High yielding synthesis of N-ethyl dehydroamino acids

Article abstract of DOI:10.1007/s00726-012-1240-z

Recently we reported the use of Asequence of alkylation and dehydration methodologies to obtain N-ethyl-α, β-dehydroamino acid derivatives. The application of this N-Alkylation procedure to several methyl esters of β,β-dibromo and β-bromo, β-substituted dehydroamino acids protected with standard amine protecting groups was subsequently reported. The corresponding N-ethyl, β-bromo dehydroamino acid derivatives were obtained in fair to high yields and some were used as substrates in Suzuki cross-coupling reactions to give N-ethyl, β,β-disubstituted dehydroalanine derivatives. Herein, we further explore N-ethylation of β-halo dehydroamino acid derivatives using triethyloxonium tetrafluoroborate as alkylating agent, but substituting N,N-diisopropylethylamine for potassium tert-butoxide as auxiliary base. In these conditions, for all β-halo dehydroamino acid derivatives, reactions were complete and the N-ethylated derivative could be isolated in high yield. This method was also applied for N-ethylation of non-halogenated dehydroamino acids. Again, with all compounds the reactions were complete and the N-ethyl dehydroamino acid derivatives could be isolated in high yields. Some of these N-ethyl dehydroamino acid methyl ester derivatives were converted in high yields to their corresponding acids and coupled to an amino acid methyl ester to give N-ethyl dehydrodipeptide derivatives in good yields. Thus, this method constitutes Ageneral procedure for high yielding synthesis of N-ethylated dehydroamino acids, which can be further applied in peptide synthesis.

Full text of DOI:10.1007/s00726-012-1240-z

Amino Acids (2012) 43:1643–1652
DOI 10.1007/s00726-012-1240-z
ORIGINAL ARTICLE
High yielding synthesis of N-ethyl dehydroamino acids
•
Luıs S. Monteiro Ana S. Suarez
´
´
Received: 9 November 2011 / Accepted: 31 January 2012 / Published online: 15 February 2012
Ó Springer-Verlag 2012
Abstract Recently we reported the use of a sequence
of alkylation and dehydration methodologies to obtain
N-ethyl-a, b-dehydroamino acid derivatives. The applica-
tion of this N-alkylation procedure to several methyl esters
of b,b-dibromo and b-bromo, b-substituted dehydroamino
acids protected with standard amine protecting groups
was subsequently reported. The corresponding N-ethyl,
b-bromo dehydroamino acid derivatives were obtained in
fair to high yields and some were used as substrates
in Suzuki cross-coupling reactions to give N-ethyl,
b,b-disubstituted dehydroalanine derivatives. Herein, we
further explore N-ethylation of b-halo dehydroamino acid
derivatives using triethyloxonium tetrafluoroborate as
alkylating agent, but substituting N,N-diisopropylethyl-
amine for potassium tert-butoxide as auxiliary base. In
these conditions, for all b-halo dehydroamino acid deriv-
atives, reactions were complete and the N-ethylated
derivative could be isolated in high yield. This method was
also applied for N-ethylation of non-halogenated dehyd-
roamino acids. Again, with all compounds the reactions
were complete and the N-ethyl dehydroamino acid deriv-
atives could be isolated in high yields. Some of these
N-ethyl dehydroamino acid methyl ester derivatives were
converted in high yields to their corresponding acids and
coupled to an amino acid methyl ester to give N-ethyl
dehydrodipeptide derivatives in good yields. Thus, this
method constitutes a general procedure for high yielding
synthesis of N-ethylated dehydroamino acids, which can be
further applied in peptide synthesis.
Keywords Amino acids Á Nonnatural amino acids Á
Alkylation Á N-ethyl dehydroamino acids Á
Protecting groups
Introduction
Non-proteinogenic amino acids are an important class of
organic compounds with a large application spectrum in
medicinal chemistry, since they can have intrinsic biolog-
ical activity or can be found in peptides with antiviral,
antitumor, anti-inflammatory or immunosuppressive
activities. Among non-proteinogenic amino acids are
N-alkylamino acids and dehydroamino acids, both of which
can be found in many biologically important peptides
(Gilon et al. 2003; Kotha 2003; Rilatt et al. 2005).
N-Alkylamino acids are constituents of various important
naturally occurring peptides and proteins (Gilon et al.
2003; Kotha 2003; Rilatt et al. 2005). Several N-alkylated
peptides show antibiotic, anticancer or antiviral activity
(Wenger 1985). N-Alkylation of the peptide bond causes
changes in volume and conformation of peptides, resulting
in reduced flexibility, increase of permeability for the
membrane (increased lipophilicity) and prevention of
cleavage by proteolytic enzymes (Aurelio et al. 2004).
Dehydroamino acids are outstanding compounds as they
are key intermediates in amino acid and peptide synthesis,
having been found in naturally occurring peptides of fungal
and microbial origin (Aydin et al. 1985) and from marine
organisms (Hamann and Scheuer 1993; Goetz et al. 1997),
in which they contribute with a catalytic role in the active
sites of some enzymes. They are also found in a variety
of peptide antibiotics of bacterial origin that include the
lantibiotics (nisin, epidermin, subtilin, gallidermin) (Palmer
et al. 1992; Jung 1991; Chatterjee et al. 2005).
´
L. S. Monteiro (&) Á A. S. Suarez
Chemistry Centre, University of Minho,
Gualtar 4710-057, Braga, Portugal
e-mail: monteiro@quimica.uminho.pt
123

´
L. S. Monteiro, A. S. Suarez
1644
Many methods of synthesis of N-alkylamino acids have
recorded with a Varian Unity Plus spectrometer at 300 and
75.4 MHz, respectively, or with a Bruker Avance II?
operating at 400 and 100.6 MHz, respectively. 1H-1H spin–
spin decoupling, DEPT (h 458), HMQC and HMBC were
used to attribute some signals. The stereochemistry of the
dehydroamino acids derivatives was determined by NOE
difference experiments. Chemical shifts are given in ppm
and coupling constants (J) in Hz. HRMS data were
obtained by the mass spectrometry service of the Univer-
sity of Vigo, Spain. Elemental analysis was performed with
a LECO CHNS 932 elemental analyzer. The reactions were
monitored by thin layer chromatography (TLC). Petroleum
ether refers to fractions with the boiling range 40–60°C.
Solvents were used without purification except for aceto-
nitrile and dichloromethane which were dried according to
standard procedures.
been developed, most of them are N-methylation (Aurelio
et al. 2004). Recently, Belsito et al. (2010) proposed the
ethylation of several 4-nitrobenzenesulfonyl (Nosyl) pro-
tected amino acids using triethyloxonium tetrafluoroborate
(Et3OBF4) as alkylating agent and N,N-diisopropylethyl-
amine (DIPEA) as a base to give N-ethylamino acid
derivatives in high yields. Subsequently, we reported the
use of a combination of this alkylation procedure and
dehydration methodologies previously developed by us
(Ferreira et al. 1999, 2007) to obtain new non-proteino-
genic amino acids namely, N-(4-nitrophenylsulfonyl),
N-ethyl-a,b-dehydroamino acids (Monteiro et al. 2010).
Thus, it was possible to obtain for the first time, new non-
natural amino acids which incorporate both the N-ethyl and
a,b-dehydro moieties.
In the case of N-alkylation of dehydroamino acids, an
alternative to the electron withdrawing effect of the amine
protecting group could be the presence of electron with-
drawing substituents on the b-carbon atom. Thus, we tested
the triethyloxonium tetrafluoroborate/N,N-diisopropyleth-
ylamine N-alkylation procedure with several methyl esters
of b,b-dibromo and b-bromo, b-substituted dehydroamino
acids protected with standard amine protecting groups such
as tert-butyloxycarbonyl (Boc), benzyloxycarbonyl (Z) and
4-nitrobenzyloxycarbonyl [Z(NO2)] as well as acyl and
sulfonyl amine protecting groups (Monteiro et al. 2011).
Depending on the nature of the halogenated dehydroamino
acid and of the protecting group, variable ratios of product to
reactant were obtained. These ranged from total conversion
to product [b,b-dibromo, dehydroalanine derivatives pro-
tected with Z(NO2) and Z, b-bromo dehydroaminobutyric
acid and dehydrophenylalanine derivatives protected with
Nosyl and Tos] to complete absence of reaction [the methyl
ester of N-(tert-butyloxycarbonyl), b-bromo dehydroamin-
obutyric acid]. Some of these N-ethylated dehydroamino
acid derivatives were used as substrates in cross-coupling
reactions to give the methyl esters of N-protected, N-ethyl,
b,b-disubstituted dehydroalanines.
