Synthesis of novel nonproteinogenic amino acids: N-ethyl-α,β- dehydroamino acid methyl esters

Article abstract of DOI:10.1002/ejoc.201001302

Two routes for the synthesis of N-ethyl-N-(4-nitrophenylsulfonyl)-α, β-dehydroamino acid derivatives from serine, threonine and phenylserine derivatives are presented. One route consists of dehydration of N-(4-nitrophenylsulfonyl)-β-hydroxyamino acid este

Full text of DOI:10.1002/ejoc.201001302

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201001302
Synthesis of Novel Nonproteinogenic Amino Acids: N-Ethyl-α,β-dehydroamino
Acid Methyl Esters
Luís S. Monteiro,*[a] Joanna Kołoman´ska,[a] and Ana C. Suarez[a]
Keywords: Amino acids / Elimination / α,β-Dehydroamino acids / Alkylation / N-Ethyl-α,β-dehydroamino acids
Two routes for the synthesis of N-ethyl-N-(4-nitrophenyl-
sulfonyl)-α,β-dehydroamino acid derivatives from serine,
threonine and phenylserine derivatives are presented. One
route consists of dehydration of N-(4-nitrophenylsulfonyl)-β-
methyl esters obtained with triethyloxonium tetrafluoro-
borate. The second strategy applied the same procedures but
in inverse order: alkylation followed by dehydration. These
methods made it possible to obtain for the first time, new
hydroxyamino acid esters with di-tert-butyl dicarbonate cat- non-natural amino acids, which incorporate both an N-ethyl
alyzed by 4-(dimethylamino)pyridine, followed by alkylation
and an α,β-dehydro moiety.
of the N-(4-nitrophenylsulfonyl)-α,β-dehydroamino acid
chemical detosylation step could be overcome by reductive
electrochemical cleavage of the protecting group.[7] The
stronger electron-withdrawing effect of nitroarylsulfon-
amides further enhances the acidity of the α-amide hydro-
gen atom, making these groups unique for the preparation
of N-alkyl peptides.[8] N-(Nitrophenylsulfonyl)amino acid
chlorides have also been used to couple to extremely hin-
dered amines on a solid support giving better results than
analogous Fmoc-amino acid chlorides.[8] Recently, Liguori
et al. proposed the ethylation of several 4-nitrophenyl-
sulfonyl (nosyl) protected amino acids using triethyl-
oxonium tetrafluoroborate (Et3OBF4) as alkylating agent to
give N-ethylamino acid derivatives in high yields.[9b] These
authors demonstrated the compatibility of the procedure
with standard Fmoc chemistry.[9] Thus, removal of the
nosyl group was accomplished by an aromatic nucleophilic
substitution (mercaptoacetic acid/sodium methoxide), and
the amino function was reprotected with the Fmoc group
by treatment with Fmoc chloride.
Dehydroamino acids can be found in several yeasts and
bacteria, in which they play a catalytic role in the active
sites of some enzymes, as well as in a variety of peptide
antibiotics of bacterial origin that include the lantibiotics
(nisin, epidermin, subtilin, gallidermin).[10] Since they affect
both chemical reactivity and conformation, dehydroamino
acids have been introduced into peptides for structure–func-
tion relationship studies and have also been used as linkers
in solid-phase peptide synthesis.[11] The chemical synthesis
of α,β-dehydroamino acids and their derivatives has been
attempted through several methods. Those that follow the
biosynthetic routes involving elimination reactions of β-hy-
droxyamino acids, β-mercaptoamino acids and N-hy-
droxyamino acids are the most important ones. In our
laboratories we developed an efficient method for the syn-
thesis of N,N-diacyl-α,β-dehydroamino acid derivatives by
Introduction
Nonproteinogenic amino acids are an important class of
organic compounds that can have intrinsic biological ac-
tivity or can be found in peptides with antiviral, antitumor,
anti-inflammatory or immunosuppressive activities. This
type of compounds is also important in drug development,
in the elucidation of biochemical pathways and in confor-
mational studies. Incorporation of non-natural amino acids
in peptide chains can change the properties of these com-
pounds, for example increasing enzymatic stability and
bioavailability. Among nonproteinogenic amino acids are
the N-alkylamino acids and dehydroamino acids, which can
be found in many biologically important peptides.[1]
