Synthesis of fluorescein derivatives containing metal-coordinating heterocycles

Article abstract of DOI:10.1016/j.tetlet.2004.07.131

Fluorescein derivatives that contain Lewis basic heterocycles have been synthesized by concise reaction sequences. The preparation of these compounds proceeds by functionalization of an electron-rich aromatic precursor and subsequent condensation to form the fluorophore. These compounds are envisioned as components of metal-based sensors.

Full text of DOI:10.1016/j.tetlet.2004.07.131

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7129–7131
Synthesis of fluorescein derivatives containing
metal-coordinating heterocycles
Matthew A. Clark, Scott A. Hilderbrand and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 23 March 2004; revised 17 July 2004; accepted 22 July 2004
Abstract—Fluorescein derivatives that contain Lewis basic heterocycles have been synthesized by concise reaction sequences. The
preparation of these compounds proceeds by functionalization of an electron-rich aromatic precursor and subsequent condensation
to form the fluorophore. These compounds are envisioned as components of metal-based sensors.
Ó 2004 Elsevier Ltd. All rights reserved.
As part of a program in our laboratory to investigate the
interactions of metal ions with fluorophores,1 we were
interested to synthesize fluorescein derivatives that con-
tain Lewis basic sites for metal coordination. Such
metal–fluorophore complexes have considerable poten-
tial for sensing applications by analyte-induced ligand
release with concomitant fluorescence Ôturn-onÕ. The
preparation of functionalized fluoresceins has received
relatively little attention, however, and finding efficient
routes to such molecules remains a challenge. We report
here the preparation of several fluorescein and naphtho-
fluorescein derivatives that contain the requisite N-
donor sites.
atom with a more electron-releasing nitrogen atom,
shifts the emission to longer wavelengths.
The synthesis of one such piperazine-modified rhoda-
fluor2 is outlined in Scheme 1. First, a palladium-cata-
lyzed coupling of 3-bromoanisole and Boc-piperazine
was performed to give the arylamine 1.3 Simultaneous
Boc-deprotection and demethylation was then achieved
by using boron tribromide. The resulting aminophenol
2 was next condensed with benzophenone derivative 34
in hot TFA to give the desired rhodafluor 4.5 As
expected, this compound is fluorescent, with an excita-
tion maximum of 520nm and an emission wavelength
of 545nm in MeOH solution.
To maximize interaction of the metal ion with the fluoro-
phore, we sought to position the donor atom as close
as possible to the fluorophore nucleus, without locating
the donor so close as to quench the emission by photo-
induced electron transfer (PET). There is considerable
precedence that benzylic amines, at a two-bond separa-
tion from the aromatic system, are efficient PET quench-
ers. Our goal therefore was to design dyes with three
bonds intervening between the donor amine and the aro-
matic nucleus. This goal was efficiently achieved by
replacing the fluorescein aryl hydroxyl group with a
piperazine ring. Such a perturbation not only situates
the donor near the fluorophore on a tether with minimal
structural flexibility, it also, by substituting the oxygen
In order to access compounds with longer emission
wavelengths, we prepared analogs of 4 containing a
more extensive aromatic p system. Such naphthofluores-
ceins emit ca. 100nm to the red of the corresponding
fluoresceins. A preparative route similar to that used
to synthesize 4 was considered, but the unavailability
of 6-bromo-1-naphthol caused us to seek an alternative
pathway (Scheme 2). Since 6-amino-1-naphthol 5 is
commercially available,6 it was employed as a starting
material to prepare the aryl piperazine by a double con-
densation. When 5 was heated with bis(2-chloro-
ethyl)amine under microwave irradiation, an
acceptable yield of the desired naphthyl piperazine 6
was achieved.7 Condensation with the benzophenone
proceeded smoothly in hot TFA to give the desired
naphthorhodol 7.8 This annulated rhodafluor did indeed
display longer wavelength fluoresence, emitting at
600nm in MeOH when excited at 525nm.
*
Corresponding author. Tel.: +1 1617 253 1892; fax: +1 617 258
8150; e-mail: lippard@lippard.mit.edu
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.07.131

