Synthesis and preliminary antibacterial evaluation of hydroxamic acid and N-formyl hydroxylamine derivatives bearing oxazole ring

Article abstract of DOI:10.1007/s00044-012-0141-8

Three hydroxamic acids and seven N-formyl hydroxylamine derivatives containing oxazole ring have been designed and synthesized. The structures of target compounds were characterized on the basis of spectral (FT-IR, ¹H NMR, and HRMS) analysis. A

Full text of DOI:10.1007/s00044-012-0141-8

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2013) 22:1426–1437
DOI 10.1007/s00044-012-0141-8
ORIGINAL RESEARCH
Synthesis and preliminary antibacterial evaluation of hydroxamic
acid and N-formyl hydroxylamine derivatives bearing oxazole
ring
•
Datong Zhang Lingyan Huo Laichun Lu Qiong Yu
•
•
•
•
Jianwu Wang Yanyan Yang
Received: 17 February 2012 / Accepted: 31 May 2012 / Published online: 17 June 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Three hydroxamic acids and seven N-formyl
hydroxylamine derivatives containing oxazole ring have
been designed and synthesized. The structures of target
compounds were characterized on the basis of spectral
DMSO Dimethylsulfoxide
PE Petroleum ether
EtOAc Ethyl acetate
1
(FT-IR, H NMR, and HRMS) analysis. All the final syn-
thesized compounds were evaluated in vitro against
Staphylococcus aureus, Streptococcus pneumonia, Esche-
richia coli, and Pseudomonas aeruginosa. Among them,
10a exhibited good activity against E. coli compared with
the reference agent ciprofloxacin hydrochloride.
Introduction
The emergence of bacterial resistance makes antibiotics a
unique class of drugs with a limited life cycle and causes
curing bacterial infections a real challenge. Consequently,
the search for new antibiotics with novel modes of action is
paramountly important (Aarons et al., 1997). Actinonin, a
hydroxamic acid pseudopeptide, was found to show a weak
inhibitory activity against Gram-positive and Gram-nega-
tive bacteria (Gordon et al., 1962; Chen et al., 2000). The
action mechanism of actinonin is believed to inhibit the
peptide deformylase (PDF), which is a metalloenzyme and
essential for bacterial survival (Chan et al., 1997; Giglone
et al., 2000). It has been demonstrated that the hydroxa-
mate group of actinonin can complex with the metal ion in
the active pocket of the peptide deformylase and is nec-
essary for its activity (Clements et al., 2001). Although
actinonin showed potent inhibitory activity against PDF in
vitro, it lacked in vivo efficacy due to the poor bioavail-
ability which was attributed to its apparent peptidic char-
acteristics (Chen et al., 2000; Clements et al., 2001;
Broughton et al., 1975). BB-3497, sharing a similar scaf-
fold with actinonin except that it employed N-formyl
hydroxylamine as the chelating group, exhibited potent
antibacterial activity both in vitro and in vivo (Davies
et al., 2003). A lot of actinonin and BB-3497 analogs (Jain
et al., 2003; Hackbarth et al., 2002; Davies et al., 2003)
have been developed to test their bioactivities, and the
generic structure of these compounds is composed of a
metal-chelating group and various amidation fragments.
Keywords Synthesis ꢀ Hydroxamic acid derivatives ꢀ
N-formyl hydroxylamine derivatives ꢀ Oxazole ꢀ
Antibacterial activity
Abbreviations
THF
Tetrahydrofuran
DMF
N,N-dimethylformamide
D. Zhang (&) ꢀ L. Huo ꢀ Y. Yang
School of Chemistry and Pharmaceutical Engineering,
Shandong Polytechnic University, Jinan 250353, Shandong,
People’s Republic of China
e-mail: dt_zhang@yahoo.com.cn
L. Lu ꢀ Q. Yu
Department of Pharmacy, The Third Affiliated Hospital of Third
Military Medical University, Chongqing 400042,
People’s Republic of China
L. Lu ꢀ Q. Yu
School of Horticulture and Landscape Architecture, Southwest
University, Chongqing 400715, People’s Republic of China
J. Wang
School of Chemistry and Chemical Engineering, Shandong
University, 27 Shanda Nanlu, Jinan 250100, Shandong,
People’s Republic of China
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Med Chem Res (2013) 22:1426–1437
1427
Fig. 1 Chemical structures of
actinonin, BB-3497, and target
compounds
OH
OH
N
O
O
O
H
N
H
N
O
HO
N
N
N
H
H
O
O
BB-3497
Actinonin
R1
OH
N
O
O
R1
R2
O
H
N
H
N
O
HO
N
N
N
N
R2
N
H
H
O
O
O
O
Type 1 of target compounds
Type 2 of target compounds
We have reported the synthesis of several series of inhib-
itors with non-peptide scaffolds, in which electron-rich
heterocyclics were incorporated to replace the amide
fragment of lead compounds to work as an amide isostere,
and evaluated their antibacterial activities (Zhang et al.,
2009, 2010; Yin et al., 2010). Some of the heterocycle-
containing compounds were found to exhibit moderate to
potent inhibitory activities against the tested bacteria in
vitro. The motif of oxazole is found in many biologically
active products. Referring to the lead structures of acti-
nonin and BB-3497, we introduced the oxazole function-
ality to serve as an amide isostere. This strategy has also
been employed to develop potent, competitive, and
reversible inhibitors of tyrosine phosphatase 1B (Sparks
et al., 2007). Two series of target compounds, which were
different in the chelating group, were synthesized and their
in vitro antibacterial activities were evaluated (Fig. 1).
with an Agilent-6510-Q-TOF spectrometer. The elemental
analysis (C, H, and N) of compounds was performed on
Vario EL III elemental analyzer. Optical rotations were
measured with a WZZ-2B auto-polarimeter.
Preparation of compound 2
Compound 1 was prepared according to the literature
procedure (Downing et al., 1999).
Compound 1 (3.84 g, 12.9 mmol) and LiOHꢀH2O
(1.14 g, 27.0 mmol) were stirred in 60 mL of methanol/
water (3:1, by v/v) at room temperature for 16 h. The reac-
tion mixture was concentrated to remove methanol and the
residue was diluted with water (20 mL). The mixture was
extracted with ethyl acetate (20 mL) and the aqueous phase
was acidified to pH \2 with concentrated HCl. The acidic
aqueous solution was then extracted with dichloromethane
(4 9 30 mL), and the combined extracts were dried over
anhydrous Na2SO4 and evaporated to afford compound 2.
Materials and methods
Chemistry
(S)-2-(1-(tert-butoxycarbonylamino)-2-
methylpropyl)oxazole-4-carboxylic acid (2)
General experimental considerations
White crystal, yield of 98 %; mp 155–157 °C (Lit mp
20
154–155 °C); [a]
D
= -30.3 (c 1.5, CHCl3) (Lit [a]
D
= -28.9 (c 1.5, CHCl3))[9] 1H NMR (CDCl3,
;
23
All reagents were used as purchased from commercial
suppliers without further purification unless otherwise
noted. Melting points were determined by a WRS-1B
Digital Melting-Point apparatus and are uncorrected. Thin-
layer chromatography (TLC) was performed on Silica Gel
400 MHz) d 0.89 (d, J = 6.5 Hz, 3H); 0.95 (d, J = 6.7 Hz,
3H); 1.35 (s, 9H), 2.15–2.23 (m, 1H), 4.83 (t, J = 8.3 Hz,
1H), 6.43 (d, J = 9.5 Hz, 1H), 8.28 (s, 1H), 12.43 (brs,
1H); ESI-MS m/z 285 [M ? H]?.
1
F254 plates with visualization by UV or iodine vapor. H
NMR spectra of CDCl3/DMSO-d6 solutions (TMS as an
internal standard) were recorded on a Bruker AVANCE II
400 spectrometer. The IR spectra were measured on a
Bruker Vector FT-IR spectrophotometer as KBr pellets or
film. Mass spectra were obtained with an API 4000 spec-
trometer. The high-resolution mass spectra were obtained
General procedure for the synthesis of compounds 3a, 3b
and 3d–g
1-Hydroxybenzotriazole (6.0 mmol) was added to a solution of
compound 2 (2.9 mmol) and various amines (3.5 mmol) in dry
THF (15 mL). After cooling to 0 °C, dicyclohexylcarbodiimide
123

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Med Chem Res (2013) 22:1426–1437
(3.1 mmol) was added in portions to the reaction mixture. The
mixture was stirred under 0 °C for 0.5 h and then stirred at room
temperature for 20 h. The resulting mixture was filtered and
evaporated under vacuum. The residue was dissolved in
dichloromethane (60 mL) and washed with saturated NaHCO3,
1 N HCl, and saturated saline successively and dried with
sodium sulfate. The solvent was concentrated, and the residue
was purified by column chromatography on silica gel
(PE/EtOAc, 3:1) to afford the target compounds.
