Synthesis and screening of (E)-1-(β-D-galactopyranosyl)-4-(aryl)but-3- ene-2-one against Mycobacterium tuberculosis

Article abstract of DOI:10.1016/j.bmcl.2011.05.037

A series of galactose-derived aryl enones were synthesised and screened against Mycobacterium tuberculosis H₃₇Rv. Preliminary results were promising with MIC values in the range 1.56-12.5 μg/mL.

Full text of DOI:10.1016/j.bmcl.2011.05.037

Bioorganic & Medicinal Chemistry Letters 21 (2011) 3947–3950
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and screening of (E)-1-(b-D-galactopyranosyl)-4-(aryl)but-3-ene-2-one
against Mycobacterium tuberculosis q
G. Mugunthan a, Kuppala Ramakrishna a, Dharmarajan Sriram b, Perumal Yogeeswari b,
K. P. Ravindranathan Kartha a,
⇑
a Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research, S.A.S. Nagar, Punjab 160062, India
b Medicinal Chemistry Research Laboratory, Pharmacy Group, Birla Institute of Technology and Science, Hyderabad 500078, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 December 2010
Revised 5 May 2011
Accepted 7 May 2011
Available online 15 May 2011
A series of galactose-derived aryl enones were synthesised and screened against Mycobacterium tubercu-
losis H37Rv. Preliminary results were promising with MIC values in the range 1.56–12.5 lg/mL.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Mycobacterium tuberculosis
D-Galactose
Enones
Anti-tubercular activity
Introduction: According to the WHO estimate approximately 1.7
million lives were wiped off by TB in 2009 alone. Even though
there is a marginal decrease in the TB death rate, there were 9.4
million new cases in 2009, including the 1.1 million cases of co-
infection with HIV. The more alarming fact is that the extensively
drug resistant cases (XDR-TB) have been reported from 58 coun-
tries and are reportedly resistant to a majority of drugs in use
against TB.2 In order to tackle the issue of resistance there is an ur-
gent need for new drugs with novel modes of action and hence less
susceptible to resistance.
C-Glycosides are stable surrogates of O-glycosides and are often
used as investigative tool in chemical biology and medicinal chem-
istry, in particular C-glycoside analogs are often used as substrate
mimics for different enzymes involved in the carbohydrate metab-
olism.3,4 Carbohydrate-rich cell wall architecture of Mycobacterium
tuberculosis (M.tb) makes it susceptible to different sugar deriva-
tives targeting different enzymatic processes involved in the cell
wall biosynthesis.
increasingly used for the synthesis of bioactive compounds. Alka-
noyl glycosides derived from b-C-glycosidic ketones were shown
to posses
a-glucosidase, glucose-6-phosphatase and glycogen
phosphorylase inhibition properties.10 Likewise, enones derived
from C-furyl glycosides were shown to have anti-microbial effect11
(Fig. 1). Since the anti-microbial activity and the safety issues of
the enone-containing chalcones have been estabilsed in the litera-
ture we decided to make anaolgs of uridine 5c-diphosphate galac-
topyranose (UDP-Galp), by replacing the phosphate bridge with an
enone-linker and the uridine component with variously substi-
tuted aromatic moeities (Fig. 2), which can potentially act as cell
wall biosynthesis inhibitor in mycobacteria, particularly the
mutase enzyme which converts the galactopyranose to a galactofu-
ranose residue (which is essential for the cell wall biosynthesis,
and hence the viability of the microbe). Absence of Galf, and hence
Galp-mutase in mammals, makes Galf metabolism an attractive
and potential target for antimycobacterial chemotherapy.12
In order to validate this hypothesis a small library of sugar-
derived enones bearing different aromatic moieties (potential mi-
mic of the uridine part) were synthesized and screened against
M.tb. H37Rv.
Chalcones are well known for their anti-infective properties,5 in
particular they are reported to have anti-tubercular activity.6,7 Lo-
pez et al. reported a series of anti-fungal chalcones acting against
the cell wall biosynthesis by inhibiting the b-(1,3)-glucan synthase
and/or chitin synthase.8 Following the Lubineau’s method for the
synthesis of b-C-glycosidic ketones,9 C-glycosides have been
The C-glycoside of D-galactose was prepared by the reaction of
reducing sugar with acetylacetone. Out of the different procedures
reported for this process, the reaction condition reported by
Bragnier and Schermann13 was found to give the desired 1-C-
(b-D-galactopyranosyl)-propan-2-one (5, Scheme 1) in high yields.
q
See Ref. 1.
The 1H NMR spectrum of the product typically showed a signal for
⇑
Corresponding author. Tel.: +91 (0) 172 2214682 86; fax: +91 (0) 172 2214692.
E-mail address: rkartha@niper.ac.in (K.P. Ravindranathan Kartha).
the methyl (singlet at d 2.20 ppm) and methylene (double doublet
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.05.037

