Synthesis of fluorescent alanines by a rhodium-catalysed conjugate addition of arylboronic acids to dehydroalanine derivatives

Article abstract of DOI:10.1002/ejoc.201201198

Several β-arylalanine derivatives containing fluorescent groups were prepared in good yields using a rhodium-catalysed conjugate addition of arylboronic acids to N,N-diprotected and N-monoprotected dehydroalanines. The best conditions for these reactions required the use of an excess of arylboronic acid (4 equiv.), [Rh(COD)₂]BF₄ as catalyst, and CsF as base in dioxane/H₂O (10:1) at 110 °C. These conditions were also applied to several dipeptides with dehydroalanine residues. The photophysical properties of some of the β-arylalanines were studied in three solvents with different polarities. Due to the absence of the α,β-double bond, the absorption and fluorescence emission of the new compounds are dominated by the photophysical properties of the polycyclic aromatic fluorophores (naphthalene, phenanthrene, and pyrene). Considering the relatively high fluorescence quantum yield of these compounds, some of them may be useful as fluorescent markers for peptides and proteins. Fluorescent β-arylalanine derivatives were prepared in good yields using a rhodium-catalysed conjugate addition of arylboronic acids to N-protected dehydroalanines. The photophysical properties of some of the β-arylalanines were studied in solvents of different polarities. Considering the high fluorescence quantum yield of these compounds, some of them may be useful as fluorescent markers. Copyright

Full text of DOI:10.1002/ejoc.201201198

FULL PAPER
DOI: 10.1002/ejoc.201201198
Synthesis of Fluorescent Alanines by a Rhodium-Catalysed Conjugate Addition
of Arylboronic Acids to Dehydroalanine Derivatives
Paula M. T. Ferreira,*[a] Luís S. Monteiro,[a] Goreti Pereira,[a]
Elisabete M. S. Castanheira,[b] and Christopher G. Frost*[c]
Keywords: Amino acids / Rhodium / Homogeneous catalysis / Arylboronic acids / Conjugate addition / Fluorescent probes
Several β-arylalanine derivatives containing fluorescent
of the β-arylalanines were studied in three solvents with dif-
groups were prepared in good yields using a rhodium-cata- ferent polarities. Due to the absence of the α,β-double bond,
lysed conjugate addition of arylboronic acids to N,N-dipro- the absorption and fluorescence emission of the new com-
tected and N-monoprotected dehydroalanines. The best con- pounds are dominated by the photophysical properties of the
ditions for these reactions required the use of an excess of
arylboronic acid (4 equiv.), [Rh(COD)2]BF4 as catalyst, and
CsF as base in dioxane/H2O (10:1) at 110 °C. These condi-
tions were also applied to several dipeptides with de-
hydroalanine residues. The photophysical properties of some
polycyclic aromatic fluorophores (naphthalene, phen-
anthrene, and pyrene). Considering the relatively high
fluorescence quantum yield of these compounds, some of
them may be useful as fluorescent markers for peptides and
proteins.
these compounds in large amounts and to use them success-
fully as substrates in Michael addition reactions. In this pa-
per, we describe the synthesis of fluorescent alanines from
serine derivatives by a sequential dehydration and rhodium-
catalysed conjugate addition of arylboronic acids.
Introduction
The transition-metal-catalysed conjugate addition of or-
ganometallics to activated alkenes constitutes an important
methodology for the construction of new C–C bonds. In
particular, the rhodium-catalysed conjugate addition of or-
ganoboron compounds to α,β-unsaturated carbonyl com-
pounds has become an important tool in organic synthe-
sis.[1] This type of reactions has been used for the synthesis
of new non-proteinogenic amino acids using α,β-dehy-
droamino acid derivatives as substrates, and gave different
enantioselectivities depending on the ligand.[2] In recent
years, we have been interested in the synthesis of α,β-dehy-
droamino acid derivatives and their application as sub-
strates in several types of reactions to obtain new amino
acids.[3] Thus, an efficient synthetic procedure was devel-
oped that allowed the preparation of N,N-diacylde-
hydroamino acid derivatives from the corresponding β-hy-
droxyamino acids. As a result of the high reaction yields
and the simple workup procedures, we were able to prepare
Results and Discussion
Several fluorescent alanine derivatives were prepared
using a rhodium-catalysed conjugate addition of aryl-
boronic acids to dehydroalanines. This reaction was initially
screened using several sets of reaction conditions and sev-
eral types of substrates. Thus, N,N-diprotected and N-mono-
protected dehydroalanines were synthesized from the cor-
responding serine derivatives using a methodology pre-
viously developed in our research group.[3e] These com-
pounds were then treated with (naphthalen-1-yl)boronic
acid under different conditions (Scheme 1, Table 1).
Table 1. Results obtained in the synthesis of β-naphthylalanine de-
rivatives using different reaction conditions.
