A palladium-catalyzed synthesis of isoalliin, the main cysteine sulfoxide in onions (Allium cepa)

Article abstract of DOI:10.1055/s-2006-950216

The natural sulfur compound isoalliin, (+)-(R)-3-[(E)-prop-1-enylsulfinyl]- 2-aminopropanoic acid, was synthesized as a mixture of diastereomers in five steps from the hydrochloride of L-cysteine ethyl ester comprising an efficient - albeit unusual - Pd-catalyzed C-S coupling as the key step. Isoalliin is in demand in the field of Allium chemistry and serves as a standard for different determination methods in breeding research on Alliaceae, like onions, leek, or garlic. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-950216

SHORT PAPER
3367
A Palladium-Catalyzed Synthesis of Isoalliin, the Main Cysteine Sulfoxide in
Onions (Allium cepa)
P
alladium
a
-Catalyze
d
nSynthesis of
Isoall
C
Institut für Organische Chemie, TU Clausthal, Leibnizstr. 6, 38678 Clausthal-Zellerfeld, Germany
Fax +49(5323)722834; E-mail: namyslo@ioc.tu-clausthal.de
Received 25 April 2006; revised 20 June 2006
To the best of our knowledge, isoalliin can be obtained
Abstract: The natural sulfur compound isoalliin, (+)-(R)-3-[(E)-
prop-1-enylsulfinyl]-2-aminopropanoic acid, was synthesized as a
mixture of diastereomers in five steps from the hydrochloride of
L-cysteine ethyl ester comprising an efficient – albeit unusual –
from the natural plant (about 3 grams from 2 kilograms of
dry onion powder by means of several ion exchange and
recrystallization steps),9 or by a known synthesis12 that in-
Pd-catalyzed C–S coupling as the key step. Isoalliin is in demand in cludes a low-yield coupling (15%) of 3-chloro-L-alanine
the field of Allium chemistry and serves as a standard for different
with (E)-propenyl benzyl sulfide.
determination methods in breeding research on Alliaceae, like on-
ions, leek, or garlic.
drochloride of L-cysteine ethyl ester (1), which was ini-
Key words: palladium, C–S coupling, natural product, amino acid,
Our synthesis started from the commercially available hy-
tially Boc-protected by means of di-tert-butyl dicarbonate
thiol
in analogy to the protocol of Franceschi and co-workers.13
We found that the subsequent Pd(0)-catalyzed thiol cou-
pling of the resulting N-Boc-L-cysteine ethyl ester (2)
Homogeneous catalysis, especially with palladium spe-
with (E)-1-bromoprop-1-ene should be carried out with
cies, nowadays is one of the most highly developed tools
tetrahydrofuran as the solvent at a slightly elevated tem-
in organic synthesis. However, in this context the carbon–
perature (Scheme 1). Thus, the isomerically pure trans-
sulfur bond formation by means of palladium-catalyzed
(prop-1-enyl) derivative 3 was obtained providing the
reactions still seems to be rather paradoxical due to the
characteristic large NMR coupling of the trans-olefinic
well-known catalyst poisoning properties of sulfur. In-
protons (J = 14.9 Hz). The chemical yield was 82% after
deed, the simultaneous use of both of these elements in
flash column chromatography on silica gel. However, it
homogenous catalysis is infrequently reported in the liter-
should be mentioned that first attempts to submit unpro-
ature.1–4
tected 1 to the Pd-coupling in NMP as the solvent gave no
conversion, whereas in DMF and THF the expected cou-
In 1995, Ortar and co-workers5 employed a special cata-
lyst which previously had been applied in cyanation
chemistry by Takagi et al.6 and seemed to be sulfur-com-
patible, i.e. the catalyst/co-catalyst system consisting of
tris(dibenzylideneacetone)dipalladiumchloroform com-
plex combined with 1,1¢-bis(diphenylphosphino)fer-
rocene. Whereas Ortar’s group prepared S-aryl
derivatives of L-cysteine, recently Moreau and
Campagne7 adapted this protocol to substituted as well as
long-chained alkenyl iodides or, in one single case, to a
bromide. In this paper, we describe a mild synthetic vari-
ant which allows for a short and efficient synthesis, as a
mixture of diastereomers, of isoalliin {(R)-3-[(E)-prop-1-
enylsulfinyl]-2-aminopropanoic acid}, an odorless, non-
volatile sulfoxide, naturally found in onions (Allium cepa)
as its L-(+)-isomer with a R-configured sulfur center.8–10
This compound is the precursor of the lachrymatory factor
of onions and a member of a group of sulfur containing
substances from onions, leek, and garlic with a variety of
health care applications.11 Among these, everyone knows
grandma’s freshly prepared and heated onion juice against
coughing and croakiness, an application of the antibacte-
rial effect of Allium species.
