Mechanistic studies on regioselective dephosphorylation of phosphate prodrugs during a facile synthesis of antitumor phosphorylated 2-phenyl-6,7-methylenedioxy-1H-quinolin-4-one

Article abstract of DOI:10.3390/molecules18078028

Phosphorylation of 2-(3-hydroxy-5-methoxyphenyl)-6,7-methylenedioxy- 1Hquinolin- 4-one (1) afforded diphosphate 2. We found that, upon treatment with methanol under mild conditions, 2 can undergo facile and highly regioselective dephosphorylation to give

Full text of DOI:10.3390/molecules18078028

Molecules 2013, 18, 8028-8045; doi:10.3390/molecules18078028
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Mechanistic Studies on Regioselective Dephosphorylation of
Phosphate Prodrugs during a Facile Synthesis of Antitumor
Phosphorylated 2-Phenyl-6,7-methylenedioxy-1H-quinolin-4-one
Yung-Yi Cheng 1, Chin-Yu Liu 1, Li-Jiau Huang 1, Chi-Hung Huang 2, Kuo-Hsiung Lee 3,4
,
Cheng-Tung Lin 5,* and Sheng-Chu Kuo 1,4,
*
1
Graduate Institute of Pharmaceutical Chemistry, China Medical University,
No.91 Hsueh-Shih Road, Taichung, 40402, Taiwan
2
3
4
5
Graduate School of Biotechnology, Hung Kuang University, Taichung, No. 1018, Sec. 6,
Taiwan Boulevard, Shalu District, Taichung, 43302, Taiwan
Natural Products Research Laboratories, UNC Eshelman School of Pharmacy,
University of North Carolina, Chapel Hill, NC 27599, USA
Chinese Medicine Research and Development Center, China Medical University and Hospital,
2 Yuh-Der Road, Taichung, 40447, Taiwan
Department of Chemistry, Science College, Tunghai University, No.1727, Sec.4,
Taiwan Boulevard, Xitun District, Taichung, 40704, Taiwan
* Authors to whom correspondence should be addressed; E-Mails: sckuo@mail.cmu.edu.tw (S.-C.K.);
ctlin@mail.thu.edu.tw (C.-T.L.); Tel./Fax: +886-4-2203-0760 (S.-C.K.);
Tel.: +886-4-2359-0248 (ext. 401) (C.-T. L.).
Received: 13 May 2013; in revised form: 5 June 2013 / Accepted: 1 July 2013 /
Published: 8 July 2013
Abstract: Phosphorylation of 2-(3-hydroxy-5-methoxyphenyl)-6,7-methylenedioxy-1H-
quinolin-4-one (1) afforded diphosphate 2. We found that, upon treatment with methanol
under mild conditions, 2 can undergo facile and highly regioselective dephosphorylation to
give the monophosphate 3, with a phosphate group remaining on the phenyl ring. The
details of the dephosphorylation process were postulated and then probed by LC-MS and
HPLC analyses. Furthermore, as a preliminary study, the water soluble monophosphate
prodrug 4 was tested for antitumor activity against a MCF-7 xenograft nude mice model.
Keywords: MCF-7; prodrugs; dephosphorylation; antitumor activity

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1. Introduction
Numerous phosphoric esters have been developed as potential water-soluble prodrugs [1–12]. Many
of them are marketed as injectable dosage forms, while only a few are used as oral dosage forms [11–13].
The phosphoric moiety is introduced into a parent drug molecule, which contains at least one hydroxyl
functional group, in order to improve water solubility of the parent drug [9–13]. After administration,
this phosphoric substituent will be cleaved enzymatically by endogenous phosphatases relatively
easily [7–14]. In general, a parent drug incorporating a single phosphoric moiety has sufficient water
solubility for use in an injectable dosage. Incorporation of more than one phosphoric moiety has no
advantage, since the extra phosphoric moieties must also be removed by phosphatases. Furthermore,
any parent drug still containing a residual phosphoric moiety will be highly polar and easily excreted
prematurely through the GI or urinary system.
We recently reported a series of substituted 2-phenyl-1H-quinolin-4-ones and identified them as a
new class of anticancer drugs [15–19]. In the course of synthesis, we found that phosphorylation of
2-(3-fluorophenyl)-1H-quinolin-4-one (A) with tetrabenzylpyrophosphate and NaH or K2CO3 in
tetrahydrofuran solution afforded diphosphate C. The monophosphate B was obtained from the
regioselective dephosphorylation of diphosphate C in methanol solution (Figure 1). Consequently, we
were prompted to investigate the importance of methanol in the cleavage process and further evaluate
the scope and mechanistic aspects of the reaction.
Figure 1. Structures of compounds A–C.
Previously, we reported that 1 acts as an antitumor agent by inducing both extrinsic and intrinsic
apoptotic pathways via ROS-mediated activation of p38 MAPK signaling in HL-60 human leukemia
cells in vitro [20]. In the present study, we selected the hydrophilic monophosphate of 1 as a target
compound. This prodrug should be converted readily to the parent molecule in the bloodstream or GI
tract by reaction with phosphatases. A similar strategy has successfully improved the clinical usage of
etoposide, estramustine, and combretastin A-4 [21–25].
2. Results and Discussion
2.1. Chemistry
As part of our ongoing research on phosphate prodrugs, we wanted to prepare
3-(6,7-methylenedioxy-4-oxo-1,4-dihydroquinolin-2-yl)-5-methoxyphenyl dihydrogen phosphate (4),
which contains a phosphate substituent on the 2-phenyl ring. We expected that this kind of analogue

