Novel fluorescent aminoxy acids and aminoxy hybrid peptides

Article abstract of DOI:10.1055/s-0030-1258345

α-Aminoxy acid conjugates and hybrid α-aminoxy dipeptide conjugates showing blue to green fluorescence (with quantum yields 0.23-0.71) were synthesized (75-92% yield) from N-acylbenzotriazoles. 7-Methoxycoumarin derivatives were obtained (quantum yields φ

Full text of DOI:10.1055/s-0030-1258345

PAPER
83
Novel Fluorescent Aminoxy Acids and Aminoxy Hybrid Peptides
F
luorescent
Amino
l
xy Aci
a
ds and Am
n
inoxy Peptides R. Katritzky,*a Abdelmotaal Abdelmajeid,a,b Srinivasa R. Tala,a M. S. Amine,b Peter J. Steelc
a
Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611, USA
Fax +1(352)3920554; E-mail: katritzky@chem.ufl.edu
b
c
Department of Chemistry, Faculty of Science, Benha University, Benha, Egypt
Chemistry Department, University of Canterbury, Christchurch, New Zealand
Received 2 September 2010; revised 20 October 2010
Dedicated to Professor Ferenc Fülöp on the occasion of his 60th birthday
in peptidomimetic foldamers21,22 and could provide useful
labels.
Abstract: a-Aminoxy acid conjugates and hybrid a-aminoxy
dipeptide conjugates showing blue to green fluorescence (with
quantum yields 0.23–0.71) were synthesized (75–92% yield) from
N-acylbenzotriazoles. 7-Methoxycoumarin derivatives were ob-
tained (quantum yields j 0.35–0.71) that could be suitable for pep-
tide assays. The wavelengths of the absorption maxima (lAbs) and
the fluorescence emission maxima (lEm) and the quantum yields (j)
of some coumarin-3-ylcarbonyl aminoxy acid conjugates were
comparable to those of the analogous amino acid derivatives.
Coumarins make up a large class of fluorescent laser dyes
for the ‘blue-green’ region, and can be highly sensitive to
the environment.23–28 Coumarins constitute the majority
of commercially important fluorescent dyes with advan-
tages of extended spectral ranges, high emission quantum
yields, photostability, and good solubility in many sol-
vents. We here report syntheses and absorption and fluo-
rescence data of new fluorescent coumarin-labeled
aminoxy acids as well as aminoxy hybrid peptides.
Key words: fluorescence, acylations, amino acids, aminoxy acids,
peptides
Coumarin-3-carboxylic acids 1a–c were converted by re-
action with 1H-benzotriazole (BtH) and thionyl chloride
in tetrahydrofuran at 20 °C into the corresponding (cou-
The labeling of proteins with fluorescent reporter groups
has enabled noninvasive imaging of gene expression, pro-
tein trafficking, and other dynamic biochemical signals in
cells of living organisms.1 This technique is now widely
applied in biology, biotechnology, medicine,2–4 and in-
creasingly useful in combinatorial chemistry, both to en-
code individual library members and to monitor
reactivity.5–7
marin-3-ylcarbonyl)benzotriazoles
2a–c
(82–86%)
(Table 1),26 characterized by 1H-NMR, 13C-NMR and el-
emental analysis.
Table 1 Preparation of (Coumarin-3-ylcarbonyl)benzotriazoles
2a–c
R
O
O
O
R
O
O
Versatile biochemical techniques for the position-specific
SOCl2, BtH
THF, r.t.
introduction of non-natural amino acids into biosynthetic
proteins have enabled the insertion of a wide variety of ar-
tificial functions by protein engineering.8–10 Appropriate-
ly labeled proteins enable the detection of ligands,
antigens, and inhibitors at low concentrations. Methods
are sought for the incorporation of non-natural fluorescent Entry
amino acids that do not suppress the binding ability of the
OH
Bt
O
1a–c
2a–c
1a R = H, 1b R = OMe, 1c R = NEt2
R
2
Yield
(%)a
protein, while being sensitive to small changes of the mi-
1
H
N-(coumarin-3-ylcarbonyl)-Bt (2a)
82
croenvironment.4
2
OMe N-(7-methoxycoumarin-3-ylcarbonyl)-Bt (2b) 83
a-Aminoxy acids are analogues of b-amino acids in which
the b-carbon atom is replaced by an oxygen atom. An a-
3
NEt2 N-[7-(diethylamino)coumarin-3-ylcarbonyl]-Bt 86
(2c)
aminoxy acid is more rigid than the corresponding b-ami-
no acid,11 moreover, aminoxy amide bonds resist enzy-
a Isolated yield.
matic degradation; this has led to the exploration of a-
aminoxy acids as peptidomimetics.12 a-Aminoxy peptides
have also attracted considerable interest as novel foldam-
ers,13–15 because of unusual conformations and interesting
bioactivity.16–20 Aminoxy peptides may exhibit strong in-
tramolecular hydrogen bonds between adjacent residues
Treatment of (coumarin-3-ylcarbonyl)benzotriazoles
2a–c with various amino acids 3a–d in aqueous acetoni-
trile at 20 °C at room temperature gave coumarin-labeled
aminoxy acids 4a–h (76–92%) (Table 2), characterized
by NMR and elemental analysis.
The structure of a representative example, 4c, was estab-
lished unambiguously by a single crystal X-ray structure
determination (Figure 1). In the solid state the carboxylic
acid group folds around in an eight-membered ring to
SYNTHESIS 2011, No. 1, pp 0083–0090
Advanced online publication: 30.11.2010
DOI: 10.1055/s-0030-1258345; Art ID: M05910SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
1
0

84
A. R. Katritzky et al.
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Table 2 Preparation of Coumarin-Labeled Aminoxy Acids 4a–h
O
O
O
O
R2
O
Bt
OH
Et3N
N
OH
O
+
H
R2
MeCN–H2O
r.t., 1 h
R1
O
O
NH2
O
R1
O
2a–c
3a–d
4a–h
4a R1 = H, R2 = H
3a R2 = H
2a R1 = H
4b R1 = OMe, R2 = H
4c R1 = NEt2, R2 = H
4d R1 = H, R2 = Me
4e R1 = OMe, R2 = Me
4f R1 = H, R2 = Bn
3b R2 = Me
3c R2 = Bn
3d R2 = i-Pr
2b R1 = OMe
2c R1 = NEt2
4g R1 = OMe, R2 = Bn
4h R1 = OMe, R2 = i-Pr
Entry
2
Aminoxy acid 3
AO-Gly-OH (3a)
AO-Gly-OH (3a)
AO-Gly-OH (3a)
AO-Ala-OH (3b)
AO-Ala-OH (3b)
AO-Phe-OH (3c)
AO-Phe-OH (3c)
AO-Val-OH (3d)
4
Yield (%)a
1
2a
2b
2c
2a
2b
2a
2b
2b
4a
4b
4c
4d
4e
4f
83
87
92
80
89
79
76
81
2
3
4
5
6
7
4g
4h
8
a Isolated yield.
