1,5-Diphenylpyrrole derivatives as antimycobacterial agents. Probing the influence on antimycobacterial activity of lipophilic substituents at the phenyl rings

Article abstract of DOI:10.1021/jm701560p

Synthesis and biological evaluation of new derivatives of 1,5-bis(4-chlorophenyl)-2-methyl-3-(4-methylpiperazin-1-yl)methyl-1H-pyrrole (BM 212, 16) are reported. Variously substituted phenyl rings with different substitution pattern and lipophilicity were

Full text of DOI:10.1021/jm701560p

3644
J. Med. Chem. 2008, 51, 3644–3648
Brief Articles
1,5-Diphenylpyrrole Derivatives as Antimycobacterial Agents. Probing the Influence on
Antimycobacterial Activity of Lipophilic Substituents at the Phenyl Rings
Mariangela Biava,*,† Giulio Cesare Porretta,† Giovanna Poce,† Alessandro De Logu,‡ Manuela Saddi,‡ Rita Meleddu,‡
Fabrizio Manetti,*,§ Edda De Rossi,| and Maurizio Botta§
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente AttiVe, UniVersita` “La Sapienza”, Piazzale A. Moro 5, I-00185
Rome, Italy, Dipartimento di Scienze e Tecnologie Biomediche, Sezione di Microbiologia Medica, UniVersita` degli Studi di Cagliari, Viale
Sant’Ignazio 38, I-09123 Cagliari, Italy, Dipartimento Farmaco Chimico Tecnologico, UniVersita` degli Studi di Siena, Via Aldo Moro, I-53100
Siena, Italy, and Dipartimento di Genetica e Microbiologia, UniVersita` degli Studi di PaVia, Via Ferrata 1, I-27100 PaVia, Italy
ReceiVed December 13, 2007
Synthesis and biological evaluation of new derivatives of 1,5-bis(4-chlorophenyl)-2-methyl-3-(4-methylpip-
erazin-1-yl)methyl-1H-pyrrole (BM 212, 16) are reported. Variously substituted phenyl rings with different
substitution pattern and lipophilicity were added to the pyrrole nucleus to evaluate their influence on the
activity toward Mycobacterium tuberculosis (MTB) and atypical mycobacteria. The most active derivatives
showed activity between 0.125-0.5 µg/mL (better than 16 and streptomycin) and protection index (64-256)
higher than 16 (4) and similar to isoniazid and streptomycin (128).
Introduction
revealed that a hydrophobic feature was a crucial key for
determining antitubercular activity, in agreement with literature
reports showing that a more hydrophobic character of com-
pounds was required to increase the antitubercular potency.5
On this basis, attempts to find the optimal substituents for
fulfilling the pharmacophoric features and thus improving
antitubercular activity led us to the identification of five new
hits.6–10 Compound 1 (Table 1), bearing a p-F-phenyl ring at
N1 of the pyrrole, a thiomorpholinomethyl chain at C3, and a
p-methylphenyl moiety at C5, showed the best antimycobacterial
activity.11
Moreover, log P values calculated for pyrrole derivatives
previously synthesized showed that more active compounds had
a marked hydrophobic character, corresponding to log P values
from 5.58 to 6.70. To check the hypothesis that a further increase
of lipophilicity could improve activity of compounds, facilitating
their entrance through the lipid-rich mycobacterial membrane,
we planned to design and synthesize new pyrrole derivatives
2-15 by modification of the molecular structure of 1 (Table
1). In particular, we chose to increase the size of the p-methyl
group of 1 to an ethyl, propyl, and isopropyl chain. On the other
hand, the p-F-phenyl group was maintained and the correspond-
ing chloro analogue was also taken into account because it was
reminiscent of N1 and C5 substituents of 16. Each of the p-alkyl-
and p-halophenyl substituents was alternately placed at position
1 or 5, respectively, following a suggestion coming from
pharmacophore modeling (further details in Supporting Informa-
tion). The new compounds were all tested against MTB and
atypical mycobacteria. The most active of them toward MTB
were also assayed for their activity toward the H37Rv strain of
MTB, toward resistant and MDR clinical isolate strains, and
toward intramacrophagic MTB.