NCH2CH3), 3.71 (brs, 2H, NCH2CH3), 2.15–2.20(m, 1H,
CH(CH3)2), 1.60–1.68 (m, 6H, N(CH2)2(CH2)3), 1.43 (s,
9H, C(CH3)3), 0.92 (d, J = 6.7 Hz, 6H, CH(CH3)2). IR
(KBr, m/cm-1) 3293, 3166, 2950, 1711, 1607, 1510.
(S)-tert-butyl-[4-(3,4-difluorophenylcarbamoyl)oxazol-
2-yl]-2-methylpropylcarbamate (3f)
Yellow solid, yield 86 %; m.p. 114.8–115.4 °C, [a]D10
=
1
-44.6 (c 1, CHCl3). H NMR (400 MHz, DMSO-d6) d:
8.75(s, 1H, OxazoleH), 7.93 (m, 1H, ArH), 7.61 (m, 2H,
ArH ? CONH), 7.40 (m, 1H, ArH), 4.50 (t, J = 8.4 Hz,
1H, NHCH(CH)C), 2.15(m, 1H, CH(CH3)2), 1.37 (s, 9H,
C(CH3)3), 0.94 (d, J = 6.4 Hz, 3H, CH(CH3)CH3), 0.78(d,
J = 6.4 Hz, 3H, CH(CH3)CH3). IR (KBr, m/cm-1) 3390,
3325, 3090, 2950, 1705, 1680, 1520, 1165.
(S)-tert-butyl 2-methyl-1-[4-(p-tolylcarbamoyl)oxazol-
2-yl]propylcarbamate (3a)
White solid, yield 75 %; m.p. 154–155 °C, [a]2D0 = -53.5
1
(c 1, CHCl3). H NMR (400 MHz, CDCl3) d: 8.70 (s, 1H,
OxazoleH), 7.66 (d, J = 8.4 Hz, 2H, ArH), 7.14 (d,
J = 8.4 Hz, 2H, ArH), 4.52 (t, J = 8.2 Hz, 1H,
NHCH(CH)C), 2.52(s, 3H, PhCH3), 2.14–2.20(m, 1H,
CH(CH3)2), 1.39 (s, 9H, C(CH3)3), 0.96 (d, J = 6.6 Hz, 3H,
CH(CH3)CH3), 0.81 (d, J = 6.7 Hz, 3H, CH(CH3)CH3). IR
(KBr, m/cm-1) 3359, 3333, 3126, 2953, 1686, 1661, 1597,
1521. ESI-MS m/z 374 [M ? H]?.
(S)-tert-butyl-[4-(benzo[d][1,3]dioxol-5-
ylcarbamoyl)oxazol-2-yl]-2-methylpropylcarbamate (3g)
Off-white solid, yield 68 %; m.p. 160.0–160.1 °C,
1
[a]1D0 = -48.0 (c 1, CHCl3). H NMR (400 MHz, DMSO-
d6) d: 8.68 (s, 1H, OxazoleH), 7.56 (d, J = 8.4 Hz, 1H,
ArH), 7.42 (d, J = 2.4 Hz, 1H, ArH), 7.23 (dd, J = 2,
8.4 Hz, 1H, ArH), 5.99 (s, 2H, OCH2O), 4.49 (t, J = 8.2 Hz,
1H, NHCH(CH)C), 2.14(m, 1H, CH(CH3)2), 1.37 (s, 9H,
C(CH3)3), 0.83 (d, J = 6.8 Hz, 3H, CH(CH3)CH3), 0.78 (d,
J = 6.8 Hz, 3H, CH(CH3)CH3). IR (KBr, m/cm-1) 3365,
3348, 2995, 1700, 1670, 1540, 1500, 1240.
(S)-tert-butyl 1-[4-(4-fluorophenylcarbamoyl)oxazol-2-yl]-
2-methylpropylcarbamate (3b)
Off-white solid, yield 77 %; m.p. 121.7–122.2 °C,
1
[a]1D0 = -51.2 (c 1, CHCl3). H NMR (400 MHz, DMSO-
d6) d: 8.72(s, 1H, OxazoleH), 7.79 (m, 2H, ArH), 7.17 (t,
J = 8.4 Hz, 2H, ArH), 4.50 (t, J = 8.4 Hz, 1H,
NHCH(CH)C), 2.15(m, 1H, CH(CH3)2), 1.37 (s, 9H,
C(CH3)3), 0.94 (d, J = 6.4 Hz, 3H, CH(CH3)CH3), 0.79 (d,
J = 6.4 Hz, 3H, CH(CH3)CH3). IR (KBr, m/cm-1) 3350,
3325, 3090, 2950, 1682, 1650, 1500, 1155.
Preparation of compound 3c
N,N0-Diisopropylethylamine (DIEA) (2.25 g, 17.4 mmol)
was added to a solution of compound 2 (1.5 g, 5.3 mmol)
in dry DMF (32 mL) and the mixture was stirred at room
temperature for 10 min, 2-amino-5-fluoropyridine (0.65 g,
5.8 mmol) was added to the mixture followed by O-(7-
azabenzotriazol-1-yl)-N,N,N0,N0-tetramethyluronium hexa-
fluorophosphate (2.21 g, 5.8 mmol). The resulting mixture
was stirred at room temperature for 24 h under nitrogen.
The mixture was poured into 10 % citric acid (40 mL) and
extracted with EtOAc (3 9 50 mL). The combined organic
layers were washed with saturated saline and saturated
NaHCO3, dried over Na2SO4, and concentrated. The yel-
low oil thus obtained was purified by column chromatog-
raphy on silica gel (PE/EtOAc, 5:1) to afford compound 3c.
(S)-tert-butyl 1-[4-(diethylcarbamoyl)oxazol-2-yl]-2-
methylpropylcarbamate (3d)
Light yellow solid, yield 87 %; m.p. 52.5–54.2 °C,
1
[a]1D0 = -48.8 (c 1, CHCl3). H NMR (400 MHz, DMSO-
d6) d: 8.41 (s, 1H, OxazoleH), 4.43 (t, J = 8 Hz, 1H,
NHCH(CH)C), 3.60 (m, 2H, NCH2CH3), 3.33 (m, 2H,
NCH2CH3), 2.11 (m, 1H, CH(CH3)2), 1.36 (s, 9H,
C(CH3)3), 1.10 (m, 6H, N(CH2CH3)2), 0.91 (d, J = 6.4 Hz,
3H, CHCH3), 0.77 (d, J = 6.4 Hz, 3H, CHCH3). IR (KBr,
m/cm-1) 3284, 3120, 2935, 1690, 1585, 1490.
(S)-tert-butyl 2-methyl-1-[4-(piperidine-1-carbonyl)oxazol-
2-yl]propylcarbamate (3e)
(S)-tert-butyl 1-[4-(5-fluoropyridin-2-ylcarbamoyl)oxazol-
2-yl]-2-methylpropylcarbamate (3c)
1
Colorless oil, yield 95 %; [a]2D0 = -40.0 (c 1, CHCl3). H
NMR (400 MHz, CDCl3) d: 8.02 (s, 1H, OxazoleH), 4.76
(app t, J = 5.7 Hz, 1H, NHCH(CH)C), 3.80 (brs, 2H,
White solid, yield 65 %; m.p. 44.8–55.1 °C, [a]2D0 = -59.2 (c
1, CHCl3). H NMR (400 MHz, DMSO-d6) d: 8.85 (s, 1H,
OxazoleH), 8.37 (d, J= 2.4 Hz, 1H, ArH), 8.17 (dd, J = 4,
1
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Med Chem Res (2013) 22:1426–1437
1429
9.2 Hz, 1H, ArH), 7.81 (dt, J = 2.4, 8.6 Hz, 1H, ArH), 4.51 (t,
J = 7.8 Hz, 1H, NHCH(CH)C), 2.15 (m, 1H, CH(CH3)2), 1.38
(s, 9H, C(CH3)3), 0.93 (d, J = 6.4 Hz, 3H, CH(CH3)CH3),
OxazoleH), 7.65 (m, 2H, ArH), 7.07 (t, J = 8.8 Hz, 2H,
ArH), 5.14 (dd, J = 6.4, 8.8 Hz, 1H, NHCH(CH)C), 2.66
(m, 2H, CH2CH(CH2)CO ? COCH2CH), 2.41 (dd, J = 8.2,
13.2 Hz, 1H, COCH2CH), 2.25 (m, 1H, CH(CH3)2), 1.45 (s,
9H, C(CH3)3), 1.25–1.34 (m, 6H, CH3CH2CH2CH2), 0.98
(d, J = 6.8 Hz, 3H, CH(CH3)CH3), 0.94 (d, J = 6.8 Hz, 3H,
CH(CH3)CH3), 0.84 (t, J = 7.0 Hz, 3H, CH3CH2CH2CH2).
