Practical synthesis of DQ-113, a new quinolone antibacterial agent, by using the intramolecular Horner-Wadsworth-Emmons reaction

Article abstract of DOI:10.3987/COM-03-9972

A practical route was developed for synthesizing the C-7 substituent of DQ-113 (6, 5-amino-7-[(3S,4R)-4-(1-aminocycloprop-1-yl)-3-fluoropyrrolidin-1-yl] -6-fluoro-1-[(1R,2S)-2-fluorocyclopropan-1-yl]-1, 4-dihydro-8-methyl-4-oxoquinolin-3-carboxylic acid),

Full text of DOI:10.3987/COM-03-9972

HETEROCYCLES, Vol. 63, No. 3, 2004, pp. 699 - 706
Received, 1st December, 2003, Accepted, 8th January, 2004, Published online, 16th January, 2004
PRACTICAL SYNTHESIS OF DQ-113, A NEW QUINOLONE
ANTIBACTERIAL AGENT, BY USING THE INTRAMOLECULAR
HORNER-WADSWORTH-EMMONS REACTION
Hiroaki Inagaki, Toshiyuki Takeda, Rie N. Miyauchi, Katsuhiro Kawakami,
Hisashi Takahashi, and Makoto Takemura*
Medicinal Chemistry Research Laboratory, Daiichi Pharmaceutical Co. Ltd.,
16-13, Kita-Kasai 1-Chome, Edogawa-ku, Tokyo 134-8630, Japan
E-mail: inagaeeb@daiichipharm.co.jp
Abstract – A practical route was developed for synthesizing the C-7 substituent
of
DQ-113
(6,
5-amino-7-[(3S,4R)-4-(1-aminocycloprop-1-yl)-3-fluoro-
pyrrolidin-1-yl]-6-fluoro-1-[(1R,2S)-2-fluorocyclopropan-1-yl]-1,4-dihydro-
8-methyl-4-oxoquinolin-3-carboxylic acid), a new quinolone antibacterial agent
for serious infections caused by Gram-positive pathogens. The key step was the
intramolecular Horner-Wadsworth-Emmons reaction. In addition, the yield of the
final aromatic nucleophilic substitution reaction was improved.
DQ-113
(5-amino-7-[(3S,4R)-4-(1-aminocycloprop-1-yl)-3-fluoropyrrolidin-1-yl]-6-fluoro-1-[(1R,2S)-
2-fluorocyclopropan-1-yl]-1,4-dihydro-8-methyl-4-oxoquinolin-3-carboxylic acid, 6) is a new, potent
antibacterial agent for the treatment of infections caused by Gram-positive pathogens, including
multi-drug resistant strains, and is presently being developed and prepared for clinical evaluation by
Daiichi.1,2 The C-7 substituent of DQ-113, (3S,4R)-4-(1-aminocycloprop-1-yl)-3-fluoropyrrolidine, has a
complex structure: it has two continuous asymmetric stereo centers, one of which contains a fluorine
atom. We previously reported its preparative synthesis, for which the key steps were fluorination of ethyl
1-{(3S)-5-oxo-1-[(S)-1-phenylethyl]pyrrolidin-3-yl}cyclopropanecarboxylate (2) by using lithium
diisopropylamide (LDA) and N-fluorobenzensulfonimide at –78 °C, and the following isomerization of
the resultant trans-fluorinated pyrrolidine (3) by using LDA and 2,6-di-tert-butylphenol at –78 °C
(Scheme 1).1 But this synthetic route included a lot of reaction steps and the total yield was very low. And
these two key reactions using the strong base and the expensive fluorinating agent at very low
temperature were not suitable for large scale synthesis.