[a] Centre of Chemistry, University of Minho,
Campus de Gualtar, 4710-057 Braga, Portugal
Fax: +351-253604382
Entry
P1
P2
Catalyst
Ligand
Product Yield [%]
[a]
1
2
3
4
5
6
7
8
9
Boc
Z(NO2)
Boc-Phe
Boc
Boc [Rh(acac)(C2H4)2] (S)-BINAP
H
H
–
–
–
E-mail: pmf@quimica.uminho.pt
[a]
[Rh(acac)(C2H4)2] (S)-BINAP
[Rh(acac)(C2H4)2] (S)-BINAP
–
Homepage: http://www.quimica.uminho.pt/1189.page
[b] Centre of Physics, University of Minho,
Campus de Gualtar, 4710-057 Braga, Portugal
[c] Department of Chemistry, University of Bath,
Bath, BA2 AY, UK
4a
3a
5
40
70
60
Boc [Rh(COD)2]BF4
–
–
–
–
–
–
–
Z(NO2) Boc [Rh(COD)2]BF4
Tos
Z(NO2)
Boc-Phe
Boc2-Ala Boc [Rh(COD)2]BF4
Boc-Ala [Rh(COD)2]BF4
[a]
Boc [Rh(COD)2]BF4
H
H
–
–
[Rh(COD)2]BF4
[Rh(COD)2]BF4
6
4a
8
85
50
17
72
Fax: +44-1225-386231
E-mail: c.g.frost@bath.ac.uk
Homepage: http://www.bath.ac.uk/chemistry/contacts/
academics/chris_frost/
10
H
7
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201198.
[a] No product isolated.
550
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Synthesis of Fluorescent Alanines
Scheme 1. Synthesis of β-naphthylalanine derivatives.
From the results obtained, it is possible to conclude that are significantly lower for those compounds with a naphthyl
the catalytic system [Rh(COD)2]BF4/(S)-BINAP failed to group (3a, 4a) or a phenanthrenyl group (3b, 4b). In fact,
give the β-naphthylalanines when the substrates were the pyrene has a much higher molar absorption coefficient (ε =
methyl esters of N,N-diprotected and N-monoprotected de- 54000 m–1 cm–1 at 335 nm[4]) than naphthalene (ε
hydroalanines. In the case of the methyl ester of the dipept- 6000 m–1 cm–1 at 275 nm[4]) and phenanthrene (ε
ide
≈
≈
N-(tert-butoxycarbonyl)phenylalanyldehydroalanine 14000 m–1 cm–1 at 292 nm[5]), which explains this behaviour.
(Boc-Phe-ΔAla-OMe) it was possible to isolate the corre- A methyl ester group is present in all compounds, and it is
sponding β-naphthylalanine derivative in 40% yield. In known that many carbonyl compounds have a low-lying
view of these results, it was decided to change the catalyst nǞπ* state. The πǞπ* and nǞπ* electronic transitions can
to [Rh(COD)2]BF4. The β-naphthylalanines were obtained be close in energy, resulting in state mixing.[6] The relatively
in good yields using the methyl esters of N,N-diacyl- and N- high values of the molar absorption coefficient can be ex-
acyldehydroalanines as substrates, except when one of the plained by a predominance of πǞπ* character in these
protecting groups was the p-toluylsulfonyl group. N,N-Di- compounds (Tables 3 and 4).
acyldehydrodipeptides were found to be poor acceptors in
The emission spectra of compounds 3a–c and 4a–c also
this type of reactions. In view of these results, several β- resemble those of the pure fluorophores.[4] In fact, the ab-
arylalanines were prepared using as substrates the methyl sence of an α,β-double bond in these compounds does not
ester of N,N-tert-butoxycarbonyldehydroalanine (Boc2- allow conjugation between the aromatic moieties (in 4a–
ΔAla-OMe) and Boc-Phe-ΔAla-OMe, and several aryl- c) and the carbonyl groups (in all compounds), which had
boronic acids, and using [Rh(COD)2]BF4 as the catalyst previously been observed for other β,β-diarylalanine com-
(Scheme 2, Table 2).
pounds.[8]
All compounds show reasonable to high fluorescence
The absorption and fluorescence properties of β-aryl-
alanine derivatives 3a–c and 4a–c were studied in three sol- quantum yields in all of the solvents studied (Tables 3 and
vents of different polarity (cyclohexane, acetonitrile, and 4). The ΦF values are similar to those of the pure polycyclic
ethanol). The absorption (λabs) and emission maximum aromatic hydrocarbons (PAHs) at room temperature (ΦF
wavelengths (λem), molar absorption coefficients (ε) and values are 0.23 in cyclohexane and 0.21 in ethanol for
fluorescence quantum yields (ΦF) are presented in Tables 3 naphthalene; 0.13 for phenanthrene and 0.65 for pyrene in
and 4. The normalized absorption and fluorescence spectra both ethanol and cyclohexane[4,9]). A decrease in ΦF values
of compounds 3a–c and 4a–c are shown in Figures 1 and relative to the PAHs was expected, considering the flexibil-
2.
ity of the chain linked to the fluorescent moieties in the new
All compounds show relatively high molar absorption compounds, which should favour non-radiative deactiva-
coefficients at the lowest energy maximum (ε
Ն
tion pathways. As expected, compounds 4a–c show lower
3.4ϫ103 m–1 cm–1) in all of the solvents studied (Tables 3 fluorescence quantum yields than their corresponding ana-
and 4), which is indicative of πǞπ* transitions. The ε values logues in the 3a–c series, which indicates a greater deactiva-
Scheme 2. Synthesis of β-arylalanine derivatives.