Synthesis of the methyl esters of N-protected
b-hydroxyamino acids
Synthesis of compounds Z(NO2)-L-Ser-OMe (Ferreira
et al. 1999), Z-L-Ser-OMe (Ferreira et al. 1999), Boc-L-
Ser-OMe (Ferreira et al. 1999), Z(NO2)-L-Thr-OMe
(Ferreira et al. 1999), Z-L-Thr-OMe (Ferreira et al. 1999),
Boc-L-Thr-OMe (Ferreira et al. 1999), Tos-L-Thr-OMe
(Ferreira et al. 1999), Z(NO2)-D,L-Phe(b-OH)-OMe
(Ferreira et al. 1999), Z-D,L-Phe(b-OH)-OMe (Monteiro
et al. 2011), Boc-D,L-Phe(b-OH)-OMe (Ferreira et al.
2001), Tos-D,L-Phe(b-OH)-OMe (Ferreira et al. 2007).
The synthesis of these compounds was described in the
references given above.
Synthesis of the methyl esters of N-protected
dehydroamino acids
Synthesis of compounds Z(NO2)-DAla-OMe (1a) (Ferreira
et al. 2007), Z-DAla-OMe (1b) (Bertj et al. 1992), Boc-
DAla-OMe (1c) (Ferreira et al. 1999), Z(NO2)-Z-DAbu-
OMe (Z-2a) (Ferreira et al. 1999), Z–Z-DAbu-OMe (Z-2b)
(Ferreira et al. 1999), Boc-Z-DAbu-OMe (Z-2c) (Ferreira
et al. 1999), Tos-Z-DAbu-OMe (Z-2d) (Ferreira et al.
2007), Z(NO2)-Z-DPhe-OMe (Z-3a) (Monteiro et al.
2011), Z-Z-DPhe-OMe (Z-3b) (Monteiro et al. 2011), Boc-
Z-DPhe-OMe (Z-3c) (Ferreira et al. 2001), Tos-Z-DPhe-
OMe (Z-3d) (Ferreira et al. 2007).
Herein, we further explore N-ethylation of b-halo dehyd-
roamino acid derivatives substituting DIPEA for potassium
tert-butoxide as auxiliary base. Contrary to the reactions
carried out using DIPEA in which no reaction or incomplete
reactions were observed, with the use of tert-butoxide, all
reactions were complete giving N-ethyl dehydroamino acid
derivatives in high yields. The method is further applied to
non-halogenated dehydroamino acid derivatives.
The synthesis of these compounds was described in the
references given above.
Synthesis of the methyl esters of N-protected
b-halo dehydroamino acids
Materials and methods
Melting points were determined with a Gallenkamp appa-
1
Synthesis of compounds Boc-DAla(b,b-Br)-OMe (4c)
(Silva et al. 2002), Z(NO2)-E-DAbu(b-Br)-OMe (E-5a)
ratus and are uncorrected. H and 13C NMR spectra were
123

High yielding synthesis of N-ethyl dehydroamino acids
1645
Synthesis of Boc-N(Et)-DAla(b,b-Br)-OMe (8c) The
general procedure described above was followed using
Boc-DAla(b,b-Br)-OMe (4c) (0.066 g, 0.174 mmol) as
reactant to afford 8c (Monteiro et al. 2011) (0.066 g, 93%).
(Ferreira et al. 2007), Z(NO2)-Z-DAbu(b-Br)-OMe (Z-5a)
(Ferreira et al. 2007), Z-Z-DAbu-(b-Br)OMe (Z-5b)
(Ferreira et al. 2007), Boc-E-DAbu(b-Br)-OMe (E-5c)
(Silva et al. 2002), Boc-Z-DAbu(b-Br)-OMe (Z-5c) (Silva
et al. 2002), Z(NO2)-Z-DPhe(b-Br)-OMe (Z-6a) (Monteiro
et al. 2011), Z-Z-DPhe(b-Br)-OMe (Z-6b) (Monteiro et al.
2011), Boc-Z-DPhe(b-Br)-OMe (Z-6c) (Abreu et al. 2004),
Boc-Z-DPhe(b-I)-OMe (Z-7c) (Queiroz et al. 2008).
The synthesis of these compounds was described in the
references given above.
Synthesis of Z(NO2)-N(Et)-E-DAbu(b-Br)-OM, (E-9a)
The general procedure described above was followed using
Z(NO2)-E-DAbu(b-Br)-OMe (E-5a) (0.065 g, 0.175 mmol)
as reactant to afford E-9a (Monteiro et al. 2011) (0.066 g,
94%).
Synthesis of Z(NO2)-N(Et)-Z-DAbu(b-Br)-OMe (Z-9a)
The general procedure described above was followed using
Z(NO2)-Z-DAbu(b-Br)-OMe (Z-5a) (0.039 g, 0.105 mmol)
as reactant to afford Z-9a (Monteiro et al. 2011) (0.035 g,
82%).
Synthesis of Z(NO2)-Z-DPhe(b-I)-OMe (Z-7a) Z(NO2)-Z-
DPhe-OMe (Z-3a) (1.069 g, 3.000 mmol) was dissolved in
dichloromethane (0.1 mol dm-3) and 1.2 equiv. of N-iodo-
succinimide was added with vigorous stirring. After reacting
for 16 h, triethylamine (1.5 equiv.) was added and stirring
continued for an additional hour. An additional 100 cm3 of
dichloromethane was added and the organic phase was
Synthesis of Z-N(Et)-Z-DAbu(b-Br)-OMe (Z-9b) The
general procedure described above was followed using
Z-Z-DAbu(b-Br)-OMe (Z-5b) (0.033 g, 0.100 mmol) as
reactant to afford Z-9b (Monteiro et al. 2011) (0.031 g,
86%).
washed with KHSO4 (1 mol dm-3), NaHCO3 (1 mol dm-3
)
and brine (3 times 30 cm3 each). After drying over MgSO4
the extract was taken to dryness at reduced pressure to afford
Z-7a (1.230 g, 85%) as a light yellow solid. M.p. 135.0–
136.0°C (from ethyl acetate/n-hexane). 1H NMR (300 MHz,
CDCl3): d = 3.46 (br. s, 3H, CH3 OMe), 5.28 [s, 2H, CH2
Z(NO2)], 6.69 (br. s, 1H NH), 7.27–7.35 (m, 5H, ArH), 7.54
(d, J = 8.4 Hz, 2H, ArH), 8.24 (d, J = 8.4 Hz, 2H, ArH)
ppm. 13C NMR (75.4 MHz, CDCl3): d = 52.53 (OCH3),
66.33 [CH2 Z(NO2)], 95.39 (C), 123.79 (CH), 128.23 (CH),
128.37 (CH), 128.61 (CH), 129.01 (CH), 132.95 (C), 140.34
(C), 142.53 (C), 147.77 (C), 152.69 (C=O), 163.93 (C=O)
ppm. C18H15N2O6I (482.23): calcd. C 44.83, H 3.14, N 5.81;
found C 44.70, H 3.15, N 6.22.
Synthesis of Boc-N(Et)-E-DAbu(b-Br)-OMe (E-9c) The
general procedure described above was followed using
Boc-E-DAbu(b-Br)-OMe (E-5c) (0.221 g, 0.750 mmol) as
reactant to afford E-9c (0.215 g, 89%) as a colorless oil
that failed to crystallize. 1H NMR (400 MHz, CDCl3):
(rotamers) d = 1.00–1.21 (m, 3H, CH2CH3), 1.39–1.48 (m,
9H, CH3 Boc), 2.40 (s, 3H, cCH3), 3.40 (br. q, J = 7.2 Hz,
2H, CH2CH3), 3.75, 3.77 (2s, 3H, CH3 OMe) ppm. 13C
NMR (100.6 MHz, CDCl3): (rotamers) d = 12.98, 13.47
(CH3), 26.80 (cCH3), 28.22 [C(CH3)3], 43.36 (CH2), 52.01,
52.78 (OCH3), 80.16, 80.72 [OC(CH3)3], 130.40 (C),
130.53 (C), 153.78 (C=O), 164.49 (C=O) ppm. m/z
(HRESI-MS) 344.04692, ([M ? Na]?, C12H20NNaO4Br,
requires 344.04734).
