Short protecting-group-free synthesis of 5-acetylsulfanyl-histidines in water: Novel precursors of 5-sulfanyl-histidine and its analogues

Article abstract of DOI:10.1039/c6ob01870j

The discovery of a non-enzymatic oxidative introduction of sulfur to the 5-position of histidine is reported, by activation with bromine or NBS followed by reaction with thioacetic acid forming novel 5-acetylsulfanyl-histidine. Complementing the previously developed regioselective oxidative S-introduction to the 2-position of histidine by reaction with cysteine, this surprising finding provides straightforward access in multi-gram quantities to naturally occurring 5-sulfanyl-histidine and its N-methylated analogues, including a hitherto unknown regioisomer of l-ergothioneine.

Full text of DOI:10.1039/c6ob01870j

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Short protecting-group-free synthesis of
-acetylsulfanyl-histidines in water: novel
5
Cite this: DOI: 10.1039/c6ob01870j
precursors of 5-sulfanyl-histidine and its
analogues†
Sylvain Daunay, Remi Lebel, Laurence Farescour, Jean-Claude Yadan and
Irene Erdelmeier*
The discovery of a non-enzymatic oxidative introduction of sulfur to the 5-position of histidine is
reported, by activation with bromine or NBS followed by reaction with thioacetic acid forming novel
5-acetylsulfanyl-histidine. Complementing the previously developed regioselective oxidative
S-introduction to the 2-position of histidine by reaction with cysteine, this surprising finding provides
straightforward access in multi-gram quantities to naturally occurring 5-sulfanyl-histidine and its
N-methylated analogues, including a hitherto unknown regioisomer of L-ergothioneine.
Received 25th August 2016,
Accepted 11th October 2016
DOI: 10.1039/c6ob01870j
www.rsc.org/obc
L-ergothioneine in multi-gram quantities, and also to the
whole 2-thiohistidine family. Briefly, hercynine (2), or histidine
Introduction
Ergothioneine (1) is a sulfur-containing betaine with an derivatives (6a–c), react after activation by bromine with
unusual imidazole–thione structure, found widely in plants, cysteine to form a thioether (4, 7a–c), which can be cleaved
animals and humans. Discovered and isolated in the early last smoothly to ergothioneine (1) and thiohistidine derivatives
1
century from Claviceps purpurea, it has regained much inter- 8a–c, respectively (Scheme 1).
est in the last decade, following the discovery of a specific
During the screening of other sulfur reagents in this new
2
transporter which regulates its tissue-specific incorporation. oxidative C–S bond formation, we evaluated thioacetic acid
While its physiological role is still under intense investigation, (9) as a potential sulfur donor. Interestingly, this reagent
accumulating evidence points towards a significant role in cell appeared to be suitable for introducing sulfur on histidine
and tissue protection, which suggests its use for health care (6a), but – contrary to cysteine – thioacetic acid reacts with the
3
–6
12
applications.
hypothetical bromolactone intermediate 10a on the 5-posi-
7
Only certain microorganisms such as fungi and specific tion, and not on the 2-position, to form a thioester (11a,
8
bacteria are able to synthesize 1 from hercynine (2), involving Scheme 2).
a cascade of enzymatic reactions, which are required in
This hitherto unknown and surprising reactivity towards
particular for C–S bond formation. The groups of Seebeck and thioacetic acid is even more interesting, as it could open a
Liu highlighted high specificity not only for the substrate direct access to 5-sulfur-substituted histidine derivatives,
structure, but also for the sulfur donor, which can be either which were up to now not directly accessible by chemical syn-
cysteine (3) or γ-glutamyl-cysteine, depending on the thesis from histidine via C–S bond formation.
9
,10
microorganism.
Here we report our first findings about this new oxidative
A completely new chemical synthesis, relying also on C–S bond-forming reaction and the subsequent transform-
1
1
cysteine as a sulfur-donor, was developed by our group, and ation of new 5-acetylsulfanyl-histidines to 5-sulfanyl-histidines
1
3
allowed a simple and convenient access to the natural product using a straightforward one-pot two-step procedure.
TETRAHEDRON, 14 Avenue de l’Opéra, 75001 Paris, France.
Results and discussion
E-mail: irene.erdelmeier@tetrahedron.fr
†
Electronic supplementary information (ESI) available: Typical procedure for
Applying the optimal conditions identified for sulfur introduc-
1
optimisation of the synthesis of 11a in solution (S1), figure of the H NMR ana-
1
4
1
13
tion using cysteine as a reagent, an ice-cold solution of histi-
dine (6a) was treated first with a slight excess of bromine
(1.3 equiv.), followed immediately by the addition of an excess
lysis of the reaction mixture (S2) and copies of spectral data ( H NMR, C NMR,
APCI-MS) for compounds 11c, 12, 16a, 16d, 17b–17e. See DOI: 10.1039/
c6ob01870j
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Scheme 1 Oxidative introduction of sulfur to the 2-position of histidine and its analogues (ref. 11).