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would have similar antitumor activity to related 2-phenyl-1H-quinolin-4-one anticancer drugs. In
principle, phosphate 4 can be prepared easily from the appropriately substituted benzoic ester by a
similar synthetic method as previously reported for 2-phenylquinolin-4-one derivatives [15−19].
Herein, we wish to detail our observations dealing with the dephosphorylation mechanism of
diphosphate 2 in methanol solution, including the time period for the release of the phosphoric
substituent from the dibenzyl 3-(4-((bis(benzoxy)phosphoryl)oxy)-6,7-methylenedioxyquinolin-4-yl)-
5-methoxyphenyl phosphate (2) (Figure 2).
Figure 2. Structures of compounds 1−4.
The syntheses of 1–4 are illustrated in Scheme 1. Initially, benzoic ester 5 was synthesized
according to reported methods [26–28]. Saponification of 5 followed by acidification gave the
corresponding benzoic acid 6. Subsequent chlorination with thionyl chloride afforded 3-benzoxy-5-
methoxybenzoyl chloride (7). Without purification, the resulting compound 7 was reacted with 2-
amino-4,5-methlenedioxyacetophenone (8) to give the desired amide 9, which was then cyclized in the
presence of sodium hydroxide in refluxing dioxane solution to yield 2-(3-benzoxy-5-methoxyphenyl)-
6,7-methylenedioxy-1H-quinolin-4-one (10). Catalytic hydrogenolysis of the resulting 10 with
palladium on activated charcoal gave the key intermediate 2-(3-hydroxy-5-methoxyphenyl)-6,7-
methylenedioxy-1H-quinolin-4-one (1). As expected, phosphorylation of 1 with two equivalents of
tetrabenzyl pyrophosphate and sodium hydride in tetrahydrofuran solution provided the crystalline
diphosphate 2 in 74% yield.
Scheme 1. Synthesis of compounds 1–4.

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Bis(dibenzyl phosphate) 2 was stable in acetonitrile/acetone solution at 25 ± 2 °C. However, it
decomposed slowly in anhydrous methanol to form dibenzyl phosphate 3, bis-phosphate derivatives 13
and 14, and benzyl phosphate 15, together with dibenzyl methyl phosphate (11) and benzyl methyl
ether (12), as shown in Scheme 2.
Scheme 2. The pathways for dephosphorylation of diphosphate 2.
To investigate the reaction mechanism, all products were isolated in sufficient purity from the
reaction mixture by using semi-preparative reversed phase column chromatography. LC-MS analyses
of the reaction products are shown in Table 1. A close examination of molecular ion differences from
the LC-ESIMS spectra over time revealed that benzyl groups are removed by methanol by the
conversion of 3→15 in pathway A and of 2→13 plus 13→14 in pathway B. Additionally, a
dibenzylphosphoryl group is removed in the transformation of 2→3. To further establish the
dephosphorylation pathways of diphosphate 2, the product distribution was then monitored at different
time intervals using HPLC analysis. The results are shown in Figure 3. The methanolysis of 2 was
complete after 96 h and gave dibenzyl phosphate 3 and benzyl hydrogen phosphate 15 as the major
detectable products. Dibenzyl phosphate 3 reached its highest detected distribution of 60% after 48 h.
However, the isolated yield of 3 using flash column chromatography was 49% after 48 h. The
formation of intermediate 13 increased at the beginning of the reaction and then decreased after 24 h.
Both 13 and 14 disappeared completely after 156 h. Compound 15 is not as soluble as 2 and 3 in

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methanol, and after a few days, 15 precipitates slowly and influences the yields detected by HPLC.
Therefore, after the reaction has proceeded for 96 h, a reduction of 3 does not cause an increase of 15.
Table 1. LC-MS analyses of the reaction products.
Compounds tR (min) HRESIMS a (m/z) ESIMS a (m/z)
Formula
C45H40NO11P2
C31H27NO8P
C15H17O4P
2
63.4
46.9
46.5
49.8
39.8
36.3
38.3
832.1916
572.1449
ND b
832
572
293
123
742
652
482
3
11
12
13
14
15
ND b
C8H10O
742.1568
652.1158
482.0988
C38H34NO11P2
C31H28NO11P2
C24H21NO8P
a
Electrospray ionization mass spectrometer (ESIMS) and micrOTOF (HRESIMS) were operated in the
positive mode [M+H]+ with full scan. b Not detected.
Figure 3. Distribution of products in the decomposition of 2 at various reaction times.
When compound 3 was isolated and re-subjected to the same reaction conditions, the
decomposition products 12 and 15 were detected after 3 days, confirming that 15 was sufficiently
stable in methanol solution. In an independent experiment, decomposition of 13 with methanol gave
dephosphorylated product 15 regioselectively and phosphate 11 in 85% yield, accompanied with a
small amount of 14. Therefore, the removal of benzyl methyl hydrogen phosphate 16 via the
decomposition of 14 was not detected successfully.
The structures of the isolated products 3, 13, 14, and 15, which were derived from decomposition of
2, were established by mass and NMR spectroscopy analyses. Since the chemical shifts of the benzylic
hydrogens in structure 2 are very close (absorptions at δ 5.28 and 5.22), we are unable to distinguish

Molecules 2013, 18
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the dibenzylphosphoryl group attached on the 1H-quinolin-4-one skeleton from that on the phenyl ring
by proton NMR spectroscopy. This same phenomenon happens in the carbon NMR absorptions; the
benzylic carbons appear at δ 70.1 and 70.6. These results limited our ability to determine the position
of the dibenzylphosphoryl groups in structure 13. Cleavage of a benzyl group from either one of the
dibenzylphosphoryl groups of 2 to form a benzyl hydrogen phosphoryl group will shift the benzylic
hydrogens’ absorption from δ 5.28 or δ 5.22 downfield to δ 5.07. Similarly, the benzylic carbon will
shift from δ 70.1 or δ 70.6 downfield to δ 67.6. With benzylic absorptions at δ 67.6 and 67.7, the
structure of 14 is confirmed to have two benzyl hydrogen phosphoryl groups attached on the
2-phenylquinolin-4-one skeleton.
The uncertainty of determining whether a dibenzylphosphoryl group is attached on the quinoline
moiety, as in 13, or on the phenyl ring, as in 13a, prompted us to use 31P-NMR spectroscopy.
31
Comparison of the P-NMR proton decoupled spectra of 3 and 15 showed that, if a benzyl group is
cleaved from the dibenzylphosphoryl group attached on the 2-phenyl ring to leave a benzyl hydrogen
phosphoryl group, the phosphorus-31 signal will shift from δ −6.61 to δ −4.73. The 31P-NMR spectrum
obtained for 14 exhibited two phosphorus signals at δ −4.82 and δ −5.44, suggesting that the
phosphorus atom of the benzyl hydrogen phosphoryl group attached on the quinolin-4-one ring absorbs
at δ −5.44. On the basis of these data, we were able to confirm that a signal at δ −5.14 in the 31P-NMR
spectrum of 13 belongs to the phosphorus atom in the benzyl hydrogen phosphoryl group attached
on 2-phenyl ring, while a second absorption at δ −6.87 must be due to the dibenzylphosphoryl
group attached on the quinolin-4-one ring. Therefore, the structure of 13 was identified as
benzyl
3-(4-((bis(benzyloxy)phosphoryl)oxy)-6,7-methylenedioxyquinolin-2-yl)-5-methoxyphenyl)
hydrogen phosphate.
With this information in hand, a possible mechanism is shown in Scheme 2. Two pathways leading
to the formation of 15 are suggested. Due to the electronic and resonance effects, a dibenzylphosphoryl
group attached on the quinolin-4-one ring has been reported to undergo decomposition much faster
than one on the phenyl ring in methanol solution [6]. Thus, the decomposition of diphosphate 2 in
methanol solution is expected to proceed by an addition-elimination reaction on the
dibenzylphosphoryl group of the quinolin-4-one skeleton and give monophosphate 3 as the only
product. One of the benzyl groups in monophosphate 3 is then slowly cleaved by methanol to give 15,
which is highly stable in solution at ambient temperature. The attack of methanol on the benzylic
carbon atom of a phosphate ester is unusual and has never been reported to our knowledge. The
proposed mechanism of nucleophilic substitution was supported by the detection of benzyl methyl
ether in the reaction mixture.
In the second pathway leading to 15, one of the two benzyl groups in the dibenzylphosphoryl group
attached on the phenyl ring of diphosphate 2 undergoes nucleophilic substitution by methanol to form
compound 13. Because benzyl methyl hydrogen phosphate 16 was not detected in the HPLC data and
the yield of 14 was low (Figure 3), compound 15 should be derived predominately from 13 by
dephosphorylation. Finally, hydrogenolysis of 3 or 15, as well as diphosphate 2, in the presence of
palladium on charcoal produces 3-(6,7-methylenedioxy-4-oxo-1,4-dihydroquinolin-2-yl)-5-methoxyphenyl
dihydrogen phosphate (4) in high yield as a stable and water soluble prodrug of the antitumor agent 1.

