Synthesis and screening of a combinatorial library of naphthalene substituted chalcones: Inhibitors of leukotriene B4

Article abstract of DOI:10.1016/S0968-0896(99)00047-4

A combinatorial mini library of naphthalene substituted chalcones has been prepared by solution phase chemistry. Screening of these mixtures for leukotriene B₄ inhibitory activity using human whole blood assay (HWBL) afforded a lead compound, 1

Full text of DOI:10.1016/S0968-0896(99)00047-4

Bioorganic & Medicinal Chemistry 7 (1999) 1237±1240
Synthesis and Screening of a Combinatorial Library of
Naphthalene Substituted Chalcones: Inhibitors of Leukotriene B4
y
Anil M. Deshpande,a Narshinha P. Argade,a Arvind A. Natua,* and Joseph Eckmanb
aDivision of Organic Chemistry (Synthesis), National Chemical Laboratory, Pune- 411 008, India
bCytoMed, Inc. 840 Memorial Drive, Cambridge, MA 02139 USA
Received 27 November 1998; accepted 29 January 1999
AbstractÐA combinatorial mini library of naphthalene substituted chalcones has been prepared by solution phase chemistry.
Screening of these mixtures for leukotriene B4 inhibitory activity using human whole blood assay (HWBL) a€orded a lead com-
pound, 1-(6-butoxy-2-naphthyl)-3-(4-nitrophenyl)-prop-2-en-1-one (K4A3) with an IC50 value of 18.5 mM. # 1999 Elsevier Science
Ltd. All rights reserved.
2-naphthyl methyl ketones (K1 to K4) with substituted
benzaldehydes (A1 to A5) to furnish combinatorial mix-
tures (KnAn) free of byproducts (Scheme 1). Twenty
compounds were synthesized in two sets as nine combi-
natorial mixtures (Table 1). In the ®rst set each pure
ketone (K1 to K4) was reacted with stoichiometric
amount of equimolar mixture of aldehydes (A1±5) and in
another set each pure aldehyde (A1 to A5) was reacted
with a stoichiometric amount of equimolar mixture of
ketones (K1±K4). Although the reactivities of aldehydes
(A1±A5) and ketones (K1±K4) are di€erent, the reaction
conditions employed for the synthesis of a combinator-
ial library were adequate for quantitative conversion
(tlc, 48 h) of a mixture of ketones and aldehydes to
obtain a diverse menu of chalcones for biological
screening. The HPLC analysis revealed that the con-
centration of anticipated components in combinatorial
mixtures are almost in the same proportion.
Introduction
Leukotrienes (LTs) are important mediators of smooth
muscle constriction,1 increased vascular permeability2
and leukocyte chemotaxis.3 The enzyme 5-lipoxygenase
(5-Lo) catalyzes the initial step in arachidonic acid cas-
cade leading to LTA4, the precursor to the family of
LTs i.e.LTB4, C4, D4 and E4.3,4 Limiting the synthesis
of LTs through inhibition of 5-Lo has provided a new
therapeutic approach for treating a variety of in¯am-
matory conditions including asthma, allergic rhenitis,
rheumatoid arthritis, psoriasis and ulcerative colitis. In
the past decade, a large number of organic compounds
have been reported5 as 5-Lo inhibitors, but the majority
of them exhibit either insucient bioavailability or redox
properties.6 Recently the chalcone derivatives7a±d have
also shown a promising 5-Lo inhibition with anti-in¯am-
matory and anti-allergic activity. Based on the suggestion8
by Summer et al. on precise ®tting of naphthalene ske-
leton in hypothetical arachidonic acid conformation, we
report a combinatorial synthesis and biological evalua-
tion of substituted naphthalene based chalcones.
Biological Evaluation
The combinatorial mixtures were evaluated for their
LTB4 inhibitory activity by human whole blood assay11
(HWBL). Inhibition for each combinatorial mixture at
di€erent concentrations was calculated and compared
with the results of standard LTB4 inhibitor Zileuton.
IC50 value was determined by regression analysis for the
compound represented in hit mixture.
Chemistry and Library Synthesis
The required ketones (K2±K4) were synthesized
employing Friedel±Crafts acylation of corresponding
6-alkoxynaphthalenes according to the literature
procedure.9 The library synthesis is based on simple
and rapid Claisen Schimidt condensation reaction10 of
Library Screening
Key words: Naphthalene chalcones; antiin¯ammatories; enzyme inhi-
bitors; leukotrienes.
The library mixtures were screened using the method
described in biological evaluation and results are depic-
ted in Table 2. Two combinatorial mixtures (K4A1±5
*Corresponding author. Tel.: +91-20-393153; fax: +91-20-393153;
ye-mail: aan@ems.ncl.res.in
NCL Communication No. 6453.
0968-0896/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(99)00047-4

