1,3,4-Oxadiazoline derivatives as novel potential inhibitors targeting chitin biosynthesis: Design, synthesis and biological evaluation

Article abstract of DOI:10.1016/j.bmcl.2008.11.095

Two series of 1,3,4-oxadiazoline heterocycle derivatives were designed, synthesized and identified. Bioactivity assays showed that all synthesized compounds inhibited chitin synthesis in yeast, suggesting they might be a novel class of potential inhibitors against chitin biosynthesis. The structure-activity relationships (SAR) of these compounds are discussed.

Full text of DOI:10.1016/j.bmcl.2008.11.095

Bioorganic & Medicinal Chemistry Letters 19 (2009) 332–335
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
1,3,4-Oxadiazoline derivatives as novel potential inhibitors targeting chitin
biosynthesis: Design, synthesis and biological evaluation
a,
Shaoyong Ke a, Fengyi Liu b, Ni Wang b, Qing Yang b, , Xuhong Qian
*
*
a Shanghai Key Laboratory of Chemical Biology, Institute of Pesticides and Pharmaceuticals, School of Pharmacy, East China University of Science and Technology,
Shanghai 200237, China
b Department of Bioscience and Biotechnology, Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two series of 1,3,4-oxadiazoline heterocycle derivatives were designed, synthesized and identified. Bio-
activity assays showed that all synthesized compounds inhibited chitin synthesis in yeast, suggesting
they might be a novel class of potential inhibitors against chitin biosynthesis. The structure–activity rela-
tionships (SAR) of these compounds are discussed.
Received 12 August 2008
Revised 21 November 2008
Accepted 24 November 2008
Available online 27 November 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
1,3,4-Oxadiazolines
Potential inhibitors
Chitin biosynthesis
Chitin (C8H13O5N)n is a linear polysaccharide (Fig. 1) chain con-
sisting of more than thousands of N-acetylglucosamine (GlcNAc,
2-acetamido-2-deoxy-D-glucose) residues joined by b-1,4 glyco-
compounds are presumed to function through non-competitive
inhibition, meaning that they bind reversibly to the sites away
from the catalytic site of chitin synthase.27 All the novel com-
pounds are potential chitin synthase inhibitors whereas they are
not chemically related. A common structural feature of these
inhibitors is benzene rings or aromatic nitrogenated moieties, but
fine details of the inhibitory mechanism of these candidates re-
main elusive and general rules cannot be drawn. On the other
hand, their modest antifungal activities imply that further investi-
gation would be needed.
sidic bonds.1–3 It is widely distributed in invertebrates, anthropods
and is an integral component in cell walls of fungi. Since it is absent
from plants and vertebrates, the biosynthesis of chitin is believed
to be a target pathway for the development of novel fungicides.4,5
Biosynthesis of chitin is dependant on chitin synthase, which use
uridine diphosphoryl-N-acetylglucosamine (UDP-GlcNAc) as sub-
strate donor to synthesize b-1,4-linked N-acetyl-D-glucosamine
(GlcNAc) (Fig. 1).6–9 Chitin synthase is undoubtedly an ideal target
for the development of drugs, pesticides and fungicides. However,
the structure of the active site of membrane-bound chitin synthase
is unknown due to its difficulties of the isolation and purification.
Therefore most ‘rational designed’ drugs claimed to be chitin
synthase inhibitors are more likely acting toward chitin
biosynthesis.4,10
The investigation on chitin biosynthesis inhibitors could be da-
ted to the discovery of fungicides such as the peptidyl nucleosides
polyoxins and nikkomycins,11,12 which are analogs of substrate do-
nor, UDP-GlcNAc (Fig. 2). Structural modifications of these com-
pounds have been reported, but with few success.13–21 Very
recently, other small organic molecules (Fig. 3) without any struc-
tural resemblance to UDP-GlcNAc, have been reported to have
inhibitory activity toward chitin biosynthesis.22–26 Most of these
An oxadiazole ring that improves pharmacokinetic and in vivo
efficacy is often observed, which is known to be due to its higher
hydrolytic and metabolic stability. Oxadiazole heterocycles are
therefore important structural motifs in the design of novel
drugs.28–32 One example is DOW416 (Fig. 4),33 which interferes
with the formation of insect tissue and epicuticle and thus is con-
sidered to be a chitin synthase inhibitor. Other examples such as
oxadiazolyl 3(2H)-pyridazinones derivatives (Fig. 4) with antifee-
dant activity, are presumed to affect chitin biosynthesis. It is be-
lieved that the oxadiazole moiety may block the incorporation of
N-acetylglucosamine into newborn chitin chain.34–36
In this paper, two series of 1,3,4-oxadiazolines derivatives
C1–10 and C11–20 (Scheme 1) were designed and synthesized.
