2, 3-Diaryl-5-ethylsulfanylmethyltetrahydrofurans as a new class of COX-2 inhibitors and cytotoxic agents

Article abstract of DOI:10.1039/b803608j

2,3-Diaryl-5-ethylsulfanylmethyltetrahydrofuran-3-ols were designed and synthesized by the allylations of benzoins followed by iodocyclization and nucleophilic replacement reactions with ethanthiol. These molecules exhibit IC₅₀ for COX-2 at <10 nM concentration and exhibit average GI ₅₀ over all the 59 human tumor cell lines at μ M concentration. The Royal Society of Chemistry.

Full text of DOI:10.1039/b803608j

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www.rsc.org/obc | Organic & Biomolecular Chemistry
2, 3-Diaryl-5-ethylsulfanylmethyltetrahydrofurans as a new class of COX-2
inhibitors and cytotoxic agents†
Palwinder Singh,*a Anu Mittal,a Satwinderjeet Kaur,b Wolfgang Holzerc and Subodh Kumara
Received 3rd March 2008, Accepted 16th April 2008
First published as an Advance Article on the web 22nd May 2008
DOI: 10.1039/b803608j
2,3-Diaryl-5-ethylsulfanylmethyltetrahydrofuran-3-ols were designed and synthesized by the allylations
of benzoins followed by iodocyclization and nucleophilic replacement reactions with ethanthiol. These
molecules exhibit IC50 for COX-2 at <10 nM concentration and exhibit average GI50 over all the 59
human tumor cell lines at lM concentration.
associated with the presently available drugs for blocking this
Introduction
enzyme demand more investigations in this field.
Starting with the steroids and passing through the use of non-
Based upon the common structural feature of diaryl based
COX-2 inhibitors24 most of which have three interacting sites
present on a central template (pyrazole in celecoxib; 1, Scheme 1),
we have chosen tetrahydrofuran (THF) as the template (struc-
turally similar to the template of rofecoxib but devoid of oxidation)
and introduced appropriately substituted phenyl rings at its C-2,
C-3 positions and ethylsulfanyl methyl group at C-5 (3, Scheme 1).
The dockings of molecules 3 (Scheme 1) in the active site of
COX-2 show that C-5 substituent interacts with R120 through
S and unlike compound 2, the presence of substituents on the
phenyl rings enhances their interactions with the active site amino
acid residues. The in-vitro COX-2 inhibition activities of 3 are
significantly higher in comparison to compound 230 (Scheme 1).
selective and selective non-steroids, the process of treatment of
inflammation has travelled a long way. The modern era of anti-
inflammatory drugs dates back to 1897 when aspirin was intro-
duced for the treatment of inflammation, fever and pain, which
was followed by the launch of many other drugs like ibuprofen,
diclofenac, and indomethacin. However, the rationalization of the
mechanism of inflammation in 1971, with the identification of the
enzyme cyclooxygenase1 as being responsible for the formation of
prostaglandins during arachidonic acid metabolism, has focused
attention on the inhibition of this enzyme for the treatment of
inflammation. Moreover, the cause of undesirable side effects of
inflammatory drugs was unraveled by the identification of two
isoforms of enzyme cyclooxygenase viz. COX-1 and COX-2.2–5
Whereas COX-1 performs desirable roles in the protection of the
gastrointestinal wall, induction of labor etc., the over-expression of
COX-2, in response to stimuli like inflammatory cytokines, growth
factors, tumor promoters, peroxisomal proliferators, hypoxia,
ionizing radiations and carcinogens, is responsible for various
inflammatory diseases,6,7 promotion of cancer8–14 and induction of
multi drug resistance.15–18 The close structural similarities between
COX-1 and COX-2 have heralded the era of selective COX-2
inhibitors,19–24 the coxibs,25,26 which were considered to be safer
than the conventional non-steroidal anti-inflammatory drugs.
However, the withdrawal of rofecoxib, due to its cardiac toxicity,
was a major setback to the use of selective COX-2 inhibitors for
inflammatory diseases. It has been proposed that the oxidation27
of rofecoxib is a possible contributor to its toxicity, while the
diminished synthesis of prostacyclin, a vasodilator, has also been
considered to be a limitation of selective COX-2 inhibitors.28,29
Therefore, the multiple role of enzyme COX-2 and the side effects
Scheme 1
aDepartment of Chemistry, Guru Nanak Dev University, Amritsar, 143005,
India. E-mail: palwinder_singh_2000@yahoo.com; Fax: +91-183-2258819;
Tel: +91-183-2258802-3495
Results and discussion
bDepartment of Botanical & Environmental Sciences, Guru Nanak Dev
University, Amritsar, 143005, India
A detailed account of the synthesis of target molecules has been
shown in Scheme 2. The condensation of benzaldehydes 4–9 in
the presence of NaCN (benzoin condensation) provided the cor-
responding symmetrical benzoins 10–15, while the unsymmetrical
benzoins (16, 17) have been obtained by the crossed benzoin
condensation (Scheme 2). In the crossed benzoin reactions, small
cInstitute of Pharmaceutical Chemistry, University of Vienna, Althanstrasse-
14, Wein, Austria
† Electronic supplementary information (ESI) available: Experimental
data for compounds 24–36, Linpinski values of compounds 38–46 and
detailed growth inhibitory data for compounds 38, 40 and 41 are given.
See DOI: 10.1039/b803608j
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Scheme 2
amounts of symmetrical benzoins 14, 15 and traces of benzoin 10
are also formed which were separated by column chromatography.
On the basis of the placement of the substituted/unsubstituted
phenyl ring at C-2/C-3 of tetrahydrofurans 35 and 36 (coming
from benzoins 16 and 17), a plausible mechanism has been written
for cross benzoin condensation (Scheme 3).
diastereoselective iodocyclisation of homoallylic alcohols has been
explained on the basis of the formation of two transition states in
this reaction.32 In the case of tetrahydrofurans 35 and 36, the
presence of a substituted phenyl ring at C-3 of tetrahydrofuran,
has been proved on the basis of HMBC and long range INEPT
NMR experiments. Treatment of compounds 28A and 28B-
36B with ethanethiol provided the corresponding compound 37–
46 (Scheme 2). The relative stereochemistries at C-2 and C-5
carbons of tetrahydrofurans have been ascertained on the basis
of observation of NOE between 2-H and 5-H in case of 37 and
no NOE between 2-H and 5-H in case of compounds 38–46. An
energy minimised structure of 38 (Fig. 1) shows anti-orientation
of the phenyl rings at C-2 and C-3 and similar geometries have
been obtained after energy minimisations of compounds 39–46.