Eur. J. Org. Chem. 2013, 550–556
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551

P. M. T. Ferreira, C. G. Frost et al.
FULL PAPER
Table 2. Yields obtained in the synthesis of several β-arylalanine
derivatives.
Figure 1. Normalized absorption and fluorescence spectra of solu-
tions of compounds 3a–c (10–5 m for absorption and 10–6 m for
emission) in cyclohexane, acetonitrile, and ethanol (λexc = 270 nm
for 3a and 3b, and λexc = 345 nm for 3c).
tion by non-radiative transitions, as would also be expected.
This effect is especially significant for compound 4c, due to
the extremely long excited-state lifetime of the pyrene
moiety (450 ns for pyrene in cyclohexane and 410 ns in eth-
anol[9]).
It is well known that the ratio of intensities between the
first and the third pyrene vibronic bands, I1/I3, is solvent-
dependent (the pyrene polarity scale), and that it rises with
increasing solvent polarity.[10] Similarly, compounds 3c and
4c have a solvent-sensitive I1/I3 ratio (Table 5) that follows
the behaviour of the pyrene fluorescence spectrum, albeit
with a lower degree of sensitivity. In fact, the groups in
close proximity to the pyrenyl moiety in compounds 3c and
4c influence the local polarity, and thus the relative inten-
sities of the vibronic bands in the emission spectra.
Figure 2. Normalized absorption and fluorescence spectra of solu-
tions of compounds 4a–c (10–5 m for absorption and 10–6 m for
emission) in cyclohexane, acetonitrile, and ethanol (λexc = 270 nm
for 4a and 4b, and λexc = 345 nm for 4c).
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Synthesis of Fluorescent Alanines
Table 3. Maximum absorption (λabs) and emission wavelengths (λem), molar absorption coefficients (ε), and fluorescence quantum yields
(ΦF) for compounds 3a, 3b, and 3c in cyclohexane (CyHx), acetonitrile (MeCN), and ethanol (EtOH).
[a]
Solvent
λabs [nm] (ε [104 m–1 cm–1])
λem [nm]
ΦF
3a
3b
3c
3a
3b
3c
3a
3b
3c
CyHx
292 (0.41),
283 (0.57),
273 (0.49),
225 (6.27)
298 (0.53), 286 343 (3.08), 327
317, 327, 352, 359 sh,
340, 352, 370, 390, 411 sh 393, 397, 404
371 sh
376, 383, 387,
0.13
0.10
0.19
0.14
0.64
(0.47), 275
(0.67), 254
(3.10), 249
(2.68) sh, 209
(2.19)
(2.07), 313
(0.83), 276
(3.06), 266
(1.76), 256
(0.80), 243
(4.83), 236
(3.03) sh
sh, 422 sh
MeCN
EtOH
292 (0.44),
282 (0.64),
273 (0.56),
224 (6.22)
298 (0.59), 286 342 (3.75), 326
319, 325, 352, 359 sh,
340, 353, 370, 389, 411 sh 393, 397, 404
372 sh
376, 383, 387,
0.08
0.15
0.61
0.57
(0.54), 275
(0.73), 253
(3.44), 248
(2.97) sh, 211
(2.00)
(2.56), 313
(1.06), 276
(4.25), 265
(2.87), 253
(3.79), 243
(8.04), 235
(4.99) sh
sh, 424 sh
292 (0.52),
282 (0.68),
272 (0.59),
224 (6.56)
297 (0.79), 285 342 (2.96), 326
318, 326, 352, 359 sh,
340, 351, 369, 389, 412 sh 393, 397, 405
372 sh sh, 422 sh
376, 383, 387,
(0.79), 275
(1.01), 253
(3.74), 248
(3.25) sh, 211
(2.59)
(2.01), 313
(0.86), 276
(3.14), 265
(1.90), 255
(1.21), 242
(5.07), 235
(3.20) sh
[a] Relative to naphthalene in cyclohexane (ΦF = 0.23 at 25 °C)[4] for 3a and 3b, and relative to anthracene in ethanol (ΦF = 0.27 at
25 °C)[7] for 3c; sh: shoulder.
Table 4. Maximum absorption (λabs) and emission wavelengths (λem), molar absorption coefficients (ε), and fluorescence quantum yields
(ΦF) for compounds 4a, 4b, and 4c in cyclohexane (CyHx), acetonitrile (MeCN), and ethanol (EtOH).
