Methyl mercapturate synthesis: An efficient, convenient and simple method

Article abstract of DOI:10.3390/molecules13102394

A safe and simple method for methyl S-arylmercapturate synthesis is described. Thirteen such compounds, to be used afterwards in metabolism studies, have been obtained with yields ranging from 71 to 99.6%. These compounds were obtained using a sulfa-Michael addition and synthesized by adding the corresponding thiophenols to a mixture composed of methyl 2-acetamidoacrylate (MAA), potassium carbonate and a phase transfer catalyst, Aliquat 336. MAA, the initial synthon, was itself isolated in quasi quantitative yield following a fully described synthesis.

Full text of DOI:10.3390/molecules13102394

Molecules 2008, 13, 2394-2407; DOI: 10.3390/molecules13102394
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.org/molecules
Article
Methyl Mercapturate Synthesis: An Efficient, Convenient and
Simple Method
Benoît Cossec *, Frédéric Cosnier and Manuella Burgart
Institut National de Recherche et de Sécurité, Département Polluants et Santé, Rue du Morvan CS
60027, Vandoeuvre, 54519 cedex, France
* Author to whom correspondence should be addressed. E-mail: benoit.cossec@inrs.fr.
Received: 25 July 2008; in revised form: 9 September 2008 / Accepted: 26 September 2008 /
Published: 1 October 2008
Abstract: A safe and simple method for methyl S-arylmercapturate synthesis is described.
Thirteen such compounds, to be used afterwards in metabolism studies, have been obtained
with yields ranging from 71 to 99.6%. These compounds were obtained using a sulfa-
Michael addition and synthesized by adding the corresponding thiophenols to a mixture
composed of methyl 2-acetamidoacrylate (MAA), potassium carbonate and a phase
transfer catalyst, Aliquat 336. MAA, the initial synthon, was itself isolated in quasi
quantitative yield following a fully described synthesis.
Keywords: Mercapturic acids, Sulfa-Michael addition, Phase Transfer Catalysis,
Biomarker
Introduction
Aromatic hydrocarbons belong to a broad family of chemical compounds which are widely used in
many industrial applications, alone or in mixtures, and as solvents and starting products for chemical
synthesis [1]. Along with the ever increasing awareness and knowledge about the toxicity of aromatic
hydrocarbons [2-4], there is a growing need for assessing reliable and specific biomarkers and for the
development of sensitive analytical methods dedicated to occupational exposure biomonitoring [5-10].
Mercapturic acids (MA), which are N-acetyl-L-cysteine-S-conjugates (or 2-acetamido-3-sulfanyl-
propionic acids according to IUPAC Nomenclature) are end products of the glutathione detoxification

Molecules 2008, 13
2395
pathway (GSH). Since Van Doorn’s preliminary studies [11], increasing attention has been paid to
these minor metabolites. Over the last decades, several MA derived from aromatic hydrocarbons have
been identified and their corresponding metabolic pathways have been explained [9-13]. In this
respect, Angerer et al. [13,14] demonstrated that toluene and xylenes are metabolised into MA using
two different oxidation pathways (Scheme 1). As an example, toluene has been shown to interact with
GSH by conjugation with reactive electrophilic intermediates. The nature of these intermediates
depends on the first oxidation step catalysed by cytochrome P-450 enzymes and which may occur
either at the side chain or at the aromatic ring. Side chain oxidation produces benzylic alcohol, which
is enzymatically transformed into a sulphate ester [11], then to a S-benzylmercapturic acid (A) once
conjugated with GSH. On the other hand, aromatic ring oxidation forms an arene oxide which leads to
S-p-tolylmercapturic acid (B) after conjugation with GSH [13].
Scheme 1.
CH2OH
sulphatase
side-chain
oxidation
Premercapturic
acid
ring
oxidation
Benzyl
sulphate
GSH
O
GSH
NHCOCH3
NHCOCH3
COOH
S
S
COOH
(A)
(B)
Several research teams have concluded that MAs are more selective and sensitive than traditional
biomarkers, which are generally the major metabolites [11, 15, 16]. During the same period, the
American Conference of Govermental Industrial Hygienists (ACGIH) and the Deutsche Forschungs
Gemeinschaft (DFG) decided to publish a biological exposure index for S-phenylmercapturic acid as a
biomarker of exposure to benzene [17, 18].
For the aforementioned reasons, a growing interest in the glutathione pathway and MA synthesis
has emerged. While MAs coming from side chain oxidation are well described in the literature [19-
21], MAs resulting from ring oxidation are still being investigated in order to improve their synthesis.
Scheme 2 reports the five main procedures for S-arylmercapturic acids (AMA) (or ester derivative)
synthesis: the reaction of aryl diazonium salts with cysteine cuprous mercaptides or with N-acetyl-L-
cysteine [13, 22, 23], the copper-assisted or palladium-assisted nucleophilic substitution of aryliodides
[24, 25], the nucleophilic substitution of aryl nitrate or nitroarylhalide [26-28], a Mitsunobu-type
coupling reaction [29], and finally the process of Behringer et al. [30, 31]
Most of the processes already described gave low yields of complicated mixtures [13, 22, 23], or
needed highly activated aromatics [28, 29, 31]. Moreover, they generally suffer from different
drawbacks (longer reaction time, high temperature reaction, high starting material purity, rigorous
dioxygen exclusion, tedious work-up) [30, 31].

Molecules 2008, 13
2396
Scheme 2.
OH
SCu complex
+
+
AcHN CO2CH3
+ ArN2
(Thienyl)2S +
AcHN CO2H
SAr
SH
AcHN CO2R
R = H, CH3
+ ArN2
+ ArI
ArSH + AcHN CO2H
AcHN CO2H
+ ArNO2
The aim of the present study was to provide gas chromatographic standards having a methyl S-aryl-
mercapturate structure (AME) in order to use them in biological monitoring and metabolism studies.
Thirteen compounds have thus been obtained in high yields from the procedure described in Scheme 3.
This process is a two-step reaction starting from 2-acetamidoacrylic acid (product 1) and involving a
smooth esterification of 1 followed by a sulfa-Michael addition [32] of the adequate thiophenol on the
produced acrylate (product 2, MAA) in the presence of a phase-transfer catalyst [33].
Scheme 3.
O
O
O
CH3I
ArSH
OH
O
O
Ar S
O
Cs2CO3
Aliquat 336
K2CO3
HN
HN
HN
O
O
1
2
3
Results and Discussion
Methyl 2-acetamidoacrylate synthesis (MAA; 2)
Few procedures have been described in the past for MAA synthesis, and conventional acidic
esterification attempts to obtain this ester often failed [34, 35]. This was probably due to the poor
stability of the unsaturated structure. Rothstein et al. [34] first obtained MAA using the nucleophilic
action of lead or sodium salts of compound 1 on iodomethane or dimethylsulphate, but yields never
exceeded 52%. Later on, Bueno et al. [36], using the potassium salt of 1, produced MAA in yields of
up to 80%, but in crude form. Another conventional reagent, diazomethane, is known to be inadequate
for unsaturated carboxylic acid esterification, yielding quantitatively pyrazolines structures [37]. Other
procedures reported in the literature involve methyl alaninate, cysteinate or serinate in more
complicated retrosynthetic mechanisms [38-40].
As far as we are concerned, numerous methylation and esterification methods were investigated in
the laboratory [41, 42], but only one reagent, cesium carbonate, was applied with success. Using
experimental conditions described in the literature [43], the yield was gradually improved by
increasing the ratio of iodomethane to compound 1 (Table 1). Finally, MAA was obtained
quantitatively by carefully controlling the nucleophilic substitution and the time reaction.

