis of isoquinolinone alkaloid marinamide 1 and its methyl ester 2 was described.
The key steps involved a regioselective Friedel–Crafts reaction of 1-benzyl-1H-pyrrole to form the
intermediate 8.
Received 7 March 2013
Received in revised form 16 April 2013
Accepted 18 April 2013
Available online 30 May 2013
ß 2013 Min Ji. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords:
Marinamide
Methyl ester of marinamide
Isoquinolinone alkaloid
Total synthesis
1. Introduction
2. Experimental
All reagents were purchased from commercial sources and used
without further purification. Melting points were determined
on a RY-1 hot stage microscope and are uncorrected. 1H NMR and
13C NMR spectra were recorded on a Bruker Avance DPX-300 MHz
instrument in CDCl3 or DMSO-d6, chemical shifts are given in part
per million (ppm) relative to TMS as an internal standard. The
HRMS spectra were obtained on a Thermo Finnigan spectrometer,
model MAT 95XP.
Marinamide
1 and its methyl ester 2 were two novel
isoquinolinone alkaloids, which were isolated from the metabolite
of mixed fermentation of two mangrove endophytic fungi (strains
Nos. 1924 and 3893) from the South China Sea. These two
compounds were identified as 4-(2-pyrrolyl)-1-isoquinolone-3-
carboxylic acid (1) and methyl 4-(2-pyrrolyl)-1-isoquinolone-3-
carboxylate (2), respectively. Preliminary studies on the biological
activity of these two compounds revealed that 1 and 2 both display
significant antibacterial activity against Escherichia coli (diameter
of bacteriostatics ring/cm: 1.4, 1; 2.0, 2), Pseudomonas pyocyanea
(0.9, 1; 1.7, 2) and Staphylococcus auereus (1.0, 1; 1.3, 2) at the
concentration of 1 mg/mL [1]. And they further demonstrated
2.1. Preparation of methyl 2-(1-benzyl-1H-pyrrole-2-carbonyl)benzoate
(8)
2-(Methoxycarbonyl)benzoic acid (4, 12.6 g, 70 mmol) was
dissolved in anhydrous dichloromethane (100 mL) and then
thionyl chloride (12.5 g, 105 mmol) was added and the mixture
was heated under reflux for 45 min. The solvent was evaporated
under reduced pressure, toluene (50 mL) was added and the
mixture was re-concentrated. The obtained residue was added into
a mixture of 1-benzyl-1H-pyrrole (7, 7.5 g, 50 mmol), Zn powder
(5.2 g, 80 mmol) and toluene (50 mL). The mixture was stirred at
room temperature (Scheme 1). After the completion of the reaction
as indicated by TLC analysis, the mixture was quenched with a
saturated sodium bicarbonate solution (50 mL) and extracted with
ethyl acetate (2Â 100 mL). Evaporation of the solvent followed by
purification on silica gel afforded colorless liquid 8 (12 g, 78%
significant antitumor activity against HepG2 (IC50 = 2.52
m
g/mL, 1
g/mL,
g/mL, 2) and Hela
and 0.007
2), MGC832 (IC50 = 0.013
cell lines (IC50 = 0.110
m
g/mL, 2), 95-D (IC50 = 1.54
m
g/mL, 1 and 0.004 m
m
g/mL, 1 and 0.091
m
m
g/mL, 1 and 0.529 mg/mL, 2) [2]. Because of
their promising biological activities, they can serve as ideal lead
compounds for the development of drugs to combat cancer and
other diseases. In order to provide an access to sufficient quantities
of these materials for further pharmacological studies, we
developed a facile and efficient approach to synthesize marina-
mide and its methyl ester.
yield). IR (cmÀ1):
1331, 1263, 1122, 1078, 1030, 913, 871, 714. 1H NMR (300 MHz,
CDCl3): 3.38 (s, 3H,), 5.70 (s, 2H), 6.18 (dd, 1H, J = 2.52, 4.05 Hz),
6.33 (dd, 1H, J = 1.71, 4.05 Hz), 7.23–7.46 (m, 7H), 7.60–7.66
n 2950, 2921, 1721, 1630, 1457, 1397, 1323,
d
* Corresponding author.
E-mail address: jimin@seu.edu.cn (M. Ji).