λabs [nm] (ε [104 m–1 cm–1])
λem[nm]
ΦF
[a]
Solvent
4a
4b
4c
4a
4b
4c
4a
4b
4c
CyHx
293 (0.34),
283 (0.53),
274 (0.49),
226 (5.86)
299 (0.96), 288
(0.87), 277
(1.12), 255
(4.92), 249 (4.10) (1.95), 256 (0.85),
sh, 221 (2.04) sh, 244 (5.10), 236
211 (2.92)
298 (0.90), 286
(0.81), 276
(1.02), 254
(4.84), 249 (4.20) (2.02), 256 (0.95),
sh, 221 (2.28) sh, 243 (5.58), 235
344 (3.52), 327
(2.37), 313 (0.94), 340, 351
277 (3.41), 266 sh, 374 sh
317, 326, 352, 361 sh, 370, 376, 383, 388,
389, 410 sh 393, 397, 421 sh
0.12 0.13 0.48
0.06 0.06 0.47
0.13 0.10 0.43
(3.13) sh
343 (3.50), 327
(2.41), 313 (0.99), 340, 354
276 (3.52), 265 sh, 372 sh
MeCN
EtOH
291 (0.39),
283 (0.57),
273 (0.48),
224 (6.35)
319, 325, 351, 360 sh, 369, 376, 382, 387,
389, 409 sh 393, 398, 419 sh
211 (3.35)
(3.57) sh
343 (3.27), 327
(2.30), 313 (1.00), 340, 352
276 (3.42), 265 sh, 375 sh
291 (0.50),
282 (0.71),
273 (0.61),
225 (7.07)
298 (1.02), 286
(0.94), 276
(1.19), 254
(5.25), 248 (4.44) (2.07), 256 (1.06),
sh, 222 (2.66) sh, 243 (5.33), 235
319, 326, 351, 357 sh, 369, 376, 382, 387,
389, 409 sh 393, 397, 420 sh
211 (3.91)
(3.46) sh
[a] Relative to naphthalene in cyclohexane (ΦF = 0.23 at 25 °C)[4] for 4a and 4b, and relative to anthracene in ethanol (ΦF = 0.27 at
25 °C)[7] for 4c; sh: shoulder.
The generally high fluorescence quantum yields of these tation of other aromatic amino acids (phenylalanine, tyr-
compounds make them good candidates to be used as osine, and tryptophan), which absorb light at wavelengths
fluorescence markers. In particular, compounds 3c and 4c lower than 300 nm.[11,12] The solvent sensitivity of the vi-
may be useful as probes for peptides and proteins, since brational structure of their emission spectra make com-
both compounds can be excited without simultaneous exci- pounds 3c and 4c also promising potential polarity probes.
Eur. J. Org. Chem. 2013, 550–556
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P. M. T. Ferreira, C. G. Frost et al.
FULL PAPER
pound. The excitation wavelengths of the samples were chosen to
ensure that there was a linear relationship between the intensity of
emitted light and the concentration of the absorbing/emitting spe-
cies (A Յ 0.05).
Table 5. Intensity ratio between the first and third vibronic bands,
I1/I3, of the fluorescence spectrum for compounds 3c and 4c.
Values for pyrene are also shown for comparison.
Solvent
I1/I3
3c
4c
Pyrene[10]
Synthesis of Dehydroalanine Derivatives: The synthesis of these
compounds was described elsewhere.[3a,3e]
CyHx
EtOH
MeCN
0.92
1.28
1.61
1.18
1.56
2.08
0.58
1.18
1.79
General Procedure for the Rhodium-Catalysed Addition of Aryl-
boronic Acids to Dehydroalanine Derivatives: An oven-dried Schlenk
tube was charged with dehydroalanine derivative (0.2 mmol),
boronic acid (4 equiv., 0.8 mmol), [Rh(COD2)]BF4 (5 mol-%,
0.01 mmol), CsF (3 equiv., 0.6 mmol), anhydrous 1,4-dioxane
(2 mL), and water (0.2 mL). The reaction mixture was heated to
110 °C under nitrogen for 24 h. The reaction mixture was cooled
to room temperature, and the solvent was evaporated. The resulting
residue was dissolved in ethyl acetate (20 mL) and washed with
water (2ϫ20 mL) and brine (2ϫ20 mL). The organic layer was
dried with MgSO4 and concentrated in vacuum. The resulting resi-
due was purified by column chromatography.
Conclusions
Several fluorescent β-arylalanines have been prepared in
good to high yields using a rhodium-catalysed conjugate
addition of arylboronic acids to N,N-diprotected dehy-
droalanine derivatives. This reaction was also applied suc-
cessfully to dipeptides with a dehydroalanine residue. The
photophysical properties of these compounds were studied
in solvents with different polarities. The results show that
all of the compounds prepared can be used as fluorescent
markers, due to their generally high fluorescence quantum
yields. The β-pyrenylalanine derivatives may be used as
probes for peptides and proteins since these compounds can
be excited without simultaneous excitation of other aro-
matic amino acids.