Molecules 2008, 13
2397
Table 1. MAA (2) synthesis.
CH3I
Cs2CO3
Entry
(equiv Time (h) Yield of 2a (%)
(equiv)
)
1
2
3
4
0.5
0.5
0.5
0.5
1.2
2.0
2.5
2.5
15
15
15
3
50
65
76
99.7
a isolated yield
Methyl S-p-toluylmercapturate (3a) synthesis: optimisation
Toluene was the first aromatic hydrocarbon to be examined when our biomonitoring studies began.
Optimisation (Scheme 2, step 2) was therefore applied to obtain 3a, a mercapturate stemming from
toluene ring oxidation (Scheme 1) [13]. The results concerning this particular optimisation are
summarized in Table 2. Tetrabutylammonium hydrogensulfate (TBAHS), which generally performs
successfully in all kinds of phase transfer catalysis (PTC) reactions [44, 45], was initially chosen as a
catalyst. Using liquid-liquid conditions (entry 1), a complete deesterification of 2 unfortunately
resulted. This negative result prompted us on the one hand to suppress water and to use a solid-liquid
process [46-48] and on another hand to replace NaOH by KOH. Three reaction-condition parameters,
i.e. the organic phase, the basic anion and the catalyst used, were then successively modified to
improve the yields obtained.
Three solvents were thus investigated and put through the procedure. From the results of runs 3, 4
and 5, toluene may be considered as the reference solvent, other solvents appearing less suitable. This
result is not in accordance with Herriott et al. who suggested that the reaction rate in PTC should be
directly related to solvent polarity [49]. When the results of runs 9 and 10 were analysed, the
relationship between solvent and yield did not appear so simple. The results observed with THF
(tetrahydrofuran), which is a polar solvent, were similar to those obtained with toluene.
The investigations then moved on to screening two different bases [50]. The results obtained in runs
2 and 3, and comparatively in runs 8 and 9, clearly demonstrate that K2CO3 is superior to KOH in
terms of compound 3a formation. Furthermore, the reactions with this latter base were by far the
cleanest, producing only a very minor amounts of by-products.
Two other PTC catalysts, cetyltrimethyl ammonium bromide (CTAB) and methyltrioctyl
ammonium chloride (Aliquat 336), were then successively tested to conduct the sulfa-Michael addition
(entries 3, 7, 8 and 9). Aliquat 336 emerged as the most active catalyst for our process (lowest
quantities, highest rate).
Finally, the reaction was performed with a excess of reagent (entry 11). In fact, it is well known that
arylmercaptans are particularly sensitive to oxidation. With an excess of p-cresol a quasi complete
conversion was observed. To confirm the catalytic effect, the reaction was then carried out using the
conditions described in entry 11 and avoiding the use of the selected PTC catalyst. In contrast with the
preceding results, no reaction at all was observed at room temperature over a 24-hour stirring period.

Molecules 2008, 13
2398
In summary, appropriate solid-liquid phase-transfer catalysis conditions have been determined to
carry out the addition of p-thiophenol to MAA, THF and toluene emerging as the solvents of choice.
The addition takes place rapidly by successively adding a catalytic quantity of the basic anion (K2CO3)
and then the catalyst (Aliquat 336) to the reaction mixture, which yields 3a, the desired product, quasi
quantitatively.
Table 2. Optimisation of the reaction conditions for the addition of p-thiocresol to compound 2.
p-Thiocresol
Base
(mmol)
PTC
Time
(h)
48
48
18
18
18
48
18
18
18
18
5
%Yield
Entry
Solvent
(mmol)
(mmol)
for 3a
1
2
1
1
NaOH 1M/H2O
NaOH (1.4)
KOH (1.2)
KOH (1.4)
KOH (1.2)
KOH (1.2)
KOH (1.2)
KOH (1.2)
K2CO3 (0.3)
K2CO3 (0.3)
K2CO3 (0.3)
TBAHS (0.2)
TBAHS (0.2)
TBAHS (0.2)
TBAHS (0.2)
TBAHS (0.2)
CTAB (0.2)
CH2Cl2/H2O
THF
100% of 2
1
22
22
33
31
49
6
3
1
THF
4
1
Acetonitrile
Toluene
Toluene
THF
5
1
6
1
7
1
CTAB (0.2)
8
1
Aliquat 336 (0.08)
Aliquat 336 (0.08)
Aliquat 336 (0.08)
Aliquat 336 (0.08)
THF
9
1
THF
63
61
99.6
10
11
1
Toluene
Toluene
1.4
Amount of 2 used: 1 mmol
Application to the synthesis of various AMEs (3b-m)
Using the optimised conditions established for 3a, the Michael addition was applied to numerous
thiophenols (Table 3). As expected from previous results, thiophenols bearing para but also ortho
electron donating groups reacted quickly, giving the corresponding AMEs 3b-d in remarkable yields
in less than 5 hours. Similarly, thiophenols bearing two electron donating groups afforded AMEs 3e-h
in satisfactory yields, but needed generally slightly longer reaction times.
In order to complete the knowledge of this procedure, several other thiophenols were then
investigated. It was successively demonstrated that a donating group was not required (3i), and, that
despite the electron withdrawing group presence, 3j-l can be obtained in relatively satisfactory yields
with reaction times ranging from 5 to 18 hours. In addition, and unexpectedly, a thiophenol bearing
five electron withdrawing groups (3m) gave similar results.
Finally, our procedure is safe, simple, convenient, and compatible with numerous substrates and, by
and large, produced AMEs in higher yields compared to other protocols [22-31]. Furthermore,
availability and low cost of Aliquat 336 are an additional favourable factor to consider.

