N-(4-nitrophenylsulfonyl)- And N-(fluorenylmethoxycarbonyl)-N-ethyl amino acid methyl esters - A practical approach

Article abstract of DOI:10.1002/ejoc.201000256

An efficient one-pot preparation of N-ethyl-N-4-nitrophenylsulfonyl (nosyl) amino acid methyl esters was accomplished by a simple N-ethylation reaction by using triethyloxonium tetrafluoroborate in the presence of N,N- diisopropylethylamine, The N-ethylated amino acid methyl esters are obtained with total retention of stereochemistry at the original chiral centers. To further broaden the scope of this methodology, the N-ethylated nosyl-protected compounds are easily converted in the more practical fluorenylmethyloxycarbonyl (Fmoc)-protected derivatives. The cleavage of methyl ester by using a mild and neutral method enables the preparation of N-ethyl amino acids that are building blocks suitable for introduction into a peptide chain. The methodology works well with both nosyl- and Fmoc-based solution-phase peptide synthesis.

Full text of DOI:10.1002/ejoc.201000256

FULL PAPER
DOI: 10.1002/ejoc.201000256
N-(4-Nitrophenylsulfonyl)- and N-(Fluorenylmethoxycarbonyl)-N-ethyl Amino
Acid Methyl Esters – A Practical Approach
Emilia Lucia Belsito,[a] Rosaria De Marco,[a] Maria Luisa Di Gioia,[a] Angelo Liguori,*[a]
Francesca Perri,[a] and Maria Caterina Viscomi[a]
Keywords: Amino acids / Protecting groups / Alkylation / One-pot synthesis
An efficient one-pot preparation of N-ethyl-N-4-nitrophen- verted in the more practical fluorenylmethyloxycarbonyl
ylsulfonyl (nosyl) amino acid methyl esters was accomplished
by a simple N-ethylation reaction by using triethyloxonium
tetrafluoroborate in the presence of N,N-diisopropylethyl-
(Fmoc)-protected derivatives. The cleavage of methyl ester
by using a mild and neutral method enables the preparation
of N-ethyl amino acids that are building blocks suitable for
amine. The N-ethylated amino acid methyl esters are obtained introduction into a peptide chain. The methodology works
with total retention of stereochemistry at the original chiral
centers. To further broaden the scope of this methodology,
the N-ethylated nosyl-protected compounds are easily con-
well with both nosyl- and Fmoc-based solution-phase pep-
tide synthesis.
cedure is a drawback of this method. The N-ethylation of
amino acids has been generally achieved by transforming
the N–H function of amino acids into iminic or amino-
acetalic systems. In recent years, the reductive amination,
by using sodium cyanoborohydride, of imines derived from
amino acids and acetaldehyde has constituted an efficient
approach for the N-ethylation of amino acids,[9,10] but over
alkylation readily occurs and is of major concern.[11,12] In
addition, the N-ethylation can be achieved smoothly by
starting from α-amino acids with hexafluoroacetone as the
protecting and activating agent; the subsequent reaction
with a cuprate prepared from copper(I) cyanide and one
equivalent of methyllithium gave the corresponding N-ethyl
compounds.[13] Nevertheless, traces of the N-methyl ana-
logues were also detected. In the attempt to obtain pro-
tected N-ethyl amino acids, an important development has
been the reduction of the N-Fmoc-oxazolidinones (Fmoc =
fluorenylmethyloxycarbonyl) of various amino acids to
their corresponding N-ethyl N-Fmoc derivatives.[14] An al-
ternative route for the synthesis of N-ethyl amino acids was
provided by the reduction of O-alkyl acetohydroxamate ob-
tained from amino acids.[15] Another approach describes
the N-ethylation of carbobenzyloxy (Z)- and tert-butoxy-
carbonyl (Boc)-protected amino acids by starting from the
generation of a dianion derivative at the oxygen and nitro-
gen atoms that is subsequently alkylated with ethyl
iodide.[16] However, Hansen and co-workers[17] suggest that
this procedure affords only a trace of ethylated product be-
cause a β-elimination reaction of the alkylating agent pref-
erentially occurs. Moreover, the necessity of maintaining
the reaction temperature at –78 °C may make the reaction
inconvenient. Thus, they posit that for a successful reaction
the dianion had to be initially generated by using tert-butyl-
Introduction
N-Alkyl α-amino acids are constituents of a large
number of naturally occurring peptides and proteins.[1] The
substitution of N-alkyl α-amino acids into biologically
active peptides and, in particular, N-methylation has re-
sulted in analogues with enhanced pharmacological proper-
ties as a consequence of conformational modifications.[2,3]
Enhanced potencies have been observed when higher N-
alkyl substitutions are employed.[4] N-Ethyl amino acids
can be widely applied as building blocks for the synthesis
of N-ethylated peptides. A substitution of N-methyl-leucine
of cyclosporin A by various N-ethyl amino acids was per-
formed with the aim of blocking the main metabolic degra-
dation pathways. The corresponding N-ethyl derivative re-
sulted in analogues of cyclosporine A exhibiting nonimmu-
nosuppressive and anti-HIV activity.[5] Various protocols
have been developed for the synthesis of N-methyl amino
acids.[2] At the present time, however, only a few methods
for the synthesis of N-ethylated amino acids and their deriv-
atives are available in the literature.[6] Furthermore, the ge-
neral N-alkylation of amino acids with nonmethyl alkylat-
ing agent seems tricky mostly due to the steric hindrance of
longer carbon chains.[7] Papaioannou and co-workers have
described a Mitsunobu-type N-ethylation of tosylamino es-
ters with excess ethanol.[8] However, the detosylation pro-
[a] Department of Pharmaceutical Sciences, University of
Calabria, Edificio Polifunzionale,
87036 Arcavacata di Rende (CS), Italy
Fax: +39-0984-493265
E-mail: A.Liguori@unical.it
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000256.
Eur. J. Org. Chem. 2010, 4245–4252
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Liguori et al.
FULL PAPER
lithium as a strong non-nucleophilic base and then treated Table 1. Synthesis of N-ethyl-N-nosyl amino acid methyl esters 2a–
l.
with the powerful alkylating agent triethyloxonium tetra-
Entry
R1
R2
% Yield[a]
fluoroborate.
In the light of this, a novel and concise method for the
synthesis of N-ethylated amino acids is needed.
2a
2b
2c
2d
2e
2f
CH3
CH(CH3)2
CHCH2(CH3)2
CH(CH3)CH2CH3
H
CH2Ph
CH2CH2CO2tBu
CH2S(Bn)
CH2C6H4O(Bn)
(CH2)4NH(Boc)
H
H
H
H
96
89
97
95
96
99
85
87
91
94
CH(CH3)CH2CH3
Results and Discussion
H
H
H
H
H
2g
2h
2i
Our attention was devoted to the development of a syn-
thetic procedure that had to respond to the following
requirements: (1) A one-pot process without the need of a
low reaction temperature would be preferred. (2) Any pos-
sible side reactions responsible for a decrease in yield
should be avoided. (3) The use of a weak or dilute base is
preferred to avoid racemization. (4) A proper protecting
group for the aminic function had to be selected to suppress
the formation of diethylated product and to enhance the
acidity of the N–H proton. (5) A synthetic method compat-
ible with widely used Fmoc-based chemistry[18] would com-
plement this approach, especially for amino acids contain-
ing functionalities incompatible with Boc chemistry.
