Electrochemical reduction of dehydroamino acids: Synthesis and photophysical properties of β,β-diarylalanines

Article abstract of DOI:10.1016/j.tet.2010.10.087

Several β,β-disubstituted dehydroalanines were prepared from β,β-dibromo or β-bromo, β-substituted dehydroalanines and aryl boronic acids using a Suzuki-Miyaura cross-coupling reaction. The electrochemical behaviour of these compounds was studied by cyclic voltammetry. All compounds studied showed similar reduction potentials and these were similar to the peak potential of the methyl ester of N-tert-butoxycarbonyl dehydrophenylalanine. Thus, the presence of a second aryl moiety in the dehydroalanine scaffold does not significantly change the reduction potential. Controlled potential electrolyses were performed at the cathodic peak potential in the presence of triethylammonium chloride as proton donor. The only products isolated in good to high yields were the corresponding β,β- diarylalanines. This reaction was also carried out using a dipeptide containing a β,β-diaryldehydroalanine to give a 1:1 diastereomeric mixture of the reduction product. The photophysical properties of two of the β,β-diaryldehydroalanines and of the corresponding β,β-diarylalanines were studied in three solvents of different polarity. The β,β-diaryldehydroalanines show low fluorescent quantum yields (Φ_F<9%) due to the conjugation of the aromatic moieties with the α,β-double bond and with the carbonyl group, which favours the non-radiative deactivation pathways. The absence of conjugation in the reduction products leads to a significant increase in the fluorescence quantum yields. These results show that the β,β-disubstituted alanines could be used as fluorescent markers.

Full text of DOI:10.1016/j.tet.2010.10.087

Tetrahedron 67 (2011) 193e200
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Electrochemical reduction of dehydroamino acids: synthesis and photophysical
properties of
b
,b
-diarylalanines
,
a
,
b
a
a
Paula M.T. Ferreira a , Luís S. Monteiro , Elisabete M.S. Castanheira , Goreti Pereira , Carla Lopes ,
*
*
Helena Vilac¸ a a
a Chemistry Centre (CQ-UM), University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
b Centre of Physics (CFUM), University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2010
Received in revised form 20 October 2010
Accepted 29 October 2010
Available online 5 November 2010
Several b,b-disubstituted dehydroalanines were prepared from b,b-dibromo or b-bromo, b-substituted
dehydroalanines and aryl boronic acids using a SuzukieMiyaura cross-coupling reaction. The electro-
chemical behaviour of these compounds was studied by cyclic voltammetry. All compounds studied
showed similar reduction potentials and these were similar to the peak potential of the methyl ester of
N-tert-butoxycarbonyl dehydrophenylalanine. Thus, the presence of a second aryl moiety in the dehy-
droalanine scaffold does not significantly change the reduction potential.
Keywords:
Dehydroamino acids
Reduction
Cyclic voltammetry
Electrolysis
b,b-diarylalanines
Controlled potential electrolyses were performed at the cathodic peak potential in the presence of
triethylammonium chloride as proton donor. The only products isolated in good to high yields were the
corresponding
diaryldehydroalanine to give a 1:1 diastereomeric mixture of the reduction product.
The photophysical properties of two of the -diaryldehydroalanines and of the corresponding
arylalanines were studied in three solvents of different polarity. The -diaryldehydroalanines show low
fluorescent quantum yields ( F<9%) due to the conjugation of the aromatic moieties with the -double
b,b-diarylalanines. This reaction was also carried out using a dipeptide containing a b,b-
b
,b
b,b-di-
b,b
F
a,b
bond and with the carbonyl group, which favours the non-radiative deactivation pathways. The absence
of conjugation in the reduction products leads to a significant increase in the fluorescence quantum
yields. These results show that the
b
,b
-disubstituted alanines could be used as fluorescent markers.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
For some years now we have been interested in the synthesis of
dehydroamino acid derivatives and in using these compounds as
substrates in several types of reactions.3 As part of this work we
have been studying the electrochemical behaviour of dehy-
droamino acid derivatives. Thus, it was possible to develop an ef-
ficient method for the synthesis of 2,5-diaminoadipic acid
derivatives by controlled potential electrolysis of N,N-diprotected
dehydroalanines.4 Recently we investigated the electrochemical
The reduction of an isolated double bond is not possible using
electrochemical methods, however when the double bond is con-
jugated with an electron-withdrawing group, such as a carbonyl it
becomes reducible in the accessible potential range.1 This can be
explained by the fact that conjugation lowers the energy of the
lowest unoccupied
duction potential. There are a few reports on the electrochemical
reduction of activated alkenes, such as
-unsaturated ketones.2 In
p-orbital, which results in a less negative re-
behaviour of
b-halodehydroamino acids and found that these
a,
b
compounds show higher reduction potentials when compared with
those of the corresponding dehydroamino acids.5 Controlled po-
tential electrolysis of the former afforded the E-isomers of the
dehalogenated dehydroamino acids. Although there are many re-
these studies it was demonstrated that the electrochemical re-
duction of unsaturated ketones involves an initial electron transfer
to the ketone producing a radical anion, which undergoes pro-
tonation followed by other chemical or electrochemical processes.2
Thus, the radical anion may dimerize or it can be reduced to give
the saturated ketone.1,2
ports concerning the chemical reduction of
acids,6 to the best of our knowledge the electrochemical reduction
of -disubstituted dehydroamino acids has not yet been de-
a,b-dehydroamino
b,b
scribed. Since dehydroamino acids can be considered activated
alkenes we decided to study the electrochemical reduction of
* Corresponding authors. Tel.: þ0 351 253 604055; fax: þ0 351 253 604382;
e-mail addresses: pmf@quimica.uminho.pt (P.M.T. Ferreira), monteiro@quimica.u-
minho.pt (L.S. Monteiro).
