Heteroaromatic alanine derivatives bearing (oligo)thiophene units: synthesis and photophysical properties

Article abstract of DOI:10.1016/j.tetlet.2008.06.109

A series of new benzoxazolylalanine derivatives bearing (oligo)thiophene units at the side chain were synthesized in good yields. The photophysical characterization of these amino acids was performed by UV-vis absorption and fluorescence emission studies and revealed that some of the compounds display high fluorescent quantum yields, making them good candidates for application as fluorescent probes.

Full text of DOI:10.1016/j.tetlet.2008.06.109

Tetrahedron Letters 49 (2008) 5258–5261
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Heteroaromatic alanine derivatives bearing (oligo)thiophene units:
synthesis and photophysical properties
b
b,
a
Susana P. G. Costa a, , Elisabete Oliveira , Carlos Lodeiro , M. Manuela M. Raposo
*
*
a Centro de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
b REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new benzoxazolylalanine derivatives bearing (oligo)thiophene units at the side chain were
synthesized in good yields. The photophysical characterization of these amino acids was performed by
UV–vis absorption and fluorescence emission studies and revealed that some of the compounds display
high fluorescent quantum yields, making them good candidates for application as fluorescent probes.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 30 May 2008
Revised 19 June 2008
Accepted 24 June 2008
Available online 28 June 2008
Keywords:
Amino acids
(Oligo)thiophene
Benzoxazole
Alanine
Fluorophores
Fluorescent markers
The insertion of coded and unnatural amino acids into the back-
bone of both natural and synthetic polymers is a very appealing
area of research since it can lead to the development of macromol-
ecules possessing biomimetic characteristics, with unique struc-
tural and biological properties. By synthetic manipulation at the
side chain of the coded amino acids, new functions and functional
relationships can be generated as well as altered physicochemical
properties, such as luminescence, conducting ability, higher ther-
mal stability and metal ion and other analytes recognition ability,
amongst other properties. Furthermore, the chirality of the amino
acid should make these polymers suitable for chiral recognition
and induce chain helicity in aggregated phases.1 The fact that ami-
no acids and peptides are known to bind a variety of metal ions as
they contain a large number of potential donor atoms through the
peptide backbone and amino acid side chains has encouraged their
application to the solution-based detection of metals2 and as solid-
state metal-ion biosensors.3
2-Benzoxazole derivatives have long been recognized as very
interesting biologically active compounds as they display high
lipophilicity and broad range biological activity. In recent reports,
several derivatives have been presented as antifungal, antimicro-
bial and anticancer agents.4 Moreover, benzoxazoles also have
excellent optical properties (broad spectral windows, high molar
absorption coefficients and fluorescence quantum yields) and they
have been described as fluorescent probes and sensing materials,
namely as fluorescent and/or colorimetric sensors for metals, anio-
nic species and pharmaceutical analysis.2c,5 Thiophene and its
derivatives also display interesting biological activity and impor-
tant electroluminescent properties with wide applications in poly-
mer science, which has prompted their application as energy
transfer and light-harvesting systems, for optical and electronic de-
vices,6 as sensors and as fluorescent markers.7 Furthermore, the
combination of amino acid building blocks and thiophene deriva-
tives has resulted in several examples of heteroaromatic amino
acids with application as ligands of the NMDA receptor, monomers
for conducting polymers with complexing ability, paramagnetic
amino acids for probing protein dynamics, as UV- and pharmaceu-
tically active amino acids for incorporation into mutant proteins
with changed optical and thermodynamic properties.8
The importance of the several abovementioned applications of
thiophene, benzoxazole and amino acid derivatives reveals that
there is a practical interest on the synthesis of new and more
complex systems that combine these units. Therefore, in order
to contribute and expand the body of work published in this
area in the last years, we decided to design new amino acid-
based systems consisting of functionalized alanines containing
these heterocyclic nuclei. The (oligo)thienyl-benzoxazole amino
acid derivatives, because of the presence of amino and carboxyl
groups, can be incorporated into peptide chains and as such used
as an energy donor in conformational studies of peptides by
means of fluorescence or be used as fluorescence markers.