form an intramolecular hydrogen bond to the amide oxy- yl)amino]acyl}benzotriazoles 7a–c (Table 4) as previous-
gen atom, with the following geometry: H···O 1.80(1) Å, ly described.26
O···O 2.657(1) Å, O-H···O 166(1)°. In the crystal packing
there is extensive p-p stacking between coumarin groups,
wherein the pyrone ring stacks over the benzene ring of an
Coumarin-labeled aminoxy hybrid peptides 8a–c (78–
82%) were prepared by treatment of N-{a-[(coumarin-3-
ylcarbonyl)amino]acyl}benzotriazoles 7a,b with ami-
adjacent molecule with a separation of 3.37(1) Å.
noxy acids 3a,b in the presence of triethylamine at 20 °C
in acetonitrile–water (Table 5).
The wavelengths of the absorption maxima (lAbs) and
fluorescence emission maxima (lEm), together with their
molar extinction coefficients (e), were measured in aceto-
nitrile for compounds 4a–h and 8a–c (Figures 2, 3, 5 and
6, Table 6). The fluorescent aminoxy acids 4a–h and hy-
brid aminoxy dipeptide conjugates 8a–c showed absorp-
tion maxima in the wavelength region of 293–346 nm.
The absorption maxima at 293–300 nm of N-(coumarin-3-
ylcarbonyl)aminoxy conjugates 4a,d,f and 8a,c were less
intense than those of 7-methoxy-N-(coumarin-3-ylcarbo-
nyl)aminoxy conjugates 4b,e,g,h and 8b at 342–346 nm.
The fluorescence emission maxima of the aminoxy acids
4a–h and hybrid aminoxy dipeptide conjugates 8a–c were
observed in acetonitrile at 20 °C in the range 398–464 nm
(Table 6, Figures 2, 3, and 6). These emission and absorp-
tion wavelengths are longer than those of tryptophan
(lAbs = 278 nm, lEm = 352 nm) or tyrosine (lAbs = 274
nm, lEm = 303 nm). The coumarin-3-ylcarbonyl aminoxy
acids 4a–h and hybrid aminoxy dipeptide conjugates 8a–
c showed intense blue to green fluorescence in acetonitrile
at 20 °C upon irradiation with a UV lamp (Figure 4).
Figure 1 X-ray crystallographic structure of 4c
We first prepared coumarin-labeled amino acids 6a–f and
(6c+6c¢) by treatment of (coumarin-3-ylcarbonyl)benzo-
triazoles 2a–c with amino acids 5a–c in the presence of
triethylamine in acetonitrile–water at 20 °C (76–94%)
(Table 3).26
Coumarin-labeled amino acids 6a,b,e were reacted with
1H-benzotriazole and thionyl chloride in THF at 20 °C to
give the corresponding N-{a-[(coumarin-3-ylcarbon-
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Fluorescent Aminoxy Acids and Aminoxy Peptides
85
Table 3 Preparation of Coumarin-Labeled Amino Acids 6a–f
R1
O
O
O
R1
O
O
O
O
H2N
Et3N
+
H
OH
Bt
N
MeCN–H2O
r.t., 1 h
OH
R3
5a–c
R3
O
2a–c
6a–f
5a R3 = H
2a R1 = H
5b R3 = Bn
2b R1 = OMe
2c R1 = NEt2
5c R3 = CH2CH2CH2CH2NH-Cbz
Entry
2
5
6
Yield (%)a
1
2
3
4
5
6
7
2a
2b
2a
2a
2b
2c
2c
5a
N-(coumarin-3-ylcarbonyl)-Gly-OH (6a)
86
94
79
82
84
76
88
5a
N-(7-methoxycoumarin-3-ylcarbonyl)-Gly-OH (6b)
N-(coumarin-3-ylcarbonyl)-L-Phe-OH (6c)
5b
5b+5b¢
5b
N-(coumarin-3-ylcarbonyl)-DL-Phe-OH (6c+6c¢)
N-(7-methoxycoumarin-3-ylcarbonyl)-L-Phe-OH (6d)
N-[7-(diethylamino)coumarin-3-ylcarbonyl]-L-Phe-OH (6e)
N-[7-(diethylamino)coumarin-3-ylcarbonyl]-(Z)-Lys-OH (6f)
5b
5c
a Isolated yield.
Table 4 Preparation of N-{a-[(Coumarin-3-ylcarbonyl)amino]acyl}benzotriazoles 7a–c
R1
O
O
O
R1
O
O
O
H
O
BtH, SOCl2
N
H
OH
N
THF, r.t., 2 h
Bt
R3
O
R3
6a,b,e
7a–c
7a R1 = H, R3 = H
7b R1 = OMe, R3 = H
7c R1 = NEt2, R3 = Bn
Entry
6
7
Yield (%)a
1
2
3
6a
6b
6e
N-(coumarin-3-ylcarbonyl)-Gly-Bt (7a)
76
74
77
N-(7-methoxycoumarin-3-ylcarbonyl)-Gly-Bt (7b)
N-[7-(diethylamino)coumarin-3-ylcarbonyl]-L-Phe-Bt (7c)
a Isolated yield.
Table 5 Preparation of Coumarin-Labeled Aminoxy Hybrid Peptides 8a–c
R1
O
O
O
R2
R1
O
O
O
O
O
O
Et3N
H
OH
H
+
N
O
MeCN–H2O
r.t., 1 h
N
O
Bt
N
OH
NH2
O
H
R3
R3
8a–c
R2
7a,b
3a,b
3a R2 = H
3b R2 = Me
8a R1 = H, R2 = H, R3 = H
8b R1 = OMe, R2 = H, R3 = H
8c R1 = H, R2 = Me, R3 = H
Entry
7
3
8
Yield (%)a
1
2
3
7a
7b
7a
3a
3a
3b
N-(coumarin-3-ylcarbonyl)-Gly-AO-Gly-OH (8a)
82
75
78
N-(7-methoxycoumarin-3-ylcarbonyl)-Gly-AO-Gly-OH (8b)
N-(coumarin-3-ylcarbonyl)-Gly-AO-Ala-OH (8c)
a Isolated yield.