IR (KBr, m/cm-1) 3355, 3080, 2995, 1713, 1685, 1610, 1525.
0.78 (d, J = 6.8 Hz, 3H, CH(CH3)CH3). IR (KBr, m/cm-1
)
3410, 3365, 2995, 1710, 1685, 1595, 1520, 1400.
General procedure for the synthesis of compound 5a–c
Acetyl chloride (55.6 mmol) was added to dry ethanol
(30 mL) at 0 °C. After stirring at 0 °C for 10 min, compound
3 (2.14 mmol) was added to the acidic solution and the
reactionmixturewasstirredatroomtemperature for10 h.The
reaction mixture was concentrated and the residue was diluted
with methylene dichloride (60 mL). The methylene dichlo-
ride solution was washed with saturated NaHCO3, dried with
sodium sulfate, and concentrated to produce the desired
intermediate, which was used without further purification.
4,6-Dimethoxy-2-chloro-1,3,5-triazine (CDMT) (2.5 mmol)
and (R)-(?)-2-butylbutanedioic acid-4-tert-butyl monoes-
ter (2.36 mmol) were dissolved in dry methylene chloride
(23 mL). N-methylmorpholine (2.6 mmol) was added at
-5 to 0 °C. After 4 h, the intermediate obtained above was
added to the reaction mixture. The mixture was stirred at
the same temperature for 2 h and then overnight at room
temperature. The precipitate produced was removed by
suction filtration and washed with a small amount of
methylene chloride. The filtrate was washed with water,
0.5 N HCl, saturated NaHCO3, and brine successively and
dried with sodium sulfate. The solvent was concentrated,
and the residue was purified by column chromatography on
silica gel (PE/EtOAc, 4:1).
(R)-tert-butyl 3-[(S)-1-[4-(5-fluoropyridin-
2-ylcarbamoyl)oxazol-2-yl]-2-methylpropylcarbamoyl]
hep-tanoate (5c)
White solid, yield 94.5 %; m.p. 74–76 °C, [a]2D0 = -52.0 (c
1, CHCl3). 1H NMR (400 MHz, CDCl3) d: 8.36 (dd, J = 4.4,
8.8 Hz, 1H, ArH), 8.26 (s, 1H, OxazoleH), 8.19 (d,
J = 2.8 Hz, 1H, ArH), 7.49 (dt, J = 2.8, 8.4 Hz, 1H, ArH),
5.15 (dd, J = 6, 8.8 Hz, 1H, NHCH(CH)C), 2.62–2.68 (m,
2H, CH2CH(CH2)CO ? COCH2CH), 2.38 (m, 1H,
COCH2CH), 2.24 (m, 1H, CH(CH3)2), 1.42 (s, 9H,
C(CH3)3), 1.21–1.26 (m, 6H, CH3CH2CH2CH2), 0.89–0.92
(m, 6H, CH(CH3)2), 0.82 (t, J = 7.2 Hz, 3H, CH3CH2
CH2CH2). IR (KBr, m/cm-1) 3308, 3205, 3060, 2900, 1700,
1662, 1608, 1480.
General procedure for the synthesis of compounds 6
A solution of compound 5 (1.69 mmol) in formic acid
(98 %, 10.5 mL) was stirred at room temperature for 9 h.
Removal of the solvent under vacuum gave a white solid.
The white solid was added to CH2Cl2 (15 mL), followed by
methanol
(2.9 mL)
and
4-dimethylaminopyridine
(R)-tert-butyl 3-[(S)-2-methyl-1-[4-(p-tolylcarbamoyl)
oxazol-2-yl]propylcarbamoyl]heptanoate (5a)
(0.153 mmol). After cooling to 0 °C, DCC (1.69 mmol)
was added and the solution was stirred at the same tem-
perature for 0.5 h. Then the reaction was continued at room
temperature for 8 h. The precipitate produced was removed
by suction filtration. The solvent was concentrated and the
residue was purified on silica gel with PE/EtOAc (3:1).
Colorless oil, yield 91.7 %; [a]2D0 = -49.5 (c 1, CHCl3). 1H
NMR (400 MHz, DMSO-d6) d: 8.70 (s, 1H, OxazoleH), 7.64
(d, J = 8.4 Hz, 2H, ArH), 7.14(d, J = 8.4 Hz, 2H, ArH), 4.83
(t, J = 8.3 Hz, 1H, NHCH(CH)C), 2.70–2.74 (m, 1H,
CH2CH(CH2)CO), 2.45 (dd, J = 9, 16 Hz, 1H, COCH2CH),
2.27 (s, 3H, PhCH3), 2.18–2.26 (m, 2H, COCH2CH ?
CH(CH3)2), 1.37(s, 9H, C(CH3)3), 1.02–1.35 (m, 6H,
CH3CH2CH2CH2), 0.98 (d, J = 6.7 Hz, 3H, CH(CH3)CH3),
0.83 (d, J = 6.7 Hz, 3H, CH(CH3)CH3), 0.76 (t, J = 6.8 Hz,
3H, CH3CH2CH2CH2). IR (KBr, m/cm-1) 3311, 3100, 1728,
1655, 1602, 1525. ESI-MS m/z 486 [M ? H]?.
(R)-methyl 3-[(S)-2-methyl-1-[4-(p-tolylcarbamoyl)oxazol-
2-yl]propylcarbamoyl]heptanoate (6a)
White solid, yield 75 %; m.p. 122–123 °C, [a]2D0 = -60.0
1
(c 1, CHCl3). H NMR (400 MHz, DMSO-d6) d: 8.71 (s,
1H, OxazoleH), 7.64 (d, J = 8.2 Hz, 2H, ArH), 7.14 (d,
J = 8.2 Hz, 2H, ArH), 4.84 (t, J = 8.3 Hz, 1H,
NHCH(CH)C), 3.55 (s, 3H, CH3O), 2.72–2.79 (m, 1H,
CH2CH(CH2)CO), 2.55 (dd, J = 9.4, 16 Hz, 1H,
COCH2CH), 2.35 (dd, J = 5.1, 16 Hz, 1H, COCH2CH),
2.27 (s, 3H, PhCH3), 2.16–2.23 (m, 1H, CH(CH3)2),
1.04–1.44 (m, 6H, CH3CH2CH2CH2), 0.98 (d, J = 6.7 Hz,
3H, CH(CH3)CH3), 0.84 (d, J = 6.7 Hz, 3H,
CH(CH3)CH3), 0.76 (t, J = 7.2 Hz, 3H, CH3CH2CH2CH2).
(R)-tert-butyl 3-[(S)-1-[4-(4-
fluorophenylcarbamoyl)oxazol-2-yl]-
2-methylpropylcarbamoyl]hepta-noate (5b)
White solid, yield 91.8 %; m.p. 88–90 °C, [a]2D0 = -47.2 (c
1, CHCl3). 1H NMR (400 MHz, CDCl3) d: 8.23(s, 1H,
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Med Chem Res (2013) 22:1426–1437
IR (KBr, m/cm-1) 3400, 3276, 3051, 1737, 1683, 1644,
1602, 1528. ESI-MS m/z 444 [M ? H]?.
CH3CH2CH2CH2), 0.98 (d, J = 6.8 Hz, 3H, CH(CH3)CH3),
0.82 (d, J = 6.8 Hz, 3H, CH(CH3)CH3), 0.75 (t, J = 7.0 Hz,
3H, CH3CH2CH2CH2). IR (KBr, m/cm-1) 3400, 3276, 3051,
1737, 1683, 1644, 1602, 1528. HRMS (m/z) calculated for
C23H32N4O5: 445.2451 [M ? H]?. found: 445.2430. Anal.