Mycobacterium tuberculosis (MTBa), responsible for tuber-
culosis (TB) in humans, causes the death of almost 3 million
people each year, and it is positioned as the leading bacterial
infectious agent.1,2 Moreover, today more people die from
tuberculosis than ever before;2 therefore, the development of
new drugs with activity against multidrug-resistant (MDR) TB,
extensively drug-resistant (XDR) TB, and latent TB is a priority
task, although new agents that will shorten the duration of
current chemotherapy are also needed. Furthermore, the new
compounds should have a novel mode of action, since such
agents are likely to be more effective against drug resistant
strains, and they must be selective for mycobacteria and
compatible with HIV-1 drugs (further details in Supporting
Information).
In the past, we have identified 1,5-diarylpyrrole derivatives
with good activity toward MTB and non-tuberculosis myco-
bacteria, also showing low cytotoxicity. Compound 16 (BM 212)
was the best hit compound of this new class of derivatives3
and one of the most promising future antimycobacterial drug.4
Moreover, structure-activity relationship (SAR) studies and a
pharmacophore-based ligand design approach allowed us to
identify chemical groups and their substitution pattern on the
pyrrole, responsible for the activity. Analysis of the superposi-
tion pathway of compounds on the pharmacophoric model
* To whom correspondence should be addressed. For M.B.: phone, +39
06 4991 3812; fax, +39 06 4991 3133; e-mail, mariangela.biava@uniroma1.it.
For F.M.: phone, +39 0577 234330; fax, +39 0577 234333; e-mail,
manettif@unisi.it.
†
Universita` “La Sapienza”.
Universita` degli Studi di Cagliari.
Universita` degli Studi di Siena.
Universita` degli Studi di Pavia.
‡
§
|
a Abbreviations: MTB, Mycobacterium tuberculosis; TB, tuberculosis;
MDR, multidrug-resistant; XDR, extensively drug-resistant; BM 212, 1,5-
(4-chlorophenyl)-2-methyl-3-(4-methylpiperazin-1-yl)methyl-1H-pyrrole; SAR,
structure-activity relationship; MIC, minimum inhibitory concentration;
MNTD, maximum nontoxic dose; PI, protection index.
Chemistry
Synthesis of the target compounds is shown in Scheme 1.
Briefly, reaction of a suitable benzaldehyde 17a-f with methyl
vinyl ketone 18 was performed according to the Stetter reaction,
10.1021/jm701560p CCC: $40.75
2008 American Chemical Society
Published on Web 05/22/2008

Brief Articles
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 12 3645
Table 1. Structure, in Vitro Activity, Cytotoxicity, Protection Index
(PI), and Calculated log P Values of the New Pyrrole Derivatives 2-15
and Reference Compounds
clinical isolates of MTB (Table 3), while 14 and 15 were also
tested toward intracellular MTB (Table 4).
Results and Discussion
Compounds 2-15 were tested in vitro against MTB and
atypical mycobacteria, and for the most active of them, activity
toward MTB H37Rv and resistant and multidrug-resistant
clinical isolate strains was also evaluated. Biological data
showed a general trend that confirmed previous suggestions on
(i) the importance of the (thiomorpholin-4-yl)methyl moiety as
a substituent of the pyrrole ring, (ii) the nature of the substituent
present in the two aromatic rings at N1 and C5, and (iii) their
substitution pattern.9–11
In principle, 2-15 showed a very good in vitro biological
profile (Table 1). In fact, they had an impressive activity toward
MTB (along with poor cytotoxicity) with a consequent high
protection index. Moreover, in several cases, they were also
active toward atypical mycobacteria (Table 2). As examples,
M. smegmatis was sensitive to 5 and 6 (whose MIC values were
32- and 16-fold better than that of isoniazid and significantly
improved with respect to that of 1), while 4, 10, and 11 had
interesting activity toward M. aVium and M. marinum, compa-
rable to 1 and better than isoniazid.