pling product was obtained in low or moderate yield, i.e.
5% or 31%, respectively. From a mechanistic point of
view such Pd-couplings with aryl or vinyl halides have
been investigated by Campagne and Jutand14 as well as by
Hartwig and co-workers.15
HCl⋅H2N
O
Boc NH
O
a
98%
HS
OEt
HS
OEt
2
1
Boc NH
Boc NH
O
O
b
c
94%
82%
S
OEt
S
OH
3
4
H2N
O
H2N
S
O
d
e
quant.
78%
S
OH
OH
O
5
6
Scheme 1 Reagents and conditions: a) Boc2O, Et3N, THF, 0–
20 °C, 2 h; b) Pd2(dba)3·CHCl3, dppf, (E)-1-bromoprop-1-ene, Et3N,
THF, 40 °C, 4 h; c) aq 1 M LiOH, MeOH, 4 °C, 12 h; d) 50%
CF3CO2H in CH2Cl2, r.t., 20 h; e) 3% H2O2, 0–20 °C, r.t., overnight.
SYNTHESIS 2006, No. 20, pp 3367–3369
Advanced online publication: 21.08.2006
DOI: 10.1055/s-2006-950216; Art ID: T07006SS
x
x.
x
x
.
2
0
0
6
© Georg Thieme Verlag Stuttgart · New York

3368
J. C. Namyslo, C. Stanitzek
SHORT PAPER
on TLC showed complete conversion. After adding H2O (40 mL)
and extraction with EtOAc (3 × 50 mL), the combined organic
phases were washed with brine (2 ×) and dried (MgSO4). Then the
organic solvent was evaporated under reduced pressure. The result-
ing crude product was purified by flash column chromatography on
silica gel (petroleum ether–EtOAc, 7:1) to give 4.90 g (98%) of the
N-protected amino ester 2; mp 25.4 °C (DSC).
The subsequent deprotection of the acid moiety within 3
was accomplished by means of an aqueous solution of
lithium hydroxide which initially gave the corresponding
lithium carboxylate of 4. Furthermore, after treatment
with DOWEX cation-exchange resin the free acid 4 was
obtained in 94% yield from 3. The N-Boc deprotection
with a solution of 50% trifluoroacetic acid in dichlo-
romethane afforded 5 with the unprotected amino acid
moiety in 78% yield. Finally, the careful oxidation of the
sulfide 5 was accomplished quantitatively applying 3%
aqueous hydrogen peroxide.
IR (KBr): 513, 540, 565, 783, 860, 1032, 1065, 1167, 1211, 1252,
1346, 1369, 1510, 1716, 1747, 2939, 2981, 3379 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.29 (t, J = 7.1 Hz, 3 H, CH3CH2),
1.45 (s, 9 H, t-C4H9), 2.97 (dd, J = 4.2, 4.2 Hz, 1 H, SCHa) 2.98 (dd,
J = 4.2, 4.2 Hz, 1 H, SCHb), 4.23 (q, J = 7.1 Hz, 2 H, CH3CH2), 4.57
(ddd, J = 7.8, 4.2, 4.2 Hz, 1 H, CHN), 5.42 (br d, J = 7.8 Hz, 1 H,
NH).