Calcd for C23H32N4O5: C, 62.14; H, 7.26; N, 12.60; Found:
C, 62.03; H, 7.20; N, 12.69.
(R)-methyl 3-[(S)-1-[4-(4-fluorophenylcarbamoyl)oxazol-
2-yl]-2-methylpropylcarbamoyl]heptanoate (6b)
White solid, yield 74.7 %; m.p. 160–162 °C, [a]D20
=
1
-57.6 (c 1, CHCl3). H NMR (400 MHz, CDCl3) d: 8.22
(s, 1H, OxazoleH), 7.62 (m, 2H, ArH), 7.01 (t, J = 8 Hz,
2H, ArH), 5.08 (t, J = 7.2 Hz, 1H, NHCH(CH)C), 3.64 (s,
3H, CH3O), 2.72–2.79 (m, 2H, CH2CH(CH2)CO ? -
COCH2CH), 2.42 (m, 1H, COCH2CH), 2.19–2.24 (m, 1H,
CH(CH3)2), 1.23–1.64 (m, 6H, CH3CH2CH2CH2), 0.95 (d,
J = 6.4 Hz, 3H, CH(CH3)CH3), 0.90 (d, J = 6.4 Hz, 3H,
CH(CH3)CH3), 0.64 (t, J = 7.0 Hz, 3H, CH3CH2CH2CH2).
IR (KBr, m/cm-1) 3395, 3100, 1755, 1672, 1530, 835.
(R)-2-butyl-N1-[(S)-1-[4-(4-
fluorophenylcarbamoyl)oxazol-2-yl]-2-methylpropyl]-
N4-hydroxysuccin-amide (7b)
White solid, yield 57 %; m.p. 202–205 °C, [a]2D0 = -50.1 (c
1
0.67, MeOH). H NMR (400 MHz, DMSO-d6) d: 8.70 (s,
1H, OxazoleH), 7.77 (m, 2H, ArH), 7.16 (t, J = 8.4 Hz, 2H,
ArH), 4.79 (t, J = 7.8 Hz, 1H, NHCH(CH)C), 2.75 (m, 1H,
CH2CH(CH2)CO), 2.16–2.21 (m, 2H, COCH2CH ?
CH(CH3)2), 2.01 (dd, J = 7.0, 14.4 Hz, 1H, COCH2CH),
1.11–1.34 (m, 6H, CH3CH2CH2 CH2), 0.98 (d, J = 6.4 Hz,
3H, CH(CH3)CH3), 0.83 (d, J = 6.4 Hz, 3H, CH(CH3)CH3),
0.74 (t, J = 6.4 Hz, 3H, CH3CH2CH2CH2); IR (KBr, m/
cm-1) 3388, 3261, 3144, 1674, 1641, 1535, 839; HRMS (m/
z) calculated for C22H29FN4O5: 449.2200 [M ? H]?.found:
449.2198. Anal. Calcd for C22H29FN4O5: C, 58.92; H, 6.52;
N, 12.49; Found: C, 58.83; H, 6.58; N, 12.41.
(R)-methyl 3-[(S)-1-[4-(5-fluoropyridin-
2-ylcarbamoyl)oxazol-2-yl]-2-methylpropylcarbamoyl]
hep-tanoate (6c)
White solid, yield 78.3 %; m.p. 124–127 °C, [a]2D0 = -64.0
(c 1, CHCl3). 1H NMR (400 MHz, CDCl3) d: 8.36–8.40 (dd,
J = 4.4, 9.2 Hz, 1H, ArH), 8.27 (s, 1H, OxazoleH), 8.19 (d,
J = 2.8 Hz, 1H, ArH), 7.51 (dt, J = 2.8, 8.4 Hz, 1H, ArH),
5.16 (t, J = 7.2 Hz, 1H, NHCH(CH)C), 3.69 (s, 3H, CH3O),
2.74–2.80 (m, 2H, COCH2CH ? CH2CH(CH2)CO), 2.44–
2.49 (m, 1H, COCH2CH), 2.26 (m, 1H, CH(CH3)2), 1.25–
1.42 (m, 6H, CH3CH2CH2CH2), 0.93–0.98 (m, 6H,
CH(CH3)2), 0.82 (t, J = 7.2 Hz, 3H, CH3CH2CH2CH2). IR
(KBr, m/cm-1) 3400, 3290, 2970, 1742, 1650, 1600, 1520,
1400.
(R)-2-butyl-N1-[(S)-1-[4-(5-fluoropyridin-
2-ylcarbamoyl)oxazol-2-yl]-2-methylpropyl]-N4-
hydroxy-succinamide (7c)
White solid, yield 60 %; m.p. 186–187 °C, [a]2D0 = -79.2
(c 0.5, MeOH). 1H NMR (400 MHz, DMSO-d6) d: 8.86 (s,
1H, OxazoleH), 8.37 (d, J = 2 Hz, 1H, ArH), 8.17 (dd,
J = 4, 8 Hz, 1H, ArH), 7.79–7.84 (dt, J = 2.4, 8 Hz, 1H,
ArH), 4.79 (t, J = 8 Hz, 1H, NHCH(CH)C), 2.77 (m, 1H,
CH2CH(CH2)CO), 2.15–2.20 (m, 2H, COCH2CH ? CH
(CH3)2), 2.02 (dd, J = 7.2, 14.4 Hz, 1H, COCH2CH), 1.33
(m, 2H, CH3CH2CH2CH2CH), 1.08–1.21 (m, 4H, CH3CH2
CH2CH2), 0.96 (d, J = 6.4 Hz, 3H, CH(CH3)CH3), 0.83 (d,
J = 6.4 Hz, 3H, CH(CH3)CH3), 0.73 (t, J = 7.2 Hz, 3H,
CH3CH2CH2CH2). IR (KBr, m/cm-1) 3383, 3261, 3080,
2954, 1693, 1641, 1512, 1392. HRMS (m/z) calculated for
C21H28FN5O5: 450.2153 [M ? H]?. found: 450.2131.
Anal. Calcd for C21H28FN5O5: C, 56.12; H, 6.28; N, 15.58;
Found: C, 56.20; H, 6.20; N, 1566.
General procedure for the synthesis of compounds 7
To a methanol/tetrahydrofuran (5 mL, 1:1 by v/v) solution
of compound 6 (1.21 mmol) 50 % aqueous hydroxylamine
(12.1 mmol) was added, followed by NaCN (0.97 mmol).
After 8 h, several drops of acetic acid were added to adjust
the pH of the reaction mixture to 6. The solution was con-
centrated and the residue was purified by column chroma-
tography on silica gel (methylene chloride/methanol, 20:1).
(R)-2-butyl-N4-hydroxy-N1-[(S)-2-methyl-1-[4-
(p-tolylcarbamoyl)oxazol-2-yl]propyl]succinamide (7a)
White solid, yield 75 %; m.p. 184–186 °C, [a]2D0 = -30.2
(c 0.18, MeOH). 1H NMR (400 MHz, DMSO-d6) d: 8.70 (s,
1H, OxazoleH), 7.65 (d, J = 8.4 Hz, 2H, ArH), 7.14 (d,
J = 8.4 Hz, 2H, ArH), 4.80 (t, J = 8.4 Hz, 1H,
NHCH(CH)C), 2.76 (m, 1H, CH2CH(CH2)CO), 2.27 (s, 3H,
PhCH3), 2.18 (m, 2H, COCH2CH ? CH(CH3)2), 2.02 (dd,
1H, J = 7.5, 14.4 Hz, COCH2CH), 1.05–1.32 (m, 6H,
General procedure for the synthesis of compounds 9
Acetyl chloride (84 mmol) was added to dry ethanol
(40 mL) at 0 °C. After stirring at 0 °C for 10 min, com-
pound 5 (3.24 mmol) was added to the acidic solution and
the reaction mixture was stirred at room temperature for
123

Med Chem Res (2013) 22:1426–1437
1431
2-[(S)-1-[(R)-2-[(N-(benzyloxy)formamidomethyl]
hexanamido]-2-methylpropyl]-N-p-tolyloxazole-
4-carboxamide (9c)
10 h. The reaction mixture was concentrated and the resi-
due was diluted with methylene dichloride (80 mL). The
methylene dichloride solution was washed with saturated
NaHCO3, dried with sodium sulfate, and concentrated to
produce the desired intermediate, which was used without
further purification.