b,
b-disubstituted dehydroalanines obtained by a SuzukieMiyaura
cross-coupling from -halogenated dehydroamino acids and aryl
b
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.10.087

194
P.M.T. Ferreira et al. / Tetrahedron 67 (2011) 193e200
Table 1
boronic acids. The photophysical properties of some of the satu-
rated amino acids prepared were evaluated in solvents of different
Cathodic peak potentials obtained by cyclic voltammetry of
b,b-diary-
ldehydroalanine derivativesa
polarity and compared with those of the corresponding
diaryldehydroalanines.
b
,b
-
Compound
ꢀEp (V vs SCE)
H
N
CO2CH3
2. Results and discussion
Boc
4a
1.81
Several
b,b-diaryldehydroalanines were prepared from a b,b
-
dibromoalanine7 or from a -bromodehydrophenylalanine3d and aryl
b
boronic acids by a SuzukieMiyaura coupling reaction (Scheme 1).8
H
H
N
CO2CH3
R1
N
CO2CH3
Boc
Boc
R1
4b
1.75
1
H
N
4a-d
CO2CH3
R
R1-B(OH)2
Pd cat.
P
Br
H
N
H
N
P=Boc, R=Br; 1
P=Boc, R=Ph; 2
CO2CH3
Ph
2, 3
CO2CH3
P
Boc
P=Boc-L-Phe, R=Ph; 3
R1
4c8
1.80
P=Boc; 5c
P=Boc- -Phe; 6a
L
R1 =
c;
a;
b;
d.
H
N
CO2CH3
Boc
4d8
1.76
Scheme 1.
Cathodic peak potentials of compounds 4aed, 5c and 6a were
measured by cyclic voltammetry at a vitreous carbon electrode in
dimethylformamide containing 0.1 mol dmꢀ3 of Bu4NBF4 as sup-
porting electrolyte (Table 1). All cyclic voltammetry plots were
irreversible in sweep rates between 0.1 and 2.0 V sꢀ1, which may be
due to a rapid chemical reaction of the radical anion formed in
the first electron transfer. An identical feature was reported for the
reduction of 1,3-diphenyl-2-propen-1-ones (chalcones).2 From the
cathodic peak potentials presented in Table 1 it is possible to
observe that all compounds studied have similar reduction poten-
tials. Also when these potentials are compared with that of the
methyl ester of N-tert-butoxycarbonyl dehydrophenylalanine
H
N
CO2CH3
Boc
5c8
1.88
(Boce
D
PheeOMe, ꢀ1.84 V vs SCE)4 it can be concluded that the
H
N
6a
CO2CH3
1.83
Boc
N
presence of a second aryl moiety at the b-carbon atom does not
significantly influence the reduction potential.
The next step in our investigation was to attempt the electro-
chemical reduction of the a,b-double bond in the dehydroamino
acid framework. Thus, compounds 4aed, 5c and 6a were sub-
mitted to controlled potential electrolysis in acetonitrile using
Et4NCl (0.1 mol dmꢀ3) as supporting electrolyte and Et3NHCl
(0.04 mol dmꢀ3) as proton donor (Scheme 2). It was found that the
H
O
Ph
Ph
a
Cathode: vitreous carbon. Solvent: dimethylformamide. Supporting electrolyte:
Bu4NBF4 0.1 mol dmꢀ3. Substrate concn: z0.005 mol dmꢀ3
.
H
H
N
only products isolated were the corresponding
(7aed, 8c and 9a) in good to high yields (Table 2). This result is
different from those reported for -unsaturated ketones and
N,N-diprotected dehydroalanines. In the case of -unsaturated
b,b-diarylalanines
N
CO2CH3
R1
CO2CH3
R1
P
-Ep
P
a,b
R
R
a,b
P=Boc; 4a-d, 5c.
P=Boc-L-Phe; 6a.
P=Boc; 7a-d, 8c.
P=Boc-L-Phe; 9a.
ketones the reduction products include the saturated ketone and
a dimer.1,2 N,N-diprotected dehydroalanines give upon electro-
chemical reduction the corresponding diamino dicarboxylic acids.4
Scheme 2.

P.M.T. Ferreira et al. / Tetrahedron 67 (2011) 193e200
195
Table 2
corresponding dehydroalanines. Hence the absorption and emis-
sion properties of compounds 4b, 4c, 7b and 7c were studied in
three solvents of different polarity (cyclohexane, acetonitrile and
ethanol). The normalized absorption and fluorescence spectra of
the four compounds are presented in Figs. 1 and 2. The maximum
absorption (labs) and emission (lem) wavelengths, molar absorption
Results obtained in the controlled potential electrolysis of
b
,
b
-diaryldehydroalanine
derivativesa
Substrate
Product
Yield(%)
85
4a
Methyl 2-(tert-butoxycarbonylamino)-
3,3-diphenylpropanoate, 7a
4b
4c
4d
Methyl 3,3-di(biphenyl-4-yl)-2-(tert-
butoxycarbonylamino)propanoate, 7b
Methyl 2-(tert-butoxycarbonylamino)-
3,3-di(naphthalen-1-yl)propanoate, 7c
Methyl 2-(tert-butoxycarbonylamino)-
79
81
78
coefficients (3) and fluorescence quantum yields (FF) of the four
compounds are presented in Table 3.