H
H
F
a)
b)
EtOOC
EtOOC
EtOOC
N
N
Ph
Ph
O
H
O
O
1
2
3
NH2
Me
O
N
F
COOH
H
H
H
Boc
H
EtOOC
N
N
NH
H
N
Ph
F
F
H2N
F
H
O
F
H
5
4
6: DQ-113
Scheme 1. Preparative synthesis of DQ-113 (fluorination/isomerization strategy): a) LDA (1.3 eq.),
(PhSO2)2NF (1.6 eq.) / THF, –78°C, 2 h, then rt, 20 min; b) LDA (1.1 eq.), 2,6-di-tert-butylphenol (1.2
eq.) / THF, –78°C, 10 min, then rt, 1 h; total yield: 0.6% (16 steps).
Recently, we reported its improved synthesis by using the Reformatsky reaction between ethyl
1-acetylcyclopropanecarboxylate (1) and ethyl bromofluoroacetate (Scheme 2), which allowed us to
avoid using strong bases or expensive fluorinating agents at low temperature.3 Nevertheless, the route still
requires a multi-step sequence, and the separation of (E)-7 from (Z)-7 and the isomerization of (Z)-7 to
(E)-7 was laborious and inadequate for large scale synthesis.
a), b), c), d)
EtOOC
F
COOEt
F
COOEt
COOEt
+
EtOOC
O
Br
Br
1
(Z)-7
(E)-7
e)
g)
f)
EtOOC
N
4
5
6: DQ-113
(E)-7
Ph
F
O
8
Scheme 2. Improved synthesis of DQ-113 (the Reformatsky reaction/isomerization strategy): a)
BrFCHCO2Et (1.0 eq.), Zn (3.0 eq.), cat. I2 / benzene, reflux, 2 h; b) SOCl2 (1.2 eq.) / pyridine, –10°C, 3
h; c) DBU (1.1 eq.) / CH2Cl2, rt, 17 h; d) NBS (1.0 eq.), cat. AIBN / CHCl3, reflux, 16 h; e) cat. NBS, cat.
AIBN / benzene, reflux, 16 h; f) (S)-1-phenylethylamine (1.1 eq.), NaHCO3 (2.5 eq.) / EtOH, reflux, 3 h;
g) H2 (5 kg/cm2), Raney Ni / EtOH, rt, 3.5 h; total yield: 2.1% (14 steps).
So as not to include such separation or isomerization steps, we designed a new practical synthetic route
for the C-7 substituent (5) by using the intramolecular Horner-Wadsworth-Emmons (HWE) reaction of
fluoroketophosphonate (14) (Scheme 3).

O
P
O
P
O
P
c)
b)
a)
EtO
EtO
EtO
EtO
EtO
EtO
COCl
COOH
COOEt
BrFCHCOOEt
F
F
F
11
10
9
O
O
EtO
EtO
d)
e)
f)
g)
P
Br
COOEt
COOEt
N
1
HN
Ph
COOEt
8
O
F
O
O
Ph
12
13
14
Scheme 3. The new practical synthesis of DQ-113 (intramolecular HWE reaction strategy): a) P(OEt)3
(2.7 eq.), 160°C, 39 h (78%); b) aq. 2 N NaOH (1.2 eq.) / EtOH, rt, 2.5 h (89%); c) (COCl)2 (3.0 eq.), cat.
DMF / benzene, rt, 6 h (98%); d) Br2 (1.3 eq) / EtOH, 30°C, 80 min (quant.); e) (S)-1-phenylethylamine
(1.0 eq.), Et3N (1.05 eq.) / MeCN, 0°C, 2 h (77%); f) 11 (1.0 eq), Et3N (1.0 eq) / THF, 0°C then rt, 12 h
(69%); g) t-BuOK (1.05 eq.), MS-4A (50% w/w) / toluene, 0°C then rt, 15 h (83%).
In the intramolecular HWE reaction route (Scheme 3), fluoroketophosphonate (14), the precursor of the
key intermediate (8), was planned to be synthesized from amino keto ester (13) and acid chloride (11).
The acid chloride (11) was derived from ethyl bromofluoroacetate by an improved method of Coutrot.4
The amino keto ester (13) was synthesized from ethyl 1-acetylcyclopropanecarboxylate (1).5 Bromination
of 1 by bromine in ethanol gave α-bromo keto ester (12) (quant.). Treatment of the resultant 12 with
(S)-1-phenylethylamine in acetonitrile in the presence of triethylamine provided the amino keto ester (13)
(77%). Slow addition of 11 into a solution of 13 and triethylamine in THF below 0°C afforded the
1
condensation product (14) in 69% yield as a diastereomixture (ca. 1:1 by H NMR spectrum). The
subsequent intramolecular Horner-Wadsworth-Emmons reaction was proceeded by using potassium
tert-butoxide and powdered molecular sieves (4A) in toluene below 0°C to yield cyclic fluoro enamide
(8) (83%). Thus, we succeeded in deriving the key intermediate (8) by 4 steps of facile reactions from the
known ethyl 1-acetylcyclopropanecarboxylate (1) without separation or isomerization of isomers.
In addition to improving the synthesis of 8, we improved the synthesis of 5 from 4. We previously
reported that compound (5) could be derived from 4 by a series of reactions including thioamidation,
desulfurization, benzyloxycarbonylation, and Curtius rearrangement reaction (6 steps, total 51% yield).1
To decrease the number of reaction steps, we designed a new reaction scheme including a reduction step
with the BH3-THF complex (Scheme 4). tert-Butoxycarbonylamino compound (15)1 was derived from 4
in 73% yield by hydrolysis of the ester moiety of 4 and the Curtius rearrangement reaction of the resultant
carboxylic acid with diphenylphosphoryl azide (DPPA) and tert-butyl alcohol. Reduction of the amide
moiety of 15 with the BH3-THF complex provided N-(1-phenylethyl)pyrrolidine derivative (16) (99%).
Deprotection of the 1-phenylethyl group of 16 by catalytic hydrogenation in ethanol at 40 °C yielded the