13C NMR (100 MHz, CDCl3): d = 14.2 (1 C, CH3CH2), 27.4 (1 C,
CH2S), 28.3 [3 C, OC(CH3)3], 54.8 (1 C, CHN), 61.9 (1 C,
CH3CH2), 80.2 [1 C, OC(CH3)3], 155.1 (1 C, NC=O), 170.3 (1 C,
OC=O).
Accordingly, the desired diastereomeric (R)-3-[(E)-prop-
1-enylsulfinyl]-2-aminopropanoic acid (6) was obtained
in almost quantitative yield after mild removal of the sol-
vent, tolerating a maximum of 28 °C bath temperature at
the rotary evaporator. Only under these work-up condi-
tions, the trans-propenyl group was unmolested, which
DCP-MS (EI, 70 eV): m/z (%) = 250 (26, [M + 1]), 194 (100, [M +
1 – C4H9]), 150 (74, [M + 1 – C5H9O2]), 132 (31), 102 (13).
1
was confirmed by H NMR spectroscopy (J = 14.3 Hz).
As expected, the oxidation of the prochiral sulfur by
means of diluted hydrogen peroxide gave two diaste-
reomers. However, if necessary, an enantioselective sul-
tert-Butyl (R)-2-[(E)-Prop-1-enylsulfanyl)-1-(ethoxycarbon-
yl)ethylcarbamate (3)
fur oxidation should be feasible, e. g. applying modified At r.t., Pd2(dba)3·CHCl3 (280 mg, 0.27 mmol), 1,1¢-bis(diphe-
nylphosphino)ferrocene (499 mg, 0.90 mmol), (E)-1-bromoprop-1-
ene (0.85 mL, 9.90 mmol) and anhyd Et3N (2.50 mL, 18.9 mmol)
were added to anhyd THF (50 mL). Subsequently, the mixture was
Sharpless methods or other techniques, probably includ-
ing biotransformations.16,17
In conclusion, we have found an efficient synthetic path-
way to isoalliin (6), a natural cysteine sulfoxide in onions
(Allium cepa) and other Alliaceae, applying a palladium-
catalyzed thiol coupling. This molecule is a key com-
pound for studies on Allium chemistry including
metabolism11 as well as biological investigations, such as
hybrid research.18
stirred for 20 min at r.t., then further 20 min at 40 °C. At this slight-
ly elevated temperature, the N-Boc protected cysteine ester 2 (2.246
g, 9.01 mmol), dissolved in anhyd THF (2 mL), was added dropwise
within 10 min. The mixture was kept at this temperature for 4 h. Af-
ter this period, H2O (50 mL) was added and the resulting mixture
was extracted with CHCl3 (3 × 50 mL). The combined organic lay-
ers were dried (MgSO4) and the solvent was removed in vacuo.
Flash column chromatography on silica gel (petroleum ether–
EtOAc, 7:1) gave the pure trans-derivative 3 as a slightly yellow
oil; yield: 2.144 g (82%).
Interestingly, a sample of the isoalliin obtained by our
new synthesis served as a standard for an efficient deter-
mination of cysteine sulfoxides in Allium plants by a new
HPLC-MS2 method, which was developed at the Federal
Centre for Breeding Research on Cultivated Plants in
Quedlinburg, Germany.19
IR (KBr): 548, 567, 781, 858, 879, 941, 1024, 1057, 1169, 1213,
1252, 1369, 1520, 1686, 1711, 1744, 2939, 2981, 3371 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.29 (t, J = 7.1 Hz, 3 H, CH3CH2),
1.45 (s, 9 H, t-C4H9), 1.73 (dd, J = 6.4, 1.2 Hz, 3 H, CH3CH=), 3.08
(d, J = 4.8 Hz, 2 H, CH2S), 4.20 (q, J = 7.1 Hz, 2 H, CH3CH2), 4.54
(ddd, J = 7.8, 4.8, 4.4 Hz, 1 H, CHN), 5.36 (br d, J = 7.8 Hz, 1 H,
NH), 5.77 (dq, J = 14.9, 6.4 Hz, 1 H, CH3CH=), 5.86 (dq, J = 14.9,
1.2 Hz, 1 H, CH=CHS).
13C NMR (100 MHz, CDCl3): d = 14.1 (1 C, CH3CH2), 18.4 (1 C,
CH3CH=), 28.1 [3 C, C(CH3)3], 35.7 (1 C, CH2S), 53.4 (1 C, CHN),
61.6 (1 C, CH3CH2), 80.0 [1 C, C(CH3)3], 122.7 (1 C, CH=CHS),
128.9 (1 C, CH3CH=), 155.0 (1 C, NC=O), 170.7 (1 C, OC=O).