1238
A. M. Deshpande et al. / Bioorg. Med. Chem. 7 (1999) 1237±1240
Scheme 1.
Table 1. Combinatorial synthesis of naphthalene based chalcones
K1A1
K1A2
K1A3
K1A4
K1A5
K2A1
K2A2
K2A3
K2A4
K2A5
K3A1
K3A2
K3A3
K3A4
K3A5
K4A1
K4A2
K4A3
K4A4
K4A5
and A3K1±4) each from one set, showed the highest
LTB4 inhibitory activity (31% and 37% at 30 mM con-
centration respectively), whereas the other mixtures
gave an insigni®cant response as compared to the stan-
dard inhibitor, Zileuton. These mixtures showed either
low or no inhibition with rather normal biological var-
iation that is present in this assay. Considerable LTB4
inhibitory activity of the combinatorial mixtures K4A1±5
and A3K1±4 indicated K4A3 as expected lead compound.
The lead compound, 1-(6-butoxy-2-naphthyl)-3-(4-nitro-
phenyl)-prop-2-en-1-one (K4A3) was synthesized by
condensing 6-butoxy-2-acetonaphthone with p-nitro-
benzaldehyde, which on evaluation for LTB4 inhibitory
assay showed 62% inhibition at 30 mM concentration
with an IC50 of 18.5 mM. It appears that further
manipulation of substituents, position of substituents
and derivatisation of functional group may provide
highly potent LTB4 inhibitors and further studies are in
progress.
combinatorial chemistry and the biological evaluation
results have provided a lead compound (K4A3) for fur-
ther exploration to obtain a potent LTB4 inhibitor.
Experimental
Aromatic aldehydes used for synthesis were procured
from Aldrich Chemical Co. HPLC analysis was per-
formed using C-18 reverse phase column in acetonitrile:
water (7:3) mixture and measuring the absorbance at
254 nm.
Method for library preparation
0.5 M Stock solutions of individual reactant (K and A)
were prepared in MeOH (20 mL). Solutions (10 mL) of
all components from same reactant i.e. ketones (K1±K4)
and aldehydes (A1±A5) were mixed separately to obtain
K1±4 & A1±5. In case of ketones the solution of mixed
components K1±4 was diluted to 50 mL with MeOH to
get 0.1 M of each reactant in solution. Solution (10 mL,
In summary, we have synthesized a mini library of
substituted naphthalene chalcones by solution phase