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A. R. Katritzky et al.
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Figure 6 Emission spectra of 4c in different solvents
Figure 2 Emission spectra of different coumarin-labeled aminoxy
acids and aminoxy peptides
Table 6 Absorption and Fluorescence Data of Coumarin-3-ylcarbo-
nyl Aminoxy Acids 4a–h and Peptides 8a–c in Acetonitrile
Compound lAbs (nm)
l
Em (nm) e (cm–1·M–1) × 104
j
4a
4b
4c
4d
4e
4f
300
343
420
298
344.5
293
346
345
294
342
296
417
405
464
407
408
415
402
400
405
398
409
0.89
4.60
6.67
2.85
3.17
3.39
4.90
5.70
3.98
6.58
2.13
0.10
0.71
0.07
0.03
0.46
0.07
0.35
0.67
0.04
0.23
0.06
4g
4h
8a
8b
8c
Figure 3 Emission spectra of different 7-methoxycoumarin-labeled
aminoxy acids and aminoxy peptides
therefore, conjugation of an electron-donating methoxy
group to the coumarin backbone at this position increases
the quantum yield, the wavelength of absorbance, and the
molar absorption coefficient values relative to N-(cou-
marin-3-ylcarbonyl) conjugates. N-(7-Methoxycoumarin-
3-ylcarbonyl) aminoxy conjugates 4b,e,g,h have higher
quantum yields than N-(coumarin-3-ylcarbonyl) aminoxy
conjugates 4a,d,f, and 8a,c (Table 6).
Figure 4 Photographs of 4a–h and 8a–c in acetonitrile under UV ir-
radiation
The absorption (lAbs) and fluorescence emission (lEm
)
wavelength maxima of compound 4c (Figures 5 and 6)
were recorded to examine the environmental sensitivity of
this fluorophore in different solvents; the quantum yield is
highest in dichloromethane and lowest in water (Table 7).
Compound 4c showed green fluorescence in dichlo-
romethane, when it was irradiated with a UV lamp and
had a higher quantum yield than with the other three sol-
vents (Table 7, Figure 7).
The wavelengths of the absorption maxima (lAbs) and
fluorescence emission maxima (lEm) and the quantum yields
of coumarin-3-ylcarbonyl amino acid conjugates 6a–d in
acetonitrile showed no significant differences to those of
the analogous aminoxy derivatives 4a,b,f,g (Table 8).
Figure 5 UV absorption spectra of 4c in different solvents
Electron-donating groups at the 7-position in the cou-
marin strongly modulate fluorescence by affecting the en-
ergy of the molecule’s two lowest excited states;27,28
In conclusion, we have synthesized novel coumarin-
labeled aminoxy acid and aminoxy hybrid dipeptide con-
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Fluorescent Aminoxy Acids and Aminoxy Peptides
87
tained on a Thermo-Finnigan (San Jose, CA) LCQ spectrophotom-
eter with electrospray ionization (ESI).
Table 7 Quantum Yield j of 4c in Different Solvents
Solvent
CH2Cl2
MeCN
MeOH
H2O
j
N-(Coumarin-3-ylcarbonyl)benzotriazoles 2a–c; General Pro-
cedure
0.22
0.07
0.03
0.01
SOCl2 (7.5 mmol) was added to a soln of 1H-benzotriazole (BtH; 25
mmol) in anhyd THF (30 mL) at r.t. and the reaction mixture was
stirred for 20 min. The appropriate coumarin-3-carboxylic acid
1a–c (5 mmol) was added to the reaction mixture, which was stirred
for 4–5 h at 25 °C. The white precipitate formed during the reaction
was removed by filtration, and the filtrate was concentrated under
reduced pressure. The residue was diluted with EtOAc (150 mL)
and the soln was washed with sat. aq Na2CO3 (3 × 50 mL) and sat.
aq NaCl (50 mL) and dried (Na2SO4). Removal of the solvent under
reduced pressure gave the corresponding 2a–c, which were recrys-
tallized from the appropriate solvent.
N-(Coumarin-3-ylcarbonyl)benzotriazole (2a)
White microcrystals (CH2Cl2–hexanes); mp 134.0–136.0 °C (Lit.26
186.0–188.0 °C).
1H NMR (300 MHz, CDCl3): d = 8.40–8.33 (m, 2 H), 8.17 (d, J =
8.3 Hz, 1 H), 7.78–7.64 (m, 3 H), 7.60–7.56 (m, 1 H), 7.47 (d, J =
8.4 Hz, 1 H), 7.41(t, J = 7.5 Hz, 1 H).
13C NMR (75 MHz, CDCl3): d = 162.6, 157.5, 154.8, 147.2, 146.2,
134.6, 131.2, 130.9, 129.7, 126.8, 125.3, 121.8, 120.5, 117.7, 117.2,
114.4.
Figure 7 Photographs of 4c in (from left to right) dichlorometh-
ane→water under UV irradiation
Anal. Calcd for C16H9N3O3: C, 65.98; H, 3.11; N, 14.43. Found: C,
65.98; H, 3.15; N, 14.07.
Table 8 Absorption and Fluorescence Data of Coumarinoylamino
Acids 6a–d in Acetonitrile
N-(7-Methoxycoumarin-3-ylcarbonyl)benzotriazole (2b)
White microcrystals (CH2Cl2); mp 232.0–234.0 °C.
Compound
l
Abs (nm)
l
Em (nm)
j
1H NMR (300 MHz, DMSO-d6): d = 8.76 (s, 1 H), 8.29 (d, J = 8.4
Hz, 2 H), 7.88–7.80 (m, 2 H), 7.67 (t, J = 7.8 Hz, 1 H), 7.16 (s, 1 H),
7.08 (d, J = 8.6 Hz, 1 H), 3.93 (s, 3 H).