1
The H and 13C NMR spectra were recorded on a Bruker Avance
NMR spectrometer at 400 MHz. All spectra were taken in CDCl3
with TMS as internal standard or were measured in D2O (HDO at
4.72 ppm). FT-IR spectra were obtained on a Bruker Vector 22 in-
strument in the 400–4000 cm–1 range (2.5% pellets in KBr). The
mass spectra (EI) were recorded on a Hewlett Packard MS 5989 B
in DCP mode. Melting points were determined on a PerkinElmer
Differential Scanning Calorimeter, DSC 6. All reactions were car-
ried out under N2 and were monitored by analytical TLC on precoat-
ed plates (silica gel 60 F254). The TLC spots were detected either
by UV absorption or by treatment with Mo-stain. All commercially
available chemicals were purchased from Fluka or Aldrich and were
used as received without further purification. THF was carefully
dried over Na/K alloy and freshly distilled prior to use. Petroleum
ether used had bp 60–70 °C.
DCP-MS (EI, 70 eV): m/z (%) = 289 (25, [M]), 233 (18, [M + 1 –
C4H9]), 190 (100, [M + 1 – C5H9O2]), 172 (61), 160 (31).
HRMS (ESI, M + H+): m/z calcd for C13H24NO4S: 290.14260;
found: 290.14230.
(R)-2-tert-Butyloxycarbonylamino-3-[(E)-prop-1-enylsulfa-
nyl)propanoic Acid (4)
A solution of the ethyl ester 3 (563 mg, 1.95 mmol) in anhyd MeOH
(0.5 mL) was treated with 1 M LiOH in MeOH (3.89 mL) at 4 °C
for 12 h. After that, DOWEX 50X8 (freshly protonated, 7 mL) was
added portionwise at the same temperature with vigorous stirring.
Subsequently, the cation exchanger was filtered off and was washed
with EtOAc (3 × 70 mL). Additionally, the aqueous layer was ex-
tracted with EtOAc (3 × 15 mL). The combined organic phases
were dried (MgSO4) and evaporated in vacuo under very mild con-
N-Boc-L-cysteine Ethyl Ester (2)
At 0 °C, a suspension of L-cysteine ethyl ester hydrochloride (1;
3.71 g, 20.0 mmol) in anhyd THF (25 mL) was treated with Et3N
(6.11 mL, 44.0 mmol). Subsequently, di-tert-butyl dicarbonate
(4.80 g, 22.0 mmol) dissolved in anhyd THF (25 mL) was added
dropwise. The resulting mixture was stirred for 2 h at r.t., whereup-
Synthesis 2006, No. 20, 3367–3369 © Thieme Stuttgart · New York

SHORT PAPER
Palladium-Catalyzed Synthesis of Isoalliin
3369
ditions (28 °C). The product 4 with the free acid group was obtained
as a slightly yellow oil; yield: 477 mg (94%). The product was ana-
lytically pure as confirmed by 1H NMR spectrum.