Derivatives bearing a halogen at the N1 phenyl ring were
generally more active than the corresponding C5 analogues,
confirming our previous findings based on the pharmacophoric
model. It is also important to note that compounds with a
chlorine atom on the phenyl ring at N1 or, alternatively, at C5
were in principle more active and less cytotoxic than the
corresponding fluoro analogues, thus showing a high PI. As an
example, 9 and 10 showed a biological profile in terms of MIC
and PI better than that found for the corresponding fluoro
analogues 2 and 3, respectively (Table 1). The only exception
to this trend was 11, showing a PI lower than that of 4 (64
versus 128, respectively), although it was characterized by a
better MIC value (0.125 versus 0.25 µg/mL, respectively).
The most interesting microbiological results were represented
by activity values found toward MTB H37Rv and its clinical
strain resistant to rifampicin. In fact, for all the new compounds
tested, MIC values ranging from 0.125 to 0.5 µg/mL were found
(Table 2), comparable to 1 and, in several cases, better than
(10 and 11) or comparable to (4, 9, 14, and 15) that of isoniazid
(0.25 µg/mL). On the other hand, compounds were inactive
against isoniazid-resistant strains, with the sole exception of 11,
which retained some activity (8 µg/mL). Also in this case, 8-11,
bearing a N1 p-halo (namely, chloro) substituent, were generally
more active than the corresponding C5 p-halo (namely, chloro)
substituted analogues 12-15. The experimental evidence makes
these compounds extremely interesting when compared to the
compounds now used in therapy, which tend to be less active
against drug-resistant mycobacteria.
c
compda
R1
R2
MICb MNTD50
PId
log Pe
1
2
3
4
5
6
7
8
F
F
F
F
methyl
ethyl
propyl
0.25
1
2
64
16
4
256
16
2
128
1
5.86
6.32
6.77
6.57
6.32
6.77
isopropyl 0.25
32
2
4
ethyl
propyl
isopropyl
Cl
F
F
F
2
1
4
16
8
0.5 6.57
128
64
64
64
methyl
ethyl
propyl
0.5
0.25
0.25
64
16
16
8
6.32
6.77
7.23
7.03
6.32
6.77
7.23
7.03
6.02
9
Cl
Cl
Cl
10
11
12
13
14
15
16
isopropyl 0.125
methyl
ethyl
propyl
Cl
Cl
Cl
0.5
0.5
0.25
0.25
1
16
16
32
32
4
32
32
128
128
4
128
>128
256
isopropyl Cl
isoniazid
streptomycin
rifampicin
0.125
0.50
0.25
32
>64
64
a Compounds 1-15 have been submitted to a PCT patent (ref 14). b MIC
(µg/mL) toward M. tuberculosis 103471. c MNTD50 (µg/mL) toward Vero
cells. d PI ) protection index, as the ratio of cytotoxicity (MNTD50) to in
vitro activity (MIC). e Calculated by the AlogP98 method (ref 15).
by employing the Discovery microwave system apparatus
(optimizing time and yield of reaction),11 leading to 1,4-
diketones 19a-f. In the presence of the appropriate amine,
following the Paal-Knoor condensation, 19a-f cyclized to yield
the expected 1,5-diarylpyrroles 20a-n. Construction of the side
chain at C3 was achieved in good yield by reaction of
compounds 20a-n with formaldehyde and thiomorpholine,
according to the Mannich reaction conditions, to give the final
compounds 2-15.
Biology
Compounds showing minimum inhibitory concentration
(MIC) values of 16 µg/mL or lower in a preliminary assay were
then studied for their inhibitory activity toward MTB 103471
and a panel of atypical mycobacteria, such as M. marinum 6423,
M. aVium 103317, and M. smegmatis 10359. MIC values were
determined for each compound toward each of the test
mycobacteria.