3948
G. Mugunthan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3947–3950
O
1
Chitin synthase inhibitor
OH
O
O
HO
O
Br
O
O
HO
HO
HO
3
OH
2
O
O
Br
α-Glucosidase inhibitor
Anti-microbial
Figure 1. Bioactive chalcones and sugar-derived enones.
OH
OH
HO
O
O
OH
OH
HO
O
O
NH
O
O
HO
O
Ar
O
O
P
N
P
O
OH
O
O
Ar = Aromatic moeity
O
HO
OH
Analog with simple enone linker
UDP-Galp: Original substrate for mutase
Figure 2. Comparison of UDP-Galp and its potential mimic with enone linker.
OH
OH
OH
OH
HO
OH
OH
HO
b
a
O
O
O
Ar
HO
OH
6 a-s
OH
OH
OH
4
Ar = Aromatic moiety
5
O
O
Scheme 1. Synthesis of
30–60 min, 60–70%.
D-galactose-derived enones. Reagents and conditions: (a) 2,4-Pentanedione, NaHCO3, H2O–THF (2:1), 90 °C, 24 h, 70%; (b) ArCHO, LiOHꢀH2O, MeOH,
at d 2.64 ppm) of the propan-2-one moiety as expected. It was fur-
MIC in the range 1.56–12.5
screened five molecules (6a, 6e, 6f, 6j, and 6k, Table 1) were found
to be promising with MIC of 1.56 g/mL which is equal to that of
lg/mL. Out of the 19 compounds
ther supported by the matching physical constants (mp 169–
171 °C, literature 175–176 °C13; [
a]
+19.5 (c 1, H2O), literature
l
D
[a
]
D
+20 (c 1, H2O)13).
one of the first line drugs ethambutol (one of the positive controls
Simple chalcones can be conveniently prepared by a base-
used) under the test condition. Also four other compounds (6b, 6m,
catalysed condensation reaction. For carbohydrate-derived enone
6o, and 6p) showed a good MIC value of 3.13 lg/mL (Table 1).
formation, organic bases like pyrrolidine14 and a combination of
Apart from the distant structural similarity of this analog to the
original substrate, the electron withdrawing and donating property
of the substituents and the hydrogen bonding ability of the hetero-
atom of the substituent on the aromatic ring, appeared to have a
possible influence on the activity. Compound 6a containing the
furan moiety turned out to be one of the most active molecules,
possibly due to its structural similarity to the ribofuranosyl moiety
of the uridine and its hydrogen bonding properties. In compound
6b, the electron donating and the hydrogen bond accepting meth-
oxy substituent in the para position of the phenyl ring appeared to
play a determining role in the activity, as possibly evident from the
fact that absence of such a group and presence of an electron with-
drawing nitro group in the ortho position of the phenyl ring made
the compounds 6c and 6d almost inactive. The positive role of an
electron donating, hydrogen bond accepting substituent in the para
position of the phenyl ring can also be observed in compounds 6e,
6f, and 6k all the three compounds are most active with MIC
L
-proline and triethylamine15 have been reported. These reactions
catalysed by organic bases were found to be slow, low yielding
and prone to the isomerisation of the glycosidic linkage. Base-
catalysed epimerization of the C-glycoside bearing a keto group
has been studied earlier,16,17 and has been shown to attain an equi-
librium. The epimerization (anomerisation) was observed even
when the peracetylated C-glycoside was subjected to deacetylation
using sodium methoxide.18 LiOHꢀH2O is a known catalyst for chal-
cone formation,19 and when the enone formation was attempted in
the presence of this catalyst; a quicker reaction was observed, but
again it was found to be accompanied by anomerisation though to
a smaller extent. Thus, sugar-derived enones 6a–6s were prepared
in the presence of LiOHꢀH2O20 (Scheme 1) and were subjected to
chromatographic purification to get the respective pure b-isomers
(6a–6s). The trans double bond in the enone was established by the
large coupling constant (J = 8.0 Hz) between the two olefinic pro-
tons.21 Thus, a small library of sugar-derived enones were success-
fully synthesized using different aromatic aldehydes and were
screened against M.tb H37Rv.22 Assay results are promising with
1.56 lg/mL. Also in compounds 6g, 6h, and 6l this effect was evi-
dent, though to a lesser extent. In the case of compound 6i, though
it possessed an electron donating and hydrogen bond accepting