7130
M. A. Clark et al. / Tetrahedron Letters 45 (2004) 7129–7131
Br
MeO
NBoc
NH
a
+
N
N
MeO
HO
b
H
N
2
1
N
Boc
HN
N
O
O
c
Cl
COOH
O
COOH
Cl
HO
4
3
OH
Scheme 1. Reagents and conditions: (a) Pd2(dba)3, NaOtBu, 2-dicyclohexylphosphino-20-dimethylaminobiphenyl, THF, 80°C, 90%; (b) BBr3,
CH2Cl2, 3d, 65%; (c) compound 3, TFA, reflux, 71%.
HN
N
OH
OH
O
O
b
a
Cl
N
H2N
CO2H
HN
7
6
5
Scheme 2. Reagents and conditions: (a) bis(2-chloroethyl)amine, DMF, microwave irradiation, 5min, 47%; (b) compound 3, TFA, 125°C, 3h, 49%.
Our success in synthesizing fluoresceins containing ali-
phatic heterocycles encouraged us to extend the chemis-
try to aromatic heterocycles. Specifically, we sought to
install an imidazole ring onto the fluorescein nucleus.
We anticipated that a direct linkage between an imid-
azole nitrogen atom and the fluorescein would be prob-
lematic, since the N-lone pair, necessary for fluores-
cence, would be largely unavailable due to aromaticity.
We therefore decided to attach the imidazole ring through
an amino group at the 2-position. Also, we expected the
presence of the electron-withdrawing imidazole to shift
the emission of the fluorophore toward the blue. To com-
pensate, we concentrated our efforts on the synthesis of
longer wavelength naphtho-type products.
pled with 6-aminonaphthol 5 by heating in toluene in
the presence of TsOH.10 The resulting arylaminoimid-
azole 9 was condensed under the usual conditions to give
the desired fluorescein 10.11 Analysis of the reaction by
LCMS showed a small amount of a byproduct, the mass
of which indicated loss of the imidazole ring. Presum-
ably, this product will form in unacceptably high levels
if the reaction is allowed to proceed for too long. The
product is fluorescent as anticipated, with an excitation
wavelength of 525nm and emission at 550nm in MeOH.
To conclude, we report here the synthesis of several
fluorescein derivatives that contain Lewis donor sites.
The interactions of these molecules with metal ions are
currently being studied. We anticipate that these and
similar compounds will be useful in metal-based sensing
systems.
As shown in Scheme 3, 2-bromo-1-methylimidazole 8
served as starting material.9 This molecule could be cou-
Me
H
OH
N
N
OH
N
O
O
Me
H2N
a
b
5
N
Me
Cl
N
N
N
CO2H
10
+
Br
H
9
N
8
Scheme 3. Reagents and conditions: (a) TsOH, toluene, reflux, 24h, 88%; (b) compound 3, TFA, 125°C, 24h, 44%.

