Pyrenylamino acids: Synthesis, photophysical and electrochemical studies

Article abstract of DOI:10.1002/ejoc.200800640

Several β-pyrenyldehydroamino acids and a pyrenylalanine derivative have been synthesized in good-to-high yields from dehydroamino acids by several types of reactions. A β-[(pyren-1-yl)methylamino]alanine was prepared by treating the methyl ester of N,N-(di-tert-butoxycarbonyl)dehydroalanine with (pyren-1-yl)methylamine hydrochloride in the presence of an excess of potassium carbonate. The E isomer of the methyl esters of N-(tert-butoxycarbonyl)-β- (1,2,4-triazol-1-yl)dehydroalanine and dehydroaminobutyric acid were treated with (pyren-1-yl)methylamine hydrochloride in the presence of triethylamine to give the E isomers of the β-aminomethylpyrene dehydroalanine and dehydroaminobutyric acid derivatives. β-(Pyren-1-yl)dehydrophenylalanine and dehydroaminobutyric acid derivatives were obtained from the pure stereoisomer of the corresponding β-bromodehydroamino acids and (pyren-1-yl)boronic acid by Suzuki cross-coupling. This reaction was also applied successfully to β-bromodehydrodipeptides. The electrochemical behaviour of some of the compounds prepared was studied by cyclic voltammetry. The peak potentials for the oxidation and reduction of pyrenylalanines were similar to those reported for pyrene. However, it was found that when the pyrene ring was conjugated with the dehydroamino acid moiety, the compounds had higher oxidation and reduction peak potentials than pyrene. The fluorescence properties of four of the pyrenylamino acids synthesized were evaluated in cyclohexane and alcohols. The methyl ester of N,N-bis(tert-butoxycarbonyl)- β-[(pyren-1-yl)methylamino]alanine presented high fluorescence quantum yields in cyclohexane and ethanol (Φ_F = 0.45 and 0.35, respectively). Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800640

FULL PAPER
DOI: 10.1002/ejoc.200800640
Pyrenylamino Acids: Synthesis, Photophysical and Electrochemical Studies
Ana S. Abreu,[a,b] Elisabete M. S. Castanheira,[b] Paula M. T. Ferreira,*[a]
Luís S. Monteiro,[a] Goreti Pereira,[a] and Maria-João R. P. Queiroz[a]
Keywords: Dehydroamino acids / Pyrene / Cross-coupling / Cyclic voltammetry / Fluorescence
Several β-pyrenyldehydroamino acids and a pyrenylalanine
derivative have been synthesized in good-to-high yields from
dehydroamino acids by several types of reactions. A β-
[(pyren-1-yl)methylamino]alanine was prepared by treating
cross-coupling. This reaction was also applied successfully to
β-bromodehydrodipeptides. The electrochemical behaviour
of some of the compounds prepared was studied by cyclic
voltammetry. The peak potentials for the oxidation and re-
the methyl ester of N,N-(di-tert-butoxycarbonyl)dehy- duction of pyrenylalanines were similar to those reported for
droalanine with (pyren-1-yl)methylamine hydrochloride in
the presence of an excess of potassium carbonate. The E iso-
mer of the methyl esters of N-(tert-butoxycarbonyl)-β-(1,2,4-
triazol-1-yl)dehydroalanine and dehydroaminobutyric acid
were treated with (pyren-1-yl)methylamine hydrochloride in
the presence of triethylamine to give the E isomers of the β-
aminomethylpyrene dehydroalanine and dehydroaminobu-
tyric acid derivatives. β-(Pyren-1-yl)dehydrophenylalanine
and dehydroaminobutyric acid derivatives were obtained
from the pure stereoisomer of the corresponding β-bromo-
dehydroamino acids and (pyren-1-yl)boronic acid by Suzuki
pyrene. However, it was found that when the pyrene ring
was conjugated with the dehydroamino acid moiety, the
compounds had higher oxidation and reduction peak poten-
tials than pyrene. The fluorescence properties of four of the
pyrenylamino acids synthesized were evaluated in cyclohex-
ane and alcohols. The methyl ester of N,N-bis(tert-butoxy-
carbonyl)-β-[(pyren-1-yl)methylamino]alanine presented high
fluorescence quantum yields in cyclohexane and ethanol (ΦF
= 0.45 and 0.35, respectively).
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
amino acid.[6] With this in mind we decided to synthesize
pyrenylamino acids by using several types of reactions de-
veloped by our group.
Introduction
In our group we have been interested in the synthesis of
non-proteinogenic amino acids such as β-substituted ala-
nines,[1] α-aminoglycines,[2] furanic amino acids,[3] 4-imid-
azolidinones[4] and β,β-diaryl- or heteroaryldehydroamino
acids.[5] The latter were synthesized by Suzuki cross-coup-
ling reactions of a β,β-dibromodehydroalanine[5a] or of β-
bromodehydrophenylalanine derivatives[5b–5d] with several
aryl- or heteroarylboronic acids. The study of the photo-
physical properties of some of the new amino acid deriva-
tives indicated their possible use as fluorescent probes.
Previously, we found that N,N-diprotected dehydroalan-
ine derivatives are excellent substrates in Michael addition
reactions, allowing the preparation of a wide variety of new
β-substituted alanines in high yields.[1,3b] Thus, a similar
strategy was proposed for the synthesis of a pyrenylalanine
derivative using the methyl ester of N,N-(di-tert-butoxycar-
bonyl)dehydroalanine as the Michael acceptor and (pyren-
1-yl)methylamine hydrochloride as the nucleophile.