3,3-bis(1,2-dihydroacenaphthylen-5-yl)
propanoate, 7d
5c
6a
Methyl 2-(tert-butoxycarbonylamino)-
76
3-(naphthalen-1-yl)-3-phenylpropanoate, 8c
Methyl 2-[(S)-2-(tert-butoxycarbonyl
amino)-3-phenylpropanamido]-3,3-
diphenylpropanoate, 9a
86
a
Cathode: vitreous carbon. Solvent: acetonitrile. Supporting electrolyte: Et4NCl
(0.1 mol dmꢀ3); Et3NHCl (0.04 mol dmꢀ3).
1H NMR spectra of compounds 7aed clearly show the presence
of two signals, one doublet between 4.38 and 6.12 ppm and a broad
triplet between 5.08 and 5.31 ppm, originated from the b- and a-CH
protons of the alanine derivatives. Also in the 13C NMR spectra it is
possible to observe the presence of two signals between 42.40 and
57.01 ppm that correspond to the a-C and b-C atoms of the satu-
rated system. In the case of compounds 5c and 6a, 1:1 di-
astereomeric mixtures were obtained showing that, as expected,
the reaction is not stereoselective.
The results obtained are consistent with the mechanism proposed
for the reduction of
saturated ketones in protic media.1 According to this mechanism the
first step in the reduction of the -diaryldehydroalanines would be
a,b-unsaturated ketones to give the corresponding
b,b
the transfer of an electron to give the radical anion. This radical rapidly
protonates and tautomerizes to give an alkyl radical, that is, more
easily reduced than the reagent and is converted into the
lalanine (Scheme 3). Although reported for -unsaturated ketones
and esters, we did not detect the formation of dimers. This can be
attributed to the sterical hindrance at the -carbon atom of the
b,b-diary-
a,b
b
b,b-
disubstituted dehydroalanines, as reported by other authors for the
reduction of norbornenedicarboxylate.9
O
O
H
H
N
N
Fig. 1. Normalized absorption (in the lowest energy band) and fluorescence spectra
(
lexc¼300 nm) of solutions (2ꢂ10ꢀ5 mol dmꢀ3 for absorption and 2ꢂ10ꢀ6 mol dmꢀ3 for
Boc
Boc
OCH3
e-
Boc
OCH3
fluorescence) of compounds 4b and 4c in several solvents.
R
R1
H+
R
R1
O
These compounds present high molar absorption coefficients at
the lowest energy band (
solvents studied (Table 3), pointing to
3
ꢁ1.1ꢂ104 molꢀ1 dm3 cmꢀ1) in the three
OH
p/p transitions. These 3
*
H
H
N
N
values are significantly lower for compounds 4c and 7c than for 4b
and 7b, due to the different chromophore groups present (two
naphthyl groups in the former case and two biphenyl groups in the
latter). In fact, biphenyl has a higher molarabsorptioncoefficientthan
naphthalene (z16,000 Mꢀ1 cmꢀ1 at 247.5 nm and z6000 Mꢀ1 cmꢀ1
at 275 nm, respectively).10,11 A methyl ester group is present in all
compounds and it is known that many carbonyl compounds present
Boc
OCH3
OCH3
R1
R
R1
R
e-/H+
O
H
a low-lying n/
p
*
state. The
p/p
*
and n/
p electronic transitions
*
N
canbenearbyinenergy, resultinginstate-mixing.12 Thehighvaluesof
Boc
OCH3
the molar absorption coefficient can be explained by a predominance
R
R1
of
p/p
*
character in these compounds (Table 3).
The influence of the solvent in the absorption spectral shape is
negligible for all compounds. The absence of the -double bond in
Scheme 3.
a,b
Considering that the compounds prepared are
b
,
b
-diarylamino
compounds 7b and 7c, which does not allow conjugation between
the aromatic moieties and with the carbonyl group, causes a shift of
the lowest energy absorption band to lower wavelengths, when
compared with 4b and 4c, respectively (Figs. 1 and 2).
acids that could be used as fluorescent markers, we decided to
study the photophysical properties of some of the electrolysis
products. The results obtained were compared with those of the

196
P.M.T. Ferreira et al. / Tetrahedron 67 (2011) 193e200
and the aromatic moieties as acceptors. A strong excited state CT
was observed in pyrene-dimethylaniline derivatives.14,15 However,
in these compounds, the electron-withdrawing properties of the
tert-butoxycarbonyl and of the methyl ester groups may contribute
to a decrease in the CT character.
From ab initio molecular quantum chemistry calculations,
obtained with Gaussian 09 software16 and use of a 3-21þG(d) basis
set,17 the optimized geometries of compounds 4b, 4c, 7b and 7c
were obtained (Fig. 3). In compounds with the a,b-double bond (4b,
4c) the two aromatic substituents are roughly perpendicular to
Fig. 3. Optimized geometries of compounds 4b, 4c, 7b and 7c obtained by Gaussian 09
software (grey: C atoms; white: H atoms; red: O atoms; blue: N atoms). For in-
terpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.
