Oxazolo[3,2-a]pyridine. A new structural scaffold for the reversal of multi-drug resistance in Leishmania

Article abstract of DOI:10.1016/j.bmcl.2012.07.100

Compounds belonging to three different classes of fused heterocyclic systems, structurally related to Calcium-channel blockers of the 1,4-dihydropyridine family, were evaluated in their ability to overcome leishmanial resistance to common drugs in a MDR Leishmania tropica strain. Compounds with the skeletal basis of oxazolo[3,2-a]pyridine displayed significant reversion of resistance to daunomycin and miltefosine, with reversion indexes up to 6.7-fold and 8.7-fold, respectively. Most interestingly, the enantiopure compound 20S attained to revert the resistance to both drugs and fairly more significantly than its enantiomer 20R.

Full text of DOI:10.1016/j.bmcl.2012.07.100

Bioorganic & Medicinal Chemistry Letters 22 (2012) 6272–6275
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Oxazolo[3,2-a]pyridine. A new structural scaffold for the reversal of
multi-drug resistance in Leishmania
Esther Caballero a, , José Ignacio Manzano b, Pilar Puebla a, Santiago Castanys b, Francisco Gamarro b,
⇑
⇑
,
Arturo San Feliciano a
a Departamento de Química Farmacéutica, Facultad de Farmacia, CIETUS, IBSAL, Universidad de Salamanca, 37007-Salamanca, Spain
b Instituto de Parasitología y Biomedicina ‘López Neyra’, IPBLN-CSIC, Granada, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Compounds belonging to three different classes of fused heterocyclic systems, structurally related to Cal-
cium-channel blockers of the 1,4-dihydropyridine family, were evaluated in their ability to overcome
leishmanial resistance to common drugs in a MDR Leishmania tropica strain. Compounds with the skeletal
basis of oxazolo[3,2-a]pyridine displayed significant reversion of resistance to daunomycin and miltefo-
sine, with reversion indexes up to 6.7-fold and 8.7-fold, respectively. Most interestingly, the enantiopure
compound 20S attained to revert the resistance to both drugs and fairly more significantly than its enan-
tiomer 20R.
Received 12 July 2012
Revised 26 July 2012
Accepted 30 July 2012
Available online 10 August 2012
Keywords:
Oxazolo[3,2-a]pyridines
MDR-reversal
Leishmaniasis
Daunomycin
Ó 2012 Elsevier Ltd. All rights reserved.
Miltefosine
Leishmaniasis is a poverty-related neglected disease character-
ized by its high morbidity, being the most important emerging and
uncontrolled infectious disease and the second cause of death
among the protozoan parasitic diseases, after Malaria. Globally
there are an estimated 70,000 deaths each year and 350 million
people are at risk of infection. To date there are no protective vac-
cines, and chemotherapy is the only effective weapon, limited to a
few first-line drugs, including miltefosine, paromomycin, ampho-
tericin B and its derivatives, along with pentavalent antimonials,
although the latter are no longer effective in endemic areas as in
Bihar (India) due to drug resistance. One of the major problems
of leishmaniasis chemotherapy is the intrinsic or acquired resis-
tance of parasites to the available drugs. ATP-binding cassette
(ABC) transporters are commonly involved in the resistance to dif-
ferent antiparasitic drugs such as antimonials,1 pentamidine,2 mil-
tefosine,3 and sitamaquine.4 Leishmania P-glycoprotein-like
transporter (Pgp-like) is included in the ABC subfamily B and its
overexpression confers a multi-drug resistance (MDR) phenotype
involved in a significantly reduction in intracellular accumulation
of drugs, similar to that characterised in mammalian cells over-
expressing Pgp.5,6 Inhibitors of Pgp have thus been advocated as
promising candidates for overcoming MDR.