Synthesis of the methyl esters
of N-(tert-butyloxycarbonyl), N-ethyl-b,
b-dibromodehydroalanine and N-protected, N-ethyl,
b-substituted, b-halodehydroamino acids
Synthesis of Boc-N(Et)-Z-DAbu(b-Br)-OMe (Z-9c) The
general procedure described above was followed using
Boc-Z-DAbu(b-Br)-OMe (Z-5c) (0.147 g, 0.500 mmol) as
reactant to afford Z-9c (0.137 g, 85%) as a colorless oil that
General procedure for the synthesis of the methyl
esters of N-(tert-butyloxycarbonyl), N-ethyl-b,
b-dibromodehydroalanine and N-protected, N-ethyl,
b-substituted, b-halodehydroamino acids
1
failed to crystallize. H NMR (400 MHz, CDCl3): (rota-
mers) d = 1.15, 1.17 (2t, J = 7.2 Hz, J = 7.2 Hz, 3H,
CH2CH3), 1.39 (s, 9H, CH3 Boc), 2.79 (s, 3H, cCH3), 3.40
(br. q, J = 7.2 Hz, 2H, CH2CH3), 3.75 (s, 3H, CH3 OMe)
ppm. 13C NMR (100.6 MHz, CDCl3): (rotamers) d =
13.00, 13.67 (CH3), 26.52 (cCH3), 27.84, 28.16 [C(CH3)3],
42.99, 44.35 (CH2), 52.06, 52.18 (OCH3), 80.22, 80.62
[OC(CH3)3], 130.91, 131.60 (C), 140.10, 140.72 (C),
153.60, 154.15 (C=O), 164.03, 164.23 (C=O) ppm. m/z
(HRESI-MS) 344.04679, ([M ? Na]?, C12H20NNaO4Br,
requires 344.04734).
The methyl ester of the N-protected, b-halodehydroamino
acid was dissolved in anhydrous dichloromethane
(0.05 mol dm-3) followed by addition of 3.5 equiv. of
potassium tert-butoxide and 2.5 equiv. of triethyloxonium
tetrafluoroborate, under inert atmosphere. The reaction
mixture was stirred at room temperature for 30 min. Then
dichloromethane was added (50 cm3) and the organic
phase was washed with KHSO4 (1 mol dm-3), NaHCO3
(1 mol dm-3) and brine (3 times 20 cm3 each), dried over
MgSO4 and the solvent evaporated at reduced pressure.
Synthesis of Z(NO2)-N(Et)-Z-DPhe(b-Br)-OMe (Z-10a)
The general procedure described above was followed using
123

´
L. S. Monteiro, A. S. Suarez
1646
Z(NO2)-Z-DPhe(b-Br)-OMe (Z-6a) (0.131 g, 0.300 mmol)
as reactant to afford Z-10a (Monteiro et al. 2011) (0.133 g,
96%).
[OC(CH3)3], 117.61 (C), 127.62 (CH), 127.97, 128.10
(CH), 128.94 (CH), 137.16 (C), 142.72 (C), 153.25 (C=O),
163.18 (C=O) ppm. C17H22NO4I (431.27): calcd. C 47.34,
H 5.14, N 3.25; found C 47.65, H 5.27, N 3.50.
Synthesis of Z-N(Et)-Z-DPhe(b-Br)-OMe (Z-10b) The
general procedure described above was followed using
Z-Z-DAbu(b-Br)-OMe (Z-6b) (0.079 g, 0.203 mmol) as
reactant to afford Z-10b (Monteiro et al. 2011) (0.069 g,
82%).
Synthesis of the methyl esters of N-protected,
N-ethyl dehydroamino acids
General procedure for the synthesis of the methyl esters
of N-protected, N-ethyl dehydroamino acids
Synthesis of Boc-N(Et)-Z-DPhe(b-Br)-OMe (Z-10c) The
general procedure described above was followed using
Boc-Z-DPhe(b-Br)-OMe (Z-6c) (0.178 g, 0.500 mmol) as
reactant to afford Z-10c (0.170 g, 88%) as a light yellow
oil. M.p. 59.5–60.0°C (from ethyl acetate/n-hexane).
1H NMR (400 MHz, CDCl3): (rotamers) d = 1.28 (t, J =
7.6 Hz, 3H, CH2CH3), 1.45, 1.52 (2s, 9H, CH3 Boc), 3.44,
3.48 (2s, 3H, CH3 OMe), 3.63 (q, J = 7.5 Hz, 2H,
CH2CH3), 7.32–7.36 (m, 5H, ArH) ppm. 13C NMR
(100.6 MHz, CDCl3): (rotamers) d = 13.29, 13.99 (CH3),
28.16 [C(CH3)3], 42.50, 44.19 (CH2), 51.95 (OCH3), 80.83,
81.17 [OC(CH3)3], 128.16 (CH), 128.22 (CH), 129.42
(CH), 132.29 (C), 135.66 (C), 139.18 (C), 153.37 (C=O),
164.66 (C=O) ppm. C17H22NO4Br (384.26): calcd. C
53.14, H 5.77, N 3.65; found C 53.29, H 5.77, N 3.86.
The methyl ester of the N-protected dehydroamino acid
was dissolved in anhydrous dichloromethane (0.05 mol
dm-3) followed by addition of 3.5 equiv. of potassium tert-
butoxide and 2.5 equiv. of triethyloxonium tetrafluorobo-
rate, under inert atmosphere. The reaction mixture was
stirred at room temperature for 30 min. Then dichloro-
methane was added (50 cm3) and the organic phase
was washed with KHSO4 (1 mol dm-3), NaHCO3
(1 mol dm-3) and brine (3 times 20 cm3 each), dried over
MgSO4 and the solvent evaporated at reduced pressure.
Synthesis of Z(NO2)-N(Et)-DAla-OMe (12a) The general
procedure described above was followed using Z(NO2)-
DAla-OMe (1a) (0.211 g, 0.750 mmol) as reactant to
afford 12a (0.204 g, 88%) as an orange oil that failed to
Synthesis of Z(NO2)-N(Et)-Z-DPhe(b-I)-OMe (Z-11a)
The general procedure described above was followed using
Z(NO2)-Z-DPhe(b-I)-OMe (Z-7a) (0.121 g, 0.250 mmol)
as reactant to afford Z-11a (0.121 g, 95%) as a light yellow
solid. M.p. 134.0–135.0°C (from ethyl acetate/n-hexane).
1H NMR (300 MHz, CDCl3): (rotamers) d = 1.35 (t, J =
7.2 Hz, 3H, CH2CH3), 3.40, 3.43 (2s, 3H, CH3 OMe),
3.68–3.73 (m, 2H, CH2CH3), 5.30, 5.35 [2s, 2H, CH2
Z(NO2)], 7.26–7.34 (m, 5H, ArH), 7.53 (d, J = 8.8 Hz,
2H, ArH), 8.21 (d, J = 8.8 Hz, 2H, ArH) ppm. 13C NMR
(75.4 MHz, CDCl3): (rotamers) d = 13.15 (CH3), 44.16,
44.63 (CH2), 52.25, 53.68 (OCH3), 66.18 [CH2 Z(NO2)],
123.65, 123.79 (CH), 127.53, 127.76 (CH), 127.93, 128.06
(CH), 128.21 (CH), 129.20, 129.36 (CH), 136.20 (C),
142.13 (C), 142.16 (C), 143.71 (C), 147.51 (C), 153.71
(C=O), 162.82 (C=O) ppm. C20H19N2O6I (510.28): calcd.
C 47.08, H 3.75, N 5.49; found C 46.83, H 4.00, N 5.49.
1
crystallize. H NMR (400 MHz, CDCl3): d = 1.19 (t, J =
6.9 Hz, 3H, CH2CH3), 3.60 (q, J = 6.9 Hz, 2H, CH2CH3),
3.72 (s, 3H, CH3 OMe), 5.21 [s, 2H, CH2 Z(NO2)], 5.63 (s,
1H, bCH2), 6.19 (s, 1H, bCH2), 7.46 (d, J = 8.4 Hz, 2H,
ArH), 8.20 (d, J = 8.4 Hz, 2H, ArH) ppm. 13C NMR
(100.6 MHz, CDCl3): d = 13.39 (CH3), 45.02 (CH2),
52.44 (OCH3), 65.94 [CH2 Z(NO2)], 121.61 (bCH2),
123.67 (CH), 127.90 (CH), 138.68 (aC), 143.66 (C),
147.53 (C), 154.19 (C=O), 164.66 (C=O) ppm. C14H16
N2NaO6 (308.29): calcd. C 54.54, H 5.23, N 9.09; found C
54.43, H 5.13, N 9.11. m/z (HRESI-MS) 331.09006,
([M ? Na]?, C14H16N2NaO6, requires 331.09060).