Scheme 2 Discovery of a regioselectivity switch using thioacetic acid.
1
Fig. 1 H NMR spectrum (D
2
O, 400 MHz, aromatic region). a. Reaction
mixture obtained according to Scheme 3, by activation of an aqueous
acidic solution of L-histidine (6a) with Br , followed by addition of thio-
acetic acid. b. Reaction mixture obtained according to ref. 11, by acti-
vation of an aqueous acidic solution of L-histidine with Br , followed by
addition of cysteine. c. Acidic aqueous solution of 2-thiohistidine (8a).
2
2
Scheme 3 Reaction of activated L-histidine with thioacetic acid.
of thioacetic acid (Scheme 3). The reaction was monitored by could be identified in the reaction mixture; as the main
1
H NMR of the reaction mixture (Fig. 1 and entry 1, Table S3, product, 5-substituted 5-acetylsulfanyl-histidine 11a (58%),
1
ESI†).
was characterized in the H NMR by the presence of an aro-
As previously observed in the reaction with cysteine, histi- matic downfield proton (H-2) in the region of 8.9 ppm.
dine was almost completely transformed, and two products Moreover, the presence of the acetylsulfanyl-fragment was
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a
confirmed by a typical singlet at 2.5 ppm for the acetyl group, Table 1 Synthesis of 5-acetylsulfanyl-histidine and its analogues
and by mass spectrometry. A minor amount of 2-thiohistidine
(
8a) was also detectable besides non-identified byproducts, but
surprisingly, no 2-acetylsulfanyl-histidine was detected by
1
5
LC-MS in the reaction mixture.
Several attempts to isolate compound 11a in pure form
from the reaction mixture by chromatography or crystallization
were hampered by its instability, and resulted in complex dark
reaction mixtures and degradation.
NMR (δ) H-2/
α-H/AcS (ppm)
b
Entry Substr. R/R′
Product/yield
Therefore, our main efforts were first directed to optimise
the reaction conditions in terms of conversion of histidine and
reaction yield of 11a, followed by developing a procedure to
transform this unstable S-acetyl derivative into stable final
products (vide infra).
1
6a
2 2
NH /CO H
8.94/4.34/2.57
Variation of different reaction parameters showed that the
best results were obtained in the concentration range for 6a
from 0.15–0.25 M, with a reaction yield of 61–70% of the new
product using 4 equivalents of thioacetic acid (9). The same
preference towards 5-substitution by thioacetic acid is
observed with N-bromosuccinimide (NBS) instead of bromine
to activate histidine, with the best results in the presence of
2
6b
2
NHMe/CO H
8.91/4.13/2.55
2
equivalents of acid and a slightly longer activation time
1
6
before addition of thioacetic acid (see Table S3, ESI†).
3
4
6c
NMe /CO H
8.92/4.20/2.57
8.91/4.13/2.53
2
2
To our delight, N-methylated histidine derivatives (6b, 6c, 2)
can also be converted to hitherto unknown 5-acetylsulfanyl
derivatives (11b, 11c and 12) in satisfactory to good reaction
yields, depending on the methylation grade of the α-amino
1
function (see Table 1, entries 2–4). H NMR analysis of the
corresponding reaction mixtures indicated in all cases, the
characteristic downfield shift for the remaining aromatic H-2
proton, as well as a thioacetyl singlet at about 2.5 ppm in
all 5-acetylsulfanyl compounds. Moreover, mass analysis con-
firmed the occurrence of acetylsulfanyl-substitution. As experi-
enced for compound 11a, 5-acetylsulfanyl-histidines 11b, 11c
and 12 are instable, but small amounts of the α-N,N-dimethyl
and α-N,N,N-trimethylammonium derivatives 11c and 12,
respectively, could be obtained with increased purity by rapid
column chromatography immediately after their formation in
the reaction mixture. These two compounds were stable
enough in aqueous acidic solution for complete analytical
+
3
−
2
NMe
/CO
2
5
13
H/CO H
2
8.69/—/2.45
1
13
characterization by H and C NMR-spectroscopy and mass
spectrometry (see Experimental).
a
b
1
Interestingly, even imidazolepropionic acid 13 can be trans-
formed in good yield to its 5-acetylsulfanyl-derivative 14
Reaction conditions: see Experimental. Calculated yields by
H
c
NMR of the crude reaction mixture, see the ESI. Optimised con-
ditions, see entry 11, Table S3, ESI. n = 4. n = 5. n = 8. n = 6.
d
e
f
g
(Table 1, entry 5), illustrating that the α-amino-function is not
required for bromolactone formation and sulfur introduction.