* Corresponding authors. Tel.: +351 253 604054; fax: +351 253 604382 (S.P.G.C.).
E-mail address: spc@quimica.uminho.pt (S. P. G. Costa).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.109

S. P. G. Costa et al. / Tetrahedron Letters 49 (2008) 5258–5261
5259
Table 1
Following our previous research in the synthesis and character-
ization of unnatural amino acids,9 benz-X-azole derivatives with
interesting optical properties10 and heterocyclic colorimetric/
fluorescent chemosensors containing (oligo)thiophene, benzoxaz-
ole and amino acid moieties,2c,11 we now report the synthesis
and characterization of a series of benzoxazolyl alanine deriva-
tives bearing (oligo)thiophene units at the side chain. With the
study of these new (oligo)thienyl-benzoxazolyl-alanines we in-
tend to evaluate the effect of the length of the (oligo)thienyl
group at the position 2 of the benzoxazole moiety in the photo-
physical properties of the resulting compounds.
The new(oligo)thienyl-benzoxazolyl-alanines 4–6 with ethyl-
enedioxythiophene, bithiophene and terthiophene units linked to
the benzoxazolyl alanine system were synthesized in good yields,
by a multistep synthesis, in order to compare the photophysical
properties of compounds 4–6 with our recently reported system
with thiophene.2c Several (oligo)thiophene moieties were used in
order to study the influence of the structure modification (e.g.,
the rigidification of the thiophene derivative by adding a ethylene-
Yields, UV–vis absorption and emission data for (oligo)thienylbenzoxazolyl-alanines
4–6 in absolute ethanol
Entry Compd. Yield Absorption kmax Emission
Stokes’
shift (nm) yield UF
Quantum
(%)
(nm) (log
e
)
kmax (nm)
1
2
3
4
5
6
7
8
9
4a
4b
4c
5a
5b
5c
6a
6b
6c
56
84
54
74
95
55
72
96
60
314 (4.37)
320 (4.12)
320 (4.28)
365 (4.32)
365 (4.39)
365 (4.29)
400 (4.57)
400 (4.49)
398
400
386
445
442
448
490
480
84
80
66
80
77
83
90
80
0.03
0.03
0.05
0.46
0.37
0.55
0.10
0.13
a
a
a
a
—
—
—
—
a
The evaluation of the photophysical properties of compound 6c was not pos-
sible due to insolubility.
dioxy bridge in compound 4a and increase of the p-conjugated sys-
tem in compounds 5a and 6a) on the overall optical properties of
compounds 4–6.
Starting from commercially available 3-nitro-L-tyrosine, N-Boc-
3-amino-L-tyrosine methyl ester 1 was obtained by using simple
synthetic procedures.2c The synthesis of 2-formyl-(oligo)thio-
phenes 2a12 and 2c13 has been previously reported by us. Conden-
Scheme 2. Synthesis of N- and C-terminal deprotected (oligo)thienylbenzoxazolyl-
alanine derivatives 4a–c to 6a–c.
sation of compounds
1 and 2a–c in ethanol afforded the
corresponding imino derivatives 3a–c, as stable solids which
allowed their characterization. By reaction with lead tetraacetate
(LTA) in DMSO, the imino compounds were oxidized to the [2-(oli-
gothienyl)benzoxazol-5-yl] alanine derivatives 4a–6a, in 56–74%
yield (Scheme 1, Table 1).14
In order to study the effect of the N- and C-terminal protecting
group of these amino acids in the photophysical properties, com-
pound 4a was then selectively deprotected at its C- and N-termi-
nus, yielding the corresponding N-Boc protected compound 4b15
and the fully deprotected ethylenodioxythienyl-benzoxazolyl-ala-
nine derivative 4c.16 By a similar procedure, compounds 5a–6a
were selectively deprotected, resulting in the corresponding N-
Boc protected 5b–6b and the fully deprotected alanine derivatives
5c–6c (Scheme 2, Table 1). The structures of the new (oligo)thie-
nyl-benzoxazolyl-alanines 4–6 were unambiguously confirmed
by their analytical and spectral data.