1H NMR (400 MHz, D2O):
sulfoxide) = 1.87 (d, 3 H, J = 7.1 Hz, CH3CH=), 3.34 (m, 2 H,
SCH2), 4.09 (dd, J = 8.0, 4.0 Hz, 1 H, CHN), 6.42 (d, J = 14.3 Hz,
1 H, CH=CHS), 6.58 (dq, J = 14.3, 7.1 Hz, 1 H, CH3CH=).
13C NMR (100 MHz, D2O): d = 17.2 (1 C, CH3CH=), 49.5 (1 C,
CHN), 50.6 (1 C, CH2S), 128.6 (1 C, CH=CHS), 142.6 (1 C,
CH3CH=), 171.2 (1 C, OC=O).
d
(other diastereomeric
IR (KBr): 777, 854, 939, 1026, 1055, 1165, 1248, 1369, 1394, 1512,
1717, 2916, 2935, 2980, 3003, 3333 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.46 (s, 9 H, t-C4H9), 1.73 (dd,
J = 6.4, 1.2 Hz, 3 H, CH3CH=), 3.11 (d, J = 4.9 Hz, 2 H, CH2S),
4.59 (ddd, J = 7.7, 4.9, 4.0 Hz, 1 H, CHN), 5.38 (br d, J = 7.7 Hz,
1H, NH), 5.81 (dq, J = 14.8, 6.4 Hz, 1 H, CH3CH=), 5.90 (dq,
J = 14.8, 1.2 Hz, 1 H, CH=CHS).
13C NMR (100 MHz, CDCl3): d = 18.4 (1 C, CH3CH), 28.3 [3 C,
C(CH3)3], 35.2 (1 C, CH2S), 53.3 (1 C, CHN), 80.5 [1 C, C(CH3)3],
122.3 (1 C, CH=CHS), 129.8 (1 C, CH3CH=), 155.3 (1 C, NC=O),
175.5 (1 C, OC=O).
Analytical data were consistent with literature values.8–10,22
Acknowledgment
We gratefully acknowledge Dr. H. Frauendorf, University of Göt-
tingen, for high-resolution mass spectral data. J.C.N. thanks Prof.
D. E. Kaufmann for helpful discussions.
DCP-MS (EI, 70 eV): m/z (%) = 261 (10, [M]), 205 (25, [M –
C4H9]), 144 (41, [M – NHBoc]), 132 (8), 87 (100,
[CH3CH=CHSCH2]), 57 (89, [C4H9]).
References
HRMS (ESI, M + H+): m/z calcd for C11H20NO4S: 262.11130;
found: 262.11088.
(1) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852.
(2) Kondo, T.; Mitsudo, T. Chem. Rev. 2000, 100, 3205.
(3) Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E.; de Meijere, A., Eds.; Wiley: New
York, 2002.
(4) Liebeskind, L. S.; Srogl, J.; Savarin, C.; Polanco, C. Pure
Appl. Chem. 2002, 74, 115.
(5) Ciattini, P. G.; Morera, E.; Ortar, G. Tetrahedron Lett. 1995,
36, 4133.
(6) Takagi, K.; Sasaki, K.; Sakakibara, Y. Bull. Chem. Soc. Jpn.
1991, 64, 1118.
(7) Moreau, X.; Campagne, J.-M. J. Organomet. Chem. 2003,
687, 322.
(8) Spåre, C. G.; Virtanen, A. I. Acta Chem. Scand. 1963, 17,
641.
(9) Carson, J. F.; Lundin, R. E.; Lukes, T. M. J. Org. Chem.
1966, 31, 1634.
(10) Krest, I.; Glodek, J.; Keusgen, M. J. Agric. Food Chem.
2000, 48, 3753.
(11) Block, E. Angew. Chem., Int. Ed. Engl. 1992, 31, 1135;
Angew. Chem. 1992, 104, 1158.
(12) Parry, R. J.; Sood, G. R. J. Am. Chem. Soc. 1989, 111, 4514;
and references cited therein.
(13) Bissolino, P.; Alpegiani, M.; Borghi, D.; Perrone, E.;
Franceschi, G. Heterocycles 1993, 36, 1529.
(14) Moreau, X.; Campagne, J. M.; Meyer, G.; Jutand, A. Eur. J.
Org. Chem. 2005, 3749.
(R)-3-[(E)-prop-1-enylsulfanyl]-2-aminopropanoic Acid (5)
The N-Boc-protected amino acid 4 (300 mg, 1.15 mmol) was treat-
ed with a solution of trifluoroacetic acid (0.17 mL, 2.30 mmol) in
anhyd CH2Cl2 (0.17 mL) at r.t. and the mixture was stirred for 20 h.