Since the oxadiazoline moiety may affect chitin biosynthesis, their
inhibitory activities against yeast chitin synthase were expected.
Their bioactivities indicated that they might be novel chitin bio-
synthesis inhibitors.
* Corresponding authors. Fax: +86 21 64252603.
E-mail addresses: qingyang@dlut.edu.cn (Q. Yang), xhqian@ecust.edu.cn (X.
Qian).
The target compounds 3-acetyl-5-aryl-2,3(2H)-1,3,4-oxadiaz-
oles C1–10 and 5-aryl-1,3,4-oxadiazole-3(2H)-carboxamide
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.11.095

S. Ke et al. / Bioorg. Med. Chem. Lett. 19 (2009) 332–335
333
OH
O
N
OH
OH
O
NHAc
O
NHAc
OH
NH
HO
HO
O
Chitin Synthase
O
HO
O
HO
HO
HO
O
O
HO
O
AcHN
O
O
O
O
UDP
O
NHAc
P
P
NHAc
O
O O
O
OH
OH
n
OH OH
UDP-GlcNAc
Chitin
Figure 1. The biosynthesis of chitin. UDP-GlcNAc is as GlcNAc donor and chitin synthase is responsible for glycosidic bond.
O
N
O
N
OHC
COOH
HOOC
COOH
NH
NH
NH
HO
HO
CH3
O
O
O
OH
O
CH3
O
COOH
N
O
O
O
O
O
N
N
H2N
O
N
H
N
N
H
H
OH NH2
OH NH2
OH NH2
OH OH
OH OH
OH OH
Polyoxin D
Nikkomycin Z
Nikkomycin X
Figure 2. Representative structures of naturally occurring polyoxins and nikkomycins.
CHO
NH
N
N
R1
O
O
N
O
N
O
N
H
O
N
O
R2
H
H
RO-41-0986
O
RO-09-3143
RO-09-3024
R1
O
O
O
HO
HO
OH
OH
R2
HO
R1
N
H
R3
COOH
Phellinsin A
OH
Obovatols
R2
Figure 3. Some non-nucleoside compounds as chitin biosynthesis inhibitors.
O
O
Cl
Cl
O
O
N
N
Cl
X
N
N
O
N
N
O
O
R
Cl
Cl
N
N
Oxadiazolyl 3(2
H)-pyridazinones
DOW416
Metoxadiazone
Figure 4. Bioactive compounds containing 1,3,4-oxadiazole moiety.
O
NH2NH2 H O
·
Nonan-5-one
EtOH
EtOH
2
N
Ar
CONHNH2
Ar COOH
Ar COOEt
Ar
N
H
H2SO4
1
2
3
4
e
t
a
n
a
Ac2O Method A
y
c
o
s
i
l
B
y
d
o
o
r
A
h
t
e
M
O
O
H
N
R
Ar
Ar
N
N
C11-20
C1-10
N
N
O
O
O
Scheme 1. Synthesis of 1,3,4-oxadiazolines derivatives C1–10 and C11–20.