N-Alkylation of the peptide bond causes changes in the
volume and conformation of peptides. N-Alkylation results
in reduced flexibility, increase of permeability for the mem-
brane (increased lipophilicity) and prevention of cleavage
by proteolytic enzymes.[2] Several N-alkylated peptides show
antibiotic, anticancer or antiviral activity. For example N-
methylleucine is found in cyclosporines.[3] A replacement of
N-methylleucine of cyclosporine A by various N-ethyl-
amino acids was performed, and the corresponding N-eth-
ylated derivatives resulted in analogues exhibiting immuno-
suppressive and anti-HIV activity.[4] Many methods of syn-
thesis of N-alkylamino acids have been developed, most of
them are N-methylations.[2] However, only a few methods
for the synthesis of N-ethylated amino acids and their deriv-
atives are available in the literature.[5] Papaioannou and co-
workers described a Mitsunobu-type N-ethylation of tosyl-
amino esters with excess ethanol.[6] Difficulties found in the
[a] Chemistry Centre, University of Minho,
Gualtar, 4710-057 Braga, Portugal
Fax: +351-253-604382
E-mail: monteiro@quimica.uminho.pt
Eur. J. Org. Chem. 2010, 6731–6735
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6731

L. S. Monteiro, J. Kołoman´ska, A. C. Suarez
SHORT COMMUNICATION
using 2 equiv. of di-tert-butyl dicarbonate (Boc2O) and 4- Table 1. Yields obtained in the synthesis of the methyl esters of N-
ethyl-N-(4-nitrophenylsulfonyl)-α,β-dehydroamino acids.[a]
(dimethylamino)pyridine (DMAP) as catalyst in dry aceto-
nitrile.[12] For the synthesis of N-acyl-α,β-dehydroamino
Compound Yield Compound Yield Compound Yield
[%]
[%]
[%]
acid derivatives, a modification of this method was sub-
sequently reported.[13] Thus, by treating β-hydroxyamino
acid derivatives with 1 equiv. of di-tert-butyl dicarbonate
and DMAP, followed by treatment with N,N,NЈ,NЈ-tet-
ramethylguanidine (TMG), N-monoprotected α,β-dehy-
droamino acid derivatives could be obtained in high yields.
In this paper we report the use of a combination of the
alkylation procedure reported by Liguori et al.[9b] and our
dehydration methodologies[12,13] to obtain new nonpro-
teinogenic amino acids, namely, N-ethyl-α,β-dehydroamino
acids.
2a
2b
2c
–
81
99
3a
3b
3c
–
84
83
4a
4b
4c
–
70
78
[a] Route A (Scheme 1).
However, in the case of the reaction with coupound 1a
the major product obtained was the methyl ester of N-(tert-
butoxycarbonyl)-β-(4-nitrophenylsulfinyl)-α,β-dehydro-
serine.[14] The introduction of the tert-butoxycarbonyl
group makes necessary a deprotection step prior to alkyl-
ation; thus, compounds 2b and 2c were treated with a 4%
solution of trifluoroacetic acid (TFA) in dichloromethane
to give compounds 3b and 3c in high yields. These N-nosyl-
α,β-dehydroamino acid derivatives were subjected to ethyl-
ation by using the conditions proposed by Liguori et al.[9b]
[2.5 equiv. of triethyloxonium tetrafluoroborate, 3.5 equiv.
of N,N-diisopropylethylamine (DIPEA) in dry dichloro-
methane] to give the corresponding N-ethyl-N-nosyl-α,β-de-
hydroamino acid derivatives in good yields (compounds 4b
and 4c, Scheme 1, Table 1).
In order to avoid the tert-butoxycarbonyl group removal
step required by Route A, an alternative strategy, in which
alkylation occurs prior to dehydration, was attempted
(Route B, Scheme 1). Thus, compounds 1a–c were treated
directly without side-chain protection with 1 equiv. of tri-
ethyloxonium tetrafluoroborate. Fortunately, the reaction
was regioselective giving in high yield (92–94%) the corre-
sponding N-ethyl-N-nosyl-β-hydroxyamino acid derivative
(compounds 5a–c, Scheme 1, Table 2).
Results and Discussion
The methodology proposed by Liguori et al. for N-ethyl-
ation of N-nosyl-protected amino acid derivatives using
2.5 equiv. of the alkylating agent triethyloxonium tetrafluo-
roborate, requires the use of side-chain protection in the
case of side-chain-functionalized amino acids.[9b] To avoid
the need for side-chain protection and subsequent deprotec-
tion, our initial approach for the synthesis of N-ethyl-α,β-
dehydroamino acid derivatives was a two-step procedure in
which the first step was dehydration followed by the alkyl-
ation reaction (Route A, Scheme 1).