In the course of our work concerning the reactivity of
Pyrene possesses unique spectroscopic properties such as dehydroamino acids towards nucleophiles, it was found that
a high fluorescence quantum yield, a long excited-state life- it is possible to synthesize β-aminodehydroalanine and β-
time and the ability to form excimers when two pyrene moi- aminodehydroaminobutyric acid derivatives[3b,7] by treating
eties (one excited and the other in the ground state) are in the E isomer of β-(1,2,4-triazol-1-yl)dehydroamino acids
close proximity. Pyrenylalanine has been used as a fluores- with amines. This reaction proceeds by the replacement of
cent probe in peptides and proteins and there are several the triazole group by the amine, giving the E isomer of the
reports describing the synthesis and applications of this corresponding β-aminodehydroamino acids. By using the
same strategy we were able to synthesize β-[(pyren-1-yl)-
methylamino]dehydroamino acids in good-to-high yields.
We have also been interested in the synthesis of β-aryl-
[a] Department of Chemistry, University of Minho,
or -heteroaryldehydroamino acids. These compounds were
prepared by Suzuki cross-coupling of β-bromodehydro-
amino acids with several aryl- or heteroarylboronic acids.[5]
Several β-pyrenyldehydroamino acid derivatives have been
Gualtar, 4710-057 Braga, Portugal
Fax: +351-253604382
E-mail: pmf@quimica.uminho.pt
[b] Department of Physics, University of Minho,
Gualtar, 4710-057 Braga, Portugal
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P. M. T. Ferreira et al.
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obtained by the Suzuki cross-coupling of β-bromodehydro- 6, (Z)-7 and (E)-7] were treated with (pyren-1-yl)boronic
phenylalanines and β-bromodehydroaminobutyric acids acid to give, with retention of configuration, the corre-
with (pyren-1-yl)boronic acid. The electrochemical and sponding β-(pyren-1-yl)dehydroamino acid in yields rang-
photophysical properties of some of the β-pyrenylamino ing from 72 to 96% [compounds (Z)-8, (E)-8, (Z)-9 and
acid derivatives synthesised were evaluated.
(E)-9, Scheme 3, Table 1].
Results and Discussion
The methyl ester of N,N-bis(tert-butoxycarbonyl)-β-
[(pyren-1-yl)methylamino]alanine (2) was obtained in 94%
yield as a racemic mixture by Michael addition reaction
between N,N-diprotected dehydroalanine derivative 1 and
(pyren-1-yl)methylamine hydrochloride (Scheme 1).
Scheme 3. Synthesis of the methyl esters of N-(tert-butoxycar-
bonyl)-β-(pyren-1-yl)dehydroamino acids.
Table 1. Results obtained in the synthesis of β-(pyren-1-yl)dehy-
droamino acids.
Scheme 1. Synthesis of the methyl ester of N,N-bis(tert-butoxycar-
bonyl)-β-[(pyren-1-yl)methylamino]alanine.
β-[(Pyren-1-yl)methylamino]dehydroamino acid deriva-
tives were synthesized by using a strategy previously devel-
oped by us. The E isomer of the β-(1,2,4-triazol-1-yl)dehy-
droamino acids (E)-3a[7] and (E)-3b[7] were treated with
(pyren-1-yl)methylamine hydrochloride in methanol to give
compounds (E)-4 and (E)-5 in 84 and 45% yields, respec-
tively (Scheme 2). The stereochemistries of compounds (E)-
4 and (E)-5 were determined by NOE difference experi-
ments by irradiating the α-NH proton and observing NOE
enhancements of the signals of the β-CH or γ-CH3 protons.
The same type of reaction was applied to the synthesis
of β-(pyren-1-yl)dehydrodipeptides [compounds (E)-12 and
(Z)-13, Scheme 4]. Compound (E)-10 was prepared by de-
hydration followed by bromination with N-bromosuc-
cinimide (NBS) of the corresponding threonine dipeptide
Scheme 2. Synthesis of the E isomer of the methyl esters of N-(tert-
butoxycarbonyl)-β-[(pyren-1-yl)methylamino]dehydroamino acids.
By using our Suzuki cross-coupling conditions[5] we were and isolation of the isomers by column chromatography.
able to prepare several β-(pyren-1-yl)dehydroaminobutyric The electrochemical behaviour and photophysical prop-
acid and -dehydrophenylalanine derivatives. Thus, the pure erties of compounds 2, (E)-4, (Z)-8, (E)-8, (Z)-9 and (E)-12
stereoisomers of the methyl esters of N-(tert-butoxycar- (Scheme 5) were studied by cyclic voltammetry and absorp-
bonyl)-β-bromodehydroamino acids [compounds (Z)-6, (E)- tion and emission spectroscopy.
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Eur. J. Org. Chem. 2008, 5697–5703

Pyrenylamino Acids
Table 2. Peak potentials obtained by cyclic voltammetry of com-
pounds 2, (E)-4, (Z)-8 and (Z)-9.[a]
Compound
–Ep [V] vs. SCE[b]
Ep [V] vs. SCE[b]
2
1.96
2.06
1.82
1.70
1.05
1.12
1.58
1.64
(E)-4
(Z)-8
(Z)-9
[a] Cathode: vitreous carbon; solvent: DMF; supporting electro-
lyte: Bu4NBF4; substrate concentration:
0.1 moldm–3
0.005 moldm–3. [b] SCE: standard calomel electrode.
The absorption and fluorescence properties of com-
pounds 2, (E)-8, (Z)-9 and (E)-12 were studied in cyclohex-
ane and methanol (and also in ethanol for compound 2).