Scheme 3
A solution of benzoin 10, allyl bromide and indium metal
(suspension) (1 : 1.5 : 1) in THF–H2O (2 : 1) on stirring at 30
2
◦C for 6–8 h, after usual workup provided 18 and under the
same reaction conditions the substituted benzoins 11–17 furnished
respective homoallylic alcohols 19–25 with diastereoselectivity
>99. High diastereoselectivity at this step has been explained on
the basis of Cram’s chelation model.31 Treatment of compounds
23 and 24 with oxone transformed the SCH3 group to SO2CH3 in
compounds 26 and 27, respectively.
◦
Stirring a solution of 18, iodine, NaHCO3 in CH3CN at 0 C
gave a mixture of two diastereomers in the ratio 1 : 4 (1H
NMR spectrum) which were separated by column chromatog-
raphy and identified as 28A and 28B (Scheme 2). Likewise,
the iodocyclisations of homoallylic alcohols 19–22 and 24–27
resulted in the formation of respective tetrahydrofurans 29–36
(A and B) in the diastereomeric ratio shown in Scheme 2. The
Fig. 1 Energy minimized structure of 38. Anti-orientations of 2-H, 5-H
and C-2, C-3 phenyl rings are in parallel with those observed from NOE
experiments.
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This particular structure of the molecules has been used during
their dockings in the active site of COX-2.
Therefore, substituted/unsubstituted benzaldehydes through
benzoin formation followed by diastereoselective allylation and
diastereoselective iodocyclisation provided the target molecules in
appreciable yields.
COX-2, pdb ID 6COX, Fig. 2) indicate that these molecules fit
in the active site of COX-2 (Fig. 3) in the same fashion as Sc-
558. It is noteworthy that the sulfur atom present with the C-5
substituent of compounds 40 and 41 approaches at a distance of
˚
2.40 A to the NH of guanidine moiety of R120, the residue active
during the metabolic phase of COX-2. The substituents present
at C-2 and C-3 phenyl rings of tetrahydrofurans interact through
H-bonding with W387 and H90, respectively. For compound 41,
the distance between F present at C-2 Ph and ArH of W387 is
In-vitro COX-2 inhibiting activities of these compounds have
been evaluated using a ‘COX (ovine) inhibitor screening assay’ kit
with 96 well plates following the standard procedure. The growth
inhibitory activities have been tested at 59 human tumor cell
lines representing leukemia, melanoma and cancers of the lung,
colon, brain, ovary, breast, prostate as well as kidney, following
the standard procedure of NCI, NIH, Bethesda, USA.33–35
All the ten compounds (37–46) evaluated for COX-1, COX-2
inhibitory activities (Table 1) carry a CH2SCH2CH3 group at C-5
of tetrahydrofuran and differ from one another by the substituent
at the two aryl rings. A very nice discrimination between the two
diastereomers of tetrahydrofuran by COX-2 has been observed
in the case of compounds 37 (2R*, 3S*, 5R*) and 38 (2R*,
3S*, 5S*) where compound 38 with 5S* configuration exhibits
significant inhibition of COX-2 with IC50 0.25 lM while 37 shows
poor inhibition of COX-2 exhibiting IC50 7.56 lM. On this basis,
the compounds 38–46 with 5S* configuration were selected for
investigations.
˚
0.987 A.
It has been observed that the compounds 39–46 exhibit
high COX-2 inhibition with IC50 < 0.01 lM which is bet-
ter than celecoxib and rofecoxib. Compounds 39–43 with o-
chlorophenyl, p-chlorophenyl, p-fluorophenyl, p-methoxyphenyl
and p-methansulfonylphenyl group, respectively, at C-2, C-3 of
Fig. 2 Validation of docking programme. Close overlapping of docked
structure of Sc-558 with its crystal structure (rms error is 0.48).
tetrahydrofuran show almost equal inhibition (90%, 10−8
M
concentration) of COX-2 and poor inhibition of COX-1. Similarly,
Moreover, a strong H-bond has been observed between fluorine
present at C-3 Ph of compound 41 and NH of H90. Compound
43 when docked in the active site of COX-2 shows an H-bond with
W387 through oxygen of SO2Me group present at C-2 Ph.
As observed during the dockings of these compounds in COX-
1, they do not enter into the active site of COX-1 and all of them
exhibit a positive docking score.
The role of COX-2 in promotion of cancer has been established
and many of its inhibitors like aspirin, rofecoxib, celecoxib etc.
have been investigated for their use as cancer chemo-preventives
compounds 45 and 46 with a substituent at C-3 phenyl ring
only, also exhibit 88% and 86% inhibition of COX-2 at 10−8
M
concentration. Compound 44 with a SCH3 group at C-3 phenyl
shows 89% inhibition of COX-2 at 10−8 M concentration and 66%
inhibition of COX-1 at 10−5 M concentration. Moreover, this class
of highly selective COX-2 inhibitors, unlike rofecoxib, is devoid of
air oxidation.
The docking studies36 (docking programme was validated by
performing the docking of Sc-558 in the crystal structure of
Table 1 In-vitro COX-2 inhibitory activities of tetrahydrofuran derivatives with SCH2CH3 group at C-5 (37–46)
COX-2
COX-1
% Inhibition
0.01 lM
% Inhibition
10 lM
Compound
0.1 lM
1 lM (10 lM)
IC50 (lM)
100 lM
IC50 (lM)
COX-2 selectivitya
37
−2
30
90
89
83
88
89
89
88
86
−2
46
97
94
78
87
85
91
85
89
−6 (68)
67
91
97
92
88
87
86
—
7.56
0.25
11.5
22
22
24.4
−12
22
46.5
66
28
40
65
75
30
55
52
57
—
54
70
80
63
68
>100
85
93
>15
340
>9300
>7800
—
>8700
>2700
∼1000
>6600
>4200
141
38
39
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.076
0.5
40
78
41
—
42
87
27
<10
66
42
10.75
>100
43
44
45
46
88
50
75
Celecoxib
Rofecoxib
>200
a COX-2 selectivity = IC50(COX-1)/IC50(COX-2).