Thus, the amine function of the β-hydroxyamino acids
serine, threonine and phenylserine was protected with the
nosyl group by reaction of their methyl esters with 4-nitro-
benzenesulfonyl chloride (compounds 1a–c, Scheme 1).
Dehydration was initially attempted by reaction with
1 equiv. of di-tert-butyl dicarbonate by using DMAP as cat-
alyst, followed by treatment with TMG.[13] However, this
method led to complex mixtures resulting from tert-butoxy-
carbonylation of the hydroxy group and also of the sulfon-
amide function. Thus, the alternative reaction with 2 equiv.
of di-tert-butyl dicarbonate was carried out.[12] In the case
of reaction with compounds 1b and 1c the corresponding
N-(tert-butoxycarbonyl)-N-(4-nitrophenylsulfonyl)-α,β-de-
hydroamino acid derivatives were obtained (compounds 2b
and 2c, Scheme 1, Table 1).
Table 2. Yields obtained in the synthesis of the methyl esters of N-
ethyl-N-(4-nitrophenylsulfonyl)-α,β-dehydroamino acids.[a]
Compound
Yield [%]
Compound
Yield [%]
5a
5b
5c
92
94
92
4a
4b
4c
73
–
62
[a] Route B (Scheme 1).
Scheme 1. Synthesis of N-ethyl-α,β-dehydroamino acid derivatives from N-(4-nitrophenylsulfonyl)-β-hydroxyamino acid methyl esters.
6732 Eur. J. Org. Chem. 2010, 6731–6735
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N-Ethyl-α,β-dehydroamino Acid Methyl Esters
due was partitioned between ethyl acetate (100 cm3) and KHSO4
(1 moldm–3) (30 cm3), and washed with KHSO4 (1 moldm–3) and
brine (2 times, 30 cm3 each). After drying with MgSO4, the extract
was taken to dryness at reduced pressure to give 1b (1.309 g, 82%)
as a white solid. M.p. 120.0–122.0 °C (from ethyl acetate/petroleum
These could now be dehydrated by reaction with 1 equiv.
of di-tert-butyl dicarbonate followed by treatment with
TMG. In the case of the reaction with compounds 5a and
5c, good yields of the corresponding N-ethyl-N-nosyl-α,β-
dehydroamino acid derivatives were obtained. Attempts in
dehydration of compound 5b resulted in long reaction times
and complex mixtures, which did not allow isolation of the
desired product.
1
ether). H NMR (400 MHz, CDCl3): δ = 1.33 (d, J = 6.6 Hz, 3 H,
γCH3), 3.58 (s, 3 H, CH3 OMe), 3.93 (dd, J = 2.4, J = 6.8 Hz, 1
H, βCH), 4.26–4.33 (m, 1 H, αCH), 5.61 (d, J = 9.9 Hz, 1 H, NH),
7.66 (d, J = 8.4 Hz, 2 H, ArH), 8.05 (d, J = 8.7 Hz, 2 H, ArH)
ppm. 13C NMR (100.6 MHz, CDCl3): δ = 20.03 (γCH3), 52.89
(OCH3), 60.96 (βCH), 68.27 (αCH), 124.21 (CH), 128.42 (CH),
145.79 (C), 150.09 (C), 170.45 (C=O) ppm. C11H14N2O7S (318.30):
calcd. C 41.36, H 4.48, N 8.72, S 10.11; found C 41.51, H 4.43, N
Conclusions
Two routes to obtain N-ethyl-α,β-dehydroamino acid de- 8.80, S 10.07.
