A novel fluoride-sensing scaffold by a peculiar acid-promoted trimerization of 5,6-dihydroxyindole

Article abstract of DOI:10.1016/j.tet.2009.01.003

An unusual rearranged trimer, 2-(2-amino-4,5-dihydroxybenzyl)-6,7-dihydroxy-3-(5,6-dihydroxyindol-3-yl)quinoline (1a), was obtained as the acetyl derivative (1b) by mild acid-promoted polymerization of 5,6-dihydroxyindole at pH 2. Compound 1b proved to be

Full text of DOI:10.1016/j.tet.2009.01.003

Tetrahedron 65 (2009) 2032–2036
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Tetrahedron
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A novel fluoride-sensing scaffold by a peculiar acid-promoted trimerization
of 5,6-dihydroxyindole
,
Lucia Panzella a, Alessandro Pezzella a, Marianna Arzillo a b, Paola Manini a, Alessandra Napolitano a,
,
Marco d’Ischia a
*
a Department of Organic Chemistry and Biochemistry, University of Naples ‘Federico II’, Via Cinthia 4, I-80126 Naples, Italy
b ‘Paolo Corradini’ Department of Chemistry, University of Naples ‘Federico II’, Via Cinthia 4, I-80126 Naples, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
An unusual rearranged trimer, 2-(2-amino-4,5-dihydroxybenzyl)-6,7-dihydroxy-3-(5,6-dihydroxyindol-
3-yl)quinoline (1a), was obtained as the acetyl derivative (1b) by mild acid-promoted polymerization of
5,6-dihydroxyindole at pH 2. Compound 1b proved to be a selective fluoride-sensing compound,
transducing Fꢀ binding into a distinct absorption at 414 nm and a marked fluorescence enhancement at
489 nm.
Received 25 September 2008
Received in revised form 28 November 2008
Accepted 5 January 2009
Available online 8 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
5,6-Dihydroxyindole
Fluoride
Sensor
Fluorescence
Rearrangement
1. Introduction
the marked Fꢀ-induced changes in the absorption and fluorescence
spectra. This observation suggested a potential of the trimer as
5,6-Dihydroxyindoles are a unique group of naturally occurring,
catechol-containing heterocyclic compounds, which arise bio-
genetically by the oxidative cyclization of catecholamines and re-
lated tyrosine-derived metabolites. A marked facility to oxidation,
leading to black insoluble polymeric materials, is the distinctive
chemical feature underlying the biological importance of 5,6-
dihydroxyindoles.1 This is well illustrated by their role as primary
building blocks of eumelanins,2 the key components of the human
pigmentary system.3 5,6-Dihydroxyindoles have also been exploi-
ted in cosmetics and medicinal chemistry, e.g., as active moieties in
antiviral agents and antibiotics.4
a novel prototype of fluoride-sensing scaffolds. Despite the vast
literature that accumulated during the past few years, the quest for
easily accessible and efficient fluoride-sensing molecular systems is
still an active area of research6 because of the significant biological,
medical, industrial, and environmental relevance of fluoride
chemistry.
In this paper we report details of the acid-promoted trimeri-
zation of 5,6-dihydroxyindole and describe the selective effects of
the fluoride anion on the chromophoric and fluorescence proper-
ties of the acetylated trimer.
Recently, while pursuing a program aimed at designing novel
5,6-dihydroxyindole-based functional materials,5 we came across
an unexpected behavior of this indole when left to polymerize
under mildly acidic conditions. The noticeable outcome of this re-
action was the formation of a rearranged trimer featuring an un-
usual 2-benzyl-3-indolylquinoline skeleton. Interestingly, the
acetylated derivative of the trimeric product was found to exhibit
selective binding properties toward fluoride anions, as revealed by
2. Results and discussion
When 5,6-dihydroxyindole was dissolved in phosphate buffer at
pH 2 and left at room temperature a smooth reaction occurred,
leading after ca. 24 h to a main trimeric species (LC/MS analysis).
This was obtained as the heptaacetyl derivative ([MþH]þ m/z 740)
in 10% yield by a simple work-up procedure involving acetylation of
the crude mixture followed by a chromatographic step, and was
identified as 2-(2-acetamido-4,5-diacetoxybenzyl)-6,7-diacetoxy-
3-(5,6-diacetoxyindol-3-yl)quinoline (1b) following complete
spectral characterization (see Supplementary data). Inspection of
the reaction mixture in the early stages showed the formation of
dimer 2 and trimer 3a as minor isolable products.7
* Corresponding author. Tel.: þ39 081 674132; fax: þ39 081 674393.
E-mail address: dischia@unina.it (M. d’Ischia).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.01.003