Transformation into 5-sulfanyl-histidine
detailed analysis of the complex mixture by HPLC and
1
As reported, all attempts to isolate 5-acetylsulfanyl-histidine
H NMR indicated slow formation of the N-acetylated analogue
1
7
1
1a in pure form from the reaction resulted in the formation 15 by acetyl S/N-migration (Scheme 4).
of dark reaction mixtures and various degradation products.
Complete cleavage of this intermediate was achieved by
While thioesters are known to rapidly hydrolyse in aqueous heating the reaction mixture for several hours. The desired
solution to the corresponding thiol and acetic acid, the mass 5-sulfanyl-histidine 16a could be obtained and isolated con-
of the expected 5-sulfanyl-histidine (or its disulfide, vide infra) veniently when hydrolysis was performed in the presence of a
could be detected only in traces by HPLC in the aqueous reac- reducing agent such as 3-mercaptopropionic acid (5) to protect
tion mixture. Upon prolonged stirring at room temperature, against oxidation and/or oxidative degradation.
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Scheme 5 Biosynthesis of ovothiol A (ref. 23).
Scheme 4 Transformation of 5-acetylsulfanyl-histidine into 5-sulfanyl-
histidine.
a
Table 2 One-pot synthesis of 5-sulfanyl-histidines and its analogues
Based on these observations, a straightforward one-pot two-
step procedure for the synthesis and isolation of 5-sulfanyl-
histidine 16a from histidine 6a was developed, in which an ice-
cold aqueous acidic solution of L-histidine (6a) is first activated
by the addition of 1.3 equivalents of bromine or NBS, rapidly
followed by the addition of thioacetic acid (9) in excess. After a
b
short reaction time of 60 min, 6 equivalents of 3-mercapto- Entry
propionic acid (5) are added to the reaction mixture containing
Substrate
R/R′
NH
NHMe/CO H
NMe
NMe
Isolated product
Yield (%)
1
6a
6b
6c
2
2
/CO
2
H
2
16a
17b
17c
59%
59%
34%
23%
55%
the 5-acetylsulfanyl compound 11a, and the cleavage is com-
plete by heating for 16 h at 100 °C. After multiple optimis-
ations and fine-tuning of a convenient isolation procedure,
pure 5-sulfanyl-histidine 16a could be obtained directly in
multi-gram quantities without chromatography from the
reaction mixture by precipitation in a global yield of 59% over
the 2 steps.
2
3
4
5
2
/CO
2
H
+
−
c
3
/CO
H
2
16d/17d
13
H/CO
2
17e
a
b
c
Reaction conditions: see Experimental. Isolated global yield. 16d
was obtained by crystallization, and further oxidised quantitatively to
the corresponding stable disulfide 17d.
This remarkably simple procedure provides 5-sulfanyl-
histidine 16a as a beige powder, which quite surprisingly
showed to be stable against air-oxidation in the solid state.
This compound is a rare natural amino acid, isolated from sea
nucleophilic substitution of brominated precursors under
harsh conditions.
20b,25
This one-pot two-step procedure could be extended to the
1
8
urchin ovaries. To our knowledge, no chemical synthesis for
this sulfur-containing amino acid has been described up to
preparation of novel α-N-methylated 5-sulfanyl-histidines
26
1
6b–d. Due to apparently higher sensitivity towards air oxi-
1
9
now, contrary to the related ovothiol A, for which two multi-
dation compared to 5-sulfanyl-histidine 16a, these derivatives
were isolated as disulfides 17b–d after controlled oxidation
2
0
step chemical syntheses had been developed.
With this short one-pot two-step procedure, multi-gram
quantities of 16a are for the first time readily available. This
will hopefully encourage the study of the biological activity of
27
using DMSO (see Table 2, entries 2–4). Moreover, the pre-
viously unknown disulfide 17e could be prepared and isolated
in a similar way with a satisfactory global yield of 55% (see
Table 2, entry 5).
2
1,22
this multi-functional amino acid,
and its application as a
precursor for ovothiol A. Interestingly, biosynthetic studies on
ovothiol A showed that enzyme-catalysed imidazole-N-methyl-
ation proceeds after C–S bond formation and cleavage to 16a
Conclusion
2
3
(see Scheme 5).