13C NMR (75 MHz, DMSO-d6): d = 164.9, 162.8, 157.3, 156.6,
156.2, 148.7, 145.5, 131.6, 131.1, 126.9, 120.2, 116.7, 113.8, 111.2,
100.9, 56.4.
6a
6b
6c
6d
293
344
297
345
404
401
414
406
0.13
0.72
0.08
0.42
Anal. Calcd for C17H11N3O4: C, 63.55; H, 3.45; N, 13.08. Found: C,
63.66; H, 3.31; H, 13.10.
jugates that have high fluorescence, in yields of 76–92%
and 75–82%, respectively, by using our standard proto-
cols of benzotriazole methodology. In the new fluorescent
aminoxy acid and aminoxy hybrid dipeptide conjugates,
compounds with an electron-donating 7-methoxy group
showed high quantum yields (0.23–0.71), which could be
suitable for use in peptide assays. The absorption maxima
(lAbs) and fluorescence emission maxima (lEm) wave-
lengths and quantum yields (j) of coumarin-3-ylcarbonyl
aminoxy acid conjugates were similar to the analogous
amino acid conjugates.
N-[7-(Diethylamino)coumarin-3-ylcarbonyl]benzotriazole (2c)
Yellow microcrystals (CH2Cl2–hexanes); mp 230.0–232.0 °C.
1H NMR (300 MHz, DMSO): d = 8.59 (s, 1 H), 8.28 (d, J = 7.8 Hz,
1 H), 8.24 (d, J = 8.1 Hz, 1 H), 7.82 (t, J = 7.5 Hz, 1 H), 7.70–7.60
(m, 2 H), 6.86 (d, J = 8.7 Hz, 1 H), 6.70–6.65 (m, 1 H), 3.60–3.50
(m, 4 H), 1.20–1.10 (m, 6 H).
13C NMR (75 MHz, DMSO): d = 163.2, 157.9, 153.1, 149.6, 145.3,
131.9, 130.9, 130.7, 126.5, 120.0, 113.7, 110.3, 106.9, 96.3, 44.5,
12.4.
Anal. Calcd for C20H18N4O3: C, 66.29; H, 5.01; N, 15.46. Found: C,
66.00; H, 5.01; N, 15.18.
Melting points were determined on a capillary point apparatus
equipped with a digital thermometer and are uncorrected. 1H NMR
(300 MHz) and 13C NMR (75 MHz) spectra were recorded of sam-
ples prepared in CDCl3 or DMSO-d6 with TMS as an internal refer-
ence. Elemental analyses were performed on a Carlo Erba-1106
instrument. Absorption spectra were recorded on a Lambda-25
(Perkin Elmer) instrument. Fluorescence spectra were recorded on
a FluoroMax-3 jobin Yuon Horiba spectrofluorimeter. Emission
spectra of all compounds were measured on the same instrument.
CH2Cl2 was dried and distilled over CaH2, whereas THF was used
after distillation over Na/benzophenone. Mass spectra were ob-
Coumarin-Labeled Aminoxy Acids 4a–h; General Procedure
The appropriate (coumarin-3-ylcarbonyl)benzotriazole 2a–c (1
mmol) was added to a soln of the appropriate a-aminoxy acid 3a–d
(1 mmol) in MeCN–H2O (3:1) in the presence of Et3N (1.1 mmol).
The reaction mixture was stirred at 20 °C for about 1 h until TLC
showed the absence of 2a–c, and then the solvent was removed un-
der reduced pressure. The residue was dissolved in EtOAc (150
mL), washed with 4 N aq HCl (3 × 50 mL) and sat. aq NaCl (50
mL), and dried (Na2SO4). After evaporation of the solvent, the res-
idue was recrystallized to give the corresponding 4a–h.
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A. R. Katritzky et al.
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N-(Coumarin-3-ylcarbonyl)-AO-Gly-OH (4a)
White microcrystals (MeOH); mp 208.0–210.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 13.25 (s, 1 H), 11.45 (s, 1 H),
8.86 (s, 1 H), 7.99 (d, J = 7.7 Hz, 1 H), 7.76 (t, J = 7.3 Hz, 1 H), 7.52
(d, J = 8.2 Hz, 1 H), 7.45 (t, J = 7.7 Hz, 1 H), 4.52 (s, 2 H).
13C NMR (75 MHz, DMSO-d6): d = 172.1, 160.6, 153.9, 148.2,
136.6. 134.3, 130.4, 130.1, 129.3, 128.3, 126.7, 125.2, 124.8, 118.3,
117.9, 116.1, 53.7, 36.8.
Anal. Calcd for C19H15NO6: C, 64.59; H, 4.28; N, 3.96. Found: C,
64.29; H, 4.83; N, 3.98.
13C NMR (75 MHz, DMSO-d6): d = 170.6, 159.2, 153.9, 147.8,
N-(7-Methoxycoumarin-3-ylcarbonyl)-AO-Phe-OH (4g)
White microcrystals (CH2Cl2); mp 186.0–188.0 °C.
134.4, 130.3, 125.3, 118.3, 116.3, 72.0.
Anal. Calcd for C12H9NO6: C, 54.76; H, 3.45; N, 5.32. Found: C,
54.63; H, 3.41; N, 5.10.
1H NMR (300 MHz, DMSO-d6): d = 8.98 (d, J = 7.3 Hz, 1 H), 8.85
(s, 1 H), 7.90 (d, J = 8.9 Hz, 1 H), 7.35–7.15 (m, 5 H), 7.10 (d, J =
2.1 Hz, 1 H), 7.03 (dd, J = 8.6, 2.3 Hz, 1 H), 4.80–4.72 (m, 1 H),
3.89 (s, 3 H), 3.25–3.15 (m, 1 H), 3.14–3.05 (m, 1 H).
13C NMR (75 MHz, DMSO-d6): d = 172.2, 164.6, 161.0, 156.3,
148.4, 136.6, 131.7, 129.3, 128.3, 126.7, 113.8, 112.1, 100.3, 56.3,
53.6, 36.9.
N-(7-Methoxycoumarin-3-ylcarbonyl)-AO-Gly-OH (4b)
White microcrystals (n-BuOH); mp 216.0–218.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 13.24 (s, 1 H), 11.45 (s, 1 H),
8.84 (s, 1 H), 7.91 (d, J = 8.6 Hz, 1 H), 7.13 (s, 1 H), 7.05 (d, J = 8.8
Hz, 1 H), 4.51 (s, 2 H), 3.90 (s, 3 H).
