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C.I.C. Esteves et al. / Dyes and Pigments xxx (2016) 1e9
as increased UV absorption and fluorescence. UV-active amino
acids are, therefore, valuable tools for biochemistry, cellular biology
and cellular imaging applications.
Having these facts in mind, and following our research interests
that include the synthesis and evaluation of fluorimetric chemo-
sensors for anions and cations based on heterocycles and amino
acids [6], new imidazo-benzocrown ether functionalised amino
acids 3 and 4 were synthesized and their evaluation as fluorescent
ester 1b was obtained as a white solid (0.101 g, 55%); 1H NMR
(400 MHz, CDCl3):
¼ 1.41 (s, 9H, C(CH3)3), 3.08e3.19 (m, 1H,
d
b-
CH2), 3.73 (s, 3H, OCH3), 4.56e4.61 (m,1H, a-H), 5.07 (d, J 7.2 Hz,1H,
NH Boc), 7.19 (d, J 8.2 Hz, 2H, H2 and H6), 7.37 (d, J 4.0 Hz, 1H, H-30),
7.59 (d, J 8.2 Hz, 2H, H3 and H5), 7.72 (d, J 4.0 Hz, 1H, H40), 9.87 (br s,
1H, CHO) ppm; 13C NMR (100.6 MHz, CDCl3):
d
¼ 28.21 (C(CH3)3),
38.12 (b-CH2), 52.28 (OCH3), 54.25 (a-C), 79.92 (C(CH3)3), 123.92
(C30), 126.41 (C3 and C5), 130.17 (C2 and C6), 131.70 (C4), 137.38
(C1), 141.25 (C40), 142.27 (C50), 153.90 (C20), 154.97 (C]O Boc),
171.54 (C]O ester), 182.70 (CHO) ppm.
chemosensors is now reported. The different
p-bridges linked to
the benzimidazole coordinating/reporting unit are intended to
improve the intramolecular electron delocalization, which will
tune the photophysical properties of new sensors and optimize the
recognition of target analytes through a greater sensitivity of
fluorescence. Additionally, the introduction of different crown
ether moieties is intended to improve the selectivity for the
recognition of the targets.
2.2. Synthesis of 40-amino-50-nitrobenzo-18-crown-6, 2b
40-Nitrobenzo-18-crown-6 (0.107 g, 0.3 ꢁ 10ꢂ3 mol) was dis-
solved in methanol/acetic acid (10:1) (5 mL), Pd/C (10 mg) was
added and the mixture was stirred in H2 atmosphere at room
temperature for 20 h. The solvent was removed in a rotary evapo-
rator and 40-aminobenzo-18-crown-6 was obtained as a yellow oil
2. Experimental
(0.096 g, 98%); 1H NMR (400 MHz, CDCl3):
6 ꢁ CH2), 3.81e3.82 (m, 4H, 2 ꢁ CH2), 4.02e4.03 (m, 4H, 2 ꢁ CH2),
6.33e6.37 (m, 2H, H30 and H60), 6.65e6.67 (m, 3H, H50 and NH2)
ppm.
d
¼ 3.60e3.66 (m, 12H,
All melting points were measured on a Stuart SMP3 melting
point apparatus. TLC analyses were carried out on 0.25 mm thick
precoated silica plates (Merck Fertigplatten Kieselgel 60F254) and
spots were visualised under UV light. Chromatography on silica gel
was carried out on Merck Kieselgel (230e240 mesh). NMR spectra
were obtained on a Bruker Avance III 400 at an operating frequency
of 400 MHz for 1H and 100.6 MHz for 13C using the solvent peak as
internal reference at 25 ꢀC. All chemical shifts are given in ppm
using dH Me4Si ¼ 0 ppm as reference and J values are given in Hz.
Assignments were made by chemical shifts, peak multiplicities and
J values and were supported by spin decoupling-double resonance
and bidimensional heteronuclear correlation techniques. Low and
high resolution mass spectrometry analyses were performed at the
“C.A.C.T.I. e Unidad de Espectrometria de Masas”, at University of
40-Aminobenzo-18-crown-6 (0.078 g, 0.24 ꢁ 10ꢂ3 mol, 1 equiv)
was suspended in acetic anhydride (5 mL) and Cu(NO3)2.3H2O
(0.058 g, 0.24 ꢁ 10ꢂ3 mol, 1 equiv) was added. The mixture was
stirred at room temperature during 1 h. The mixture was diluted
with CHCl3 (10 mL) and saturated Na2CO3 solution (10 mL) was
added. After complete acetic anhydride hydrolysis, the organic
layer was dried over anhydrous MgSO4 and the solvent was
removed under vacuum. The crude was submitted to silica gel
column chromatography using mixtures of dichloromethane and
methanol of increasing polarity as eluent. The fractions containing
the purified product were collected and evaporated under vacuum,
to yield N-(40-(50-nitrobenzo-18-crown-6))nitrous amide as a yel-
Vigo, Spain. Fluorescence spectra were collected using
a
FluoroMax-4 spectrofluorometer. UVevisible absorption spectra
(200e700 nm) were obtained using a Shimadzu UV/2501PC spec-
trophotometer. Luminescence quantum yields were measured us-
ing 9,10-diphenylanthracene in ethanol as standard (FF ¼ 0.95) [7].