Fig. 2. Normalized absorption (in the lowest energy band) and fluorescence spectra
(
fluorescence) of compounds 7b and 7c in several solvents.
lexc¼270 nm) of solutions (2ꢂ10ꢀ5 mol dmꢀ3 for absorption and 2ꢂ10ꢀ6 mol dmꢀ3 for
Table 3
Maximum absorption (labs) and emission wavelengths (lem), molar absorption coefficients (
3
) and fluorescence quantum yields (FF) for compounds 4b, 4c, 7b and 7c
/104 Mꢀ1 cmꢀ1
)
lem (nm)
FF
a
Solvent
labs (nm) (
3
4b
4c
7b
7c
4b
4c
7b
7c
4b
4c
7b
7c
Cyclohexane
296 (0.87)sh
285 (1.12)
274 (0.90) sh
222 (8.27)
283 (3.21)
203 (6.38)
298 (1.47)
216 (7.90)
260 (3.32)
206 (6.39)
368
382 sh
363
318
328
339
0.067
0.025
0.043
0.086
0.37
0.13
295 (0.90)sh
285 (1.12)
275 (0.91) sh
220 (8.52)
Acetonitrile
281 (3.41)
204 (6.06)
297 (1.54)
216 (8.39)
262 (3.45)
203 (6.93)
368
385 sh
367
365
318
319
328
339
0.044
0.035
0.32
0.40
0.15
0.23
295 (0.91)sh
285 (1.14)
275 (0.93) sh
220 (8.45)
Ethanol
281 (3.09)
201 (6.87)
298 (1.51)
216 (8.31)
261 (3.41)
205 (6.56)
369
387 sh
327
339
a
Relative to anthracene in ethanol (
F
¼0.27 at 25 ꢃC)13 for 4b and 4c; relative to naphthalene in cyclohexane (FF¼0.23 at 25 ꢃC)11 for 7b and 7c; sh: shoulder.
The emission bands of the
(4b and 4c) are located at lower energies than the corresponding
-diarylalanine compounds (7b and 7c). A band enlargement and
loss of vibrational structure is observed for 4b and 4c and also for
compound 7b (Figs. 1 and 2).
Compounds 4b and 4c present low fluorescence quantum yields
F<9%, Table 3). The conjugation of the dehydroamino acid double
bond with the aromatic electrons of the -systems (two biphenyl
groups in 4band twonaphthyl moieties inthe caseof 4c)and withthe
b
,
b
-disubstituted dehydroalanines
each other. In compounds 7b and 7c there is a change in the relative
position of the two aromatic moieties.
b
,
b
Figs. 4 and 5 display the representation of HOMO and LUMO
molecularorbitals for the same fourcompounds. In both compounds
4b and 4c (Fig. 4), the HOMOeLUMO transition causes a significant
increase in the electronic density of the carbonyl groupof the methyl
carboxylate substituent, more significant for 4c. It can also be ob-
served that the tert-butoxycarbonyl moiety does not participate in
the HOMOeLUMO transition of these two compounds.
(F
p
carbonyl group favours the non-radiative deactivation pathways. For
the b,b-diarylalanines 7b and 7c the fluorescence quantum yield
In compound 4c, the HOMOeLUMO transition implies notable
differences in the electronic density of the naphthyl substituents
(Fig. 4, below), with a charge transfer from one naphthyl to the
other and a decrease in the electronic density of the nitrogen atom,
confirming the significant charge transfer (CT) character of the
excited state. This can justify the complete loss of vibrational
structure in the fluorescence emission of compound 4c, together
values increase significantly, due to the suppression of the conjuga-
tion, especially for compound 7b. Compound 7c exhibits absorption
and emission spectra that roughly resemble those of naphthalene.11
Some charge transfer (CT) character of the excited state is pos-
sible to occur, the conjugated NH group acting as the electron donor

P.M.T. Ferreira et al. / Tetrahedron 67 (2011) 193e200
197
cathodic peak potentials showed that these compounds can be
considered activated alkenes and thus could be electrochemically
reduced. Controlled potential electrolysis in the presence of an
excess of a proton donor afforded the corresponding
lalanines in good to high yields. The electrochemical reduction of
the -double bond of dehydroamino acids constitutes an alter-
b,b-diary-
a,b
native to the conventional chemical methods, which use metals as
catalysts.
The photophysical properties of two of the
b,b-disubstituted
dehydroalanines and of the corresponding reduction products
were studied. It was found that although the dehydroalanines
show low quantum yields their reduction products show reason-
ably high fluorescent yields and could be useful as fluorescent
markers.
In summary, we have developed a new methodology for the
synthesis of
b,b-diarylalanines from brominated dehydroalanines
using a sequential SuzukieMiyaura cross-coupling followed by the
electrochemical reduction of the a,b-double bond.
Fig. 4. Representation of HOMO and LUMO molecular orbitals of compounds 4b
(above) and 4c (below).
4. Experimental section
Melting points (ꢃC) were determined in a Gallenkamp apparatus
and are uncorrected. 1H and 13C NMR spectra were recorded on
a Varian Unity Plus at 300 and 75.4 MHz, respectively, or on
a Bruker Avance IIþ at 400 and 100.6 MHz, respectively. 1He1H
spinespin decoupling and DEPTq
45ꢃ were used. Chemical shifts
are given in parts per million and coupling constants in hertz. MS
and HRMS data were recorded by 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 Macher-
eyeNagel silica gel 230e400 mesh. Petroleum ether refers to the
boiling range 40e60 ꢃC. When solvent gradient was used, the in-
crease of polarity was made from neat petroleum ether to mixtures
of diethyl ether/petroleum ether, increasing 10% of diethyl ether
each time until the isolation of the product.