Besides providing 5-sulfanyl-histidine via the cleavage of First applications of a new oxidative C–S bond-forming reac-
the 5-acetylsulfanyl intermediate 11a, it is noteworthy that this tion are reported, which is based on a strikingly different
new sulfur introduction represents to our knowledge the first regioselectivity in the reaction of thioacetic acid vs. cysteine.
non-enzymatic example of direct oxidative C–S bond formation While further studies are needed to unveil the mechanistic
in the 5-position of histidine (and its derivatives), corres- background for this finding, it is the key for a short one-pot
ponding formally to an insertion of sulfur in a non-electro- two-step synthesis of 5-sulfanyl-histidine 16a and derivatives
2
4
philic aromatic C–H bond. Non-enzymatic sulfur introduc- 17b–e presented here, starting directly from the parent histi-
tion in this position was previously exclusively performed by dine compounds. This new protecting-group-free approach
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2
provides for the first time a convenient access to 5-sulfanyl- of the reaction mixture were diluted in 500 µL D O, filtered,
1
histidine 16a, a naturally occurring amino acid, in multigram and the solution analysed by H NMR (400 MHz) as described
quantities without chromatographic purification, as well as to in the typical procedure S1 (ESI, and Fig. S2†).
the whole α-N-methylated 5-sulfanyl-histidine family, includ-
Compounds 11c and 12 could be obtained in small quan-
ing the hitherto unknown 5-sulfanyl-betaine 16d, a regio- tities by rapid purification on a silica gel column (ethyl
3
1
isomer of the ubiquitous amino acid, ergothioneine (1).
acetate/ethanol/water) for analysis, but decomposed on
storage.
L-5-Acetylsulfanyl-α,N,N-dimethyl-histidine (11c). α-N,N-
Dimethyl-L-histidine hydrochloride hydrate (6c, 6.0 g,
Experimental
General
25 mmol, 1 equiv.) and concentrated hydrochloric acid
(
(
2.1 mL, 2.46 g, 25 mmol, 1 equiv.) were dissolved in water
135 mL) and the solution was cooled to 0 °C. Under vigorous
Chemicals were purchased from commercial suppliers and
1
used as received. H NMR spectra were recorded at 300 MHz
stirring, N-bromosuccinimide (5.87 g, 33 mmol, 1.3 equiv.) was
and 400 MHz with D
2 2
O (4.79 ppm) or D O/DCl as the solvent
added. After 1 minute, thioacetic acid (7.2 mL, 7.61 g,
and internal standard (‘NH’ protons did not appear in
1
00 mmol, 4 equiv.) was added quickly and the mixture was
1
H NMR spectra because of D O exchange, for D O/DCl-solu-
2
2
stirred at 0 °C for 30 minutes. Compound 11c is obtained with
a reaction yield of 68 mol% as calculated from H NMR. The
product was purified on a silica gel column (gradient of ethyl
1
tions). Chemical shifts are reported in parts per million
δ) downfield from TMS. Spin multiplicities are indicated by
(
the following symbols: s (singlet), d (doublet), t (triplet),
q (quartet), and m (multiplet). Coupling constants are reported
acetate/ethanol/water) for analysis.
1
H NMR (300 MHz, D O pH = 2–3): δ 2.54 (s, 3H), 2.96
2
1
3
in hertz (Hz). C NMR spectra were obtained at 75 MHz with
D O or D O/DCl as the solvent (2-(trimethylsilyl)-1-propanes-
(
s, 6H), 3.28 (dd, J = 14.7, 10.4 Hz, 1H), 3.39 (dd, J = 14.7,
2
2
4.4 Hz, 1H), 3.87 (dd, J = 10.4, 4.4 Hz, 1H), 8.81 (s, 1H).
Traces of the hydrolysed product are detected at δ 2.96 (s),
2
8
ulfonic acid sodium salt was used as internal standard ).
Mass spectra were recorded by atmospheric pressure chemical
ionization (APCI-MS). Melting points were determined on a
Stuart Scientific apparatus. HPLC analyses (see the ESI†) were
performed with two column types: Column A: Hypercarb 5µ
3
.20 (m), 4.05 (m), 8.30 (sl).
1
3
2
C NMR (75 MHz, D O): δ (ppm) = 22.4, 30.0, 41.7, 68.4,
1
17.2, 134.3, 136.8, 170.9, 195.9.
APCI-MS: m/z 258.9 (M + H) .