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anticancer agents. All these molecules follow Lipinski’s rule of
five38 (electronic supplementary information).†
Conclusions
In parallel with the structural features of celecoxib and rofecoxib,
2,3-diaryl-5-ethylsulfanylmethyl tetrahydrofurans were designed.
The benzoins, obtained from substituted/unsubstituted ben-
zaldehydes, undergo indium mediated diastereoselective allylation
followed by iodocyclisation and nucleophilic replacement of the
iodo group with ethanthiol to furnish the target molecules. These
molecules exhibit IC50 for COX-2 in the nM range and a high
selectivity for COX-2 over COX-1. Compounds 40 and 41 also
show considerable growth inhibitory activities at various cancer
cell lines.
Fig. 3 Compound 40, 41 and 42 docked in the active site of COX-2 (pdb
ID 6COX). H’s are omitted for clarity. ‘S’ of C-5 substituent approaches to
Experimental
˚
the guanidine moiety of R120 at a distance of 2.40 A for compounds 40, 41
General
˚
˚
and 3.22 A for compound 42. H-bond (2.01 A) between F present at C-3
phenyl of compound 41 and NH of H90 is visible while the corresponding
substituents of compounds 40 and 42 have a distance of 2.34 A and 2.42 A,
Melting points were determined in capillaries and are uncorrected.
1H and 13C NMR spectra were run on a JEOL JNM AL 300 MHz
and 75 MHz NMR spectrometer, respectively, using CDCl3 as
solvent. Chemical shifts are given in ppm with TMS as an internal
reference. J values are given in Hertz. Chromatography was
performed with silica 100–200 mesh and reactions were monitored
by thin layer chromatography (TLC) with silica plates coated with
silica gel HF-254. The bioassay kit was purchased from Cayman
Chemical. Experimental procedure and spectroscopic data of
benzoins (10–17) and allylated products (18–23, 26) (Scheme 2)
has already been reported39 and that for compounds 24, 25, 27–36
has been given as supplementary data in the ESI.†
˚
˚
respectively, from NH of H90.
along with other cytotoxic drugs.9 Due to the high COX-2
inhibitory activities of tetrahydrofurans discussed above, some
of the molecules (38, 40 and 41; picked by NIH from a list of
ten compounds) were subjected to 59 human tumor cell lines
for screening their growth inhibitory activities (Table 2). The
average GI50 of compound 38 over all the 59 human tumor
cell lines is 5.49 × 10−5 M. It exhibits GI50 < 1.00 × 10−8
M
for the SR cell line of leukemia showing 51% and 60% growth
inhibitions of tumor cells at 10−7 M and 10−8 M concentrations,
respectively. Compounds 40 and 41 show significant growth
inhibitory activities with average GI50 over all the 59 cancer cell
lines as 1.73 × 10−5 M and 1.31 × 10−5 M, respectively. Compound
41 exhibits GI50 3.65 × 10−8 M and <1.00 × 10−8 M at CCRF-CEM
and SR cell lines of leukemia, respectively. Moreover, the growth
inhibitory activities of compounds 40 and 41 at the PC3 cell line
of prostate cancer are better than those reported for celecoxib37
(Table 2). High LC50 values of compounds 38, 40 and 41 indicate
the poor toxicity of these compounds.
(2R*, 3S*, 5R*)-5-Ethylsulfanylmethyl-2,3-diphenyl-
tetrahydrofuran-3-ol (37)
To the ice cold solution of NaH (0.12 g, 5.5 mmol) in DMF
(2–3 ml) was added ethanethiol (0.34 g, 5.5 mmol) and stirred
for 2 min, followed by the addition of ice cold solution of 28A
(2.24 g, 5 mmol). On completion of reaction (20–30 min, TLC
monitoring), the reaction mixture was extracted with diethylether.
The organic layer was dried over anhydrous sodium sulfate and the
solvent was distilled off. The residue was column chromatographed
(silica gel 100–200) using ethyl acetate, hexane as eluents to isolate
37 as thick liquid. Yield 72%; (Found: C, 72.63; H, 7.19. C19H22O2S
requires C, 72.57; H, 7.05%). mmax (CHCl3/cm−1): 3600 (OH); dH
(300 MHz, CDCl3, Me4Si): 1.34 (3H, t, J = 7.5 Hz, CH3), 2.17
Therefore, the model proposed for tetrahydrofuran based COX-
2 inhibitors, with three interacting sites, has proved well and
the molecules show better COX-2 inhibitory activities than that
of celecoxib and rofecoxib. The high COX-2 inhibition and
significant growth inhibitory activities of these molecules at
various cancer cell lines indicate that they could be used as
lead molecules for their development into anti-inflammatory and
2
3
(1H, bs, exchanges with D2O), 2.38 (1H, dd, J = 14.1 Hz, J =
4.8 Hz, H-4), 2.76 (2H, q, J = 7.5 Hz, SCH2), 2.83 1H, (dd,
Table 2 Comparison of growth inhibitory activities (GI50) of compounds 38, 40, 41 with celecoxiba
Compound
Average GI50 over all the 59 tumor cell lines
Activity at PC3 cancer cell line, GI50/lM
Average LC50 over all the cell lines
38
5.49 × 10−5
1.73 × 10−5
1.31 × 10−5
—
M
M
M
44.5
20.0
16.2
47.0
9.5 × 10−5
7.2 × 10−5
6.6 × 10−5
M
M
M
40
41
Celecoxib
a Rofecoxib exhibits 15% inhibition (at 10−6 M concentration) of tumor cells of PC3 cell line of prostate cancer.37
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2J = 14.1 Hz, 3J = 9.0 Hz, H-4), 2.97 (1H, dd, 2J = 13.8 Hz, 2J =
4.8 Hz, 5-CH2), 3.09 (1H, dd, 1J = 13.8 Hz, 2J = 5.4 Hz, 5-CH2),
CH2S), 44.77 (−ve, C-4), 79.33 (+ve, C-5), 82.29 (+ve, C-2), 82.95
(ab, C-3), 126.73 (+ve, ArCH), 126.85 (+ve, ArCH), 128.08 (+ve,
ArCH), 128.91 (+ve, ArCH), 129.31 (+ve, ArCH), 129.64 (+ve,
ArCH), 129.93 (+ve, ArCH), 131.38 (+ve, ArCH), 131.57 (ab,
ArC), 133.04 (ab, ArC), 134.34 (ab, ArC), 138.28 (ab, ArC); m/z
(FAB) 364.9 (M+-H2O), 364.9 (M+-OH).