Anal. Calcd for C20H17NO7: C, 62.66; H, 4.47; N, 3.65. Found: C,
62.30; H, 4.34; N, 3.53.
13C NMR (75 MHz, DMSO-d6): d = 164.6, 164.1, 157.2, 156.8,
149.0, 131.5, 113.7, 113.3, 111.6, 100.2, 56.2.
Anal. Calcd for C13H11NO7: C, 53.25; H, 3.78; N, 4.78. Found: C,
52.90; H, 3.69; N, 4.56.
N-(7-Methoxycoumarin-3-ylcarbonyl)-AO-Val-OH (4h)
White microcrystals (CH2Cl2–hexanes); mp 189.0–190.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 9.29 (d, J = 8.0 Hz, 1 H), 8.85
(s, 1 H), 7.58 (dd, J = 8.8, 1.3 Hz, 1 H), 6.98–6.90 (m, 1 H), 6.87 (s,
1 H), 4.74–4.64 (m, 1 H), 3.91 (s, 3 H), 2.40 (q, J = 6.3 Hz, 1 H),
1.07 (d, J = 6.0 Hz, 6 H).
13C NMR (75 MHz, DMSO-d6): d = 175.7, 165.3, 162.9, 161.9,
157.0, 149.1, 131.3, 114.4, 112.5, 100.5, 58.2, 56.3, 30.8, 19.6,
18.0.
N-[7-(Diethylamino)coumarin-3-ylcarbonyl]-AO-Gly-OH (4c)
Yellow microcrystals (CH2Cl2); mp 190.0–192.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 13.20 (s, 1 H), 11.37 (s, 1 H),
8.63 (s, 1 H), 7.66–7.60 (m, 1 H), 6.76 (t, J = 7.5 Hz, 1 H), 6.56 (d,
J = 7.3 Hz, 1 H), 4.47 (s, 2 H), 3.46 (d, J = 6.8 Hz, 4 H), 1.13 (t, J =
6.9 Hz, 6 H).
13C NMR (75 MHz, DMSO-d6): d = 170.7, 160.9, 160.5, 157.3,
152.7, 148.2, 131.7, 110.2, 107.7, 107.6, 95.8, 72.0, 44.4, 12.3.
Anal. Calcd for C16H17NO7·2H2O: C, 51.75; H, 4.61; N, 3.77.
Found: C, 51.71; H, 4.79; N, 3.53.
Anal. Calcd for C16H18N2O6: C, 57.48; H, 5.43; N, 8.38. Found: C,
57.34; H, 5.40; N, 8.31.
Coumarin-Labeled Amino Acids 6a–f; General Procedure
The appropriate (coumarin-3-ylcarbonyl)benzotriazole 2a–c (1
mmol) was added to a soln of the appropriate amino acid 5a–c (1
mmol) in MeCN–H2O (1:3) in the presence of Et3N (1.1 mmol). The
reaction mixture was stirred at 20 °C until TLC showed the absence
of 2a–c. After removal of the solvent, the residue was dissolved in
EtOAc (150 mL); the soln was washed with 4 N aq HCl (3 × 50 mL)
and sat. aq NaCl (50 mL) and then dried (Na2SO4). After evapora-
tion of the solvent, the residue was recrystallized to give the corre-
sponding 6a–f.
N-(Coumarin-3-ylcarbonyl)-AO-Ala-OH (4d)
Pale yellow microcrystals (CH2Cl2–hexanes); mp 196.0–198.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 13.04 (s, 1 H), 9.11 (d, J = 7.0
Hz, 1 H), 8.91 (s, 1 H), 8.00 (d, J = 7.8 Hz, 1 H), 7.77 (t, J = 7.2 Hz,
1 H), 7.52 (d, J = 8.2 Hz, 1 H), 7.45 (t, J = 7.5 Hz, 1 H), 4.46 (q,
J = 6.9 Hz, 1 H), 1.42 (d, J = 7.1 Hz, 3 H).
13C NMR (75 MHz, DMSO-d6): d = 173.5, 160.5, 153.9, 148.0,
134.3, 130.4, 125.2, 118.4, 118.2, 116.2, 48.2, 17.9.
Anal. Calcd for C13H11NO6: C, 56.32; H, 4.00; N, 5.05. Found: C,
56.12; H, 4.49; N, 4.76.
N-(Coumarin-3-ylcarbonyl)-Gly-OH (6a)
White microcrystals; mp 238.0–240.0 °C (Lit.23 238.0–239.0 °C).
1H NMR (300 MHz, DMSO-d6): d = 12.82 (s, 1 H), 9.04 (br s, 1 H),
8.92 (br s, 1 H), 8.00 (d, J = 7.5 Hz, 1 H), 7.77 (t, J = 7.3 Hz, 1 H),
7.51 (d, J = 8.1 Hz, 1 H), 7.45 (t, J = 7.1 Hz, 1 H), 4.05 (d, J = 5.3
Hz, 2 H).
13C NMR (75 MHz, DMSO-d6): d = 170.8, 161.1, 160.3, 153.9,
148.1, 134.3, 130.4, 125.1, 118.4, 118.1, 116.1, 41.5.
N-(7-Methoxycoumarin-3-ylcarbonyl)-AO-Ala-OH (4e)
Pale yellow microcrystals (MeOH); mp 204.0–206.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 13.00 (s, 1 H), 9.05 (d, J = 6.8
Hz, 1 H), 8.87 (s, 1 H), 7.92 (d, J = 8.6 Hz, 1 H), 7.14 (s, 1 H), 7.05
(dd, J = 8.6, 2.2 Hz, 1 H), 4.46 (t, J = 7.1 Hz, 1 H), 3.90 (s, 3 H),
1.41 (d, J = 7.1 Hz, 3 H).
13C NMR (75 MHz, DMSO-d6): d = 173.6, 164.6, 160.9, 160.8,
156.2, 148.2, 131.6, 114.1, 113.7, 112.0, 100.2, 56.3, 48.1, 18.0.
N-(7-Methoxycoumarin-3-ylcarbonyl)-Gly-OH (6b)
White microcrystals (MeOH); mp 266.0–268.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 12.82 (s, 1 H), 8.99 (s, 1 H),
8.86 (s, 1 H), 7.90 (d, J = 8.6 Hz, 1 H), 7.11 (s, 1 H), 7.03 (d, J = 8.6
Hz, 1 H), 4.04 (d, J = 4.9 Hz, 2 H), 3.89 (s, 3 H).