rivatives involving alkylation and dehydration of N-nosyl-
Synthesis of Nosyl-D,L-Phe(β-OH)-OMe (1c): The same procedure
β-hydroxyamino acid derivatives are proposed. The route,
in which alkylation occurs prior to dehydration, results in
one step less, giving the N-ethyl derivatives of dehydroalan-
ine and dehydrophenylalanine in overall yields of 67% and
57%, respectively. The corresponding dehydroaminobutyric
as described for the preparation of 1b was applied, substituting
HCl·H-d,l-Phe(βOH)-OMe (1.158 g, 5 mmol) for HCl·H-l-Thr-
OMe to give 1c (1.693 g, 89%) as a yellow oil. M.p. 166.0–168.0 °C
(from ethyl acetate/petroleum ether). 1H NMR (300 MHz, CDCl3):
δ = 3.44 (s, 3 H, CH3 OMe), 4.14 (dd, J = 5.6, J = 5.2 Hz, 1 H,
acid and dehydrophenylalanine derivatives could also be βCH), 5.08 (s, 1 H, αCH), 5.81 (s, 1 H, NH), 7.09–7.16 (m, 3 H,
ArH Phe), 7.23–7.26 (m, 2 H, ArH Phe), 7.72 (d, J = 9.0 Hz, 2 H,
ArH nosyl), 8.18 (d, J = 9.0 Hz, 2 H, ArH nosyl) ppm. 13C NMR
(75.4 MHz, CDCl3): δ = 51.94 (OCH3), 62.72 (βCH), 72.45 (αCH),
123.94 (CH), 126.27 (CH), 127.08 (CH), 127.60 (CH), 127.66 (CH),
140.88 (C), 146.78 (C), 148.97 (C), 169.97 (C=O) ppm.
C16H16N2O7S (380.37): calcd. C 50.52, H 4.24, N 7.36, S 8.43;
found C 50.05, H 4.23, N 7.22, S 8.50.
obtained by the alternative route (dehydration prior to
alkylation) in overall yields of 48% and 64%, respectively.
Thus, it was possible to obtain for the first time new non-
natural amino acids, which incorporate both the N-ethyl
and α,β-dehydro moieties. These can be interesting precur-
sors of new peptides with potential pharmacological ac-
tivity.
Synthesis of the Methyl Esters of N-(tert-Butoxycarbonyl)-N-(4-ni-
trophenylsulfonyl)-α,β-dehydroamino Acids
Experimental Section
Synthesis of Nosyl-ΔAbu(N-Boc)-OMe (2b): Nosyl-l-Thr-OMe (1b)
(0.955 g, 3 mmol) was dissolved in dry acetonitrile (0.5 moldm–3),
followed by the addition DMAP (0.1 equiv.) and di-tert-butyl di-
carbonate (2.2 equiv.). The reaction mixture was stirred for 18 h.
The solvent was evaporated at reduced pressure. Diethyl ether
(100 cm3) was added to the extract. The organic phase was washed
with KHSO4 (1 moldm–3), NaHCO3 (1 moldm–3) and brine
(3ϫ30 cm3) and then dried with MgSO4. Removal of the solvent
afforded 2b (0.973 g, 81%) as a brown oil that solidified on stand-
General: Melting points [°C] were determined with a Gallenkamp
apparatus and are uncorrected. 1H and 13C NMR spectra were
recorded with a Varian Unity Plus at 300 and 75.4 MHz, respec-
tively, or with a Bruker Avance II+ at 400 and 100.6 MHz, respec-
tively. 1H-1H spin-spin decoupling, DEPT θ 45°, HMQC and
HMBC were used to attribute some signals. Chemical shifts are
given in ppm and coupling constants in Hz. MS and HRMS data
were recorded by the mass spectrometry service of the University
of Vigo, Spain; elemental analysis was performed with a LECO
CHNS 932 elemental analyzer. The reactions were monitored by
thin layer chromatography (TLC). Column chromatography was
performed on Macherey–Nagel silica gel 230–400 mesh. Petroleum
ether refers to the fraction with boiling range 40–60 °C. When a
solvent gradient was used, the increase of polarity was made from
neat petroleum ether to mixtures of diethyl ether/petroleum ether
by increasing the percentage of diethyl ether by 10% each time until
the isolation of the product. Solvents were used without purifica-
tion except for acetonitrile and dichloromethane, which were dried
by using standard procedures.
1
ing. M.p. 95.0–96.0 °C. H NMR (300 MHz, CDCl3): δ = 1.33 (s,
9 H, CH3 Boc), 2.05 (d, J = 7.2 Hz, 3 H, γCH3), 3.80 (s, 3 H, CH3
OMe), 7.35 (q, J = 7.2 Hz, 1 H, βCH), 8.37 (s, 4 H, ArH) ppm.
13C NMR (100.6 MHz, CDCl3): δ = 14.92 (γCH3), 27.73 [C-
(CH3)3], 52.52 (OCH3), 85.44 [C(CH3)3], 123.46 (CH), 127.12 (C)
130.81 (CH), 144.66 (C), 145.03 (CH), 150.51 (C), 163.91 (C=O),
171.14 (C=O) ppm. HRMS (ESI): calcd. for C16H20N2NaO8S
423.0838; found 423.0833.
