Synthesis of the intermediate for fumimycin: A natural peptide deformylase inhibitor

Article abstract of DOI:10.1007/s11164-012-0817-1

Synthetic efforts toward the potential bacterial peptide deformylase inhibitor fumimycin are reported. The synthetic approach features a tandem Friedel-Crafts alkylation/lactonization access as key reaction to the α, α-disubstituted amino acid unit, and r

Full text of DOI:10.1007/s11164-012-0817-1

Res Chem Intermed
DOI 10.1007/s11164-012-0817-1
Synthesis of the intermediate for fumimycin: a natural
peptide deformylase inhibitor
•
Zhi-Wang Zhou Wei-Chao Li Yu Hu Bin Wang
•
•
•
•
Gang Ren Li-Hua Feng
Received: 22 July 2012 / Accepted: 12 September 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract Synthetic efforts toward the potential bacterial peptide deformylase
inhibitor fumimycin are reported. The synthetic approach features a tandem Friedel–
Crafts alkylation/lactonization access as key reaction to the a, a-disubstituted amino
acid unit, and results in the synthesis of an advanced racemic intermediate with an
Z configuration propenyl group starting from vanillin with 18 % total yield in five steps.
Keywords Fumimycin Á Peptide deformylase Á Inhibitors Á Friedel–Crafts
alkylation Á Synthesis
Introduction
Bacterium infection is a common and frequently-occurring disease in our daily lives,
and various bacterium infections can be a fatal threat to mankind. Despite the past
success of antibiotic drug discovery, at least in the industrially developed world,
infectious diseases remain the second-leading cause of death. Bacterial infections cause
17 million deaths globally, particularly in children and the elderly [1]. Since penicillin
was discovered at the beginning of the twentieth century, the antibiotic family has
expanded day by day to become the most effective approach to cure bacterium infection
Z.-W. Zhou (&) Á W.-C. Li Á B. Wang Á L.-H. Feng
Department of Pharmacy, Nanchang University, 461 Bayi Road, Nanchang 330006, Jiangxi,
People’s Republic of China
e-mail: zzw_308@163.com
Y. Hu
Department of Chemistry, Nanchang University, 999 Xuefu Road, Nanchang 330031, Jiangxi,
People’s Republic of China
G. Ren
Key Laboratory of Modern Preparation of TCM, Jiangxi University of Traditional Chinese
Medicine, Ministry of Education, Nanchang 330004, Jiangxi, People’s Republic of China
123

Z.-W. Zhou et al.
diseases. However, the emergence of bacterial resistance has gradually lessened the
effectiveness during long-term clinical use. Especially during the past 10 years, the
drug-resistant bacteria has brought lots of difficulties to clinical treatment such as
the abuse of antibiotics and the rapid growth of various drug-resistant germs [2]. With
the emergence of bacterial resistance to all known classes of antibiotics, the search for
new antibiotics with novel modes of action is of utmost urgency.
One novel antibacterial target that has attracted more and more attention lately is
the bacterial peptide deformylase (PDF) (EC 3.5.1.31) [3, 4]. PDF is an essential
enzyme for bacterial growth that catalyzes the removal of the formyl group at the
N-terminus of bacterial proteins, while it is not required in mammalian cells. PDF
inhibitors are potentially non-toxic, broad-spectrum and selective mechanism-based
antibacterial agents [5, 6]. Although PDF offers a new target for antibacterial agent
discovery, only a few PDF inhibitors have been reported so far, and most of them
are peptidic [7–11]. In the course of screening for new PDF inhibitors, a novel
nonpeptidic metabolite named fumimycin (1, Fig. 1) was isolated from the
fermentation broth of Aspergillus fumisynnematus F746 by Kim and coworkers
[12]. It possesses an unusual skeleton incorporating an aromatic ring fused to a five-
membered lactone with an alanine unit of which the amino group is linked to a fumaric
acid residue. Fumimycin exhibits potent inhibition on S. aureus PDF with an IC50 of
4.1 lM and also displays antibacterial activity against methicillin-resistant S. aureus
(MRSA) and quinolone-resistant S. aureus (QRSA). Therefore, fumimycin may
represent a lead structure of PDF inhibitors for further structural optimization.
Due to our research interests in discovering new PDF inhibitors, the significant
biological properties and the distinctive structural features of fumimycin prompted us to
synthesize this compound. The challenge for the synthesis of fumimycin is to establish
¨
the stereogenic quaternary carbon center of the a,a-disubstituted amino acid unit. Brase
and coworkers [13, 14] carried out considerable work on the synthesis of fumimycin
and its analogue, including racemic synthesis of fumimycin and methoxyfumimycin
and enantioselective synthesis of (?)-fumimycin [15], as well as an organocatalytic
amination approach to an advanced intermediate [16]; however, the key step in most of
these reactions was the 1, 2-addition of a Grignard reagent to ketimine, which makes the
reaction steps too long and the reaction conditions too tedious. Here, we report a
racemic synthetic effort toward fumimycin and a concise approach to give rise to the
benzofuran-2-one skeleton with a a,a-disubstituted amino unit, with commercial
vanillin as the starting material, which undergoes allylation, Dakin oxidation, tandem
Friedel–Crafts alkylation/lactonization [17, 18], Claisen rearrangement, and olefin
isomerization to get the key racemic intermediate, 3-acetylamino-5-hydroxy-6-
methoxy-3-methyl-4-[(Z)-propenyl]-3H-benzofuran-2-one 10.
OH
O
Fig. 1 (-)Fumimycin, a
potential lead to antibacterial
agents
O
NH
HO
HO
O
O
(-)Fumimycin (1)
123