absorption and emission bands of compounds 4–6. The wavelength
of maximum absorption was shifted to longer wavelengths as the
number of thiophene units increased, ca. 50 nm for each added
thiophene (Table 1, entries 1, 4 and 7). The same trend was
observed in the emission spectra as the position of the wavelength
of maximum emission was red-shifted with the increase of the
number of thiophene units, which varied from kem = 398 nm for
4a to kem = 490 nm for 6a (Fig. 1).
The fluorescence quantum yields were determined using a
0.1 M solution of 1-naphthylamine in cyclohexane as standard
(
UF = 0.46)17 and (oligo)thienylbenzoxazolyl-alanines 4–6 exhib-
ited good to excellent relative fluorescence quantum yields. Ala-
nines 5a–c, with a bithiophene at position 2 of the benzoxazole,
were found to be strongly emissive (0.37 < UF < 0.55) while com-
pounds 6a–c, with a terthiophene substituent, displayed much
lower quantum yields (0.10 < UF < 0.13). The ethylenedioxythi-
enylbenzoxazolyl-alanines 4a–c displayed residual fluorescence
(Table 1, entries 1–3). For this compound it was found that the
rigidity introduced by the ethylenedioxy bridge on the thiophene
ring had practically no effect on the wavelengths of maximum
absorption and emission, although a strongly deleterious effect
The absorption and emission spectra of (oligo)thienylbenzoxaz-
olyl-alanines 4–6 were measured in absolute ethanol (1–
2 ꢀ 10ꢁ5 M solution) (Table 1). The nature of the thiophenic substi-
tuent at position 2 of the benzoxazole had a clear influence on the
1.2
4a
5a
6a
4a
1
0.8
0.6
0.4
0.2
0
5a
6a
250
350
450
Wavelength (nm)
550
650
Figure 1. Normalized UV–vis absorption and emission spectra of compounds 4a, 5a
and 6a in absolute ethanol at T = 298 K (4a, kexc = 314 nm; 5a, kexc = 365 nm; 6a,
kexc = 400 nm) (absorption, full line; emission, dotted line).
Scheme 1. Synthesis of fully protected (oligo)thienylbenzoxazolyl-alanine deriva-
tives 4a–6a.

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S. P. G. Costa et al. / Tetrahedron Letters 49 (2008) 5258–5261
on the fluorescence quantum yield was observed, when compared
to 2-thienylbenzoxazolyl-alanine which was reported to have a
fluorescence quantum yield of 0.80.2c A related alanine derivative
containing a benzoxazole bearing a methylthiophene was reported
by other authors,18 with the wavelength of maximum emission at
354 nm and a relative fluorescence quantum yield of 0.64.
References and notes
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F. M.; Pham, S. M.; Leblanc, R. M. Anal. Chem. 2003, 75, 1706–1712; (c) Costa, S.
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W. Mater. Sci. Eng. 2001, 32, 1–40.
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Melucci, M.; Sotgiu, G. Adv. Mater. 2005, 17, 1581–1593.
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L. Protein Sci. 2001, 10, 1281–1292; (b) Lima, P. G.; Caruso, R. R. B.; Alves, S. O.;
Pessoa, R. F.; Mendonça-Silva, D. L.; Nunes, R. J.; Noël, F.; Castro, N. G.; Costa, P.
R. R. Bioorg. Med. Chem. Lett. 2004, 14, 4399–4403; (c) Brembilla, A.; Collard, A.;
Henry, B.; Jadamiec, M.; Lapkowski, M.; Matlengiewicz, M.; Rodehüser, L.