The absorption (λabs) and emission maxima (λem), molar
extinction coefficients (ε) and fluorescence quantum yields
(ΦF) are presented in Table 3. The normalized absorption
and fluorescence spectra of compounds 2, (E)-8, (Z)-9 and
(E)-12 are shown in Figure 1.
Scheme 4. Synthesis of the methyl esters of N-(tert-butoxycar-
bonyl)-β-(pyren-1-yl)dehydrodipeptides.
The absorption spectra of all the compounds (Figure 1)
show intense bands with high molar extinction coefficients
at the lowest energy peak (logεՆ4.5, Table 3) typical of
a π–π* transition.[9] Compound 2 exhibits absorption and
emission spectra that resemble those of pyrene,[10] with high
fluorescence quantum yields in cyclohexane and ethanol
(ΦF = 0.58 for pyrene in cyclohexane[11]).
The pyrenylamino acids (E)-8 and (E)-12 show very sim-
ilar absorption and fluorescence spectra (Figure 1). In the
emission spectra, a redshift, loss of vibrational structure,
and stronger tailing above 500 nm are observed relative to
compound 2. The fluorescence quantum yields are mark-
edly lower (Table 3). The conjugation of the dehydroamino
acid double bond with the pyrene π electrons leads to an
increase in the non-radiative deactivation pathways. Some
charge transfer (CT) character of the excited state may also
be present, the conjugated NH group acting as the electron
donor and the pyrenyl moiety as the acceptor. A strong
excited-state CT has been observed in pyrene–dimethylani-
Scheme 5. β-(Pyrenyl)amino acid derivatives submitted to cyclic
voltammetry and photophysical studies.
The peak potentials obtained by cyclic voltammetry of line derivatives.[12,13] In compounds (E)-8 and (E)-12, the
compounds 2, (E)-4, (Z)-8 and (Z)-9 are presented in electron-withdrawing properties of the tert-butoxycarbonyl
Table 2. The peak potentials obtained for the oxidation and group and of the methyl ester may contribute to a decrease
reduction of compounds 2 and (E)-4 are similar to those in the CT character. The similarity of the spectral properties
reported in the literature for pyrene (Ep = –2.07 V and Ep = of the two pyrenylamino acids (E)-8 and (E)-12 indicates a
+1.10 V vs. Ag/AgCl, respectively).[8] For compounds (Z)-8 negligible influence of the phenyl ring in compound (E)-12.
and (Z)-9, both the reduction and oxidation potentials were
Compound (Z)-9 exhibits the highest loss of vibrational
higher than those reported for pyrene. The results obtained structure in both absorption and fluorescence spectra (Fig-
for compounds 2 and (E)-4 were expected because com- ure 1). This behaviour can be attributed to the interaction
pound 2 is a pyrenylalanine derivative and in compound between the π systems of pyrene and the phenyl ring, which
(E)-4 there is no conjugation between the pyrene moiety are close together in this compound, as previously detected
and the dehydroalanine. By comparing the results obtained for 1-phenylpyrene.[12,14]
for compound (E)-4 with those for compounds (Z)-8 and
The ΦF values are lower in alcohols for all compounds
(Z)-9 in which the pyrene ring is conjugated with the α,β- (Table 3). Besides the greater excited-state CT character ex-
double bond of the dehydroamino acid, we can conclude pected in polar solvents, solute–solvent hydrogen-bonding
that this conjugation makes the pyrene nucleus easier to interactions may play an important role, leading to en-
reduce and more difficult to oxidize. Also, compound (Z)- hanced singletǞtriplet intersystem crossing efficiency.[9] All
9, due to the further conjugation with the phenyl group, four pyrenylamino acids have the possibility of establishing
shows higher reduction and oxidation peak potentials than hydrogen bonds with protic solvents. Nevertheless, com-
compound (Z)-8.
pound 2 has a very reasonable ΦF value in ethanol.
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P. M. T. Ferreira et al.
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Table 3. Absorption and emission maxima (λabs and λem), molar extinction coefficients (ε) at absorption maxima and fluorescence quan-
tum yields (ΦF) relative to anthracene in ethanol for compounds 2, (E)-8, (Z)-9 and (E)-12 in cyclohexane (CyHx), ethanol (EtOH) and
methanol (MeOH).
Compound
Solvent
λabs [nm] (ε) [104 –1 cm–1]
λem [nm]
ΦF
2
CyHx
EtOH
MeOH
CyHx
MeOH
CyHx
MeOH
CyHx
MeOH
343 (5.23), 327 (3.59), 276 (4.78), 243 (7.46)
342 (6.08), 326 (4.21), 276 (5.53), 242 (9.10)
341 (5.04), 325 (3.46), 275 (4.01), 242 (7.71)
343 (3.62), 328 (2.55), 277 (4.01), 243 (6.01)
342 (3.14), 327 (2.21), 276 (3.45), 241 (5.53)
346 (3.08), 278 (4.14), 243 (6.97)
348 (3.40), 278 (4.29), 241 (7.90)
343 (3.82), 328 (2.67), 277 (4.28), 243 (6.38)
342 (4.20), 327 (2.96), 276 (4.68), 241 (7.60)
418, 396, 387, 381, 375
418, 396, 387, 382, 376
416, 396, 387, 381, 375
444, 419, 395
420, 398, 388
414, 401, 392
421, 397
445, 420, 396
419, 398, 389
0.45
0.35
0.07
0.04
0.02
0.09
0.01
0.02
0.01
(E)-8
(Z)-9
(E)-12
yields. Cyclic voltammetry studies showed that pyrenyl-
amino acids exhibit oxidation and reduction potentials sim-
ilar to those of pyrene. However, when the pyrene moiety
is conjugated with the α,β-double bond of the dehydro-
amino acid derivative, the compounds are easier to reduce
and more difficult to oxidize than pyrene. Fluorescence
studies showed that when deprotected these pyrenylamino
acids can be useful for conformational studies in peptides
and proteins, especially the highlyfluorescent β-[(pyren-1-yl)-
methylamino]alanine.