In this regard, the 4-nitrophenylsulfonyl (nosyl) protect-
ing group, firstly described by Fukuyama,[19] seemed to be
the most promising group for our purpose. Thanks to its
strong electron-withdrawing character, the nosyl group acts
as both an activating and protecting group and enhances
the reactivity of the N–H function towards various alkylat-
ing agents.[20,21] In addition, the compatibility of the nosyl
group with the more practical Fmoc protecting group, com-
monly used in peptide and amino acid synthesis has been
well documented.[20–22]
On the basis of the above considerations, we initially sub-
jected the N-nosyl-alanine methyl ester, (1a) chosen as a
model system, to treatment with 2.5 equivalents of trieth-
yloxonium tetrafluoroborate and 3.5 equivalents of N,N-di-
isopropylethylamine (DIPEA) in dichloromethane at room
temperature (Scheme 1, Table 1). To our delight, the reac-
tion was complete in only ten minutes and TLC analysis
clearly showed the total conversion of 1a into a single prod-
uct; subsequently, a simple workup afforded the corre-
sponding N-ethylated product 2a in excellent yield. No ad-
ditional chromatographic purification procedure was re-
quired.
2l
[a] Isolated yield.
tigated the scope of the reaction with respect to amino acids
containing functionalized side chains with acid-labile pro-
tecting groups (e.g. Boc, tBu, benzyl) to make the adopted
procedure more general. The N-nosyl-glutamic acid methyl
ester protected on the side-chain carboxylic function with a
tert-butyl group (1g), was chosen as a model system. It was
subjected to the ethylation reaction as described above and
gave the N-ethylated derivative 2g in 85% yield. The
method works well also for preparing N-ethyl derivatives of
other amino acids with functionalized side chains as indi-
cated in Table 1 by entries 2h–l.
It was observed that the use of a stoichiometric quantity
of base could halve the reaction yield. To demonstrate this
assumption, one equivalent of N-nosyl-alanine methyl ester
1a was treated with 2.5 equivalents of triethyloxonium tet-
rafluoroborate in the presence of an equivalent amount of
base. The starting compound 1a was still detected in the
reaction mixture even after 3 h. Thus, the mixture was
washed with water at first and then with a NaOH aqueous
solution. Final evaporation of the solvent gave the N-ethyl-
N-nosyl-alanine methyl ester 2a in 50% yield. The reaction
conducted on the starting compounds 1b–f proceeded anal-
ogously affording the N-ethylated derivatives 2b–f in 50%
yields.
The necessity of conducting the ethylation reaction by
adding an excess of base is justified by the presence of BF3
and F– arising from the decomposition of the tetrafluo-
roborated anion.[23] The fluoride ion represents the counter-
anion of the diisopropylammonium species, whereas BF3
coordinates the aminic function of the nosyl-protected
amino acid restraining its ability to interact with the electro-
phile.
Our next effort was devoted to the demonstration that
the chiral integrity of the starting α-amino acids had been
maintained throughout the synthetic process. To address
Scheme 1. Direct N-ethylation of 1a–l.
Encouraged by this promising result, we tested the N- this issue, the alkylating reaction described above was re-
ethylation reaction with other amino acids. Compounds 1b– peated with both N-nosyl-isoleucine methyl ester (1d) and
f were subjected to the above described reaction conditions N-nosyl--alloisoleucine methyl ester (1e). The resulting N-
to afford the N-ethylated derivatives in almost quantitative ethylated derivatives 2d and 2e were obtained in 95 and
yields (Scheme 1, Table 1).
96% yields, respectively. The two crude reaction products
It is worth noting that the triethyloxonium tetrafluoro- were analyzed by GCMS analysis. The GCMS analysis
borate could react with the functional groups of side-chain- (Figure 1) revealed the detection of only one diastereoiso-
functionalized amino acids. For this purpose, we next inves- mer in each of the gas chromatograms, thus confirming that
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Protected N-Ethyl Amino Acid Methyl Esters
Figure 1. GCMS analyses of the N-ethyl-N-nosyl-amino acid methyl esters: (A) N-Ethyl-N-nosyl--Ile-OMe (2d, r.t., 11.80 min). (B) N-
Ethyl-N-nosyl--alloIle-OMe (2e, r.t., 11.88 min). (C) A mixture of 2d and 2e.
the alkylation reaction occurred with retention of configu-
ration of the original stereocenters. The complete separa-
tion of 2d and 2e was confirmed by performing the GCMS
analysis (Figure 1) of an opportunely prepared mixture of
2d (25 mg) and 2e (25 mg).
The final test of this methodology required the demon-
stration of the compatibility of our developed procedure
with standard Fmoc chemistry. Thus, we planned the re-
moval of the nosyl protective group and the subsequent in-
troduction of the Fmoc group to obtain the desired N-
ethyl-N-Fmoc-amino acid methyl esters.
The removal of the nosyl group from the intermediate
N-ethylated sulfonamide 2a–h was accomplished by an aro-
matic nucleophilic substitution (SNAr) by treatment with
Scheme 2. Removal of nosyl group and reprotection of the amino
the reagent system mercaptoacetic acid/sodium methoxide
in acetonitrile/methanol (Scheme 2) by following a pro-
cedure already described in our previous paper for N-meth-
ylated amino acids.[22] Lastly, the amino function was re-
protected with the Fmoc group by treatment with Fmoc
chloride.[22]
The reaction was complete in 90 min and the Fmoc-N-
ethyl-amino acid methyl esters 3a–h were afforded in 71–
94% overall yields (Table 2).
function with a Fmoc group.
Table 2. Synthesis of N-Fmoc-N-ethyl amino acid methyl esters 3a–
h.
Entry
R1
R2
% Yield[a]
3a
3b
3c
3d
3f
3g
3h
CH3
CH(CH3)2
CHCH2(CH3)2
CH(CH3)CH2CH3
CH2Ph
CH2CH2CO2tBu
CH2S(Bn)
H
H
H
H
H
H
H
90
91
80
94
90
76
71
At this point methyl ester cleavage was required to make
these compounds available as building blocks for Fmoc-
based chemistry. To avoid racemization during the saponifi-
cation step,[24] methyl ester cleavage was easily realized by [a] Isolated yield.
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A. Liguori et al.