The progress of the reaction was monitored by 1H NMR spectrosco-
py. After complete conversion, twice distilled H2O (30 mL) as well
as activated Amberlyst A-21 (15 mL) were added portionwise. Af-
ter vigorous stirring for 10 min, the ion exchanger was filtered off
and washed with H2O (10 × 25 mL) until ninhydrin test was nega-
tive. The collected solutions were concentrated to dryness in vacuo
at 28 °C to give the propenylated amino acid 5; yield: 144 mg
(78%); mp 172.1 °C (DSC).
1H NMR (400 MHz, D2O): d = 1.64 (br s, 3 H, CH3CH=), 2.96 (dd,
J = 14.9, 7.9 Hz, 1 H, SCHa), 3.15 (dd, J = 14.9, 3.0 Hz, 1 H, SCHb),
3.82 (m, 1 H, CHN), 5.83–5.91 (m, 2 H, CH=CH).
13C NMR (100 MHz, D2O): d = 17.6 (1 C, CH3CH=), 33.4 (1 C,
CH2S), 53.6 (1 C, CHN), 119.8 (1 C, CH=CHS), 132.1 (1 C,
CH3CH=), 172.7 (1 C, OC=O).
The analytical data were in accordance with literature values.20,21
(R)-3-[(E)-Prop-1-enylsulfinyl)-2-aminopropanoic Acid (Isoal-
liin, 6)
The propenylsulfanyl compound 5 (132 mg, 0.82 mmol) was sus-
pended in 3% aq H2O2 (0.98 mL, 0.86 mmol). Subsequently, the
mixture was stirred overnight at r.t. 1H NMR spectrum in D2O of the
resulting clear solution confirmed complete conversion. Careful re-
moval of the solvent in vacuo at 28 °C afforded the pure product in
quantitative yield (145 mg).
1H NMR (400 MHz, D2O): d (one diastereomeric sulfoxide) = 1.86
(d, J = 7.1 Hz, 3 H, CH3CH=), 3.17 (dd, J = 13.7, 8.2 Hz, 1 H,
SCHa), 3.56 (dd, J = 13.7, 5.6 Hz, 1 H, SCHb), 4.03 (dd, J = 8.2, 5.6
Hz, 1 H, CHN), 6.42 (d, J = 14.3 Hz, 1 H, CH=CHS), 6.63 (dq, 1 H,
J = 14.3, 7.1 Hz, CH3CH=).
(15) Fernández-Rodríguez, M. A.; Shen, Q.; Hartwig, J. F. J. Am.
Chem. Soc. 2006, 128, 2180.
(16) Fernández, I.; Khiar, N. Chem. Rev. 2003, 103, 3651.
(17) Legros, J.; Dehli, J. R.; Bolm, C. Adv. Synth. Catal. 2005,
347, 19.
(18) Storsberg, J.; Schulz, H.; Keusgen, M.; Tannous, F.;
Dehmer, K. J.; Keller, E. R. J. J. Agric. Food. Chem. 2004,
52, 5499.
(19) Ziegert, K.; Schütze, W.; Schulz, H.; Keusgen, M.; Gun, F.;
Keller, E. R. J. J. Appl. Bot. Food Qual. 2006, 80, 31.
(20) Nishimura, H.; Mizuguchi, A.; Mizutani, J. Tetrahedron
Lett. 1975, 3201.
13C NMR (100 MHz, D2O): d = 17.2 (1 C, CH3CH=), 50.1 (1 C,
CHN), 52.0 (1 C, CH2S), 130.0 (1 C, CH=CHS), 143.6 (1 C,
CH3CH=), 171.1 (1 C, OC=O).
(21) Sugii, M.; Suzuki, T.; Nagasawa, S. Chem. Pharm. Bull.
1963, 548.
(22) Breu, W. Phytomedicine 1996, 3, 293.
Synthesis 2006, No. 20, 3367–3369 © Thieme Stuttgart · New York