Molecules 2008, 13
2399
Table 3. Methyl S-arylmercapturates (AME) synthesis.
Time
Yieldd
(%)
c
Entry
AMEa
Solvent
Tol
RF
(h)
1
2
3a
3b
3c
3d
3e
3f
5
0.28
0.31
0.29
0.32
0.32
0.32
0.26
0.31
0.26
0.27
0.25
99.6
99
94
96
95
90
82
98
96
76
88
SR
SR
SR
SR
Tol
5
5
3
Tol
4
Tol
5
5
Tol
5
SR
6
Tol
13
18
18
5
SR
SR
7
3g
3h
3i
Tol
8
Tol
SR
SR
9
Tol
Cl
10
11
3j Tol/THFb
18
5
SR
Br
3k
3l
Tol
Tol
SR
SR
H3CO
12
13
18
18
0.23
0.26
76
71
Cl
Cl
Cl
Cl
3m Tol/THFb
SR
Cl
a R = -CH2-CH(NHCOCH3)(CO2CH3)
bTol/THF: 1/1; v/v
c
The reaction was monitored by thin layer chromatography using
diethyl ether as eluent.
d Isolated yield
Conclusions
Methyl S-arylmercapturates have been synthesized according to a two-step reaction starting from
compound 1 (Scheme 2) and involving a sulfa-Michael addition whose reactivity was enhanced by

Molecules 2008, 13
2400
adding a phase transfer catalyst. A simple and efficient synthesis of MAA (2), the intermediary
synthon, is described. The complete method proceeds smoothly with typical yields ranging from 71 to
99.6%, and may be applied to various thiophenols bearing electron donating groups or electron
withdrawing groups. These compounds were then used as standards in gas chromatographic analysis
and in metabolism studies.
Experimental
General
2-Acetamidoacrylic acid (1), thiophenol, chlorothiophenol, pentachlorothiophenol, 4-toluenethiol,
2-ethylthiophenol, 2-toluenethiol, cesium carbonate, tetrabutylammonium hydrogensulfate (TBAHS),
cetyltrimethyl ammonium bromide (CTAB) and methyltrioctyl ammonium chloride (Aliquat 336)
were purchased from Sigma-Aldrich-Fluka. 4-bromothiophenol, 2,4-dimethylthiophenol, 2,5-
dimethyl-thiophenol, 3,4-dimethylthiophenol and 3,5-dimethylthiophenol were all available from
Acros Organics (Fisher Bioblock, France). Finally, 4-ethylthiophenol was obtained from Avocado
1
13
(Interchim, France). All the other chemicals were of analytical grade. H-NMR and C-NMR spectra
were obtained on a Bruker AM 400 spectrometer operated at 400 and 100 MHz, respectively. The
chemical shifts are reported in parts per million relative to TMS. The melting points (m.p.) were
determined with an Electrothermal digital melting point apparatus and were uncorrected. The
elemental analyses (EA) were performed by the Central Analysis Department of CNRS (Vernaison,
France). The mass spectra (MS) were collected in electronic impact mode (70 eV) on a Varian 1200
MS/MS mass spectrometer coupled to a Varian 3800 gas chromatograph (GC). A Varian DB-5MS
column (30 m; 0.25 mm; 0.25 μm) was used for the AME chromatography. The carrier gas was helium
(1 mL/min). The injector (split ratio = 30) was held at 250 °C. The GC oven program was: 70 °C
initial temperature, isothermal for 1.5 min, then 15 °C/min to 250 °C, and finally isothermal for 7 min.
Thin-layer chromatography (TLC) was carried out on Merck 60 F254 precoated silica gel plates
(0.25mm) using diethyl ether (100%) as eluent (RF in Table 3). Spots were visualized successively
with a UV lamp (λ=254 nm) and iodine vapors.
Methyl 2-acetamidoacrylate synthesis (MAA; 2)
Cs2CO3 (4.2 mmol) was added to 1 (1368 mg, 8.4 mmol) dissolved in MeOH (20 mL), and the
mixture was stirred at room temperature for one hour. The methanolic solution was concentrated until
dryness. Iodomethane (42 mmol; 2.7 mL), in controlled portions and undergoing constant stirring, was
added over a two-hour period to the residue dissolved in DMF (30 mL). The reaction mixture was
finally concentrated to dryness, and the crude residue, purified over a silica gel column using diethyl
ether (Et2O 100%, RF = 0.55) as eluent, gave 1199 mg of 2 as white needles, i.e. a yield of 99.7 %
1
Methyl 2-acetamidoacrylate (2). White solid, mp: 52-53°C; H-NMR (CDCl3): δ (ppm) 2.11 (s, 3 H,
COCH3), 3.82 (s, 3 H, OCH3), 5.85 (s, 1H, CH2), 6.57 (s, 1H, CH2), 7.72 (s, 1H, NH); 13C- NMR
(CDCl3): δ (ppm) 24.56 (CH3), 52.88 (CH3), 108.63 (CH2), 130.85 (C=CH2), 164.56 (CO2), 168.74