FULL PAPER
an SN2 dealkylation according to the efficient procedure de- N-ethyl amino acids prepared by our procedure represent
scribed by Biron and Kessler.[25] Treatment of N-ethyl-N- building blocks that are useful for solution-phase peptide
Fmoc--isoleucine methyl ester (3d), chosen as a model sys- synthesis.
tem, with lithium iodide in refluxing ethyl acetate, afforded
The present procedure has the advantages of mild reac-
after 20 h the corresponding N-ethyl-N-Fmoc--isoleucine tion conditions, short reaction times, high yields of prod-
(4) in 94% yield, which could be directly used without fur- ucts, and simple experimental workup procedures.
ther purification. The cleavage reaction did not involve ra-
1
cemization as shown by H NMR spectroscopic analysis of
Experimental Section
4. The N-ethyl-N-Fmoc amino acid 4 was now suitable for
incorporation into a peptide chain. To demonstrate the ap-
plication of this method, we prepared a dipeptide under the
standard peptide coupling conditions by using N,N-di-
General: Solvents were purified and dried by standard procedures
and distilled prior to use. Commercially available reagents were
purchased from Aldrich Chemical Co. The 1H and 13C NMR spec-
cyclohexylcarbodiimide
(DCC)/1-hydroxybenzotriazole tra were recorded at 300 and 75 MHz, respectively, on a Bruker
Avance 300 spectrometer by using CDCl3 as the solvent. Chemical
(HOBt).[26] The N-ethyl-N-Fmoc-isoleucine 4 was coupled
with the amino-free -valine methyl ester hydrochloride by
using DCC, HOBt, and N-methylmorpholine in THF for
1 h. The resulting dipeptide N-ethyl-N-Fmoc--ile--val-
OMe (5) was isolated in 90% yield and only one dia-
stereoisomer was observed by 1H NMR spectroscopic
analysis.
shifts (δ) are reported in ppm. Coupling constants (J) are reported
in Hertz (Hz). Reaction mixtures were monitored by TLC by using
Merck Silica gel 60-F254 precoated glass plates.
GCMS analyses were performed by HP-5MS (30 mϫ0.25 mm,
PhMesiloxane capillary column). The mass detector was operated
in the electron-impact ionization mode (EIMS) with an electron
energy of 70 eV. The injection port was heated to 250 °C. The oven-
temperature program was initially set at 100 °C with a hold of
2 min and ramped to 280 °C at 14 °C/min with a hold of 10 min.
Methane gas at a pressure of ca. 2 Torr was used as the CI reagent
gas. The N-nosyl α-amino acid methyl esters were prepared as de-
scribed previously.[20]
Our methodology can readily be accommodated to the
standard Fmoc and nosyl-based peptide synthetic strategy.
In an additional experiment, in fact, we successfully pre-
pared a nosyl-protected dipeptide containing an N-ethyl
amino acid. Thus, the N-ethyl-N-nosyl--isoleucine methyl
ester (2d) was subjected to the above described SN2 dealky-
lation reaction with LiI. Also in this case, the corresponding
N-ethyl-N-nosyl--isoleucine (6) was afforded in excellent
yield (96%) and without the need for chromatographic pu-
rification. The N-ethylated nosyl-protected amino acid 6
was finally activated and coupled with valine methyl ester
hydrochloride by means of DCC/HOBt as the coupling rea-
gents.
Synthesis of N-Ethyl-N-nosyl Amino Acid Methyl Esters 2a–l. Gene-
ral Procedure: DIPEA (3.5 mmol) and solid triethyloxonium tetra-
fluoroborate (2.5 mmol) were added to a solution of 1a–l (150 mg,
1 mmol) in CH2Cl2 (20 mL) under an inert atmosphere. The reac-
tion mixture was stirred at room temperature for 10 min. The mix-
ture was quenched with a 1  HCl solution (or a 10% citric acid
solution for compounds 1g and l) and then washed with a 1 
NaOH solution. The organic layer was extracted with CH2Cl2
(2ϫ10 mL) and dried with Na2SO4. Evaporation of the solvent
afforded the corresponding N-ethyl-N-nosyl amino acid methyl
esters 2a–l as yellow oils in 85–99% overall yields.
The dipeptide N-ethyl-N-nosyl--ile--valOMe (7) was
recovered after 1 h in high yield (92%) and without purifi-
cation. Also in this case, no racemization was observed as
1
N-Ethyl-N-nosyl-L-alanine Methyl Ester (2a): By following the ge-
shown by the H NMR spectrum of the crude product of
neral procedure, treatment of a solution of N-nosyl-alanine methyl
ester (1a) (150 mg, 0.52 mmol) in dry CH2Cl2 (20 mL) with DIPEA
(0.32 mL, 1.82 mmol) and triethyloxonium tetrafluoroborate
(222 mg, 1.3 mmol) afforded 2a (144 mg, 96% yield) as a yellow
7.
1
oil. H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.34 (d, J = 9 Hz, 2
Conclusions
H, o-NO2), 8.01 (d, J = 9 Hz, 2 H, m-NO2), 4.71 (q, J = 7.2 Hz, 1
H, α-CH), 3.54 (s, 3 H, OCH3), 3.39 (m, 1 H, NCH2CH3), 3.19 (m,
1 H, NCH2CH3), 1.51 (d, J = 7.5 Hz, 3 H, CH3), 1.25 (t, J =
7.1 Hz, 3 H, NCH2CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C):
δ = 171.2, 149.9, 145.7, 127.8, 123.9, 55.7, 52.3, 41.2, 17.1, 16.6
ppm. MS (EI) (rel. intensity%): m/z = 257 (100), 229 (10), 186 (25),
122 (17), 56 (14). C12H16N2O6S (316.07): calcd. C 45.56, H 5.10,
N 8.86, O 30.35, S 10.14; found C 45.47, H 5.08, N 8.88.
A mild and efficient method for the preparation of N-
ethylated amino acids has been described. The N-ethyl
amino acids were obtained by a one-pot process of mono-
ethylation of amino acids specifically acquired by using the
nosyl group as an excellent protecting group. The chiral in-
tegrity of nosyl-protected amino acids during base-induced
N-alkylation has also been investigated. The thus-obtained
N-ethyl derivatives of isoleucine and -alloisoleucine do not
show any detectable racemization.
The N-ethyl-N-nosyl-amino acid methyl esters prepared
by this method can be further converted into the corre-
sponding N-Fmoc protected derivatives. The de-esterifica-
tion is easily performed by using an SN2 dealkylation reac-
tion. The corresponding N-protected N-ethyl amino acids
are efficiently incorporated into dipeptides under standard
N-Ethyl-N-nosyl-L-valine Methyl Ester (2b): By following the gene-
ral procedure, treatment of a solution of N-nosyl-valine methyl
ester (1b) (150 mg, 0.47 mmol) in dry CH2Cl2 (20 mL) with DIPEA
(0.29 mL, 1.64 mmol) and triethyloxonium tetrafluoroborate
(200 mg, 1.2 mmol) afforded 2b (134 mg, 89% yield) as a yellow
1
oil. H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.33 (d, J = 8.7 Hz,
2 H, o-NO2), 8.03 (d, J = 8.7 Hz, 2 H, m-NO2), 4.13 (d, J =
10.5 Hz, 1 H, α-CH), 3.51 (m 1 H, NCH2CH3), 3.45 (s, 3 H,
OCH3), 3.30 (m, 1 H, NCH2CH3), 2.10 [m, 1 H, CH(CH3)2], 1.23
coupling conditions. The N-nosyl- and N-Fmoc-protected (t, J = 7.7 Hz, 3 H, NCH2CH3), 1.07 [d, J = 6.6 Hz, 3 H, CH-
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Protected N-Ethyl Amino Acid Methyl Esters
(CH3)2], 0.95 [d, J = 6.6 Hz, 3 H, CH(CH3)2] ppm. 13C NMR
(CDCl3, 75 MHz, 25 °C): δ = 170.6, 149.5, 145.9, 128.7, 123.9, 65.9,
51.6, 40.6, 28.6, 19.7, 16.2 ppm. MS (EI) (rel. intensity%): m/z =
301 (28), 285 (100), 186 (19), 158 (5), 122 (10), 56 (12).