Experimental Section
General Methods: Melting points [°C] were determined with a Gal-
lenkamp apparatus and are uncorrected. 1H and 13C NMR spectra
were recorded with a Varian Unity Plus spectrometer at 300 and
75.4 MHz, respectively, and on a Bruker Avance II+ spectrometer
at 400 and 100.6 MHz, respectively. 1H–1H spin–spin decoupling
and DEPT θ 45° methods were used. Chemical shifts are given
in ppm and coupling constants in Hz. HRMS (EI or ESI) spectra
were recorded at the Mass Spectrometry Service of the University
of Vigo, Spain. Elemental analysis was performed on a LECO
CHNS 932 elemental analyser.
The reactions were monitored by thin-layer chromatography
(TLC). Column chromatography was performed on Macherey–Na-
gel silica gel (230–400 mesh). Petroleum ether refers to the boiling
range 40–60 °C. When a solvent gradient was used, the polarity was
increased from neat petroleum ether to mixtures of diethyl ether/
petroleum ether, with an increase of 10% diethyl ether in each step,
until isolation of the product. [PdCl2dppf·CH2Cl2] (1:1) refers to
[1,1Ј-bis(diphenylphosphanyl)ferrocene]dichloropalladium(II)
complexed to dichloromethane.
Figure 1. Normalized absorption (dashed lines) and fluorescence
(solid lines) (λexc = 342 nm) spectra of compounds 2, (E)-8, (Z)-9
and (E)-12 in cyclohexane (concentrations are 2ϫ10–5  for ab-
sorption and 3ϫ10–6  for emission).
These results show that the compounds studied, after de-
protection, can be inserted into peptides and proteins and
be excited without simultaneous excitation of tryptophan
or other aromatic amino acids (tyrosine and phenylalanine)
that absorb light at λ Ͻ 300 nm.[9,15] Therefore, when depro-
tected, these compounds may be useful for conformational
studies, especially the highly fluorescent β-[(pyren-1-yl)-
methylamino]alanine.
Cyclic voltammetry experiments were carried out by using a
Hi-Tek DT 2101potentiostat and a Hi-Tek PPRl wave generator
connected to a Philips PM 8043 recorder and a three-electrode,
home-built glass cell.
Absorption spectra were recorded with a Shimadzu UV-3101PC
UV/Vis/NIR spectrophotometer. Fluorescence measurements were
performed by using a Fluorolog 3 spectrofluorimeter equipped with
double monochromators for both excitation and emission. Fluores-
cence spectra were corrected for the instrumental response of the
system. All spectroscopic measurements were performed at 25 °C.
Conclusions
Several types of reactions, namely, Michael addition,
substitution and Suzuki cross-coupling reactions, have been
used to prepare pyrenylamino acid derivatives from α,β-de-
hydroamino acids. Thus, a variety of pyrenyl amino and
To determine the fluorescence quantum yields, the solutions were
previously bubbled for 1 h with ultrapure nitrogen. The fluores-
dehydroamino acids have been obtained in good-to-high cence quantum yields (Φs) were determined by using the standard
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Pyrenylamino Acids
method [Equation (1), where A is the absorbance at the excitation
wavelength, F is the integrated emission area and n is the refraction
index of the solvents used; subscripts r and s refer to the reference
and sample compounds].[16,17] Anthracene in ethanol was used as
the reference (Φr = 0.27[18] at 25 °C).
DMSO): δ = 1.38 (s, 18 H, CH3 Boc), 2.05 (s, 3 H, γCH3), 3.51 (s,
3 H, OCH3), 5.21 (br. s, 2 H, CH2), 7.34 (s, 1 H, NH), 8.08–8.36 (m,
9 H, ArH), 9.33–9.36 (m, 1 H, NH) ppm. 13C NMR (100.6 MHz,
DMSO): δ = 14.21 (γCH3), 28.24 [C(CH3)3], 44.67 (CH2), 50.16
(OCH3), 77.64 [OC(CH3)3], 93.09 (C), 122.74 (CH), 122.87 (CH),
123.87 (C), 124.14 (C), 124.94 (CH), 125.05 (CH), 125.33 (CH),
125.46 (CH), 126.05 (CH), 126.41 (CH), 127.36 (CH), 127.75 (C),
127.92 (CH), 130.29 (C), 130.38 (C), 130.80 (C), 132.24 (C), 132.66
(C), 155.69 (C=O), 168.62 (C=O) ppm. HRMS (EI): calcd. for
C27H28N2O4 444.2049; found 444.2039.
Φs = |(ArFsns2)/(AsFrnr2)|Φr
(1)
Synthesis of Compounds 1, (E)-3a, (E)-3b, (Z)-6, (E)-6, (Z)-7, (E)-
7 and (Z)-11: The synthesis of these compounds has been described
elsewhere.[4,5,7,19]
General Procedure for the Synthesis of β-(Pyren-1-yl)dehydroamino
Synthesis of Methyl 2-[Bis(tert-butoxycarbonyl)amino]-3-[(pyren-1-
yl)methylamino]propanoate (2): (Pyren-1-yl)methylamine hydro-
chloride (0.500 mmol), triethylamine (1 mmol) and K2CO3
(6 equiv.) were added to a solution of 1 (0.500 mmol, 151 mg) in
acetonitrile (5 mL). The reaction mixture was stirred at room tem-
perature for 48 h (the reaction was followed by 1H NMR). The
reaction mixture was filtered and the solvent removed under re-
duced pressure to give 2 (249 mg, 94%) as a white solid; m.p.