Synthesis of the intermediate for fumimycin
Results and discussion
The retrosynthetic analysis of ( )-fumimycin is shown in Scheme 1. Our retrosynthetic
strategy employs a tandem Friedel–Crafts alkylation/lactonization of 5 as the key
reaction [17, 18]. Foremost, we hypothesized that ( )-fumimycin could derive
from amine 2 through amidation and deprotection; and 2 could be obtained by
olefin isomerization from 3. The Claisen rearrangement precursor for amine 3, the
benzofuran-2-one 4, can be prepared from phenol 5 through Friedel–Crafts alkylation
and subsequent lactonization, respectively, while 5 can be synthesized through Dakin
oxidation and allylation by employing the readily available vanillin 6 as starting
material.
As illustrated in Scheme 2, the route from vanillin 6 to phenol 5 followed the
¨
protocol of Brase [13, 14] with slight modifications. Firstly, vanillin was converted to its
allyl ether 7 which underwent a Dakin oxidation to give the phenol 5. A cascade
lactonization followed by the Friedel–Crafts alkylation adduct of 5 with commercially
available methyl 2-acetamidoacrylate gave rise to the benzofuranone 8 with 87 % yield.
Claisen rearrangement of allyl ether 8 smoothly provided 9. Isomerization of the
¨
terminal double bondwas carried out as described byBrase [13]. Disappointingly, when
9 was heated in the presence of RhCl3Á3H2O in ethanol under 45 °C, only
decomposition could be observed. When we increased the reaction temperature to
60 °C, a trace isomer product can be observed. Under refluxing after 4 h, the olefin
isomerization surprisingly gave the Z configuration product 10, exclusively evident
from the 3JHH coupling constant of the double bond (3JHH = 11.2 Hz).
Conclusions
In summary, a tandem Friedel–Crafts alkylation/lactonization was used as the key
reaction step to establish the a-arylalanine skeleton of fumimycin, which gave a
yield as high as 87 %. The racemic advanced intermediate, 3-acetylamino-5-
hydroxy-6-methoxy-3-methyl-4-[(Z)-propenyl]-3H-benzofuran-2-one, was synthe-
sized with 18 % total yield in five steps with vanillin as the starting material.
OH
O
O
NH
Olefin
isomerization
NHPG
O
NHPG
O
HO
amidation
HO
HO
HO
O
O
MeO
O
O
O
MeO
3
2
( )-Fumimycin
Claisen
rearrangement
HO
MeO
Friedel-Crafts alkylation
Lactonization
NHPG
O
Allylation
Dakin oxidation
O
O
H
O
MeO
MeO
OH
Vanillin(6)
5
4
Scheme 1 The retrosynthetic analysis of ( )fumimycin
123