Synthesis of Nosyl-ΔPhe(N-Boc)-OMe (2c): The same procedure as
described for the preparation of 2b was applied, substituting nosyl-
d,l-Phe(β-OH)-OMe (1c) (0.761 g, 2 mmol) for 1b to give 2c
(0.916 g, 99%) as a yellow oil. M.p. 135.0–136.0 °C (from ethyl
acetate/petroleum ether). 1H NMR (400 MHz, CDCl3): δ = 1.29 (s,
9 H, CH3 Boc), 3.87 (s, 3 H, CH3 OMe), 7.42–7.45 (m, 3 H, ArH
Phe), 7.65–7.67 (m, 2 H, ArH Phe), 7.93 (s, 1 H, βCH), 8.23 (d, J
= 9.2 Hz, 2 H, ArH nosyl), 8.30 (d, J = 9.2 Hz, 2 H, ArH nosyl)
ppm. 13C NMR (100.6 MHz, CDCl3): δ = 27.64 [C(CH3)3], 52.81
(OCH3), 85.49 [C(CH3)3], 123.26 (C), 123.36 (CH), 129.04 (CH),
130.14 (CH), 131.06 (CH), 131.17 (CH), 132.09 (C), 142.79 (βCH),
144.28 (C), 149.50 (C), 150.50 (C=O), 164.96 (C=O) ppm.
C21H22N2O8S (462.48): calcd. C 54.54, H 4.79, N 6.06, S 6.93;
found C 54.22, H 4.88, N 6.00, S 6.62.
Synthesis of the Methyl Esters of N-(4-Nitrophenylsulfonyl)-β-hy-
droxyamino Acids
Synthesis of Nosyl-
L-Ser-OMe (1a): The synthesis of this com-
pound was described elsewhere.[14]
Synthesis of Nosyl- -Thr-OMe (1b): HCl·H-l-Thr-OMe (0.848 g,
L
5 mmol) was dissolved in dichloromethane (0.1 moldm–3) followed
by addition of 2.2 equiv. of triethylamine and 1 equiv. of 4-nitro-
benzenesulfonyl chloride with vigorous stirring and cooling in an
ice bath. The reaction mixture was stirred at room temperature for
4 h. The solvent was then evaporated at reduced pressure. The resi-
Eur. J. Org. Chem. 2010, 6731–6735
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6733

L. S. Monteiro, J. Kołoman´ska, A. C. Suarez
SHORT COMMUNICATION
Synthesis of the Methyl Esters of N-(4-Nitrophenylsulfonyl)-α,β-de-
hydroamino Acids
CDCl3): δ = 13.32 (CH3), 44.70 (CH2), 52.37 (OCH3), 123.74 (CH),
124.50 (C), 128.84 (CH), 129.31 (CH), 131.18 (CH), 131.31 (CH),
132.14 (C), 144.35 (CH), 145.11 (C), 149.98 (C), 165.19 (C=O)
ppm. C18H18N2O6S (390.41): calcd. C 55.38, H 4.65, N 7.18, S
8.21; found C 55.28, H 4.70, N 7.11, S 7.93.
Synthesis of Nosyl-ΔAbu-OMe (3b): Nosyl-ΔAbu(N-Boc)-OMe (2b)
(0.961 g, 2.4 mmol) was dissolved in dichloromethane (25 cm3) fol-
lowed by the addition of TFA (1 cm3). The reaction mixture was
stirred for 18 h. Then dichloromethane (75 cm3) was added, and
the organic phase was washed with KHSO4 (1 moldm–3) and brine
(3ϫ30 cm3) and dried with MgSO4. Removal of the solvent af-
forded 3b (0.605 g, 84%) as a white solid. M.p. 116.0–118.0 °C. 1H
NMR (400 MHz, CDCl3): δ = 2.10 (d, J = 7.2 Hz, 3 H, γCH3),
3.49 (s, 3 H, CH3 OMe), 6.18 (s, 1 H, NH), 7.09 (q, J = 7.2 Hz, 1
H, βCH), 8.01 (d, J = 9.2 Hz, 2 H, ArH), 8.33 (d, J = 9.2 Hz, 2
H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 15.24 (γCH3),
52.56 (OCH3), 123.98 (CH), 124.83 (C), 128.83 (CH), 141.92
(βCH), 144.83 (C), 150.23 (C), 164.11 (C=O) ppm. C11H12N2O6S
(300.29): calcd. C 44.00, H 4.03, N 9.33, S 10.68; found C 43.46,
H 4.08, N 9.21, S 10.60. HRMS (ESI): calcd. for C11H12N2NaO6S
323.0314; found 323.0308.