114.5–116.0 °C (from diethyl ether/n-hexane). 1H NMR (400 MHz,
DMSO): δ = 1.34 (s, 18 H, CH3 Boc), 3.05 (dd, J = 12.8, J =
9.2 Hz, 1 H, βCH2), 3.33 (dd, J = 12.8, J = 5.2 Hz, βCH2) 3.62 (s,
3 H, OCH3), 4.37 (d, J = 13.6 Hz, 1 H, CH2), 4.46 (d, J = 13.6 Hz,
Acid
Derivatives:
(Pyren-1-yl)boronic
acid
(1.5 equiv.),
[PdCl2dppf·CH2Cl2] (1:1) (10 mol-%) and Cs2CO3 (1.4 equiv.) were
added to a solution of the β-bromodehydroamino acid derivative
in THF/H2O (1:1). The reaction mixture was heated at 70 °C (the
reaction was followed by TLC). The solvent was removed under
reduced pressure and the residue dissolved in ethyl acetate
(100 mL). The organic layer was washed with water and brine
(2ϫ30 mL each), dried with MgSO4 and the solvent removed. The
residue was submitted to column chromatography.
Synthesis of Methyl (Z)-2-(tert-Butoxycarbonylamino)-3-(pyren-1-
yl)but-2-enoate [(Z)-8]: Compound (Z)-8 was prepared from com-
1 H, CH2), 5.11 (dd, J = 9.2, J = 5.2 Hz, 1 H, αCH), 8.05–8.28 (m, pound (Z)-6 (0.500 mmol, 147 mg) according to the general pro-
8 H ArH), 8.43 (d, J = 9.2 Hz, 1 H, ArH) ppm. 13C NMR
cedure described above, heating for 1 h. Column chromatography
(100.6 MHz, DMSO): δ = 27.43 [C(CH3)3], 48.24 (3-CH2), 50.23 using diethyl ether/petroleum ether (4:1) gave (Z)-8 (161 mg, 78%)
(NHCH2), 51.93 (OCH3), 57.54 (2-CH), 82.27 [OC(CH3)3], 123.63 as a white solid; m.p. 183.0–184.0 °C (from diethyl ether/petroleum
1
(CH), 123.98 (C), 124.07 (C), 124.51 (CH), 124.92 (CH), 124.98 ether). H NMR (400 MHz, CDCl3): δ = 1.26 (s, 9 H, CH3 Boc),
(CH), 126.09 (CH), 126.75 (CH), 126.86 (CH), 127.06 (CH), 127.39 2.47 (s, 3 H, γCH3), 3.98 (s, 3 H, OCH3), 5.51 (br. s, 1 H, NH),
(CH), 128.50 (C), 129.85 (C), 130.34 (C), 130.77 (C), 134.45 (C),
151.70 (C=O), 170.05 (C=O) ppm. C31H36N2O6 (532.63): calcd. C NMR (100.6 MHz, CDCl3): δ = 21.54 (γCH3), 27.97 [C(CH3)3],
7.82 (d, J = 6.8 Hz, 1 H, ArH), 8.03–8.23 (m, 8 H, ArH) ppm. 13C
69.90, H 6.81, N 5.26; found C 69.86, H 6.76, N 5.21.
52.12 (OCH3), 80.67 [OC(CH3)3], 123.83 (CH), 124.76 (C), 124.96
(C), 125.19 (CH), 125.35 (CH), 125.49 (CH), 126.08 (C), 126.21
(CH), 127.10 (C), 127.17 (CH), 127.82 (CH), 128.50 (CH), 130.93
(C), 131.95 (C), 131.21 (C), 132.14 (C), 134.53 (C), 153.00 (C=O),
165.68 (C=O) ppm. HRMS (EI): calcd. for C26H25NO4 415.17836;
found 415.17843.