Fig. 5. Representation of HOMO and LUMO molecular orbitals of compounds 7b
(above) and 7c (below).
Cyclic voltammetry experiments were carried out using a Hi-
Tek potentiostat type DT 2101 and a Hi-Tek wave generator type
PPRl, connected to a Philips recorder type PM 8043 coupled to
a two-compartment, three-electrode, home-built glass cell. The
working electrode was a vitreous carbon disc (area 0.05 cm2)
made by sealing a rod (Sigri Elektrographit GmbH) into a glass
support and grinding the face of the disc. The counter electrode
was a platinum spiral and the reference electrode a mercury pool
separated from the working electrode by a Luggin capillary. For
controlled potential electrolysis a conventional three-compart-
ment, three-electrode, home-built cell made of PTFE was used.
The working and counter electrodes were vitreous carbon discs
from Sigri Elektrographit GmbH (area 4.9 cm2) placed in a holder
so that only one face was exposed to the solution. The working
and the secondary electrode compartments were separated by
a glass frit. The reference electrode was a mercury pool con-
nected to the working electrode compartment by a Luggin
capillary.
Spectroscopic measurements: All the solutions were prepared
using spectroscopic grade solvents. Absorption spectra were
recorded in a Shimadzu UV-3101PC UVevis-NIR spectrophotome-
ter. Fluorescence measurements were performed using a Fluorolog
3 spectrofluorimeter, equipped with double monochromators in
both excitation and emission and a temperature controlled cuvette
holder. Fluorescence spectra were corrected for the instrumental
response of the system.
with the red shift in polar solvents (4 nm between cyclohexane and
acetonitrile, Fig. 1). A similar behaviour (with a significant loss of
vibrational structure in the emission bands) was already observed
for another
b,b-diaryldehydroalanine, the methyl (Z)-2-[(tert-
butoxycarbonyl)-3-phenyl-3-pyren-1-yl]acrylate that exhibits
a strong conjugation between adjacent aromatic moieties.18
The HOMOeLUMO transition of compound 7b (Fig. 5, above)
causes strong changes in the electron density of several atoms of this
molecule. While the HOMO is completely located at both biphenyl
groups, upon HOMOeLUMO transition there is a charge transfer
from the biphenyls to the a-carbon and nitrogen atoms and also to
the carbonyl of Boc group. This charge transfer justifies the almost
complete loss of vibrational structure in fluorescence emission.
A completely different feature is observed for the HOMO and
LUMO orbitals of compound 7c (Fig. 5) that is completely localized
in the naphthyl moieties. For this reason, both absorption and
fluorescence spectra of this molecule are similar to those of
naphthalene (vd. Fig. 2).
The fluorescence studies reported here show that the latter two
compounds (7b and 7c), due to their reasonable fluorescence
quantum yields and distinct photophysical behaviour, can be useful
as fluorescent markers.
3. Conclusions
The electrochemical behaviour of several
hydroalanine derivatives was studied by cyclic voltammetry. The
b
,
b
-diarylde-
Fluorescence quantum yields: For fluorescence quantum yield
determination, the solutions were previously bubbled for 30 min

198
P.M.T. Ferreira et al. / Tetrahedron 67 (2011) 193e200
with ultrapure nitrogen. The fluorescence quantum yields (Fs) were
give 6a (176 mg, 78%) as a white solid. Mp 99.0e100.0 ꢃC (from
diethyl ether/petroleum ether). 1H NMR (400 MHz, CDCl3): 1.35 (s,
9H, CH3 Boc), 3.10 (d J¼6.4 Hz, 2H, CH2), 3.57 (s, 3H, OCH3),
4.33e4.39 (m, 1H, CH), 4.84 (br s, 1H, NH), 7.26e7.30 (m, 16H,
determined using the standard method (Eq. 1),19
ArFsn2
AsFrn2r
Fs
¼
s Fr
(1)
ArHþNH) ppm. 13C NMR (100.6 MHz, CDCl3):
¼28.17 [C(CH3)3],
d
37.68 (CH2), 52.07 (OCH3), 55.45 (CH), 80.38 [OC(CH3)3], 124.60 (C),
127.04 (CH), 128.10 (CH), 128.11 (CH), 128.59 (CH), 128.72 (CH),
128.74 (CH),129.14 (CH),129.48 (CH),129.75 (CH),136.18 (C),136.28
(C), 137.80 (C), 139.21 (C), 155.20 (C]O), 165.91 (C]O), 169.64 (C]
O) ppm. Anal. Calcd for C30H32N2O5 (500.59): C 71.98, H 6.44, N
5.60; found C 72.14, H 6.57, N 5.60.
where A is the absorbance at the excitation wavelength, F the
integrated emission area and n the refraction index of the sol-
vents used. Subscripts refer to the reference (r) or sample (s)
compound.
4.1. Synthesis of compounds 1, 2 and 3
4.3. General procedure for the controlled potential
electrolysis of b,b-diaryldehydroamino acids
The synthesis of compounds 1,7 23d and 35 was described
elsewhere.
A solution of Et4NCl (0.1 mol dmꢀ3; supporting electrolyte) and
Et3NHCl (0.04 mol dmꢀ3; proton donor) in acetonitrile (15 cm3) was
added to a three-electrode cell. To its cathodic compartment the
4.2. General procedure for the synthesis of
diaryldehydroamino acid derivatives
b,b-
b,b-diaryldehydroamino acid derivative was added and a cyclic
To a solution of the
b
-brominated dehydroamino acid derivative
voltammogram recorded. The potential was adjusted to a value
50 mV more negative than that corresponding to the CV peak and
the electrolysis started, the reaction being monitored by HPLC.