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Balog, M.; Fogassy, E.; Hideg, K. Tetrahedron 2008, 64, 1094–1100.
9. (a) Costa, S. P. G.; Maia, H. L. S.; Pereira-Lima, S. M. M. A. Org. Biomol. Chem.
2003, 1, 1475–1479; (b) Jiang, W.-Q.; Costa, S. P. G.; Maia, H. L. S. Org. Biomol.
Chem. 2003, 1, 3804–3810.
10. (a) Batista, R. M. F.; Costa, S. P. G.; Raposo, M. M. M. Tetrahedron Lett. 2004, 45,
2825–2828; (b) Costa, S. P. G.; Batista, R. M. F.; Sousa, A. M. R. C.; Raposo, M. M.
M. Mater. Sci. Forum 2006, 514–516, 147–151; (c) Costa, S. P. G.; Batista, R. M.
F.; Cardoso, P.; Belsey, M.; Raposo, M. M. M. Eur. J. Org. Chem. 2006, 17, 3938–
3946; (d) Batista, R. M. F.; Costa, S. P. G.; Malheiro, E. L.; Belsey, M.; Raposo, M.
M. M. Tetrahedron 2007, 63, 4258–4265; (e) Batista, R. M. F.; Costa, S. P. G.;
Belsley, M.; Raposo, M. M. M. Tetrahedron 2007, 63, 9842–9849; (f) Pina, J.;
Seixas de Melo, J.; Burrows, H. D.; Batista, R. M. F.; Costa, S. P. G.; Raposo, M. M.
M. J. Phys. Chem. A 2007, 111, 8574–8578.
The fluorescence quantum yield of nonsubstituted (oligo)thio-
phenes is expected to increase as the oligothiophene chain length
increases, due to a further extension of the conjugated
p-system.
On the other hand, the heavy atom induced spin–orbit coupling
by the sulfur atoms can give rise to a very efficient intersystem
crossing mechanism, thus lowering the emission.19 Moreover, azo-
methine nitrogens contribute to the heavy atom effect concomi-
tant with the increased degree of conjugation.19c Also, the
different chains (thiophene, bithiophene and terthiophene) should
exhibit different degrees of torsion between the thiophene units,
which leads to variations in the effectivity conjugation length,
affecting the planarity of the whole heteroaromatic system.19d In
our case, we believe that a combination of the abovementioned
effects could be responsible for the trend observed in our results.
Keeping in mind further applications of these amino acids as
emissive probes for energy transfer or FRET (Fluorescence Reso-
nance Energy Transfer) studies in more complex structures as pep-
tides chains, in Figure 1 can also be seen the good superposition of
some absorption and emission spectra (e.g., compounds 6a and
4a), which opens up a very wide range of potential interesting
applications to be explored.
As can be seen by the results, the presence of C- and N-terminal
protecting groups does not affect the position of the absorption
and emission bands, nor the fluorescence quantum yield, which
does not vary significantly, within the same series of compounds
(e.g., compare entries 1–3 in Table 1). In order to assess the influ-
ence of the solvent polarity in the photophysical properties of ala-
nines 4–6, spectra were run in cyclohexane, dioxane, acetonitrile
and ethanol/water (1:1) and the obtained results showed that no
solvatochromic effect was observed, suggesting that these com-
pounds are solvent polarity independent.
In summary, we have achieved the synthesis of new fluorescent
heteroaromatic amino acid derivatives 4–6 containing (oligo)thio-
phene and benzoxazole moieties combined with an alanine residue
by simple procedures in good yields and their photophysical prop-
erties were evaluated. Due to their interesting photophysical prop-
erties these heterocyclic alanine derivatives could find application
as useful building blocks for peptide-based fluorimetric chemosen-
sors, as fluorescent markers and probes for FRET studies in pep-
tides. Studies on the application of these new derivatives as
chemosensors are currently ongoing and it is expected that their
metal ion sensing ability can be comparable to that of 2-thienyl-
benzoxazolylalanine, which has been found to respond via a fluo-
rescence quenching effect to the presence of Cu(II), Ni(II) and
Hg(II) in a very efficient manner.2c
11. Batista, R. M. F.; Oliveira, E.; Costa, S. P. G.; Lodeiro, C.; Raposo, M. M. M. Org.
Lett. 2007, 9, 3201–3204.