Synthesis of Methyl (E)-2-(tert-Butoxycarbonylamino)-3-[(pyren-1-
yl)methylamino]acrylate [(E)-4]: (Pyren-1-yl)methylamine hydro-
chloride (0.550 mmol, 147 mg) and triethylamine (1 mmol) were
added to a solution of (E)-3a (0.500 mmol, 134 mg) in methanol
(5 mL). The reaction mixture was stirred at room temperature for
48 h (the reaction was followed by 1H NMR). Ethyl acetate
(50 mL) was added to the reaction mixture and the organic phase
washed with water and brine (2ϫ20 mL). The organic layer was
dried with MgSO4 and the solvent removed under reduced pressure
to give (E)-4 (181 mg, 84%) as a white solid; m.p. 151.0–152.0 °C
(from diethyl ether/n-hexane). 1H NMR (400 MHz, DMSO): δ =
Synthesis of Methyl (E)-2-(tert-Butoxycarbonylamino)-3-(pyren-1-
yl)but-2-enoate [(E)-8]: Compound (E)-8 was prepared from com-
pound (E)-6 (0.250 mmol, 73.5 mg) according to the general pro-
cedure described above, heating for 1 h. Column chromatography
using diethyl ether/petroleum ether (1:4) gave (E)-8 (100 mg, 96%)
as a white solid; m.p. 205.0–206.0 °C (from diethyl ether/petroleum
1
1.40 (s, 18 H, CH3 Boc), 3.49 (s, 3 H, OCH3), 5.08 (br. s, 2 H, CH2), ether). H NMR (400 MHz, CDCl3): δ = 1.60 (s, 9 H, CH3 Boc),
7.12–7.50 (m, 3 H, ArH + NH), 8.06–8.29 (m, 9 H, ArH) ppm. 13
NMR (100.6 MHz, DMSO): δ = 28.24 [C(CH3)3], 47.91 (CH2),
C
2.37 (s, 3 H, γCH3), 3.18 (s, 3 H, OCH3), 6.35 (br. s, 1 H, NH),
7.77 (d, J = 7.6 Hz, 1 H, ArH), 8.01 (t, J = 7.6 Hz, 1 H, ArH),
50.22 (OCH3), 77.72 [OC(CH3)3], 96.78 (C), 123.04 (CH), 123.88 8.07–8.20 (m, 7 H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3): δ
(C), 123.99 (C), 124.77 (CH), 125.15 (CH), 125.25 (CH), 126.08 = 23.34 (γCH3), 28.31 [C(CH3)3], 51.61 (OCH3), 80.76 [OC-
(CH), 126.24 (CH), 127.03 (CH), 127.38 (CH), 127.55 (CH), 127.71 (CH3)3], 124.50 (CH), 124.61 (CH), 124.66 (CH), 124.83 (C),
(C), 130.12 (C), 130.29 (C), 130.78 (C), 133.57 (C), 146.25 (CH),
124.95 (CH), 125.02 (CH), 125.26 (C), 125.90 (CH), 127.20 (CH),
155.16 (C=O), 166.41 (C=O) ppm. C26H26N2O4 (430.50): calcd. C 127.37 (CH), 127.58 (C), 127.66 (CH), 130.42 (C), 131.00 (C),
72.54, H 6.09, N 6.51; found C 71.92, H 5.64, N 6.69.
131.31 (C), 136.93 (C), 141.81 (C), 153.46 (C=O), 165.06
(C=O) ppm. C26H25NO4 (415.48): calcd. C 75.16, H 6.06, N 3.37;
found C 75.19, H 6.16, N 3.45.
Synthesis of Methyl (E)-2-(tert-Butoxycarbonylamino)-3-[(pyren-1-
yl)methylamino]but-2-enoate [(E)-5]: (Pyren-1-yl)methylamine hy-
drochloride (0.55 mmol, 147 mg) and triethylamine (1 mmol) were
added to a solution of (E)-3b (0.500 mmol, 141 mg) in methanol
(5 mL). The reaction mixture was stirred at room temperature for
Synthesis of Methyl (Z)-2-(tert-Butoxycarbonylamino)-3-phenyl-3-
(pyren-1-yl)acrylate [(Z)-9]: Compound (Z)-9 was prepared from
compound (Z)-7 (0.250 mmol, 89.0 mg) according to the general
procedure described above, heating for 1.5 h. Column chromatog-
raphy using diethyl ether/petroleum ether (1:1) gave (Z)-9 (110 mg,
92%) as a white solid; m.p. 175.0–176.0 °C (from diethyl ether/pe-
troleum ether). 1H NMR (400 MHz, CDCl3): δ = 1.24 (s, 9 H, CH3
Boc), 3.72 (s, 3 H, OCH3), 5.71 (br. s, 1 H, NH), 7.24–7.28 (m, 5
1
48 h (the reaction was followed by H NMR spectroscopy). Ethyl
acetate (50 mL) was added to the reaction mixture and the organic
phase washed with water and brine (2ϫ20 mL). The organic layer
was dried with MgSO4 and the solvent removed under reduced
pressure to give (E)-5 (100 mg, 45%) as a white solid; m.p. 159.0–
160.0 °C (from diethyl ether/n-hexane). 1H NMR (400 MHz, H, ArH), 7.75 (d, J = 7.6 Hz, 1 H, ArH), 8.03–8.27 (m, 8 H,
Eur. J. Org. Chem. 2008, 5697–5703
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5701

P. M. T. Ferreira et al.
FULL PAPER
ArH) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 27.99 [C(CH3)3],
diethyl ether/petroleum ether (1:4) afforded Boc--Phe-E-∆Abu(β-
52.25 (OCH3), 81.27 [OC(CH3)3], 124.32 (CH), 124.68 (C), 125.05 Br)-OMe [(E)-10] as a white solid; m.p. 119.0–119.5 °C (from di-
1
(C), 125.11 (CH), 125.48 (CH), 125.65 (CH), 126.26 (CH), 127.17
(CH), 127.82 (CH), 128.04 (CH), 128.14 (CH), 128.19 (2ϫCH),
ethyl ether/n-hexane). H NMR (300 MHz, CDCl3): δ = 1.40 (s, 9
H, CH3 Boc), 2.21 (s, 3 H, γCH3 ∆Abu), 3.01–3.17 (m, 2 H, βCH2
128.45 (C), 128.67 (2ϫCH), 128.75 (CH), 130.90 (C), 131.20 (C), Phe), 3.77 (s, 3 H, OCH3), 4.41–4.45 (m, 1 H, αCH Phe), 5.12 (s,
131.33 (C), 132.68 (C), 140.06 (C), 152.62 (C=O), 166.41 1 H, NH), 7.21–7.34 (m, 5 H, ArH), 7.86 (s, 1 H, NH) ppm. 13C
(C=O) ppm. C31H27NO4 (477.55): calcd. C 77.97, H 5.70, N 2.93;
found C 77.60, H 5.76, N 3.03.