When all starting material had disappeared, the content of the ca-
thodic compartment was concentrated at reduced pressure and the
residue partitioned between 100 cm3 of diethyl ether and 25 cm3 of
water. The organic phase was washed with water and brine
(2ꢂ25 cm3 each). After drying over MgSO4 the extract was taken to
dryness at reduced pressure.
in THF/H2O (1:1) the boronic acid, PdCl2dppf$CH2Cl2 (1:1) (10 mol%)
and Cs2CO3 (1.4 equiv) were added. The reaction mixture was heated
at 90 ꢃC, and the reaction was monitored by TLC until all the
b,b-dibromodehydroalanine derivative was consumed (1e3 h). The
solvent was removed under reduced pressure and the residue was
dissolved in ethyl acetate (100 mL). The organic layer was washed
with water and brine (2ꢂ30 mL each), dried with MgSO4 and the
solvent was removed. The residue was submitted to column
chromatography.
4.3.1. Controlled potential electrolysis of 4a. The general procedure
described above using 4a (74.8 mg, 0.212 mmol, 0.014 mol dmꢀ3) as
substrate was followed to give 7a as a white solid (63.7 mg, 85%).
Mp 89.0e90.0 ꢃC (from diethyl ether/petroleum ether). 1H NMR
(400 MHz, CDCl3): 1.38 (s, 9H, CH3 Boc), 3.51 (s, 3H, OCH3), 4.38 (d,
4.2.1. Synthesis of compounds 4c, 4d and 5c. Synthesis of com-
pounds 4c, 4d and 5c was described elsewhere8.
4.2.2. Synthesis of Methyl 2-(tert-butoxycarbonylamino)-3,3-diphe-
nylacrylate, 4a. The general procedure described above using
compound 1 (160.3 mg, 0.45 mmol, 0.05 mol dmꢀ3) and phenyl-
boronic acid (3 equiv) was followed to give 4a as a white solid
(92.24 mg, 58%). Mp 122.0e123.0 ꢃC (from diethyl ether/petroleum
ether). 1H NMR (300 MHz, CDCl3): 1.46 (s, 9H, CH3 Boc), 3.54 (s, 3H,
OCH3), 6.06 (br s, 1H, NH), 7.11e7.14 (m, 2H, ArH), 7.22e7.40 (m, 8H,
ArH) ppm. 13C NMR (100.6 MHz, CDCl3): 28.15 [C(CH3)3], 52.00
(OCH3), 81.23 [OC(CH3)3],125.74 (C),127.83 (CH),128.08 (CH),128.30
(CH), 128.75 (CH), 129.14 (CH), 129.86 (CH), 134.12 (C), 138.57 (C),
139.79 (C), 152.81 (C]O), 166.55 (C]O) ppm. Calcd for C21H23NO4
(353.42): C 71.37, H 6.56, N 3.96; found C 71.28, H 6.62, N 4.10.
J¼6.3 Hz, 1H,
1H,
CH), 7.25e7.32 (m, 10H, ArH) ppm. 13C NMR (100.6 MHz,
CDCl3): 28.18 [C(CH3)3], 52.00 (OCH3), 53.68 ( CH), 56.80 ( CH),
bCH), 4.85 (br d, J¼6.3 Hz,1H, NH), 5.08 (br t, J¼6.6 Hz,
a
b
a
80.12 [OC(CH3)3], 127.03 (CH), 127.14 (CH), 128.25 (CH), 128.48 (CH),
128.58 (CH), 128.70 (CH), 139.58 (C), 140.21 (C), 155.18 (C]O),
172.65 (C]O) ppm. Anal. Calcd for C21H25NO4 (355.43): C 70.96, H
7.09, N 3.94; found C 70.55, H 7.05, N 4.02.
4.3.2. Controlled potential electrolysis of 4b. The general procedure
described above using 4b (63.5 mg, 0.126 mmol, 0.008 mol dmꢀ3
)
as substrate was followed to give 7b as a white solid (50.3 mg, 79%).
Mp 162.0e163.0 ꢃC (from diethyl ether/petroleum ether). 1H NMR
(400 MHz, CDCl3): 1.39 (s, 9H, CH3 Boc), 3.57 (s, 3H, OCH3), 4.49 (d,
4.2.3. Synthesis of Methyl 3,3-di(biphenyl-4-yl)-2-(tert-butox-
ycarbonylamino)acrylate, 4b. The general procedure described
J¼6.3 Hz, 1H,
1H,
CH), 7.34e7.46 (m, 10H, ArH), 7.54e7.59 (m, 8H, ArH) ppm. 13
NMR (100.6 MHz, CDCl3): 28.21 [C(CH3)3], 52.13 (OCH3), 53.08
CH), 56.80 ( CH), 80.24 [OC(CH3)3], 126.97 (CH), 126.99 (CH),
127.22 (CH), 127.28 (CH), 127.32 (CH), 127.44 (CH), 128.68 (CH),
128.74 (CH), 128.76 (CH), 129.04 (CH), 129.73 (CH), 138.58 (C),
139.24 (C), 139.92 (C), 140.11 (C), 140.57 (C), 155.24 (C]O), 172.62
(C]O) ppm. Anal. Calcd for C33H33NO4 (507.63): C 78.08, H 6.55, N
2.76; found C 77.68, H 6.51, N 2.84.
