Inter-and intramolecular photochemical reactions of fleroxacin

Article abstract of DOI:10.1021/ol900189v

In the cation formed by photoinduced C-F bond cleavage in fleroxacin, intramolecular reaction with the W-ethyl chain is prevented by the electron-withdrawing effect of fluorine and intermolecular attack by nucleophiles is facilitated.2009 American Chemica

Full text of DOI:10.1021/ol900189v

ORGANIC
LETTERS
2009
Vol. 11, No. 9
1875-1878
Inter- and Intramolecular Photochemical
Reactions of Fleroxacin
Elisa Fasani,*,† Sandra Monti,*,‡ Ilse Manet,‡ Fedele Tilocca,† Luca Pretali,†
Mariella Mella,† and Angelo Albini*,†
Department of Organic Chemistry, UniVersity of PaVia, V. Taramelli 10, 27100 PaVia,
Italy, and Istituto per la Sintesi Organica e la FotoreattiVita` ISOF-CNR,
V. Gobetti 101, 40127 Bologna, Italy
elisa.fasani@unipV.it, monti@isof.cnr.it, angelo.albini@unipV.it
Received January 29, 2009
ABSTRACT
In the cation formed by photoinduced C-F bond cleavage in fleroxacin, intramolecular reaction with the N-ethyl chain is prevented by the
electron-withdrawing effect of fluorine and intermolecular attack by nucleophiles is facilitated.
Some components of the important class of fluoroquinolone
(FQ) antibacterials are characterized by an uncommonly high
photolability and phototoxicity.1-3 Indeed, some “2nd gen-
eration” derivatives bearing a second fluoro atom substituent
in 8, besides the ubiquitous fluoro in 6, had inhibited or
limited the therapeutic use due to this fact.
assumed to be responsible for such effects. In fact, compound
1 outbalances other FQs not as O2 sensitizer [Φ(1O2) ) 0.07]5
but for the efficient photodecomposition (Φ ) 0.55,2 for most
FQs < 0.1). Excitation of this drug is known to cause in
Vitro DNA cleavage,4 which suggests irreversible binding
to DNA.6
Determining the nature of such chemical binding would
help in understanding the photogenotoxic effect and thus to
devise better drugs, but more importantly may suggest how
to tune the effect so that related molecules could be envisaged
that have a phototherapeutic effect. In the case of 1, extensive
experimental and computational studies have been carried
out3 and support the mechanism depicted in Scheme 1 with
Clarifying the chemistry underlying these negative side
effects has proven to be a hard task. Much work has been
devoted to the case of lomefloxacin (1, Scheme 1) because
it is strongly photogenotoxic.4 Apparently, this cannot be
traced back to the generation of singlet oxygen, generally
† University of Pavia.
‡ FRAE-CNR Bologna.
3
triplet aryl cation 2+ as the intermediate. As it generally
(1) (a) Jeffrey, A. M.; Shao, L.; Brendler-Schwaab, S. Y.; Schlu¨ter, G.;
Williams, G. M. ArchiV. Toxicol. 2000, 74, 555. (b) Agrawal, N.; Ray,
R. S.; Farooq, M.; Pant, A. B.; Hans, R. K. Photochem. Photobiol. 2003,
87, 1226. (c) Albini, A.; Monti, S. Chem. Soc. ReV. 2003, 32, 238. (d)
Cuquerella, M. C.; Miranda, M. A.; Bosca`, F. J. Phys. Chem. B 2006, 110,
6441. (e) de Vries, H.; Beijersbergen van Henegouwen, G. J. Photochem.
Photobiol. B: Biol. 2000, 58, 6. (f) Lorenzo, F.; Navaratnam, S.; Allen,
occurs with triplet aryl cations, this is similar to a triplet
carbene and reacts via hydrogen abstraction from the N-ethyl
chain giving a distonic diradical that cyclizes (path a). A
weak nucleophile such as water does not interfere, but anions
Cl- and Br- trap the cation and attack at position 8 (path
N. S. J. Am Chem. Soc. 2008, 130, 12238
(2) Fasani, E.; Barberis Negra, F. F.; Mella, M.; Monti, S.; Albini, A.
J. Org. Chem. 1999, 64, 5388
(3) Freccero, M.; Fasani, E.; Mella, M.; Manet, I.; Monti, S.; Albini, A.
.
.
(5) (a) Martinez, L. J.; Sik, R. H.; Chignell, C. Photochem. Photobiol.
1998, 67, 399. (b) Wagai, N.; Tawara, K. Arch. Toxicol. 1992, 66, 392.
(6) Notice, however, that photoinactivation of topoisomerase IIR has
been supported for related FQs, not for 1, see: Perrone, C. E.; Takahashi,
K. C.; Williams, G. M. Toxicol. Sci. 2002, 69, 16.
Chem.-Eur. J. 2008, 14, 653
.
(4) Marrot, L.; Belaidi, J. P.; Jones, C.; Perez, P.; Riou, L.; Sarasin, A.;
Meunier, J. R. J. InVest. Dermatol. 2003, 121, 596.
10.1021/ol900189v CCC: $40.75
Published on Web 03/30/2009
 2009 American Chemical Society