White solid, yield 70 %; m.p. 106.3–111.5 °C, [a]D20
=
1
-30.0 (c 0.5, CHCl3). H NMR (400 MHz, DMSO-d6) d:
8.63–8.66 (m, 2H, CONH ? OxazoleH), 8.01 (s, 0.4H,
CHO), 7.84 (s, 0.6H, CHO), 7.66 (m, 2H, ArH), 7.32–7.39
(m, 5H, PhH), 7.13 (m, 2H, ArH), 4.72–4.92 (m, 3H,
PhCH2 ? NHCH(CH)C), 3.42–3.63 (m, 2H, NCH2CH),
2.74–2.87 (m, 1H, CH2CH(CH2)CO), 2.26 (s, 3H, PhCH3),
2.17 (m, 1H, CH(CH3)2), 1.23–1.43 (m, 6H, CH3CH2
CH2CH2), 0.77–0.83 (m, 9H, CH (CH3)2 ? CH3CH2
CH2CH2). IR (KBr, m/cm-1) 3340, 3050, 2970, 2945, 2875,
1685, 1660, 1605, 1435.
4,6-Dimethoxy-2-chloro-1,3,5-triazine (CDMT) (4.02
mmol) and (R)-2-[[N-(benzyloxy) formamido]methyl]hex-
anoic acid (3.7 mmol) were dissolved in dry methylene
chloride (38 mL). N-methylmorpholine (4.64 mmol) was
added at -5 to 0 °C. After 4 h, the intermediate obtained
above (3.1 mmol) was added to the reaction mixture. The
mixture was stirred at the same temperature for 2 h and
then overnight at room temperature. The precipitate pro-
duced was removed by suction filtration and washed with a
small amount of methylene chloride. The filtrate was
washed with water, 0.5 N HCl, saturated NaHCO3, and
brine successively and dried with sodium sulfate. The
solvent was concentrated, and the residue was purified by
column chromatography on silica gel (PE/EtOAc, 1:2).
2-[(S)-1-[(R)-2-[[N-(benzyloxy)formamido]methyl]
hexanamido]-2-methylpropyl]-N-(4-fluorophenyl)oxazole-
4-carboxamide (9d)
White solid, yield 63 %; m.p. 126.4–127.6 °C, [a]D20
=
1
-45.8 (c 1, CHCl3). H NMR (400 MHz, DMSO-d6) d:
8.68 (m, 2H, CONH ? OxazoleH), 8. 04 (s, 0.45H, CHO),
7.78 (m, 2.55H, 0.55CHO ? 2ArH), 7.31–7.39 (m, 5H,
PhH), 7.17 (t, J = 8.4 Hz, 2H, ArH), 4.71–4.90 (m, 3H,
PhCH2 ? NHCH(CH)C), 3.39–3.63 (m, 2H, NCH2CH),
2.73–2.87 (m, 1H, CH2CH(CH2)CO), 2.20 (m, 1H,
CH(CH3)2), 1.15–1.43 (m, 6H, CH3CH2CH2CH2), 0.76–
0.93 (9H, m, CH (CH3)2 ? CH3CH2CH2CH2). IR (KBr,
m/cm-1) 3310, 3075, 2985, 2945, 1690, 1660, 1600, 1520.
2-[(S)-1-[(R)-2-[[N-(benzyloxy)formamido]methyl]
hexanamido]-2-methylpropyl]-N,N-diethyloxazole-
4-carboxamide (9a)
Colorless oil, yield 80 %; [a]2D0 = -48.2 (c 2, CHCl3). H
1
NMR (400 MHz, DMSO-d6) d: 8.41 (s, 1H, OxazoleH), 8.20
(s, 0.5H, CHO), 7.81(s, 0.5H, CHO), 7.31–7.57 (m, 5H, PhH),
4.73–4.90 (m, 3H, PhCH2 ? NHCH(CH)C), 3.59–3.64 (m,
3H, NCH2CH3 ? NCHCHCO), 3.36–3.37(m, 3H, NCH2
CH3 ? NCHCHCO), 2.66–2.80 (m, 1H, CH2CH(CH2)
CO), 2.08 (m, 1H, CH(CH3)2), 1.31(m, 2H, CH3CH2CH2
CH2), 1.10–1.22 (m, 10H, N(CH2CH3)2 ? CH3CH2CH2
CH2), 0.85 (d, J = 6.4 Hz, 3H, CHCH3), 0.76 (m, 6H,
CHCH3 ? CH3CH2CH2CH2). IR (KBr, m/cm-1) 3305, 3060,
2975, 2945, 2880, 1680, 1620, 1580.
2-[(S)-1-[(R)-2-[[N-(benzyloxy)formamido]
methyl]hexanamido]-2-methylpropyl]-N-(5-fluoropyridin-
2-yl)oxazole-4-carboxamide (9e)
White solid, yield 45 %; m.p. 101–102 °C, [a]2D0 = -28.11
(c 0.37, CHCl3). 1H NMR (400 MHz, DMSO-d6) d:
8.72–8.87 (m, 2H, CONH ? OxazoleH), 8.36 (d, J =
2.8 Hz, 1H, ArH), 8.17 (dd, J = 4, 8.8 Hz, 1H, ArH), 8.10 (s,
0.5H, CHO), 7.80 (dt, J = 3, 8.6 Hz, 1H, ArH), 7.75 (s, 0.5H,
CHO), 4.79–4.88 (m, 3H, PhCH2 ? NHCH(CH)C), 3.39–
3.63 (m, 2H, NCH2CH), 2.76–2.84 (m, 1H, CH2CH(CH2)
CO), 2.17 (m, 1H, CH(CH3)2), 1.23–1.43 (m, 6H,
CH3CH2CH2CH2), 0.73–0.95 (m, 9H, CH (CH3)2 ? CH3
CH2CH2CH2). IR (KBr, m/cm-1) 3340, 3050, 2950, 2880,
1680, 1660, 1540, 1400.
(R)-2-[[N-(benzyloxy)formamido]methyl]-N-[(S)-
2-methyl-1-[4-(piperidine-1-carbonyl)oxazol-
2-yl]propyl]hexanamide (9b)
Colorless oil, yield 58 %; [a]2D0 = -45.0 (c 0.4, CHCl3).
1H NMR (400 MHz, DMSO-d6) d: 8.42 (s, 1H, OxazoleH),
8.20 (s, 0.5H, CHO), 7.84 (s, 0.5H, CHO), 7.37–7.44 (m,
5H, PhH), 4.74–4.90 (m, 3H, PhCH2 ? NHCH(CH)C),
3.34–3.71(m, 6H, N(CH2)2 ? NCH2CH), 2.69–2.85 (m,
1H, CH2CH (CH2)CO), 2.08 (m, 1H, CH(CH3)2), 1.06–
1.60 (m, 12H, N(CH2)2(CH2)3 ? CH3CH2CH2CH2), 0.84
(d, J = 6.8 Hz, 3H, CHCH3), 0.76 (m, 6H, CHCH3 ?
CH3CH2CH2CH2). IR (KBr, m/cm-1) 3210, 3065, 2850,
1640, 1575.
2-[(S)-1-[(R)-2-[[N-(benzyloxy)formamido)]methyl]
hexanamido]-2-methylpropyl]-N-(3,4-difluorophe-nyl)
oxazole-4-carboxamide (9f)
White solid, yield 62 %; m.p. 107.4–109.5 °C, [a]D20
-42.4 (c 1, CHCl3). H NMR (400 MHz, DMSO-d6) d:
=
1
123

1432
Med Chem Res (2013) 22:1426–1437
(R)-2-[(N-hydroxyformamido)methyl]-N-[(S)-2-methyl-1-
[4-(piperidine-1-carbonyl)oxazol-2-yl]pro-pyl]hexanamide
(10b)
8.68 (m, 2H, CONH ? OxazoleH), 8. 04 (s, 0.4H, CHO),
7.93(m, 1H, ArH), 7.77 (s, 0.6H, CHO), 7.62(m, 1H, ArH),
7.31–7.44 (m, 6H, 5PhH ? 1ArH), 4.70–4.90 (m, 3H,
PhCH2 ? NHCH(CH)C), 3.39–3.63 (m, 2H, NCH2CH),
2.73–2.86 (m, 1H, CH2CH(CH2)CO), 2.20 (m, 1H,
CH(CH3)2), 1.16–1.43 (m, 6H, CH3CH2CH2CH2), 0.82–
0.93 (9H, m, CH(CH3)2 ? CH3CH2CH2CH2). IR (KBr, m/
cm-1) 3310, 3090, 2995, 2950, 2875, 1680, 1600, 1528.