b
CH), 4.93 (br d, J¼6.6 Hz, 1H, NH), 5.17 (br t, J¼6.6 Hz,
above using compound 1 (161.6 mg, 0.45 mmol, 0.05 mol dmꢀ3
)
a
C
and biphenylboronic acid (3 equiv) was followed to give 4b
(184.3 mg, 81%) as a white solid. Mp 108.0e109.0 ꢃC (from diethyl
ether/petroleum ether). 1H NMR (400 MHz, CDCl3): 1.49 (s, 9H, CH3
Boc), 3.61 (s, 3H, OCH3), 6.18 (br s, 1H, NH), 7.25e7.27 (m, 2H, ArH),
7.34e7.41 (m, 4H, ArH), 7.44e7.49 (m, 4H, ArH), 7.56e7.58 (m, 2H,
ArH), 7.62e7.65 (m, 6H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3):
28.16 [C(CH3)3], 52.07 (OCH3), 81.29 [OC(CH3)3], 125.75 (C), 126.74
(CH),126.95 (CH),127.00 (CH),127.39 (CH),127.44 (CH),127.65 (CH),
128.77 (CH), 128.86 (CH), 129.70 (CH), 130.44 (CH), 133.16 (C),
137.46 (C), 138.74 (C), 140.20 (C), 140.39 (C), 140.57 (C), 141.13 (C),
152.84 (C]O), 166.59 (C]O) ppm. Anal. Calcd for C33H31NO4
(505.61): C 78.39, H 6.18, N 2.77; found C 77.87, H 6.64, N 2.73.
(b
a
4.3.3. Controlled potential electrolysis of 4c. The general procedure
described above using 4c (135.4 mg, 0.296 mmol, 0.020 mol dmꢀ3) as
substrate was followed to give 7c of a colourless oil (109.6 mg, 81%).
Thecompoundwaspurifiedbycolumnchromatographyusingdiethyl
ether/petroleum ether 1/9 and then 1/8 as eluent. Mp 55.0e56.0 ꢃC.
1H NMR (400 MHz, CDCl3): 1.34 (s, 9H, CH3 Boc), 3.29 (s, 3H, OCH3),
4.2.4. Synthesis of (S)-methyl 2-(2-(tert-butoxycarbonylamino)-3-
phenylpropanamido)-3,3-diphenylacrylate, 6a. The general pro-
cedure described above using compound 3 (226.5 mg, 0.45 mmol,
0.05 mol dmꢀ3) and phenylboronic acid (1.5 equiv) was followed to
5.03 (br d, J¼6.9 Hz, 1H, NH), 5.31 (br t, J¼6.3 Hz, 1H,
J¼5.7 Hz,1H, CH), 7.41e7.48 (m, 6H, ArH), 7.53 (d, J¼5.1 Hz,1H, ArH),
7.62 (br d, J¼5.1 Hz,1H, ArH), 7.77 (t, J¼6.3 Hz, 2H, ArH), 7.84e7.88 (m,
aCH), 6.12 (d,
b

P.M.T. Ferreira et al. / Tetrahedron 67 (2011) 193e200
199
2H, ArH), 8.10e8.18 (m, 2H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3):
28.13 [C(CH3)3], 43.10 ( CH), 52.08 (OCH3), 57.01 ( CH), 80.07 [OC
and 139.27 (C), 139.82 and 139.87 (C), 155.14 (C]O) 170.91 and
171.13 (C]O), 171.54 and 171.66 (C]O) ppm.
b
a
(CH3)3], 122.85 (CH), 122.96 (CH), 125.26 (CH), 125.43 (CH), 125.51
(CH), 125.62 (CH), 125.85 (CH), 126.26 (CH), 126.45 (CH), 126.61 (CH),
127.85 (CH), 127.97 (CH), 128.99 (CH), 129.14 (CH), 131.24 (C), 131.95
(C), 133.93 (C), 134.13 (C), 135.54 (C), 136.79 (C), 155.16 (C]O), 172.67
(C]O) ppm. Anal. Calcd for C29H29NO4 (455.54): C 76.46, H 6.42, N
3.07; found C 76.44, H 6.60, N 3.14.
Acknowledgements
~
Foundation for the Science and Technology for Fundasao para
ˇ
a Ciencia e Tecnologia (FCT)dPortugal and FEDER (Fundo Europeu
de Desenvolvimento Regional) for financial support to Centro de
Química (CQ-UM) and Centro de Física (CFUM) of University of
Minho and through research project PTDC/QUI/81238/2006, cofi-
nanced by FCT and program FEDER/COMPETE (FCOMP-01-0124-
FEDER-007467). The NMR spectrometer Bruker Avance II 400 is
part of the National NMR Network and was purchased in the
framework of the National Programme for Scientific Re-equip-
4.3.4. Controlled potential electrolysis of 4d. The general procedure
described above using 4d (58.1 mg, 0.115 mmol, 0.008 mol dmꢀ3) as
substrate was followed to give 7d as a white solid (45.4 mg, 78%). The
compound was purified by column chromatography using diethyl
ether/petroleum ether 1/9 and then 1/8 as eluent. Mp 110.0e111.0 ꢃC.
1H NMR (400 MHz, CDCl3): 1.34 (s, 9H, CH3 Boc), 3.35 (s, 3H, OCH3),
3.35e3.37 (m, 4H, 2 CH2), 5.00 (br d, J¼8.4 Hz, 1H, NH), 5.25 (br t,
ment, contract REDE/1517/RMN/2005, with funds from POCI 2010
(FEDER) and Fundac¸ ao para a Ciencia e a Tecnologia (FCT). G.P.
acknowledges FCT for a Ph.D. grant SFRH/BD/38766/2007.
