The photochemistry of lomefloxacin. An aromatic carbene as the key intermediate in photodecomposition

Article abstract of DOI:10.1039/a703226i

Irradiation of the 6,8-difluoroquinolone antibiotic lomefloxacin causes selective heterolytic defluorination from position 8 and leads to products rationalized as arising from the cation either via nucleophile addition or intramolecular carbene C-H insertion and hydrogen transfer.

Full text of DOI:10.1039/a703226i

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The photochemistry of lomefloxacin. An aromatic carbene as the key
intermediate in photodecomposition
E. Fasani, M. Mella, D. Caccia, S. Tassi, M. Fagnoni and A. Albini*
Department of Organic Chemistry, University of Pavia, Via Taramelli 10, 27100 Pavia, Italy
Irradiation of the 6,8-difluoroquinolone antibiotic lome-
floxacin causes selective heterolytic defluorination from
position 8 and leads to products rationalized as arising from
the cation either via nucleophile addition or intramolecular
carbene C–H insertion and hydrogen transfer.
to irradiation than all the monofluoro FQs we examined under
these conditions (Fdec < 0.03) and also than compounds 2–5,
which are monofluoro derivatives. Thus, these products arose
from a monophotonic reaction, and both processes (i) and (ii)
occurred in a single reaction rather than via two sequential
photoinitiated steps. Furthermore the product distribution was
again similar in air-equilibrated solution. On the other hand,
irradiation of the monohydrochloride omitting the buffer gave
product 6, conserving the piperazine ring intact and with
substitution of a chloro group for the 8-fluoro atom.
The strong antibacterial activity of fluoroquinoles (FQ) has led
1
to their large-scale use in medicine. A major drawback of this
relatively new class of antibiotics is their phototoxic effects.2
There are indications that reactive oxygen species, such as
singlet oxygen, superoxide anion or hydroxyl radical, play a
–8
The mechanism we propose is indicated in Scheme 2. The
reaction proceeds from the short-lived singlet excited state and
is by far the main decay path (Freact ≈ 0.65, tfluo < 1 ns, little
influence of dissolved oxygen, thus Fisc is low and the triplet
has little role in the reaction). Homolysis of the C–F bond is
obviously out of question (bond dissociation energy C–F = 125
9–14
role in some cases.
However, this is not necessarily the main
mechanism underlying such an effect. As an example, it has
recently been reported that in the case of lomefloxacin 1, which
is not only more phototoxic than other FQs but also more
photomutagenic and carcinogenic,1
5,16
the phototoxic effect is
not related to singlet oxygen and superoxide anion generation1
7
kcal mol , energy of signlet excited 1 = 81 kcal mol ). Thus
21
21
2
and furthermore photoinduced single strand breaks in plasmid
heterolytic loss of F is involved, and the mechanism proposed
18
DNA are actually decreased in the presence of oxygen. This
suggests that oxygen activation is not involved, and that a
different mechanism operates; it has also been noted that
fluoride ion is liberated during the photolysis of 1.18 An
investigation of the photochemistry of 1 was thus warranted. As
is shown in the following, a detailed product study demonstrates
the unusual photochemical behaviour of this molecule and
suggests a mechanism for the toxic and particularly the
carcinogenic/mutagenic activity.
must explain also the other modification occurring con-
currently. The singlet state has an internal charge transfer
2
1
character (strong Stokes shift, 6000 cm ) and the zwitterionic
form is important in 1*. Loss of fluoride leads to cation 7, for
which mesomeric carbocationic and carbene forms can be
considered. In phosphate buffer we isolated no product arising
from the carbocation (such as phenols), but that product 6
formed when the nucleophile is chloride substantiates this path.
On the other hand a striking indication for carbene reactivity
comes from the structure of product 4, obviously resulting from
well precedented carbene C–H insertion19 into the conforma-
2
4
Irradiation (l > 310 nm) of lomefloxacin 1 (10 m) in
argon-flushed phosphate buffer (pH 7.4) up to 90% conversion
gave several products. These were transformed into the
corresponding NA-ethylcarbamates by treatment with ethyl
chloroformate, extracted with chloroform, esterified by treat-
ment with diazomethane and chromatographically separated. In
this way, the main photolysis products were recognised as the
two diamines 2 and 3, accompanied by minor amounts of the
tricyclic derivative 4 and a trace of the imidazolonyl derivative
O
F
CO2H
N
N
N
F
Et
H
5
(Scheme 1). Irradiation of the hydrochloride of 1 in argon-
flushed water (pH 4.3) gave the 8-chloro derivative 6 as one of
Me
1
the main products.†
hν
O
O
O
F
CO2H
F
CO2H
F
CO2H
N
N
Me
N
H
N
N
H
N
N
Cl
Et
H
NH2
Et
NH2
Et
Me
2
3
6
O
N
O
Inspection of 2–5 shows that two processes occurred in all
cases, viz (i) reductive defluorination selectively at position 8,
and (ii) modification of an N-alkyl chain, either the piperazinyl
group (products 2, 3 and 5) or the 1-ethyl group (product 4).
Since the photoproducts are themselves FQs, competitive light-
absorption by the products and the starting material occurred.
However, when the irradiation was limited to a lower
conversion (35%) the ratio of products was essentially the same.
Furthermore, we found that 1 (Fdec 0.65) was far more sensitive
F
CO2H
F
CO2H
N
N
N
Me
N
Et
N
H
O
H
Me
4
5
Scheme 1
Chem. Commun., 1997
1329