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2.2. In Vitro Biological Evaluation
The synthesized 2-(3-hydroxy-5-methoxyphenyl)-6,7-methylenedioxy-1H-quinolin-4-one (1) was
evaluated for cell antiproliferative activity against human Hep3B hepatoma, Colo205 colon carcinoma,
A498 renal carcinoma, NCI-H460 lung cancer, and Detroit 551 embryonic fibroblast cell lines.
Compound 1 exhibited no significant cytotoxic activity against these five cell lines (IC50 > 50 μM).
Compound 1 was submitted to US-NCI for evaluation of growth inhibitory activity against the NCI
human cancer cell line panel. The results are shown in Table 2. The mean of logGI50 value of
compound 1 was −4.73, indicating weak inhibitory activity against most cancer cell lines. In
preliminary screening against 60 human cell lines, 1 demonstrated high selective inhibitory activity
against NCI-H522 (non-small cell lung cancer), OVCAR-3 (ovarian cancer), K562 (leukemia) and
MCF-7 (breast cancer) cell lines, showing logGI50 values of −6.70, −6.55, −6.47, and −6.39 (Figure 4,
Supplementary Data).
Table 2. Inhibition of in vitro tumor cell growth by compound 1.
Cell lines
K562
logGI50
−6.47
−6.70
−6.55
−6.39
logTGI
>−4.00
−6.33
logLC50
>−4.00
>−4.00
>−4.00
>−4.00
NCI-H522
OVCAR-3
MCF-7
−6.13
>−4.00
K562, leukemia cell line; NCI-H522, non-small cell lung cancer cell line; OVCAR-3, ovarian cancer cell
line; MCF-7, breast cancer cell line. 2. GI50 = the concentration that causes 50% growth inhibition; TGI = the
concentration that causes total growth inhibition; LC50 = the dosage of a given drug required to kill 50% of a
test population.
Figure 4. Dose-response curves of compound 1 against the sensitive cell lines.
2.3. In Vivo Antitumor Activity
Based on the NCI-60 cell line screening results of 1, we selected the MCF-7 xenograft model using
dosing at 90 mg/kg (i.v.) to evaluate the in vivo antitumor activity of monophosphate prodrug 4
(Figure 5A−C). According to the results shown in Figure 5A, prodrug 4 induced time-dependent

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inhibition of MCF-7 tumor growth. During the course of antitumor evaluation, no significant body
weight changes were detected in either the tested or the control mice (Figure 5C).
Figure 5. (A) Mean tumor volume-time profiles (B) Mean tumor weight-time profiles
(C) Mean body weight-time profiles in MCF-7 xenograft nude mice (n = 4) following i.v.
dosing of 4 at 90 mg/kg 5 days per week for 4 consecutive weeks. * p < 0.05 vs. control.
3. Experimental
3.1. Synthesis
3.1.1. General
Commercial reagents were used without purification. TLC was performed on precoated silica gel 60
F254 (Merck) and spots were visualized by UV light at 254 nm. Silica gel 60 (Merck 70–230 mesh) was
used for flash column chromatography. Melting points were determined on a Yanaco MP-500D
apparatus and uncorrected. IR spectra were recorded on a Shimadzu IR Prestige-21 FTIR-8400S
spectrophotometer. The one- and two-dimensional NMR spectra were recorded on either Bruker 500
AV II or Avance DPX-200 FT-NMR spectrometers at room temperature on solutions in CDCl3, D2O,
1
CD3OD, or DMSO-d6. TMS was used as the internal standard for the H and 13C-NMR, and
phosphoric acid was used as the external standard for 31P-NMR. HPLC was performed on a Shimadzu
LC-10AT apparatus equipped with Shimadzu SPD-M10AVP diode-array detector and Shimadzu and
Shimadzu SIL-10A auto-injector. Phenomenex Prodigy ODS3 100A, (5 μm, 250 × 4.6 mm i.d),