Scheme 1
.
Photochemical Reactions of Lomefloxacin (1) in
Water
b). On the other hand, softer anions such as I- or π
nucleophiles such as pyrrole act as electron donors and the
ensuing radical coupling leads to products functionalized at
the side chain (path c).
The aggressive cation 32+ may be responsible for phototox-
icity, but this implies that an intermolecular reaction with a
biomolecule competes with the intramolecular trapping by the
N-ethyl group (path a). The association of the drug with DNA
in ViVo is obviously determining (the association with bacterial
gyrase and topoisomerase explains the FQs action), but tackling
the mechanism in solution by some structural change may
evidence any chemoselectivity. In view of the above, an obvious
move is a modification of the N-ethyl group.
Figure 1. (a) Transients following excitation at 355 nm of a N2O
saturated 1.4 × 10-4 M solution of 3 in 1.0 × 10-3 M NaHCO3
buffer of pH 7.2 at 25 °C.; upper trace, 40 ns, lower trace, 420 ns
after the flash. (b) Calculated components by deconvolution of the
previous transient absorption; for the attribution, see text. (Inset)
Decays at 360 and 490 nm and biexponential best fit with τ1 ) 41
ns and τ2 ) 154 ns.
The signal could be analyzed as involving three subsequent
transients (Figure 1b). The initial signal (9, λmax ) 370 nm)
was safely attributed to triplet 3 (τ ) 40 ns, a short lifetime
3
just as for 1, due to the fast C8-F bond cleavage). This
evolved into a second transient (4, λmax ) 475 nm, τ ) 150
Fleroxacin (3) appeared a suitable candidate. This is a FQ
known to be even more phototoxic than 1, while producing
3
ns), attributed to cation 4+. This had a spectrum similar to
3
that of 2+ but was somewhat longer-lived, consistent with
5,7
less O2 [Φ(1O2) ) 0.03]. 3 differs from 1 for bearing a
N-(2-fluoroethyl) group. Thus, with 3 the photodefluorination
from C8 would not be affected, since the chromophore is
unchanged, but the chemistry of the cation would, since
fluorine substitution at the side chain hinders electrophilic
H abstraction (path a). As it will be shown below this change
did indeed dramatically affect the photochemistry occurring
and the results strengthened the mechanistic picture.
1
the idea that H abstraction from the N-ethyl chain was now
suppressed. Coherently, the products isolated after irradia-
tion9 involved different paths.
Two of these, amide 5 and amine 6, reasonably resulted
from the alternative hydrogen abstraction from the piperazine
side-chain (geometrically less convenient, see Scheme 2, path
a). Analoguous products had been previously found from
other FQs and rationalized as involving C-C bond cleavage
Fleroxacin 3 turned out to be rather photoreactive (Φ )
0.22). Flash photolysis in water showed a differential
absorption in the region 350-500 nm that underwent
biexponential decay (Figure 1a).8
(8) For the conditions of the flash photolysis experiments, see ref 2.
(9) Preparative irradiations were carried out on 6 × 800 mL 2.5 × 10-4
M aqueous solutions of 3 by using a 125 high-pressure mercury arc through
quartz. The solutions were evaporated, the residue taken up by benzene-
methanol 8:2 and treated by (trimethylsilyl)diazomethane (2.0 M solution
in hexanes). The products were separated by chromatography (eluent:
chloroform/methanol) as the methyl esters and characterized. Key analytic
and spectroscopic data are listed in the Supporting Information.
(7) Yabe, K.; Goto, K.; Jindo, T.; Sekiguchi, M.; Furuhama, K. Toxicol.
Lett. 2005, 157, 203
.
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Org. Lett., Vol. 11, No. 9, 2009