2
3
4.65 (1H, dq, J = 9.9 Hz, J = 4.8 Hz, H-5), 5.11 (1H, s, H-
2), 6.98–7.43 (10H, m, ArH); dC (75 MHz, CDCl3, Me4Si): 14.95
(+ve, CH3), 27.48 (−ve, SCH2), 37.11(−ve, 5-CH2), 48.17 (−ve,
C-4), 77.19 (+ve, C-5), 82.04 (ab, C-3), 90.38 (+ve, C-2), 125.40
(+ve, CH), 126.65 (+ve, CH), 127.02 (+ve, CH), 127.93 (+ve, CH),
127.96 (+ve, CH), 128.21 (+ve, CH), 135.09 (ab, C), 142.01 (ab,
C). NOE experiments: irrradiation of singlet at d 5.18 (H-2) shows
positive NOE with signals at d 4.65 (H-5), and 6.94, 7.43 (ArH)
and irradiation of dq at d 4. 65 (H-5) shows positive NOE with dd
at d 2.83 (5.46%); m/z (FAB) 313 (M+-1).
(2R*, 3S*, 5S*)-2,3-Bis-(4-chlorophenyl)-5-ethylsulfanyl-
methyltetrahydrofuran-3-ol (40)
According to the preparation of 39, 40 was obtained from 30B as
thick liquid. Yield 78%; (Found: C, 59.41; H, 5.13; S, 8.21. C19H20
Cl2O2S requires C, 59.53; H, 5.26; S, 8.37%). mmax (CHCl3)/cm−1:
3450 (OH); dH (300 MHz, CDCl3, Me4Si): 1.30 (3H, t, J = 7.5 Hz,
CH3), 1.69 (1H, bs, OH, exchanges with D2O), 2.52 (2H, d, J =
8.1 Hz, 4-H), 2.68 (2H, q, J = 7.5 Hz, SCH2), 2.90 (1H, dd,
(2R*, 3S*, 5S*)-5-Ethylsulfanylmethyl-2,3-diphenyl-
tetrahydrofuran-3-ol (38)
3
2
According to the preparation of 37, 38 was obtained from 28B
as thick liquid. Yield 72%; (Found: C, 72.36; H, 6.89. C19H22O2S
requires C, 72.57; H, 7.05%). mmax (CHCl3/cm−1): 3620 (OH). dH
(300 MHz, CDCl3, Me4Si): 1.31 (3H, t, J = 7.5 Hz, CH3), 2.46
(1H, bs, exchanges with D2O), 2.55 (2H, d, J = 7.8 Hz, H-4), 2.69
(2H, q, J = 7.5 Hz, SCH2), 2.96 (2H, q, J = 2.4 Hz, 5-CH2),
4.86 (1H, m, H-5), 5.43 (1H, s, H-2), 7.02–7.44 (10H, m, ArH); dC
(75 MHz, CDCl3, Me4Si): 14.98 (+ve, CH3), 27.09 (−ve, SCH2),
37.19 (−ve, 5-CH2), 47.92 (−ve, C-4), 78.44 (+ve, C-5), 83.25 (ab,
C-3), 89.53 (+ve, C-2), 125.30 (+ve, CH), 125.87 (+ve, CH), 126.60
(+ve, CH), 127.26 (+ve, CH), 128.28 (+ve, CH), 128.35 (+ve, CH),
135.48 (ab, C), 141.66 (ab, C). NOE experiments: irradiation of
singlet at d 5.43 (H-2) shows positive NOE with signals at d 7.03
(19.8%), 7.38 (11.86%) (ArH) and shows no positive NOE with
multiplet at d 4.86 (H-5); m/z (FAB) 313 (M+-1).
2J = 13.8 Hz, J = 4.8 Hz, CH2S), 2.98 (1H, dd, J = 13.5 Hz,
3J = 5.7 Hz, CH2S), 4.79–4.88 (1H, m, 5-H), 5.31 (1H, s, 2-H),
6.96 (2H, d, J = 8.4 Hz, ArH), 7.22 (2H, d, J = 8.4 Hz, ArH)
7.35 (4H, m, ArH); Decoupling of triplet at d 1.30 converts q
at d 2.68 into singlet. Decoupling of doublet at d 2.52 converts
multiplet at d 4.79–4.88 into a double doublet; dC (75.4 MHz,
CDCl3, Me4Si): 14.97 (+ve, CH3), 27.18 (−ve, SCH2), 37.17 (−ve,
CH2S), 47.73 (−ve, C-4), 78.29 (+ve, C-5), 82.93 (ab, C-3), 88.85
(+ve, C-2), 126.79 (+ve, ArCH), 127.95 (+ve, ArCH), 128.48 (+ve,
ArCH), 128.60 (+ve, ArCH), 133.36 (ab, ArC), 133.81 (ab, ArC),
134.14 (ab, ArC), 139.91 (ab, ArC); In 1H-13C HETCOR spectrum,
carbon signal at d 14.97 shows correlation with signal at d 1.30 in IH
spectrum, carbon signal at d 27.18 shows correlation with quartet
at d 2.68 in IH NMR spectrum which assigns it to be CH2 of the
SC2H5 group, carbon signal at d 37.17 shows correlation with two
double doublets at d 2.90 and 2.98 in IH NMR spectrum, carbon
I
signal at d 47.73 shows correlation with doublet at d 2.52 in H
(2R*, 3S*, 5S*)-2,3-Bis-(2-chlorophenyl)-5-ethylsulfanyl-
NMR spectrum, carbon signal at d 78.29 shows correlation with
multiplet at d 4.79–4.88 in IH NMR spectrum, carbon signal at d
88.85 shows correlation with signal at d 5.31 in IH NMR spectrum
and therefore confirm the assignments of hydrogens and carbons
in this compound; NOE experiments: irradiation of singlet at d
5.31 (2-H) shows NOE with signals at d 7.35, 6.96 (ArHs), 2.68
(SCH2), 2.52 (4-Hs) and no NOE has been observed with the signal
at d 4.79–4.88 (5-H); m/z (FAB) 382.9 (M+).
methyltetrahydrofuran-3-ol (39)
To the ice cold solution of NaH (0.12 g, 5.5 mmol) in DMF
(2–3 ml) was added ethanethiol (0.34 g, 5.5 mmol) and stirred
for 2 min, followed by the addition of ice cold solution of 29B
(2.24 g, 5 mmol). On completion of reaction (20–30 min, TLC
monitoring), the reaction mixture was extracted with diethylether.