Methyl 2-[Bis(tert-butoxycarbonyl)amino]-3-(naphthalen-1-yl)prop-
anoate (3a): The general procedure described above was applied
with compound 1 (0.2 mmol, 60.5 mg) and 1-naphthaleneboronic
acid (0.8 mmol, 138 mg) to give compound 3a (60.2 mg, 70%) as
an oil. 1H NMR (300 MHz, CDCl3): δ = 1.12 (s, 18 H, CH3), 3.48–
3.56 (dd, J = 11.1 and 3.3 Hz, 1 H, CH2), 3.73 (s, 3 H, OCH3),
3.96–4.02 (dd, J = 3.6 and 10.8 Hz, 1 H, CH2), 5.18–5.23 (dd, J =
3.9 and 6.9 Hz, 1 H, CH), 7.21 (d, J = 9.6 Hz, 1 H, HAr), 7.31 (t,
J = 8.1 Hz, 1 H, HAr), 7.37–7.47 (m, 2 H, HAr), 7.67 (d, J =
8.1 Hz, 1 H, HAr), 7.77 (d, J = 6.9 Hz, 1 H, HAr), 7.96 (d, J =
7.8 Hz, 1 H, HAr) ppm. 13C NMR (75.5 MHz, CDCl3): δ = 27.60
(CH3), 33.07 (CH2), 52.42 (OCH3), 59.10 (CH), 82.89 [C(CH3)3],
123.40 (CH), 125.57 (CH), 125.61 (CH), 126.21 (CH), 127.51 (CH),
128.14 (CH), 128.86 (CH), 132.14 (C), 133.87 (C), 133.91 (C),
151.27 (C=O), 170.95 (C=O) ppm. HMRS (ESI): calcd. for
C24H31NNaO6 452.20491; found 452.2032.
Experimental Section
General Remarks: Melting points [°C] were determined with a
Büchi 535 melting point apparatus. 1H and 13C NMR spectra were
recorded with a Bruker AV300 spectrometer at 300 and 75.4 MHz,
respectively, or with a Bruker Avance 400 spectrometer at 400 and
1
100.6 MHz, respectively. H–1H spin–spin decoupling and DEPT-
45 were used. Chemical shifts are given in ppm and coupling con-
stants in Hz. MS and HRMS data were recorded with Bruker ESI-
TOF or ESI-QTOF instruments at the Mass Spectrometry Service
at the University of Bath. Reactions were monitored by thin-layer
chromatography (TLC). Column chromatography was performed
on Macherey–Nagel silica gel 230–400 mesh. Petroleum ether refers
to a boiling range of 40–60 °C. When a solvent gradient was used,
an increase of polarity was made from neat petroleum ether to
mixtures of diethyl ether/petroleum ether, increasing the amount of
diethyl ether by 10% at a time until the product was isolated.
Methyl 2-[Bis(tert-butoxycarbonyl)amino]-3-(phenanthren-9-yl)prop-
anoate (3b): The general procedure described above was applied
with compound 1 (0.2 mmol, 60.5 mg) and 9-phenanthreneboronic
acid (0.8 mmol, 177 mg) to give compound 3b (66.0 mg, 69%) as
an oil. 1H NMR (300 MHz, CDCl3): δ = 1.07 (s, 18 H, CH3), 3.51–
3.59 (dd, J = 10.8 and 3.6 Hz, 1 H, CH2), 3.75 (s, 3 H, OCH3),
4.03–4.09 (dd, J = 3.6 and 10.8 Hz, 1 H, CH2), 5.23–5.28 (dd, J =
3.9 and 6.9 Hz, 1 H, CH), 7.47–7.59 (m, 5 H, HAr), 7.72 (d, J =
7.5 Hz, 1 H, HAr), 8.00–8.04 (m, 1 H, HAr), 8.57 (d, J = 7.8 Hz,
1 H, HAr), 8.64–8.67 (m, 1 H, HAr) ppm. 13C NMR (75.5 MHz,
CDCl3): δ = 27.54 (CH3), 33.56 (CH2), 52.48 (OCH3), 58.63 (CH),
82.88 [C(CH3)3], 122.33 (CH), 123.33 (CH), 123.98 (CH), 126.37
(CH), 126.39 (CH), 126.66 (CH), 126.90 (CH), 128.28 (CH), 128.79
(CH), 130.04 (C), 130.74 (C), 131.06 (C), 131.61 (C), 131.98 (C),
151.47 (C=O), 170.97 (C=O) ppm. HMRS (ESI): calcd. for
C28H33NNaO6 502.22056; found 502.2245.