An analysis of the lipophilic character of the new compounds
showed that their calculated log P values ranged from 6.32 to
7.23 (Table 1). In general, the increase of log P values (in
comparison to the previous series of pyrrole derivatives)11 was
found to be associated with an improvement of activity. In fact,
9 out of the 14 new compounds (4, 8-15, about 64%) had MIC
values toward MTB 103471 in the range between 0.5 and 0.125
µg/mL, with respect to only 4 out of 28 congeneric derivatives
(about 14%) described in our previous work.11 Four of the
remaining new pyrroles (2, 3, 5, and 6) had activity between 1
and 2 µg/mL, while 7 was the sole exception (MIC of 16 µg/
mL).
Cytotoxic activity assays were performed in Vero cells to
determine the maximum nontoxic dose (MNTD, expressed as
µg/mL), defined as the drug concentration that decreases cell
multiplication less than 50% of the control.
In vitro activity of 2-15, as well as cytotoxicity and
protection index (PI), are reported in Tables 1 and 2. Compounds
that were particularly active were also tested against MTB
H37Rv and strains resistant to isoniazid and rifampicin (Table
2). Isoniazid, streptomycin, rifampicin, and 16 were used as
reference compounds.
Selected compounds (namely, 4, 14, and 15), which showed
an interesting activity against reference strains and the highest
PI values, were also evaluated for their activity against 32
Moreover, it is also very important to point out that two of
the most active compounds (10 and 11), showing the best MIC

3646 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 12
Brief Articles
Scheme 1a
a Compounds. 17a, R2 ) F; 17b, R2 ) Cl; 17c, R2 ) CH3; 17d, R2 ) C2H5; 17e, R2 ) C3H7; 17f, R2 ) i-C3H7; 19a, R2 ) F; 19b, R2 ) Cl; 19c, R2
) CH3; 19d, R2 ) C2H5; 19e, R2 ) C3H7; 19f, R2 ) i-C3H7; 20a, R1 ) F, R2 ) C2H5; 20b, R1 ) F, R2 ) C3H7; 20c, R1 ) F, R2 ) i-C3H7; 20d, R1
)
Cl, R2 ) CH3; 20e, R1 ) Cl, R2 ) C2H5; 20f, R1 ) Cl, R2 ) C3H7; 20g, R1 ) Cl, R2 ) i-C3H7; 20h, R1 ) C2H5, R2 ) F; 20i, R1 ) C3H7, R2 ) F; 20j,
R1 ) i-C3H7, R2 ) F; 20k, R1 ) CH3, R2 ) Cl; 20l, R1 ) C2H5, R2 ) Cl; 20m, R1 ) C3H7, R2 ) Cl; 20n, R1 ) i-C3H7, R2 ) Cl; 2, R1 ) F, R2 ) C2H5;
3, R1 ) F, R2 ) C3H7; 4, R1 ) F, R2 ) i-C3H7; 5, R1 ) C2H5, R2 ) F; 6, R1 ) C3H7, R2 ) F; 7, R1 ) i-C3H7, R2 ) F; 8, R1 ) Cl, R2 ) CH3; 9, R1 )
Cl, R2 ) C2H5; 10, R1 ) Cl, R2 ) C3H7; 11, R1 ) Cl, R2 ) i-C3H7; 12, R1 ) CH3, R2 ) Cl; 13, R1 ) C2H5, R2 ) Cl; 14, R1 ) C3H7, R2 ) Cl; 15, R1
) i-C3H7, R2 ) Cl. Reagents: (a) (1) 3-ethyl-5-(2-hydroxyethyl)-4-methylthiazolium bromide, NEt3, mw, 15 min, (2) 2 N HCl; (b) H2NC6H4R1, p-toluensulfonic
acid, EtOH, mw, 30 min; (c) (1) thiomorpholine, CH3CN, HCHO, CH3COOH, room temp, 3 h; (2) NaOH 20% w/v.
Table 2. In Vitro Activity of the New Pyrrole Derivatives 2-15 and
Reference Compounds toward Atypical Mycobacteria, M. tuberculosis
H37Rv, and Its Rifampicin-Resistant and Isoniazid-Resistant Strains
that an increase of lipophilicity could improve antimycobacterial
activity of such a class of compounds.