L. Panzella et al. / Tetrahedron 65 (2009) 2032–2036
2033
A comparative study showed that none of the other indoles
examined, i.e., indole, 5-hydroxyindole, 6-hydroxyindole, 5,6-
dimethoxyindole, and 5,6-dihydroxy-N-methylindole, give the
corresponding 3-(indol-3-yl)quinoline trimer under the same
conditions. Use of stronger acids, e.g., HCl, or organic acids, e.g.,
acetic acid, was not productive, furnishing invariably ill-defined
mixtures. The facile formation of 1a from 5,6-dihydroxyindole is
therefore attributed to the specific reactivity of this indole via the 2-
position,8 steering in part the acid-promoted polymerization
pathway through the usually less favorable 2-(2-amino-4,5-dihy-
droxyphenyl)-1-(5,6-dihydroxyindol-2-yl)-1-(5,6-dihydroxyindol-
3-yl)ethane (4) (Scheme 1). Formation of the quinoline system of 1a
from 4 may proceed through a rearrangement step akin to that
described for indole trimers in acidic media.9
The 2-(2-aminobenzyl)-3-(indol-3-yl)quinoline system featured
by 1a has previously been obtained only by harsh treatment of
indole under Friedel–Crafts acylation conditions10 or in the pres-
ence of p-toluenesulfonic acid followed by complex work-up, ex-
traction, and chromatographic separation steps.9
Figure 1. Changes in the UV/vis spectra of 1b (1ꢁ10ꢀ5 M) in CH3CN after addition of 0,
10, 15, 20, 25, 50, 100, 150, and 200 equiv of tetrabutylammonium fluoride (TBAF).
Inset: Absorbance at 414 nm versus equiv of Fꢀ.
The absorption properties of 1b are shown in Figure 1. The
compound exhibited a distinct maximum at 330 nm in CH3CN.
Upon the addition of increasing concentrations of Fꢀ, a yellow
coloration became apparent, due to the development of an ab-
sorption at 414 nm. Examples of colorless-to-yellow color changes
associated with Fꢀ binding to an organic compound have already
been reported in the literature.11 No clear isosbestic point is
apparent from data in Figure 1, which suggests that color de-
velopment involves more complex equilibria than a simple 1:1
substrate–anion binding process.
The acetylated derivative 1b exhibited a remarkable fluores-
cence enhancement upon the addition of Fꢀ (Fig. 2). The fluores-
cence response of 1b (5ꢁ10ꢀ7 M) upon addition of up to 300 equiv
F
ꢀ is shown in Figure 3. In the absence of Fꢀ, fluorescence of the free
compound was weak and barely detectable. Addition of the anion
to the solution caused the emergence of a distinct emission band at
489 nm following excitation at 414 nm. This effect is worthy of note
since anion binding causes fluorescence quenching for most of the
HO
OH
HO
HO
HO
HO
x2
H+
reported sensors,12 with only
enhancement.11c,13
a
few exhibiting fluorescence
NH
N
H
N
H
2
The recognition process was selective for Fꢀ since in the pres-
ence of other anions, including Clꢀ, Brꢀ, Iꢀ, AcOꢀ, NOꢀ2 , HSO4ꢀ, no
significant changes in the fluorescence spectra were observed.
Complete fluorescence quenching was noted however in the
presence of water (>20%). The stoichiometry of the fluoride–1b
interaction was determined to be 2:1 from the Job’s plot (Fig. 3).
In subsequent experiments, the effects of Fꢀ binding on the
parent 5,6-diacetoxyindole and the acetylated trimer 3b were in-
vestigated. Trimer 3b was a suitable model to identify the chro-
mogenic and fluorogenic systems in 1b, since it exhibited the
H+
RO
OR
NH
OH
HO
HO
HO
NH
H+
RO
RO
+
NH2
HN
NHR
OR
OR
OH
N
H
4
OH
3a, R=H
Ac2O
3b, R=Ac
rearrangement
RO
OR
HO
OH
RO
RO
NH
HO
HO
[O]
NH
OH
N
RHN
N
H2N
OR
OR
OH
1a, R=H
Ac2O
Scheme 1.
1b, R=Ac
Figure 2. Fluorescence changes of 1b (5ꢁ10ꢀ5 M) in CH3CN upon addition of 25 equiv
of TBAF. (A) no additive; (B) þTBAF (under a UV lamp at 366 nm).