Molecules 2008, 13
2401
(CO); IR: ν (cm-1) 3364, 3155, 3057, 2963, 1813, 1708, 1678, 1629, 1512, 1428, 1369, 1327, 1246,
1208, 1172, 1038, 995, 977, 905, 856, 809; MS (m/z, EI): 143 (M+, 66), 111 (44), 101 (100), 71 (27),
43 (88), 42 (63); Anal. Calcd for C6H9NO3: C, 50.35; H, 6.34; N, 9.79; O, 33,53; Found: C, 50.41; H,
6.37; N, 9.39; O, 33,30.
General procedure for methyl S-arylmercapturate (AME) synthesis: compounds 3a-m
Compounds 3b to 3m were synthesized using the parameters defined for 3a in Table 2 entry 11 and
in Table 3. Briefly, adequate thiophenol (1.4 mmol), Aliquat 336 (32 mg, 0.08 mmol) and finally
powdered potassium carbonate (41 mg, 0.3 mmol) were added respectively to 2 (143 mg, 1 mmol)
dissolved in toluene (30 mL, except if otherwise indicated in Table 3). The reaction mixture was
stirred at room temperature and regularly monitored by thin layer chromatography using Et2O as
eluent (RF, Table 3). The mixture filtered, the crude product, isolated after solvent removal, was
subjected to silica gel column chromatography using Et2O as eluent. Yields are given in Table 3.
1
2-Acetamido-3-p-tolylsulfanylpropionic acid methyl ester (3a). White solid, mp: 84-85°C; H-NMR
(CD3OD): δ (ppm) 1.88 (s, 3H, COCH3), 2.31 (s, 3H, CH3-Ar), 3.28-3.44 (m, 2H, SCH2), 3.56 (s, 3H,
O-CH3), 4.60-4.63 (m, 1H, CH-CH2), 4.81-4.85 (m, 1H), 7.09-7.32 (m, 4H, ArH); 13C-NMR
(CD3OD): δ (ppm) 20.1 (CH3), 21.4 (CH3), 36.2 (CH2), 51.8 (CH3), 52.7 (CH), 129.9 (CHAr), 131.4
(CHAr), 131.6 (C), 137.5 (C), 171.4 (CO), 172.2 (CO); IR: ν (cm-1) 3322, 3075, 2984, 2944, 2931,
2843, 1727, 1645, 1543, 1495, 1423, 1344, 1321, 1248, 1179, 1089, 1036, 803; MS (m/z, EI): 267
(M+, 15), 208 (80), 149 (30), 43 (100); Anal. Calcd for C12H17NO3S; C 58.40, H 6.41, N 5.24, O 17.95,
S 11.99; Found: C 58.66, H 6.48, N 5.14, O 18.40, S 12.16.
2-Acetamido-3-(4-ethylphenylsulfanyl)-propionic acid methyl ester (3b). White solid, mp: 64-65°C;
3
3
1H-NMR (CD3OD): δ (ppm) 1.19 (t, J = 7.6 Hz, 3 H, CH3), 1.92 (s, 3 H, COCH3), 2.59 (q, J = 7.6
Hz, 2 H, CH2-CH3), 3.18 (dd, 2J = 14 Hz, 3J = 7.8 Hz, 1 H, SCH2), 3.34 (dd, 2J = 14 Hz, 3J = 4.8 Hz, 1
H, SCH2), 3.60 (s, 3 H, OCH3), 4.55 (dd, J = 7.8 Hz, 3J = 4.8 Hz, 1 H, CH2-CH), 4.84 (s, 1 H, NH),
3
7.13-7.16 (m, 2 H, 2-Ar-H), 7.32-7.35 (m,2 H, 2 Ar-H); 13C-NMR (CD3OD): δ (ppm) 16.2 (CH3), 22.4
(CH3), 29.5 (CH2), 37.3 (CH2), 53.0 (CH3), 53.8 (CH), 129.8 (CHAr), 131.0 (CAr), 132.7 (CHAr), 145.0
(CAr), 172.4 (CO), 173.2 (CO); IR: ν (cm-1) 3318, 3076, 2969, 2946, 2934, 1723, 1637, 1546, 1496,
1438, 1422, 1347, 1321, 1248, 1219, 1180, 1091, 1036, 1017, 814, 696; MS (m/z, EI): 281 (M+, 26),
222 (100), 207 (10), 190 (6), 163 (53), 151 (55), 136 (78), 123 (29), 105 (25), 88 (52), 77 (60), 59
(51); Anal. Calcd for C14H19NO3S; C 59.76, H 6.81, N 4.98, O 17.06, S 11.40; Found: C 59.82, H
6.79, N 4.90, O 17.35, S 11.46.
1
2-Acetamido-3-o-tolylsulfanylpropionic acid methyl ester (3c). White solid, mp: 66-67°C; H-NMR
(CD3OD): δ (ppm) 1.95 (s, 3 H, COCH3), 2.38 (s, 3 H, CH3-Ar), 3.21 (dd, 2J = 14.0 Hz, 3J = 8.0 Hz, 1
H, SCH2), 3.39 (dd, 2J = 14.0 Hz, 3J = 4.9 Hz, 1 H, SCH2), 3.64 (s, 3 H, OCH3), 4.58 (dd, 3J = 8.0 Hz,
3J = 4.9 Hz, 1 H, CH-CH2), 7.12 – 7.22 (m, 3 H, Ar-H), 7.41 – 7.43 (m, 1 H, Ar-H); 13C-NMR
(CD3OD): δ (ppm) 19.6 (CH3), 21.2 (CH3), 34.8 (CH2), 52.0 (CH3), 52.7 (CH), 126.9 (CHAr), 127.4
(CHAr), 130.5 (CHAr), 130.7 (CHAr), 134.1 (CAr), 139.1 (CAr), 171.3 (CO), 172.2 (CO); IR: ν (cm-1)