12. Costa, F.; Silva, C. J. R.; Raposo, M. M. M.; Fonseca, A. M.; Neves, I. C.; Carvalho,
A. P.; Pires, J. Micropor. Mesopor. Mater. 2004, 72, 111–118.
13. Batista, R. M. F.; Costa, S. P. G.; Belsley, M.; Lodeiro, C.; Raposo, M. M. M.
Tetrahedron, in press.
14. General procedure for the synthesis of alanines 4a–6a: compound 1 (1 equiv)
was stirred with the corresponding 2-formyl (oligo)thiophene 2 (1 equiv) and
heated in ethanol at reflux (5 mL/equiv) for 3 h. The solvent was evaporated
and the crude imine 3 used in the next step without further purification.
N-tert-Butyloxycarbonyl 3-(bithien-20-yl)imino-
L
-tyrosine methyl ester 3b
(crude): yellow solid. Mp 162.5–163.6 °C. 1H NMR (CDCl3) d 1.43 (s, 9H,
C(CH3)3), 2.96–3.12 (m, 2H, b CH2), 3.73 (s, 3H, CH3), 4.52–4.60 (m, 1H, -H),
a
5.01 (d, J = 7.8 Hz, 1H, NH), 6.92–6.95 (m, 2H, H-2 + H-5), 7.06–7.10 (m, 2H, H-
6 + H-400), 7.22 (d, J = 4.2 Hz, 1H, H-30), 7.31–7.34 (m, 2H, H-300 + H-500), 7.41 (d,
J = 3.9 Hz, 1H, H-40), 8.70 (s, 1H, N@CH) ppm. 13C NMR (CDCl3)
d
28.31
(C(CH3)3), 37.90 (b CH2), 52.26 (CH3), 54.46 (
aC), 80.01 (C(CH3)3), 115.03 (C2),
116.45 (C6), 124.18 (C30), 125.18 (C300), 126.02 (C500), 127.61 (C1), 128.21 (C400),
129.49 (C5), 133.51 (C40), 134.95 (C3), 136.86 (C20 or C200), 141.09 (C50), 142.88
(C20 or C200), 149.37 (N@CH), 151.25 (C4), 155.12 (C@O Boc), 172.46 (C@O
ester) ppm. MS (FAB) m/z (%): 487 ([M+H]+, 63), 486 (28), 431 (26), 307 (38),
298 (36), 155 (30), 154 (100). HRMS: (FAB) m/z for C24H27N2O5S2; calcd
487.1361, found 487.1368. Calcd for C24H26N2O5S2: C, 59.24; H, 5.39; N, 5.76; S,
13.18. Found: C, 59.41; H, 5.48; N, 5.76; S, 13.23.
Acknowledgements
Thanks are due to Fundação para a Ciência e Tecnologia (Portu-
gal) for financial support through project PTDC/QUI/66250/2006
and a Ph.D. grant to E.O. (SFRH/BD/35905/2007).
The corresponding crude imine 3 (1 equiv) and lead tetraacetate (1.5 equiv)
were stirred at room temperature in DMSO (5 mL/equiv) for 3 days. The
mixture was poured over water and extracted with ethyl acetate (3 ꢀ 10 mL).
After drying with anhydrous magnesium sulfate and evaporation of the solvent
under reduced pressure, the crude compound was submitted to column
chromatography with silica gel by elution with dichloromethane.
Supplementary data
Characterization data (1H and 13C NMR, MS and HRMS and UV–
vis) for derivatives 4a–c and 6a–c are available. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2008.06.109.