2034
L. Panzella et al. / Tetrahedron 65 (2009) 2032–2036
Figure 3. Changes in the emission spectra of 1b (5ꢁ10ꢀ7 M) in CH3CN after addition of
TBAF from 0 to 300 equiv (excitation wavelength 414 nm). Inset A: Fluorescence in-
tensity at 489 nm versus equiv of Fꢀ. Inset B: Job’s plot for 1b–fluoride interaction in
CH3CN (total [1b]þ[Fꢀ]¼5ꢁ10ꢀ5 M).
diacetoxyindole and acetamido moieties placed at a comparable
distance by a non-conjugating spacer chain replacing the rigid
quinoline ring. Upon the addition of Fꢀ neither diacetoxyindole nor
3b developed significant fluorescence. This observation suggests
that the quinoline ring is an essential constituent of the fluoride-
sensing fluorophore.
To gain a deeper insight into the mechanism of Fꢀ coordination
by 1b, 1H NMR titration experiments were carried out in DMSO-d6.
Figure 4. Partial 1H NMR spectra of 1b in DMSO-d6 (A) in the absence or in the
presence of (B) 0.5 or (C) 2.5 equiv of TBAF.
With 0.5 equiv Fꢀ, the indole NH proton signal at
d 11.6 (Fig. 4A)
disappeared (Fig. 4B). A similar behavior has previously been
reported for several systems containing NH protons, indicating
interaction with Fꢀ anions.11a,c,13a,14 Addition of further amounts of
the anion caused disappearance of the amide NH proton signal at
attributed to fast proton exchange with the water impurity present
in the DMSO solvent.13c
Verification of the proposed Brønsted acid–base reaction by ti-
tration experiments with the strong base [(Bu)4N]OH (TBAOH) was
precluded by deacetylation of the catechol functions under such
d
9.81, accompanied by a slight upfield shift of the other proton
signals (Fig. 4C).13c,14,15 A visible fluorescence with concomitant
color change was well apparent at this stage.
No alteration of the signals due to the acetyl groups was ob-
served, ruling out deacetylation during Fꢀ coordination.
The ROESY spectrum of 1b in the absence of Fꢀ (see Sup-
plementary data) showed distinct cross peaks between: (a) the
quinoline H-4 and the indole H-4 resonances; (b) the quinoline
H-8 and the amide NH signals; and (c) the benzylic proton sin-
glet and the indole H-2 proton doublet. These contacts suggested
a preferential conformation with the indole NH group close to
the benzylic methylene and the amide group facing the quino-
line nitrogen, as previously described for related systems on the
basis of X-ray analysis.10 Interestingly, in the ROESY spectrum
recorded after addition of 0.5 equiv of Fꢀ, at a stage when only
the indole NH has disappeared, the cross peak between the
amide NH and the quinoline H-8 signals was no longer
detectable.
AcO
a
OAc
7''
AcO
4''
OAc
H
H
9''
8''
5
4
10
AcO
AcO
NH
1''
F-
3''
AcO
AcO
N
9
2
H
H
N
8
N
H
N
F-
c
H
H
1'
2'
N
b
Ac
AcO
Ac
OAc
OAc
OAc
1b
F-
AcO
Based on the above NMR data and the 2:1 stoichiometry infer-
red from the titration experiments, it is suggested that the initial
coordination of Fꢀ to the indole NH hydrogen induces a rotation of
180ꢂ of the benzylic ring. Upon the addition of further aliquots of
fluoride, significant deprotonation may occur,16 and the deproto-
nated indole can be stabilized by an intramolecular hydrogen bond
with the amide NH group (Scheme 2).17 A locked coplanar dispo-
sition of the indole and quinoline rings would then ensue, with
consequent fluorescence enhancement and UV bathochromic
shift.13a,17b Attempts to detect the HFꢀ2 signal in the 1H NMR spec-
= ROESY contacts
OAc
AcO
AcO
N
-
[F-H-F]-
H
+
N
N
Ac
AcO
OAc
trum at ca.
d 16 were however unsuccessful. This has been
Scheme 2.