Anal. Calcd for C14H13NO7: C, 54.73; H, 4.26; N, 4.56. Found: C,
54.77; H, 4.19; N, 4.47.
N-(Coumarin-3-ylcarbonyl)-AO-Phe-OH (4f)
White microcrystals (EtOH); mp 214.0–216.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 9.06 (d, J = 7.5 Hz, 1 H), 8.90
(s, 1 H), 7.99 (dd, J = 6.4, 1.5 Hz, 1 H), 7.80–7.65 (m, 1 H), 7.50 (d,
J = 8.3 Hz, 1 H), 7.45–7.37 (m, 1 H), 7.35–7.20 (m, 5 H), 4.80–4.75
(m, 1 H), 3.25–3.16 (m, 1 H), 3.15–3.08 (m, 1 H).
13C NMR (75 MHz, DMSO-d6): d = 170.9, 164.5, 161.5, 160.8,
156.2, 148.2, 131.6, 114.0, 113.6, 112.0, 100.2, 56.3, 41.5.
Anal. Calcd for C13H11NO6: C, 56.32; H, 4.00; N, 5.05. Found: C,
56.11; H, 4.01; N, 5.01.
N-(Coumarin-3-ylcarbonyl)-L-Phe-OH (6c)
White microcrystals (CHCl3–hexanes); mp 178.0–179.0 °C (Lit.26
178.0–179.0 °C).
Synthesis 2011, No. 1, 83–90 © Thieme Stuttgart · New York

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Fluorescent Aminoxy Acids and Aminoxy Peptides
89
1H NMR (300 MHz, CDCl3): d = 9.28 (d, J = 6.6 Hz, 1 H), 8.87 (s,
1 H), 7.67 (t, J = 7.5 Hz, 2 H), 7.38 (d, J = 8.2 Hz, 2 H), 7.35–7.20
(m, 5 H), 4.97 (q, J = 7.1 Hz, 1 H), 4.50–4.43 (m, 2 H), 3.36 (dd,
J = 13.8, 5.1 Hz, 1 H), 3.20 (dd, J = 13.8, 7.9 Hz, 1 H).
13C NMR (75 MHz, CDCl3): d = 174.2, 161.9, 161.1, 154.4, 149.0,
135.9, 134.4, 130.1, 129.4, 128.7, 127.2, 125.4, 118.4, 117.5, 116.6,
54.4, 37.5.
N-{a-[(Coumarin-3-ylcarbonyl)amino]acyl}benzotriazoles
7a–c; General Procedure
SOCl2 (1.2 mmol) was added to a soln of BtH (4 mmol) in anhyd
THF (15 mL) at 20 °C and the reaction mixture was stirred for 20
min. The appropriate N-(coumarin-3-ylcarbonyl) amino acid 6a,b,e
(1 mmol) was added and the mixture was stirred for 4 h at r.t. The
white precipitate formed during the reaction was removed by filtra-
tion, the filtrate was diluted with additional CH2Cl2 (80 mL), and
the soln was washed with 4 N HCl (3 × 50 mL), sat. aq Na2CO3
(3 × 50 mL), and sat. aq NaCl (50 mL) and then dried (Na2SO4). Re-
moval of the solvent under reduced pressure gave the corresponding
product 7.
N-(Coumarin-3-ylcarbonyl)-DL-Phe-OH (6c+6c¢)
White microcrystals (EtOH); mp 200.0–202.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 13.16 (s, 1 H), 9.04 (d, J = 7.1
Hz, 1 H), 8.92 (s, 1 H), 8.00 (d, J = 7.9 Hz, 1 H), 7.77 (t, J = 7.8 Hz,
1 H), 7.52 (d, J = 8.4 Hz, 1 H), 7.45 (t, J = 7.5 Hz, 1 H), 7.30–7.15
(m, 5 H), 4.79–4.72 (m, 1 H), 3.25–3.00 (m, 2 H).
N-(Coumarin-3-ylcarbonyl)-Gly-Bt (7a)
Pale yellow microcrystals (CH2Cl2); mp 206.0–208.0 °C.
13C NMR (75 MHz, CDCl3): d = 172.1, 160.6, 153.9, 148.2, 136.,
134.3, 130.4, 129.3, 128.3, 126.7, 125.2, 118.4, 117.8, 116.1, 53.7,
36.8.
1H NMR (300 MHz, DMSO-d6): d = 9.39 (t, J = 5.3 Hz, 1 H), 8.95
(s, 1 H), 8.30–8.17 (m, 1 H), 7.99 (d, J = 7.3 Hz, 1 H), 7.84–7.70
(m, 2 H), 7.62 (t, J = 7.6 Hz, 1 H), 7.51 (d, J = 8.2 Hz, 1 H), 7.44 (t,
J = 7.5 Hz, 2 H), 5.26 (d, J = 5.4 Hz, 2 H).
13C NMR (75 MHz, DMSO-d6): d = 168.1, 161.7, 160.4, 154.0,
148.5, 145.2, 134.4, 131.0, 130.4, 126.6, 125.2, 120.1, 118.3, 117.8,
116.1, 113.6, 43.5.
Anal. Calcd for C19H15NO5: C, 67.65; H, 4.48; N, 4.15. Found: C,
67.36; H, 4.38; N, 4.13.
N-(7-Methoxycoumarin-3-ylcarbonyl)-L-Phe-OH (6d)
White microcrystals (CH2Cl2); mp 256.0–258.0 °C.
Anal. Calcd for C18H12N4O4·H2O: C, 59.02; H, 3.30; N, 15.29.
Found: C, 59.42; H, 3.35; N, 14.82.
1H NMR (300 MHz, DMSO-d6): d = 13.12 (br s, 1 H), 8.98 (d, J =
7.8 Hz, 1 H), 8.87 (s, 1 H), 7.92 (d, J = 8.6 Hz, 1 H), 7.35–7.15 (m,
5 H), 7.13 (s, 1 H), 7.05 (d, J = 8.8 Hz, 1 H), 4.75 (q, J = 6.4 Hz, 1
H), 3.90 (s, 1 H), 3.20 (dd, J = 13.0, 5.1 Hz, 1 H), 3.10 (dd, J = 14.1,
7.2 Hz, 1 H).
13C NMR (75 MHz, DMSO-d6): d = 172.1, 164.6, 160.9, 156.2,
148.4, 136.6, 131.7, 129.2, 128.3, 126.7, 113.7, 112.0, 100.3, 56.3,
53.6, 36.9.