All commercially available reagents were purchased from Sigma-
eAldrich, ACROS, or TCI and used as received. Organic solvents used
in the spectroscopic studies were of spectroscopic grade. Com-
pound 1a was synthesised as reported elsewhere [8] and N-(tert-
low oil (0.081 g, 91%); 1H NMR (400 MHz, CDCl3):
d
¼ 3.67e3.98 (m,
16H, 8 ꢁ CH2), 4.22e4.31 (m, 4H, 2 ꢁ CH2), 7.70 (s, 1H, H30), 8.47 (s,
1H, H60) ppm.
The previous compound (0.050 g, 0.13 ꢁ 10ꢂ3 mol) was treated
with concentrated HCl (2 mL) in 1,2-dicloroethane (10 mL) and
diethyl ketone (0.4 mL, 3.9 ꢁ 10ꢂ3 mol, 30 equiv) by stirring under
pressure at 80 ꢀC for 4 h. The mixture was neutralized with trie-
thylamine. The solvent was removed under vacuum and the residue
purified by silica gel column chromatography using mixtures of
dichloromethane and methanol of increasing polarity as eluent. 40-
Amino-50-nitrobenzo-18-crown-6 2b was obtained as a yellow oil
butyloxycarbonyl)-4-bromo-
precursor for 1b) was prepared from commercially available 4-
bromo- -phenylalanine by standard protecting group chemistry.
L-phenylalanine methyl ester (the
L
(0.041 g, 48%); 1H NMR (400 MHz, CDCl3):
d
¼ 3.60e3.75 (m, 12H,
2.1. Synthesis of N-(tert-butyloxycarbonyl)-4-(50-formylthiophen-
20-yl)-
-phenylalanine methyl ester, 1b
6 ꢁ CH2), 3.85e3.87 (m, 4H, 2 ꢁ CH2), 4.06e4.22 (m, 4H, 2 ꢁ CH2),
L
6.24 (s, 1H, H30), 7.42 (s, 1H, H60) ppm.
N-(tert-Butyloxycarbonyl)-4-bromo-L-phenylalanine
methyl
2.3. Synthesis of 15-crown-5-benzimidazolyl phenylalanine, 3
ester (0.170 g, 0.47 ꢁ 10ꢂ3 mol, 1 equiv) and Pd(PPH3)4 (0.016 g,
0.014 ꢁ 10ꢂ3 mol, 0.03 equiv) were stirred in DME (10 mL) during
10 min under inert atmosphere at 80 ꢀC. 5-Formylthiophene
boronic acid (0.088 g, 0.57 ꢁ 10ꢂ3 mol, 1.2 equiv), dissolved in ab-
solute ethanol (1 mL), and Na2CO3 2 M (0.5 mL, 2 equiv) were added
to the previous reaction mixture under inert atmosphere and the
progress of the reaction was followed by TLC. Ethyl acetate (10 mL)
and saturated NaCl solution (10 mL) were added, the mixture was
transferred to an extraction funnel and the layers were separated.
The organic layer was washed with water (3 ꢁ 15 mL) and NaOH
10% aqueous solution (1 ꢁ 15 mL). After drying the organic layer
over anhydrous MgSO4, the solvent was removed under vacuum.
The crude solid was purified by column chromatography, eluting
A solution of N-(tert-butyloxycarbonyl)-4-formyl-L-phenylala-
nine methyl ester 1a [8] (0.34 g, 0.11 ꢁ 10ꢂ3 mol, 1 equiv)ꢂa3nd 40-
amino-50-nitrobenzo-15-crown-5 2a (0.035 g, 0.11 ꢁ 10
mol,
1 equiv) in absolute ethanol (3 mL) was treated with Na2S2O4
(0.057 g, 0.33 ꢁ 10ꢂ3 mol, 3 equiv), dissolved in water (1 mL), and
heated at 80 ꢀC with stirring for 15 h. The mixture was poured into
water (20 mL) and extracted with ethyl acetate (3 ꢁ 20 mL). The
organic layer was dried with anhydrous MgSO4 and evaporated
under reduced pressure to give the crude product that was sub-
mitted to silica gel column chromatography using mixtures of
dichloromethane and n-hexane of increasing polarity as eluent. The
fractions containing the purified product were collected and
evaporated under vacuum, giving crown ether benzimidazolyl
phenylalanine methyl ester 3 as a yellow oil (0.044 g, 70%); 1H NMR
with
dichloromethane-methanol
(100:1).
N-(tert-Butylox-
methyl
ycarbonyl)-4-(50-formylthiophen-20-yl)-
L-phenylalanine
Please cite this article in press as: Esteves CIC, et al., Novel functionalised imidazo-benzocrown ethers bearing a thiophene spacer as fluorimetric