L. Panzella et al. / Tetrahedron 65 (2009) 2032–2036
2035
conditions.1b Because of this deacetylation reaction and the lack of
effect of acetate and hydrogen sulfate anions,1b should be regarded
as a specific fluoride sensor and not a mere acid–base sensor.
The fluorescence titration profile in Figure 3 apparently sup-
ported the two-step process depicted in Scheme 2. Quantitative
measurements of the Fꢀ affinity from the titration data by pre-
viously reported procedures18 gave values of log K¼6.04 and 4.08,
which are comparable with those of many known fluorescence-
based sensors.13c,14,17b,19
(C-20), 136.9 (C-500), 137.5 (C-50), 138.6 (C-600), 139.0 (C-4), 140.5 (C-
40), 141.7 (C-6), 144.0 (C-9), 144.4 (C-7), 160.5 (C-2), 167.9–169.1
(COCH3).
Acknowledgements
This work was supported by a grant from Italian Ministry of
´
University (MIUR, PRIN 2006). L.P. thanks ‘L’OREAL Italia Per le
Donne e la Scienza’ for a research fellowship. We thank the ‘Centro
Interdipartimentale di Metodologie Chimico-Fisiche’ (CIMCF, Uni-
versity of Naples Federico II) for NMR facilities.
3. Conclusion
We have reported herein a novel fluoride-sensing scaffold,
which was obtained by a variant of the classic acid-promoted tri-
merization of indoles. The mild one-pot conversion of 5,6-dihy-
droxyindole to 1b provides an expedient and practical access route
to the 2-(2-amidobenzyl)-3-(indol-3-yl)quinoline system, and the
ease of preparation would offset the relatively small product yield.
Compound 1b represents a chromogenic and fluorogenic fluoride-
sensing system20 operating in the turn-on mode, and its
characterization may stimulate further studies on the potential
fluoride-sensing properties of related indolylquinoline systems.
Supplementary data
1H NMR, 13C NMR, 1H,1H COSY, ROESY, 1H,13C HSQC-DEPT, and
1H, 13C HMBC spectra of 1b are provided. Supplementary data as-
sociated with this article can be found in the online version, at
doi:10.1016/j.tet.2009.01.003.
References and notes
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4. Experimental section
4.1. General methods and materials
5,6-Dihydroxyindole was prepared as reported.21 TBAF 1.0 M
solution in THF was used as obtained. LC/MS analysis was carried
out on an instrument equipped with an ESI ion source; an octa-
decylsilane-coated column, 150 mmꢁ4.6 mm, 3.5
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at 0.4 mL/min was used. The eluant system was 0.2% formic acid,
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