Spectroscopic Measurements: The solutions were prepared using
spectroscopic grade solvents. All measurements were performed at
25.0Ϯ0.5 °C. Absorption spectra were recorded with a Shimadzu
UV-3101PC UV/Vis/NIR spectrophotometer. Fluorescence mea-
surements were performed using a Fluorolog 3 spectrofluorimeter,
equipped with double monochromators in both excitation and
emission and with a temperature-controlled cuvette holder. Fluo-
rescence spectra were corrected for the instrumental response of
the system. For the determination of fluorescence quantum yields,
the solutions were bubbled for 30 min with ultrapure nitrogen in
advance of the measurement. The fluorescence quantum yields (Φs)
were determined using the standard method [Equation (1)][13,14]
Methyl 2-[Bis(tert-butoxycarbonyl)amino]-3-(pyren-1-yl)propanoate
(3c): The general procedure described above was applied with com-
pound 1 (0.2 mmol, 60.5 mg) and 1-pyreneboronic acid (0.8 mmol,
1
197 mg) to give compound 3c (67.0 mg, 67%) as an oil. H NMR
(300 MHz, CDCl3): δ = 1.02 (s, 18 H, CH3), 3.76 (s, 3 H, OCH3),
3.84–3.92 (dd, J = 10.5 and 3.6 Hz, 1 H, CH2), 4.16–4.22 (dd, J =
4.2 and 10.2 Hz, 1 H, CH2), 5.29–5.34 (dd, J = 4.2 and 6.6 Hz, 1
H, CH), 7.78 (d, J = 7.8 Hz, 1 H, HAr), 7.89–7.95 (m, 3 H, HAr),
8.03 (d, J = 8.1 Hz, 2 H, HAr), 8.10 (d, J = 7.8 Hz, 2 H, HAr), 8.22
(d, J = 9.3 Hz, 1 H, HAr) ppm. 13C NMR (75.5 MHz, CDCl3): δ
= 27.50 (CH3), 33.42 (CH2), 52.46 (OCH3), 59.85 (CH), 82.87
[C(CH3)3], 123.13 (CH), 124.84 (CH), 124.90 (CH), 125.00 (C),
ArFsns2
AsFrnr2
Φs =
Φr
(1)
where A is the absorbance at the excitation wavelength, F is the
integrated emission area, and n is the refraction index of the sol-
vents used. Subscripts refer to the reference (r) or sample (s) com-
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Synthesis of Fluorescent Alanines
125.03 (CH), 125.84 (CH), 126.98 (CH), 127.44 (CH), 127.68 (CH), Methyl 2-[2-(tert-Butoxycarbonylamino)-3-phenylpropanamido]-3-
128.63 (CH), 129.52 (C), 130.46 (C), 130.85 (C), 131.30 (C), 131.88 (phenanthren-9-yl)propanoate (4b): The general procedure described
(C), 151.54 (C=O), 170.93 (C=O) ppm. HMRS (ESI): calcd. for above was applied with compound 2 (0.2 mmol, 70.0 mg) and 9-
C30H33NNaO6 526.22056; found 526.2205.
phenanthraneboronic acid (0.8 mmol, 177 mg) to give compound
4b (57.0 mg, 54%).[2c]
Methyl 2-{tert-Butoxycarbonyl[(4-nitrobenzyloxy)carbonyl]amino}-
3-(naphthalen-1-yl)propanoate (5): The general procedure described
above was applied with methyl 2-{tert-butoxycarbonyl[(4-nitro-
benzyloxy)carbonyl]amino}acrylate (0.2 mmol, 76 mg) and 1-
Methyl 2-[2-(tert-Butoxycarbonylamino)-3-phenylpropanamido]-3-
(pyren-1-yl)propanoate (4c): The general procedure described above
was applied with compound 2 (0.2 mmol, 70.0 mg) and 1-py-
reneboronic acid (0.8 mmol, 197 mg) to give compound 4c
naphthaleneboronic acid (0.8 mmol, 138 mg) to give compound 5
1
(61.3 mg, 60%) as an oil. H NMR (300 MHz, CDCl3): δ = 1.17 (37.6 mg, 34%) as a white solid. M.p. 185.0–186.0 °C (from ethyl
(s, 9 H, CH3), 3.48–3.57 (dd, J = 11.1 and 3.6 Hz, 1 H, CH2), 3.72
(s, 3 H, OCH3), 3.98–4.04 (dd, J = 3.9 and 10.5 Hz, 1 H, CH2),
4.93 (q, J = 13.5 Hz, 2 H, CH2), 5.26–5.31 (m, 1 H, CH), 7.12–
7.15 (m, 3 H, HAr), 7.26 (t, J = 7.2 Hz, 1 H, HAr), 7.38–7.44 (m,
acetate/n-hexane). 1H NMR (300 MHz, CDCl3): δ = 1.15 and 1.18
(s, 9 H, CH3), 2.82–2.87 (m, 2 H, CH2), 3.36 and 3.41 (s, 3 H,
OCH3), 3.60–3.63 (m, 2 H, CH2), 4.17 (br. s, 1 H, CH), 4.68–4.91
(m, 2 H, CH and NH), 6.22–6.37 (dd, J = 7.8 Hz, 1 H, NH), 6.92–
2 H, HAr), 7.65 (d, J = 8.1 Hz, 1 H, HAr), 7.75–7.79 (m, 1 H, 7.03 (m, 4 H, HAr), 7.47–7.51 (dd, J = 2.4 and 5.4 Hz, 1 H, HAr),
HAr), 7.90–7.94 (m, 1 H, HAr), 8.05 (d, J = 6.9 Hz, 2 H, HAr) 7.83–7.92 (m, 5 H, HAr), 7.97–8.14 (m, 4 H, HAr) ppm. 13C NMR
ppm. 13C NMR (75.5 MHz, CDCl3): δ = 27.58 (CH3), 32.80 (CH2),
(75.5 MHz, CDCl3): δ = 28.15 and 28.20 (CH3), 35.64 and 35.79
52.66 (OCH3), 59.49 (CH), 66.89 (CH2), 84.00 [C(CH3)3], 123.16 (CH2), 38.36 (CH2), 52.31 and 52.34 (OCH3), 53.73 and 53.82
(CH), 123.67 (CH), 125.46 (CH), 125.76 (CH), 126.30 (CH), 127.71 (CH), 55.75 (CH), 80.22 [C(CH3)3], 122.83 and 123.02 (CH),
(CH), 128.05 (CH), 128.97 (CH), 131.91 (C), 133.37 (C), 133.82 124.75 (CH), 124.84 (C), 125.08 (C), 125.15 (CH), 125.30 (CH),
(C), 142.37 (C), 150.40 (C=O), 153.20 (C=O), 170.29 (C=O) ppm. 126.04 (CH), 126.92 (CH), 127.31 (CH), 127.44 (CH), 127.91 (CH),
HMRS (ESI): calcd. for C27H28N2NaO8 531.17434; found
531.1750.