As a general trend for the activity toward MTB 103471,
chloro derivatives 8-15 were more active and less cytotoxic
with respect to the corresponding fluoro analogues and they were
also characterized by a higher lipophilicity. Moreover, com-
pounds with the p-Cl-phenyl ring at N1 (8-11) showed a very
slight improvement of activity with respect to the corresponding
C5 derivatives (12-15). This was in good agreement with what
arose from the analysis of the superposition mode of such a
class of compounds to the pharmacophoric model. In fact, 1,5-
diarylpyrrole derivatives with a substitution pattern similar to
that of 1 (as in 8-11) showed their preferred orientation
characterized by the C5 p-alkylphenyl moiety able to match a
hydrophobic (with the p-alkyl group) and an aromatic (with the
phenyl ring at C5) portion of the pharmacophore, while the N1
p-halophenyl substituent filled the remaining aromatic ring
feature of the model. Inverting the substitution pattern at
positions 1 and 5 (as in 12-15) caused a rotation of about 180°
around the pyrrole plane, leading to fit all pharmacophoric
features in a less profitable way (i.e., the fit score, which is the
sum of how well each chemical feature of compounds fits each
pharmacophoric feature, was lower in comparison to the
alternative orientation).11
MIC (µg/mL)
M.
M.
M.
MTB
R-
I-
compd
aVium marinum smegmatis H37Rv resistanta resistantb
1
2
3
4
5
6
7
8
8
>16
>16
4
>16
>16
>16
16
16
4
8
>16
16
16
16
8
>16
>16
8
>16
16
>16
>16
8
4
4
16
4
16
16
100
16
32
16
>16
>16
>16
2
4
8
>16
8
0.125
NDc
NDc
0.25
NDc
NDc
NDc
0.5
0.25
0.125
0.125
0.5
0.5
32
NDc
NDc
0.25
NDc
NDc
NDc
0.5
NDc
NDc
>16
NDc
NDc
NDc
16
9
0.25
0.125
0.125
0.5
16
>16
8
>16
16
>16
>16
2
10
11
12
13
14
15
16
>16
8
16
8
0.5
0.5
>16
>16
25
64
8
0.25
0.25
2
0.25
4
0.25
0.25
1
0.25
4
0.5
32
8
isoniazid
streptomycin
rifampicin
>16
16
0.25
0.25
0.5
32
0.25
>64
a R-resistant: rifampicin-resistant. b I-resistant: isoniazid-resistant. c ND:
not determined.
Table 3. In Vitro Antimicrobial Activity (µg/mL) of Selected
Compounds 4, 14, and 15 toward 32 Clinical Isolates of M. tuberculosis,
As Determined by the Agar Dilution Method
Among fluoro derivatives, the most interesting activity (0.25
µg/mL) was shown by 4, also characterized by one of the best
protection index (128).
compd
MIC range
MIC50
MIC90
Compounds 4, 14, and 15 were also assayed toward clinical
isolates of MTB (showing activity comparable to that of
isoniazid used as control, Table 3), while 14 and 15 were also
evaluated for their activity toward intracellular MTB (Table 4).
It is important to note that while the inhibitory activity toward
extracellular MTB accounts for the ability of test compounds
to treat active tuberculosis, assays on intracellular MTB assess
the ability of compounds to inhibit mycobacteria during the
latent phase of TB, before latent TB infection itself progresses
to active disease. Biological results reported in Table 4 indicate
that both compounds showed a concentration-dependent inhibi-
tion of the growth of intracellular mycobacteria, comparable to
that of rifampicin, used as the reference compound. This result
is very important because mycobacteria can reside for years
inside lymphoid cells and macrophages (during the latent phase
of TB) and many traditional drugs are unable to get at it.
Moreover, combating latent TB infection is one of the major
challenges mainly for reducing the high rate of progression to
active disease in immunocompromised individuals (in fact,
progression is higher in persons with concomitant HIV-1
infection).