developing compounds that overcome resistances with lower host
toxicity.7 Several types of compounds with diverse levels of MDR
reversal properties have been described.8 One of the physio-phar-
macological categories of drugs displaying such reversion ability is
that of Calcium channel blockers represented by verapamil and
nifedipine related drugs.9 Here we report on the evaluation of
the MDR reversal abilities of representative members of three
types of fused heterocyclic compounds structurally related to
nifedipine and 1,4-dihydropyridines, that had previously displayed
long-acting antihypertensive effects, presumably due to its proba-
ble Ca2+-channel blocker pro-drug character.10
The syntheses of the heterocyclic compounds shown in Table 1
were achieved following the known Hantzsch procedure as previ-
ously reported (Scheme 1).11 This methodology afforded com-
pounds 1–7 in good yields, starting with enamines of
acetoacetate esters obtained from 2-aminoethanol (compounds
1–4, 6 and 7, Table 1) and 1-amino-2-propanol (5). The use of thi-
oethanolamine enamine led to the thiazolopyridine 8 and that of 3-
aminopropanol enamine rendered the pyrido-oxazine 9.
Compounds 1–9 include three types of fused heterocyclic sys-
tems, with seven diversely substituted oxazolo[3,2-a]pyridines,
one thiazolo[3,2-a]pyridine and one pyrido[2,1-b]oxazine, which
were initially evaluated in their MDR reversal potentiality on
promastigotes of a highly daunomycin-resistant strain of Leish-
mania tropica.12 Daunomycin (DNM) resistance in this line is re-
lated to a decreased intracellular drug accumulation mainly due
to the Pgp-like transporter overexpression.13 This resistant line
Parasite resistance to drugs has emerged as a major problem in
current medicine, and therefore, there is great clinical interest in
⇑
Corresponding authors. Tel.: +34 923294528 (E.C.), +34 958181667 (F.G.).
E-mail addresses: escab@usal.es (E. Caballero), gamarro@ipb.csic.es (F. Gamarro).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.07.100

E. Caballero et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6272–6275
6273
Table 1
Structures and growth inhibition results (%) of wild-type (WT) and MDR L. tropica lines by daunomycin (DNM, 150
l
M) in the presence of selected heterocyclic compounds
R3
8
COOMe
O
(Het) Ar
MeOOC
X
9
1
Cl
1
R1
8
7
2
N
4
N
4
MeOOC
) pyrido[2,1-b]oxazine
R2
( ) oxa(thia)zolo[3,2-a]pyridines
(
Compound
X
R1
R2
R3
Ar (Het)
[Comp.] = 3
l
M
[Comp.] = 6
l
M
[Comp.] = 8
lM
WT
MDR
WT
MDR
WT
MDR
1
2
3
4
5
6
7
8
9
O
O
O
O
O
O
O
S
H
H
H
H
Me
H
H
H,H
H,H
H,H
H,H
O
H,H
H,H
H,H
CH2OH
COOMe
COOBn
COOBn
COOMe
COOMe
COOMe
COOMe
3-NO2Ph
3-NO2Ph
3-NO2Ph
3-ClPh
3-ClPh
2-furyl
<10
<10
<10
<10
<10
<10
<10
<10
<10
<10
<10
<10
16.4 0.2
<10
<10
<10
<10
<10
<10
<10
<10
26.3 0.2
97.6 0.5
93.3 0.4
<10
<10
<10
80.3 4.2
83.8 5.8
74.8 3.6
75.3 2.3
23.9 1.0
<10
<10
<10
57.3 3.3
43.2 2.5
18.1 0.2
92.3 0.9
71.6 3.7
<10
<10
<10
77.8 1.5
78.9 2.0
36.8 1.1
21.8 1.0
65.1 3.3
<10
<10
<10
<10
<10
<10
2-pyrrolyl
3-ClPh
H
Data shown are the average of three independent experiments SD, relative to the control, grown in the absence of DNM and compounds. Values above 75% inhibition are
boldfaced to facilitate comparisons.
Parasites were exposed to increasing concentrations of drugs in
the absence or presence of a 6 lM concentration of the com-
R3
XH
(Het) Ar
MeOOC
R1
i
pounds. The RI is defined as the ratio between the IC50 values found
for a certain drug in absence or in presence of each compound. In
other words, a RI value of 3 indicates a 3-fold enhancement of drug
effectiveness against the multi-resistant parasite. The calculated
maximum RI value with DNM (ratio between IC50 values for
MDR and parental drug-sensitive parasites) is 64. The maximum
RI value with MLF is 22.