Synthesis of Z-N(Et)-DAla-OMe (12b) The general pro-
cedure described above was followed using Z-DAla-OMe
(1b) (0.118 g, 0.500 mmol) as reactant to afford 12b
(0.119 g, 91%) as a colorless oil that failed to crystallize.
1H NMR (300 MHz, CDCl3): (rotamers) d = 1.18 (t, J =
6.9 Hz, 3H, CH2CH3), 3.60 (q, J = 6.9 Hz, 2H, CH2CH3),
3.63, 3.87 (2s, 3H, CH3 OMe), 5.13 (s, 2H, CH2 Z), 5.54 (s,
1H, bCH2), 6.09 (s, 1H, bCH2), 7.29–7.35 (m, 5H, ArH)
ppm. 13C NMR (75.4 MHz, CDCl3): (rotamers) d = 13.43,
13.93 (CH3), 44.77 (CH2), 52.21, 52.94 (OCH3), 66.45,
67.39 (CH2 Z), 120.06 (bCH2), 127.85 (CH), 127.96 (CH),
128.36 (CH), 136.18, 136.63 (C), 139.06 (aC), 154.60
(C=O), 164.90 (C=O) ppm. C14H17NO4 (263.29): calcd. C
63.87, H 6.51, N 5.23; found C 63.30, H 6.46, N 5.63. m/z
Synthesis of Boc-N(Et)-Z-DPhe(b-I)-OMe (Z-11c) The
general procedure described above was followed using
Boc-Z-DPhe(b-I)-OMe (Z-7c) (0.133 g, 0.328 mmol) as
reactant to afford Z-11c (0.134 g, 95%) as a light orange
oil. M.p. 69.5–70.0°C (from diethyl ether/n-hexane).
1H NMR (400 MHz, CDCl3): (rotamers) d = 1.30 (t, J =
7.2 Hz, 3H, CH2CH3), 1.48, 1.53 (2s, 9H, CH3 Boc), 3.46
(s, 3H, CH3 OMe), 3.61 (q, J = 7.2 Hz, 2H, CH2CH3),
7.27–7.35 (m, 5H, ArH) ppm. 13C NMR (100.6 MHz,
CDCl3): (rotamers) d = 13.36, 14.08 (CH3), 28.24, 28.35
[C(CH3)3], 42.89, 44.76 (CH2), 51.97 (OCH3), 81.17
123

High yielding synthesis of N-ethyl dehydroamino acids
1647
(HRESI-MS) 286.10537, ([M ? Na]?, C14H17NNaO4,
164.82, 165.11 (C=O) ppm. m/z (HRESI-MS) 300.12063,
([M ? Na]?, C15H19NNaO4, requires 300.12118).
requires 286.10553).
Synthesis of Boc-N(Et)-DAla-OMe (12c) The general
procedure described above was followed using Boc-DAla-
OMe (1c) (0.101 g, 0.500 mmol) as reactant to afford 12c
(0.090 g, 79%) as a colorless oil that failed to crystallize.
1H NMR (300 MHz, CDCl3): (rotamers) d = 1.17 (t,
J = 7.2 Hz, 3H, CH2CH3), 1.43 (s, 9H, CH3 Boc), 3.54
(br. q, J = 6.9 Hz, 2H, CH2CH3), 3.78, 3.88 (2s, 3H, CH3
OMe), 5.40 (s, 1H, bCH), 5.91, 5.93 (2s, 1H, bCH) ppm.
13C NMR (75.4 MHz, CDCl3): (rotamers) d = 13.47,
14.09 (CH3), 28.16, 28.38 [C(CH3)3], 44.32 (CH2), 52.15,
52.98 (OCH3), 80.67, 80.77 [OC(CH3)3], 116.96, 117.20
(bCH2), 139.94 (aC), 161.14 (C=O), 165.61 (C=O) ppm.
m/z (HRESI-MS) 252.12063, ([M ? Na]?, C11H19NNaO4,
requires 252.12118).
Synthesis of Boc-N(Et)-Z-DAbu-OMe (Z-13c) The gen-
eral procedure described above was followed using Boc-Z-
DAbu-OMe (Z-2c) (0.294 g, 1.364 mmol) as reactant to
afford Z-13c (0.304 g, 92%) as a colorless oil that failed to
crystallize. 1H NMR (400 MHz, CDCl3): (rotamers)
d = 1.06 (t, J = 7.2 Hz, 3H, CH2CH3), 1.35, 1.46 (2s, 9H,
CH3 Boc), 1.75 (d, J = 7.2 Hz, 3H, cCH3), 3.32, 3.40 (2q,
J = 7.2 Hz and J = 7.2 Hz, 2H, CH2CH3), 3.71, 3.72 (2s,
3H, CH3 OMe), 6.79, 6.89 (2q, J = 7.2 Hz and
J = 7.2 Hz, 1H, bCH) ppm. 13C NMR (100.6 MHz,
CDCl3): (rotamers) d = 12.99, 13.02 (CH3), 13.68, 13.81
(cCH3), 28.13, 28.25 [C(CH3)3], 42.84, 43.99 (CH2), 51.83,
51.88 (OCH3), 79.80, 80.08 [OC(CH3)3], 132.42, 132.54
(aC), 135.93, 136.17 (bCH), 154.17, 154.38 (C=O),
165.01, 165.45 (C=O) ppm. C12H21NO4 (243.30): calcd. C
59.24, H 8.70, N 5.76; found C 59.02, H 8.70, N 5.71. m/z
(HRESI-MS) 266.13655, ([M ? Na]?, C12H21NNaO4,
requires 266.13683).
Synthesis of Z(NO2)-N(Et)-Z-DAbu-OMe (Z-13a) The
general procedure described above was followed using
Z(NO2)-Z-DAbu-OMe (Z-2a) (0.221 g, 0.750 mmol) as
reactant to afford Z-13a (0.217 g, 90%) as an orange oil
that failed to crystallize. 1H NMR (400 MHz, CDCl3):
(rotamers) d = 1.14 (t, J = 7.2 Hz, 3H, CH2CH3), 1.81 (d,
J = 7.2 Hz, 3H, cCH3), 3.51 (br. q, J = 7.2 Hz, 2H,
CH2CH3), 3.73, 3.77 (2s, 3H, CH3 OMe), 5.19, 5.30 [br.
s ? s, 2H, CH2 Z(NO2)], 7.00, 7.04 (2q, J = 7.2 Hz and
J = 7.2 Hz, 1H, bCH), 7.42, 7.55 (2d, J = 8.8 Hz and
J = 8.8 Hz, 2H, ArH), 8.20, 8.24 (2d, J = 8.8 Hz
and J = 8.8 Hz, 2H, ArH) ppm. 13C NMR (100.6 MHz,
CDCl3): (rotamers) d = 12.94 (CH3), 13.76, 13.91 (cCH3),
44.08 (CH2), 52.20 (OCH3), 65.68, 65.89 [CH2 Z(NO2)],
123.64, 123.77 (CH), 127.73, 127.92 (CH), 131.59, 131.96
(C), 138.40, 139.07 (bCH), 144.00, 144.09 (C), 147.46 (C),
154.56, 154.75 (C=O), 164.54, 164.87 (C=O) ppm.
C15H18N2O6 (322.31): calcd. C 55.90, H 5.63, N 8.69;
found C 55.82, H 5.41, N 8.65. m/z (HRESI-MS)
323.12432, ([M ? H]?, C15H19N2O6, requires 323.12431).