ˇ
~
J¼7.5 Hz,1H,
a
CH), 5.87 (d, J¼6.9 Hz,1H,
bCH), 7.21e7.25 (m, 5H, ArH),
7.34e7.43 (m, 2H, ArH), 7.56e7.78 (m, 3H, ArH) ppm. 13C NMR
(100.6 MHz, CDCl3): 28.15 [C(CH3)3], 29.82 (CH2), 30.46 (CH2), 42.40
(b-CH), 52.08 (OCH3), 57.05 (a-CH), 79.95 [OC(CH3)3], 118.57 (CH),
Supplementary data
118.84 (CH), 118.97 (CH), 119.2 (CH), 119.27 (CH), 126.95 (CH), 127.47
(CH),127.97 (CH),128.21 (CH),130.01 (C),131.43 (C),132.80 (C),139.49
(C),139.71 (C),145.25 (C),145.45 (C),146.46 (C),146.70 (C),155.30 (C]
O),171.79 (C]O) ppm. Anal. Calcd for C33H33NO4 (507.62): C 78.08, H
6.55, N 2.67; found C 77.88, H 6.91, N 2.72.
Supplementary data related to this article can be found online,
at doi:10.1016/j.tet.2010.10.087. These data include MOL files and
InChIKeys of the most important compounds described in this
article.
References and notes
4.3.5. Controlled potential electrolysis of 5c. The general procedure
described above using 5c (90.2 mg, 0.224 mmol, 0.015 mol dmꢀ3) as
substrate was followed to give 8c as 1:1 mixture of diastereomers
(68.4 mg, 76%). The diastereomers were separated by column chro-
matographyusingdiethylether/petroleumether1/5aseluenttogive:
mp 134.0e135.0 ꢃC. 1H NMR (300 MHz, CDCl3): 1.36 (s, 9H, CH3 Boc),
3.43 (s, 3H, OCH3), 4.93 (br d, J¼7.8 Hz, 1H, NH), 5.10e5.22 (m, 2H,
1. Grishaw, J. Electrochemical Reactions and Mechanisms in Organic Chemistry;
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Maturana, R.; Siquella, J. A. Electroanalysis 2006, 521e525; (d) Pastor, F. T.;
Drakulic, B. J. Tetrahedron Lett. 2010, 51, 734e738.
~
aCHþbCH), 7.15e7.33 (m, 5H, ArH), 7.42e7.56 (m, 3H, ArH), 7.67 (br d,
3. (a) Ferreira, P. M. T.; Maia, H. L. S.; Monteiro, L. S.; Sacramento, J.; Sebastiao,
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J¼6.9 Hz, 1H, ArH), 7.78e7.86 (m, 2H, ArH), 8.05e8.08 (m, 1H, ArH)
ppm. 13C NMR (100.6 MHz, CDCl3): 28.17 [C(CH3)3], 49.25 (CH), 51.95
(OCH3), 57.34 (CH), 80.14 [OC(CH3)3],123.15 (CH),124.82 (CH),125.30
(CH), 125.55 (CH), 126.34 (CH), 127.17 (CH), 127.92 (CH), 128.43 (CH),
128.52 (CH), 128.97 (CH),132.01 (C), 134.17 (C), 134.69 (C), 139.66 (C),
155.09 (C]O), 173.09 (C]O) ppm. Anal. Calcd for C25H27NO4
(405.49): C 74.05, H 6.71, N 3.45; found C 73.89, H 6.75, N 3.55.
Oil, 1H NMR (400 MHz, CDCl3): 1.41 (s, 9H, CH3 Boc), 3.56 (s, 3H,
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OCH3), 4.88 (br d, J¼6.9 Hz, 1H, NH), 5.25 (br t, J¼4.8 Hz, 1H,
aCH),
5.33 (d, J¼4.8 Hz, 1H, CH), 7.22e7.31 (m, 5H, ArH), 7.41e7.51 (m,
b
3H, ArH), 7.72 (br d, J¼5.4 Hz, 1H, ArH), 7.78 (br d, J¼6.0 Hz, 1H,
ArH), 7.82e7.84 (m, 1H, ArH), 7.97e7.99 (m, 1H, ArH) ppm. 13C
NMR (100.6 MHz, CDCl3): 28.22 [C(CH3)3], 47.97 (CH), 52.25
(OCH3), 57.04 (CH), 80.19 [OC(CH3)3], 123.52 (CH), 125.03 (CH),
125.34 (CH), 125.44 (CH), 126.16 (CH), 127.21 (CH), 127.87 (CH),
128.68 (CH), 128.76 (CH), 128.84 (CH), 131.41 (C), 133.99 (C), 136.32
(C), 139.45 (C), 155.33 (C]O), 172.54 (C]O) ppm. Anal. Calcd for
C25H27NO4 (405.49): C 74.05, H 6.71, N 3.45; found C 74.61, H 7.07,
N 3.45.
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1H NMR (300 MHz, CDCl3): 1.36 (s, 9H, CH3 Boc), 2.89e3.01 (m,
2H, CH2), 3.48 and 3.50 (2s, 3H, OCH3), 4.26e4.43 (m, 1H, CH), 4.72
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(OCH3), 53.13 and 53.72 (CH), 55.23 and 55.55 (CH), 80.13 [OC
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