N-tert-Butyloxycarbonyl [2-(bithien-20-yl)benzoxazol-5-yl]-
ester 5a: yellow solid (0.198 g, 74%). Mp 107.0–108.7 °C. UV (ethanol): kmax
nm (log
) 365.0 (4.53). 1H NMR (CDCl3) d 1.43 (s, 9H, C(CH3)3), 3.16–3.29 (m,
2H, b CH2), 3.74 (s, 3H, CH3), 4.63–4.66 (m, 1H, -H), 5.03 (d, J = 8.1 Hz, 1H,
NH), 7.06–7.09 (m, 1H, H-400), 7.11 (dd, J = 1.2 and 8.4 Hz, 1H, H-6), 7.24 (d,
L-alanine methyl
e
a

S. P. G. Costa et al. / Tetrahedron Letters 49 (2008) 5258–5261
5261
J = 4.2 Hz, 1H, H-40), 7.31–7.34 (m, 2H, H-7 and H-300 or 500), 7.44–7.46 (m, 1H,
H-300 or 500), 7.48 (s, 1H, H-4), 7.80 (d, J = 4.2 Hz, 1H, H-30) ppm. 13C NMR
16. General procedure for the synthesis of alanines 4c–6c: the corresponding N-
Boc protected alanine was stirred in a trifluoroacetic acid/dichloromethane
solution (1:1, 4 mL/mmol) at room temperature for 2 h. The solvent was
evaporated, the solid residue dissolved in pH 8 aqueous buffer solution and
extracted with ethyl acetate (3 ꢀ 10 mL). After drying with anhydrous
magnesium sulfate and evaporation of the solvent, the product was isolated
as a solid.
(CDCl3)
d 28.24 (C(CH3)3), 38.26 (b CH2), 52.29 (CH3), 54.57 (aC), 79.99
(C(CH3)3), 110.20 (C7), 120.23 (C4), 124.43 (C40), 125.07 (C500), 125.90 (C300),
126.31 (C6), 127.38 (C20), 128.14 (C400), 130.66 (C30), 132.85 (C5), 136.19 (C200),
142.30 (C50), 142.35 (C3a), 149.62 (C7a), 155.05 (C@O Boc), 159.01 (C2), 172.08
(C@O ester) ppm. MS (FAB) m/z (%): 486 ([M+2H]+, 34), 485 ([M+H]+, 87), 430
(32), 429 (100), 325 (37), 297 (38), 296 (31), 293 (58), 193 (28), 185 (71), 171
(34). HRMS: (FAB) m/z for C24H25N2O5S2; calcd 485.1205, found 485.1212.
15. General procedure for the synthesis of alanines 4b–6b: the corresponding
alanine methyl ester (1 equiv) was dissolved in 1,4-dioxane (1 mL/equiv), in an
ice bath and sodium hydroxide 1 M aqueous solution (1.5 equiv) was added
dropwise. The mixture was stirred at room temperature for 3 h. The pH was
adjusted to 2–3 by addition of KHSO4 1 M aqueous solution and extracted with
ethyl acetate (3 ꢀ 10 mL). After drying with anhydrous magnesium sulfate and
evaporation of the solvent, the residue was triturated with diethyl ether and
the pure compound was obtained as a solid.