NMR (75.4 MHz, CDCl3): δ = 25.53 (γCH3 ∆Abu), 28.18
[(CH3)3C], 37.36 (βCH2 Phe), 52.36 (OCH3), 55.47 (αCH Phe),
80.78 [(CH3)3C], 122.97 (C), 125.58 (CH), 127.05 (CH), 128.72
(CH), 129.27 (CH), 136.20 (C), 155.82 (C=O), 163.82 (C=O),
169.69 (C=O) ppm. C19H25BrN2O5 (441.32): calcd. C 51.71, H
5.71, N 6.35; found C 51.34, H 5.65, N 6.69.
Synthesis of Methyl (E)-2-(tert-Butoxycarbonylamino)-3-phenyl-3-
(pyren-1-yl)acrylate [(E)-9]: Compound (E)-9 was prepared from
compound (E)-7 (0.400 mmol, 145 mg) according to the general
procedure described above, heating for 2 h. Column chromatog-
raphy using diethyl ether/petroleum ether (1:2) gave (E)-9 (140 mg,
72%) as a white solid; m.p. 200.0–201.0 °C (from diethyl ether/pe-
troleum ether). 1H NMR (400 MHz, CDCl3): δ = 1.50 (s, 9 H, CH3
Boc), 3.14 (s, 3 H, OCH3), 6.47 (br. s, 1 H, NH), 7.27–7.37 (m, 5
H, ArH), 7.85 (d, J = 8.0 Hz, 1 H, ArH), 7.98–8.19 (m, 8 H,
ArH) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 28.19 [C(CH3)3],
51.81 (OCH3), 81.40 [OC(CH3)3], 124.43 (CH), 124.66 (C), 124.92
(C), 125.05 (CH), 125.14 (CH), 125.21 (CH), 126.01 (CH), 127.35
(CH), 127.61 (CH), 127.67 (CH), 127.73 (CH), 127.79 (C), 128.25
(2ϫCH), 128.83 (2ϫCH), 129.26 (CH), 129.33 (C), 130.88 (C),
131.01 (C), 131.26 (C), 132.98 (C), 134.65 (C), 138.93 (C), 152.96
(C=O), 166.06 (C=O) ppm. HRMS (EI): calcd. for C31H27NO4
477.1940; found 477.1943.
Boc-Phe-Z-∆Abu(β-Br)-OMe [(Z)-10] was also isolated as white so-
lid; m.p. 109.0–110.0 °C. 1H NMR (300 MHz, CDCl3): δ = 1.41 (s,
9 H, CH3 Boc), 2.54 (s, 3 H, γCH3 ∆Abu), 3.05–3.19 (m, 2 H,
βCH2 Phe), 3.80 (s, 3 H, OCH3), 4.47–4.49 (m, 1 H, αCH Phe),
4.98 (d, J = 6.6 Hz, 1 H, NH), 7.22–7.35 (m, 5 H, ArH), 7.77 (s, 1
H, NH) ppm. 13C NMR (75.4 MHz, CDCl3): δ = 24.61 (γCH3
∆Abu), 28.21 [(CH3)3C], 37.35 (βCH2 Phe), 52.55 (OCH3), 55.22
(αCH Phe), 80.58 [(CH3)3C], 124.41 (C), 126.57 (C), 127.03 (CH),
128.71 (CH), 129.33 (CH), 136.12 (C), 155.47 (C=O), 162.75
(C=O), 169.64 (C=O) ppm. C19H25BrN2O5 (441.32): calcd. C
51.71, H 5.71, N 6.35; found C 51.73, H 5.60, N 6.64.
Synthesis of Compound (E)-12: Compound (E)-12 was prepared
from compound (E)-10 (0.250 mmol, 110 mg) according to the ge-
neral procedure described above, heating for 1 h. Column
chromatography using diethyl ether/petroleum ether (1:1) gave (E)-
12 (113 mg, 80%) as a white solid; m.p. 129.5–130.5 °C (from di-
ethyl ether/petroleum ether). 1H NMR (400 MHz, CDCl3): δ = 1.48
(s, 9 H, CH3 Boc), 2.19 and 2.22 (2 s, 3 H, γCH3), 3.15 (s, 3 H,
OCH3), 3.15–3.33 (m, 2 H, βCH2), 4.59–4.64 (m, 1 H, αCH), 5.13
(br. s, 1 H, NH), 7.30–7.40 (m, 5 H, ArH), 7.72–7.81 (m, 2 H,
ArH), 7.99–8.20 (m, 8 H, ArH) ppm. 13C NMR (100.6 MHz,
CDCl3): δ = 23.51 (γCH3), 28.28 [C(CH3)3], 37.79 (CH2), 51.71
(OCH3), 56.03 (CH), 80.63 [OC(CH3)3], 124.39 (C), 124.48 (CH),
124.56 (CH), 124.59 (CH), 124.67 (C), 124.81 (C), 125.04 (CH),
125.08 (CH), 125.94 (CH), 127.09 (CH), 127.29 (CH), 127.37 (CH),
127.49 (C), 127.54 (C), 127.82 (CH), 128.79 (CH), 129.43 (CH),
130.52 (C), 131.02 (C), 131.32 (C), 136.57 (C), 143.73 (C), 155.77
(C=O), 164.38 (C=O), 170.05 (C=O) ppm. HRMS (ESI): calcd. for
C35H35N2O5 [M + H]+ 563.2546; found 563.2541.