The organic layer was dried over anhydrous sodium sulfate and the
solvent was distilled off. The residue was column chromatographed
(silica gel 100–200) using ethyl acetate, hexane as eluents to isolate
39 as thick liquid. Yield 71%; (Found: C, 59.41; H, 5.13; S,
8.21. C19H20Cl2O2S requires C, 59.53; H, 5.26; S, 8.37%). mmax
(CHCl3)/cm−1): 3450 (OH); dH (300 MHz, CDCl3, Me4Si): 1.30
(3H, t, J = 7.5 Hz, CH3), 1.99 (1H, d, J = 1.8 Hz, OH, exchanges
(2R*, 3S*, 5S*)-2,3-Bis-(4-fluorophenyl)-5-ethylsulfanyl-
methyl tetrahydrofuran-3-ol (41)
According to the preparation of 39, 41 was obtained from
31B as thick liquid. Yield 79%; (Found: C, 65.02; H, 5.53; S,
9.09. C19H20F2O2S requires C, 65.12; H, 5.75; S, 9.15%). mmax
(CHCl3)/cm−1: 3431 (OH); dH (300 MHz, CDCl3, Me4Si): 1.30
(3H, t, J = 7.5 Hz, CH3), 1.73 (1H, bs, OH, exchanges with D2O),
2.53 (2H, d, J = 7.5 Hz, 4-H), 2.68 (2H, q, J = 7.2 Hz, SCH2),
2
3
with D2O), 2.34 (1H, dd, J = 12.9 Hz, J = 6.0 Hz, 4-H), 2.68
(2H, dq, J = 7.5 Hz, J = 0.9 Hz, SCH2), 2.90 (1H, dd, J =
2
3
2
3
2
3
13.2 Hz, J = 6.3 Hz, CH2S), 3.02 (1H, dd, J = 13.2 Hz, J =
2
3
4
6.0 Hz, CH2S), 3.27 (1H, ddd, J = 12.9 Hz, J = 9.6 Hz, J =
2
3
1.8 Hz, 4-H, converted into dd on D2O exchange), 4.84 (1H, ddd,
2.92 (1H, dd, J = 13.5 Hz, J = 4.8 Hz, CH2S), 2.98 (1H, dd,
3
3
3
3J = 12.0 Hz, J = 9.9 Hz, J = 6.0 Hz, 5-H), 6.47 (1H, s, 2-
2J = 13.5 Hz, J = 5.7 Hz, CH2S), 4.79–4.88 (1H, m, 5-H), 5.33
H), 7.17–7.22 (4H, m, ArH), 7.29–7.39 (2H, m, ArH), 7.51–7.54
(1H, s, 2-H), 6.89–7.09 (6H, m, ArH), 7.38 (2H, two doublets, 3J =
5.4 Hz, 3J = 5.1 Hz, ArH); dC (75.4 MHz, CDCl3, Me4Si): 14.97
(+ve, CH3), 27.16 (−ve, SCH2), 37.23 (−ve, CH2S), 47.70 (−ve,
C-4), 78.15 (+ve, C-5), 82.81 (ab, C-3), 88.87 (+ve, C-2), 115.20
3
3
(1H, m, ArH), 7.69 (1H, dd, J = 7.5 Hz, J = 1.5 Hz, ArH).
Decoupling of triplet at d 1.30 converts dq at d 2.68 into a doublet
with J = 0.9 Hz. Decoupling of double doublet at d 2.34 converts
ddd at d 3.27 into a dd 3J = 9.3 Hz, 3J = 1.8 Hz; dC (75.4 MHz,
CDCl3, Me4Si): 14.87 (+ve, CH3), 26.76 (−ve, SCH2), 36.79 (−ve,
(+ve, d, JC-F(ortho) = 21.00 Hz, ArCH), 115.24 (+ve, d, JC-F(ortho)
=
21.07 Hz, ArCH), 127.05 (+ve, d, JC-F(meta) = 8.02 Hz, ArCH),
2710 | Org. Biomol. Chem., 2008, 6, 2706–2712
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128.34 (+ve, d, JC-F(meta) = 8.02 Hz, ArCH), 131.03 (ab, d, JC-F(para)
=
(1H, s, 2-H), 7.03–7.06 (2H, m, ArH), 7.21–7.25 (5H, m, ArH),
7.33 (2H, d, J = 8.7 Hz, ArH); dC (75 MHz, CDCl3, Me4Si): 14.94
(+ve, CH3), 15.54 (+ve, SCH3), 27.04 (−ve, SCH2), 37.13 (−ve,
CH2S), 47.79 (−ve, C-4), 78.29 (+ve, C-5), 82.96 (ab, C-3), 89.29
(+ve, C-2), 125.83 (+ve, ArCH), 126.16 (+ve, ArCH), 126.56 (+ve,
ArCH), 128.17 (+ve, ArCH), 128.22 (+ve, ArCH), 135.43 (ab,
ArC), 137.34 (ab, ArC), 138.49 (ab, ArC); m/z [MALDI (TOF)]
383.69 (M++ Na+), 399.69 (M++ K+).
3.15 Hz, C), 137.19 (ab, d, JC-F(para) = 3.08 Hz, ArC), 162.02 (ab, d,
JC-F = 244.2 Hz, ArC), 162.63 (ab, d, JC-F = 244.8 Hz, ArC); m/z
[MALDI (TOF)] 373 (M+ + Na+), 389 (M+ + K+).