G. Mugunthan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3947–3950
3949
Table 1
Table 1 (continued)
In vitro assay results against M.tb H37Rv
Molecule
Ar moiety
MIC (
3.13
lg/mL)
6m
OH
OH
HO
N
O
Ar
6n
6.25
OH
N
N
Ar = Aromatic moiety
O
6o
6p
6q
6r
3.13
3.13
>12.5
6.25
Molecule
Ar moiety
MIC (
lg/mL)
OH
O
6a
1.56
OH
O
O
6b
6c
3.13
Cl
O2N
Cl
>12.5
O
O2N
6d
6e
>12.5
1.56
O
F
F
F
6s
12.5
F
F
O
F
Ethambutol
Isoniazid
Rifampicin
1.56
0.05
0.10
O
OH
6f
1.56
6.25
O
NO2
O
methoxy substituent in the para position of the phenyl ring, the
bromine present in the meta position was found to be detrimental.
One of the most active compounds 6j possessing an electron with-
drawing carboxyl group, even though did not fit in the observed
trend of increased activity with electron donating substituent in
the para position as to be expected from the foregoing observa-
tions, fit in to the hydrogen bond accepting category very well.
Among the compounds containing a quinoline residue, the link-
age-position appeared to influence the activity. Thus, compound
6m linked through C-2 of the quinoline residue was more active
than compound 6n that is linked through C-3 of the same moiety;
and the presence of a –OH group on C-8 of the quinoline seemed to
play no role in determining the activity, as compounds 6m and 6o
gave the same activity. Compounds containing lipophilic substitu-
ents performed poor in general with the exception of 6p bearing a
vinyl substituent at the meta position of the phenyl ring.
O
O
6g
O
6h
6i
6.25
O
O
O
Br
>12.5
The above results indicate that the Michael-accepting property of
the enones perhaps does not play a prominent role in determining
the activity (e.g., compound 6j containing an electron withdrawing
carboxyl group and compound 6k containing an electron donating
OH
6j
1.56
1.56
O
dimethylamino group gave similar results (MIC, 1.56 lg/mL, Table
N
1). To ascertain the mechanismof action, including the role of the en-
one moiety investigations are in progress currently using analogous
molecules with different types of linkers in the place of the enones
described here. This followed by the enzyme inhibition and the tox-
icity studies would shed more light on the nature of the action of
these potentially important molecules. Finally, evidence for the
stability of the enones under the test conditions was obtained from
blank experimentsconducted, using two of the active compounds 6e
and 6k as representative examples, in which the samples at the end
6k
OH
6l
6.25
N
OH