C14H20N2O6S (344.10): calcd. C 48.83, H 5.85, N 8.13, O 27.87, S
9.31; found C 48.71, H 5.86, N 8.10.
(m, 5 H, CH2C6H5), 4.88 (dd, J = 6.6, 8.7 Hz, 1 H, α-CH), 3.58 (s,
3 H, OCH3), 3.25–3.50 (m, 3 H, CH2Ph + NCH2CH3), 2.99 (m, 1
H, CH2Ph), 1.19 (t, J = 7.0 Hz, 3 H, NCH2CH3) ppm. 13C NMR
(CDCl3, 75 MHz, 25 °C): δ = 170.8, 149.7, 145.8, 136.4, 129.1,
128.8, 128.5, 127.1, 123.9, 61.4, 52.4, 40.9, 36.5, 15.6 ppm. MS (EI)
(rel. intensity%): m/z = 333 (25), 301 (100), 186 (17), 122 (12), 91
(14), 56 (12). C18H20N2O6S (392.10): calcd. C 55.09, H 5.14, N
7.14, O 24.46, S 8.17; found C 54.96, H 5.16, N 7.13.
N-Ethyl-N-nosyl-L-leucine Methyl Ester (2c): Following the general
procedure, treatment of a solution of N-Nosyl-leucine methyl ester
1c (150 mg, 0.45 mmol) in dry CH2Cl2 (20 mL) with DIPEA
(0.27 mL, 1.59 mmol) and triethyloxonium tetrafluoroborate
(192 mg, 1.12 mmol) afforded 2c (146 mg, 97% yield) as a yellow
N-Ethyl-N-nosyl-L-glutamic Acid γ-tert-Butyl Methyl Diester (2g):
By following the general procedure, treatment of a solution of N-
nosyl-glutamic acid (OtBu) methyl ester (1g) (150 mg, 0.37 mmol)
in dry CH2Cl2 (20 mL), with DIPEA (0.23 mL, 1.29 mmol) and
1
oil. H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.33 (d, J = 8.7 Hz,
2 H, o-NO2), 8.01 (d, J = 8.7 Hz, 2 H, m-NO2), 4.65 (m, 1 H, α- triethyloxonium tetrafluoroborate (176 mg, 0.92 mmol) afforded 2g
CH), 3.48 (s, 3 H, OCH3), 3.35 (m, 1 H, NCH2CH3), 3.20 (m, 1
(135 mg, 85% yield). 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.34
H, NCH2CH3), 1.80–1.61 [m, 3 H, CH2CH(CH3)2 + CH2CH- (d, J = 9 Hz, 2 H, o-NO2), 8.02 (d, J = 9 Hz, 2 H, m-NO2), 4.66
(CH3)2], 1.26 (t, J = 7.7 Hz, 3 H, NCH2CH3), 1.01 [d, J = 6.3 Hz, (dd, J = 10.5, 4.8 Hz, 1 H, α-CH), 3.51 (s, 3 H, OCH3), 3.38 (m, 1
3 H, CH2CH(CH3)2], 0.98 [d, J = 6.3 Hz, 3 H, CH2CH(CH3)2] H, NCH2CH3), 3.14 (m, 1 H, NCH2CH3), 2.48–2.43 (m, 2 H, γ-
ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 171.4, 149.9, 145.5,
128.7, 123.9, 58.5, 52.1, 42.2, 41.1, 39.2, 24.4, 22.9, 21.3, 16.6 ppm.
MS (EI) (rel. intensity%): m/z = 299 (100), 243 (11), 186 (11), 122
CH2), 2.33 (m, 1 H, β-CH2), 1.92 (m, 1 H, β-CH2), 1.47 (s, 9 H,
tBu), 1.24 (t, J = 7.2 Hz, 3 H, NCH2CH3) ppm. 13C NMR (CDCl3,
75 MHz, 25 °C): δ = 171.7, 170.6, 149.9, 145.5, 128.7, 123.9, 80.9,
(7), 56 (7). C15H22N2O6S (358.12): calcd. C 50.27, H 6.19, N 7.82, 59.6, 52.3, 42.2, 31.2, 28.1, 25.3, 14.1 ppm. C18H26N2O8S (430.14):
O 26.78, S 8.95; found C 50.39, H 6.17, N 7.80.
calcd. C 50.22, H 6.09, N 6.51, O 29.73, S 7.45; found C 50.32, H
6.07, N 6.49.
N-Ethyl-N-nosyl- -isoleucine Methyl Ester (2d): By following the
L
general procedure, treatment of a solution of N-nosyl-isoleucine
methyl ester (1d) (150 mg, 0.45 mmol) in dry CH2Cl2 (20 mL) with
DIPEA (0.28 mL, 1.57 mmol) and triethyloxonium tetrafluoro-
N-Ethyl-N-nosyl-S-benzyl-L-cysteine Methyl Ester (2h): By follow-
ing the general procedure, treatment of a solution of N-nosyl-cyste-
ine (SBn) methyl ester (1h) (150 mg, 0.36 mmol) in dry CH2Cl2
(20 mL), with DIPEA (0.22 mL, 1.26 mmol) and triethyloxonium
borate (192 mg, 1.12 mmol) afforded 2d (153 mg, 95%) as a yellow
1
oil. H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.33 (d, J = 9 Hz, 2 tetrafluoroborate (171 mg, 0.90 mmol) afforded 2h (138 mg, 88%
H, o-NO2), 8.01 (d, J = 9 Hz, 2 H, m-NO2), 4.21 (d, J = 10.5 Hz, yield). 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.32 (d, J = 9.0 Hz,
1 H, α-CH), 3.54 (m, 1 H, NCH2CH3), 3.43 (s, 3 H, OCH3), 3.30
(m, 1 H, NCH2CH3), 1.92–1.71 [m, 2 H, CH(CH3)CH2CH3], 1.28– SCH2C6H5), 4.62 (t, J = 7.8 Hz, 1 H, α-CH), 3.76 (s, 2 H,
1.12 [m, 4 H, NCH2CH3 + CH(CH3)CH2CH3], 0.86–0.98 [m, 6 H, SCH2Ph), 3.60 (s, 3 H, OCH3), 3.30 (m, 1 H, NCH2CH3), 3.18 (m,
CH(CH3)CH2CH3 + CH(CH3)CH2CH3] ppm. 13C NMR (CDCl3, 1 H, NCH2CH3), 3.03 (dd, J = 13.8, 7.2 Hz, 1 H, CH2SBn), 2.71
2 H, o-NO2), 8.14 (d, J = 9.0 Hz, 2 H, m-NO2), 7.40–7.25 (m, 5 H,
75 MHz, 25 °C): δ = 170.7, 149.9, 145.5, 128.6, 123.9, 64.7, 51.5,
(dd, J = 13.8, 8.2 Hz, 1 H, CH2SBn), 1.13 (t, J = 7.0 Hz, 3 H,
40.6, 34.8, 25.3, 16.4, 15.4, 10.8 ppm. MS (C.I.): m/z (%) = 387 (9) NCH2CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 169.8,
[M + C2H5]+, 359 (7) [M + H]+, 329 (2), 299 (100), 269 (4), 172 151.6, 145.4, 137.2, 129.0, 128.7, 128.6, 127.4, 124.0, 59.6, 52.5,
(3), 156 (3), 114 (2). C16H24N2O6S (358.12): calcd. C 51.60, H 6.50,
N 7.52, O 25.78, S 8.61; found C 51.72, H 6.48, N 7.54.