N-(7-Methoxycoumarin-3-ylcarbonyl)-Gly-Bt (7b)
White microcrystals (n-BuOH); mp 240.0–242.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 9.33 (t, J = 5.6 Hz, 1 H), 8.94
(s, 1 H), 8.31 (d, J = 8.3 Hz, 1 H), 8.23 (d, J = 8.2 Hz, 1 H), 7.95 (d,
J = 8.9 Hz, 1 H), 7.82 (t, J = 7.7 Hz, 1 H), 7.65 (t, J = 7.7 Hz, 1 H),
7.16 (d, J = 2.3 Hz, 1 H), 7.05 (dd, J = 8.8, 2.2 Hz, 1 H), 5.24 (d,
J = 5.3 Hz, 2 H), 3.92 (s, 3 H).
13C NMR (75 MHz, DMSO-d6): d = 171.6, 165.3, 162.3, 161.5,
157.0, 149.0, 139.0, 132.5, 128.3, 126.6, 125.1, 119.7, 115.6, 114.5,
112.8, 111.8, 101.0, 57.1, 42.1.
Anal. Calcd for C20H15NO6.H2O: C, 62.33; H, 4.45; N, 3.63 Found:
C, 62.35; H, 4.16; N, 3.54.
N-[7-(Diethylamino)coumarin-3-ylcarbonyl]-L-Phe-OH (6e)
Yellow microcrystals (CH2Cl2–hexanes); mp 98.0–100.0 °C.
Anal. Calcd for C19H14N4O5: C, 60.32; H, 3.73; N, 14.81. Found: C,
60.06; H, 3.83; N, 14.50.
1H NMR (300 MHz, CDCl3): d = 9.34 (d, J = 6.4 Hz, 1 H), 8.63 (s,
1 H), 7.40 (d, J = 8.9 Hz, 1 H), 7.35–7.15 (m, 5 H), 6.63 (dd, J = 9.2,
2.4 Hz, 1 H), 6.46 (d, J = 2.0 Hz, 1 H), 4.85–4.75 (m, 1 H), 3.49–
3.40 (m, 4 H), 3.36 (d, J = 4.9 Hz, 1 H), 3.18 (dd, J = 14.3, 8.6 Hz,
1 H), 1.24 (t, J = 6.8 Hz, 6 H).
13C NMR (75 MHz, CDCl3): d = 174.4, 164.1, 157.8, 152.9, 148.5,
136.3, 131.4, 129.4, 128.6, 127.1, 110.1, 108.9, 96.5, 54.6, 45.1,
37.3, 12.4.
N-[7-(Diethylamino)coumarin-3-ylcarbonyl]-Phe-Bt (7c)
Yellow microcrystals (CHCl-3–hexanes); mp 128.0–130.0 °C.
1H NMR (300 MHz, CDCl3): d = 9.52 (d, J = 5.6 Hz, 1 H), 8.58 (s,
1 H), 8.29 (d, J = 8.1 Hz, 1 H), 8.15 (d, J = 8.1 Hz, 1 H), 7.66 (t,
J = 7.1 Hz, 1 H), 7.54 (t, J = 7.4 Hz, 1 H), 7.40–7.20 (m, 5 H), 6.61
(d, J = 8.7 Hz, 1 H), 6.48 (s, 1 H), 6.35–6.30 (m, 1 H), 3.56 (dd, J =
12.8, 4.2 Hz, 1 H), 3.45 (t, J = 8.7 Hz, 4 H), 3.33 (dd, J = 13.0, 9.0
Hz, 1 H), 1.23 (t, J = 6.6 Hz, 6 H).
Anal. Calcd for C23H24N2O5: C, 67.63; H, 5.92; N, 6.86. Found: C,
67.43; H, 6.18; N, 6.79.
13C NMR (75 MHz, CDCl3): d = 170.7, 163.5, 162.7, 157.9, 152.9,
148.5, 146.2, 135.7, 131.4, 130.7, 129.5, 128.9, 127.4, 126.5, 120.4,
114.7, 110.1, 109.4, 108.4, 96.7, 55.2, 45.2, 38.4, 12.6.
N-[7-(Diethylamino)coumarin-3-ylcarbonyl]-L-(Z)-Lys-OH (6f)
Orange microcrystals (CH2Cl2–hexanes); mp 76.0–78.0 °C.
1H NMR (300 MHz, CDCl3): d = 9.33 (d, J = 7.0 Hz, 1 H), 8.68 (s,
1 H), 7.45–7.20 (m, 6 H), 6.64 (dd, J = 9.1, 2.4 Hz, 1 H), 6.49 (d,
J = 2.0 Hz, 1 H), 5.08 (s, 2 H), 4.98–4.92 (m, 1 H), 4.68–4.60 (m, 1
H), 3.46 (q, J = 7.1 Hz, 4 H), 3.25–3.20 (m, 2 H), 2.10–1.85 (m, 2
H), 1.60–1.50 (m, 4 H), 1.25 (t, J = 7.1 Hz, 6 H).
13C NMR (75 MHz, CDCl3): d = 174.8, 164.1, 162.7, 157.9, 156.7,
153.0, 148.8, 136.8, 131.6, 128.6, 128.1, 110.2, 109.2, 108.5, 96.7,
66.7, 52.9, 45.3, 40.9, 31.7, 29.4, 22.8, 12.6.
Anal. Calcd for C29H27N5O4: C, 68.36; H, 5.34; N, 13.74. Found: C,
68.26; H, 5.16; N, 13.69.
Coumarin-Labeled Dipeptides 8a–c; General Procedure
The appropriate N-{a-[(coumarin-3-ylcarbonyl)amino]acyl}benzo-
triazole 7a,b (0.5 mmol) was added to a soln of the appropriate a-
aminoxy acid 3a,b (0.5 mmol) in MeCN–H2O (1:3) in the presence
of Et3N (1.3 mmol). The reaction mixture was stirred until 7a,b had
disappeared. Then MeCN was removed under reduced pressure, the
residue was dissolved in EtOAc (100 mL), and the soln was washed
with 4 N aq HCl (3 × 50 mL) and sat. aq NaCl (50 mL), and subse-
quently dried (Na2SO4). Evaporation of the solvent and recrystalli-
zation gave the corresponding 8a–c.