As shown in Table 2, the selected compounds showed RI values
ranging from 3.9 to 1.3 for DNM, and from 3.5 to 1.5 for MLF. As can
be observed, among the oxazolo[3,2-a]pyridines, compound 2 dis-
played the strongest MDR reversal potential with RI values of 3.9
and 3.4 for both drugs DNM and MLF, respectively. These values
are of the same order and comparable to those obtained with Cuz-
+
O
N
HN
n
R2
R3
(Het) Ar
MeOOC
X
R1
n = 0, 1
X = O, S
n
i: MeOH rt, 24 h
R2
Scheme 1. The synthesis of oxa(thia)zolo[3,2-a]pyridines (n = 0) and pyrido[2,1-
b]oxazines (n = 1).
co
5 (1a,2a-diacetoxy-6b,9b-difuroyloxy-4b-hydroxydihydro-b-
agarofuran), a known and rather complex natural sesquiterpenoid
isolated from Maytenus canariensis, being considered up to date as
the most promising reversal compound of MDR phenotype in
Leishmania.18 Consequently, Cuzco 5 was used in this research as
the positive control of MDR reversion.
Aiming at discovering better MDR reversal agents of this type, we
evaluated a larger number of oxazolo[3,2-a]pyridines, including
several enantiomerically pure compounds. The structures of the
evaluated compounds 10–25 are shown in Table 3. The enantiose-
lective synthesis of 2,3,8,8a-tetrahydro-7H-oxazolo[3,2-a]pyridines
has an MDR phenotype similar to tumour cells, with a cross-resis-
tance profile to several unrelated drugs and an overexpressed
drug-efflux Pgp-like transporter (LMDR1).14 The wild-type (WT)
L. tropica LCR-strain was also included in the assays. To determine
the intrinsic toxicity of compounds, the WT and MDR Leishmania
lines were exposed to compounds in the absence of DNM.15 The
cytotoxicity of compounds for human myelomonocytic THP-1 cells
was also determined,16 and the results (data not shown) showed
that these compounds present a significantly low toxicity with
IC50 values higher than 400 lM.
In order to assess the chemosensitising activity of the com-
pounds, promastigotes of the L. tropica MDR line were exposed to
Table 2
both 150
lM DNM and three different concentrations (3, 6 and
Quantitative effects of compounds 2, 7, 8, and 9 on the IC50 values for daunomycin
(DNM) and miltefosine (MLF) in the MDR L. tropica line
8
l
M) of the compounds in racemic form. The preliminary evalua-
tion results included in Table 1 showed that several oxazolo[3,2-
a]pyridines were able to revert the resistance to DNM and that
compound 2 achieved a 75% parasite inhibition after 72 h at a
Compound (6 lM) IC50(lM) for DNM RIDNM IC50(lM) for MLF RIMLF
—
2
7
8
179.8 1.1
47.0 6.4
138.6 4.4
83.0 4.0
55.7 0.4
38.3 0.6
—
239.5 2.1
70.8 3.9
123.2 2.7
69.2 6.3
161.2 2.8
39.7 2.1
—
3.9
1.3
2.2
3.2
4.7
3.4
1.9
3.5
1.5
6.0
low 3 lM concentration. Additionally, compounds 7, 8 and 9 dis-
played a moderate MDR reversal activity with lower toxicity.
These encouraging results moved us to design and perform new
evaluation experiments focused on establishing IC50 values of
resistance reversion for the most active compounds. Considering
that the MDR Leishmania line showed a significant cross-resistance
to miltefosine (MLF),16 the IC50 and the reversal index (RI) values to
DNM and MLF were determined for compounds 2, 7, 8 and 9.
9
Cuzco5
IC50 values for the WT line in absence of compounds were: 2.8 0.1 (DNM), and
10.9 2.1 (MLF). RI values above 3.0 are boldfaced for comparison purposes. Data
are expressed as average SD (n = 3) of concentrations of DNM and MLF necessary
to inhibit parasite growth by 50%.