derivatives C11–20 were prepared as depicted in Scheme 1. The
condensation reaction between acylhydrazides 3 and nonan-5-
one led to the formation of the important building blocks, the
substituted aroylhydrazones 4. The intermediates aroylhydrazones
4 undergo convenient heterocyclization reaction in the presence of
acetic anhydride to provide 3-acetyl-5-aryl-2,3(2H)-1,3,4-oxadiaz-
oles C1–10 using method A. For synthesis of C11–20 by method B,
the reaction of substituted aroylhydrazone 4 with various substi-
tuted benzoylisocyanate was at about 75–80 °C in toluene for about
0.5–1.5 h to obtain the target compounds C11–20 characterized as

334
S. Ke et al. / Bioorg. Med. Chem. Lett. 19 (2009) 332–335
N-(substituted-benzoyl)-1,3,4-oxadiazole-3(2H)-carboxamide. All
the structures were determined by 1H NMR, 13C NMR, IR, EIMS
and high-resolution mass spectra (HR-MS) data.37
C1 and C4) dramatically changed the inhibition activity, suggesting
that the presence of a group in the ortho-position of benzene ring at-
tached to oxadiazoline heterocycle could introduce important steric
effects. The compound containing ortho-position substituent aro-
matic ring may increase the potent activity. On the other hand,
methyl group bound to benzene rings in compound C6 is better
for inhibitory activity than alkyl- or alkoxy-substituted analogs.
However, analogs with naphtalene and furane as aromatic ring
(compounds C9, C10) showed moderate inhibitory activities.
In order to improve the physicochemical properties, we intro-
duced a urea moiety known as an important structural element
for bioactivities,39–44 into 1,3,4-oxadiazolines heterocyclic struc-
ture. Figure 6 shows the effect on chitin biosynthesis by 1,3,4-oxa-
diazole-3(2H)-carboxamide derivatives C11–20. Compounds C13,
C15, C19 and C20 exhibited moderate inhibition activities at the
The inhibition activity against chitin biosynthesis was assayed
using yeast cell extracts by the modified procedure described by
Lecuro and Bulik.38 All these compounds were evaluated. Nikko-
mycin Z (NZ), the well-known fungicide isolated from Treptomyces
tendae, was applied as positive control. And diflubenzuron (DFB),
the well-known insecticide, was used as negative control. Figures
5 and 6 indicated all the compounds, C1–10 and C11–20, were
bioactive.
To explore the influence of structural changes on activity, we
introduced various electron-withdrawing groups (such as halogen
atoms) and electron-donating substituents (Me, MeO, Et, etc.) into
the aromatic ring. As shown in Figure 5, all the synthesized com-
pounds 3-acetyl-5-aryl-2,3(2H)-1,3,4-oxadiazoles C1–10 displayed
moderate to high inhibitory activities on chitin biosynthesis at
concentration of 250 lM, while compound C15 exhibited the high-
est inhibitory activity (about 86%). Other compounds displayed
relatively weak inhibition. Comparing with compounds C11 and
C4, the introduction of acylurea moiety in C11 significantly de-
creased the inhibitory activity. C11–20 possess structural moiety
similar to that of diflubenuron, which might explain their relatively
lower activity. As for the substituents on two aromatic rings when
the substituent R is 2,6-difluoro-substituent, the compound bear-
ing 3,5-dimethylphenyl (C20) is more favorable for inhibition
activities, which is in agreement with compound C6. Furthermore,
250
lM, and compounds C1, C4, and C6 exhibited significant inhib-
itory effect on enzyme activity up to 91%, 69%, and 68% at 250
lM,
respectively. Among all the compounds containing electron-with-
drawing groups, the compound containing o-chloro-substituent
C1 is the comparably highest potent, which led to 91% inhibition.
This may be due to the incorporation of halogen atoms into the aro-
matic ring that increases lipophilicity. Nevertheless, position
changes of the chloro-substituent within the same ring (compounds
120
A
B
Control 250 µM
100
80
60
40
20
O
N
Ar
N
C1-10
O
Substituents
Substituents
Compd.
No.
Compd.
No.
Ar
Ar
C1
C2
C3
C4
C5
3,5-Me2-C6H3
p-Et-C6H4
3,4-OCH2O-C6H3
Naphthalen-2-yl
Furan-2-yl
o
-Cl-C6H4
C6
C7
0
3,4,5-(MeO)3-C6H2
3-MeO-C6H4
NZ DFB C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
C8
C9
C10
Compd. No.
p
p
-Cl-C6H4
-F-C6H4
Figure 5. The structures and relative activity relationships of 1,3,4-oxadiazolines derivatives C1–10. (A) the activity remaining in the presence of compounds; (B) the
structures of compounds.
B
O
H
N
R
Ar
N
120
100
80
60
40
20
0
A
N
Control 250 µM
O
C11-20
O
Substituents
Ar
p-Cl-C6H4
Compd.