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Nucleodur® C18 HTec (5 µm, 250 × 4.6 mm i.d), semi-preparative Nucleodur® C18 HTec (5 µm,
250 × 10 mm i.d) and Thermo Hypersil ODS (5 µm,150 × 4.6 mm i.d) columns were used for
analytical and preparative purposes, respectively. EIMS spectra were measured with HP 5995 GC–MS.
The ESIMS spectra were obtained using the LC-ESIMS was performed on Agilent 1100 apparatus
equipped with ESI mass spectrometer and with ultraviolet detection. The LC-ESI-HRMS spectra were
obtained using LC-ESI-HRMS performed on an Agilent 1100 apparatus equipped with a Bruker
micrOTOF orthogonal ESI-TOF mass spectrometer.
3.1.2. Synthesis of Compounds 5−7 and 9
Methyl 3-(benzoxy)-5-methoxybenzoate (5, 4.45 g, 16.34 mmol) was dissolved in a solution of
95% ethanol (120 mL) and water (5 mL). Sodium hydroxide (2.00 g, 50.00 mmol) was added, and the
reaction mixture was heated under reflux for one hour. After the reaction mixture was evaporated
under vacuum, the residue was quenched with water (150 mL). The solution was neutralized with
dilute HCl, and the precipitate was collected and washed with water and acetone to give 6 (3.79 g,
90%). Compound 6 (2.57 g, 9.95 mmol), thionyl chloride (4.80 g, 28.07 mmol) and N,N-dimethyl
formamide (3 drops) were added in dry toluene (200 mL) and stirred at room temperature. The reaction
mixture was stirred for 24 h and then evaporated to dryness. The crude product was used directly in the
next step without further purification. Compounds 7 (2.77 g, 10.01 mmol) and 8 (1.79 g, 9.96 mmol)
were suspended in dry THF (200 mL) and triethylamine (10 mL). The mixture was stirred at room
temperature for 24 h and then evaporated. The residue was purified by silica gel column
chromatography with CH2Cl2/EtOAc = 3:1 to obtain 9.
N-(6-Acetylbenzo[d][1,3]dioxol-5-yl)-3'-(benzoxy)-5'-methoxy-benzamide (9). Light yellow solid
1
(3.15g, 75%). Mp: 151–152 °C. H-NMR (500 MHz, DMSO-d6): δ 2.64 (s, –COCH3,3H), 3.84 (s,
–OCH3, 3H), 5.20 (s, –OCH2Ph, 2H), 6.19 (s, –OCH2O–, 2H), 6.88 (s, H-4, 1H), 7.09 (s, H-6, 1H),
7.16 (s, H-2, 1H), 7.36 (d, J = 7.43 Hz, ArH, 1H), 7.43 (t, J = 7.43 Hz, ArH, 2H), 7.49 (d, J = 7.43 Hz,
ArH, 2H), 7.68 (s, H-5′, 1H), 8.34 (s, H-2′, 1H), 12.85 (s, -NH, 1H); 13C-NMR (125 MHz, DMSO-d6):
δ 29.3 (–COCH3), 56.0 (–OCH3), 70.1 (–OCH2Ph), 100.8 (C-2′), 102.9 (–OCH2O–), 105.0 (C-4),
105.7 (C-6), 106.5 (C-2), 111.3 (C-5′), 116.5 (C-6′), 128.5 (–OCH2Ph), 128.6 (–OCH2Ph), 129.0
(–OCH2Ph), 136.9 (C-1, and –OCH2Ph), 138.3 (C-1′), 143.1 (C-4′), 152.7 (C-3′), 160.3 (C-3), 161.2
(C-5), 165.0 (–NHCO), 201.8 (–COCH3). IR: ν 3471, 2939, 2883, 2640, 1683, 1593, 1456, 1431,
1350, 1301, 1269, 1203, 1159, 1060, 1033, 952, 846, 732 cm−1. EIMS (70eV) m/z: 419.1 [M]+;
LC-ESI-HRMS (Positive mode) m/z: [M+H]+ calcd for C24H22NO6, 420.1442; found 420.1438.
Methyl 3-(benzoxy)-5-methoxybenzoate (5). Colorless oil. 1H-NMR (200 MHz, CDCl3): δ 3.79 (s, 3H),
3.88 (s, 3H), 5.05 (s, 2H), 6.70 (t, J = 2.5 Hz, 1H), 7.18 (dd, J = 2.5, 1.2 Hz, 1H), 7.26 (dd, J = 2.5,
13
1.2 Hz, 1H), 7.29-7.46 (m, 5H); C-NMR (50 MHz, CDCl3): δ 52.2, 55.5, 70.2, 106.5, 107.5, 108.0,
127.5, 128.1, 128.6, 133.4, 136.5, 159.6, 160.6, 166.8. EIMS (70eV) m/z: 272.1 [M]+.
1
3-(Benzyloxy)-5-methoxybenzoic acid (6). Colorless oil. H-NMR (200 MHz, CDCl3): δ 3.81 (s, 3H),
13
5.07 (s, 2H), 6.76 (t, J = 2.5 Hz, 1H), 7.25 (dd, J = 2.5, 1.2 Hz, 1H), 7.30–7.50 (m, 5H); C-NMR