+
(100 × 4.6 mm, 30 °C), elution with a mixture of two eluents
L-5-Acetylsulfanyl-α,N,N,N-trimethyl-histidine (12). L-Hercynine
A1 (acetonitrile containing 0.2% formic acid) and A2 (water
containing 0.2% formic acid), applying a flow of 0.9% min−
and a gradient from 100% A2 to 100% A1 in 12 min. Column
B: diol 5µ (250 × 4.6 mm, 20 °C), elution with a mixture of two
eluents B1 (acetonitrile containing 0.1% trifluoroacetic acid)
and B2 (water containing 0.1% trifluoroacetic acid), applying a
flow of 1.5 mL min− and a gradient from 90% B1/10% B2 to
(
2, 2.0 g, 10 mmol, 1 equiv.) and concentrated hydrochloric
1
acid (1.66 mL, 1.96 g, 20 mmol, 2 equiv.) were dissolved in
water (55 mL) and the solution was cooled to 0 °C. Under
vigorous stirring, N-bromosuccinimide (2.48 g, 14 mmol,
1
4
.4 equiv.) was added. After 5 minutes, thioacetic acid (4.4 mL,
.69 g, 60 mmol, 6 equiv.) was added quickly and the mixture
1
stirred at 0 °C for 30 minutes. Compound 12 is obtained with
a reaction yield of 65 mol% as calculated from H NMR, and
1
5
0% B1/50% B2 in 12 min.
Purifications of compounds 11c and 12 were performed by
was purified rapidly on a silica gel column (gradient of ethyl
chromatography on silica gel (45–60µ). α-N-Methyl-histidine
hydrochloride (6b·HCl), α-N,N-dimethyl-histidine hydro-
acetate/ethanol/water) for analysis.
1
2
H NMR (300 MHz, D O): δ 2.53 (s, 3H), 3.30 (s, 9H), 3.37
chloride hydrate (6c·HCl·H O) and L-hercynine (2) were pre-
2
(m, 1H), 3.44 (dd, J = 14.0, 3.8 Hz, 1H), 3.88 (dd, J = 11.7,
2
9
pared according to Reinhold et al., and imidazolepropionic
3
.8 Hz, 1H), 8.72 (s, 1H).
3
0
1
3
acid (13) was prepared according to Pirrung et al. Other
2
C NMR (75 MHz, D O): δ (ppm) = 22.9, 30.0, 52.5, 76.5,
reagents, such as L-histidine hydrochloride hydrate
1
17.9, 133.1, 137.2, 169.7, 196.0.
APCI-MS: m/z 272.1 (M + H) .
+
(
2 2
6a·HCl·H O), thioacetic acid (9), Br , NBS and 3-mercapto-
propionic acid (5), were commercially available and used
One-pot synthesis of L-5-sulfanyl-histidine 16a. L-Histidine
without further purification.
hydrochloride monohydrate (6a, 20.96 g, 100 mmol, 1 equiv.)
was dissolved in water (720 mL) and concentrated hydrochloric
acid (8.35 mL, 9.85 g, 100 mmol, 1 equiv.) and the solution
was cooled to 5 °C. Under vigorous stirring and continued
Typical procedure A: synthesis of 5-acetylsulfanyl-substituted
compounds in solution (Table 1)
An aqueous solution of the starting histidine, (6a–c, 2, cooling, N-bromosuccinimide (23.37 g, 130 mmol, 1.3 equiv.)
mmol, 1 equiv.) or imidazolepropionic acid, respectively, (13, was added in one portion. Thioacetic acid (30 mL, 31.7 g,
5
for entry 5) and concentrated hydrochloric acid (1 equiv. when 400 mmol, 4 equiv.) was added at once 2.5 minutes later and
using Br as a reagent, 2 equiv. in the case of NBS) was cooled the mixture was stirred for 1 h at 5 °C. 3-Mercaptopropionic
2
in an ice-bath. Bromine or N-bromosuccinimide (1.3 equiv.) acid (53 mL, 64.3 g, 600 mmol, 6 equiv.) was added and the
was added all at once, followed by thioacetic acid (9, 4–8 slightly yellow solution heated at 100 °C for 16 h (attention:
equiv.) after 2.5 min. After stirring for 30 min at 0 °C, 200 µL H S is formed during hydrolysis of thioacetic acid used in
2
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excess. To trap this highly toxic and malodorous compound, solution (835 µl, 985 mg, 10 mmol, 1 equiv.) and the solution
the outlet of the reflux condenser must be connected with a was cooled to 1 °C. N-Bromosuccinimide (2.67 g, 15 mmol,
bleach trap!). After cooling, a white precipitate was removed by 1.5 equiv.) was added quickly under vigorous stirring. After
filtration and the filtrate, containing the desired product, was 2 minutes, thioacetic acid (5.9 mL, 6.28 g, 80 mmol, 8 equiv.)