Synthesis of Tos-N(Et)-Z-DAbu-OMe (Z-13d) The general
procedure described above was followed using Tos-Z-
DAbu-OMe (Z-2d) (0.135 g, 0.500 mmol) as reactant to
afford Z-13d (0.122 g, 82%) as a colorless oil that failed to
crystallize. 1H NMR (400 MHz, CDCl3): d = 1.10 (t,
J = 7.2 Hz, 3H, CH2CH3), 1.98 (d, J = 7.2 Hz, 3H,
cCH3), 2.42 (s, 3H, CH3 Tos), 3.21, 3.64 (2 br. s, 2H,
CH2CH3), 3.51 (s, 3H, CH3 OMe), 7.28 (d, J = 8.4 Hz,
2H, ArH), 7.32 (q, J = 7.2 Hz, 1H, bCH), 7.70 (d,
J = 8.4 Hz, 2H, ArH) ppm. 13C NMR (100.6 MHz,
CDCl3): d = 14.01 (CH3), 15.22 (c CH3), 21.49 (Tos CH3),
44.06 (CH2), 51.76 (OCH3), 127.66 (CH), 128.72 (C),
129.18 (CH), 136.99 (C), 143.10 (C), 146.47 (CH), 164.39
(C=O) ppm. m/z (HRESI-MS) 320.09270, ([M ? Na]?,
C14H19NNaO4S, requires 320.09325).
Synthesis of Z(NO2)-N(Et)-Z-DPhe-OMe (Z-14a) The
general procedure described above was followed using
Z(NO2)-Z-DPhe-OMe (Z-3a) (0.270 g, 0.758 mmol) as
reactant to afford Z-14a (0.283 g, 97%) as a light yellow
Synthesis of Z-N(Et)-Z-DAbu-OMe (Z-13b) The general
procedure described above was followed using Z-Z-DAbu-
OMe (Z-2b) (0.062 g, 0.250 mmol) as reactant to afford
Z-13b (0.064 g, 93%) as a colorless oil that failed to
crystallize. 1H NMR (400 MHz, CDCl3): (rotamers)
d = 1.13 (t, J = 7.2 Hz, 3H, CH2CH3), 1.76, 1.80 (2d,
J = 7.2 Hz and J = 7.2 Hz, 3H, cCH3), 3.46–3.50 (m, 2H,
CH2CH3), 3.66, 3.77 (2s, 3H, CH3 OMe), 5.11, 5.21 (2s,
2H, CH2 Z), 6.93, 7.02 (2q, J = 7.2 Hz and J = 7.2 Hz,
1H, bCH), 7.28-7.39 (m, 5H, ArH) ppm. 13C NMR
(100.6 MHz, CDCl3): (rotamers) d = 13.00 (CH3), 13.83,
13.92 (cCH3), 43.80, 43.91 (CH2), 52.02, 52.11 (OCH3),
67.10, 67.36 (CH2 Z), 127.66, 127.76 (CH), 127.82, 127.96
(CH), 128.33, 128.46 (CH), 131.77, 132.19 (C), 139.30,
136.66 (C), 138.01, 138.81 (bCH), 155.01, 155.27 (C=O),
1
oil that failed to crystallize. H NMR (400 MHz, CDCl3):
(rotamers) d = 1.10, 1.21 (2t, J = 7.2 Hz and J = 7.2 Hz,
3H, CH2CH3), 3.29, 3.47, 3.72 (3br. s, 2H, CH2CH3), 3.79,
3.85 (2s, 3H, CH3 OMe), 5.21, 5.37 [br. s ? s, 2H, CH2
Z(NO2)], 7.34–7.58 (m, 8H, ArH ? bCH), 8.12, 8.26 [2d,
J = 8.8 Hz and J = 8.8 Hz, 2H, Z(NO2)] ppm. 13C NMR
(100.6 MHz, CDCl3): (rotamers) d = 12.76, 13.57 (CH3),
43.83, 44.16 (CH2), 52.53, 52.55 (OCH3), 65.90, 66.02
[CH2 Z(NO2)], 123.57, 123.78 (CH), 127.89, 127.99 (CH),
128.73, 128.92 (CH), 129.78 (C), 130.04, 130.09 (CH),
130.43 (CH), 132.58, 132.78 (C), 136.43, 136.81 (CH),
143.83, 144.01 (C), 147.45, 147.62 (C), 154.94, 155.15
123

´
L. S. Monteiro, A. S. Suarez
1648
(C=O), 165.52, 166.01 (C=O) ppm. m/z (HRESI-MS)
385.13941, ([M ? H]?, C20H21N2O6, requires 385.13996).
Synthesis of N-protected, N-ethyl dehydroamino acids
General procedure for the synthesis of N-protected, N-ethyl
dehydroamino acids
Synthesis of Z-N(Et)-Z-DPhe-OMe (Z-14b) The general
procedure described above was followed using Z-Z-DPhe-
OMe (Z-3b) (0.749 g, 2.405 mmol) as reactant to afford
Z-14b (0.733 g, 90%) as a yellow oil that failed to crys-
The methyl ester of the N-protected, N-ethyl dehydroamino
acid was dissolved in dioxane (0.2 mol dm-3), followed by
addition of 3 cm3 of NaOH (1 mol dm-3). The solution
was stirred at room temperature for 2 h and then acidified
to pH 2–3 with KHSO4 (1 mol dm-3) and extracted with
ethyl acetate (5 9 10 cm3). The organic extracts were
collected, dried over MgSO4 and evaporated at reduced
pressure.
1
tallize. H NMR (400 MHz, CDCl3): (rotamers) d = 1.07
(t, J = 7.2 Hz, 3H, CH2CH3), 3.24, 3.40 (2br. s, 2H,
CH2CH3), 3.68, 3.84 (s, 3H, CH3 OMe), 5.17, 5.28 (br.
s ? s, 2H, CH2 Z), 7.23–7.52 (m, 10H, ArH ? bCH), 7.59
(s, 1H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3): (rota-
mers) d = 12.84, 13.58 (CH3), 43.55, 44.00 (CH2), 52.33,
52.47 (OCH3), 67.37, 67.50 (CH2 Z), 127.81 (CH), 127.86
(CH), 128.32, 128.50 (CH), 128.67, 128.78 (CH), 129.86
(CH), 130.01 (CH), 130.06 (C), 132.84, 132.94 (C), 135.66
(CH), 136.44, 136.66 (C), 155.52, 155.57 (C=O), 165.81,
166.24 (C=O) ppm. m/z (HRESI-MS) 362.13628,
([M ? Na]?, C20H21NNaO4, requires 362.13683).
Synthesis of Boc-N(Et)-DAla-OH (15c) The general pro-
cedure described above was followed using Boc-N(Et)-
DAla-OMe (12c) (0.134 g, 0.587 mmol) as reactant to
afford 15c (0.115 g, 91%) as a light yellow oil that failed to
crystallize. 1H NMR (400 MHz, CDCl3): d = 1.18 (t,
J = 7.2 Hz, 3H, CH2CH3), 1.44 (s, 9H, CH3 Boc), 3.56 (q,
J = 6.9 Hz, 2H, CH2CH3), 5.53 (s, 1H, bCH), 6.07 (s, 1H,
bCH) ppm. 13C NMR (100.6 MHz, CDCl3): d = 15.10
(CH3), 28.36 [C(CH3)3], 44.29 (CH2), 81.25 [OC(CH3)3],
118.90 (bCH2), 139.53 (aC), 156.33 (C=O), 169.13 (C=O)
ppm. m/z (HRESI-MS) 216.12374, ([M ? H]?, C10H17NO4,
requires 216.12356).
Synthesis of Boc-N(Et)-Z-DPhe-OMe (Z-14c) The gen-
eral procedure described above was followed using Boc-Z-
DPhe-OMe (Z-3c) (0.206 g, 0.750 mmol) as reactant to
afford Z-14c (0.208 g, 92%) as a light yellow oil that failed
to crystallize. 1H NMR (400 MHz, CDCl3): (rotamers)
d = 1.04 (t, J = 7.2 Hz, 3H, CH2CH3), 1.40, 1.54 (2s, 9H,
CH3 Boc), 3.10, 3.30, 3.67 (3br. s, 2H, CH2CH3), 3.83 (s,
3H, CH3 OMe), 7.36–7.39 (m, 4H, ArH ? bCH) 7.50–7.53
(m, 2H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3): (ro-
tamers) d = 12.94, 13.54 (CH3), 28.17, 28.37 [C(CH3)3],
42.53, 44.11 (CH2), 52.24, 52.31 (OCH3), 80.44, 80.60
[OC(CH3)3], 128.57, 128.71 (CH), 129.63, 129.73 (CH),
129.87, 129.95 (CH), 130.68 (C), 133.23, 133.28 (C),
133.99 (CH), 154.56, 154.78 (C=O), 166.11, 166.68
(C=O) ppm. m/z (HRESI-MS) 328.15193, ([M ? Na]?,
C17H23NNaO4, requires 328.15248).