Synthesis of Boc-L-Phe-Z-∆Abu-OMe: DMAP (0.1 equiv.) was
added followed by di-tert-butyl dicarbonate (1.0 equiv.) to a solu-
tion of Boc--Phe--Thr-OMe (2.00 mmol, 0.67 g) in dry acetoni-
trile (1 moldm–3) under rapid stirring at room temperature. The
reaction was monitored by TLC (diethyl ether/n-hexane, 1:1) until
all the reactant had been consumed. Then, 2% in volume of TMG
was added and stirring was continued and the reaction followed by
TLC. When all the reactant had been consumed, evaporation at
reduced pressure gave a residue that was partitioned between di-
ethyl ether (100 mL) and KHSO4 (1 moldm–3, 30 mL). The organic
phase was thoroughly washed with KHSO4 (1 moldm–3), NaHCO3
(1 moldm–3) and saturated brine (2ϫ30 mL each), and dried with
MgSO4. Removal of the solvent afforded Boc--Phe-Z-∆Abu-OMe
(660 mg, 91%) as a white solid; m.p. 93.0–94.0 °C (from diethyl
ether/n-hexane). 1H NMR (300 MHz, CDCl3): δ = 1.40 (s, 9 H,
CH3 Boc), 1.68 (d, J = 6.9 Hz, 3 H, γCH3 ∆Abu), 3.03–3.22 (m, 2
H, βCH2 Phe), 3.73 (s, 3 H, OCH3), 4.49–5.13 (m, 1 H, αCH Phe),
5.12 (d, J = 7.5 Hz, 1 H, NH), 6.79 (q, J = 6.9 Hz, 1 H, βCH
∆Abu), 7.21–7.33 (m, 5 H, ArH), 7.50 (s, 1 H, NH) ppm. 13C NMR
(75.4 MHz, CDCl3): δ = 14.47 (γCH3 ∆Abu), 28.18 [(CH3)3C],
37.96 (βCH2 Phe), 52.26 (OCH3), 55.82 (αCH Phe), 80.35
[(CH3)3C], 125.70 (C), 126.92 (CH), 128.62 (CH), 129.34 (CH),
134.54 (CH), 136.39 (C), 155.50 (C=O), 164.60 (C=O), 169.74
(C=O) ppm. C19H26N2O5 (362.42): calcd. C 62.97, H 7.23, N 7.73;
found C 62.84, H 7.33, N 7.56.
Synthesis of Compound (Z)-13: Compound (Z)-13 was prepared
from compound (Z)-11 (0.300 mmol, 124 mg) according to the ge-
neral procedure described above, heating for 1.5 h. Column
chromatography using diethyl ether/petroleum ether (1:1) gave (Z)-
13 (130 mg, 81%) as a white solid; m.p. 222.5–223.0 °C (from di-
1
ethyl ether/n-hexane). H NMR (300 MHz, CDCl3): δ = 0.93 (s, 9
H, CH3 Boc), 3.44–3.51 (m, 2 H, CH2 Gly), 3.69 (s, 3 H, OCH3),
4.68 (br. s, 1 H, NH), 7.23–7.29 (m, 6 H, ArH + NH), 7.77 (d, J
= 7.8 Hz, 1 H, ArH), 8.04–8.25 (m, 8 H, ArH) ppm. 13C NMR
(100.6 MHz, CDCl3): δ = 27.63 [C(CH3)3], 43.96 (CH2), 52.35
(OCH3), 79.90 [OC(CH3)3], 124.13 (CH), 124.65 (C), 124.98 (C),
125.11 (CH), 125.65 (CH), 125.75 (CH), 126.31 (CH), 126.68 (C),
127.17 (CH), 127.92 (CH), 128.14 (CH), 128.25 (CH), 128.27 (CH),
128.41 (C), 128.64 (CH), 128.84 (CH), 130.76 (C), 131.21 (C),
131.57 (C), 131.95 (C), 133.53 (C), 139.44 (C), 155.28 (C=O),
165.99 (C=O), 167.90 (C=O) ppm. C33H30N2O5 (534.60): calcd. C
74.14, H 5.66, N 5.24; found C 73.90, H 5.47, N 5.24.
Synthesis of Boc-L-Phe-∆Abu(β-Br)-OMe (10): Boc--Phe-Z-∆Abu-
OMe (1 mmol, 0.362 g) was dissolved in dichloromethane
(0.1 moldm–3) and N-bromosuccinimide (2.5 equiv.) was added
with vigorous stirring. After reacting for 16 h, triethylamine
(1.5 equiv.) was added and stirring was continued for 30 min. The
solvent was then evaporated at reduced pressure and the residue
partitioned between dichloromethane (100 mL) and KHSO4
(1 moldm–3, 50 mL). The organic phase was washed with KHSO4
(1 moldm–3), NaHCO3 (1 moldm–3) and brine (3ϫ30 mL each).
After drying with MgSO4 the extract was taken to dryness at re-
duced pressure to give Boc--Phe-∆Abu(β-Br)-OMe (397 mg, 90%)
as a 1:1 mixture of E and Z isomers. Column chromatography in
Acknowledgments
We thank the Portuguese Foundation for Science and Technology
(FCT) and the European Communautaire Fund (FEDER) for fin-
5702
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 5697–5703

Pyrenylamino Acids
[7]
ancial support through the Centro de Química (POCTI-SFA-3-686)
and the Centro de Física of University of Minho (POCI/QUI/
59407/2004) and for a post-doctoral grant awarded to A. S. A.
(SFRH/BPD/24548/2005).
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Published Online: October 22, 2008
Eur. J. Org. Chem. 2008, 5697–5703
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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