127.44 and 128.04 (CH), 128.60 (CH), 129.27 and 129.32 (CH),
129.41 and 129.52 (C), 129.70 and 129.79 (C), 130.69 and 130.73
(C), 130.76 and 130.79 (C), 131.36 (C), 136.48 (C), 155.28 (C=O),
155.25 (C=O), 170.90 and 171.04 (C=O), 171.59 and 171.89 (C=O)
ppm. HMRS (ESI): calcd. for C34H34N2NaO5 573.23654; found
573.2374.
Methyl 3-(Naphthalen-1-yl)-2-[(4-nitrobenzyloxy)carbonylamino]-
propanoate (6): The general procedure described above was applied
with
methyl
2-[(4-nitrobenzyloxy)carbonylamino]acrylate
(0.2 mmol, 56 mg) and 1-naphthaleneboronic acid (0.8 mmol,
138 mg) to give compound 6 (69.5 mg, 85%) as a white solid. M.p.
140.0–141.0 °C (from ethyl acetate/n-hexane). 1H NMR (300 MHz,
CDCl3): δ = 3.38–3.45 (dd, J = 6.9 and 7.2 Hz, 1 H, CH2), 3.54–
3.60 (m, 4 H, OCH3 and CH2), 4.72 (d, J = 7.8 Hz, 1 H, CH), 5.07
(s, 2 H, CH2), 5.29 (d, J = 8.1 Hz, 1 H, NH), 7.14–7.19 (m, 2 H,
HAr), 7.28–7.35 (m, 2 H, HAr), 7.42–7.46 (m, 2 H, HAr), 7.71 (d,
J = 8.4 Hz, 1 H, HAr), 7.78–7.82 (m, 1 H, HAr), 7.95–8.01 (m, 1
Methyl
2-[2-(tert-Butoxycarbonylamino)propanamido]-3-(naphth-
alen-1-yl)propanoate (7): The general procedure described above
was applied with methyl 2-[2-(tert-butoxycarbonylamino)propana-
mido]acrylate (0.2 mmol, 54.5 mg) and 1-naphthaleneboronic acid
(0.8 mmol, 138 mg) to give compound 7 (57.3 mg, 72%) as an oil.
1H NMR (300 MHz, CDCl3): δ = 1.15–1.21 (m, 3 H, CH3), 1.35
(s, 9 H, CH3), 3.46–3.54 (m, 5 H, CH2 and OCH3), 4.05 (q, J =
H, HAr), 8.12 (d, J = 8.7 Hz, 2 H, HAr) ppm. 13C NMR 7.2 Hz, 1 H, CH), 4.73 (br. s, 1 H, NH), 4.85–4.94 (m, 1 H, CH),
(75.5 MHz, CDCl3): δ = 35.58 (CH2), 54.48 (OCH3), 54.74 (CH), 6.54–6.65 (dd, J = 6.9 Hz, 1 H, NH), 7.16–7.19 (m, 1 H, HAr),
65.34 (CH2), 123.28 (CH), 123.75 (CH), 125.28 (CH), 125.88 (CH), 7.31 (t, J = 8.1 Hz, 1 H, HAr), 7.38–7.51 (m, 2 H, HAr), 7.70 (d,
126.45 (CH), 127.52 (CH), 128.00 (CH), 128.16 (CH), 128.99 (CH),
J = 8.1 Hz, 1 H, HAr), 7.78 (d, J = 8.4 Hz, 1 H, HAr), 8.00–8.05
132.00 (C), 132.08 (C), 133.92 (C), 143.67 (C), 147.58 (C), 155.15 (dd, J = 3.0 and 5.4 Hz, 1 H, HAr) ppm. 13C NMR (75.5 MHz,
(C=O), 172.20 (C=O) ppm. HMRS (ESI): calcd. for CDCl3): δ = 18.17 (CH3), 28.28 and 28.30 (CH3), 35.31 and 35.46
C22H20N2NaO6 431.12191; found 431.1220.
(CH2), 52.31 (OCH3), 53.08 and 53.20 (CH), 60.44 (CH), 80.24
[C(CH3)3], 123.43 and 123.58 (CH), 125.26 and 125.29 (CH),
125.82 (CH), 126.36 and 126.44 (CH), 127.49 (CH), 128.02 and
128.05 (CH), 128.90 (CH), 132.05 and 132.16 (C), 132.20 and
132.26 (C), 13387 (C), 155.45 (C=O), 171.98 and 172.14 (C=O),
172.26 and 171.32 (C=O) ppm. HMRS (ESI): calcd. for
C22H29N2O5 401.20765; found 401.2097.