Anal. Calcd for C28H33N3O7: C, 64.23; H, 6.35; N, 8.03. Found: C,
63.99; H, 6.65; N, 7.65.
Synthesis 2011, No. 1, 83–90 © Thieme Stuttgart · New York

90
A. R. Katritzky et al.
PAPER
(2) Mayer, A.; Neuenhofer, S. Angew. Chem., Int. Ed. Engl.
N-(Coumarin-3-ylcarbonyl)-Gly-AO-Gly-OH (8a)
Brown microcrystals (EtOH); mp 173.0–175.0 °C.
1994, 33, 1044.
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1H NMR (300 MHz, DMSO-d6): d = 11.50 (s, 1 H), 9.02 (s, 1 H),
8.87 (s, 1 H), 7.97 (d, J = 7.7 Hz, 1 H), 7.74 (t, J = 7.7 Hz, 1 H), 7.48
(d, J = 8.4 Hz, 1 H), 7.42 (t, J = 7.5 Hz, 1 H), 4.39 (s, 2 H), 3.94 (s,
2 H).
13C NMR (75 MHz, DMSO-d6): d = 170.2, 166.2, 161.2, 160.4,
154.0, 147.9, 134.3, 130.4, 125.2, 118.4, 116.2, 71.9, 40.4.
Anal. Calcd for C14H12N2O7: C, 52.51; H, 3.78; N, 8.75. Found: C,
52.22; H, 3.79; N, 8.25.
N-(7-Methoxycoumarin-3-ylcarbonyl)-Gly-AO-Gly-OH (8b)
White microcrystals (EtOH); mp 196.0–198.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 13.00 (br s, 1 H), 11.47 (s, 1
H), 8.96 (br s, 1 H), 8.86 (s, 1 H), 7.92 (d, J = 8.6 Hz, 1 H), 7.12 (d,
J = 2.2 Hz, 1 H), 7.05 (dd, J = 8.8, 2.5 Hz, 1 H), 4.38 (br s, 2 H),
3.95–3.88 (m, 5 H).
(9) Hohsaka, T.; Ashizuka, Y.; Taira, H.; Murakami, H.; Sisido,
M. Biochemistry 2001, 40, 11060.
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1994, 344, 171.
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Y.; Zhu, N.-Y.; Luo, S.-W.; Wu, Y.-D. J. Am. Chem. Soc.
2004, 126, 6956.
13C NMR (75 MHz, DMSO-d6): d = 170.2, 166.2, 164.5, 161.5,
160.7, 156.2, 148.1, 131.6, 114.3, 113.7, 112.1, 100.3, 71.9, 56.3.
(12) Briggs, M. T.; Morley, J. S. J. Chem. Soc., Perkin Trans. 1
1979, 2138.
Anal. Calcd for C15H14N2O8.H2O: C, 48.92; H, 4.38; N, 7.61.
Found: C, 48.62; H, 4.27; N, 7.11.
(13) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore,
J. S. Chem. Rev. 2001, 101, 3893.
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Zhu, N.-Y. Angew. Chem. Int. Ed. 2004, 43, 6719.
(15) Li, X.; Yang, D. Chem. Commun. 2006, 3367.
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Andreu, D. Bioorg. Med. Chem. Lett. 2007, 17, 5155.
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M. D.; Hengel, S. M. Tetrahedron Lett. 2002, 43, 5727.
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Reboud, M.; Corbier, C. J. Pept. Sci. 2003, 9, 282.
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Chem. Soc. 1999, 121, 11189.
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Kiran, M. U.; Naresh, P.; Chaitanya, G. K.; Bhanuprakash,
K.; Jagadeesh, B. J. Org. Chem. 2008, 73, 9443.
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Soc. 2008, 130, 743.
N-(Coumarin-3-ylcarbonyl)-Gly-AO-Ala-OH (8c)
Pale brown microcrystals (MeOH); mp 234.0–236.0 °C.
1H NMR (300 MHz, DMSO-d6): d = 12.65 (s, 1 H), 9.08 (br s, 1 H),
8.91 (s, 1 H), 8.42 (d, J = 6.6 Hz, 1 H), 7.99 (d, J = 7.3 Hz, 1 H),
7.76 (t, J = 7.7 Hz, 1 H), 7.51 (d, J = 8.2 Hz, 1 H), 7.44 (t, J = 7.3
Hz, 1 H), 4.25 (t, J = 6.8 Hz, 1 H), 4.05–4.00 (m, 2 H), 1.29 (d, J =
7.0 Hz, 3 H).
13C NMR (75 MHz, DMSO-d6): d = 173.9, 167.8, 160.9, 160.4,
153.9, 147.8, 134.2, 130.3, 125.1, 118.4, 116.1, 47.6, 42.4, 17.3.
ESI-HRMS: m/z calcd for C15H14N2O7 [M + H]+: 335.0875; found:
335.0860.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis. Included
are UV absorption spectra, fluorescence emission spectra, photo-
graphs of 6a–d in acetonitrile under UV irradiation and 1H and 13
C
(23) Esteves, A. P.; Rodrigues, L. M.; Silva, M. E.; Gupta, S.;
Oliveira-Campos, A. M. F.; Machalicky, O.; Mendonça, A.
J. Tetrahedron 2005, 61, 8625.
NMR spectra.
(24) Sui, G.; Kele, P.; Orbulescu, J.; Huo, Q.; Leblanc, R. M. Lett.
Pept. Sci. 2002, 8, 47.
(25) Heiner, S.; Detert, H.; Kuhn, A.; Kunz, H. Bioorg. Med.
Chem. 2006, 14, 6149.
(26) Katritzky, A. R.; Narindoshvili, T.; Angrish, P. Synthesis
2008, 2013.
Acknowledgment
The authors thank Dr. C. D. Hall for helpful discussions and Dr.
Kirk Shanze (University of Florida) for help with UV and fluore-
scence measurements. Mr. A. Abdelmajeid is grateful to the Mini-
stry of Higher Education in Egypt for a fellowship.
(27) Sivakumar, K.; Xie, F.; Cash, B. M.; Long, S.; Barnhill, H.
N.; Wang, Q. Org. Lett. 2004, 6, 4603.
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