Z.-W. Zhou et al.
HO
O
a
O
c
b
O
MeO
O
MeO
MeO
OH
H
Vanillin(6)
H
7
5
NHAc
NHAc
NHAc
O
d
e
O
HO
MeO
HO
O
O
O
O
MeO
O
MeO
10
8
9
Scheme 2 Reagents and conditions: a K2CO3, allylbromide, acetone, reflux, 6 h, 98 %; b H2O2, H2SO4,
B(OH)3, THF, r.t., 5 h, 92 %; c methyl 2-acetamidoacrylate, BF3ÁEt2O, THF, r.t., 2 d, 85 %; d DMF,
170 °C, 10 h, 84 %; e RhCl3Á3H2O, EtOH, reflux, 4 h, 28 %
Experimental section
General reagents and all solvents were analytically pure grade and were used
without further purification. Column chromatography (CC) was performed on silica
gel (200–300 mesh). Analytical thin layer chromatography (TLC) was performed on
pre-coated silica gel GF254 plates (Qingdao Haiyang Chemical, Qingdao, PR of
China). Visualization on TLC was achieved by the use of UV light (254 nm) and
1
treatment with I2. H (600 MHz) and 13C (150 MHz) NMR spectra were recorded
on a Bruker AVANCE III 600 MHz spectrometer with CDCl3 as solvent and TMS
as internal standard. Chemical shifts are reported in ppm relative to the internal
reference. ESIMS were obtained on a Waters ZQ4000/2695 HPLC–MS. HRMS
were measured on a Finnigan MAT95 mass spectrometer or a VG ZAB-HS
spectrometer.
4-Allyloxy-3-methoxybenzaldehyde (7) [13, 14]
To a suspension of vanillin 6 (2.87 g, 18.86 mmol) and K2CO3 (3.64 g, 26.4 mmol)
in acetone (28 mL), allylbromide (2.1 mL, 24.51 mmol) was added. The mixture
was heated to reflux for 6 h. After filtration, the filtrate was concentrated under
reduced pressure. The crude product was purified by chromatography on silica gel
(PE:AcOEt = 10:1) to afford the allylether 7 as brown oil (3.55 g, 98 %).
4-Allyloxy-3-methoxyphenol (5) [13, 14]
Boric acid (5.72 g, 92.3 mmol) was suspended in THF (50 mL), H2O2 (30 % in
H2O, 6 mL), and H2SO4 (2.66 mL, 98 %). After stirring for 30 min, 7 (3.55 g,
18.5 mmol) was added as solution in THF (20 mL) within 15 min. After additional
stirring for 5 h, the mixture was filtered. The filtrate was neutralized by addition of
sat. NaHCO3 solution; the aqueous layer was extracted with EtOAc (3 9 90 mL).
123