Synthesis of Z-N(Et)-Z-DAbu-OH (Z-16b) The general
procedure described above was followed using Z-N(Et)-Z-
DAbu-OMe (Z-13b) (0.277 g, 1.000 mmol) as reactant to
afford Z-16b (0.249 g, 95%) as a yellow oil that failed to
crystallize. 1H NMR (400 MHz, CDCl3): (rotamers)
d = 1.15 (t, J = 7.2 Hz, 3H, CH2CH3), 1.79 (d,
J = 7.2 Hz, 3H, cCH3), 3.50 (q, J = 7.2 Hz, 2H,
CH2CH3), 5.11 (br. s, 2H, CH2 Z), 7.08 (q, J = 7.2 Hz, 1H,
bCH), 7.25–7.40 (m, 5H, ArH) ppm. 13C NMR
(100.6 MHz, CDCl3): d = 13.03 (CH3), 14.10 (cCH3),
43.75 (CH2), 67.27 (CH2 Z), 127.57 (CH), 127.81 (CH),
128.34 (CH), 131.28 (C), 136.53 (C), 140.67 (bCH),
155.24 (C=O), 169.36 (C=O) ppm. m/z (HRESI-MS)
264.12332, ([M ? H]?, C14H17NO4, requires 264.12358).
Synthesis of Tos-N(Et)-Z-DPhe-OMe (Z-14d) The general
procedure described above was followed using Tos-Z-
DPhe-OMe (Z-3d) (0.166 g, 0.500 mmol) as reactant to
afford Z-14d (0.165 g, 92%) as a light yellow oil. M.p.
111.0–112.0°C (from diethyl ether/petroleum ether). 1H
NMR (300 MHz, CDCl3): d = 1.03 (t, J = 7.2 Hz, 3H,
CH2CH3), 2.44 (s, 3H, CH3 Tos), 3.49 (br. s, 2H, CH2CH3),
3.58 (s, 3H, CH3 OMe), 7.29 (d, J = 8.1 Hz, 2H, ArH),
7.39–7.41 (m, 3H, ArH), 7.75 (d, J = 8.1 Hz, 2H, ArH),
7.84–7.89 (m, 3H, ArH ? bCH) ppm. 13C NMR
(75.4 MHz, CDCl3): d = 13.38 (CH2CH3), 21.53 (Tos
CH3), 44.13 (CH2), 52.08 (OCH3), 125.42 (C), 128.04
(CH), 128.62 (CH), 129.17 (CH), 130.80 (CH), 131.15 (CH),
132.51 (C), 136.62 (C), 143.22 (C), 143.32 (CH), 165.76
(C=O) ppm. C19H21NO4S (359.44): calcd. C 63.49, H 5.89,
N 3.90, S 8.92; found C 63.55, H 5.97, N 3.96, S 8.73.
Synthesis of Z-N(Et)-Z-DPhe-OH (Z-17b) The general
procedure described above was followed using Z-N(Et)-Z-
DPhe-OMe (Z-14b) (0.339 g, 1.000 mmol) as reactant to
afford Z-17b (0.307 g, 94%) as a light yellow oil that failed
to crystallize. 1H NMR (400 MHz, CDCl3): (rotamers)
d = 1.11 (t, J = 7.2 Hz, 3H, CH2CH3), 3.26, 3.46 (2br. s,
2H, CH2CH3), 5.16 (s, 2H, CH2 Z), 7.23–7.25 (m, 3H,
ArH), 7.36–7.41 (m, 4H, ArH), 7.52–7.56 (m, 3H, ArH)
7.64 (s, 1H, bCH) ppm. 13C NMR (100.6 MHz, CDCl3):
d = 12.89 (CH3), 43.58 (CH2), 67.54 (CH2 Z), 127.62
(CH), 127.79 (CH), 128.30 (CH), 128.40 (C), 128.77 (CH),
128.87 (CH), 130.27 (CH), 132.57 (C), 136.32 (C), 137.75
123

High yielding synthesis of N-ethyl dehydroamino acids
1649
(CH), 155.52 (C=O), 170.63 (C=O) ppm. m/z (HRESI-MS)
326.13868, ([M ? H]?, C19H19NO4, requires 326.13923).
171.60 (C=O) ppm. m/z (HRESI-MS) 487.22309,
([M ? H]?, C29H30N2O5, requires 487.22330).
Synthesis of the methyl esters of N-protected, N-ethyl
dehydrodipeptides
Results and discussion
General procedure for the synthesis of the methyl esters
of N-protected, N-ethyl dehydrodipeptides
An alternative to the use of strong electron withdrawing
moieties to enhance the acidity of the NH proton and allow
N-ethylation of amino acid derivatives can be the use of a
strong base. In the case of amino acids, a strong base will
induce racemisation and thus limit the value of the products
obtained. With dehydroamino acids this problem does not
occur, so the strong base potassium tert-butoxide was tes-
ted as auxiliary base in N-ethylation of dehydroamino acid
derivatives with triethyloxonium tetrafluoroborate. Ini-
tially, the methyl ester of N-(tert-butyloxycarbonyl),
b-bromo, dehydroaminobutyric acid, for which no ethyla-
tion was observed using the weak base N,N-diisopropyl-
ethylamine (Monteiro et al. 2011), was subjected to the
treatment with 2.5 equiv. of triethyloxonium tetrafluoro-
borate using 3.5 equiv. of potassium tert-butoxide as base.
In these conditions the reaction was complete in approxi-
mately 30 min allowing, the isolation of the N-ethyl
dehydroamino acid derivative in 85% yield. Thus, all other
substrates that previously had given no alkylation reaction
or incomplete reaction using DIPEA (Monteiro et al. 2011)
were subjected to ethylation using potassium tert-butoxide
as base. Thus, the methyl ester of b,b-dibromo dehydro-
alanine protected with Boc and the Z-isomers of the methyl
esters of b-bromo, dehydroaminobutyric acid and dehydr-
ophenylalanine protected with Z(NO2), Z and Boc, were
reacted with 2.5 equiv. of triethyloxonium tetrafluoroborate
using 3.5 equiv. of potassium tert-butoxide as base (com-
pounds 4c, 5a–c, 6a–c, Scheme 1; Table 1).
The N-protected, N-ethyl dehydroamino acid was dissolved
in acetonitrile (0.05 mol dm-3), followed by addition of
1.0 equiv. of HOBt, 1.1 equiv. of DCC, 1.0 equiv. of
HCl.H–L-Phe-OMe and 1.0 equiv. of NEt3 in an ice bath.
After stirring for 16 h at room temperature, the mixture
was filtered and the solvent was evaporated at reduced
pressure. The residue was dissolved in ethyl acetate
(100 cm3) and washed with KHSO4 (1 mol dm-3),
NaHCO3 (1 mol dm-3) and brine (3 9 30 cm3 each). The
organic layer was dried with MgSO4 and the solvent
evaporated at reduced pressure.
Synthesis of Z-N(Et)-Z-DAbu-L-Phe-OMe (18b) The
general procedure described above was followed using
Z-N(Et)-Z-DAbu-OH (Z-16b) (0.165 g, 0.629 mmol) as
reactant to afford 18b (0.233 g, 87%) as a colorless oil that
failed to crystallize. 1H NMR (300 MHz, CDCl3):
d = 1.02 (br. s, 3H, CH2CH3), 1.62 (d, J = 7.2 Hz, 3H,
cCH3), 3.07–3.40 (m, 4H, CH2CH3 ? bCH2), 3.71 (s, 3H,
CH3 OMe), 4.85–5.20 (m, 3H, CH2 Z ? aCH), 6.33 (d,
J = 7.2 Hz, 1H, NH), 6.87 (q, J = 7.2 Hz, 1H, bCH),
6.99–7.35 (m, 10H, ArH), 7.49 (s, 1H, bCH) ppm. 13C
NMR (300 MHz, CDCl3): d = 12.91 (CH3), 13.62 (cCH3),
37.50 (bCH2), 43.89 (CH2), 52.28 (OCH3), 53.07 (aCH),
67.37 (CH2 Z), 127.15 (CH), 127.80 (CH), 128.04 (CH),
128.41 (CH), 128.59 (CH), 129.04 (CH), 133.45 (C),
134.62 (CH), 135.58 (C), 136.23 (C), 155.02 (C=O),
163.79 (C=O), 171.61 (C=O) ppm. m/z (HRESI-MS)
425.20710, ([M ? H]?, C24H28N2O5, requires 425.20765).