1001-8417/$ – see front matter ß 2013 Min Ji. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.04.039

[
C.-L. Feng et al. / Chinese Chemical Letters 24 (2013) 767–769
O
O
O
O
O
O
O
OH
O
e
b
d
a
O
O
O
O
+
O
N
Bn
7
Bn
Bn
N
N
O
OH
Cl
4
8
5
3
9
c
O
HClHN
+
O
g
11
N
H
6
O
f
O
O
O
O
O
j
NH
h
H2N
NH
N
H
O
NH
i
OH
O
OH
O
O
10
O
O
O
Bn
NH
NH
N
N Bn
1
13
12
2
Scheme 1. The synthesis of marinamide (1) and its methyl ester (2). Reagents and conditions: (a) methanol, reflux; (b) SOCl2, reflux; (c) KOH, DMSO, benzyl bromide, N2; (d)
Zn, toluene, r.t.; (e) 50% NaOH, methanol; (f) SOCl2, methanol; (g) HOBT, EDCI, THF; (h) DBU, methanol/THF; (i) TFA/H2SO4, anisole; (j) 6 mol/L HCl, CH3CN.
(m, 2H), 7.85–7.88 (m, 1H). 13C NMR (75 MHz, CDCl3):
d
51.26,
(cmÀ1):
n 3428, 3294, 3100, 3056, 3028, 2950, 2927, 1762, 1660,
51.98, 108.96, 122.58, 127.18, 128.13, 129.39, 129.39, 129.43,
129.43, 129.61, 129.67, 129.70, 130.64, 131.82, 132.01, 138.71,
1624, 1591, 1544, 1525, 1457, 1409, 1396, 1348, 1316, 1256, 1209,
1175, 1153, 1084, 1036, 1011, 968, 920, 876, 758, 752, 688; 1H
141.51, 166.60, 185.05. HRMS calcd. for
320.1287; found: 320.1289.
C
20H18NO3 [M+1]:
NMR (300 MHz, DMSO-d6): d 3.60 (s, 3H), 3.86 (d, 2H, J = 6.6 Hz),
5.65 (s, 2H), 6.14 (m, 1H), 6.34 (m, 1H), 7.37–7.18 (m, 7H), 7.59 (m,
2H), 7.70 (m, 1H), 8.87 (d, 1H, J = 6.6 Hz); 13C NMR (75 MHz,
2.2. Preparation of 2-(1H-pyrrole-2-carbonyl)benzoic acid (9)
DMSO-d6): d 41.65, 52.23, 52.64, 109.27, 124.86, 125.81, 127.34,
127.53, 128.43, 128.56, 128.61, 129.71, 129.96, 130.48, 131.91,
133.09, 134.28, 138.05, 139.64, 167.94, 169.95, 186.73; HRMS
calcd. for C22 H21N2O4 [M+1]: 377.1501; found: 377.1504.
The compound 8 (12 g) was dissolved in methanol (100 mL)
followed by the addition of a 50% NaOH solution (20 mL). The
mixture was stirred at room temperature for 1 h and neutralized
with hydrochloric acid. After methanol was evaporated under
reduced pressure, the residue was extracted with ethyl acetate (3Â
100 mL). After evaporation of the solvent, the solid was recrys-
tallized from dichloromethane/methanol to give the colorless
2.4. Preparation of 4-(1-benzyl-1H-pyrrol-2-yl)-1-oxo-1,2-dihydro-
isoquinoline-3-carboxylic acid methyl ester (13)
Compound 12 (0.38 g, 1 mmol) was dissolved in methanol
(10 mL) and THF (5 mL), and then 1,8-diazabicyclo[5.4.0]-7-
undecene (DBU, 0.3 g, 2 mmol) was added. The mixture was
heated under reflux for 12 h. The solvent was evaporated under
reduced pressure, and the residue was partitioned between ethyl
acetate and 1 mol/L hydrochloric acid, washed with aqueous
sodium bicarbonate, and brine. The solvent was evaporated under
reduced pressure, and the obtained residue was purified by silica
gel column chromatography to give 13 (0.3 g, 86% yield) as yellow
crystal 9 (9.6 g, 96% yield). Mp: 166–168 8C. IR (cmÀ1):
n 3414,
3004, 2952, 1721, 1631, 1524, 1461, 1403, 1383, 1329, 1287, 1254,
1123, 1088, 1060, 961, 917, 886, 875, 777, 752. 1H NMR (300 MHz,
DMSO-d6):
d
5.71 (s, 2H), 6.13 (m, 1H), 6.27 (m, 1H), 7.22–7.37 (m,
51.08, 108.90,
9H), 7.88 (m, 1H). 13C NMR (75 MHz, DMSO-d6):
d
122.15, 126.95, 127.09, 127.09, 127.78, 127.78, 129.35, 129.48,
130.02, 130.02, 130.38, 131.47, 131.47, 138.87, 141.87, 167.39, and
185.79. HRMS calcd. for C19H16NO3 [M+1]: 306.1131; found:
306.1130.