Molecules 2008, 13
2402
3311, 3069, 2952, 2940, 2838, 1736, 1650, 1542, 1432, 1335, 1223, 1168, 1036, 744; MS (m/z, EI):
267 (M+, 13), 208 (100), 149 (56), 137 (44), 134 (61), 91 (18), 43 (26); Anal. Calcd for C12H17NO3S;
C 58.40, H 6.41, N 5.24, O 17.95, S 11.99; Found: C 58.28, H 6.45, N 5.08, O 18.19, S 12.19.
2-Acetamido-3-(2-ethylphenylsulfanyl)propionic acid methyl ester (3d). White solid, mp: 55-56°C; 1H-
3
3
NMR (CD3OD): δ (ppm) 1,18 (t, J = 7.5 Hz, 3H, CH3-CH2), 1.94 (s, 3H, COCH3), 2.78 (q, J = 7.5
2
3
2
3
Hz, 2H, CH2-CH3), 3.19 (dd, J = 13.9 Hz, J = 8.1 Hz, 1 H, SCH2), 3.39 (dd, J = 13.9 Hz, J = 5,0
Hz, 1 H, SCH2), 3.62 (s, 3H, CH3-Ar), 4.55 (dd, 3J = 8.1 Hz, 3J = 5.0 Hz, 1 H, CH-CH2), 7.15 – 7.21
(m, 3 H, Ar-H), 7.41 – 7.44 (m, 1 H, Ar-H); 13C-NMR (CD3OD): δ (ppm) 15.7 (CH3), 22.4 (CH3), 28.2
(CH2), 36.5 (CH2), 53.0 (CH3), 53.7 (CH), 127.8 (CHAr), 128.4 (CHAr), 130.1 (CHAr), 131.9 (CHAr),
134.6 (CAr), 146.3 (CAr), 172.5 (CO), 173.3 (CO); IR: ν (cm-1) 3280, 3058, 2964, 2872, 1747, 1655,
1541, 1437, 1373, 1214, 1128, 1027, 752; MS (m/z, EI): 281 (M+, 42), 222 (88), 151 (30), 148 (100),
144 (25), 136 (45), 91 (12), 43 (21); Anal. Calcd for C14H19NO3S; C 59.76, H 6.81, N 4.98, O 17.06, S
11.40; Found: C 59.39, H 6.76, N 4.88, O 17.46, S 11.74.
2-Acetamido-3-(3,5-dimethylphenylsulfanyl)propionic acid methyl ester (3e). White solid, mp: 73-
74°C; 1H-NMR (CD3OD): δ (ppm) 1.92 (s, 3 H, COCH3), 2.25 (s, 6 H, 2 CH3-Ar), 3.18 (dd, 2J = 14.2
3
2
3
Hz, J = 7.6 Hz, 1 H, SCH2), 3.35 (dd, J = 14.2 Hz, J = 4.8 Hz, 1 H, SCH2), 3.62 (s, 3 H, OCH3),
4.57 (dd, 3J = 7.6 Hz, 3J = 4.8 Hz, 1 H, CH2-CH), 4.84 (s, 1H, NH), 6.85 (m, 1 H, 1 Ar-H), 7.01 (m, 2
13
H, 2 Ar-H); C-NMR (CD3OD): δ (ppm) 21.4 (2 x CH3), 22.4 (CH3), 34.6 (CH2), 53.0 (CH3), 52.4
(CH), 129.3 (CHAr), 131.1 (CHAr), 131.5 (CAr), 134.2 (CAr), 170.1 (CO), 171.1 (CO); IR: ν (cm-1)
3269, 3072, 2953, 2914, 2856, 1757, 1733, 1652, 1549, 1430, 1376,1249, 1216, 1125, 1031, 841; MS
(m/z, EI): 281 (M+, 25), 222 (45), 189 (8), 163 (100), 151 (42), 135 (19), 122 (14), 105 (51), 91 (36),
77 (34), 59 (35); Anal. Calcd for C14H19NO3S; C 59.76, H 6.81, N 4.98, O 17.06, S 11.40; Found: C
59.72, H 6.96, N 4.81, O 16.94, S 11.42.
2-Acetamido-3-(3,4-dimethylphenylsulfanyl)propionic acid methyl ester (3f). White solid, mp: 84-
85°C; 1H-NMR (CD3OD): δ (ppm) 1.92 (s, 3 H, COCH3), 2.20 (s, 3 H, CH3-Ar(4)), 2.21 (s, 3 H, CH3-
2
3
2
3
Ar (3)), 3.14 (dd, J = 14.0 Hz, J = 8.0 Hz, 1 H, SCH2), 3.31 (dd, J = 14.0 Hz, J = 4.8 Hz, 1 H,
SCH2), 3.61 (s, 3 H, OCH3), 4.53 (dd, 3J = 8.0 Hz, 3J = 4.8 Hz, 1 H, CH2-CH), 4.84 (s, 1H, NH), 7.05
13
(m, 1 H, Ar-H), 7.13 (m, 1 H, Ar-H), 7.19 (m, 1 H, Ar-H); C-NMR (CD3OD): δ (ppm) 19.5 (CH3),
19.9 (CH3), 22.4 (CH3),37.3 (CH2), 52.9 (CH3), 53.8 (CH3), 130.2 (CHAr), 131.4 (CHAr), 132.6 (CAr),
133.7 (CHAr), 137.2 (CAr), 138.7 (CAr), 172.5 (CO), 173.2 (CO); IR: ν (cm-1) 3284, 3061, 3015, 2951,
2928, 2867, 1747, 1659, 1539, 1488, 1436, 1372, 1213, 1129, 1022, 814; MS (m/z, EI): 281 (M+, 18),
222 (65), 191 (7), 163 (100), 151 (55), 137 (16), 105 (53), 91 (38), 77 (49), 59 (28); Anal. Calcd for
C14H19NO3S; C 59.76, H 6.81, N 4.98, O 17.06, S 11.40; Found: C 59.87, H 6.98, N 4.93, O 16.84, S
11.19.
2-Acetamido-3-(2,4-dimethylphenylsulfanyl)propionic acid methyl ester (3g). White solid, mp: 84-
85°C; 1H-NMR (CD3OD): δ (ppm) 1.95 (s, 3 H, COCH3), 2.27 (s, 3 H, CH3-Ar (4)), 2.36 (s, 3 H, CH3-
2
3
2
3
Ar (2)), 3.13 (dd, J = 14.0 Hz, J = 8.0 Hz, 1 H, SCH2), 3.30 (dd, J = 14.0 Hz, J = 5.0 Hz, 1 H,
SCH2), 3.62 (s, 3 H, OCH3), 4.52 (dd, 3J = 8.0 Hz, 3J = 5.0 Hz, 1 H, CH2-CH), 4.86 (s, 1H, NH), 6.98