41.2, 36.4, 31.5, 15.7 ppm. C19H22N2O6S2 (438.09): calcd. C 52.04,
H 5.06, N 6.39, O 21.89, S 14.62; found C 51.98, H 5.08, N 6.38.
N-Ethyl-N-nosyl-D-alloisoleucine Methyl Ester (2e): By following
N-Ethyl-N-nosyl-O-benzyl-L-tyrosine Methyl Ester (2i): By follow-
the general procedure, treatment of a solution of N-nosyl--alloiso-
leucine methyl ester (1e) (150 mg, 0.45 mmol) in dry CH2Cl2
(20 mL) with DIPEA (0.28 mL, 1.57 mmol) and triethyloxonium
tetrafluoroborate (192 mg, 1.12 mmol) afforded 2e (154 mg, 96%)
ing the general procedure, treatment of a solution of N-nosyl-tyro-
sine (OBn) methyl ester (1i) (150 mg, 0.32 mmol) in dry CH2Cl2
(20 mL) with DIPEA (0.19 mL, 1.12 mmol) and triethyloxonium
tetrafluoroborate (136 mg, 0.8 mmol) afforded 2i (136 mg, 91%
as a yellow oil. 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.34 (d, J yield) as a yellow oil. 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.23
= 9 Hz, 2 H, o-NO2), 8.04 (d, J = 9 Hz, 2 H, m-NO2), 4.23 (d, J = (d, J = 9 Hz, 2 H, o-NO2), 7.79 (d, J = 9 Hz, 2 H, m-NO2), 7.49–
7.0 Hz, 1 H, α-CH), 3.53 (m, 1 H, NCH2CH3), 3.46 (s, 3 H, OCH3), 7.32 (m, 5 H, OCH2C6H5), 7.10 (d, J = 8.7 Hz, 2 H, m-OBn), 6.88
3.31 (m, 1 H, NCH2CH3), 2.03–1.90 [m, 2 H, CH(CH3)CH2CH3], (d, J = 8.7 Hz, 2 H, o-OBn), 5.03 (s, 2 H, OCH2Ph), 4.81 (dd, J =
1.30–1.13 [m, 4 H, CH(CH3)CH2CH3 + NCH2CH3], 0.99–0.83 [m,
6 H, CH(CH3)CH2CH3 + CH(CH3)CH2CH3] ppm. 13C NMR
(CDCl3, 75 MHz, 25 °C): δ = 170.5, 149.7, 145.3, 128.7, 123.9, 63.7,
51.5, 40.2, 34.0, 25.7, 16.3, 15.2, 11.1 ppm. MS (C.I.): m/z (%) =
7.2, 8.4 Hz, 1 H, α-CH), 3.56 (s, 3 H, OCH3), 3.50–3.25 (m, 3 H,
CH2PhOBn + NCH2CH3), 2.93 (m, 1 H, CH2PhOBn), 1.20 (t, J =
7.2 Hz, 3 H, NCH2CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C):
δ = 170.8, 157.8, 149.6, 146.7, 130.2, 128.5, 128.3, 128.1, 127.6,
387 (8) [M + C2H5]+, 359 (6) [M + H]+, 329 (5), 299 (100), 269 123.9, 115.1, 70.1, 61.6, 52.3, 40.9, 35.7, 15.8 ppm. MS (EI) (rel.
(11), 172 (9), 156 (10), 114 (8). C16H24N2O6S (358.12): calcd. C
51.60, H 6.50, N 7.52, O 25.78, S 8.61; found C 51.70, H 6.51, N
7.49.
intensity%): m/z = 281 (32), 225 (100), 207 (60), 91 (8).
C25H26N2O7S (498.15): calcd. C 60.23, H 5.26, N 5.62, O 22.46, S
6.43; found C 60.37, H 5.24, N 5.63.
N-Ethyl-N-nosyl-
the general procedure, treatment of a solution of N-nosyl-phenylal-
anine methyl ester (1f) (150 mg, 0.41 mmol) in dry CH2Cl2 (20 mL) Nα-nosyl-lysine (Boc) methyl ester (1l) (150 mg, 0.34 mmol) in dry
L-phenylalanine Methyl Ester (2f): By following
Nα-Ethyl-Nα-nosyl-Nε-(tert-butyloxycarbonyl)-
(2l): By following the general procedure, treatment of a solution of
L-lysine Methyl Ester
with DIPEA (0.25 mL, 1.44 mmol) and triethyloxonium tetrafluo-
roborate (175 mg, 1.02 mmol) afforded 2f (148 mg, 99% yield) as a
yellow oil. 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.20 (d, J =
8.7 Hz, 2 H, o-NO2), 7.75 (d, J = 8.7 Hz, 2 H, m-NO2), 7.18–7.30
CH2Cl2 (20 mL), with DIPEA (0.21 mL, 1.19 mmol) and trieth-
yloxonium tetrafluoroborate (161 mg, 0.85 mmol) afforded 2l
(150 mg, 93% yield). 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 8.34
(d, J = 9.0 Hz, 2 H, o-NO2), 8.00 (d, J = 9.0 Hz, 2 H, m-NO2),
Eur. J. Org. Chem. 2010, 4245–4252
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4249

A. Liguori et al.
FULL PAPER
4.54 (dd, J = 9.9, 5.4 Hz, 1 H, α-CH), 3.49 (s, 3 H, OCH3), 3.34
(2c) (200 mg, 0.56 mmol) with the reagent system mercaptoacetic
(m, 1 H, NCH2CH3), 3.22–3.09 (m, 3 H, NCH2CH3 + ε-CH2), acid
(0.11 mL,
1.68 mmol)/sodium
methoxide
(242 mg,
1.98 (m, 1 H, β-CH2), 1.78–1.65 (m, 2 H, δ-CH2 + β-CH2), 1.56–
1.48 (m, 3 H, γ-CH2 + δ-CH2), 1.44 (s, 9 H, tBu), 1.23 (t, J =
4.48 mmol), and subsequently with Fmoc chloride (145 mg,
1
0.56 mmol), afforded 3c as a colorless oil (176 mg, 80% yield). H
7.2 Hz, 3 H, NCH2CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): NMR (CDCl3, 300 MHz, 25 °C): (rotamers) δ = 7.80–7.27 (m, 8
δ = 170.9, 168.2, 156.0, 150.3, 128.6, 123.9, 80.0, 60.2, 52.2, 41.6,
34.0, 29.8, 29.4, 28.4, 23.2, 16.4 ppm. C20H31N3O8S (473.18): calcd.