For all the b-bromo dehydroamino acid derivatives,
N-ethylation reactions were complete after approximately
30 min and the corresponding N-ethylated derivative could
be isolated in high yield (Table 1). In two cases in which
E-isomers of b-bromo dehydroamino acid derivatives were
subjected to N-alkylation, (compounds E-5a and E-5c)
high yields in N-ethyl derivatives were obtained. The same
procedure applied to two b-iodo derivatives of dehydr-
ophenylalanine protected with the Z(NO2) and Boc groups
(compounds Z-7a and Z-7c) gave the corresponding
N-ethylated derivatives in excellent yields (Table 1).
N-Ethylation using the Et3OBF4/DIPEA procedure of
non-halogenated dehydroamino acids with amine protect-
ing groups other than the 4-nitrobenzenesulfonyl group had
been completely ineffective, even after several hours of
reaction and addition of large excess of alkylating agent
(Monteiro et al. 2011). However, the results obtained in
ethylation of b-halo dehydroamino acid derivatives using
as base potassium tert-butoxide, led us to reattempt the
Synthesis of Z-N(Et)-Z-DPhe-L-Phe-OMe (19b) The
general procedure described above was followed using
Z-N(Et)-Z-DPhe-OH (Z-17b) (0.321 g, 0.988 mmol) as
reactant to afford 19b (0.386 g, 80%) as a light yellow oil
that failed to crystallize. 1H NMR (400 MHz, CDCl3):
d = 0.95 (br. s, 3H, CH2CH3), 2.95–3.21 (m, 4H,
CH2CH3 ? bCH2), 3.71 (s, 3H, CH3 OMe), 4.82–5.30 (m,
3H, CH2 Z ? aCH), 6.46 (d, J = 7.2 Hz, 1H, NH),
7.01–7.32 (m, 15H, ArH), 7.49 (s, 1H, bCH) ppm. 13C
NMR (400 MHz, CDCl3): d = 13.07 (CH3), 37.52 (bCH2),
43.68 (CH2), 52.29 (OCH3), 53.48 (aCH), 67.62 (CH2 Z),
127.22 (CH), 127.90 (CH), 128.02 (CH), 128.35
(CH), 128.69 (C), 128.78 (CH), 129.09 (CH), 129.43 (CH),
129.53 (CH), 129.64 (CH), 131.25 (C), 132.91 (CH),
135.51 (C), 136.04 (C), 155.16 (C=O), 164.95 (C=O),
123

´
L. S. Monteiro, A. S. Suarez
1650
Scheme 1 Synthesis of N-ethyl
dehydroamino acid derivatives
H C
3
CH
2
H
P
N
CO CH
2
P
N
X
CO CH
2
3
3
Et OBF
3
4
(CH ) COK
3 3
X
R
R
NXS
Et N
X= Br, R = Br, 4c;
X= Br, R = Br, 8c;
H
P
N
CO CH
2 3
X= Br, R = CH , 5a-c;
X= Br, R = CH , 9a-c;
3
3
3
X= Br, R = C H , 6a-c;
X= Br, R = C H , 10a-c;
6
5
6
5
X= I, R = C H , 7a, 7c.
X= I, R = C H , 11a, 11c.
6
5
6
5
R
R = H, 1a-c;
R = CH , 2a-d;
H C
3
3
Et OBF
CH
2
3
4
R = C H , 3a-d.
6
5
P
N
R
CO CH
2 3
(CH ) COK
3 3
R = H, 12a-c;
R = CH , 13a-d;
3
R = C H , 14a-d.
6
5
P = Z(NO ), a; Z, b; Boc, c; Tos, d.
2
Table 1 Results obtained in the
N-ethylation of the methyl
esters of N-protected b,
b-dibromo and b-halo,
b-substituted dehydroamino
acids
Reactant
Product
Yield (%)
Boc-DAla(b,b-Br)-OMe, 4c
Boc-N(Et)-DAla(b,b-Br)-OMe, 8c
93
94
82
86
89
85
96
95
82
88
95
Z(NO2)-E-DAbu(b-Br)-OMe, E-5a
Z(NO2)-Z-DAbu(b-Br)-OMe, Z-5a
Z–Z-DAbu(b-Br)-OMe, Z-5b
Boc-E-DAbu(b-Br)-OMe, E-5c
Boc-Z-DAbu(b-Br)-OMe, Z-5c
Z(NO2)-Z-DPhe(b-Br)-OMe, Z-6a
Z(NO2)-Z-DPhe(b-I)-OMe, Z-7a
Z-Z-DPhe(b-Br)-OMe, Z-6b
Z(NO2)-N(Et)-E-DAbu(b-Br)-OMe E-9a
Z(NO2)-N(Et)-Z-DAbu(b-Br)-OMe, Z-9a
Z-N(Et)-Z-DAbu(b-Br)-OMe, Z-9b
Boc-N(Et)-E-DAbu(b-Br)-OMe, E-9c
Boc-N(Et)-Z-DAbu(b-Br)-OMe, Z-9c
Z(NO2)-N(Et)-Z-DPhe(b-Br)-OMe, Z-10a
Z(NO2)-N(Et)-Z-DPhe(b-I)-OMe, Z-11a
Z-N(Et)-Z-DPhe(b-Br)-OMe, Z-10b
Boc-N(Et)-Z-DPhe(b-Br)-OMe, Z-10c
Boc-N(Et)-Z-DPhe(b-I)-OMe, Z-11c
Boc-Z-DPhe(b-Br)-OMe, Z-6c
Boc-Z-DPhe(b-I)-OMe, Z-7c
alkylation of non-halogenated dehydroamino acids with
standard amine protecting groups. Thus, the methyl esters
of dehydroalanine, dehydroaminobutyric acid and dehydr-
ophenylalanine protected with Z(NO2), Z, Boc and 4-tol-
uenesulfonyl (Tos) were subjected to N-ethylation with
triethyloxonium tetrafluoroborate using potassium tert-
butoxide as base (compounds 1a–c, 2a–d, 3a–d, Scheme 1;
Table 2). With all the dehydroamino acid derivatives the
reactions were complete and the corresponding N-ethylated
derivative could be isolated in high yield (Table 2).
treated with a mixture of an aqueous solution of NaOH and
dioxane, giving in all cases high yields in the corre-
sponding N-protected, N-ethyl dehydroamino acid (com-
pounds 15c, Z-16b and Z-17b, Scheme 2).
Two of these dehydroamino acid derivatives (com-
pounds Z-16b and Z-17b) were reacted with phenylalanine
methyl ester using the standard DCC/HOBt procedure to
give the corresponding N-ethyl dehydrodipeptide deriva-
tive in good yields (compounds 18b and 19b, Scheme 2).