No.
R
2,6-F2
2,6-F2
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
3,4,5-(MeO)3-C6H2
3,5-Me2-C6H3
3,4-OCH2O-C6H3
2,4-Cl2-C6H3
o-Cl
o-Cl
o-Cl
2,6-F2
2,6-F2
2,6-F2
3,4-OCH2O-C6H3
2,4-Cl2-C6H3
Furan-2-yl
NZ DFB C11 C12 C13 C14 C15 C16 C17 C18 C19 C20
Compd. No.
2,4-Cl2-C6H3
3,5-Me2-C6H3
2,3,5-F3-4-MeO
2,6-F2
Figure 6. The structures and relative activity relationships of 1,3,4-oxadiazole-3(2H)-carboxamides derivatives C11–20. (A) the activity remaining in the presence of
compounds; (B) the structures of compounds.

S. Ke et al. / Bioorg. Med. Chem. Lett. 19 (2009) 332–335
8. Cohen, E. Ann. Res. Entomol. 1987, 32, 71.
335
90
80
70
60
50
40
30
20
10
0
9. Merzendorfer, H. J. Comp. Physiol. B 2006, 176, 1.
10. van Eck, W. H. Insect Biochem. 1979, 9, 295.
NZ
C1
11. Zhang, D.; Miller, M. J. Curr. Pharm. Des. 1999, 5, 73.
12. Li, Y.; Cui, Z.; Hu, J.; Ling, Y.; Yang, X. Prog. Chem. 2007, 19, 535.
13. Grugier, J.; Xie, J.; Duarte, I.; Valery, J.-M. J. Org. Chem. 2000, 65, 979.
14. Schafer, A.; Thiem, J. J. Org. Chem. 2000, 65, 24.
15. Obi, K.; Uda, J.-I.; Iwase, K.; Sugimoto, O.; Ebisu, H.; Matsuda, A. Bioorg. Med.
Chem. Lett. 2000, 10, 1451.
16. Behr, J.-B.; Gautier-Lefebvre, I.; Mvondo-Evina, C.; Guillerm, G.; Ryder, N. S. J.
Enzyme Inhib. 2001, 16, 107.
17. Wang, R.; Steensma, D. H.; Takaoka, Y.; Yun, J. W.; Kajimoto, T.; Wong, C.-H.
Bioorg. Med. Chem. 1997, 5, 661.
18. Yeager, A. R.; Finney, N. S. J. Org. Chem. 2004, 69, 613.
19. Yeager, A. R.; Finney, N. S. J. Org. Chem. 2005, 70, 1269.
20. Plant, A.; Thompson, P.; Williams, D. M. J. Org. Chem. 2008, 73, 3714.
21. Behr, J.-B.; Gainvors-Claisse, A.; Belarbi, A. Nat. Prod. Res. 2007, 21, 76.
22. Sudoh, M.; Yamazaki, T.; Masubuchi, K.; Taniguchi, M.; Shimma, N.; Arisawa,
M.; Yamada-Okabe, H. J. Biol. Chem. 2000, 275, 32901.
23. Urbina, J. M.; Cortés, J. C. G.; Palma, A.; López, S. N.; Zacchino, S. A.; Enriz, R. D.;
Ribas, J. C.; Kouznetzov, V. V. Bioorg. Med. Chem. 2000, 8, 691.
24. López, S. N.; Castelli, M. V.; Zacchino, S. A.; Domínguez, J. N.; Lobo, G.; Charris-
Charris, J.; Cortés, J. C. G.; Ribas, J. C.; Devia, C.; Rodríguez, A. M.; Enriz, R. D.
Bioorg. Med. Chem. 2001, 9, 1999.
25. Masubuchi, K.; Taniguchi, M.; Umeda, I.; Hattori, K.; Suda, H.; Kohchi, Y.;
Isshiki, Y.; Sakai, T.; Kohchi, M.; Shirai, M.; Okabe, H.; Sudoh, M.; Yamazaki, T.;
Shimma, N. Bioorg. Med. Chem. Lett. 2000, 10, 1459.
26. Hwang, E. I.; Kwon, B. M.; Lee, S. H.; Kim, N. R.; Kang, T. H.; Kim, Y. T.; Park, B.
K.; Kim, S. U. J. Antimicrob. Chemother. 2002, 49, 95.