Molecules 2008, 13
2403
13
(m, 1 H, Ar-H), 7.04 (m, 1 H, Ar-H), 7.31 (m, 1 H, Ar-H); C-NMR (CD3OD): δ (ppm) 20.8 (CH3),
21.2 (CH3), 22.4 (CH3),36.5 (CH2), 53.0 (CH3), 53.7 (CH), 128.5 (CHAr), 131.3 (CAr), 132.4 (CHAr),
132.9 (CHAr), 138.6 (CAr), 140.7 (CAr), 172.5 (CO), 173.2 (CO); IR: ν (cm-1) 3293, 3063, 2959, 2916,
1749, 1652, 1546, 1435, 1411, 1376, 1292, 1258, 1218, 1170, 1127, 821; MS (m/z, EI): 281 (M+, 48),
222 (91), 207 (11), 190 (10), 163 (70), 151 (63), 138 (29), 122 (18), 105 (100), 91 (52), 77 (74), 59
(54); Anal. Calcd for C14H19NO3S; C 59.76, H 6.81, N 4.98, O 17.06, S 11.40; Found: C 59.76, H
6.86, N 4.82, O 17.40, S 11.38.
2-Acetamido-3-(2,5-dimethylphenylsulfanyl)propionic acid methyl ester (3h). White solid, mp: 84-
85°C; 1H-NMR (CD3OD): δ (ppm) 1.95 (s, 3 H, COCH3), 2.29 (s, 3 H, CH3-Ar (5)), 2.33 (s, 3 H, CH3-
2
3
2
3
Ar (2)), 3.18 (dd, J = 14.0 Hz, J = 8.0 Hz, 1 H, SCH2), 3.35 (dd, J = 14,0 Hz, J = 4.8 Hz, 1 H,
SCH2), 3.63 (s, 3 H, OCH3), 4.57 (dd, 3J = 8.0 Hz, 3J = 4.8 Hz, 1 H, CH2-CH), 4.86 (s, 1H, NH), 6.95
3
3
13
(d, J = 7.8 Hz, 1 H, 1 Ar-H), 7.07 (d, J = 7.8 Hz, 1 H, 1 Ar-H), 7.24 (s, 1 H, 1 Ar-H); C-NMR
(CD3OD): δ (ppm) 20.3 (CH3), 21.1 (CH3), 22.4 (CH3), 35.9 (CH2), 53.0 (CH3), 53.7 (CH), 129.0
(CHAr), 131.4 (CHAr), 132.2 (CHAr), 134.7 (CAr), 137.0 (CAr), 137.4 (CAr), 172.4 (CO), 173.3 (CO); IR:
ν (cm-1) 3319, 3004, 2952, 2915, 1769, 1651, 1530, 1489, 1432, 1379, 1251, 1216, 1189, 1171, 1029,
976, 800; MS (m/z, EI): 281 (M+, 25), 222 (73), 190 (10), 163 (100), 149 (98), 135 (25), 122 (15), 105
(84), 91 (58), 77 (78), 59 (58); Anal. Calcd for C14H19NO3S; C 59.76, H 6.81, N 4.98, O 17.06, S
11.40; Found: C 59.91, H 6.84, N 4.95, O 17.41, S 11.52.
1
2-Acetamido-3-phenylsulfanylpropionic acid methyl ester (3i). White solid, mp: 64-65°C; H-NMR
(CD3OD): δ (ppm) 1.96 (s, 3 H, COCH3), 3.23 (dd, 2J = 14.0 Hz, 3J = 8.0 Hz, 1 H, SCH2), 3.40 (dd, 2J
= 14,0 Hz, 3J = 5.2 Hz, 1 H, SCH2), 3.62 (s, 3 H, OCH3), 4.68 (dd, 3J = 8.0 Hz, 3J = 5.2 Hz, 1 H, CH2-
CH), 4.85 (s, 1H, NH), 7.20-7.24 (m, 1 H, 1 Ar-H), 7.28-7.32 (m, 2 H, 2 Ar-H), 7.39-7.43 (m, 2 H, 2
13
Ar-H); C-NMR (CD3OD): δ (ppm) 22.4 (CH3), 36.6 (CH2), 53.0 (CH3), 53.8 (CH), 128.1 (CHAr),
130.3 (CHAr), 131.8 (CHAr), 136.3 (CAr), 172.4 (CO), 173.3 (CO); IR: ν (cm-1) 3283, 3058, 2952,
2849, 1747, 1653, 1539, 1482, 1438, 1373, 1306, 1214, 1177, 1128, 1025, 743; MS (m/z, EI): 253
(M+,10), 194 (84), 163 (14), 135 (100), 123 (73), 109 (45), 88 (42), 77 (35), 65 (47); Anal. Calcd for
C12H15NO3S; C 56.90, H 5.97, N 5.53, O 18.95, S 12.66; Found: C, 57.35; H, 5.91; N, 5.32; O, 18.75;
S, 12.66.
2-Acetamido-3-(4-chlorophenylsulfanyl)propionic acid methyl ester (3j). White solid, mp: 106-107°C;
1H-NMR (DMSO-d6): δ (ppm) 1.83 (s, 3 H, COCH3), 3.19 (dd, 2J = 13.8 Hz, 3J = 8.0 Hz, 1 H, SCH2),
2
3
3
3
3.32 (dd, J = 13.8 Hz, J = 5.4 Hz, 1 H, SCH2), 3.57 (s, 3 H, OCH3), 4.40 (dd, J = 8.0 Hz, J = 8.0
Hz, 3J = 5.4 Hz, 1 H, CH-CH2), 7.36 (s, 4 H, Ar-H), 8.50 (d, 3J = 8.0 Hz, 1H, NH); 13C-NMR (DMSO-
d6): δ (ppm) 22.4 (CH3), 34.6 (CH2), 51.8 (CH3), 52.4 (CH), 129.3 (CHAr), 131.1 (CHAr), 131.5 (CAr),
134.2 (CAr), 170.1 (CO), 171.1 (CO); IR: ν (cm-1) 3308, 3075, 2951, 2930, 1730, 1643, 1546, 1478,
1425, 1369, 1349, 1247, 1219, 1192, 1177, 1099, 1037, 1011, 808; MS (m/z, EI): 287 (M+,11), 230
(41), 228 (100), 197 (10), 169 (50), 157 (22), 143 (15), 108 (12), 88 (29); Anal. Calcd for
C12H14ClNO3S; C 50.09, H 4.90, Cl 12.32, N 4.87, O 16.68, S 11.14; Found: C 50.80, H 5.23, Cl
11.93, N 4.68, O 16.20, S 10.54.

Molecules 2008, 13
2404
2-Acetamido-3-(4-bromophenylsulfanyl)propionic acid methyl ester (3k). White solid, mp: 121-122°C;
1H-NMR (CDCl3): δ (ppm) 1.80 (s, 3H, COCH3), 3.21 (dd, 2J = 14.2 Hz, 3J = 4.7 Hz, 1 H, SCH2), 3.35
(dd, 2J = 14.2 Hz, 3J = 4.7 Hz, 1 H, SCH2), 3.49 (s, 3H, OCH3), 4.73 (m, 1 H, CH-CH2), 6.16 (s, 1H,
13
NH), 7,14-7,16 (m, 2 H, Ar-H), 7.29 – 7.31 (m, 2 H, Ar-H); C-NMR (CDCl3): δ (ppm) 22.9 (CH3),
36.5 (CH2), 52.2 (OCH3), 52.5 (CH), 121.0 (CAr), 132.1 (CHAr), 132.3 (CHAr), 134.0 (CAr), 169.6
(CO), 170.6 (CO); IR: ν (cm-1) 3307, 3047, 2950, 1729, 1641, 1546, 1474, 1425, 1350, 1324, 1245,
1176, 1092, 1039, 1006, 806; MS (m/z, EI): 333 (18), 331 (M+, 17), 274 (99), 272 (100), 215 (10), 213
(20), 203 (13), 201 (13), 189 (11), 187 (11), 144 (18), 134 (53), 122 (67), 108 (50), 88 (74), 75 (25),
60 (60); Anal. Calcd for C12H14BrNO3S: C, 43.38; H, 4.25; Br, 24.05; N, 4.22; O, 14.45; S, 9.65.
Found: C,44.56, H, 4.41, Br, 23.59, N, 4.04, O, 14.38, S, 9.29.
1
2-Acetamido-3-(4-methoxyphenylsulfanyl)propionic acid methyl ester (3l). Colorless oil; H-NMR
(CDCl3): δ (ppm) 1.89 (s, 3H, COCH3), 3.22 (dd, 2J = 14.2 Hz, 3J = 5.0 Hz, 1 H, SCH2), 3.32 (dd, 2J =
3
14.2 Hz, J = 4.7 Hz, 1 H, SCH2), 3.54 (s, 3H, OCH3), 3.76 (s, 3 H, Ar-OCH3), 4.77 (m, 1 H, CH-
CH2), 6.81 – 6.83 (m, 2 H, Ar-H), 7.35 – 7.37 (m, 2 H, Ar-H); 13C-NMR (CDCl3): δ (ppm) 22.9 (CH3),
38.0 (CH2), 52.2 (OCH3), 52.3 (CH), 55.3 (OCH3), 114.7 (CHAr), 124.7 (CAr), 134.2 (CHAr), 159.4
(CAr), 169.7 (CO), 170.8 (CO); IR: ν (cm-1) 3289, 3065, 2953, 2838, 1746, 1656, 1592, 1541, 1494,
1437, 1373, 1286, 1246, 1175, 1029, 827; MS (m/z, EI): 283 (47), 224 (100), 165 (27), 153 (22), 144
(25), 165 (27), 109 (9), 84 (7); Anal. Calcd for C13H17NO4S: C, 55.11; H, 6.05; N, 4.94; O, 22.59; S,
11.32. Found: C, 55.27; H, 6.32; N, 4.87; O, 22.79; S, 11.30.
2-Acetamido-3-pentachlorophenylsulfanylpropionic acid methyl ester (3m). White solid, mp: 145-
1
2
3
146°C; H-NMR (DMSO-d6): δ (ppm) 11.75 (s, 3 H, COCH3), 3.17 (dd, J = 14.0 Hz, J = 8.8 Hz, 1
H, SCH2), 3.40 (dd, 2J = 14.0 Hz, 3J = 4.8 Hz, 1 H, SCH2), 3.57 (s, 3 H, OCH3), 4.36 (ddd, 3J = 8.8 Hz,
3J = 8.0 Hz, 3J = 4.8 Hz, 1 H, CH-CH2), 8.36 (d, 3J = 8.0 Hz, 1H, NH); 13C-NMR (DMSO-d6): δ (ppm)
22.4 (CH3), 36.0 (CH2), 52.5 (CH), 52.8 (CH3), 131.4 (CAr), 133.5 (CAr), 134.4 (CAr), 138.2 (CAr),
170.0 (CO), 170.8 (CO); IR: ν (cm-1) 3293, 2956, 1758, 1651, 1532, 1435, 1378, 1334, 1305, 1259,
1215, 1190, 1170, 1130, 688; MS (m/z, EI): 425 (M+, 5), 366 (100), 324 (13), 246 (18), 167 (12), 144
(37), 117 (48), 102 (18), 88 (90), 69 (34); Anal. Calcd for C12H10Cl5NO3S; C 33.87, H 2.37, Cl 41.66,
N 3.29, O 11.28, S 7.54; Found: C 34.24, H 2.58, Cl 42.01, N 2.97, O 10.75, S 6.96.
Acknowledgements
The authors gratefully acknowledge Caroline Bastien and Stéphane Grossmann for their technical
assistance.
References and Notes
1.
2.
Lauwerys, R.R.; Haufroid, V.; Hoet, P.; Lison, D. In Toxicologie industrielle et intoxications
professionnelles; Elsevier Masson S.A.S. Ed.: Issy-les-Moulineaux (France), 2007; pp. 528-557.
Lataye, R.; Campo, P.; Pouyatos, B.; Cossec, B.; Blachere, V.; Morel, G. Solvent ototoxicity in
the rat and guinea pig. Neurotox. Teratol. 2003, 25, 39-50.