C-7 substituent (5) in quantitative yield. Thus, we could derive 5 from 4 by 4 reaction steps in total 72%
yield.
H
H
H
H
Boc
Boc
d)
c)
a), b)
N
N
N
H
5
N
4
H
Ph
F
Ph
F
O
16
15
Scheme 4. The improved synthesis of the C-7 substituent (5) from 4: a) aq. 1 N NaOH (3.0 eq.) / EtOH,
40°C, 6 h; b) DPPA (1.0 eq.), Et3N (2.0 eq.) / toluene, rt, 1 h, and reflux, 2h, then tert-BuOH, reflux, 21 h
(73%, 2 steps); c) BH3-THF (3.5 eq.) / THF, rt, 17 h then Et3N (5 eq.) / 80% aq. EtOH, reflux, 2.5 h
(99%); d) H2, 10% Pd-C / EtOH, 40°C, 3 h (quant.).
Finally, we improved the final aromatic nucleophilic substitution reaction between the C-7 substituent (5)
and the quinolone nucleus (17). The reaction conditions reported previously (triethylamine / DMSO,
150 °C, 18 h) only gave 12% of 6.1 Because some impurities, which were thought to have resulted by a
decomposition of 5 or 17, were observed under these conditions, we explored whether milder conditions
would give no impurities. As a result, we found the conditions using 1.0 eq. of 17, 1.5 eq. of 5, and 1.2 eq.
of N-methylpiperidine in DMSO at 75°C for 7 days gave 56% of 6 from 17 (Scheme 5).
NH2
O
F
F
COOH
a) - c)
5
6: DQ-113
+
N
Me
F
17
Scheme 5. Improved reaction condition of the final aromatic nucleophilic substitution reaction: a)
N-methylpiperidine (1.2 eq.) / DMSO, 75°C, 7 days; b) conc. HCl, then extraction at pH = 7.4; c)
recrystallization from MeOH / i-PrOH (56%).
In conclusion, we developed a practical method to synthesize the C-7 substituent (5) of DQ-113 (6) by
using the intramolecular HWE reaction. In addition, we developed improved reaction conditions for the
final aromatic nucleophilic substitution reaction between the C-7 substituent (5) and the
5-amino-8-methylquinolone nucleus (17). Finally, by this method, the total yield for the synthesis of
DQ-113 (6) from 1 was improved to 11.9% (11 steps). In comparison, the yield for the
fluorination/isomerization strategy was 0.6% (16 steps), and that for the Reformatsky
reaction/isomerization strategy was 2.1% (14 steps).