6274
E. Caballero et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6272–6275
Table 3
Resistance reversal effects for further oxazolo[3,2-a]pyridine derivatives on the MDR line of L. tropica
R3
R3
R4-Ph
O
R4-Ph
O
8
5
7
1
8
5
R1
7
1
R1
2
N
4
2
N
4
MeOOC
MeOOC
R2
R2
2S-series
(r)-racemic
2R-series
Compound (6
l
M)
R1
R2
R3
R4
IC50
(l
M) for DNM
IC50 (lM) for MLF
—
179.8 1.1
179.7 4.2
189.0 2.0
187.6 2.1
206.7 11
34.2 0.4
187.9 2.2
54.2 0.8
56.9 2.1
50.7 0.3
173.5 3.5
172.6 5.1
79.6 4.6
26.7 4.2
172.1 8.4
42.1 2.0
57.9 3.2
51.4 2.3
45.7 1.4
61.2 0.2
88.8 0.7
38.3 0.6
239.5 2.1
131.7 0.0
154.2 4.2
185.1 5.4
124.5 1.4
71.1 2.4
238.3 8.4
58.1 0.6
38.7 0.3
55.1 2.3
133.7 7.1
171.3 2.5
84.3 2.0
45.6 3.6
151.6 0.4
55.5 2.9
45.8 2.8
27.7 2.2
50.9 3.6
46.0 3.3
62.1 4.3
39.7 2.1
10(r)
11(r)
12(r)
13(r)
14(r)
15(r)
16(r)
17(r)
18S
19(r)
19R
20R
20S
21(r)
22R
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H,H
CH2OAc
COOMe
COOEt
3-NO2
3-Cl
3-NO2
3-Cl
H,H
H,H
H,H
H,H
H,H
H,Et
H,Et
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
O
COOEt
COOAllyl
COOtBu
COOMe
COOMe
CH2OAc
COOMe
COOMe
COOMe
COOMe
COOEt
COOEt
COOEt
COOtBu
COOBn
COOBn
COOEt
3-Cl
3-NO2
3-NO2
3-Cl
3-Cl
3-NO2
3-NO2
3-Cl
3-Cl
3-NO2
3-Cl
3-Cl
3-Cl
3-Cl
3-Cl
22S
23R
24R
24S
25R
Cuzco5
3-Cl
IC50 values for the WT line in absence of compounds were: 2.8 0.1
lM (DNM) and 10.9 2.1 lM (MLF). Data are expressed as average SD (n = 3) of concentrations of DNM
and MLF necessary to inhibit parasite growth by 50%. IC50 values similar as or better than those found for Cuzco 5 are boldfaced for comparison purposes.
was carried out with enamines prepared from homochiral 1-amino-
2-propanols as previously reported by some of us.19 It is noticeable
that the formation of three new stereocenters in the bicyclic system
is highly controlled by the presence of a single chiral carbon atom in
the starting enamine reagent, and placed in the end product three,
four and five bonds away from the newly created stereocenters.
When racemic 1-amino-2-propanol and 2-aminobutanol were
used, racemic 16, 17, 19 or 21 were obviously produced.
The results of the MDR reversal evaluation are shown in Table 3.
As it can be seen, the tested compounds behave differently
depending on their respective structures and on the antileishma-
nial drug being considered. Thus, while most compounds reverted
Leishmania resistance to one or both drugs in a similar manner as
the mentioned above for compounds 2 and 8, several of them re-
sulted practically inactive and even one compound (13) increased
the resistance to DNM. These experimental facts support the exis-
tence of fair structure/resistance–reversal relationships that can
also be extended to the stereochemical aspects, as in the case of
the enantiomers 20R and 20S.
the series of oxazolo[3,2-a]pyridine derivatives tested, compound
20S which produces a significant reversion of resistance to both
drugs DNM and MLF, is considered as a good candidate for further
studies as reversal agent of Leishmania MDR. Similarly, compound
23R, with the highest RI against the modern antileishmanial agent
MLF would play a similar role. Complementarily, we attempted to
determine the MDR reversal activity of these compounds on intra-
cellular amastigotes, however due to the high intrinsic cytotoxicity
of MLF and DNM for the human THP-1 cells, with IC50 values:
26.9 3.1
lM (MLF) and 7.0 0.04 lM (DNM), it was difficult to
quantify the resistance reversion.