1
Gum, yield 88 %; [a]2D0 = -49.6 (c 1, CHCl3). H NMR
(400 MHz, DMSO-d6) d: 8.41 (s, 1H, OxazoleH), 8.22 (s,
0.4 H, CHO), 7.78 (s, 0.6H, CHO), 4.76 (t, 1H, J = 8 Hz,
NHCH(CH)C), 3.31–3.72 (m, 6H, N(CH2)2 ? NCH2CH),
2.71–2.81(m, 1H, CH2CH(CH2)CO), 2.13 (m, 1H, CH
(CH3)2), 1.08–1.60 (m, 12H, N(CH2)2(CH2)3 ? CH3
CH2CH2CH2), 0.74–0.93 (m, 9H, CH(CH3)2 ? CH3CH2
CH2CH2). IR (KBr, m/cm-1) 3286, 3175, 2955, 2935, 2860,
1664, 1661, 1612, 754; HRMS (m/z) calculated for
C21H34N4O5: 423.2607 [M ? H]?. Found: 423.2599. Anal.
Calcd for C21H34N4O5: C, 59.70; H, 8.11; N, 13.26; Found:
C, 59.73; H, 8.07; N, 13.33.
N-(benzo[d][1,3]dioxol-5-yl)-2-[(S)-1-[(R)-2-
[[N-(benzyloxy)formamido]methyl]hexanamido]-
2-me-thylpropyl]oxazole-4-carboxamide (9g)
White solid, yield 55 %; m.p. 118.5–121.5 °C, [a]D20
=
-34.7 (c 0.88, CHCl3). 1H NMR (400 MHz, DMSO-d6) d:
8.67 (m, 2H, CONH ? OxazoleH), 8. 04 (s, 0.4H, CHO),
7.77 (s, 0.6H, CHO), 7.21–7.43 (m, 7H, ArH), 6.87 (d,
J = 8.4 Hz, 1H, ArH), 6.00 (s, 2H, OCH2O), 4.71–4.96 (m,
3H, PhCH2 ? NHCH(CH)C), 3.39–3.84 (m, 2H, NCH2
CH), 2.74–2.87 (m, 1H, CH2CH(CH2)CO), 2.19 (m, 1H,
CH(CH3)2), 1.22–1.43 (m, 6H, CH3CH2CH2CH2), 0.82–
0.93 (9H, m, CH(CH3)2 ? CH3CH2CH2CH2). IR (KBr,
m/cm-1) 3310, 3070, 2975, 2950, 2880, 1690, 1660, 1540,
1500, 1210.
2-[(S)-1-[(R)-2-[(N-
hydroxyformamido)methyl]hexanamido]-2-methylpropyl]-
N-p-tolyloxazole-4-carboxamide (10c)
Off-white solid, yield 76 %; m.p. 189.8–190.2 °C,
[a]1D0 = -90.0 (c 0.5, CHCl3). 1H NMR (400 MHz, DMSO-
d6) d: 8.68 (s, 1H, OxazoleH), 8.19 (s, 0.3 H, CHO), 7.75 (s,
0.7H, CHO), 7.66 (m, 2H, ArH), 7.13 (d, J = 8 Hz, 2H,
ArH), 4.88 (t, 1H, J = 7.6 Hz, NHCH(CH)C), 3.36–3.62
(m, 2H, NCH2CH), 2.75–2.89 (m, 1H, CH2CH(CH2)CO),
2.19–2.26 (m, 4H, CH3Ph ? CH(CH3)2), 1.22–1.50 (m, 6H,
CH3CH2CH2CH2), 0.83–0.96 (m, 9H,CH (CH3)2 ? CH3
CH2CH2CH2). IR(KBr, m/cm-1) 3260, 3121, 3072, 2964,
2932, 2858, 1691, 1647, 1602, 1525, 812; HRMS (m/z) cal-
culated for C23H32N4O5: 445.2451 [M ? H]?. Found:
445.2448. Anal. Calcd for C23H32N4O5: C, 62.14; H, 7.26;
N, 12.60; Found: C, 62.09; H, 7.29; N, 12.47.
General procedure for the synthesis of compounds 10
A stirred mixture of compound 9 (0.48 mmol) and anhy-
drous ethanol (6 ml) was hydrogenated over 5 % Pd/C
(240 mg) using an H2 balloon at room temperature for 6 h.
The reaction mixture was filtered and evaporated. The
residue was purified by a silica-gel column chromatogra-
phy (dichloro-methane/methanol, 20:1).
N,N-diethyl-2-[(S)-1-[(R)-2-[(N-hydroxyformamido)
methyl]hexanamido]-2-methylpropyl]oxazole-
4-carboxamide (10a)
N-(4-fluorophenyl)-2-[(S)-1-[(R)-2-
[(N-hydroxyformamido)methyl]hexanamido]-
2-methylpropyl]oxa-zole-4-carboxamide (10d)
White solid, yield 71 %; m.p. 107.4–108.3 °C, [a]D20
=
White solid, yield 60 %; m.p. 200.1–201.4 °C, [a]D20
=
1
-106.0 (c 0.5, CHCl3). 1H NMR (400 MHz, DMSO-d6) d:
8.73 (s, 1H, OxazoleH), 8.19 (s, 0.3 H, CHO), 7.67–7.84(m,
2.7H, 0.7CHO ? 2ArH), 7.17 (t, J = 8.4 Hz, 2H, ArH),
4.89 (t, J = 7.4 Hz, 1H, NHCH(CH)C), 3.31–3.62 (m, 2H,
NCH2CH), 2.75–2.89 (m, 1H, CH2CH(CH2)CO), 2.22 (m,
1H, CH(CH3)2), 1.23–1.44 (m, 6H, CH3CH2CH2CH2),
0.84–0.92 (9H, m, CH (CH3)2 ? CH3CH2CH2CH2).
IR(KBr, m/cm-1) 3275, 3115, 2962, 2872, 1684, 1651, 1533,
1510, 835. HRMS (m/z) calculated for C22H29FN4O5:
449.2200 [M ? H]?. Found: 449.2211. Anal. Calcd for
C22H29FN4O5: C, 58.92; H, 6.52; N, 12.49; Found: C, 58.99;
H, 6.45; N, 12.43.
-57.6 (c 1, CHCl3). H NMR (400 MHz, DMSO-d6) d:
8.40–8.50 (m, 2H, CONH ? OxazoleH), 8.22 (s, 0.45H,
CHO), 7.78 (s, 0.55H, CHO), 4.76 (t, J = 7.6 Hz, 1H, NH
CH(CH)C), 3.35–3.69 (m, 6H, N(CH2CH3)2 ? NCH2CH),
2.71–2.81 (m, 1H, CH2CH(CH2)CO), 2.13 (m, 1H, CH
(CH3)2), 1.10–1.32 (m, 12H, N(CH2CH3)2 ? CH3CH2CH2
CH2), 0.91(d, J = 7.2 Hz, 3H, CHCH3), 0.77 (m, 6H, CH
CH3 ? CH3CH2CH2CH2). IR (KBr, m/cm-1) 3302, 3170,
3066, 2964, 2872, 1676, 1647, 1618. HRMS (m/z) calcu-
lated for C20H34N4O5: 411.2607 [M ? H]?. Found:
411.2596. Anal. Calcd for C20H34N4O5: C, 58.52; H, 8.35;
N, 13.65; Found: C, 58.45; H, 8.43; N, 13.70.