A. M. Deshpande et al. / Bioorg. Med. Chem. 7 (1999) 1237±1240
1239
Table 2. Results of LTB4 inhibitory activity
% Inhibition of LTB4 formation in
human whole blood assay
Compounds K2A1 to K2A5, A3K1 and A3K3 were syn-
thesized by analogus method.
1-(6-Methoxy-2-naphthyl)-3-(phenyl)-prop-2-en-1-one
(K2A1). Mp 124 ꢀC, H NMR (CDCl3, 200 MHz) d 3.95
1
Combatorial
mixture
Concentrations
(s, 3H), 7.05±7.20 (d, J=14 Hz, 1H), 7.30±7.60 (m, 9H),
7.65±7.75 (d, J=7 Hz, 1H), 7.80±7.90 (d, J=7 Hz, 1H),
7.90±8.05 (d, J=7 Hz, 1H); MS (m/e) 288, 271, 260,
245, 229, 211, 185, 170, 142, 127, 114, 103, 77, 63; Anal.
calcd. for C20H16O2: C, 88.33; H, 5.60. Found: C, 83.24;
H, 5.72.
1 mM
3 mM
10 mM
30 mM
K1A1±5
K2A1±5
K3A1±5
K4A1±5
A1K1±4
A2K1±4
A3K1±4
A4K1±4
A5K1±4
K4A3
4
25
13
±
1
12
8
10
16
5
27
12
5
25
69
2
15
13
24
26
2
25
4
14
36
93
30
20
26
31
19
8
37
2
±
6
14
20
2
1
23
48
1-(6-Methoxy-2-naphthyl)-3-(4-methoxyphenyl)-prop-2-en-
1-one (K2A2). Mp 130 ꢀC, H NMR (CDCl3, 200 MHz)
1
d 3.85 (s, 3H), 3.95 (s, 3H), 6.80±6.95 (d, J=7 Hz, 1H),
6.95±7.10 (d, J=14 Hz, 1H), 7.15±7.55 (m, 6H), 7.60±
7.80 (m, 2H), 7.80±7.90 (d, J=7 Hz, 1H), 7.90±8.00 (d,
J=7 Hz, 1H); MS (m/e) 318, 301, 290, 275, 259, 247,
211, 185, 170, 142, 127, 114, 101, 89, 77, 63; Anal. calcd.
for C21H18O3: C, 79.23; H, 5.70. Found: C, 79.34; H,
5.77.
62
±
Zileutona
Lead compound and hit combinatorial mixtures are shown by bold
face type.
a=Standard Inhibitor, (IC50 0.93 mM).
0.5 M) of individual reactant (K or A) and solution of
mixed components of other reactants (A1±5 or K1±4
1-(6-Methoxy-2-naphthyl)-3-(4-nitrophenyl)-prop-2-en-1-
1
,
one (K2A3). Mp 200 ꢀC, H NMR (CDCl3, 200 MHz) d
0.1 M, 10 mL) were mixed and aq. NaOH was added
(0.5 M, 1 mL). The reaction mixtures were stirred at rt
for 48 h, then concentrated to dryness in vacuo, neu-
tralised with 1 N HCL and extracted with CHCl3
(2Â40 mL). The combined organic layers were washed
with water, brine and dried over anhydrous Na2SO4,
concentration in vacuo furnished gummy or solid pro-
ducts, in quantitative yield.
3.95 (s, 3H), 7.20±7.30 (d, J=14 Hz, 1H), 7.30±7.60 (m,
5H), 7.60±7.90 (m, 4H), 7.90±8.05 (d, J=7 Hz, 1H),
8.15±8.30 (d, J=7 Hz, 1H); MS (m/e) 333, 316, 290,
274, 259, 243, 228, 215, 197, 170, 157, 142, 127, 114,
102, 76, 63; Anal. calcd. for C20H15O4: C, 72.06; H,
4.54; N, 4.20. Found: C, 72.00; H, 5.68; N, 4.31.
1-(6-Methoxy-2-naphthyl)-3-(4-N,N-dimethylanilino)-
ꢀ
1
prop-2-en-1-one (K2A4). Mp 174 C, H NMR (CDCl3,
200 MHz) d 3.05 (s, 6H), 4.00 (s, 3H), 6.65±6.80 (d,
J=7 Hz, 2H), 7.10±7.30 (m, 2H), 7.45±7.70 (m, 3H),
7.75±8.00 (m, 3H), 8.05±8.20 (d, J=7 Hz, 1H), 8.5 (s,
1H); Anal. calcd. for C22H21O2: C, 79.73; H, 6.39; N,
4.23. Found: C, 79.45; H, 6.68; N, 4.31.
HPLC Analysis
The combinatorial mixtures K2A1±5 and A3K1±4 were
analyzed by HPLC. The HPLC of combinatorial library
mixtures was compared with the authentic mixtures
prepared by mixing equimolar amounts of the com-
pounds synthesized individually (K2A1 to K2A5 and
A3K1 to A3K4). They showed identical HPLC pro®le.
1-(6-Methoxy-2-naphthyl)-3-(4-methylphenyl)-prop-2-en-
1-one (K2A5). Mp 165 ꢀC, H NMR (CDCl3, 200 MHz)
1
d 2.40 (s, 3H), 3.95 (s, 3H), 7.15±7.35 (m, 4H), 7.50±7.65