Molecules 2008, 13
2405
3.
4.
Gagnaire, F.; Marignac, B.; Langlais, C.; Bonnet, P. Ototoxicity in rats exposed to ortho-, meta-
and para-xylene vapours for 13 weeks. Pharmacol. Toxicol. 2001, 89, 6-14.
Korsak, Z.; Rydzynski, K. Neurotoxic effects of acute and subchronic inhalation exposure to
trimethylbenzene isomers (pseudocumene, mesitylene, hemimellitene) in rats. Int. J. Occup.
Med. Environ. Health. 1996, 9, 341-349.
5.
6.
7.
Vermeulen, N.P.E. Analysis of mercapturic acids as a tool in biotransformation, biomonitoring
and toxicological studies. Trends Pharmacol. Sci. 1989, 10, 177-181.
Nelson, E.D. Determination of mercapturic acid and excretions in exposure control to toxicants.
Crit. Rev. Toxicol. 1992, 22, 371-389.
De Rooij, B.M.; Commandeur, J.N.M.; Vermeulen, N.P.E. Mercapturic acids as biomarkers of
exposure to electrophilic chemicals: applications to environmental and industrial chemicals.
Biomarkers 1998, 3, 239-303.
8.
9.
Angerer, J.; Ewers, U.; Wilhelm, M. Human biomonitoring: State of the art. Int. J. Hyg. Environ.
Health 2007, 210, 201-228.
Perbellini, L.; Veronese, N.; Princivalle, A. Mercapturic acids in the biological monitoring of
occupational exposure to chemicals. J. Chrom. B Analyt. Technol. Biomed. Life Sci. 2002, 781,
269-290.
10. Haufroid, V.; Lison, D. Mercapturic acids revisited as biomarkers of exposure to reactive
chemicals in occupational toxicology: a minireview. Int. Arch. Occup. Environ. Health 2005, 78,
343-354.
11. Van Doorn, R.; Leijdekkers, C.M.; Bos, R.P.; Brouns, R.M.E.; Henderson, P.T. Alcohol and
sulphate intermediates in the metabolism of toluene and xylenes to mercapturic acids. J. Appl.
Toxicol. 1981, 1, 236-242.
12. Zheng, J.; Hanzlik, R.P. Bromo(monohydroxy)phenyl mercapturic acids from bromobenzene-
treated rats. Drug Metabol. Disp. 1992, 20, 688-694.
13. Angerer, J.; Schildbach, M.; Kramer, A. S-p-Toluylmercapturic acid in the urine of workers
exposed to toluene: a new biomarker for toluene exposure. Arch. Toxicol. 1998, 72, 119-123.
14. Gonzalez-Reche, L.M.; Schettgen, T.; Angerer, J. New approaches to the metabolism of xylenes:
verification of the formation of phenylmercapturic acid metabolites of xylenes. Arch. Toxicol.
2003, 77, 80-85.
15. Boogaard, P.J.; Van Sittert, N.J. Biological monitoring of exposure to benzene – A comparison
between S-phenylmercapturic acid, trans,trans-muconic acid and phenol. Occup. Environ. Med.
1995, 52, 611-620.
16. Inoue, O.; Kanno, E.; Kasai, K.; Ukai, H.; Okamoto, S.; Ikeda, M. Benzylmercapturic acid is
superior to hippuric acid and o-cresol as a urinary marker of occupational exposure to toluene.
Toxicol. Lett. 2004, 147, 177-186.
17. American Conference of Governmental Industrial Hygienists, Threshold Limit Values and
Biological Exposure Indices 1998.
18. Deutsche Forschungsgemeinschaft, List of MAK and BAT values. Commission for the
investigation of health hazards of chemical compounds in the work area, Report N° 32. VCH:
New-York, 1998.