Molecules 2013, 18
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(50 MHz, CDCl3): δ 55.9, 69.9, 106.2, 107.6, 108.2, 128.1, 128.3, 128.9, 133.3, 137.2, 159.9, 160.8,
167.3. EIMS (70eV) m/z: 258.1 [M]+.
3.1.3. Synthesis of Compound 10
A mixture of 9 (3.33 g, 7.95 mmol) and NaOH (2.50 g, 62.50 mmol) was suspended in 1,4-dioxane
(200 mL). The reaction mixture was refluxed for 12 h. After cooling to room temperature, the mixture
was evaporated and then the residue was added to 10% NH4Cl solution (100 mL). The precipitate was
collected and washed with water and acetone. The residue was purified by silica gel column
chromatography with EtOAc to obtain 10.
2-(3-Benzoxy-5-methoxyphenyl)-6,7-methylenedioxy-1H-quinolin-4-one (10). Gray-white solid
(5.21 g, 75%). Mp: 238–239 °C.1H-NMR (500 MHz, DMSO-d6): δ 3.85 (s, –OCH3, 3H), 5.22 (s,
–OCH2Ph, 2H), 6.16 (s, –OCH2O–, 2H), 6.31 (s, br, H-3, 1H), 6.79 (s, H-4', 1H), 6.95 (s, H-6', 1H),
7.04 (s, H-2', 1H), 7.21 (s, H-8, 1H), 7.36 (m, ArH, 1H), 7.40–7.44 (m, ArH and H-5, 2H), 7.49–7.50
(m, 2H, ArH), 11.50 (s, 1H, –NH); 13C-NMR (125 MHz, DMSO-d6,): δ 56.0 (–OCH3), 70.1
(–OCH2Ph), 97.7 (C-8), 101.8 (C-5), 102.4 (–OCH2O–), 103.2 (C-4'), 106.0 (C-6'), 106.6 (C-2'), 107.2
(C-3), 120.5 (C-4a), 128.3 (–OCH2Ph), 128.4 (–OCH2Ph), 129.0 (–OCH2Ph), 137.3 (C-8a, C-1', and
–OCH2Ph), 140.7 (C-2), 145.7 (C-6), 151.6 (C-7), 160.3 (C-3'), 161.2 (C-5'), 176.6 (C-4). IR: ν 3242,
3132, 3057, 2966, 1604, 1589, 1521, 1471, 1321, 1265, 1195, 1163, 1045, 941, 855, 842 cm−1. EIMS
(70 eV) m/z: 401.1 [M]+; LC-ESI-HRMS (Positive mode) m/z: [M+H]+ calcd for C24H20NO5, 402.1336;
found 420.1329.
3.1.4. Synthesis of Compound 1
A suspension of 0.50 mg (1.25 mmol) of 10 and 0.25 mg of palladium (10 wt% on activated
carbon) in methanol (60 mL) was stirred at room temperature under hydrogen gas atmosphere for 24 h.
The precipitate was collected and dissolved in 10% NaOH solution and then filtered. The
filtrate was acidified with dil HCl and the precipitate was then collected and washed with
acetone and water to obtain 1.
2-(3-Hydroxy-5-methoxyphenyl)-6,7-methylenedioxyquinolin-1H-4-one (1). White solid (0.30 mg, 77%).
1
Mp: >300 °C. H-NMR (500 MHz, DMSO-d6): δ 3.81 (s, –OCH3, 3H), 6.16 (s, –OCH2O–, 2H), 6.24
(s, br, H-3, 1H), 6.52 (s, H-4′, 1H), 6.77 (s, H-6′, 1H), 6.78 (s, H-2′, 1H), 7.22 (s, H-8, 1H), 7.40 (s,
H-5, 1H), 9.91 (s, –OH, 1H), 11.56 (s, br, –NH, 1H); 13C-NMR (125 MHz, DMSO-d6): δ 55.8
(–OCH3), 97.8 (C-8), 101.5 (C-5), 102.4 (–OCH2O–), 103.3 (C-4′), 104.3 (C-6′), 106.7 (C-2′), 107.2
(C-3), 120.8 (C-4a), 136.8 (C-1′), 137.8 (C-8a), 145.7 (C-6), 149.3 (C-2), 151.6 (C-7), 159.4 (C-3′),
161.2 (C-5′), 176.1 (C-4). IR: ν 3383, 3250, 3153, 3010, 2974, 2839, 2704, 1635, 1616, 1591, 1531,
1496, 1489, 1471, 1429, 1338, 1315, 1265, 1199, 1165, 1056, 1035, 931, 854 cm−1. LC-ESI-MS
(Positive mode) m/z: 312 [M+H]+; LC-ESI-HRMS (Positive mode) m/z: [M+H]+ calcd for C17H13NO5,
311.0794; found, 311.0800. The purity analysis was detected by reversed-phase HPLC on Thermo
Hypersil ODS column (150 × 4.6 mm i.d) using an acetonitrile/0.02 M NaHCO3 (70:30) mixture as

Molecules 2013, 18
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eluent. Flow rate was 0.3 mL/min and UV detector was set at 254 nm. The retention time of 1 was
5.4 min. The purity of 1 was 97.0%.
3.1.5. Synthesis of Compound 2
To a stirred solution of 1 (0.41 g, 1.32 mmol) in dry THF (40 mL) was added NaH (60% in mineral
oil, 0.36 g) at 0 °C. After the mixture was stirred for 5 min, tetrabenzyl pyrophosphate (1.44 g, 2.68 mmol)
was added and stirring was continued for 10 min. The reaction mixture was filtered and washed with
THF. The filtrate was concentrated under vacuum at a temperature below 30 °C. The residue was
purified by silica gel column chromatography with an eluent of n-hexane-EtOAc = 1:1 to obtain 2.
Dibenzyl
3-(4-((bis(benzoxy)phosphoryl)oxy)-6,7-methylenedioxyquinolin-4-yl)-5-methoxyphenyl
1
phosphate (2). White solid (0.82 g, 74%). Mp: 80–81 °C. H-NMR (500 MHz, CD3OD): δ 3.84 (s,
3
3
–OCH3, 3H), 5.21 (d, JHP = 9.4 Hz, –OPO(OCH2Ph)2, 2H), 5.22 (d, JHP = 9.0 Hz,–OPO(OCH2Ph)2,
3
3
2H), 5.27 (d, JHP = 10.7 Hz, –OPO(OCH2Ph)2, 2H), 5.28 (d, JHP = 9.9 Hz, –OPO(OCH2Ph)2, 2H),
6.20 (s, –OCH2O–, 2H), 6.81 (s, H-6, 1H), 7.07 (s, H-3′, 1H), 7.28–7.40 (m, ArH, H-8′, H-2 and H-4,
22H), 7.46 (s, H-5′, 1H). 1H-NMR (200 MHz, CDCl3): δ 3.77 (s, –OCH3, 3H), 5.13 (d, 3JHP = 8.3 Hz,
–OPO(OCH2Ph)2, 4H), 5.16 (d, 3JHP = 9.5 Hz,–OPO(OCH2Ph)2, 4H), 6.09 (s, –OCH2O–, 2H), 6.78 (m,
13
H-6, 1H), 7.10 (s, H-3′, 1H), 7.23–7.40 (m, ArH, H-8′, H-2, H-4, and H-5′, 22H); C-NMR (50 MHz,
2
CDCl3): δ 55.6 (–OCH3), 70.1 (–OPO(OCH2Ph)2), 70.6 (d, JC-P = 5.0 Hz, –OPO(OCH2Ph)2), 97.2
(C-5′), 101.9 (–OCH2O–), 106.1 (C-8′), 106.6 (C-3′), 106.7 (d, 3JC-P = 5.0 Hz, C-6), 110.0 (C-4), 111.5
(d, 3JC-P = 5.0 Hz, C-2), 117.45 (d, 3JC-P = 5.0 Hz, C-4a′), 128.1 (–OCH2Ph), 128.1 (–OCH2Ph), 128.6
3
(–OCH2Ph), 128.7 (–OCH2Ph), 128.9 (–OCH2Ph), 135.0 (d, JC-P = 5.0 Hz, –OCH2Ph), 135.5 (d,
3JC-P = 5.0 Hz, –OCH2Ph), 141.5 (C-3), 148.2 (C-6′, and C-8a′), 151.5 (C-7′), 151.7 (d, 3JC-P = 5.0 Hz,
C-1), 153.8 (d, 2JC-P = 5.0 Hz, C-4′), 154.9 (C-2′), 160.8 (C-5); 31P-NMR (202.4947 MHz, CD3OD): δ
−6.61, −6.77. IR: ν 3089, 3064, 3032, 2953, 2927, 2891, 2360, 1614, 1581, 1490, 1456, 1352, 1292,
1265, 1199, 1020, 962, 896, 746, 696 cm−1. LC-ESI-MS (Positive mode) m/z: 832 [M+H]+; LC-ESI-
HRMS (Positive mode) m/z: [M+H]+ calcd for C45H40NO11P2, 832.2071; found, 832.1916. The purity
analysis was detected by reversed-phase HPLC on Nucleodur® C18 HTec (5 µm, 250 × 4.6 mm i.d)
using a MeOH/0.02 M NaHCO3 (93:7) mixture as eluent. The flow rate was 0.5 mL/min and UV
detector was set at 254 nm. The retention time of 2 was 11.2 min; the purity of 2 was 99.6%.
3.1.6. Synthesis of Compounds 3 and 11
A suspension of 2 (0.92 g, 1.11 mmol) in 100 mL of methanol was stirred at 25 °C for 48 h. The
precipitate was collected and purified by silica gel column chromatography (EtOAc) to give 2 (0.05 g,
5%), 3 (0.31 g, 49%) and dibenzyl methyl phosphate 11 (0.52 g, 40%).
Dibenzyl 3-(6,7-methylenedioxyquinolin-2-yl)-5-methoxyphenyl phosphate (3). Compound 3
was obtained as a white solid. Mp: 108–109 °C. 1H-NMR (500 MHz, CDCl3): δ 3.66 (s, –OCH3, 3H),
5.10 (d, 3JHP = 9.1 Hz, –OPO(OCH2Ph)2, 4H), 6.01 (s, –OCH2O–, 2H), 6.37 (s, H-3′, 1H), 6.62 (s, H-6,
1H), 6.98 (s, H-8′, 1H), 7.16 (s, H-2, 1H), 7.28–7.40 (m, ArH and H-4, 11H), 7.61 (s, H-5′, 1H);
13C-NMR (125 MHz, CDCl3): δ 55.6 (–OCH3), 70.4 (d, 2JC–P = 6.0 Hz, –OPO(OCH2Ph)2), 97.3 (C-5′),