powders. Mp: 182–183 8C; IR (cmÀ1):
n 3433, 3169, 3109, 3049,
2948, 2917, 1737, 1655, 1602, 1495, 1463, 1431, 1336, 1294, 1249,
1230, 1154, 1124, 1067, 873, 751,724, 579. 1H NMR (300 MHz,
2.3. Preparation of [2-(1-benzyl-1H-pyrrole-2-
carbonyl)benzoylamino]acetic acid methyl ester (12)
CDCl3):
7.16 (m, 7H), 7.62 (m, 2H), 8.24 (d, 1H, J = 7.26 Hz), 11.43 (s, 1H).
13C NMR (75 MHz, CDCl3):
52.64, 53.51, 112.83, 113.11, 117.69,
126.80, 127.81, 127.81, 128.06, 128.06, 128.30, 128.47, 128.76,
129.61, 129.61, 130.06, 130.06, 133.36, 134.53, 137.38, 160.94,
161.17. HRMS calcd. for C22H19N2O3 [M+1]: 359.1396; found:
359.1405.
d 3.57 (s, 3H), 4.88 (m, 2H), 6.02 (m, 1H), 6.19 (m, 1H), 6.93–
N-Hydroxybenzotrizole (HOBT) (1.6 g, 12 mmol) and com-
pound 9 (3.5 g, 10 mmol) were dissolved in anhydrous THF
(50 mL). Then, 1-ethyl-3-(3-dimethyllaminopropyl)carbodiimide
hydrochloride (EDCI) (1.91 g, 10 mmol) was added, and the
solution was stirred for 30 min. This solution was transferred to
d
a
solution of glycine methyl ester hydrochloride 11 (1.5 g,
12 mmol) and triethylamine (1.6 g, 16 mmol) in THF (60 mL) that
had been prepared. The reaction was allowed to stir overnight at
room temperature. Then the reaction was diluted with water
(200 mL) and extracted with ethyl acetate (200 mL). The organic
layer was washed with water (100 mL), a solution of saturated
sodium bicarbonate (100 mL), 1 mol/L hydrochloric acid (100 mL),
and finally water again (100 mL). After evaporation of the solvent,
the crude product was crystallized from dichloromethane to give
the colorless crystal 12 (3.4 g, 90% yield). Mp 187–189 8C; IR
2.5. Preparation of methyl ester of marinamide (2)
A solution of 13 (0.36 g, 1 mmol), trifluoroacetic acid (1.5 mL),
methoxybenzene (0.5 mL) and concentrated sulfuric acid (0.1 mL)
was heated at 90 8C for 30 min. The reaction mixture was dissolved
in ice water and extracted with dichloromethane (3Â 50 mL) and
washed with saturated sodium bicarbonate (2Â 30 mL). The
organic layer was dried over anhydrous sodium sulfate, and
concentrated under reduced pressure to give a yellow solid

C.-L. Feng et al. / Chinese Chemical Letters 24 (2013) 767–769
769
followed by purification on silica gel to afford pure product 2
(0.12 g, 46% yield). Mp >300 8C; IR (cmÀ1): 3428, 3331, 1658. 1H
Having synthesized the key intermediate 8, the next step was to
prepare intermediate 12. This was achieved in three steps
including hydrolysis of 8 to afford 9, the synthesis of glycine
methyl ester hydrochloride 11 and acylation of 9 with 11. Thus,
hydrolysis of ester 8 with 50% NaOH afforded the acid 9, which
reacted with 11 generated from the esterification of glycine in
methanol with SOCl2 to provide compound 12 in 90% yield [7].