3950
G. Mugunthan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3947–3950
13. Bragnier, N.; Scherrmann, M.-C. Synthesis 2005, 814.
14. Bisht, S. S.; Pandey, J.; Sharma, A.; Tripathi, R. P. Carbohydr. Res. 2008, 343, 1399.
15. Wang, J.-F.; Lei, M.; Li, Q.; Ge, Z.-M.; Wang, X.; Li, R.-T. Tetrahedron 2009, 65,
4826.
16. Allevi, P.; Anastasia, M.; Ciuffreda, P.; Fiecchi, A.; Scala, A. J. Chem. Soc., Perkin
Trans. 1 1989, 1275.
17. Grugier, J.; Xie, J.; Duarte, I.; Valéry, J.-M. J. Org. Chem. 2000, 65, 979.
18. Riemann, I.; Papadopoulos, M. A.; Knorst, M.; Fessner, W.-D. Aust. J. Chem. 2002,
55, 147.
19. Bhagat, S.; Sharma, R.; Sawant, D. M.; Sharma, L.; Chakraborti, A. K. J. Mol. Catal.
A: Chem. 2006, 244, 20.
of 28 days when subjected to extraction and LC–MS analysis were
found to be intact.
Conclusion: Though the exact target is currently unknown,
galactose-derived enones were found to be highly effective as anti-
mycobacterial agents against M.tb H37Rv as expected. SAR of the
screened compounds suggests that antimycobacterial activity is
not because of the Micheal-acceptor ability of these compounds.
Detailed enzyme inhibition studies related to the cell wall biosyn-
thesis to ascertain its likely inhibition and toxicity studies are cur-
rently underway and will be published in due course.
20. Representative procedure for synthesis of 6a: To
a solution of 1-C-(b-D-
galactopyranosyl)-propan-2-one (1 mmol, 220 mg) in anhydrous methanol
(5 mL) was added LiOHꢀH2O (0.1 mmol) and was stirred for 10 min followed by
the addition of the desired piperonal (1 mmol, 150 mg). The reaction mixture
was stirred at room temperature for 45 min. After the completion of the
reaction, it was quenched with Dowex 50WX8-200 (H+) resin and the resin was
filtered off. The filtrate was concentrated under reduced pressure and was
subjected to chromatographic purification to get the pure product (230 mg,
65% b-isomer).
Acknowledgment
M.G. sincerely acknowledges financial support in the form of a
Senior Research Fellowship by NIPER.
21. Representative physical and NMR data for 6a: yield 65%; brownish solid; mp
169.5–170.2; [
a
]
ꢁ113.9 (c 0.05, MeOH); 1H NMR (400 MHz, CD3OD) d 7.58
References and notes
D
(d, J = 16 Hz, 1H, H-1), 7.42 (s, 1H, Ar-H), 7.13, 6.85 (2ꢂd, J = 8 Hz, 2H, Ar-H),
6.76 (d, J = 16 Hz, 1H, H-2), 6.01 (s, 2H, –O–CH2–O–), 3.87–3.69 (m, 1H, H-40),
3.72–3.62 (m, 3H, H-60a, b and H-10), 3.50–3.44 (m, 3H, H-50, H-30 and H-20),
3.09–3.04 (dd, J = 16 Hz, 1H, H-4a), 2.95–2.89 (dd, J = 9.2 Hz, J = 16 Hz, 1H, H-
4b); 13C NMR (100 M Hz, CD3OD) d 199.77, 149.77, 148.57, 143.51, 129.00,
125.01, 124.17, 108.10, 106.19, 101.69, 78.75, 76.70, 74.89, 70.89, 69.45, 61.14,
42.91; IR (KBr) mmax 3441, 1660, 1626, 1660, 1597, 1506, 1451 cmꢁ1; MALDI-
TOF MS: m/z calculated for C17H20O8: 352.34. Found: 375.74 [M+Na]+, 391.75
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agar tubes containing 10-fold serial dilutions of drugs per mL. The tubes were
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