(2R*, 3S*, 5S*)-5-Ethylsulfanylmethyl-2,3-bis-(4-
methoxyphenyl)-tetrahydrofuran-3-ol (42)
According to the preparation of 39, 42 was obtained from
32B as thick liquid. Yield 75%; (Found: C, 67.23; H, 6.97;
S, 8.44. C21H26O4S requires C, 67.35; H, 7.00; S, 8.56%). mmax
(CHCl3)/cm−1: 3458 (OH); dH (300 MHz, CDCl3, Me4Si): 1.29
(3H, t, J = 7.5 Hz, CH3), 1.76 (1H, bs, OH, exchanges with D2O),
2.47 (1H, dd, 2J = 12.9 Hz, 3J = 9.0 Hz, 4-H), 2.53 (1H, dd, 2J =
12.9 Hz, 3J = 6.3 Hz, 4-H), 2.68 (2H, q, J = 7.5 Hz, SCH2), 2.91
(2R*, 3S*, 5S*)-5-Ethylsulfanylmethyl-3-(4-
methanesulfonyl-phenyl)-2-phenyltetrahydrofuran-3-ol (45)
According to the preparation of 39, 45 was obtained from 35B
as white solid, mp 115 ◦C. Yield 78%; (Found: C, 61.26; H, 6.04;
S, 16.22. C20H24O4S2 requires C, 61.20; H, 6.16; S, 16.34%). mmax
2
3
2
−1
=
(1H, dd, J = 13.8 Hz, J = 5.4 Hz, CH2S), 2.96 (1H, dd, J =
(CHCl3)/cm : 3500 (OH), 1320 (S O); dH (300 MHz, CDCl3,
3
13.5 Hz, J = 5.7 Hz, CH2S), 3.76 (3H, s, OCH3), 3.82 (3H, s,
Me4Si): 1.31 (3H, t, J = 7.5 Hz, CH3), 1.61 (1H, bs, OH, exchanges
2
3
OCH3), 4.76–4.85 (1H, m, 5-H), 5.31 (1H, s, 2-H), 6.78 (2H, d,
J = 9.0 Hz, ArH), 6.88 (2H, d, J = 9.0 Hz, ArH), 6.98 (2H, d,
J = 8.4 Hz, ArH), 7.32 (2H, d, J = 8.7 Hz, ArH); dC (75.4 MHz,
CDCl3, Me4Si): 14.95 (+ve, CH3), 27.01 (−ve, SCH2), 37.22 (−ve,
CH2S), 47.54 (−ve, C-4), 55.11 (+ve, OCH3), 55.17 (+ve, OCH3),
78.01 (+ve, C-5), 82.68 (ab, C-3), 89.03 (+ve, C-2), 113.56 (+ve,
ArCH), 113.58 (+ve, ArCH), 126.46 (+ve, ArCH), 127.36 (ab,
ArC), 127.87 (+ve, ArCH), 133.74 (ab, ArC), 158.58 (ab, ArC),
159.38 (ab, ArC); m.z (FAB) 357 (M+-OH).
with D2O), 2.58 (2H, dd, J = 9.3 Hz, J = 6.6 Hz, 4-H), 2.70
2
3
(2H, q, J = 7.2 Hz, SCH2), 2.93 (1H, dd, J = 13.8 Hz, J =
4.2 Hz, CH2S), 3.03 (1H, dd, 2J = 13.5 Hz, 3J = 6.0 Hz, CH2S),
3.09 (3H, s, SO2CH3), 4.87–4.92 (1H, m, 5-H), 5.46 (1H, s, 2-H),
6.98–7.01 (2H, m, ArH), 7.24–7.01 (3H, m, ArH), 7.68 (2H, d,
J = 8.7 Hz, ArH), 7.95 (2H, d, J = 8.7 Hz, ArH); Decoupling
of multiplet at d 4.87–4.92 converts dd’s at d 2.93, 3.03 and 2.58
into doublets and can be assigned as 5-H; dC (75.4 MHz, CDCl3,
Me4Si): 14.97 (+ve, CH3), 27.18 (−ve, SCH2), 37.05 (−ve, CH2S),
44.43 (+ve, SO2CH3), 48.01 (−ve, C-4), 78.47 (+ve, C-5), 83.07 (ab,
C-3), 89.60 (+ve, C-2), 126.33 (+ve, ArCH), 126.58 (+ve, ArCH),
127.41 (+ve, ArCH), 128.53 (+ve, ArCH), 128.59 (+ve, ArCH),
134.76 (ab, ArC), 139.34 (ab, ArC), 148.45 (ab, ArC); m/z (FAB)
392.9 (M++1), 375 (M+-OH).
(2R*, 3S*, 5S*)-5-Ethylsulfanylmethyl-2,3-bis-(4-
methanesulfonylphenyl)-tetrahydrofuran-3-ol (43)
According to the preparation of 39, 43 was obtained from 33B
as white solid, mp 123 ◦C. Yield 77%; (Found: C, 53.44; H, 6.02;
S, 20.26. C21H26O6S3 requires C, 53.59; H, 5.57; S, 20.44%). mmax
(2R*, 3S*, 5S*)-5-Ethylsulfanylmethyl-3-(4-methoxyphenyl)-
tetrahydrofuran-3-ol (46)
−1
=
(CHCl3)/cm : 3400 (OH), 1300 (S O); dH (300 MHz, CDCl3,
Me4Si): 1.31 (3H, t, J = 7.5 Hz, CH3), 1.77 (1H, bs, OH, exchanges
with D2O), 2.58 (1H, dd, 2J = 13.2 Hz,3J= 6.3 Hz, 4-H), 2.67 (1H,
According to the preparation of 39, 46 was obtained from
36B as thick liquid. Yield 73%; (Found: C, 69.58; H, 6.99;
S, 9.17. C20H24O3S requires C, 69.73; H, 7.02; S, 9.31%). mmax
(CHCl3)/cm−1: 3448 (OH); dH (300 MHz, CDCl3, Me4Si): 1.29
(3H, t, J = 7.5 Hz, CH3), 1.75 (1H, bs, OH, exchanges with D2O),
2.50 (2H, d, J = 6.9 Hz, 4-H), 2.68 (2H, q, J = 7.5 Hz, SCH2),
2
3
dd, J = 13.2 Hz, J = 9.9 Hz, 4-H), 2.69 (2H, q, J = 7.5 Hz,
SCH2), 2.93 (1H, dd, 2J = 13.5 Hz, 3J = 4.5 Hz, CH2S), 3.02 (4H,
m, 3H of SO2CH3 + 1H of CH2S), 3.09 (3H, s, SO2CH3), 4.92–5.44
(1H, m, 5-H), 5.44 (1H, s, 2-H), 7.21 (2H, d, J = 8.4 Hz, ArH),
7.68 (2H, d, J = 8.4 Hz, ArH), 7.79 (2H, d, J = 8.1 Hz, ArH), 7.94
(2H, d, J = 8.7 Hz, ArH); dC (75.4 MHz, CDCl3, Me4Si): 14.97
(+ve, CH3), 27.27 (−ve, SCH2), 37.03 (−ve, CH2S), 44.32 (+ve,
SO2CH3), 44.39 (+ve, SO2CH3), 48.21 (−ve, C-4), 78.48 (+ve, C-
5), 83.45 (ab, C-3), 88.94 (+ve, C-2), 126.55 (+ve, ArCH), 127.10
(+ve, ArCH), 127.59 (+ve, ArCH), 127.65 (+ve, ArCH), 139.67
(ab, ArC), 140.08 (ab, ArC), 142.01 (ab, ArC), 147.54 (ab, ArC);
m/z (FAB) 493.1 (M++ Na+), 509.1 (M++ K+).