partially evaporated. The pH of the solution was adjusted to was added quickly. Stirring was continued vigorously at 0 °C
pH = 4 and the mixture was poured on cold ethanol to initiate for 60 minutes. The solution was extracted with ethyl acetate
precipitation. After filtration of the crude product, it was (3 × 50 mL). 3-Mercaptopropionic acid (3.5 mL, 4.24 g,
further purified by solubilisation and precipitation in water, 40 mmol, 4 equiv.) was added to the aqueous phase and
changing the pH of the mixture gradually from 1 to 5. After fil- heated at 130 °C for 3 h (attention: H
2
S is formed during the
tration and drying under vacuum, L-5-sulfanyl-histidine 16a hydrolysis of thioacetic acid used in excess. To trap this highly
(
11.15 g, 59%) was obtained as a beige solid.
toxic and malodorous compound, the outlet of the reflux con-
O): δ 3.19 (dd, J = 15.8, 7.3 Hz, 1H), denser must be connected with a bleach trap!). After cooling
.27 (dd, J = 15.8, 5.1 Hz, 1H), 4.09 (dd, J = 7.3, 5.1 Hz, 1H), and concentration, the residue was oxidized with DMSO in
1
H NMR (400 MHz, D
2
3
8
.25 (s, 1H).
acetic acid. After filtration, crude disulfide was obtained as a
1
H NMR (400 MHz, D
2
O/DCl, pH = 0): δ 3.50 (dd, J = 15.7, brown powder (2.10 g), which was purified by recrystallization
7
1
.1 Hz, 1H), 3.57 (dd, J = 15.7, 7.1 Hz, 1H), 4.51 (t, J = 7.1 Hz, in ethanol, to yield 17c·2HCl·2HBr as a white powder
H), 8.78 (s, 1H). (1.125 g, 34%).
1
3
1
C NMR (75 MHz, D
35.4, 173.6.
2
O/DCl): δ 26.3, 55.2, 122.1, 130.1,
2
H NMR (400 MHz, D O): δ 3.01 (s, 2 × 6H), 3.34 (dd, J =
1
14.6, 11.2 Hz, 2 × 1H), 3.51 (dd, J = 14.6, 4.1 Hz, 2 × 1H), 4.09
(dd, J = 11.2, 4.1 Hz, 2 × 1H), 8.86 (s, 2 × 1H).
+
APCI-MS m/z: 186 (M − H ).
Mp: 260 °C (decomp.).
1
3
C NMR (75 MHz, D
2
O): δ 21.6, 41.7, 66.1, 123.4, 133.9,
E.A: found: C, 38.10; H, 5.01; N 22.35 calculated: C, 38.49; 136.8, 169.1.
+
H, 4.84; N 22.44.
α] = +7.4° [c = 0.1, 1 N HCl].
APCI-MS: m/z 429.0 (M + H) .
E.A: found: C, 29.40; H, 4.59; N, 12.26 calculated for
[
D
Disulfide 17b. The hydrochloride salt of α-N-methyl-L-histi-
dine (6b, 1.05 g, 5 mmol, 1 equiv.) was dissolved in water
C
16
H
24
N
6
O
4
S
2
·2HCl·2HBr: C, 28.97; H, 4.25; N, 12.67.
Mp: 211 °C (decomp.).
[α] = +98° [c = 1, 0.1 N HCl].
max (0.1 N HCl) 257 nm.
Disulfide 17d. L-Hercynine (2, 1.00 g, 5 mmol, 1 equiv.) was
(
(
35 mL) containing concentrated hydrochloric acid solution
420 µl, 5 mmol, 1 equiv.) and the solution was cooled to 1 °C.
D
λ
Under
vigorous
stirring
and
continued
cooling,
N-bromosuccinimide (1.17 g, 6.5 mmol, 1.3 equiv.) was added dissolved in water (35 mL) and a concentrated hydrochloric
rapidly. Then thioacetic acid (2.57 mL, 2.74 g, 35 mmol, acid solution (420 µl, 0.49 g, 5 mmol, 1 equiv.) was added,
7
equiv.) was added very quickly. Stirring was continued vigor- then the solution was cooled to 0 °C. With vigorous stirring,
ously at 0 °C for 60 minutes. The solution is extracted with bromine (335 µl, 1.04 g, 6.5 mmol, 1.3 equiv.) was added. After
4
2
0 mL ethyl acetate and 3-mercaptopropionic acid (2.2 mL, 3 minutes, thioacetic acid (3.67 mL, 3.92 g, 50 mmol, 10
.65 g, 25 mmol, 5 equiv.) was added to the aqueous phase equiv.) was added quickly. Stirring was continued for
(
2
attention: H S is formed during the hydrolysis of thioacetic 60 minutes. 3-Mercaptopropionic acid (2.2 mL, 2.68 g,
acid used in excess. To trap this highly toxic and malodorous 25 mmol, 5 equiv.) was added and the solution was heated at
compound, the outlet of the reflux condenser must be con- 140° C for 3 h (attention: H S is formed during hydrolysis of
2
nected with a bleach trap!). After heating at 100 °C for 20 h thioacetic acid used in excess. To trap this highly toxic and
and cooling to rt, the reaction medium is extracted with ethyl malodorous compound, the outlet of the reflux condenser
acetate (4 × 35 mL). After concentration of the aqueous phase must be connected with a bleach trap!). After extraction,
and oxidation in acetic acid with DMSO (0.39 g, 5 mmol, neutralization of the aqueous phase and crystallization in
1
4
equiv.), the crude disulfide was purified on Dowex 50WX2- the presence of dithiothreitol (780 mg, 5 mmol, 1 equiv.), L-5-
00 (NH OH, 10%), to yield compound 17b (620 mg, 59%). sulfanyl-α-N,N,N-trimethyl-histidine 16d is obtained as a white
O/DCl): δ 2.69 (s, 2 × 3H), 3.19 (dd, powder.