Methyl 2-[2-(tert-Butoxycarbonylamino)-3-phenylpropanamido]-3-
(naphthalen-1-yl)propanoate (4a): The general procedure described
above was applied with compound 2 (0.2 mmol, 70.0 mg) and 1-
naphthaleneboronic acid (0.8 mmol, 138 mg) to give compound 4a
(48.0 mg, 50%) as a white solid. M.p. 139.0–140.0 °C (from ethyl
1
acetate/n-hexane). H NMR (300 MHz, CDCl3): δ = 1.30 (s, 9 H,
CH3), 2.87–2.98 (m, 2 H, CH2), 3.37–3.49 (m, 5 H, CH2 and Methyl 2-[2-Bis(tert-butoxycarbonyl)amino-N-(tert-butoxycarbon-
OCH3), 4.25 (br. s, 1 H, CH), 4.79–4.86 (m, 2 H, CH and NH), yl)propanamido]-3-(naphthalen-1-yl)propanoate (8): The general
6.23–6.39 (dd, J = 7.5 Hz, 1 H, NH), 7.00–7.30 (m, 7 H, HAr),
7.40–7.51 (m, 2 H, HAr), 7.68 (d, J = 8.4 Hz, 1 H, HAr), 7.78 (d,
J = 8.7 Hz, 1 H, HAr), 7.98 (t, J = 8.7 Hz, 1 H, HAr) ppm. 13C
NMR (75.5 MHz, CDCl3): δ = 28.24 (CH3), 35.41 and 35.46
(CH2), 38.37 (CH2), 52.24 (OCH3), 53.08 and 53.26 (CH), 55.64
(CH), 80.23 [C(CH3)3], 123.37 and 123.50 (CH), 125.28 (CH),
125.82 and 125.86 (CH), 126.38 and 126.48 (CH), 126.98 (CH),
127.41 and 127.48 (CH), 128.04 and 128.09 (CH), 128.64 and
128.67 (CH), 128.92 (CH), 129.31 and 129.37 (CH), 131.98 and
132.04 (C), 132.08 and 132.12 (C), 133.90 (C), 136.49 (C), 155.28
(C=O), 170.83 and 170.96 (C=O), 171.67 and 171.97 (C=O) ppm.
HMRS (ESI): calcd. for C28H32N2NaO5 499.22089; found
499.2251.
procedure described above was followed with methyl N-(tert-bu-
toxycarbonyl)-2-[2-bis(tert-butoxycarbonyl)amino]propanamidoac-
rylate (0.2 mmol, 95 mg) and 1-naphthaleneboronic acid
(0.8 mmol, 138 mg) to give compound 8 (20.0 mg, 17%) as an oil.
1H NMR (300 MHz, CDCl3): δ = 1.40–1.44 (m, 30 H, CH3), 3.29–
3.37 (dd, J = 5.7 and 8.7 Hz, 1 H, CH2), 3.57 (s, 3 H, OCH3), 3.66
(t, J = 10.2 Hz, 1 H, CH), 3.92–3.99 (dd, J = 7.5 and 6.9 Hz, 1 H,
CH2), 5.48–5.56 (m, 1 H, CH), 7.29–7.51 (m, 4 H, HAr), 7.66 (d,
J = 8.1 Hz, 1 H, HAr), 7.77 (d, J = 6.9 Hz, 1 H, HAr), 8.11 (d, J
= 8.1 Hz, 1 H, HAr) ppm. 13C NMR (75.5 MHz, CDCl3): δ =
15.85 (CH3), 27.65 and 28.04 (CH3), 33.55 (CH2), 52.19 (OCH3),
56.98 (CH), 57.66 (CH), 82.60 and 83.85 [C(CH3)3], 124.00 (CH),
125.13 and 125.32 (CH), 125.52 (CH), 125.90 (CH), 126.16 (CH),
Eur. J. Org. Chem. 2013, 550–556
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Supporting Information (see footnote on the first page of this arti-
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Acknowledgments
Thanks are due to the Foundation for Science and Technology
(FCT, Portugal), Quadro de Referência Estratégico Nacional
(QREN), and Fundo Europeu de Desenvolvimento Regional/
União Europeia (FEDER/EU) for financial support through the
research centers, CQ/UM [PEst-C/QUI/UI0686/2011 (FCOMP-01-
0124-FEDER-022716)] and CFUM [PEst-C/FIS/UI0607/2011 (F-
COMP-01-0124-FEDER-022711)], and project PTDC/QUI/81238/
2006 (cofinanced by Fundo Europeu de Desenvolvimento Re-
gional/Programa Operacional Fatores de Competitividade
(FEDER/COMPETE), ref. FCOMP-01-0124-FEDER-007467).
G. P. acknowledges her PhD grant from Fundação para a Ciência
e a Tecnologia (FCT), Programa Operacional Potencial Humano/
Quadro de Referência Estratégico Nacional (POPH-QREN),
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Received: September 6, 2012
Published Online: November 29, 2012
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Eur. J. Org. Chem. 2013, 550–556