2
3
2.88–2.99 (2H, two double doublets, J = 13.8 Hz, J = 6.0 Hz,
3J = 5.1 Hz, CH2S), 3.81 (3H, s, OCH3), 4.78–4.87 (1H, m, 5-H),
5.36 (1H, s, 2-H), 6.88 (2H, d, J = 9.0 Hz, ArH), 7.03–7.06 (2H, m,
ArH), 7.22–7.25 (3H, m, ArH), 7.32 (2H, d, J = 9.0 Hz, ArH); dC
(75 MHz, CDCl3, Me4Si): 14.95 (+ve, CH3), 27.02 (−ve, SCH2),
37.19 (−ve, CH2S), 47.77 (−ve, C-4), 55.17 (+ve, OCH3), 78.21
(+ve, C-5), 82.91 (ab, C-3), 89.26 (+ve, C-2), 113.59 (+ve, ArCH),
126.46 (+ve, ArCH), 126.64 (+ve, ArCH), 128.07 (+ve, ArCH),
128.15 (+ve, ArCH), 133.62 (ab, ArC), 133.65 (ab, ArC), 158.62
(ab, ArC); m/z (FAB) 327 (M+-OH).
(2R*, 3S*, 5S*)-5-Ethylsulfanylmethyl-3-(4-methylsulfanyl-
phenyl)-2-phenyltetrahydrofuran-3-ol (44)
According to the preparation of 39, 44 was obtained from
34B as thick liquid. Yield 77%; (Found: C, 66.58; H, 6.69; S,
17.66. C20H24O2S2 requires C, 66.63; H, 6.71; S, 17.79%). mmax
(CHCl3)/cm−1: 3415 (OH); dH (300 MHz, CDCl3, Me4Si): 1.29
(3H, t, J = 7.2 Hz, CH3), 1.79 (1H, bs, OH, exchanges with D2O),
2.48 (3H, s, SCH3), 2.50 (2H, dd, 2J = 14.1 Hz, 3J = 7.2 Hz, 4-H),
2.68 (2H, q, J = 7.2 Hz, SCH2), 2.93 (1H, d, J = 2.7 Hz, CH2S),
2.95 (1H, d, J = 3.6 Hz, CH2S), 4.79–4.88 (1H, m, 5-H), 5.37
In-vitro COX-1, COX-2 inhibitory activities
In-vitro COX-1, COX-2 inhibiting activities of these compounds
have been evaluated using ‘COX (ovine) inhibitor screening assay’
kit with 96 well plates. Both ovine COX-1 and COX-2 enzymes
were included. This screening assay directly measures PGF2a
produced by SnCl2 reduction of COX-derived PGH2. COX-1,
COX-2initialactivitytubes were preparedtaking 950 ll of reaction
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buffer, 10 ll of heme, 10 ll of COX-1 and COX-2 enzymes in
respective tubes. Similarly, COX-1, COX-2 inhibitor tubes were
prepared by adding 20 ll of inhibitor (compound under test) in
each tube in addition to the above ingredients. The background
tubes correspond to inactivated COX-1 and COX-2 enzymes
obtained after keeping the tubes containing enzymes in boiling
water for 3 min. Reactions were initiated by adding 10 ll of
arachidonic acid in each tube and quenched with 50 ll of 1M
HCl. PGH2 thus formed was reduced to PGF2a by adding 100 ll
of SnCl2. The prostaglandin produced in each well was quantified
using broadly specific prostaglandin antiserum that binds with
major prostaglandins and reading the 96 well plate at 405 nm.
The wells of the 96 well plate showing low absorption at 405 nm
indicate the low level of prostaglandins in these wells and hence less
activity of the enzyme. Therefore, the COX inhibitory activities of
the compounds could be quantified from the absorption values of
different wells of the 96 well plate. The results of these studies have
been represented in terms of the percentage inhibition of COX-1
and COX-2 enzymes.
7 J. L. Masferrer, B. S. Zweifel, P. T. Manning, S. D. Hauser, K. M.
Leahy, W. G. Smith, P. C. Isakson and K. Siebert, Proc. Natl. Acad.
Sci. U. S. A., 1994, 91, 3228–3232.
8 Y. T. Jeon and Y. S. Song, Mini Rev. Med. Chem., 2006, 6, 827–833.
9 J. -B. Meric, S. Rottey, K. Olaussen, J. -C. Soria, D. Khayat, O. Rixe
and J. -P. Spano, Crit. Rev. Oncol./Hematol., 2006, 59, 51–64.
10 C. H. Liu, S.-H. Chang, K. Narko, O. C. Trifan, M.-T. Wu, E. Smith,
C. Haudenschild, T. F. Lane and T. Hla, J. Biol. Chem., 2001, 276,
18563–18569.
11 K. Subbaramaiah and A. J. Dannenberg, Trends Pharmacol. Sci., 2003,
24, 96–102.
12 J. R. Brown and R. N. DuBois, J. Clin. Oncol., 2005, 23, 2840–2855.
13 L. J. Marnett and R. N. DuBois, Annu. Rev. Pharmacol. Toxicol., 2002,
42, 55–80.
14 W. Dempke, C. Rie, A. Grothey and H. -J. Schmoll, J. Cancer Res. Clin.
Oncol., 2001, 127, 411–417.
15 A. Sorokin, Curr. Pharm. Des., 2004, 10, 647–657.
16 O. Fantappie, E. Masini, I. Sardi, L. Raimondi, D. Bani, M. Solazzo,
A. Vannacci and R. Mazzanti, Hepatology, 2002, 35, 843–852.
17 D. Ratnasinghe, P. J. Daschner, M. R. Anver, B. H. Kasprzak, P. R.
Taylor, G. C. Yeh and J. A. Tangrea, Anticancer Res., 2001, 21, 2141–
2147.