C 50.73, H 6.60, N 8.87, O 27.03, S 6.77; found C 50.88, H 6.59,
N 8.89.
H, Fmoc-ArH), 4.74, 4.64 (2m, 1 H, α-CH), 4.58–4.42 (2m, 1 H,
Fmoc-CH2), 4.35–4.22 (2m, 1 H, Fmoc-CH), 3.70, 3.60 (2 s, 3 H,
OCH3), 3.48–3.25 (2 m, 1 H, NCH2CH3), 1.90–1.46 [2m, 3 H,
CH2CH(CH3)2
+ CH2CH(CH3)2], 1.24–1.12 (2 m, 3 H,
NCH2NCH3), 1.08–0.94 [2 m, 6 H, CH2CH(CH3)2] ppm. 13C
NMR (CDCl3, 75 MHz, 25 °C): (rotamers) δ = 172.3, 156.2, 144.0,
141.3, 128.3, 128.0, 127.6, 127.4, 125.0, 124.8, 120.3, 120.1, 67.3,
67.0, 57.4, 52.1, 50.3, 47.2, 46.7, 40.9, 40.1, 38.4, 38.2, 24.7, 23.1,
21.8, 14.6, 13.9 ppm. C24H29NO4 (395.21): calcd. C 72.89, H 7.39,
N 3.54, O 16.18; found C 72.93, H 7.40, N 3.55.
Synthesis of N-Ethyl-N-Fmoc Amino Acid Methyl Esters 3a–h. Ge-
neral Procedure: Mercaptoacetic acid (3 mmol) was added to a
solution of 2a–h (1 mmol) in dry acetonitrile (10 mL) and the reac-
tion mixture was maintained at 50 °C. Sodium methoxide (8 mmol)
was then gradually added to the solution with a variable amount
of methanol to facilitate the sodium methoxide solubilization. The
resulting mixture was stirred for 30 min and the conversion of the
precursors 2a–h was monitored by TLC (Et2O/petroleum ether =
6:4). A 1  HCl solution (or a 10% citric acid solution for com-
pound 2g) was then added and the mixture extracted with EtOAc
(3ϫ10 mL). The aqueous phase was basified with saturated aque-
ous NaHCO3 (pH 8). The basic phase, containing the deprotected
product, was then treated with a solution of Fmoc chloride
(1 mmol) in dry CH2Cl2 (10 mL). The reaction mixture was stirred
at room temperature for 1 h and then the organic layer was sepa-
rated. The aqueous phase was extracted with three additional por-
tions of CH2Cl2 (3ϫ10 mL). The combined organic extracts were
dried with Na2SO4 and the solvents evaporated under vacuum to
afford the corresponding N-ethyl-N-Fmoc-amino acid methyl es-
ters as oils in 71–94% overall yields.
N-Ethyl-N-Fmoc-L-isoleucine Methyl Ester (3d): By following the
general procedure, treatment of N-ethyl-N-nosyl-isoleucine methyl
ester (2d) (200 mg, 0.56 mmol) with the reagent system mercap-
toacetic acid (0.11 mL, 1.68 mmol)/sodium methoxide (242 mg,
4.48 mmol), and subsequently with Fmoc chloride (145 mg,
1
0.56 mmol), afforded 3d as a colorless oil (208 mg, 94% yield). H
NMR (CDCl3, 300 MHz, 25 °C): (rotamers) δ = 7.75–7.28 (m, 8
H, Fmoc-ArH), 4.64–4.42 (m, 2 H, Fmoc-CH2), 4.25 (t, J = 5.9 Hz,
1 H, Fmoc-CH), 4.15 (m, 1 H, α-CH), 3.68 (s, 3 H, OCH3), 3.44–
3.11 (m, 2 H, NCH2), 1.93 [m, 1 H, CH(CH3)CH2CH3], 1.42–1.32
[m, 2 H, CH(CH3)CH2CH3], 1.13 (t, J = 6.6 Hz, 3 H, NCH2CH3),
0.96–0.81 [m, 6 H, CH(CH3)CH2CH3, CH(CH3)CH2CH3] ppm.
13C NMR (CDCl3, 75 MHz, 25 °C): (rotamers) δ = 171.9, 156.9,
144.1, 141.5, 127.4, 127.3, 125.1, 119.9, 67.3, 63.1, 51.8, 47.5, 39.4,
33.8, 24.8, 15.9, 14.2, 10.9 ppm. C24H29NO4 (395.21): calcd. C
72.89, H 7.39, N 3.54, O 16.18; found C 73.08, H 7.37, N 3.53.
N-Ethyl-N-Fmoc-L-alanine Methyl Ester (3a): By following the ge-
neral procedure, treatment of N-ethyl-N-nosyl-alanine methyl ester
(2a) (200 mg, 0.63 mmol) with the reagent system mercaptoacetic
N-Ethyl-N-Fmoc-L-phenylalanine Methyl Ester (3f): By following
acid
(0.13 mL,
1.89 mmol)/sodium
methoxide
(272 mg,
the general procedure, treatment of N-ethyl-N-nosyl-phenylalanine
methyl ester (2f) (200 mg, 0.51 mmol) with the reagent system mer-
captoacetic acid (0.11 mL, 1.53 mmol)/sodium methoxide (220 mg,
4.08 mmol), and subsequently with Fmoc chloride (132 mg,
5.04 mmol), and subsequently with Fmoc chloride (163 mg,
0.63 mmol), afforded 3a as a colorless oil (200 mg, 90% yield).1H
NMR (CDCl3, 300 MHz, 25 °C): (rotamers) δ = 7.81–7.28 (m, 8
H, Fmoc-ArH), 4.61–4.10 (m, 4 H, Fmoc-CH, Fmoc-CH2, α-CH),
3.70 and 3.60 (2 s, 3 H, OCH3), 3.43–3.29, 3.24–3.05 (2 m, 2 H,
NCH2), 1.44, 1.33 (2 d, J = 7.2 Hz, 3 H, CHCH3), 1.15, 1.06 (2 t,
J = 7.2 Hz, 3 H, NCH2CH3) ppm. 13C NMR (CDCl3, 75 MHz,
25 °C): (rotamers) δ = 171.5, 155.2, 143.8, 141.4, 127.7, 127.0,
124.9, 119.9, 67.3, 54.9, 52.2, 47.3, 40.8, 15.4, 14.9 ppm.