4
1
H NMR (300 MHz, D
J = 15.2, 9.3 Hz, 2 × 1H), 3.34 (dd, J = 15.2, 5.2 Hz, 2 × 1H), 4.04
2
1
H NMR (400 MHz, D
2
O): 3.19 (dd, J = 14.2, 10.7 Hz, 1H),
(
dd, J = 9.3, 5.2 Hz, 2 × 1H), 8.78 (s, 2 × 1H).
3.29 (s, 9H), 3.36 (dd, J = 14.2, 4.1 Hz, 1H), 4.00 (dd, J = 10.7,
1
3
C NMR (75 MHz, D
35.6, 139.3, 171.9.
APCI-MS: m/z 401.0 (M + H) .
2
O/DCl): δ 26.3, 34.4, 62.1, 126.0, 4.1 Hz, 1H), 8.22 (s, 1H).
+
1
APCI-MS: m/z 230 (M + H) .
+
This compound was dissolved in water (50 mL) and stirred
under an ambient atmosphere. After filtration and freeze
drying of the filtrate, the corresponding disulfide 17d is
obtained as a yellow powder (263 mg; 23%).
Mp: 197 °C (decomp.).
[α] = +56° [c = 1, 0.1 N HCl].
D
λmax (0.1 N HCl) 255 nm.
Disulfide 17c. The hydrochloride of α-N,N-dimethyl-histidine
1
H NMR (400 MHz, MeOD): δ 3.00 (dd, J = 13.6, 10.8 Hz,
monohydrate (6c, 2.43 g, 10 mmol, 1 equiv.) was dissolved in 2 × 1H), 3.07 (dd, J = 13.6, 3.6 Hz, 2 × 1H), 3.25 (s, 2 × 9H), 3.73
5
4 mL of water containing concentrated hydrochloric acid (dd, J = 10.8, 3.6 Hz, 2 × 1H), 7.87 (s, 2 × 1H).
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1
H NMR (400 MHz, D
2
O): δ 2.68 (dd, J = 13.5, 11.0 Hz,
4 B. Halliwell, I. K. Cheah and C. L. Drum, Biochem. Biophys.
Res. Commun., 2016, 470(2), 245–250, DOI: 10.1016/j.
bbrc.2015.12.124.
5 R. Forster, F. Spezia, D. Papineau, C. Sabadie,
I. Erdelmeier, M. Moutet and J.-C. Yadan, Food Chem.
Toxicol., 2015, 80, 85–91.
2
(
× 1H), 2.75 (dd, J = 13.5, 4.3 Hz, 2 × 1H), 3.19 (s, 2 × 9H), 3.68
dd, J = 11.0 Hz, 4.3 Hz, 2 × 1H); 7.97 (s, 2 × 1H).
1
3
C NMR (75 MHz, D
41.2, 172.5.
2
O): δ 25.4, 54.5, 79.8, 126.5, 138.9,
1
+
APCI-MS: m/z 457.0 (M + H) .
Mp: 180 °C (decomp.).
6 Y. C. Hseu, H. W. Lo, M. Korivi, Y. C. Tsai, M. J. Tang and
H. L. Yang, Free Radicals Biol. Med., 2015, 86, 102–117.
7 S. D. Genghof and O. Van Damme, J. Bacteriol., 1970, 103,
475–478; G. W. Jones, S. Doyle and D. A. Fitzpatrick, Gene,
2014, 549, 161–170; T. Pluskal, M. Ueno and M. Yanagida,
PLoS One, 2014, 9(5), e97774, DOI: 10.1371/journal.
pone.0097774.
[
α] = +146° [c = 1, 0.1 N HCl].
D
λ
max (0.1 N HCl) 255 nm.