Synthesis of the Methyl Esters of N-Ethyl-N-(4-nitrophenylsulfonyl)-
β-hydroxyamino Acids
Synthesis of Nosyl-L-Ser(N-Et)-OMe (5a): Nosyl-l-Ser-OMe (1a)
(0.304 g, 1 mmol) was dissolved in dry dichloromethane
(0.05 moldm–3), and N,N-diisopropylethylamine (3.5 equiv.) and
triethyloxonium tetrafluoroborate (1.0 equiv.) were added under an
inert gas. The reaction mixture was stirred at room temperature for
30 min. Then dichloromethane (80 cm3) was added. The organic
phase was washed with KHSO4 (1 moldm–3), NaHCO3
(1 moldm–3) and brine (3ϫ25 cm3) and dried with MgSO4. Re-
1
moval of the solvent afforded 5a (0.307 g, 92%) as a yellow oil. H
NMR (300 MHz, CDCl3): δ = 1.26 (t, J = 7.2 Hz, 3 H, CH3), 3.17–
3.26 (m, 1 H, NCH2CH3), 3.39–3.48 (m, 1 H, NCH2CH3), 3.63 (s,
3 H, CH3 OMe), 3.92–3.97 (m, 1 H, βCH2), 4.11–4.16 (m, 1 H,
βCH2), 4.66 (t, J = 6.0 Hz, 1 H, αCH), 8.07 (d, J = 9.0 Hz, 2 H,
ArH), 8.36 (d, J = 9.0 Hz, 2 H, ArH) ppm. 13C NMR (100.6 MHz,
CDCl3): δ = 15.83 (CH3), 42.48 (NCH2CH3), 52.56 (OCH3), 61.59
(βCH2), 61.73 (αCH), 124.04 (CH), 128.74 (CH), 145.70 (C),
150.01 (C), 169.66 (C=O) ppm. HRMS (ESI): calcd. for
C12H16N2NaO7S 355.0576; found 355.0570.
Synthesis of Nosyl-ΔPhe-OMe (3c): The same procedure as de-
scribed for the preparation of 3b was applied, substituting nosyl-
ΔPhe(N-Boc)-OMe (2c) (0.462, 1 mmol) for 2b to give 3c (0.299 g,
83%) as a yellow oil. M.p. 149.0–150.0 °C (from ethyl acetate/pe-
troleum ether). 1H NMR (300 MHz, CDCl3): δ = 3.64 (s, 3 H, CH3
OMe), 6.33 (s, 1 H, NH), 7.35–7.37 (m, 3 H, ArH), 7.60 (s, 1 H,
βCH), 7.44–7.78 (m, 2 H, ArH), 7.96 (d, J = 8.7 Hz, 2 H, ArH
nosyl), 8.25 (d, J = 8.7 Hz, 2 H, ArH nosyl) ppm. 13C NMR
(75.4 MHz, CDCl3): δ = 52.92 (OCH3), 122.08 (C), 123.90 (CH),
128.65 (CH), 128.82 (CH), 130.69 (CH), 130.82 (CH), 132.32 (C),
138.46 (βCH), 145.14 (C), 150.17 (C), 165.19 (C=O) ppm. HRMS
(ESI): calcd. for C16H14N2NaO6S 385.0470; found 385.0467.