By removal of the Nosyl protecting group from
N-Nosyl, N-ethyl amino acid methyl esters and reprotection
of the amino function with the 9-fluorenylmethoxycarbonyl
group (Fmoc), Liguori demonstrated the compatibility of
this N-ethylation procedure with standard Fmoc chemistry
(Belsito et al. 2010). However, further conversion of the
methyl esters to acids is essential for other practical
applications of these compounds, such as incorporation
into peptides. Thus, representative methyl esters of N-ethyl
dehydroamino acid derivatives were subjected to cleavage
of the methyl ester and subsequent coupling with an amino
acid methyl ester. Three N-ethyl dehydroamino acid
derivatives (compounds 12c, Z-13b and Z-14b) were
Conclusions
A triethyloxonium tetrafluoroborate/potassium tert-butox-
ide alkylation procedure was applied for N-ethylation of
several methyl esters of b,b-dibromo dehydroalanine,
b-bromo dehydroaminobutyric acid and b-halo dehydr-
ophenylalanine, N-protected with urethane groups. All
N-ethyl, b-halo dehydroamino acid derivatives could be
obtained in high yields. This method was also applied for
N-ethylation of non-halogenated dehydroamino acids with
urethane amine protecting groups and also the 4-toluene-
sulfonyl group. With all the dehydroamino acid derivatives,
123

High yielding synthesis of N-ethyl dehydroamino acids
1651
Table 2 Results obtained in the
Reactant
Product
Yield (%)
N-ethylation of the methyl
esters of N-protected
dehydroamino acids
Z(NO2)-DAla-OMe, 1a
Z(NO2)-N(Et)-DAla-OMe, 12a
Z-N(Et)-DAla-OMe, 12b
88
91
79
90
93
92
82
97
90
92
92
Z-DAla-OMe, 1b
Boc-DAla-OMe, 1c
Boc-N(Et)-DAla-OMe, 12c
Z(NO2)-Z-DAbu-OMe, Z-2a
Z-Z-DAbu-OMe, Z-2b
Boc-Z-DAbu-OMe, Z- 2c
Tos-Z-DAbu-OMe, Z-2d
Z(NO2)-Z-DPhe-OMe, Z-3a
Z–Z-DPhe-OMe, Z-3b
Boc-Z-DPhe-OMe, Z-3c
Tos-Z-DPhe-OMe, Z-3d
Z(NO2)-N(Et)-Z-DAbu-OMe, Z-13a
Z-N(Et)-Z-DAbu-OMe, Z-13b
Boc-N(Et)-Z-DAbu-OMe, Z-13c
Tos-N(Et)-Z-DAbu-OMe, Z-13d
Z(NO2)-N(Et)-Z-DPhe-OMe, Z-14a
Z-N(Et)-Z-DPhe-OMe, Z-14b
Boc-N(Et)-Z-DPhe-OMe, Z-14c
Tos-N(Et)-Z-DPhe-OMe, Z-14d
Scheme 2 Synthesis of N-ethyl
dehydrodipeptide derivatives
H3C
H3C
H3C
CH2
CH2
CH2
CO2H
O
P
N
R
CO2CH3
P
N
R
P
N
N
H
CO2CH3
HCl.H-L-Phe-OMe
NaOH aq
Dioxane
DCC/HOBt
R
P = Boc, R = H, 15c;
P = Z, R = CH3, -16b;
P = Z, R = C6H5, -17b.
P = Z, R = CH3, 18b;
P = Z, R = C6H5, 19b.
P = Boc, R = H, 12c;
P = Z, R = CH3, -13b;
P = Z, R = C6H5, -14b.
Z
Z
Z
Z
Aydin M, Lucht N, Koenig WA, Lupp R, Jung G, Winkelmann G
(1985) Structure elucidation of the peptide antibiotics herbicolin
A and B. Liebigs Ann Chem 1985:2285–2300
Belsito EL, De Marco R, Di Gioia ML, Liguori A, Perri F, Viscomi
MC (2010) N-(4-Nitrophenylsulfonyl)- and N-(fluorenylmeth-
oxycarbonyl)-N-ethyl amino acid methyl esters—a practical
approach. Eur J Org Chem 2010:4245–4252
Bertj F, Ebert L, Gardossi L (1992) One-step stereospecific synthesis
of a, b-dehydroamino acids and dehydropeptides. Tetrahedron
Lett 33:8145–8148
Chatterjee C, Paul M, Xie L, Van Der Donk WA (2005) Biosynthesis
and mode of action of lantibiotics. Chem Rev 105:633–684
Ferreira PMT, Maia HLS, Monteiro LS, Sacramento J (1999)
High yielding synthesis of dehydroamino acid and dehydro-
peptide derivatives. J Chem Soc, Perkin Trans 1 1999:3697–
3703
Ferreira PMT, Maia HLS, Monteiro LS, Sacramento J (2001) Michael
addition of thiols, carbon nucleophiles and amines to dehydro-
amino acid and dehydropeptide derivatives. J Chem Soc Perkin
Trans 1(2001):3167–3173
Ferreira PMT, Monteiro LS, Pereira G, Ribeiro L, Sacramento J, Silva
L (2007) Reactivity of dehydroamino acids towards N-bromo-
succinimide: synthesis of b-bromo and b, b-dibromodehydroa-
mino acid derivatives and of 4-imidazolidinones. Eur J Org
Chem 2007:5934–5949
the reactions were complete and the corresponding
N-ethylated derivative could be isolated in high yields.
Some of these N-ethylated methyl ester derivatives were
converted in high yields to their corresponding acids and
coupled to an amino acid methyl ester to give N-ethyl
dehydrodipeptide derivatives in good yields.
Thus, regardless of the nature of the dehydroamino acid
and of the amine protecting group, this method constitutes
a high yielding procedure for synthesis of N-ethylated
dehydroamino acid derivatives, which can be further
applied in peptide synthesis.
Acknowledgments Foundation for Science and Technology
(FCT)—Portugal and Fundo Europeu de Desenvolvimento Regional
(FEDER) for financial support to Chemistry Centre of University of
Minho. The NMR spectrometer Bruker Avance II? 400 is part of the
National NMR Network and was purchased in the framework of the
National Program for Scientific Re-equipment; contract REDE/1517/
RMN/2005, with funds from POCI 2010, FEDER and FCT.
References
Gilon C, Dechantsreiter MA, Burkhart F, Friedler A, Kessler H
(2003) Synthesis of N-alkylated peptides. In: Goodman M, Felix
A, Moroder L, Toniolo C (eds) Houben-Weyl: methods of
organic chemistry. Synthesis of peptides and peptidomimetics;
Thieme Verlag, Stuttgart, New York, E22c:215–271
Goetz G, Nakao Y, Scheuer PJ (1997) Two acyclic kahalalides from
Abreu AS, Ferreira PMT, Monteiro LS, Queiroz MJRP, Ferreira
ICFR, Calhelha RC, Estevinho LM (2004) Synthesis of pure
stereoisomers of benzo[b]thienyl dehydrophenylalanines by
Suzuki cross-coupling. Preliminary studies of antimicrobial
activity. Tetrahedron 60:11821–11828
the sacoglossan mollusk Elysia rufescens.
60:562–567
J
Nat Prod
Aurelio L, Brownlee RTC, Hughes AB (2004) Synthetic preparation
of N-methyl-a-amino acids. Chem Rev 104:5823–5846
123

´
L. S. Monteiro, A. S. Suarez
1652
Hamann MT, Scheuer PJ (1993) Kahalalide F: a bioactive depsipep-
tide from the sacoglossan mollusk Elysia rufescens and the green
alga Bryopsis sp. J Am Chem Soc 115:5825–5826
Jung G (1991) Lantibiotics: ribosomally synthesized biologically
active polypeptides containing sulfide bridges and a, b-didehyd-
roamino acids. Angew. Chem Int Ed Engl 30:1051–1068
Kotha S (2003) The building block approach to unusual a-amino acid
derivatives and peptides. Acc Chem Res 36:342–351
Kubo A, Shiba T (1992) Effects of dehydroalanine on peptide
conformations. J Am Chem Soc 114:5634–5642
Queiroz MJRP, Begouin A, Pereira G, Ferreira PMT (2008) New
synthesis of methyl 5-aryl or heteroaryl pyrrole-2-carboxylates
by a tandem Sonogashira coupling/5-endo-dig-cyclization from
b-iododehydroamino acid methyl esters and terminal alkynes.
Tetrahedron 64:10714–10720
Rilatt I, Caggiano L, Jackson RFW (2005) Development and
applications of amino acid derived organometallics. Synlett
2005:2701–2719
Silva NO, Abreu AS, Ferreira PMT, Monteiro LS, Queiroz MJRP
(2002) Synthesis using Suzuki cross coupling of sulfur analogues
of dehydrotryptophan with definite stereochemistry. Eur J Org
Chem 2002:2524–2528
Wenger RM (1985) Synthesis of cyclosporine and analogues:
structural requirements for immunosuppressive activity. Angew
Chem Int Ed Engl 24:77–85
´
´
Monteiro LS, Kołomanska J, Suarez AC (2010) Synthesis of novel
non-proteinogenic amino acids: N-ethyl-a, b-dehydroamino acid
methyl esters. Eur J Org Chem 2010:6731–6735
´
Monteiro LS, Andrade JJ, Suarez AC (2011) Synthesis of new
N-ethyl dehydroamino acid derivatives: N-ethyl b, b-dibromo,
N-ethyl b-bromo b-substituted, N-ethyl b, b-disubstituted
N-protected dehydroamino acid methyl esters. Eur J Org Chem.
doi:10.1002/ejoc.201100907
Palmer DE, Pattaroni C, Nunami K, Chadha RK, Goodman M,
Wakamiyia T, Fukase K, Horimoto S, Kitazawa M, Fujita H,
123