In order to assess the expectancy of these oxazolo-pyridines as
potential lead compounds is important to note that several repre-
sentative compounds of this series were tested already in vivo,
having demonstrated their antihypertensive efficacy with low tox-
icity.10 However, such assays were performed under intraperito-
neal administration only, and some doubts emerged about their
bioavailability under oral administration, the ordinary route of
administering MLF and DNM. At this respect, the expectancy for
the potential clinical use of compounds 17, 20, 22 and 23 results
reinforced by fulfilling of the Lipinski rules, and complemented
with favourable theoretical physico-chemical, ADME and toxico-
logical calculations and predictions (data not shown) obtained
for those compounds. Consequently, compounds 20S and 23R
could be considered as drug leads for MDR reversal in the treat-
ment of resistant leishmaniasis. Furthermore, the good results
found for 20S, 14 and other compounds of the series in the reversal
of resistance to DNM, a well established and clinically used anti-
cancer drug, suggest the potential interest of these family of com-
pounds for the treatment of anthracycline-resistant neoplastic
diseases. In this sense, in vitro MDR reversal studies on human
neoplastic cells have been initiated.
A
first observation related to the influence of the m-
substituted phenyl group attached to position C-7 reveals that
the 3-chlorophenyl group is preferred to the nitro group at the
same position.
It can also be observed that compounds 14 and 20S significantly
revert the IC50 values for DNM, by more than 5- and 6-fold, respec-
tively, with higher RI values than the reference Cuzco5 (Table 3,
Fig. 1). Indeed, among the tested compounds, 20S showed the high-
est potency for DNM-resistance reversion. Additionally, compound
23R reverts around 8.7-folds the IC50 value for MLF, with higher RI
than Cuzco 5 for this drug (Fig. 1). Interestingly, both compounds
20S and 23R present a low toxicity for human THP-1 cells, with
IC50 values higher than 400 lM (data not shown). Therefore, within

E. Caballero et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6272–6275
6275
Figure 1. Reversal indexes of oxazolo[3,2-a]pyridine derivatives, showing their comparative effects on the resistance to DNM and MLF by the MDR line of L. tropica. Reversal
index (RI) values for a compound and drug (DNM: black; MLF: grey) correspond to the ratio IC50 (without)/IC50 (with compound). Continuous (DNM) and dotted (MLF) lines
define the RI values for Cuzco 5 to allow an easy comparison of compounds with the positive reference. Bars represent the average of three independent experiments SD (⁄p
<0.05).
8. Teodori, E.; Dei, S.; Scapecchi, S.; Gualtieri, F. Farmaco 2002, 57, 385.
9. Miri, R.; Mehdipour, A. Bioorg. Med. Chem. 2008, 16, 8329.
10. Moran, A.; Martín, E.; Velasco, C.; Martín, M. L.; San Román, L.; Caballero, E.;
As a conclusion, the results here shown constitute a first report
which justifies the interest of the oxazolo[3,2-a]pyridine moiety as
a good structural base for MDR reversal. Accordingly, it seems rec-
ommendable to continue the research with evaluation of a wider
family of compounds, including further variants in substituents
and arrangements all around the fused heterocyclic system, and
to extend the research to other therapeutic areas. In order to bring
about this task, experimental studies of in vivo efficacy and acute
toxicity, and the establishment of the mechanism of action with
identification of the target molecule(s) or the biochemical path-
way(s) of MDR reversion used by this type of compounds will be
necessary.
Puebla, P.; Medarde, M.; San Feliciano, A.; Martín, M. L. J. Pharm. Pharmacol.
1997, 49, 421; Martín, E.; Moran, A.; Martín, M. L.; San Román, L.; Puebla, P.;
Medarde, M.; Caballero, E.; San Feliciano, A. Bioorg. Med. Chem. 2000, 10, 319.
11. Caballero, E.; Puebla, P.; Medarde, M.; San Feliciano, A. Tetrahedron 1993, 49,
10079; San Feliciano, A.; Caballero, E.; Puebla, P.; Pereira, J. A.; Gras, J.; Valenti,
C. Eur. J. Med. Chem. 1992, 27, 527; Caballero, E.; Puebla, P.; Medarde, M.;
Honores, Z.; Sastre, P.; López, J. L.; San Feliciano, A. Tetrahedron Lett. 1998, 39,
455.