Synthesis of Nosyl-L-Thr(N-Et)-OMe (5b): The same procedure as
described for the preparation of 5a was applied, substituting nosyl-
l-Thr-OMe (1b) (0.318 g, 1 mmol) for 1a to give 5b (0.324 g, 94%)
as brown oil, which was crystallized from diethyl ether/petroleum
ether. M.p. 91.5–93.0 °C. 1H NMR (400 MHz, CDCl3): δ = 1.32 (t,
J = 7.2 Hz, 3 H, CH3), 1.36 (d, J = 6.4 Hz, 3 H, γCH3), 3.26–3.46
(m, 1 H, CH2), 3.41–3.48 (m, 1 H, CH2), 3.54 (s, 3 H, CH3 OMe),
4.39–4.45 (m, 1 H, βCH), 4.47 (d, J = 5.2 Hz, 1 H, αCH), 8.05 (d,
J = 8.8 Hz, 2 H, ArH), 8.36 (d, J = 8.8 Hz, 2 H, ArH) ppm. 13C
NMR (100.6 MHz, CDCl3): δ = 15.76 (CH3), 19.87 (γCH3), 43.20
(CH2), 52.31 (OCH3), 65.42 (αCH), 66.91 (βCH), 123.89 (CH),
128.87 (CH), 145.36 (C), 149.96 (C), 169.76 (C=O) ppm. HRMS
(ESI): calcd. for C13H18N2NaO7S 369.0732; found 369.0728.
Synthesis of the Methyl Esters of N-Ethyl-N-(4-nitrophenylsulfonyl)-
α,β-dehydroamino Acids by Alkylation of the Methyl Esters of N-
(4-Nitrophenylsulfonyl)-α,β-dehydroamino Acids
Synthesis of Nosyl-ΔAbu(N-Et)-OMe (4b): Nosyl-ΔAbu-OMe (3b)
(0.150 g, 0.5 mmol) was dissolved in dry dichloromethane
(0.05 moldm–3) followed by the addition of N,N-diisopropylethyl-
amine (3.5 equiv.) and triethyloxonium tetrafluoroborate
(2.2 equiv.) under an inert gas. The reaction mixture was stirred at
room temperature for 30 min. Then dichloromethane (20 cm3) was
added. The organic phase was washed with KHSO4 (1 moldm–3),
NaHCO3 (1 moldm–3) and brine (3ϫ15 cm3 each) and dried with
MgSO4. Removal of the solvent afforded 4b (0.114 g, 70%) as a
Synthesis of Nosyl-D,L-Phe(β-OH)(N-Et)-OMe (5c): The same pro-
cedure as described for the preparation of 5a was applied, substitut-
ing nosyl-d,l-Phe(β-OH)-OMe (1c) (0.380 g, 1 mmol) for 1a to give
5c (0.376 g, 92%) as a yellow oil, which was crystallized from ethyl
acetate/petroleum ether. M.p. 164.0–166.0 °C. 1H NMR (400 MHz,
CDCl3): δ = 1.26 (t, J = 6.8 Hz, 3 H, CH3), 3.47 (s, 3 H, CH3
OMe), 3.47–3.58 (m, 2 H, CH2), 4.81 (d, J = 7.2 Hz, 1 H, αCH),
5.23 (d, J = 7.2 Hz, 1 H, βCH), 7.34–7.37 (m, 5 H, ArH), 7.86 (d,
J = 8.4 Hz, 2 H, nosyl ArH), 8.24 (d, J = 8.4 Hz, 2 H, nosyl ArH)
ppm. 13C NMR (100.6 MHz, CDCl3): δ = 14.14 (CH3), 42.01
(CH2), 52.22 (OCH3), 65.32 (αCH), 72.22 (βCH), 123.83 (CH),
126.72 (CH), 128.49 (CH), 128.61 (CH), 128.77 (CH), 139.15 (C),
145.79 (C), 149.81 (C), 169.40 (C=O) ppm. HRMS (ESI): calcd.
for C18H20N2NaO7S 431.0889; found 431.0887.
1
yellow oil that solidified on standing. M.p. 90.0–91.0 °C. H NMR
(400 MHz, CDCl3): δ = 1.14 (t, J = 7.2 Hz, 3 H, CH2CH3), 2.05
(d, J = 7.2 Hz, 3 H, γCH3), 3.18 (br. s, 1 H, CH2CH3), 3.55 (s, 3
H, CH3 OMe), 3.73 (br. s, 1 H, CH2CH3), 7.39 (q, J = 7.2 Hz, 1
H, βCH), 8.01 (d, J = 9.2 Hz, 2 H, ArH), 8.34 (d, J = 9.2 Hz, 2
H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3): δ = 13.97 (CH3),
15.44 (γCH3), 44.49 (CH2), 52.06 (OCH3), 123.82 (CH), 128.03 (C),
128.93 (CH), 145.44 (C), 147.55 (βCH), 149.93 (C), 163.79 (C=O)
ppm. C13H16N2O6S (328.24): calcd. C 47.55, H 4.91, N 8.53, S
9.77; found C 47.03, H 4.99, N 8.36, S 9.52. HRMS (ESI): calcd.
for C13H16N2NaO6S 351.0627; found 351.0621.