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O
O
O
O
F
CO2H
F
CO2H
F
CO2H
H2O
F
CO2H
δ–
–F –
δ+
N
+
N
N
N
N
N
N
N
••
+
N
F
Et
N
Et
N
Et
N
OH Et
H
H
H
H
Me
Me
F
Me
Me
1*
7
Cl –
O
N
O
O
F
CO2H
CO2H
F
CO2H
6
+
N
+
N
+
N
+
N
N
N
H
N
Et
N
Et
H
H
H
Me
Me
Me
8
9
–
H +
–
H +
O
O
F
CO2H
F
CO2H
4
+
N
N
N
N
N
Et
N
Et
H
H
Me
Me
H2O
2
+ 3
Scheme 2
tionally suitable d position. Products 2 and 3 also arise from the
carbene via intramolecular hydrogen transfer to give the
rearranged cations 8 and 9. These deprotonate to enediamines,
which in turn are hydrolysed, whether in solution or during
work up, to the observed diamines. Compound 5 also results
from demolition of the piperazinyl chain coupled with de-
fluorination.
References
1
H. C. Neu, in The 4-Quinolones, ed. G. C. Crumplin, Springer Verlag,
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2
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1
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20
N-formyl-4-fluorotryptophan methyl ester and of some dime-
6
2
1
thoxyfluorobenzenes, both involving related activation by
electron-donating groups. The peculiarly high photoreactivity
of lomefloxacin with respect to other FQs (see above) is
certainly due to the selective defluorination from position 8,
apparently due to activation by both the adjacent amino groups,
which increase the zwitterionic character of the singlet excited
state and facilitate the fragmentation indicated in Scheme 2.
Studies in progress in this laboratory show however that this
type of reactivity is rather general, and defluorination accom-
panied by alkylamino chain degradation also takes place in FQs
containing only the 6-fluoro group, although with a lower
efficiency.‡
1
7
8
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1
1
The fact that cation 7 exhibits carbene chemistry dominated
by intramolecular C–H insertion and hydrogen transfer strongly
suggests that the cause of the observed carcinogenic/mutagenic
effect of this drug is formation of a covalent bond by direct
reaction of excited 1 with DNA through a related inter-
molecular process.
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4
6
Partial support of this work by the Istituto Superiore di
Sanit a` , Rome, is gratefully acknowledged.
6
5
Footnotes
8
*
†
E-mail: albini@chifis.unipv.it
All products have been fully characterised by elemental analysis and
2
2
1 G. Zhang and P. Wan, J. Chem. Soc., Chem. Commun., 1994, 19.
2 G. Condorelli, G. De Guidi, S. Giuffrida, P. Miano, A. Velardita and
S. Sortino, Photochem. Photobiol., 1996, 63, 75S.
spectroscopic means as the corresponding NA-ethylcarbamate methyl
esters.
‡
2
Release of fluoride by irradiation of enoxacin has been detected (ref.
2).
Received in Cambridge, UK, 12th May 1997; Com. 7/03226I
1330
Chem. Commun., 1997
Typeset and printed by Black Bear Press Limited, Cambridge, England