Disulfide 17e. 3-(1H-Imidazol-5-yl)propanoic acid hydro-
chloride hydrate (13, 1.5 g, 7.55 mmol, 1 equiv.) was dissolved
in 60 mL of water and the solution was cooled to 2 °C. Under
vigorous stirring and continued cooling, N-bromosuccinimide
(
2.3 g, 12.8 mmol, 1.7 equiv.) was added in one portion.
8 S. D. Genghof, J. Bacteriol., 1964, 87, 852–862.
9 A. Braunshausen and F. P. Seebeck, J. Am. Chem. Soc.,
2011, 133(6), 1757–1759, DOI: 10.1021/ja109378e;
G. T. M. Mashabela and F. P. Seebeck, J. Chem. Soc., Chem.
Commun., 2013, 49, 7714–7716.
Thioacetic acid (2.2 mL, 2.39 g, 30.2 mmol, 4 equiv.) was
added at once one minute later and the mixture was stirred for
1
h at 5 °C. 3-Mercaptopropionic acid (4.0 mL, 4.86 g,
5.3 mmol, 6 equiv.) was added and the slightly yellow solu-
4
tion was heated at 100 °C for 16 h (attention: H
during hydrolysis of thioacetic acid used in excess. To trap this
2
S is formed 10 H. Song, M. Leninger, N. Lee and P. Liu, Org. Lett., 2013,
15(18), 4854–4857, DOI: 10.1021/ol402275t.
highly toxic and malodorous compound, the outlet of the 11 I. Erdelmeier, S. Daunay, R. Lebel, L. Farescour and
reflux condenser must be connected with a bleach trap!). After J.-C. Yadan, Green Chem., 2012, 14, 2256–2265.
cooling, a precipitate was removed and the filtrate was washed 12 G. L. Schmir and L. A. Cohen, Biochemistry, 1965, 4, 533–
with ethyl acetate (5 × 25 mL). The pH of the aqueous solution 538; S. Ito, J. Org. Chem., 1985, 50, 3636–3638.
was adjusted to 4 with aqueous NaOH. The solution was par- 13 Concerning the nomenclature and numbering of histidine
tially evaporated, and ethanol was added quickly, to precipitate
the desired product. The solid was filtered, washed with
ethanol and dried under vacuum. During the isolation pro-
cedure, the initially formed compound 16e was oxidized by air
to the disulfide 17e, which was obtained as a white solid
derivatives, we apply in the following the IUPAC definition,
which assigns the number 4 to the imidazole carbon carry-
ing the amino acid side chain in histidine and its substi-
tuted derivatives, and the number 5 to the adjacent imid-
azole carbon (IUPAC. Compendium of Chemical
Technology, 2nd ed. (the “Gold Book”). Compiled by
A. D. McNaught and A. Wilkinson, Blackwell Scientific
Publications, Oxford; DOI: 10.1351/goldbook.P04890).
Moreover, according to IUPAC, the functional group –SH
should be named ‘sulfanyl’ instead of the previously used
term ‘mercapto’, while the suffix or prefix ‘thio’ is limited
specifically to the functional groups in which oxygen is
replaced by sulfur (IUPAC Provisional Recommendations.
Preferred IUPAC Names. Chapter 6, Sect 60–64. September,
(
725 mg, 55%).
1
H NMR (400 MHz, D O): δ 2.40 (t, J = 7.1 Hz, 2 × 2H), 2.63
2
(
t, J = 7.1, 2 × 2H), 8.57 (s, 2 × 1H).
1
3
C NMR (75 MHz, D
75.8.
2
O): δ 19.2, 32.5, 121.4, 136.0, 138.6,
1
+
APCI-MS: m/z 343.5 (M + H) .
E.A.: found: C, 42.0; H, 4.12; N, 16.42 calculated: C, 42.09;
H, 4.12; N, 16.36.
2004;
http://www.iupac.org/fileadmin/user_upload/publi-
cations/recommendations/nomenclature-of-organic-chem-
istry/Chapter6-Sec60-64.pdf). Therefore, we propose to
name the new compounds 5-acetylsulfanyl-histidine and
5-sulfanyl-histidine, respectively.
Acknowledgements
The authors would like to thank Dr Marc Moutet
(Tetrahedron) for helpful and stimulating discussions.
14 See ref. 11.
15 Further studies have to show if 8a is formed by immediate
hydrolysis of transient 2-acetylsulfanyl-histidine or by reac-
tion of traces of H S with the bromolactone 10a.
2
1
6 Adding acetic acid instead of thioacetic acid results in a
complex mixture of products, without formation of a
corresponding O-acetyl derivative in significant amounts,
according to mass spectroscopic analysis.
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