18 V. A. Patel, M. J. Dunn and A. Sorokin, J. Biol. Chem., 2002, 277,
38915–38920.
19 L. De Avaliacdo, M. P. Veloso, H. Verli, C. A. M. Fraga, A. L. P.
Miranda and E. J. Barreiro, Med. Chem. Rev., 2004, 1, 73–90.
20 K. Bennett, M. Teeling and J. Feely, Eur. J. Clin. Pharmacol., 2003, 59,
645–649.
In-vitro growth inhibitory activities
The detailed evaluations for growth inhibitory activities at 59
human tumor cell lines were carried out by screening unit of
NCI at NIH Bethesda, USA. The compounds were evaluated at
five concentrations viz. 10−4 M, 10−5 M, 10−6 M, 10−7 M and
10−8 M. The percentage growth of tumor cells was calculated at
each cell line for each concentration of the compound. The results
are expressed as growth inhibition of 50% (GI50) which is the
concentration of the compound causing 50% reduction in the net
protein increase (as measured by SRB staining) in control cells
during drug incubation, total growth inhibition (TGI) and LC50
indicating the net loss of cells following treatment.
21 G. Dannhardt and W. Kiefer, Eur. J. Med. Chem., 2001, 36, 109–126.
22 B. Everts, P. Wahrborg and T. Hedner, Clin. Rheumatol., 2000, 19,
331–343.
23 D. L. DeWitt, Mol. Pharmcol., 1999, 55, 625–631.
24 P. Singh and A. Mittal, Mini Rev. Med. Chem., 2008, 8, 73–90.
25 T. D. Penning, J. J. Talley, S. R. Bertenshaw, J. S. Carter, P. W. Collins,
S. Docter, M. J. Graneto, L. F. Lee, J. W. Malecha, J. M. Miyashiro,
R. S. Rogers, D. J. Rogier, S. S. Yu, G. D. Anderson, E. G. Burton, J. N.
Cogburn, S. A. Gregory, C. M. Koboldt, W. E. Perkins, K. Seibert,
A. W. Veenhuizen, Y. Y. Zhang and P. C. Isakson, J. Med. Chem., 1997,
40, 1347–1365.
26 P. Prasit, Z. Wang, C. Brideau, C. C. Chan, S. Charleson, W. Cromilish,
D. Ethier, J. F. Evans, A. W. Ford-Hutchinson, J. Y. Gauthier, R.
Gordon, J. Gyay, M. Gresser, S. Kargman, B. Kennedy, Y. Leblanc,
S. Leger, J. Mancini, G. P. O’Neill, M. Ouellet, M. D. Percival, H.
Perrier, D. Riendeau, I. Rodger, P. Tagari, M. Therien, P. Vickers, E.
Wong, L. -J. Xu, R. N. Young and R. Zamboni, Bioorg. Med. Chem.
Lett., 1999, 9, 1773–1778.
Acknowledgements
27 L. R. Reddy and E. J. Corey, Tetrahedron Lett., 2005, 46, 927–929.
28 C. Charlier and C. Michaux, Eur. J. Med. Chem., 2003, 38, 645–659
and references therein.
Department of Science & Technology, New Delhi, Council of
Scientific & Industrial Research, New Delhi and University Grants
Commission, New Delhi have been gratefully acknowledged for
financial support. Anu thanks CSIR, New Delhi for a Senior
Research Fellowship. We are thankful to Dr V. L. Narayanan
and his group at NCI, NIH, Bethesda for anticancer screening.
29 P. A. Howard and P. Delafontaine, J. Am. Coll. Cardiol., 2004, 43,
519–525.
30 P. Singh, A. Mittal, S. Kaur and S. Kumar, Bioorg. Med. Chem., 2006,
14, 7910–7916.
31 S. Kumar, P. Kaur, A. Mittal and P. Singh, Tetrahedron, 2006, 62, 4018–
4026.
32 P. Kaur, P. Singh and S. Kumar, Tetrahedron, 2005, 61, 8231–8240.
33 M. C. Ally, D. A. Scudiero, P. A. Monks, M. L. Hursy, M. J. Czerwinski,
D. A. Fine, B. J. Abbott, J. G. Mayo, R. H. Showmaker and M. R. Boyd,
Cancer Res., 1988, 48, 589–601.
34 M. R. Grever, S. A. Schepartz and B. A. Chabner, Semin. Oncol., 1992,
19, 622–638.
35 M. R. Boyd and K. D. Paull, Drug Dev. Res., 1995, 34, 91–109.
36 Dockings were performed using ‘Dock into active site’ module of
BioMed Cache 7.5.0.85 obtained form FQS Poland SP. Z.O.O.
37 N. Pommery, T. Taverne, A. Telliez, L. Goossens, C. Charlier, J.
Pommery, J.-F. Goossens, R. Houssin, F. Durant and J. -P. Henichart,
J. Med. Chem., 2004, 47, 6195–6205.
References
1 J. R. Vane, Nat. New Biol., 1971, 231, 232–235.
2 W. L. Xie, J. G. Chipman, D. L. Robertson, R. L. Erikson and D. L.
Simmons, Proc. Natl. Acad. Sci. U. S. A., 1991, 88, 2692–2696.
3 W. L. Smith, R. M. Garavito and D. L. DeWitt, J. Biol. Chem., 1996,
271, 33157–33160.
4 L. J. Marnett, S. W. Rowlinson, D. C. Goodwin, A. S. Katgutkar and
C. A. Lanzo, J. Biol. Chem., 1999, 274, 22903–22906.
5 R. G. Kurumbail, A. M. Stevens, J. K. Gierse, J. J. McDonald, R. A.
Stegeman, J. Y. Pak, D. Gildehaus, J. M. Miyashiro, T. D. Penning, K.
Seibert, P. C. Isakson and W. C. Stallings, Nature, 1996, 384, 644–648.
6 K. Seibert, Y. Zhang, K. Leahy, S. Hauser, J. Masferrer, W. Perkins,
L. Lee and P. C. Isakson, P., Proc. Natl. Acad. Sci. U. S. A., 1994, 91,
12013–12017.
38 C. A. Lipinski, F. Lombardo, B. W. Dominy and P. J. Feeney, Adv. Drug
Delivery Rev., 1997, 23, 4–25.
39 P. Singh, A. Mittal and S. Kumar, Bioorg. Med. Chem., 2007, 15, 3990–
3996.
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This journal is
The Royal Society of Chemistry 2008
©