12. Leishmania strains and culture: The wild-type (WT) L. tropica LRC-strain was a
5
clone obtained by agar plating. A L. tropica line highly resistant to DNM was
maintained in the presence of 150
previously described.
l
M DNM (or 30
lM MLF) and used as
17
Promastigote forms were grown at 28 oC in RPMI
1640-modified medium (Gibco), and supplemented with 20% heat-inactivated
fetal bovine serum (Gibco).
13. Kennedy, M. L.; Cortés-Selva, F.; Pérez-Victoria, J. M.; Jiménez, I. A.; González,
A. G.; Muñoz, O. M.; Gamarro, F.; Castanys, S.; Ravelo, A. G. J. Med. Chem. 2001,
44, 4668.
14. Núñez, M. J.; Cortés-Selva, F.; Bazzocchi, I. L.; Jiménez, I. A.; González, A. G.;
Ravelo, A. G.; Gavín, J. A. J. Nat. Prod. 2003, 66, 572.
Acknowledgments
This work was supported by the Spanish research funds
SAF2009-07440 (to FG), SAF2011-28102 (to SC), JCyL-SA063A09
(to EC), ISCIII-Red de Investigación Cooperativa en Enfermedades
Tropicales (RICET)-FEDER RD06/0021/0002 (FG) and RD 06/0021/
0022 (ASF).
15. Parasite growth control: After 72 h of incubation at 28 °C, the viability of
promastigotes was determined by the colorimetric MTT assay and absorbance
was read at 540 nm using
a microplate reader (Beckman Biomek 2000).
Growth inhibition respect to non-treated parasites was quantified by the ratio
of absorbance at a given compound concentration with respect to control.
16. Human cells toxicity: THP-1 cells were grown at 37 °C and 5% CO2 in RPMI-1640
supplemented with 10% iFBS, 2 mM glutamate, 100 U/mL penicillin and
References and notes
100
l
g/mL streptomycin. 3 Â 104 cells /well, in 96 wells-plate, were
1. Mukherjee, A.; Padmanabhan, P. K.; Singh, S.; Roy, G.; Girard, I.; Chatterjee, M.;
Ouellette, M.; Madhubala, R. J. Antimicrob. Chemother. 2007, 59, 204.
2. Coelho, A. C.; Beverley, S. M.; Cotrim, P. C. Mol. Biochem. Parasitol. 2003, 130, 83.
3. Castanys-Muñoz, E.; Alder-Baerens, N.; Pomorski, T.; Gamarro, F.; Castanys, S.
Mol. Microbiol. 2007, 64, 1141.
differenced to macrophages with 20 ng/mL of PMA treatment for 48 h
followed to 24 h of culture in fresh medium. The MTT-based assay
procedures were the same as for Leishmania parasites with the exception of
incubation temperature, 37 °C in this case.
17. Pérez-Victoria, J. M.; Pérez-Victoria, F. J.; Parodi-Talice, A.; Jiménez, I. A.;
Ravelo, A. G.; Castanys, S.; Gamarro, F. Antimicrob. Agents Chemother. 2001, 45,
2468.
4. Castanys-Muñoz, E.; Pérez-Victoria, J. M.; Gamarro, F.; Castanys, S. Antimicrob.
Agents Chemother. 2008, 52, 3573.
5. Chiquero, J. M.; Pérez-Victoria, F.; O’Valle, J. M.; González-Ros, R. G.; del Moral,
J. A.; Ferragut, S.; Castanys, S.; Gamarro, F. Biochem. Pharmacol. 1998, 55, 131.
6. Perez-Victoria, J. M.; Di Pietro, A.; Barron, D.; Ravelo, A. G.; Castanys, S.;
Gamarro, F. Curr. Drug Targets 2002, 3, 311.
18. Cortés-Selva, F.; Campillo, M.; Reyes, C. P.; Jiménez, I. A.; Castanys, S.; Bazzochi,
I. L.; Pardo, L.; Gamarro, F.; Ravelo, A. G. J. Med. Chem. 2004, 47, 576.
19. Caballero, E.; Puebla, P.; Medarde, M.; Sánchez, M.; Salvadó, M. A.; García-
Granda, S.; San Feliciano, A. J. Org. Chem. 1996, 61, 1890.
7. Eckford, P. D. W.; Sharom, F. J. Chem. Rev. 2009, 109, 2989.