M. A. Clark et al. / Tetrahedron Letters 45 (2004) 7129–7131
7131
1. Experimental procedures
1.1. Pip-rhodafluor 4
spectrometer was funded through NSF Grant CHE-
9808061. We thank Carolyn Woodroofe for supplying
compound 3 and Elizabeth Nolan for her assistance in
preparing this manuscript.
To a solution of 2 (200mg, 1.12mmol) in 10mL of TFA
was added compound 3 (1.314g, 4.50mmol), following
which the solution was heated to reflux for 3d. The
crude product was isolated by removal of the TFA
under reduced pressure. Column chromatography on
silica gel (100% acetone, 100% MeOH) yielded 4 as a
bright red solid (435mg, 70.6%).
References and notes
1. (a) Clark, M. A.; Duffy, K.; Tibrewala, J.; Lippard, S. J.
Org. Lett. 2003, 5, 2051; (b) Burdette, S. C.; Lippard, S. J.
Coord. Chem. Rev. 2001, 216–217, 333; (c) Franz, K. J.;
Singh, N.; Spingler, B.; Lippard, S. J. Inorg. Chem. 2000,
39, 4081.
2. Fluorescein derivatives that contain an oxygen and
nitrogen substituent will be referred to as rhodafluors;
they are also termed rhodols or fluorhods.
3. Zhang, X.-X.; Buchwald, S. L. J. Org. Chem. 2000, 65,
8027.
4. Burdette, S. C.; Frederickson, C. J.; Bu, W.; Lippard, S. J.
J. Am. Chem. Soc. 2003, 125, 1778.
1.2. Pip-naphthorhodafluor 7
Naphthol 6 (7mg, 0.027mmol) and benzophenone 3
(13mg, 0.046mmol) were combined in TFA (250lL)
in a sealed tube and heated to 125°C for 3h. The solu-
tion was evaporated and the residue chromatographed
on silica with 9:1 DCM/MeOH (1% TFA added during
chromatography to facilitate elution of the product).
This procedure afforded 7 (11mg, 69%) as a purple solid.
5. For 4: 1H NMR (400MHz, CD3CN) 8.71 (2H, br s),
7.97 (1H, d, J = 7.5Hz), 7.75–7.66 (2H, m), 7.16 (1H, d,
J = 7.3Hz), 6.96 (1H, s), 6.74 (1H, s), 6.69–6.63 (2H, m),
3.48 (4H, m), 3.28 (4H, m); 19F NMR (282MHz, CD3OD)
98.6; HR-ESIMS 435.1100 (M+H) calcd 435.1106.
6. TCI Chemicals, Tokyo, Japan.
1.3. Imidazole-naphthol 9
6-Amino-1-naphthol 5 (92mg, 0.58mmol), 2-bromo-N-
methylimidazole
7. Jaisinghani, H. G.; Khadilkar, B. M. Tetrahedron Lett.
1997, 38, 6875.
8 (140mg, 0.87mmol), p-toluene
8. For 7: 1H NMR (300MHz, CD3OD) 8.40 (d, 1H,
J = 9Hz), 8.08 (dd, 1H, J = 1.5, 7Hz), 7.75 (m, 2H), 7.50
(dd, 1H, J = 2, 10Hz), 7.38 (d, 1H, J = 9Hz), 7.24 (d, 1H,
J = 2Hz), 7.20 (m, 1H), 7.01 (s, 1H), 6.70 (s, 1H), 6.64 (d,
1H, J = 9Hz), 3.59 (m, 4H), 3.41 (m, 4H); ESIMS 485.3
(M + H, 100%), calcd 485.1.
9. Miller, R. D.; Lee, V. Y.; Moylan, C. R. Chem. Mater.
1994, 6, 1023. Carbon tetrabromide was used instead of
dibromoethane.
sulfonic acid (132mg, 0.70mmol), and toluene (1mL)
were combined in a tube and heated to 115°C for 24h.
The solution was then partitioned between saturated
aqueous bicarbonate and EtOAc. The organic layer
was dried and evaporated. The residue was chromato-
graphed with 9:1 CH2Cl2/MeOH to afford 9 (123mg,
88%). ESIMS 240.2 (M + H, 100%), calcd 240.3.
10. Personal communication, Dr. Ryan Schoenfeld, Roche
Palo Alto.
Acknowledgements
11. For 10: 1H NMR (300MHz, CD3OD) 8.62 (d, 1H,
J = 9Hz), 8.16 (d, 1H, J = 8Hz), 7.67 (m, 2H), 7.68 (s,
1H), 7.60 (m, 1H), 7.57 (d, 1H, J = 9Hz), 7.24 (m, 2H),
7.16 (s, 1H), 7.11 (d, 1H, J = 2Hz), 6.78 (m, 1H), 3.78 (s,
3H); ESIMS 496.3 (M+H, 100%), calcd 496.1.
This work was supported by the National Institute of
General Medical Sciences (GM-65519) and the National
Science Foundation (CHE-0234951). M.A.C. is an
NRSA Fellow (GM66501-01). The MIT DCIF NMR