EXPERIMENTAL
Unless otherwise noted, materials were obtained from commercial suppliers and used without further
purification. Optical rotations were measured in a 0.5-dm cell at 589 nm with a HORIBA SEPA-300
1
polarimeter. H NMR spectra and 13C NMR spectra were determined on a JEOL JNM-EX400
spectrometer. Chemical shifts are reported in parts per million relative to tetramethylsilane as internal
standard. High-resolution mass spectra were obtained on a JEOL JMS-700 mass spectrometer under
electron impact ionization conditions (EI), or fast atom bombardment ionization conditions (FAB).
Ethyl 1-(Bromoacetyl)cyclopropanecarboxylate (12): To a solution of ethyl 1-acetylcyclopropane-
carboxylate (1, 1.199 kg, 7.68 mol) in EtOH (6 L) was added Br2 (516 mL, 10.00 mol) dropwise over 160
min in an ice bath. After the solution was warmed at 25-30 °C for 80 min, water (6 L) was added and the
lower layer (organic layer) was separated. The upper layer was concentrated in vacuo, and extracted with
AcOEt (3 ×). The organic layers were combined and washed with aqueous 10% Na2S2O3 solution (2 ×),
saturated aqueous NaHCO3 solution, and brine. The washed organic solution was dried over anhydrous
Na2SO4 and concentrated in vacuo to give 12 (1.857 kg, quant.) as a colorless oil. 12 was used for the
next reaction without further purification. 1H NMR (CDCl3): δ 1.30 (3H, t, J = 6.6 Hz), 1.58-1.65 (4H, m),
13
4.23 (2H, q, J = 7.1 Hz), 4.50 (2H, s). C NMR (CDCl3): δ 13.9, 21.0, 35.0, 53.5, 60.5, 170.2, 197.4.
79
High-resolution MS (FAB) Calcd for C8H12O3 Br+H: 234.9970. Found: 234.9959. Calcd for
81
C8H12O3 Br+H: 234.9950. Found: 236.9948.
Ethyl 1-{N-[(S)-1-Phenylethyl]aminoacetyl}cyclopropanecarboxylate (13): To a solution of ethyl
1-(bromoacetyl)cyclopropanecarboxylate (12, 302.3 g, 1.286 mol) in acetonitrile (1.3 L) was added
(S)-1-phenylethylamine (164 mL, 1.286 mol) dropwise over 15 min below 0 °C, and then triethylamine
(188 mL, 1.350 mol) was added dropwise over 25 min below 0°C. After stirring the solution for 2 h at
0 °C, water (0.65 L) was added, and the solution was concentrated in vacuo. Then diisopropyl ether (0.8
L) was added to the concentrated solution, and the organic layer was separated. An aqueous 2 N HCl
solution (0.8 L) was added to the organic solution in order to change the desired amine compound to its
hydrochloride salt, and the aqueous layer including the salt was separated. The aqueous layer was
alkalified with an aqueous 2 N NaOH solution (ca. 0.8 L), and the resultant solution was extracted with
AcOEt (2 ×). The combined extracts were dried over anhydrous Na2SO4 and concentrated in vacuo to
give 13 (273.1 g, 77%) as a red-brown oil. 13 was used for the next reaction without further purification.
1H NMR (CDCl3): δ 1H NMR (CDCl3): δ 1.17 (3H, t, J = 7.2 Hz), 1.38 (3H, d, J = 6.6 Hz), 1.47 (4H, s),
3.72 (1H, q, J = 6.6 Hz), 3.83-3.88 (2H, m), 4.10 (2H, q, J = 7.2 Hz), 7.22-7.38 (5H, m). 13C NMR
(CDCl3): δ 13.8, 19.78, 19.83, 24.2, 33.6, 56.9, 57.9, 61.2. 126.6, 127.0, 128.4, 144.9, 170.4, 204.2.