Synthesis of the intermediate for fumimycin
The combined organic extracts were washed with brine (50 mL), dried over
Na2SO4, and concentrated under reduced pressure. Flash chromatography
1
(PE:AcOEt = 4:1) afforded the phenol 5 as brown oil (2.998 g, 92 %). H NMR
d (CDCl3, ppm): 3.68 (s, 3H), 4.49 (d, J = 6 Hz, 2H), 5.21 (dd, J = 10.2, 1.2 Hz,
1H), 5.32 (dd, J = 17.4, 1.2 Hz, 1H), 5.99–6.05 (m, 1H), 6.34 (dd, J = 8.4, 2.4 Hz,
1H), 6.46 (d, J = 2.4 Hz, 1H), 6.72 (d, J = 8.4 Hz, 1H), 7.20 (brs, 1H).
5-Allyloxy-3-acetylamino-6-methoxy-3-methyl-3H-benzofuran-2-one (8)
An amount of 1.58 mL BF3ÁEt2O was added at room temperature to a solution of
methyl 2-acetamidoacrylate (429.4 mg, 3 mmol) in 10 mL dried THF. After 30 min
of stirring, 5 (434 mg, 2.4 mmol) was added and the reaction mixture was stirred for
2 days. Work-up began by quenching with Na2CO3Á10H2O followed by filtration,
evaporation of the solvent and column chromatography (PE:EtOAc = 1:1) to obtain
1
the lactone 8 as a white solid (595 mg, 85 %). H NMR d (CDCl3, ppm): 1.60 (s,
3H), 1.95 (s, 3H), 3.87 (s, 3H), 4.54–4.56 (m, 2H), 5.28 (dd, J = 10.2, 1.2 Hz, 1H),
5.39 (dd, J = 17.4, 1.2 Hz, 1H), 6.04–6.09 (m, 1H), 6.43 (s, 1H), 6.75 (s, 1H), 6.78
(s, 1H). 13C NMR d (CDCl3, ppm): 176.4, 169.2, 151.3, 147.7, 145.3, 133.3, 119.5,
118.1, 108.9, 96.6, 71.2, 57.3, 56.3, 24.3, 22.3. (?)ESI–MS (MeOH): m/z 313.7
[M ? Na]?, 323.8 [M ? MeOH ? H]?, 345.8 [M ? MeOH ? Na]?; (-)ESI–MS
(MeOH): m/z 289.7 [M–H]-, 321.8 [M ? MeOH–H]-. HRMS (ESI): m/z
[M ? Na]? Calcd for C15H17NO5: 314.0999; found: 314.1011.
4-Allyl-3-acetylamino-5-hydroxy-6-methoxy-3-methyl-3H-benzofuran-2-one (9)
A solution of 8 (1.018 g, 3.5 mmol) in DMF (80 mL) was heated at 170 °C to reflux
for 10 h. The solvent was removed under reduced pressure, and 20 ml water was
added to the residue. The mixture was extracted with EtOAc (39 50 mL). The
organic phase was washed with saturated brine, dried over Na2SO4 and concentrated
under reduced pressure. Flash chromatography (PE:EtOAc = 1:2) afforded 9 as a
white solid (852 mg, 84 %). 1H NMR d (CDCl3, ppm): 1.67 (s, 3H), 1.93 (s, 3H), 3.38
(dd, J = 15.6, 5.4 Hz, 1H), 3.50 (dd, J = 15.6, 5.4 Hz, 1H), 3.90 (s, 3H), 4.90 (dd,
J = 17.4, 1.8 Hz, 1H), 5.02 (dd, J = 10.2, 1.8 Hz, 1H), 5.48 (brs, 1H), 5.94–5.99 (m,
1H), 6.14 (s, 1H), 6.66 (s, 1H). 13C NMR d (CDCl3, ppm): 176.4, 169.2, 146.9, 146.3,
140.7, 135.4, 121.3, 118.4, 115.3, 94.0, 57.8, 56.4, 29.6, 23.9, 22.2. (?)ESI–MS
(MeOH): m/z 313.8 [M ? Na]?, 329.9 [M ? K]?, 345.8 [M ? MeOH ? Na]?,
605.0 [2M ? Na]?; (-)ESI–MS (MeOH): m/z 289.7 [M–H]-. HRMS (ESI): m/z
[2M ? Na]? Calcd for C30H34N2O10Na: 605.2111; found: 605.2110.
3-Acetylamino-5-hydroxy-6-methoxy-3-methyl-4-[(Z)-propenyl]-3H-
benzofuran-2-one (10)
Under N2 a mixture of 9 (130 mg, 0.45 mmol) and RhCl3Á3H2O (11 mg, 40 lmol)
in EtOH (10 mL) was heated to reflux. After 4 h, the reaction was allowed to cool to
r.t. and filtered through a 3-cm pad of Celite. The fitrate was concentrated under
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Z.-W. Zhou et al.
reduced pressure. Flash chromatography (PE:EtOAc = 1:2) afforded the white solid
10 (36.4 mg, 28 %). 1H NMR d (CDCl3, ppm): 1.59 (d, J = 11.2 Hz, 3H), 1.62 (s,
3H), 1.96 (s, 3H), 3.91 (s, 3H), 5.51 (s, 1H), 6.01–6.09 (m, 2H), 6.22 (d,
J = 11.2 Hz, 1H), 6.85 (s, 1H). 13C NMR d (CDCl3, ppm): 176.2, 168.9, 147.3,
146.2, 139.4, 132.6, 119.9, 119.6, 117.7, 94.5, 70.6, 57.6, 56.4, 26.5, 22.6, 22.1,
15.1. (?)ESI–MS (MeOH): m/z 313.8 [M ? Na]?, 345.8 [M ? MeOH ? Na]?;
(-)ESI–MS (MeOH): m/z 289.7 [M–H]-. HRMS (ESI): m/z [M–H]- Calcd for
C15H17NO5: 290.1034; found: 290.1032.
Acknowledgments Financial support from the National Natural Science Foundation of China (grants
30901855) and Natural Science Foundation of Jiangxi Province, China (grants 2009GQY0143) are
gratefully acknowledged. We thank the analytical group of the Analysis and Testing Center of Nanchang
University for the spectral measurements.
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