Scheme 2
.
Photochemical Reactions of Fleroxacin (3) in Water
Scheme 3
.
Photochemistry of Fleroxacin (4) in the Presence of
Iodide and Bisulfite
I- is attack at the side chain (Scheme 1, path c). This result
is again an indication of the preference for ionic coupling at
C8 in the case of fleroxacin, which extends also to a soft
anion such as iodide.
A clean reduction to product 9 took place in the presence
of bisulfite (0.02 M), reasonably via reduction of the cation
to radical 4, followed by further reduction to the anion and
protonation.
Pyrrole was then tested and found to quench effectively
34+ [k(pyrrole) ) 5 × 108 M-1s-1 by flash photolysis].
Irradiation in the presence of 0.05 M pyrrole suppressed the
products obtained in neat water and gave a new main product,
which was isolated (62%) and found to contain two
equivalents pyrrole nuclei attached at CHCH2N group. The
and water addition at the radical level to form formyl
derivatives such as 5. This is followed by oxidative dealky-
lation to give 6, an ubiquitous phenomenon with dialkylary-
lamines.10,11
Scheme 4
.
Photochemistry of Fleroxacin (4) in the Presence of
Pyrrole
As for the third product, analytic and spectroscopic
evidence showed that it resulted from water addition and
HF elimination and supported the formula of oxazinoquino-
linone 7. Thus, cation 34+ is trapped by water, whereas this
3
is not the case with 2+. The difference is explained by the
3
greater charge localization at C8 in cation 4+, due to the
lack of the stabilizing interaction with the nucleophilic C-H
bonds characteristic of 32+, and by the cooperative effect of
the formation of the strong HF bond when the fluoride anion
is detached.
Cation 34+ was effectively quenched by iodide, with k(I-)
) 3.5 × 109 M-1s-1 as determined by flash photolysis.
Irradiation with 0.005 M KI gave the straightforward
substitution product (8, see Scheme 3). This is in contrast to
the behavior of cation 32+, for which the main process with
(10) Fasani, E.; Mella, M.; Monti, S.; Albini, A. Eur. J. Org. Chem,.
2001, 391
.
(11) Fasani, E.; Rampi, M.; Albini, A. J. Chem. Soc., Perkin Trans. 2
1999, 1901
.
Org. Lett., Vol. 11, No. 9, 2009
1877

characterization supported the assignement of structure 10
to this product (see Scheme 4).
Furthermore, the borderline between the ionic coupling
at C8 and the ET/radical coupling mechanism is shifted. This
lies between Br- and I- with 32+, but between I- and pyrrole
with 34+. Thus, with the first cation both pyrrole and iodide
act as electron donors. This is followed by a radical reaction,
intramolecular H transfer from the chain. On the contrary,
This surprising result was explained by the operation of
an electron transfer (ET) path with this π nucleophile
followed by coupling within the radical ion pair (11, Scheme
4). Elimination of HF (for a precedent, see ref 12) then led
to the stabilized pyrrolemethine cation 13 and to the final
product via electrophilic substitution.
3
with 4+ ionic attack at C8 is the exclusive process with
iodide and has some role with water. With this cation, the
ET path remains a prerogative of a π donor such as pyrrole.
Triplet phenyl cations show some selectivity toward nucleo-
philes, for example, 4-dimethylaminophenyl cation couples
three times faster with iodide than with pyrrole.13 Apparently,
the difference is larger in this case, with no detectable ionic
attack with pyrrole.14
A neutral π reagent such as pyrrole thus acts as electron
donor rather than as nucleophile. The lack of fast ionic
attack at C8 restores the role of the interaction with the
side chain, despite its weakness, so that the radical center
is formed on the chain, not at C8. It thus appears that the
mode and efficiency of intermolecular reactions of triplet
3
aryl cations can be tuned by structural variations. In 2+,
These results further document the unusual chemistry of
aryl cations and suggest that also the interaction with
biomolecules can be directed to some degree. It is hoped
that these chemical data, completed with biological studies,
lead to a better understanding of the phototoxicity of this
drug family and in perspective to the design of new
photoactivated drugs.
the geometrically favored interaction with the nucleophile
C-H bond in the chain makes intramolecular reaction fast
(path a, scheme 1).
Acknowledgment. Partial support of this work by Murst,
Rome is gratefully acknowledged.
Supporting Information Available: Experimental pro-
cedures and analytical and spectroscopic data for compounds
5-10. This material is available free of charge via the
Internet at http://pubs.acs.org.
3
The introduction of a fluorine atom in 4+ loosens the
“shielding” of C8 by the ethyl group so that the charge is
more localized at that carbon. Therefore, even with a weak
nucleophile such as water intermolecular trapping of the
cation (path c in formula 14) better competes with H
abstraction. The latter path remains majoritary (about 3 to
1), but in this case involves the piperazine side-chain, rather
than the geometrically, but not electronically, convenient
ethyl group (path b rather than path a).
OL900189V
(13) Dichiarante, V.; Fagnoni, M.; Albini, A. J. Org. Chem. 2008, 73,
1282.
(14) An alternative mechanism that can be considered is intramolecular
electron transfer from the piperazine moiety to the heterocyclic ring. We
thank a Referee for pointing to this possibility, and indeed, we think that
this may have a role with other FQs, where oxidation of the amine side-
chain results. This seem difficult to apply to the present case both because
of the difficult C-F bond cleavage from a radical anion and because it
would not lead to the observed products, compare ref 2.
(12) For related elimination reactions, see, for example: Kwong-Chip,
J. M.; Tidwell, T. T. Tetrahedron Lett. 1989, 30, 1319.
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Org. Lett., Vol. 11, No. 9, 2009