Molecules 2008, 13
2406
19. Van Bladeren, P.J.; Buys, W.; Breimer, D.D.; Van der Gen, A. The Synthesis of Mercapturic
Acids and their Esters. Eur. J. Med. Chem. 1980, 15, 495-497.
20. Hayden, P.; Schaeffer, V.H.; Larsen, G.; Stevens, J.L. Cysteine S-conjugates. Meth. Enzymol.
1987, 143, 228-234.
21. Buijs, W.; Eid, M.I.A.; Onkenhout, W.; Vermeulen, N.P.E. The use of sulfenyl halides in the
synthesis of mercapturic acids and their esters. Recl. Trav. Chim. Pays-Bas 1986, 105, 449-455.
22. West, H.D.; Mathura, G.R. Synthesis of some aryl-substituted L-cysteines and their fate in the
animal body. J. Biol. Chem. 1954, 208, 315-318.
23. Parke, D.V.; Williams, R.T. Studies in detoxication 38. The metabolism of benzene: the
determination of phenylmercapturic acid in urine; mercapturic acid excretion by rabbits
receiving benzene. Biochem. J. 1951, 48, 624-629.
24. Ciattini, P.G.; Morera, E.; Ortar, G. A new, palladium-catalyzed synthesis of aromatic
mercapturic acid derivatives. Tetrahedron Lett. 1995, 36, 4133-4136.
25. Hickman, R.J.S.; Christie, B.J.; Guy, R.W.; White, T.J. Synthesis of aromatic S-substituted
derivatives of N-acetyl-L-cysteine. Aust. J. Chem. 1985, 38, 899-904.
26. Kondoh, A.; Yorimitsu, H.; Oshima, K. Nucleophilic aromatic substitution reaction of
nitroarenes with alkyl- or arylthio groups in dimethyl sulfoxide by means of cesium carbonate .
Tetrahedron 2006, 62, 2357-2360.
27. Pombrio, J.M.; Giangreco, A.; Li, L.Q.; Wempe, M.F.; Anders, M.W.; Swett, D.H.; Pritchard,
J.B.; Ballatori, N. Mercapturic acids (N-acetylcysteine S-conjugates) as endogenous substrates
for the renal organic anion transporter-1. Mol. Pharmacol. 2001, 60, 1091-1099.
28. Saxena, M.; Henderson, G.B. MOAT4, a novel multispecific organic-anion transporter for
glucuronides and mercapturates in mouse L1210 cells and human erythrocytes. Biochem. J.
1996, 320, 273-281.
29. Nakamura, S.; Goto, K.; Kondo, M.; Naito, S.; Tsuda, Y.; Shishido, K. Enantioselective total
synthesis and structure determination of the mercapturic acid sulfoxide conjugate. Bioorg. Med.
Chem. Lett. 1997, 7, 2033-2036.
30. Behringer, H.; Fackler, E. Eine einfache synthese der racemischen Mercaptursäuren. Justus
Liebigs Ann. Chem. 1949, 564, 73-78.
31. Hanzlik, R.P.; Weller, P.E.; Desai, J.; Zheng, J.; Hall, L.R; Slaughter, D.E. Synthesis of
mercapturic acid-derivatives of putative toxic metabolites of bromobenzene. J. Org. Chem. 1990,
55, 2736-2742.
32. Enders, D.; Lüttgen, K.; Narine, A.A. Asymmetric sulfa-Michael additions. Synthesis 2007, 7,
959-980.
33. Montanari, F.; Landini, D.; Rolla, F. Phase transfer catalyzed reactions. Top. Curr. Chem. 1982,
101, 147-200.
34. Rothstein, E. J. Experiments in the synthesis of derivatives of α,α-aminoacrylic acid from serine
and N-substituted serines. Chem. Soc. 1949, 1968-1971.
35. Cardillo, G.; Gentilucci, L.; Tolomelli, A.; Tomasini, C. Asymmetric 1,4 addition of Grignard
reagents to chiral alpha,beta-unsaturated esters in the presence of Lewis acids. Tetrahedron
1999, 55, 6231-6242.

Molecules 2008, 13
2407
36. Bueno, M.P.; Cativiela, C.; Finol, C.; Mayoral, J.A.; Jaime, C. Diels-Alder reactions of methyl-
N-acyl-alpha,beta-dehydroalaninates with cyclopentadiene. Can. J. Chem. 1987, 65, 2182-2186.
37. Roper, R.; Ma, T.S. Diazomethane as a reagent for microsynthesis. Microchemical J. 1957, 1,
245-260.
38. Kolar, A.J.; Olsen, R.K. Convenient, large-scale preparation of 2-acetamidoacrylic acid and its
methyl ester. Synthesis 1977, 457-459.
39. Rich, D.H.; Tam, J.P. Synthesis of didehydropeptides from peptides containing 3-alkylthio-
amino acid residues. Tetrahedron Lett. 1975, 211-212.
40. Srivastava, V.P.; Roberts, M.; Holmes, T.; Stammer, C.H. Synthesis of (+/-) 2,3-methanovaline
and (+/-)-2,3-methanoleucine. J. Org. Chem. 1989, 54, 5866-5870.
41. Placidi-Rampont, V. Synthèse, séparation et analyse de métabolites soufrés du toluène. PhD
Thesis, Henri Poincaré University Nancy 1: Nancy, France, 1997
42. Bastien, C. Synthèse et identification de métabolites soufrés du toluène. DEA report, Henri
Poincaré University Nancy 1: Nancy, France, 2000
43. Wang, S.S.; Gisin, B.F.; Winter, D.P.; Makofske, R.; Kulesha, I.D.; Tzougraki, C.; Meienhofer,
J. Facile synthesis of amino-acid and peptide esters under mild conditions via cesium salts. J.
Org. Chem. 1977,42, 1286-1290.
44. Chidambaran, M.; Sonavane, S.U.; de la Zerda, J.; Sasson, Y. Didecyldimethylammonium
bromide (DDAB): a universal, robust, and highly potent phase-transfer catalyst for diverse
organic transformations. Tetrahedron 2007, 63, 7696-7701.
45. Makosza, M. Phase-transfer catalysis. A general green methodology in organic synthesis. Pure
Appl. Chem. 2000, 72, 1399-1403.
46. Sirovski, F.; Reichardt, C.; Garokhova, M.; Ruban, S.; Stoikova, E. Solid liquid phase-transfer
catalysis. Some models and solvent influence. Tetrahedron 1999, 55, 6363-6374.
47. Yang, H.M.; Lin, C.L. Phase-transfer catalyzed benzylation of sodium benzoate using aliquat
336 as catalyst in liquid-liquid system. J. Mol. Catal. A: Chem. 2003, 206, 67-76.
48. Lopez, A.; Moreno-Manas, M.; Pleixats, R.; Roglans, A.; Ezquerra, J.; Pedregal, C. Ethyl N-
(diphenylmethylene)glycinate as anionic glycine equivalent. Monoalkylation, dialkylation and
Michael additions under solid-liquid phase-transfer catalysis. Tetrahedron 1996, 52, 8385-8386.
49. Herriott, A.W.; Picker, D. Phase transfer catalysis – evaluation of catalysts. J. Am. Chem. Soc.
1975, 97, 2345-2349.
50. Superchi, S.; Nardiello, M.; Donnoli, M.I.; Scafato, P.; Menicagli, R.; Rosini, C.
Enantioselective synthesis of the fragrance trans-magnolione under asymmetric phase transfer
catalysis. C.R. Chimie 2005, 8, 867-874.
Sample Availability: Samples of the compounds mentioned in this paper are available from the
corresponding author (B.C.)
© 2008 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.
This article is an open-access article distributed under the terms and conditions of the Creative
Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).