27. Behr, J.-B. Curr. Med. Chem. Anti-Infective Agents 2003, 2, 173.
28. Abadi, A. H.; Eissa, A. A. H.; Hassan, G. S. Chem. Pharm. Bull. 2003, 51, 838.
29. Khanum, S. A.; Shashikanth, S.; Umesha, S.; Kavitha, R. Eur. J. Med. Chem. 2005,
40, 1156.
30. Almasirad, A.; Vousooghi, N.; Tabatabai, S. A.; Kebriaeezadeh, A.; Shafiee, A.
Acta Chim. Slov. 2007, 54, 317.
31. Hall, A.; Brown, S. H.; Chowdhury, A.; Giblin, G. M. P.; Gibson, M.; Healy, M. P.;
Livermore, D. G.; McArthur Wilson, R. J.; Naylor, A.; Anthony Rawlings, A.;
Roman, S.; Ward, E.; Willay, C. Bioorg. Med. Chem. Lett. 2007, 17, 4450.
32. Ali, O. M.; Amer, H. H.; Abdel-Rahman, A. A.-H. Synthesis 2007, 18, 2823.
33. Arrington, J. P.; Wade, L. L. U.S. Patent 4215129, 1980; Chem. Abstr. 1980, 94,
59794f.
34. Cao, S.; Qian, X. H.; Song, G. H.; Chai, B.; Jiang, Z. S. J. Agric. Food Chem. 2003, 51,
152.
35. Huang, Q. C.; Qian, X. H.; Song, G. H.; Cao, S. Pest Manag. Sci. 2003, 59, 933.
36. Cao, S.; Wei, N.; Zhao, C. M.; Li, L. N.; Huang, Q. C.; Qian, X. H. J. Agric. Food
Chem. 2005, 53, 3120.
C15
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6
LogC
Figure 7. Semi-logarithmic plot of the inhibitory activity of compounds C1 and C15
against chitin biosynthesis.
compounds containing electron-withdrawing group such as
2,4-dichlorophenyl (C15 and C19) is more efficient than those with
alkoxy-substituents on the benzene ring. However, compound C11
containing p-chlorophenyl showed lower activity, suggesting that
the ortho-position steric effects of aromatic ring attached to
oxadiazoline ring were important for inhibitory activity.
Since compounds C1 and C15 displayed the highest inhibitory
activity, five serial dilutions were further tested for enzyme activ-
ity. As indicated in Figure 7, their inhibitory effects on chitin bio-
synthesis were concentration-dependent. Compounds C1 and C15
exhibited significant inhibition against chitin biosynthesis with
the IC50 values of 7.93 and 5.60 lM, respectively.
In conclusion, the novel 1,3,4-oxadiazolines derivatives C1–10
and C11–20 were designed as potential antifungal reagents, which
interfered with chitin biosynthesis. Compounds C1 and C15
showed the highest inhibitory activity at lower concentration.
The understanding of structure–activity relationship and assay of
these compounds may provide some insights into the rational de-
sign of new inhibitors of chitin biosynthesis.
37. The selected analytical data of representative compounds C1 and C15 were as
follows: C1. This compound was obtained as light yellow liquid following the
standard procedures, yield 76%, IR (KBr):
m ;
= 1665 (C=O), 1617 (C=N) cmÀ1 1H
NMR (400 MHz, CDCl3): d = 7.78 (dd, 3J = 7.6 Hz, 4J = 2 Hz, 1H, Ph-H), 7.50 (dd,
3J = 8.2 Hz, 4J = 1.2 Hz, 1H, Ph-H), 7.33–7.43 (m, 2H, Ph-H), 2.42–2.46 (m, 2H,
CH2), 2.34 (s, 3H, COCH3), 1.94–2.02 (m, 2H, CH2), 1.27–1.44 (m, 8H, CH2), 0.90
(t, J = 7 Hz, 6H, CH3); 13C NMR (100 MHz, CDCl3): d = 167.29, 152.93, 133.30,
131.67, 131.22, 130.40, 126.71, 124.00, 104.86, 36.06, 24.70, 22.39, 22.26,
13.98; MS: m/z = 336 (M+), 279, 237, 210, 139, 111; EI-HRMS: calcd for
C18H25ClN2O2 (M+), 336.1605; found, 336.1605; C15. This compound was
obtained as white solid following the standard procedures, yield 64%, mp
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (20502006Á20572024), the National
High Technology Research and Development Program of China
(2006AA10A201) and the National Key Technology R&D Program
of China (2006BAE01A01-8). The authors also thank for the partial
support from Shanghai Leading Academic Discipline Project (B507)
and Shanghai Foundation of Science and Technology (064319022).