Molecules 2013, 18
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3
101.8 (–OCH2O–), 102.3 (C-8′), 107.6 (C-3′), 107.8 (d, JC–P = 4.2 Hz, C-2), 109.9 (C-6), 110.9 (d,
3JC–P = 4.2 Hz, C-4), 121.0 (C-4a′), 128.1 (–OCH2Ph), 128.6 (–OCH2Ph), 128.9 (–OCH2Ph), 135.0 (d,
3
3JC–P = 6.0 Hz, –OCH2Ph), 136.8 (C-8a′), 145.9 (C-2′), 148.1 (C-6′), 151.4 (d, JC–P = 6.7 Hz, C-1),
151.9 (C-7′), 160.9 (C-5), 177.4 (C-4′); 31P-NMR (202.4947 MHz, CD3OD): δ −6.61. IR: ν 3234,
3095, 3066, 3008, 2960, 2927, 2846, 1635, 1602, 1589, 1521, 1471, 1259, 1155, 1035, 1012, 966, 854,
734, 696 cm−1. LC-ESI-MS (Positive mode) m/z: 572 [M+H]+; LC-ESI-HRMS (Positive mode) m/z:
[M+H]+ calcd for C31H27NO8P, 572.1469; found, 572.1449. The purity analysis was detected by
reversed-phase HPLC on Thermo Hypersil ODS column (150 × 4.6 mm i.d.) using a MeOH/0.01 M
NaHCO3 (95:5) mixture as eluent. The flow rate was 0.5 mL/min. UV detector was set at 254 nm. The
retention time of 3 was 3.9 min; the purity of 3 was 93.1%.
Dibenzyl methyl phosphate (11). Compound 11 was obtained as colorless oil.1H NMR (200 MHz,
3
CDCl3): δ 3.67 (d,3JHP = 11.2 Hz, –OCH3, 3H), 5.02 (d, JHP = 8.1 Hz, –OCH2Ph, 4H), 7.33 (s, br,
13
2
2
ArH, 10H). C-NMR (50 MHz, CDCl3,): δ54.3 (d, JC–P = 5.0 Hz, –OCH3), 69.3 (d, JC–P = 5.0 Hz,
–OCH2Ph), 127.9 (–OCH2Ph), 128.6 (–OCH2Ph), 135.8 (d, JC–P = 6.6 Hz, –OCH2Ph). 31P-NMR
3
(202.4947MHz, CD3OD): δ −0.16. LC-ESI-MS m/z: 293.0 [M+H]+.
3.1.7. Synthesis of Compound 15
A suspension of 2 (0.20 g, 0.24 mmol) in methanol (100 mL) was stirred at 25°C for 96 h. The
precipitate was collected to give benzyl hydrogen phosphate 15 (0.04 mg, 33%).
Benzyl 3-(6,7-methylenedioxyquinolin-2-yl)-5-methoxyphenylhydrogen phosphate (15). Light yellow
solid. Mp: 60–61 °C; 1H-NMR (500 MHz D2O + NaOH): δ 3.70 (s, –OCH3, 3H), 4.90 (d, 3JHP = 7.9 Hz,
-OPO(OCH2Ph)OH, 2H), 5.89 (s, –OCH2O–, 2H), 6.59 (s, H-3′, 1H), 6.64 (s, H-2, 1H), 6.98 (s, H-4,
1H), 7.00 (s, H-6, 1H), 7.05 (s, H-8′, 1H), 7.12 (d, J = 7.2 Hz, ArH, 1H), 7.16 (d, J = 7.2 Hz, ArH,
13
2H), 7.23 (d, J = 7.4 Hz, ArH, 2H), 7.34 (s, H-5′, 1H); C-NMR (125 MHz, D2O + NaOH): δ 55.7
2
(–OCH3), 68.4 (d, JC-P = 5.4 Hz, –OPO(OCH2Ph)OH), 99.3 (C-5′), 101.4 (–OCH2O–), 103.6 (C-8′),
105.4 (C-3′), 106.3 (d, 2JC-P = 3.6 Hz, C-2), 108.9 (C-6), 112.2 (d, 3JC-P = 4.5 Hz, C-4), 120.9 (C-4a′),
3
127.9 (–OCH2Ph), 128.3 (–OCH2Ph), 128.7 (–OCH2Ph), 136.7 (d, JC-P = 7.3 Hz, –OCH2Ph), 142.8
2
(C-3), 145.2 (C-6′), 147.0 (C-8a′), 150.1 (C-7′), 152.7 (d, JC-P = 7.3 Hz, C-5), 156.9 (C-2′), 159.7
(C-1), 172.4 (C-4′); 31P-NMR (202.4947 MHz, CD3OD): δ −4.73. IR: ν 3396, 3070, 2922, 2835, 1869,
1645, 1608, 1479, 1421, 1273, 1157, 1085, 1035, 1012, 871, 851 cm−1. LC-ESI-MS (Positive mode)
m/z: 482 [M+H]+; LC-ESI-HRMS (Positive mode) m/z: [M+H]+ calcd for C24H21O8NP, 482.0999;
found, 482.0988.
3.1.8. Synthesis of Compound 4
A suspension of dibenzyl phosphate 3 (0.04 g, 0.07 mmol) in anhydrous methanol (20 mL) was
hydrogenated in the presence of 10% Pd/C (0.02 g) at 25 °C for 15 min. The precipitate was collected
and dissolved in 10% NaHCO3 solution and then filtered. The filtrate was acidified with dilute HCl
and the precipitate was then collected and washed with acetone to obtain 4.