Finally, the intermediate 13 was successfully synthesized by an
intramolecular condensation of 12 promoted by DBU in methanol/
THF (2/1) [8]. Then, deprotection of 13 with acid (TFA/H2SO4) and
anisole at 90 8C successfully afforded the targeted methyl ester of
marinamide 2 [9]. Hydrolysis of 2 with 6 mol/L hydrochloric acid
afforded marinamide 1.
NMR (300 MHz, CDCl3):
7.06 (m, 1H), 7.41–7.58 (m, 3H), 7.89 (m, 1H), 12.01 (s, 1H), 11.45
(s, 1H). 13C NMR (75 MHz, CDCl3):
52.17, 108.54, 110.67, 112.48,
d 3.54 (s, 3H), 6.11 (m, 1H), 6.24 (m, 1H),
d
116.98, 122.23, 122.45, 123.61, 124.50, 124.63, 131.44, 140.07,
141.14, 167.45, 172.28. HRMS calcd. for C15H13N2O3 [M+1]+:
269.0926; found: 269.0931.
2.6. Preparation of marinamide(1)
Compound 2 (0.1 g) was dissolved in acetonitrile (5 mL)
followed by the addition of a 6 mol/L hydrochloric acid solution
(5 mL). The mixture was stirred and refluxed for 10 h. After the
methanol was evaporated under reduced pressure, the residue was
extracted with ethyl acetate (3Â 20 mL). The solvent was
evaporated under reduced pressure; the obtained residue was
recrystallized from dichloromethane to give marinamide 1 (0.06 g,
The spectral data (IR, 1H NMR, 13C NMR, HRMS) of all new
intermediates, marinamide, and its methyl ester were satisfactory.
The spectral data of 1 and 2 were identical to those reported in the
literature [1].
66% yield). Mp > 300 8C; IR (cmÀ1):
(300 MHz, DMSO-d6): 6.18 (m, 1H), 6.23 (m, 1H), 6.98 (m, 1H),
7.35–7.56 (m, 3H), 7.88 (m, 1H), 9.76 (s, 1H), 11.79 (s, 1H), 14.95 (s,
1H). 13C NMR (75 MHz, DMSO-d6):
106.48, 109.17, 113.56,
n
3428, 3330, 1650. 1H NMR
4. Conclusion
d
In conclusion, marinamide 1 and its methyl ester 2 have been
successfully synthesized by employing a regioselectivity Friedel–
Crafts acylation of a protected pyrrole as the key step. The
syntheses described here will allow further investigations on their
pharmacological properties and structure–activity relationship.
d
120.18, 121.96, 122.54, 123.31, 123.48, 125.46, 134.34, 136.78,
146.72, 167.56, and 178.34. HRMS calcd. for C14H11N2O3 [M+1]+:
255.0770; found: 255.0773.
3. Results and discussion
Acknowledgment
As illustrated in Scheme 1, the synthesis of intermediate 8 is the
key step for the synthesis of marinamide and its methyl ester,
which involved a regioselective Friedel–Crafts acylation of 1-
benzyl-1H-pyrrole to form a 2-acylpyrrole derivative. The Friedel–
Crafts acylation of pyrrole and its derivatives often produced a
mixture of 2- and 3-acylated products because of their high
reactivity in the electrophilic substitution reactions and general
sensitivity to acid-catalyzed polymerizations [3]. Frequently, it
was difficult to separate the mixtures. Therefore, it is crucial to only
produce the 2-acylpyrrole derivative. Yadav once reported a
general method for the selective 2-acylation of a range of pyrrole
derivatives by exposing them to an acid chloride and zinc powder
in toluene [6]. We attempted to adopt this method to synthesize
the key intermediate 8. The synthesis of 8 commenced with the
synthesis of 4 from the monoesterification of phthalic anhydride 3
in methanol based on the procedures reported in the literature [4].
Then the intermediate 7 was synthesized by treating pyrrole 6 with
benzyl bromide in anhydrous DMSO [5]. Based on what the
literature described, reaction of compound 4 with SOCl2 gave the
acyl chloride 5, which reacted with 7 through the catalysis of zinc
powder in toluene at room temperature to successfully provide the
key intermediate 8 in 68% yield [6].
We are grateful to the National Basic Research of China (No.
2011CB933503) for financial support.
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