C21H23NO4 (353.16): calcd. C 71.37, H 6.56, N 3.96, O 18.11;
found C 71.56, H 6.54, N 3.97.
1
0.51 mmol), afforded 3f as a colorless oil (198 mg, 90% yield). H
NMR (CDCl3, 300 MHz, 25 °C): (rotamers) δ = 7.82–7.13 (m, 13
H, Fmoc-ArH + C6H5), 4.81–3.99 (m, 4 H, Fmoc-CH, Fmoc-CH2,
α-CH), 3.74, 3.47 (2 s, 3 H, OCH3), 3.36 (dd, J = 14.1, 5.4 Hz, 1
H, CH2Ph), 3.19 (2 m, 1 H, NCH2CH3), 2.97 (dd, J = 14.1, 5.4 Hz,
1 H, CH2Ph), 2.79 (2 m, 1 H, NCH2CH3), 0.87–0.79 (2 m, 3 H,
NCH2CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): (rotamers)
δ = 171.4, 155.8, 144.4, 141.5, 137.9, 129.2, 128.5, 127.6, 127.0,
126.6, 125.1, 120.1, 67.2, 65.2, 62.1, 61.6, 52.3, 50.3, 47.3, 43.3,
42.9, 35.6, 35.3, 13.7, 13.0 ppm. C27H27NO4 (429.51): calcd. C
75.50, H 6.34, N 3.26, O 14.90; found C 75.36, H 6.35, N 3.24.
N-Ethyl-N-Fmoc-L-valine Methyl Ester (3b): By following the gene-
ral procedure, treatment of N-ethyl-N-nosyl-valine methyl ester (2b)
(200 mg, 0.58 mmol) with the reagent system mercaptoacetic acid
(0.12 mL, 1.74 mmol)/sodium methoxide (250 mg, 4.64 mmol), and
subsequently with Fmoc chloride (150 mg, 0.58 mmol), afforded 3b
as a colorless oil (201 mg, 91% yield). 1H NMR (CDCl3, 300 MHz,
25 °C): (rotamers) δ = 7.78–7.30 (m, 8 H, Fmoc-ArH), 4.59–4.40
(2 m, 2 H, Fmoc-CH2), 4.34 (d, J = 10.5 Hz, 1 H, α-CH), 4.26 (t,
J = 6 Hz, 1 H, Fmoc-CH), 3.71, 3.65 (2 s, 3 H, OCH3), 3.40–3.07
(2 m, 2 H, NCH2), 2.18, 2.06 [2 m, 1 H, CH(CH3)2], 1.10 (t, J =
7.0 Hz, 3 H, NCH2CH3), 0.99–0.83 [m, 6 H, CH(CH3)2] ppm. 13C
NMR (CDCl3, 75 MHz, 25 °C): (rotamers) δ = 173.4, 157.2, 144.0,
142.7, 141.4, 128.2, 127.7, 127.3, 125.1, 124.8, 120.0, 119.9, 71.6,
64.0, 51.9, 47.4, 39.2, 27.7, 19.8, 18.8, 14.1, 13.5 ppm. C23H27NO4
(381.19): calcd. C 72.42, H 7.13, N 3.67, O 16.78; found C 72.51,
H 7.14, N 3.65.
N-Ethyl-N-Fmoc-L-glutamic Acid γ-tert-Butyl Methyl Diester (3 g):
By following the general procedure, treatment of N-ethyl-N-nosyl-
glutamic acid (OtBu) methyl ester (2g) (200 mg, 0.46 mmol) with
the reagent system mercaptoacetic acid (0.097 mL, 1.39 mmol)/so-
dium methoxide (198 mg, 3.68 mmol), and subsequently with
Fmoc chloride (119 mg, 0.46 mmol), afforded 3g as a colorless oil
(164 mg, 76% yield). 1H NMR (CDCl3, 300 MHz, 25 °C): (rota-
mers) δ = 7.80–7.28 (m, 8 H, Fmoc-ArH), 4.66–4.38 (2 m, 3 H,
Fmoc-CH2 + α-CH), 4.29–4.18 (2 m, 1 H, Fmoc-CH), 3.70, 3.52
(2 s, 3 H, OCH3), 3.48–3.27 (2 m, 2 H, NCH2CH3), 3.15–3.01 (2
m, 2 H, NCH2CH3), 2.36–2.24 (m, 2 H, β-CH2), 2.11–2.02 (m, 2
H, γ-CH2), 1.45 (s, 9 H, tBu), 1.12 (2 t, J = 7.1 Hz, 3 H, NCH2CH3)
ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): (rotamers) δ = 172.2,
168.0, 156.7, 143.9, 141.2, 136.2, 135.1, 127.9, 127.6, 127.3, 127.0,
125.2, 124.9, 120.0, 119.9, 80.6, 67.3, 65.1, 59.0, 58.5, 52.3, 52.2,
N-Ethyl-N-Fmoc-L-leucine Methyl Ester (3c): By following the ge-
neral procedure, treatment of N-ethyl-N-nosyl-leucine methyl ester
4250
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 4245–4252

Protected N-Ethyl Amino Acid Methyl Esters
47.3, 41.7, 31.9, 31.7, 28.1, 24.9, 24.7, 14.2, 13.7 ppm. C27H33NO6
(467.23): calcd. C 69.36, H 7.11, N 3.00, O 20.53; found C 69.17,
H 7.13, N 2.98.
nosyl-isoleucine were dissolved in dry THF (20 mL). The solution
was stirred and cooled in an ice/water bath while DCC (1.15 mmol)
was added. Stirring was continued for 1 h at 0 °C and an additional
hour at room temperature whilst monitoring by TLC. N,NЈ-Dicy-
clohexylurea was removed by filtration and the solvent evaporated
in vacuo. A mixture of ethyl acetate (30 mL) and a saturated solu-
tion of NaHCO3 in water (10 mL) was added to the residue and
the organic phase extracted with a 10% solution of citric acid in
water (10 mL), again with saturated NaHCO3 (10 mL), and then
brine. The organic layer was dried with Na2SO4, filtered, and the
solvents evaporated to dryness in vacuo to afford the N-ethylated
dipeptides 5 and 7 as oils in 90 and 92% yields, respectively.
N-Ethyl-N-Fmoc-(S)-benzyl-L-cysteine Methyl Ester (3h): By fol-
lowing the general procedure, treatment of N-ethyl-N-nosyl-cyste-
ine (SBn) methyl ester (2h) (200 mg, 0.45 mmol) with the reagent
system mercaptoacetic acid (0.095 mL, 1.37 mmol)/sodium meth-
oxide (194 mg, 3.59 mmol), and subsequently with Fmoc chloride
(116 mg, 0.45 mmol), afforded 3h as a colorless oil (154 mg, 71%
yield). 1H NMR (CDCl3, 300 MHz, 25 °C): (rotamers) δ = 7.82–
7.24 (m, 13 H, Fmoc-ArH + SCH2C6H5), 4.58–4.16 (m, 4 H,
Fmoc-CH2 + FmocCH + α-CH), 4.06, 4.03 (2 s, 2 H, SCH2Ph),
3.96 (s, 3 H, OCH3), 3.54–3.04 (m, 3 H, NCH2CH3 + CH2SBn),
2.93 (m, 1 H, CH2SBn), 0.91 (t, J = 6.9 Hz, 3 H, NCH2CH3) ppm.