(d, J=7 Hz, 2H), 7.65±7.75 (s, 1H), 7.75±7.95 (m, 3H),
8.05±8.15 (d, J=7 Hz, 1H), 8.5 (s, 1H); MS (m/e) 302,
287, 259, 243, 211, 197, 158, 127, 105, 91, 77, 65; Anal.
calcd. for C21H18O2: C, 83.41; H, 6.00. Found: C, 83.24;
H, 6.37.
1-(6-Butoxy-2-naphthyl)-3-(4-nitrophenyl)-prop-2-en-1-one
(K4A3). To a stirred solution of 6-butoxy-2-acet-
onaphthone (2.42 gm, 10 mmol) and p-nitrobenzalde-
hyde (1.51 gm, 10 mmol) in MeOH (10 mL) was added
aq. NaOH (10 mmol, 2 mL). Reaction mixture was stir-
red at rt for 24 h. It was concentrated to dryness in
vacuo, neutralized with 1 N HCL and extracted with
CHCl3 (2Â25 mL). The combined organic layer was
washed with water, brine and dried over Na2SO4 and
concentration of organic layer in vacuo followed by the
silica gel column chromatographic puri®cation of the
1-(2-naphthyl)-3-(4-nitrophenyl)-prop-2-en-1-one (K1A3).
Mp 139 ꢀC, H NMR (CDCl3, 300 MHz) d 6.65±6.75
1
(d, J=7 Hz, 2H), 7.30±7.75 (m, 6H), 7.75±8.00 (m, 3H),
8.05±8.15 (d, J=9 Hz, 1H), 8.15±8.25 (d, J=9 Hz, 1H);
Anal. calcd. for C19H13O3: C, 75.24; H, 4.32; N, 4.62.
Found: C, 75.45; H, 4.68; N, 4.56.
1
residue furnished pure K4A3 in quantitative yield. H
NMR (CDCl3, 200 MHz) d 0.85 (t, J=8 Hz, 3H), 1.20±
1.50 (m, 2H), 1.50±1.80 (m, 2H), 4.15 (t, J=7 Hz, 2H),
7.20±7.55 (m, 5H), 7.65 (d, J=8 Hz, 2H), 7.75 (d,
J=8 Hz, 1H), 7.84 (d, J=8 Hz, 1H), 7.95 (d, J=8 Hz,
1H), 8.23 (d, J=8 Hz, 2H); MS (m/e) 375, 318, 291, 270,
255, 244, 227, 215, 197, 183, 170, 155, 142, 126, 115,
102, 89, 83, 71, 63, 57; IR nmax 1655, 1600 cm 1. Anal.
calcd. for C23H21NO4: C, 73.60; H, 5.60; N, 3.73.
Found: C, 73.48; H, 5.73; N, 3.57.
1-(6-Ethoxy-2-naphthyl)-3-(4-nitrophenyl)-prop-2-en-1-one
(K3A3). Mp 156 ꢀC, 1H NMR (CDCl3, 200 MHz) d 1.30±
1.45 (t, J=6 Hz, 3H), 4.15±4.35 (q, J=6 Hz, 2H), 7.15±
7.35 (d, J=14 Hz, 1H), 7.35±7.60 (m, 5H), 7.60±7.90 (m,
4H), 7.90±8.05 (d, J=7 Hz, 1H), 8.15±8.30 (d, J=7 Hz,
1H); MS (m/e) 347, 290, 272, 255, 226, 197, 171, 143,
115, 102, 76, 63; Anal. calcd. for C21H17NO4: C, 72.61;
H, 4.93; N, 4.03. Found: C, 72.85; H, 5.08; N, 3.86.

1240
A. M. Deshpande et al. / Bioorg. Med. Chem. 7 (1999) 1237±1240
5-Lo human whole blood assay. 11 Human blood was
collected into heparinised blood collection tubes and
aliquoted in 1 mL portion into 1.5 mL microfuge tubes.
5 mL of test compound in DMSO was added to the
blood sample and incubated for 15 min. at 37 ꢀC. Cal-
cium ionophore A23187 (in DMSO, 50 mM ®nal con-
centration) and the sample were incubated for 30 min. at
37 ꢀC. The samples were centrifuged (1100 X g, 10 min.
at 4 ꢀC) and supernatant was assayed for LTB4 using an
EIA kit (Cayman Chemical). All results are mean of
duplicates and in most cases triplicate determination.
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Acknowledgements
A.M.D. thanks CSIR, New Delhi, for the award of a
fellowship. We are grateful to Dr. C. Grace Yeh, Dr.
Ralph T. Scannell and Dr. Mukund Chorghade,
CytoMed, Inc. Cambridge, MA 02139 USA for providing
the biological evaluation data. We thank Dr. K. N.
Ganesh, Head, Division of Organic Chemistry (Synthesis),
for constant encouragement.
References and Notes
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