Synthesis of the Methyl Esters of N-Ethyl-N-(4-nitrophenylsulfonyl)-
α,β-dehydroamino Acids by Dehydration of the Methyl Esters of N-
Ethyl-N-(4-nitrophenylsulfonyl)-β-hydroxyamino Acids
Synthesis of Nosyl-ΔPhe(N-Et)-OMe (4c): The same procedure as
described for the preparation of 4b was applied, substituting nosyl-
ΔPhe-OMe (3c) (0.050 g, 0.138 mmol) for 3b to give 4c (0.042 g,
78%) as a yellow oil. M.p. 96.0–98.0 °C (from diethyl ether/petro-
leum ether). H NMR (400 MHz, CDCl3): δ = 1.08 (t, J = 7.2 Hz, (5a) (0.166 g, 0.5 mmol) was dissolved in dry acetonitrile
3 H, CH2CH3), 3.46 (br. s, 1 H, CH2CH3), 3.64 (s, 3 H, CH3 OMe),
3.77 (br. s, 1 H, CH2CH3), 7.43–7.45 (m, 3 H, ArH Phe), 7.88–7.90 carbonate (1.1 equiv.) were added. The reaction mixture was stirred
Synthesis of Nosyl-ΔAla(N-Et)-OMe (4a): Nosyl-l-Ser(N-Et)-OMe
1
(0.1 moldm–3), and DMAP (0.1 equiv.) followed by di-tert-butyl di-
(m, 3 H, ArH Phe, βCH), 8.03 (d, J = 9.2 Hz, 2 H, ArH nosyl),
for 30 min and then TMG added. After stirring for additional
15 min, the solvent was evaporated at reduced pressure. Diethyl
8.34 (d, J = 9.2 Hz, 2 H, ArH nosyl) ppm. 13C NMR (100.6 MHz,
6734
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Eur. J. Org. Chem. 2010, 6731–6735

N-Ethyl-α,β-dehydroamino Acid Methyl Esters
ether (100 cm3) was added to the extract. The organic phase was
washed with KHSO4 (1 moldm–3), NaHCO3 (1 moldm–3) and
brine (3ϫ30 cm3) and then dried with MgSO4. Removal of the
solvent afforded compound 4a (0.115 g, 73%) as a yellow oil. M.p.
69.0–70.0 °C (from ethyl acetate/petroleum ether). 1H NMR
(400 MHz, CDCl3): δ = 1.16 (t, J = 7.2 Hz, 3 H, CH3), 3.50 (q, J
= 7.2 Hz, 2 H, NCH2CH3), 3.68 (s, 3 H, CH3 OMe), 5.96 (s, 1 H,
βCH2), 6.54 (s, 1 H, βCH2), 8.01 (d, J = 9.2 Hz, 2 H, ArH), 8.35
(d, J = 9.2 Hz, 2 H, ArH) ppm. 13C NMR (100.6 MHz, CDCl3): δ
= 13.99 (CH3), 44.30 (NCH2CH3), 52.58 (OCH3), 124.04 (CH),
128.84 (CH), 130.48 (βCH2), 134.43 (αC), 145.00 (C), 150.06 (C),
163.63 (C=O) ppm. HRMS (ESI): calcd. for C12H14N2NaO6S
337.0470; found 337.0465.
[4] R. T. Shuman, E. L. Smithwick, R. C. A. Frederickson, P. D.
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Synthesis of Nosyl-ΔPhe(N-Et)-OMe (4c): The same procedure as
described for the preparation of 4a was applied, substituting nosyl-
d,l-Phe(β-OH)(N-Et)-OMe (5c) (0.380 g, 0.93 mmol) for 5a to give
4c (0.226 g, 62%).
Acknowledgments
The Foundation for Science and Technology (FCT) – Portugal and
the Fundo Europeu de Desenvolvimento Regional (FEDER) are
acknowledged for financial support to the Chemistry Centre of the
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 Programme for Scientific Re-
equipment (contract REDE/1517/RMN/2005) with funds from the
Programa Operacional Ciência e Inovação (POCI 2010), FEDER
and FCT.
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Received: September 17, 2010
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Published Online: October 28, 2010
Eur. J. Org. Chem. 2010, 6731–6735
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6735