13C NMR (CDCl3, 75 MHz, 25 °C): (rotamers) δ = 170.5, 165.8,
145.0, 144.5, 142.7, 141.5, 141.3, 128.5, 128.2, 127.3, 127.2, 125.1,
125.0, 123.8, 120.2, 120.0, 71.6, 65.1, 50.4, 44.7, 38.8, 36.7, 32.1,
13.3 ppm. C28H29NO4S (475.18): calcd. C 70.71, H 6.15, N 2.95,
O 13.46, S 6.74; found C 70.58, H 6.13, N 2.96.
N-Ethyl-N-Fmoc-L-isoleucyl-L-valine (OMe) (5): By following the
general procedure, treatment of N-ethyl-N-Fmoc-isoleucine (4)
(200 mg, 0.52 mmol) with valine methyl ester hydrochloride (88 mg,
0.52 mmol), HOBt (77 mg, 0.57 mmol), N-methylmorpholine
(52 mg, 0.52 mmol), and DCC (124 mg, 0.60 mmol) afforded 5 as
a yellow oil (232 mg, 90% yield). 1H NMR (CDCl3, 300 MHz,
25 °C): (rotamers) δ = 7.87–7.28 (m, 9 H, Fmoc-ArH + NH), 4.60–
4.03 (m, 5 H, Fmoc-CH2 + FmocCH + α-CHIle + α-CHVal), 3.78,
3.60 (2 s, 3 H, OCH3), 3.43–3.09 (m, 2 H, NCH2), 2.16 [m, 1 H,
CH(CH3)2], 1.69–1.60 [m, 3 H, CH(CH3)CH2CH3 + CH(CH3)-
Synthesis of N-Ethyl-N-Fmoc-L-isoleucine (4) and N-Ethyl-N-nosyl-
L-isoleucine (6). General Procedure: The N-ethyl-N-Fmoc-isoleucine
methyl ester (3d) or the N-ethyl-N-nosyl--isoleucine methyl ester
(2d) (1 mmol) and LiI (5 mmol) were dissolved in ethyl acetate
(5 mL). The reaction mixture was heated at reflux for 24 h and the
conversion of the precursors 3d and 2d was monitored by TLC. A
saturated aqueous Na2CO3 solution was then added and extracted
with ethyl acetate. The aqueous phase was acidified with 1  HCl
and extracted with ethyl acetate (3ϫ10 mL). The organic phase
was washed with brine. The organic phase was dried with Na2SO4
and the solvents evaporated under vacuum to afford the corre-
sponding amino acids 4 and 6 as oils in 94 and 96% yields, respec-
tively.
CH2CH3], 1.03–0.80 [m, 12 H, NCH2CH3
+ CH(CH3)2 +
CH(CH3)CH2CH3] ppm. C29H38N2O5 (494.62): calcd. C 70.42, H
7.74, N 5.66, O 16.17; found C 70.59, H 7.72, N 5.67.
N-Ethyl-N-nosyl-L-isoleucyl-L-valineOMe (7): By following the ge-
neral procedure, treatment of N-ethyl-N-nosyl-isoleucine
6
(200 mg, 0.58 mmol) with valine methyl ester hydrochloride (97 mg,
0.58 mmol), HOBt (86 mg, 0.64 mmol), N-methylmorpholine
(59 mg, 0.58 mmol), and DCC (137 mg, 0.67 mmol) afforded 7 as
a yellow oil (243 mg, 92% yield). 1H NMR (CDCl3, 300 MHz,
25 °C): δ = 8.35 (d, J = 9 Hz, 2 H, o-NO2), 8.02 (d, J = 9 Hz, 2 H,
m-NO2), 6.70 (d, J = 8.4 Hz, NH), 4.45 (dd, J = 8.4, 4.8 Hz, 1 H,
NHCH), 3.85 (d, J = 10.8 Hz, CHCONH), 3.73 (s, 3 H, OCH3),
3.55 (m, 1 H, NCH2CH3), 3.30 (m, 1 H, NCH2CH3), 2.18 [m, 1 H,
CH(CH3)2], 1.91 [m, 1 H, CH(CH3)CH2CH3], 1.29 (t, J = 7.2 Hz,
3 H, NCH2CH3), 1.06 [m, 1 H, CH(CH3)CH2CH3], 0.97–0.90 [m,
6 H, CH(CH3)2], 0.83 [d, J = 6.3 Hz, 3 H, CH(CH3)CH2CH3], 0.72
[t, J = 7.3 Hz, 3 H, CH(CH3)CH2CH3], 0.45 [m, 1 H, CH(CH3)-
CH2CH3] ppm. C20H31N3O7S (457.54): calcd. C 52.50, H 6.83, N
9.18, O 24.48, S 7.01; found C 52.29, H 6.85, N 9.16.
N-Ethyl-N-Fmoc-L-isoleucine (4): By following the general pro-
cedure, treatment of N-ethyl-N-Fmoc-isoleucine methyl ester (3d)
(200 mg, 0.51 mmol) with LiI (339 mg, 2.53 mmol) afforded 4 as a
colorless oil (182 mg, 94% yield). 1H NMR (CDCl3, 300 MHz,
25 °C): (rotamers) δ = 7.82–7.27 (m, 8 H, Fmoc-ArH), 4.57 (d, J
= 6.0 Hz, 2 H, Fmoc-CH2), 4.25 (t, J = 5.5 Hz, 1 H, Fmoc-CH),
4.10 (m, 1 H, α-CH), 3.31 (2 m, 1 H, NCH2), 3.06 (m, 1 H, NCH2),
2.15 [m, 1 H, CH(CH3)CH2CH3], 1.45–1.31 [m, 2 H, CH(CH3)-
CH2CH3], 1.30–1.08 (2 m, 3 H, NCH2CH3), 1.01–0.82 [m, 6 H,
CH(CH3)CH2CH3, CH(CH3)CH2CH3] ppm. 13C NMR (CDCl3,
75 MHz, 25 °C): (rotamers) δ = 177.2, 155.8, 141.4, 134.3, 127.8,
127.1, 124.7, 120.0, 67.7, 65.1, 47.3, 43.3, 33.2, 25.2, 15.7, 13.8, 10.8
ppm. C23H27NO4 (381,19): calcd. C 72.42, H 7.13, N 3.67, O 16.78;
found C 72.23, H 7.12, N 3.66.
Supporting Information (see also the footnote on the first page of
1
this article): H and 13C NMR spectra of all compounds.
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Received: February 24, 2010
Published Online: June 18, 2010
4252
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 4245–4252