104.3–105.7 °C; IR (KBr):
m ;
= 3362 (N–H), 1710, 1691 (C@O), 1625 (C@N) cmÀ1
1H NMR (400 MHz, CDCl3): d = 9.36 (s, 1H, N–H), 7.74 (d, J = 8.8 Hz, 1H, Ph-H),
7.66 (d, J = 7.2 Hz, 1H, Ph-H), 7.54 (d, 4J = 2 Hz, 1H, Ph-H), 7.41–7.44 (m, 2H, Ph-
H), 7.35–7.40 (m, 2H, Ph-H), 2.34–2.42 (m, 2H, CH2), 1.96–2.05 (m, 2H, CH2),
1.25–1.41 (m, 8H, CH2), 0.89 (t, J = 7 Hz, 6H, CH3); 13C NMR (100 MHz, CDCl3):
d = 165.79, 152.10, 146.04, 137.79, 134.76, 134.14, 131.78, 131.30, 131.08,
130.47, 129.99, 129.91, 127.34, 127.08, 121.62, 105.08, 36.33, 24.62, 22.31,
13.97; MS: m/z = 509, 328, 286, 271, 244, 180, 139, 111, 75; EI-HRMS: calcd for
C24H26Cl3N3O3 (M+), 509.1040; found, 509.1025.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2008.11.095.
38. Lucero, H. A.; Kuranda, M. J.; Bulik, D. A. Anal. Biochem. 2002, 305, 97.
39. Kaymakçiog˘lu, B. K.; Rollas, S.; Körceg˘ez, E.; Ariciog˘lu, F. Eur. J. Pharm. Sci.
2005, 26, 97.
References and notes
40. Takahashi, T.; Sakuraba, A.; Hirohashi, T.; Shibata, T.; Hirose, M.; Haga, Y.;
Nonoshita, K.; Kanno, T.; Ito, J.; Iwaasa, H.; Kanatani, A.; Fukami, T.; Sato, N.
Bioorg. Med. Chem. 2006, 14, 7501.
41. Ban, H.; Muraoka, M.; Ioriya, K.; Ohashi, N. Bioorg. Med. Chem. Lett. 2006, 16, 44.
42. Galiano, S.; Ceras, J.; Cirauqui, N.; Pérez, S.; Juanenea, L.; Rivera, G.; Aldana, I.;
Monge, A. Bioorg. Med. Chem. 2007, 15, 3896.
43. Kim, Y. J.; Ryu, J.-H.; Cheon, Y. J.; Lim, H. J.; Jeon, R. Bioorg. Med. Chem. Lett.
2007, 17, 3317.
44. Berglund, M.; Dalence-Guzemán, M. F.; Skogvall, S.; Sterner, O. Bioorg. Med.
Chem. 2008, 16, 2529.
1. Groll, A. H.; De Lucca, A. J.; Walsh, T. J. Trends Microbiol. 1998, 6, 117.
2. Xie, J.; Thellend, A.; Becker, H.; Vidal-Cros, A. Carbohydrate Res. 2001, 334, 177.
3. Hwang, E. I.; Lee, Y. M.; Lee, S. M.; Yeo, W. H.; Moon, J. S.; Kang, T. H.; Park, K. D.;
Kim, S. U. Plant Med. 2007, 73, 679.
4. Ruiz-Herrera, J.; San-Blas, G. Curr. Drug Targets Infect. Disord. 2003, 3, 77.
5. Maertens, J. A.; Boogaerts, M. A. Curr. Pharm. Des. 2000, 6, 225.
6. Gooday, G. W. J. Gen. Microbiol. 1977, 99, 1.
7. Cabib, E. Adv. Enzymol. 1987, 59, 59.