Molecules 2013, 18
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3-(6,7-Methylenedioxy-4-oxo-1,4-dihydroquinolin-2-yl)-5-methoxyphenyl
dihydrogen
phosphate
1
(4).White solid (0.02 g, 90%). Mp: > 300 °C. H-NMR (500 MHz, D2O + NaOH): δ3.87 (s,
–OCH3, 3H), 6.02 (s, –OCH2O–, 2H), 6.77 (s, H-3′, 1H), 6.91 (s, H-6, 1H), 7.12 (s, H-2, 1H),
13
7.16 (s, H-8′, 1H), 7.23 (s, H-4, 1H), 7.44 (s, H-5′, 1H); C-NMR (125 MHz, D2O + NaOH):
δ 55.7 (–OCH3), 99.4 (C-5′), 101.5 (–OCH2O–), 103.6 (C-8′), 105.4 (C-3′), 106.7 (C-6),
107.1 (C-2), 112.4 (C-4), 120.9 (C-4a′), 142.3 (C-3), 145.4 (C-6′), 147.1 (C-8a′), 150.3
31
(C-7′), 155.3 (C-5), 157.8 (C-2′), 159.8 (C-1), 172.6 (C-4′); P-NMR (202.4947 MHz, CD3OD):
δ −3.86. IR: ν 3365, 3107, 2918, 2411, 1845, 1645, 1606, 1577, 1498, 1475, 1421, 1390, 1317, 1271,
1197, 1163, 1091, 1058, 1035, 935, 867, 839, 802, 528 cm−1. LC-ESI-HRMS (Negative mode) m/z:
[M−H]− calcd for C17H13O8NP, 390.0373; found, 390.0389. The purity analysis was detected by
reversed-phase HPLC on a Thermo Hypersil ODS column (5 µm, 150 × 4.6 mm i.d) using a
MeOH/9% NaHCO3 (70:30) mixture as eluent. The flow rate was 0.5 mL/min and UV detector was set
at 254 nm. The retention time of 4 was 2.71 min; the purity of 4 was 99.4%.
3.2. Isolation Reaction Products from Reaction Mixture
After stirring for 24 h, the starting material 2 was dissolved in MeOH. The reaction mixture was
separated by semi-preparative HPLC on Rp-18 column (Nucleodur®, C18 HTec, 5 μm, 250 × 10 mm
i.d.) using a MeOH/0.02 M NaHCO3 (90:10) mixture solution as the mobile phase with flow rate
0.5 mL/min to obtain compounds 2, 3, 13, 14, and 15.
Benzyl
3-(4-((bis(benzoxy)phosphoryl)oxy)-6,7-methylenedioxyquinolin-2-yl)-5-methoxyphenyl
hydrogen phosphate (13). Colorless oil. 1H-NMR (500 MHz, CD3OD): δ 3.83 (s, –OCH3, 3H), 5.07 (d,
3JHP = 8.0 Hz, –OPO(OCH2Ph)OH, 2H), 5.20 (d, JHP = 8.8 Hz, –OPO(OCH2Ph)2, 4H), 6.16 (s,
3
–OCH2O–, 2H), 6.80 (s, H-4′, 1H), 7.30–7.40 (m, ArH, 19H), 7.45 (s, H-5, 1H), 7.80 (s, H-3, 1H);
13C-NMR (125 MHz, CD3OD): δ 54.9, 67.6, 70.3, 97.5, 102.0, 104.1, 106.0, 106.3, 109.7, 111.4,
117.3, 127.2, 127.4, 127.9, 128.0, 128.3, 128.5, 134.7, 136.5, 141.3, 146.5, 147.0, 150.3, 150.8, 154.5,
155.8, 160.0; 31P-NMR (202.4947MHz, CD3OD): δ −5.14, −6.87. IR: ν 3441, 3034, 2955, 2895, 1962,
1892, 1815, 1750, 1636, 1603, 1468, 1456, 1269, 1250, 1150, 1092, 1038, 1015, 966, 864, 736, 696
cm−1. LC-ESI-MS (Positive mode) m/z: 742 [M+H]+; LC-ESI-HRMS (Positive mode) m/z: [M+H]+
calcd for C38H34NO11P2, 742.1602; found, 742.1568.
Benzyl 3-((4-((benzoxy)phosphoryl)oxy)-6,7-methylenedioxyquinolin-2-yl)-5-methoxyphenyl hydrogen
phosphate (14). Colorless oil.1H-NMR (500 MHz, CD3OD): δ 3.83 (s, 3H, –OCH3), 5.05 (d,
3
3JHP = 8.8 Hz, –OPO(OCH2Ph)OH, 2H), 5.06 (d, JHP = 7.4 Hz, –OPO(OCH2Ph)OH, 2H), 6.15 (s,
–OCH2O–, 2H), 7.04 (s, H-4′, 1H), 7.16–7.23 (m, 3H), 7.26–7.37 (m, 4H), 7.37–7.43 (m, 2H), 7.45 (s,
13
H-5, 1H), 7.78 (s, H-3, 1H); C-NMR (125 MHz, CD3OD): δ 54.5, 67.6, 67.8, 97.4, 101.8, 104.1,
106.3, 106.4, 107.9, 111.7, 117.7, 126.7, 127.1, 127.4, 127.8, 137.3, 137.5, 141.6, 147.4, 147.7, 151.3,
154.1, 156.2, 156.5, 160.7; 31P-NMR (202.4947 MHz, CD3OD): δ -4.82, -5.44. IR: ν 3441, 3065,
3034, 2955, 2895, 2852, 1603, 1558, 1522, 1470, 1456, 1250, 1150, 1092, 1036, 1022, 968, 864, 736, 698
cm−1. LC-ESI-MS (Positive mode) m/z: 652 [M+H]+; LC-ESI-HRMS (Positive mode): m/z [M+H]+
calcd for C31H28NO11P2, 652.1132; found, 652.1158.