4
14
15
isoniazid
0.25–0.5
0.125–2
0.25–1
0.50
0.50
0.25
0.125
0.5
1
0.5
0.25
0.125–1
Table 4. Activity of Decreasing Concentrations of Compounds 14 and
15 in Macrophages J774 Infected with M. tuberculosis H37Rva
concentrations (µg/mL)
compd
14
15
1
0.5
0.25
98.79
98.74
99.79
85.79
84.21
98.42
21.05
47.37
95.79
rifampicin
a Results are expressed as % of reduction of the number of surviving
bacteria with respect to the untreated control.
values, also had the highest log P values (7.23 and 7.03,
respectively). In a similar way, their corresponding regioisomers
14 and 15 were the most active among compounds belonging
to the subclass consisting of 12-15. These results, showing that
antimycobacterial activity was in general improved with the
increase of lipophilicity, provided support for the hypothesis

Brief Articles
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 12 3647
3.24 (t, 2H), 2.85 (t, 2H), 2.69 (q, 2H), 2.47 (s, 3H), 1.22 (t, 3H).
Anal. (C13H16O2) C, H.
In summary, biological results on the new pyrroles showed
an antimycobacterial activity generally improved with respect
to analogues previously described. On the basis of the low
cytotoxicity, several compounds also showed a high PI value,
comparable to that of current antitubercular drugs such as
isoniazid. Compounds were active toward rifampicin-resitant
strains, as well as toward a wide panel of clinical isolates, further
confirming the great significance reached in recent years by the
pyrrole class of compounds in the field of new antimycobacterial
compounds.2
General Procedure for the Preparation of 1,5-Diaryl-
pyrroles 20a-n. Following the Paal-Knorr reaction, the proper
diketone 19 (2.28 mmol) was dissolved in ethanol (2 mL) into
a round-bottom flask equipped with a stir bar. The suitable amine
(2.28 mmol) and p-toluensulfonic acid (30 mg, 0.17 mmol) were
added. The flask was inserted into the cavity of the Discovery
microwave system apparatus and heated (150 W for 30 min,
internal temperature 160 °C, and internal pressure 150 psi).13
The reaction mixture was cooled and concentrated. The crude
material was purified by chromatography on aluminum oxide
(activity II-III, according to Brockmann) with cyclohexane as
the eluant to give the expected 1,5-diarylpyrroles 20a,c-n as
solids in satisfactory yield and compound 20b as an oil. Example,
2-methyl-1-(4-fluorophenyl)-5-(4-ethylphenyl)-1H-pyrrole (20a):
mp 118 °C (yield 80%); 1H NMR (CDCl3) δ 7.12 (m, 2H), 7.05
(m, 2H), 6.99 (m, 2H), 6.96 (m, 2H), 6.36 (d, 1H), 6.31 (d,
1H), 2.56 (q, 2H), 2.13 (s, 3H), 1.17 (t, 3H). Anal. (C19H18FN)
C, H, N, F.
General Procedure for the Preparation of Compounds
2-15. Following the Mannich reaction, to a stirred solution of an
appropriate pyrrole 20a-n (5.6 mmol) in acetonitrile (20 mL), a
mixture of thiomorpholine (0.57 g, 5.6 mmol), formaldehyde (0.18
g, 5.6 mmol) (40% in water), and 5 mL of acetic acid was added
dropwise. After the addition was complete, the mixture was stirred
at room temperature for 3 h. The mixture was then treated with a
solution of sodium hydroxide (20%, w/v) and extracted with ethyl
acetate. The organic extracts were combined, washed with water,
and dried. After removal of solvent, the residue was purified by
column chromatography, using silica gel and petroleum ether/ethyl
acetate (3:1 v/v). The eluates were combined after TLC control
and the solvent was removed to give 2-15 as solids in satisfactory
yield. Recrystallization from diethyl ether gave the final products.