[2-(Bithien-20-yl)benzoxazol-5-yl]-
55%). Mp 248.6–250.2 °C. UV (ethanol): kmax nm (log
(DMSO-d6) 3.05–3.15 (m, 2H,
L
-alanine 5c: light yellow solid (0.031 g,
) 365.0 (4.04). 1H NMR
CH2), 4.10–4.17 (m, 1H, -H), 7.15–
e
d
b
a
7.19 (m, 1H, H-400), 7.30 (dd, J = 1.2 and 8.4 0H0 z, 1H, H-6), 7.48 (d, J = 3.9 Hz,
1H, H-40), 7.54 (dd, J = 1.2 and 3.6 Hz, 1H, H-3 or H-500), 7.65 (d, J = 1.2 Hz, 1H,
H-4), 7.66 (dd, J = 1.8 and 5.4 Hz, 1H, H-300 or H-500), 7.72 (d, J = 8.4 Hz,
1H, H-7), 7.91 (d, J = 3.9 Hz, 1H, H-30) ppm. 13C NMR (DMSO-d6) d 30.52 (b CH2),
53.11 (a
C), 118.92 (C7), 124.41 (C40), 125.24 (C500), 125.80 (C300), 126.57 (C6),
127.33 (C20), 128.10 (C400), 130.18 (C4), 130.30 (C2), 130.45 (C30), 132.41
(C5), 136.35 (C200), 141.03 (C3a), 142.33 (C50), 151.62 (C7a), 180.65 (CO2H)
ppm. MS (FAB) m/z (%): 372 ([M+2H]+, 11), 371 ([M+H]+, 32), 278 (19), 186
(70), 154 (100). HRMS: (FAB) m/z for C18H15N2O3S2; calcd 371.0524, found
371.0522.
N-tert-Butyloxycarbonyl
yellow solid (0.064 g, 95%). Mp 183.5–185.0 °C. UV (ethanol): kmax nm (log
365.0 (4.53). 1H NMR (CDCl3) d 1.46 (s, 9H, C(CH3)3), 3.34 (d, J = 4.5 Hz, 2H, b
[2-(bithien-20-yl)benzoxazol-5-yl]-
L-alanine
5b:
e
)
CH2), 4.65–4.72 (m, 1H,
a
-H), 5.15 (d, J = 7.5 Hz, 1H, NH), 7.05–7.08 (m, 1H,
17. Berlman, B. In Handbook of Fluorescence Spectra of Organic Molecules, 2nd ed.;
Academic Press: New York, 1971.
18. Guzow, K.; Szmigiel, D.; Wróblewski, D.; Milewska, M.; Karolczak, J.; Wiczk, W.
J. Photochem. Photobiol. A: Chem. 2007, 187, 87–96.
19. (a) Becker, R. S.; Seixas de Melo, J.; Maçanita, A. L.; Elisei, F. J. Phys. Chem. 1996,
100, 18683–18695; (b) Seixas de Melo, J.; Silva, L. M.; Arnaut, L. G.; Becker, R. S.
J. Chem. Phys. 1999, 111, 5427–5433; (c) Dufresne, S.; Bourgeaux, M.; Skene, W.
G. J. Mater. Chem. 2007, 17, 1166–1177; (d) Seixas de Melo, J.; Burrows, H. D.;
Svensson, M.; Andreson, M. R.; Monkman, A. P. J. Chem. Phys. 2003, 118, 1550–
1556.
H-400), 7.18–7.22 (m, 2H, H-6 + H-40), 7.29–7.33 (m, 2H, H-300 + H-500), 7.47 (d,
J = 8.4 Hz, 1H, H-7), 7.61 (d, J = 1.2 Hz, 1H, H-4), 7.81 (d, J = 3.9 Hz, 1H, H-30)
ppm. 13C NMR (CDCl3) d 28.30 (C(CH3)3), 37.96 (b CH2), 54.50 (
aC), 80.10
(C(CH3)3), 110.22 (C7), 120.18 (C4), 124.47 (C40), 125.13 (C500), 125.79 (C300),
126.58 (C6), 127.35 (C20), 128.25 (C400), 130.43 (C30), 133.01 (C5), 136.10 (C200),
142.20 (C50), 142.03 (C3a), 149.42 (C7a), 155.05 (C@O Boc), 159.01 (C2), 174.65
(CO2H) ppm. MS (FAB) m/z (%): 472 ([M+2H]+, 6), 471 ([M+H]+, 18), 470 (M+,
10), 415 (11), 307 (32), 289 (16), 166 (11), 155 (30), 154 (100). HRMS: (FAB)
m/z for C23H23N2O5S2; calcd 471.1048, found 471.1054.