123

Med Chem Res (2013) 22:1426–1437
1433
N-(5-fluoropyridin-2-yl)-2-[(S)-1-[(R)-2-
[(N-hydroxyformamido)methyl]hexanamido]-
2-methylpropyl]oxazole-4-carboxamide (10e)
ArH), 6. 87 (d, 1H, J = 8.4 Hz, ArH), 5.99 (s, 2H, OCH2
O), 4.90 (t, J = 7.6 Hz, 1H, NHCH(CH)C), 3.32–3.63 (m,
2H, NCH2CH), 2.78–2.89 (m, 1H, CH2CH(CH2)CO), 2.23
(m, 1H, CH(CH3)2), 1.22–1.48 (m, 6H, CH3CH2CH2CH2),
0.82–0.96 (9H, m, CH (CH3)2 ? CH3CH2CH2CH2). IR
(KBr, m/cm-1) 3248, 3121, 3084, 2966, 2856, 1687, 1641,
1526, 1489, 1244, 1209. HRMS (m/z) calculated for
C23H30N4O7: 475.2193 [M ? H]?. Found: 475.2191. Anal.
Calcd for C23H30N4O7: C, 58.22; H, 6.37; N, 11.81; Found:
C, 58.17; H, 6.28; N, 11.91.
Yellow solid, yield 67 %; m.p. 169–170 °C, [a]1D0 = -51.6
(c 0.5, CHCl3). 1H NMR (400 MHz, DMSO-d6) d: 8.84 (s,
1H, OxazoleH), 8.70 (d, J = 8.2 Hz, 1H, CONH), 8.38 (s,
1H, ArH), 8.30 (s, 0.3 H, CHO), 8.18 (dd, J = 4.6, 9.0 Hz,
1H, ArH), 7.82 (t, J = 8.4 Hz, 1H, ArH), 7.73 (s, 0.7H,
CHO), 4.86 (t, J = 8.0 Hz, 1H, NHCH(CH)C), 3.32–3.63
(m, 2H, NCH2CH), 2.78–2.89 (m, 1H, CH2CH(CH2)CO),
2.19 (m, 1H, CH(CH3)2), 1.28–1.46 (m, 6H, CH3CH2
CH2CH2), 0.70–0.95 (9H, m, CH (CH3)2 ? CH3CH2CH2
CH2). IR (KBr, m/cm-1) 3261, 3118, 2959, 2928, 2858,
1693, 1670, 1514, 1392; HRMS (m/z) calculated for
C21H28FN5O5: 450.2153 [M ? H]?. Found: 450.2145.
Anal. Calcd for C21H28FN5O5: C, 56.12; H, 6.28; N, 15.58;
Found: C, 56.19; H, 6.35; N, 15.53.
In vitro antibacterial activity evaluation
All the target compounds (7a–c and 10a–g) were evaluated
for their in vitro antibacterial activities by the broth mic-
rodilution method as per NCCLS guidelines (National,
2000). Tested microorganism strains were Escherichia coli
(ATCC 25922), Pseudomonas aeruginosa (ATCC 27853),
Staphylococcus aureus (ATCC 25923), and Streptococcus
pneumoniae (NCTC 7466). The test compounds were dis-
solved in dimethyl sulfoxide (DMSO) and diluted with
culture medium at the required final concentration. Cipro-
floxacin hydrochloride was used as the control drug. The
MIC values were defined as the lowest concentration of the
test sample allowing no visible growth after incubation at
37 °C for 24 h. All experiments were performed in triplicate.
N-(3,4-difluorophenyl)-2-[(S)-1-[(R)-2-
[(N-hydroxyformamido)methyl]hexanamido]-
2-methylpropyl]oxazole-4-carboxamide (10f)
White solid, yield 62 %; m.p. 201.3–201.5 °C, [a]D20
=
-111.6 (c 0.5, CHCl3). 1H NMR (400 MHz, DMSO-d6) d:
8.73 (s, 1H, OxazoleH), 8.18 (s, 0.3H, CHO), 7.95 (m, 1H,
ArH), 7.75 (s, 0.7H, CHO), 7.65 (m, 1H, ArH), 7.40 (m,
1H, ArH), 4.89 (t, J = 7.6 Hz, 1H, NHCH(CH)C),
3.32–3.63 (m, 2H, NCH2CH), 2.87–2.89 (m, 1H, CH2CH
(CH2)CO), 2.22 (m, 1H, CH(CH3)2), 1.22–1.44 (m, 6H,
CH3CH2CH2CH2), 0.82–0.96 (9H, m, CH (CH3)2 ?
CH3CH2CH2CH2). IR (KBr, m/cm-1) 3286, 3117, 2964,
2860, 1689, 1649, 1537, 1516, 874. HRMS (m/z) calculated
for C22H28F2N4O5: 467.2106 [M ? H]?. Found: 467.2101.
Anal. Calcd for C22H28F2N4O5: C, 56.64; H, 6.05; N,
12.01; Found: C, 56.70; H, 5.98; N, 12.05.
Results and discussion
Chemistry
The synthetic routes to the target compounds are outlined
in Schemes 2 and 3. The synthesis of the oxazole inter-
mediates is illustrated in Scheme 1. (S)-Methyl 2-[1-(tert-
butoxy-carbo-nylamino)-2-methylpropyl]oxazole-4-car-
boxylate 1, prepared according to the literature procedure
(Downing et al., 1999), was hydrolyzed by lithium
hydroxide to give the free acid.
N-(benzo[d][1,3]dioxol-5-yl)-2-[(S)-1-[(R)-2-
[(N-hydroxyformamido)methyl]hexanamido]-
2-methyl-propyl]oxazole-4-carboxamide (10g)
The coupling of 2 with various substituted amines to
afford compound 3 was carried out successfully with
dicyclohexylcarbodiimide (DCC) and 1-hydroxybenzotri-
azole (HOBt) except 3c. As for the reaction with 5-flu-
oropyridin-2-amine, lots of by-products were formed under
this condition. Finally, O-(7-azabenzotriazol-1-yl)-N,N,N0,
White solid, yield 65 %; m.p. 194.2–194.5 °C, [a]D20
=
1
-92.0 (c 0.5, CHCl3). H NMR (400 MHz, DMSO-d6) d:
8.66 (s, 1H, OxazoleH), 8.19 (s, 0.3 H, CHO), 7.75 (s,
0.7H, CHO), 7.44 (s, 1H, ArH), 7.28 (d, 1H, J = 8.4 Hz,
O
O
O
R1
R2
H
N
H
N
H
N
O
O
O
OH
O
N
b or c
a
N
N
N
O
O
O
O
O
O
3a-g
2
1
Scheme 1 Synthesis of compounds 3a–g. Reagents and conditions: a LiOHꢀH2O, CH3OH/H2O, rt; b substituted amine, DCC, HOBT, THF,
0 °C to rt; and c 5-fluoropyridin-2-amine, HATU, DIEA, DMF, N2, rt
123

1434
Med Chem Res (2013) 22:1426–1437
Scheme 2 Synthesis of
compounds 7a–c. Reagents and
conditions: a CH3COCl, EtOH,
0 °C to rt; b (R)-2-butyl-
butanedioic acid 4-tert-butyl
ester 4, CDMT, NMM, CH2Cl2,
0 °C to rt; c HCOOH, rt;
d CH3OH, DCC, DMAP,
CH2Cl2, 0 °C to rt; and
e NH2OH, NaCN, CH3OH/THF
O
O
O
R1
H
N
R1
R2
H
N
a, b
O
c, d
N
N
N
O
O
N
R2
O
O
O
OH
O
O
4
5a-c
3a-c
O
R1
O
O
O
R1
R2
H
N
O
H
N
e
HO
N
N
N
H
O
N
N
R2
O
O
O
O
6a-c
7a-c
OBn
N
R1
O
O
H
N
R1
H
N
a, b
O
O
O
N
N
N
N
O
R2
R2
H
O
O
O
OH
N
O
8
9a-g
3a-g
OBn
R1
R2
OH
N
O
H
c
O
N
N
N
H
O
O
10a-g
Scheme 3 Synthesis of compounds 10a–g. Reagents and conditions: a CH3COCl, EtOH, 0 °C to rt; b (R)-2-[[N-(benzyloxy)form-
amido]methyl]hexanoic acid 8, CDMT, NMM, CH2Cl2, rt; and c H2, 5 % Pd/C, EtOH, rt
1
N0-tetramethyluronium hexafluorophosphate (HATU) and
N,N0-diisopropyl ethylamine (DIEA) (Arkadius et al.,
2008) were employed to promote the reaction of 2 and
5-fluoropyridin-2-amine to give 3c. The chiral intermediate
(R)-2-butylbutanedioic acid 4-tert-butyl ester 4 was pre-
pared in high ee value according to the literature method
(Zhang et al., 2006). As shown in Scheme 2, compounds
3a–c were deprotected with acetyl chloride in ethanol and
then coupled with the chiral succinate with the acceleration
of 1-chloro-3,5-dimethoxy-2,4,6-triazine (CDMT) and N-
methyl morpholine (NMM). After removal of the tert-butyl
group followed by the esterification step, the hydroxamates
7a–c were obtained through a cyanide-catalyzed hydrox-
ylamination of the esters (Ho et al., 2005).