Example, 2-methyl-1-(4-fluorophenyl)-3-(thiomorpholin-4-yl)-
methyl-5-(4-ethylphenyl)-1H-pyrrole (2): mp 98 °C (yield 40%);
1H NMR (CDCl3) δ 7.29 (m, 2H), 7.06 (m, 2H), 6.92 (m, 4H),
6.26 (s, 1H), 3.42 (s, 2H), 2.74 (s broad, 4H), 2.68 (s broad,
4H), 2.58 (q, 2H), 2.03 (s, 3H), 1.15 (t, 3H). Anal. (C24H27FN2S)
C, H, N, S, F.
Conclusions
New pyrrole derivatives related to 16 were designed, syn-
thesized, and tested to probe the influence of lipophilicity on
their antimycobacterial activity, based on the hypothesis that
an increase of lipophilicity could improve inhibitory activity
toward mycobacteria. In general, the new compounds showed
a very high antimycobacterial activity, and the best results were
found for compounds with higher lipophilicity (with respect to
the parent compounds),11 in agreement with recent literature
showing that the most active compounds among a set of
oxadiazole derivatives were those characterized by the highest
log P values (namely, 8.14 and 9.20).12 Further efforts are
required to design and synthesize new derivatives with ap-
propriate lipophilicity. In fact, although a relationship between
MIC values and log P seemed to exist for the current pyrrole
derivatives, we are aware that in general molecules with too
high a lipophilicity are destined to have a very low solubility
(not enough for assays to be performed) and to be trapped in
hydrophobic media (i.e., membranes) that avoid compounds
themselves to reach their site of action. On this basis, suitable
formulations should be studied to improve pharmacokinetic
properties of compounds and allow them to reach their site of
action.
Finally, considering that no new antitubercular drug has been
introduced in the market during the past 50 years, results
reported in this paper are of crucial importance in the field of
antitubercular agents, and they further support the evidence that
this class of compounds can be considered very promising in
this field. However, additional derivatives have been designed
to further improve the biological profile and to allow for in vivo
experiments, while studies on the mechanism of action of pyrrole
derivatives at the molecular level are ongoing in our laboratories
to shed light on their molecular target.
Microbiology. Antimycobacterial activity and cytotoxic activity
assays were performed following a protocol previously reported11
(further details in Supporting Information).
Acknowledgment. Financial support from the Italian Min-
istero dell’Universita` e della Ricerca (Grant PRIN 2005037820)
and CARIPLO (Grant Rif. 2006.0880/10.8485 Bando 2006) is
gratefully acknowledged.
Supporting Information Available: Additional information on
tuberculosis and pyrrole compounds as antimycobacterial agents,
experimental details (chemistry and microbiology, spectroscopic,
and elemental analysis data), and details of computational methods.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Experimental Section
Chemistry.GeneralProcedureforthePreparationofPentane-
1,4-diones 19a-f. Following the Stetter reaction, a mixture of the
appropriate benzaldehyde 17a-f (0.09 mol), triethylamine (19.5
mL, 0.14 mol), methyl vinyl ketone 18 (0.09 mol), and 3-ethyl-5-
(2-hydroxyethyl)-4-methylthiazolium bromide (3.53 g, 0.014 mol)
(Scheme 1) was put into a round-bottom flask equipped with a stir
bar. The flask was inserted into the cavity of a Discovery microwave
system apparatus and heated (150 W for 15 min, internal temper-
ature 70 °C, and internal pressure 60 psi). The residue was treated
with 2 N HCl (10 mL). After extraction with ethyl acetate, the
organic layer was washed with aqueous sodium bicarbonate and
water. The organic fractions were dried over Na2SO4, filtered, and
concentrated to give a crude orange liquid. After chromatography
on aluminum oxide (activity II-III, according to Brockmann)
(cyclohexane/ethyl acetate, 3:1 v/v), the desired 19a-c were isolated
as a light-yellow solids which, after recrystallization from cyclo-
hexane, gave an analytical sample as white needles, and compounds
19d-f as oils. Example, 1-(4-ethylphenyl)-pentane-1,4-dione (19d):
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