N-[(1-Ethoxycarbonylcycloprop-1-yl)carbonylmethyl]-N-[(S)-1-phenylethyl]-2-(diethoxy-
phosphoryl)-2-fluoroacetamide (14): To a solution of ethyl 1-{N-[(S)-1-phenylethyl]aminoacetyl}-
cyclopropanecarboxylate (13, 212.3 g, 0.771 mol) in THF (1.2 L) was added a solution of
diethoxyphophorylfluoroacetyl chloride (11, 176.2 g, 0.771 mol) in THF (0.2 L) dropwise over 20 min
below 0 °C, and then triethylamine (108 mL, 0.771 mol) solution in THF (0.1 L) was added dropwise
over 20 min below 0 °C. After stirring the solution for 12 h at ambient temperature, the solution was
filtered through Celite, diluted with AcOEt, and washed with aqueous 1 N HCl solution, water, aqueous
saturated NaHCO3 solution, and brine. The resultant solution was dried over anhydrous Na2SO4 and
concentrated in vacuo. The crude product was purified by silica gel column chromatography, eluting with
1
AcOEt/hexane = 2:1 to yield 14 (244.8 g, 69%, ca. 1:1 diastereomixture) as a red-orange oil. H NMR
(CDCl3): δ 1.09-1.73 (16H, m), 4.00-4.46 (7H, m), 4.57-4.98 (1H, m), 5.07-5.31 (0.5H, m), 5.50-5.70 (1H,
m), 5.99-6.11 (0.5H, m), 7.23-7.42 (5H, m). 13C NMR (CDCl3): δ 13.75, 13.78, 13.82, 15.8, 15.9, 16.17,
16.21, 17.7, 17.9, 18.6, 18.7, 19.0, 19.9, 20.18, 20.21, 20.5, 32.8, 33.0, 33.51, 33.54, 51.8 51.9, 52.16,
52.19, 52.3, 52.46, 54.51, 54.62, 54.68, 54.79, 54.84, 61.06, 61.10, 61.4, 61.5, 64.10, 64.14, 64.26, 64.37,
64.44, 64.5, 64.6, 83.9, 84.6, 85.5, 85.9, 86.2, 86.5, 87.4, 88.1, 126.7, 127.39, 127.45, 127.48, 127.65,
127.68, 128.0, 128.2, 128.4, 128.6, 138.9, 139.7, 170.1, 170.6, 197.5, 197.7, 200.1, 200.5. High-
resolution MS (EI) Calcd for C22H31NO7FP: 471.1822. Found: 471.1838.
Ethyl 1-{4-Fluoro-2,5-dihydro-5-oxo-1-[(S)-1-phenylethyl]-1H-pyrrol-3-yl}cyclopropanecarboxylate
(8): To a suspension of N-[(1-ethoxycarbonylcycloprop-1-yl)carbonylmethyl]-N-[(S)-1-phenylethyl]-
2-(diethoxyphosphoryl)-2-fluoroacetamide (14, 244.8 g, 0.519 mol) and powdered molecular sieves 4A
(123 g) in toluene (1.1 L) was added potassium tert-butoxide (61.2 g, 0.545 mol) portionwise over 1 h
below 0 °C. After the solution was stirred for 15 h at ambient temperature, an aqueous 10% citric acid
solution was added dropwise to the solution at 0 °C. The resultant suspension was diluted with AcOEt
and filtered through Celite, and then the organic layer of the filtrate was separated and washed with water
and brine. The resultant solution was dried over anhydrous Na2SO4 and concentrated in vacuo. The crude
product was purified by silica gel column chromatography, eluting with AcOEt/hexane = 1:2 to yield 8
(135.8 g, 83%) as a red-yellow oil. The spectral data of 8 obtained by this procedure were identical to
those reported.3
(3S,4R)-4-[1-(tert-Butoxycarbonylamino)cycloprop-1-yl]-3-fluoro-1-[(S)-1-phenylethyl]pyrrolidine
(16):
To a solution of
(3S,4R)-4-[1-(tert-butoxycarbonylamino)cycloprop-1-yl]-3-fluoro-2-oxo-
1-[(S)-1-phenylethyl]pyrrolidine (15, 122.8 g, 0.339 mol) in THF (2.4 L) was added a 1 N BH3-THF
complex solution in THF (1.2 L, 1.200 mol) dropwise over 1.5 h in an ice-salt cooling bath. After the
solution was stirred for 17 h at ambient temperature, the solvent was evaporated off. Water (0.5 L), EtOH
(2 L), and triethylamine (0.25 L) were added to the residue, and the mixture was heated for reflux for 2.5