Molecules 2013, 18
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3.3. Reaction Products Analysis by HPLC-ESI-MS
On-line coupling of HPLC with ESI mass spectrometry has been used to analyze reaction
mechanisms [29,30]. After stirring starting material 2 (4 mg/mL) in MeOH for 72 h, the, separation
was achieved on ODS analytical column (Phenomenex Prodigy ODS3 100A, 5 μm, 250 × 4.6 mm i.d)
using a gradient elution of acetonitrile and 0.04 mM NH4OH/H2O (pH = 9) in the following ratios 95%
of water at 0 min, 88% of water at 5 min, 80% of water at 25 min, 54% of water at 30 min, 34% of
water at 35−50 min, 1% of water at 55−65 min, 95% of water at 66−76 min. The flow rate was
0.5 mL/min and the column temperature was maintained at 25–28 °C. The reaction was then
monitored by LC-MS. An electrospray ionization (ESI) mass spectrometer and micrOTOF were
operated in the positive mode with full scan.
3.4. Bioassay
3.4.1. Cell Culture
Human renal carcinoma A498, colon cancer (Colo205) and non-small-cell-lung cancer (NCI-H460)
cells were maintained in RPMI-1640 medium supplemented with 10% fetal bovine serum
(GIBCO/BRL), penicillin (100 U/mL)/streptomycin (100 μg/mL)(GIBCO/BRL) and 1% L-glutamine
(GIBCO/BRL) at 37 °C in a humidified atmosphere containing 5% CO2. Human hepatocellular
carcinoma cell line Hep3B was obtained from America Type Culture Collection (Manassas, VA,
USA). Hep3B cells were cultured in DMEM/F12 medium supplemented with 10% FBS, and penicillin
(100 U/mL)/streptomycin (100 μg/mL) and maintained in a humidified incubator containing 5% CO2.
Normal skin Detroit 551 cells were maintained in DMEM medium supplemented with 10% fetal
bovine serum (GIBCO/BRL), penicillin (100 U/mL)/streptomycin (100 μg/mL) (GIBCO/BRL)
and 1% L-glutamine (GIBCO/BRL) at 37 °C in a humidified atmosphere containing 5% CO2.
Logarithmically growing cancer cells were used for all experiments.
3.4.2. In vitro Cell Viability Assay
Cell viability was detected by 3(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)
assay. Cells were cultured in 96-well plates at 37 °C and incubated with complete medium containing
the vehicle (DMSO) or compounds for indicated times and concentrations. After treatment, cells were
incubated with MTT solution (1 mg/mL in 1× PBS) at 37 °C for 2 h. The absorbance of the samples
was read at wavelength of 570 nm and corrected for inference at 630 nm.
3.4.3. In Vivo Antitumor Activity Assay
The animals used in this experiment had access to food and water ad libitum. Experimental
procedures using animals were approved by the Institutional Animal Care and Use Committees of The
National Health Research Institutes. Nude female BALB/c mice (18–20 g; 6–8 weeks of age) were
purchased from The National Laboratory Animal Center, Taipei, Taiwan, and maintained in
pressurized ventilated cages according to institutional regulations. Human breast cancer MCF-7
(ATCC HTB-22) cells were cultured in DMEM with 10% heat-inactivated FBS and incubated at 37 °C

Molecules 2013, 18
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in a humidified atmosphere containing 5% CO2. Each nude mouse was subcutaneously inoculated with
3 × 106 MCF-7 cells in 0.2 mL PBS via a 24-gauge needle. After the injection of tumor cells, the
animals were injected with beta-estradiol every 2 days. After the appearance of a 100 mm3 tumor
nodule, the tumor-bearing mice were randomly divided into 2 groups. Compound 4 was administered
by i.v. injection at 90 mg/kg on 5 days every week for 4 consecutive weeks. The animals were weighed
and the tumors were measured using calipers twice a week before, during, and after drug treatment.
The tumor volume was calculated with the following formula: 1/2 (L×W2), where L is the length and
W is the width of the tumor [31]. At the end of the experiments, the animals were euthanized with
carbon dioxide followed by cervical dislocation.
4. Conclusions
Our present studies have demonstrated that dephosphorylation of an O-dibenzylphosphate moiety
on quinolin-4-one rings can be carried out in a methanol solution at ambient temperature resulting in
the formation of various phenyl dihydrogen phosphate systems. The reaction mechanism for the
decomposition of 2 is readily explained by a sequence of addition-elimination reactions. The results
reported herein indicate that dephosphorylation of an O-dibenzylphosphate or an O-benzyl hydrogen
phosphate moiety on the quinolin-4-one ring is much faster than that on the phenyl ring. Finally,
hydrogenolysis of 3 produces monophosphate 4 in high yield as a stable and water soluble prodrug of
the antitumor agent 1. Based on the NCI-60 cell line screening data of 1, we selected the MCF-7
xenograft model to demonstrate that the monophosphate prodrug 4 inhibited the growth of MCF-7
tumor by about 44%, at 90 mg/kg/day dosage, without causing significant toxicity.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/18/7/8028/s1.
Acknowledgments
The investigation was supported by research grants from the National Science Council of the
Republic of China awarded to S.-C.K. (NSC 99-2323-B-039-001). This study also was supported by
Taiwan Department of Health, China Medical University Hospital Cancer Research Center of
Excellence (DOH101-TD-C-111-005).
Conflict of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of all compounds are available from the authors.
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