structures of 7a–c and 10a–g were determined by IR, H
NMR, and HRMS spectroscopy. For example, (R)-2-butyl-
N4-hydroxy-N1-[(S)-2-methyl-1-[4-(p-tolylcarbamoyl)oxa-
zol-2-yl]propyl]succinamide 7a gave an [M ? H]-ion peak
at m/z 445.2430 in the HRMS, in good agreement with the
molecular formula C23H32N4O5. In the IR spectrum of 7a,
the abroad absorption bands around 3,400 and 3,270 cm-1
indicated the presence of active hydrogen groups in the
compound. The amide carbonyl absorptions were observed
at about 1737, 1683, and 1644 cm-1. The other prominent
absorption bands observed in the IR spectrum were 3,051
cm-1 (Ar–H) and 1,528 cm-1 (C=N). 1H NMR spectrum of
7a indicated the chemical shift of the CH3 protons at
terminal of butyl group as a triplet at d = 0.75 ppm. The
two CH3 protons of isopropyl appeared as two doublets
centered at d = 0.82 ppm and d = 0.98 ppm with a vicinal
coupling constant J= = 6.3 Hz. The six methylene protons
signals in butyl group appeared at the range of d =
1.05–1.32 ppm as complex multiplet peaks. One of the
methylene protons of the succinate moiety appeared as
a double doublet centered at d = 2.02 ppm (J= = 7.5,
14.4 Hz). The other methylene proton, mixed with the ter-
tiary hydrogen signal of the isopropyl, resonated as multiplet
peaks centered at d = 2.18 ppm. The spectra indicated the
The intermediate (R)-2-[[N-(benzyloxy)formamido]methyl]
hexanoic acid 8, prepared following the published way (Slade
et al., 2006), was activated with CDMT and NMM, and then
coupled with the oxazole intermediates to give compounds 9.
The O-benzyl protecting group in compound 9 was removed by
catalytic hydrogenation to afford the target compounds 10a–
g (Scheme 3).
Three hydroxamic acids and seven N-formyl hydroxyl-
amines were synthesized. Some characteristics of the syn-
thesized target compounds are outlined in Table 1. The
123

Med Chem Res (2013) 22:1426–1437
1435
Table 1 The characteristics of the synthesized target compounds
R1
R2
O
H
N
MBG
N
N
O
O
(MBG = Metal Binding Group)
Compound
MBG
R1=
H
R2=
Mol. formula
C23H32N4O5
m.p. (°C)
Yield (%)
75
7a
CONH(OH)
184.0–186.0
CH3
7b
7c
CONH(OH)
CONH(OH)
H
H
C22H29FN4O5
C21H28FN5O5
202.0–205.0
186.0–187.0
57
60
F
N
F
10a
10b
N(OH)CHO
N(OH)CHO
C20H34N4O5
C21H34N4O5
107.4–108.3
Gum
71
88
10c
10d
10e
10f
N(OH)CHO
N(OH)CHO
N(OH)CHO
N(OH)CHO
H
H
H
H
C23H32N4O5
189.8–190.2
200.1–201.4
169.0–170.0
201.3–201.5
76
60
67
62
CH3
F
C22H29FN4O5
C21H28FN5O5
C22H28F2N4O5
N
F
F
F
O
O
10g
N(OH)CHO
H
C23H30N4O7
194.2–194.5
65
chemical shift of the CH3 protons of p-toluidino group at
d = 2.27 ppm in the form of singlet peak. The chiral proton
signal of succinate appeared as multiplet centered at
2.76 ppm. The chiral proton adjacent to the oxazole reso-
nated at d = 4.80 ppm as triplet (J = 8.4 Hz). The proton
signal in oxazole moiety appeared at d = 8.70 ppm as sin-
glet. All the other aromatic and active hydrogen were
observed at expected regions.
P. aeruginosa. Ciprofloxacin hydrochloride was used as the
positive drug. Some of the compounds exhibited moderate
to good activity against these bacteria compared to the
positive drug. The observed data on the antibacterial
activity of the compounds and positive drug are given in
Table 2.
From the data, it can be seen that the nature of the chelating
group has a marked effect on antibacterial activity. Com-
pounds 7a, 7b, and 7c with hydroxamic acid functionality
possessed increased potency compared to 10c, 10d, and 10e
which had N-formyl hydroxylamine as the chelating group.
For example, compound 7a had the MIC value of 25 lg/mL
against E. coli, whereas 10c showed no activity. Compounds
10a and 10b (MIC = 12.5–100 lg/mL) were found to be
more active than 10c–g (MIC[ 200 lg/mL), indicating that
the compounds with aliphatic substituents at the amidation
Evaluation of the antibacterial activity
All the final synthesized compounds (7a–c and 10a–
g) were evaluated for their antibacterial activities by
the broth microdilution method against the gram-positive
organisms Staphylococcus aureus and Streptococcus
pneumonia and the gram-negative E. coli and
123

1436
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Arkadius P, Jeyaraj D, Zheng Yin (2008) Peptide deformylase
inhibitors of Mycobacterium tuberculosis: synthesis, structural
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KRH, Wright DE (1975) Studies concerning the antibiotic
actinonin. Part VIII. Structure-activity relationships in the
actinonin series. J Chem Soc Perkin Trans 1:857–860
Chan MK, Gong WM, Rajagopalan PTR (1997) Crystal structure of
the Escherichia coli peptide deformylase. Biochemistry 36:
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National Committee for Clinical Laboratory Standards (2000)
Methods for dilution antimicrobial susceptibility tests for
bacteria that grow aerobically, approved standard M7-A5, 4th
edn. NCCLS, Wayne
Slade J, Parker D, Girgis M, Mueller M, Vivelo J, Liu H, Bajwa J,
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Table 2 Antibacterial activities of 7a–c and 10a–c (MIC, lg/mL)
Compd Escherichia Pseudomonas Staphylococus Streptococcus
coli
ATCC
25922
aeruginosa
ATCC 27853 ATCC 25923 NCTC 7466
aureus
pneumoniae
7a
25
100
50
200
7b
100
100
100
[200
[200
50
7c
50
100
100
10a
10b
10c
10d
10e
10f
10g
12.5
25
100
25
100
50
[200
[200
[200
[200
[200
[200
6.25
[200
[200
[200
[200
[200
[200
[200
[200
[200
[200
6.25
[200
[200
[200
[200
[200
6.25
Positive 6.25
druga
a
Ciprofloxacin hydrochloride is used as positive drug
group were preferable for the in vitro activity to those with the
aromatic substituents in this series. Among all detected
compounds, compound 10a was the most potent against the
four tested bacteria. Especially, 10a showed the MIC of
12.5 lg/mL against E. coli, which was comparable to that of
the positive drug. Additionally, 10a exhibited moderate
activity (MIC = 25 lg/mL) against Staphylococcus aureus.
Conclusion
Two series of novel compounds bearing oxazole ring have
been designed based on the peptidomimetic idea and then
synthesized. The SAR analysis showed that chelating groups
played an important role in antibacterial activity. Replace-
ment of N-formyl hydroxylamine with hydroxamic acid
group resulted in increased activity. Modification of the
substituents on the amidation group of compound 10
revealed that aliphatic substituents improved the antibacte-
rial activity significantly. Compound 10a with the dieth-
ylaminocarbonyl group attached to oxazole ring exhibited
good activity. These preliminary results are beneficial for
further studies in developing new lead compounds.
Acknowledgments This research was supported by the Foundation
for Outstanding Young Scientist in Shandong Province (BS20
09SW032), the Natural Science Foundation of Shandong Province
(2009ZRB01128) and the Scientific Technological Project in Shan-
dong Province (2009GG20002027).
A
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deformylase inhibitor. Org Process Res Dev 10:78–93
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E, Wei M, Ala PJ, Gonneville L, Taylor N, Li Y, Wynn R, Burn
TC, Liu PC, Combs AP (2007) Benzothiazole benzimidazole
(S)-isothiazolidinone derivatives as protein tyrosine phospha-
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