h. The solvent was evaporated off, and saturated aqueous NaHCO3 solution was added to the residue. The
resultant mixture was extracted with CHCl3 (3 ×), and the combined organic layer was washed with brine,
dried over anhydrous Na2SO4, and concentrated in vacuo. The crude product was purified by silica gel
column chromatography, eluting with AcOEt/hexane = 1:1 to yield 16 (116.3 g, 99%) as a colorless gum.
1H NMR (CDCl3): δ 0.55-0.60 (1H, m), 0.71-0.95 (3H, m), 1.34 (3H, d, J = 6.3 Hz), 1.42 (9H, s),
2.28-2.43 (2H, m), 2.52 (1H, t, J = 7.8 Hz), 2.69 (1H, dd, J = 32.7, 11.5 Hz), 3.00-3.12 (1H, m), 3.28 (1H,
13
q, J = 6.3 Hz), 5.07 (0.8H, br s), 5.13 (1H, dm, J = 56.2 Hz), 7.20-7.29 (5H, m). C NMR (CDCl3): δ
12.0, 12.8, 22.7, 28.3, 30.7 (d, J = 7.5 Hz), 48.3 (d, J = 17.6 Hz), 52.4, 59.1 (J = 22.6 Hz), 65.1, 79.0,
94.9 (J = 178.1 Hz), 127.0, 127.1, 128.3, 144.8, 155.6. High-resolution MS (EI) Calcd for C20H29N2O2F:
348.2213. Found: 348.2218. [α]D -57.3° (c 0.908, CHCl3).
5-Amino-7-[(3S,4R)-4-(1-aminocycloprop-1-yl)-3-fluoropyrrolidin-1-yl]-6-fluoro-1-[(1R,2S)-
2-fluorocycloprop-1-yl]-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic Acid (3 = DQ-113): To
a solution of (3S,4R)-4-[1-(tert-butoxycarbonylamino)cycloprop-1-yl]-3-fluoro-1-[(S)-1-phenylethyl]-
pyrrolidine (16, 523 mg, 1.50 mmol) in EtOH (15 mL) was added 10% Pd/C (520 mg, containing 50.2%
water), and the suspension was stirred vigorously at 40 °C for 3 h under hydrogen atmosphere. The
catalyst was filtered off, and the filtrate was concentrated in vacuo to give a crude of 7 (366 mg, quant.)
as a colorless gum. The crude 5 was used for the next reaction without further purification.
The crude 5 (366 mg, 1.50 mmol), 5-amino-6,7-difluoro-1-[(1R,2S)-2-fluorocycloprop-1-yl]-1,4-dihydro-
8-methyl-4-oxoquinoline-3-carboxylic acid (17, 312 mg, 1.00 mmol), N-methylpiperidine (0.146 mL,
1.20 mmol), and DMSO (1.7 mL) were mixed and the mixture was heated at 75 °C for 7 days under
nitrogen atmosphere. The solvent was removed in vacuo, and the residue was dissolved with CHCl3, and
the solution was washed with aqueous 10% citric acid solution. The resultant solution was dried over
anhydrous Na2SO4 and concentrated in vacuo. The obtained crude Boc-DQ-113 was roughly purified by
silica gel chromatography, eluting with CHCl3/MeOH = 10:1 to give 520 mg of Boc-DQ-113.
To the roughly purified Boc-DQ-113 (520 mg) was added concentrated aqueous HCl solution (10 mL) at
0 °C, and the mixture was stirred for 20 min at ambient temperature. The solution was washed with
CHCl3, then alkalified with saturated aqueous NaOH solution at 0 °C, and the solution was washed with
CH2Cl2. After the pH of the solution was adjusted to 7.4 with concentrated aqueous HCl solution and
aqueous 1 N HCl solution, the aqueous solution was extracted with CHCl3 (2 ×), and the combined
organic layer was dried over anhydrous Na2SO4 and concentrated in vacuo. The obtained crude product
was purified by recrystallization from MeOH/i-PrOH to yield 6 (= DQ-113, 243 mg, 56%) as a yellow
powder. The spectral data of 6 obtained by this procedure were identical to those reported.1

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