The Forgotten Pyrazines: Exploring the Dakin–West Reaction

Article abstract of DOI:10.1002/chem.202000475

Pyrazines are an underreported class of N-heterocycles available from nitrogen-rich biomass presenting an interesting functional alternative for current aromatics. In this work, access to pyrazines obtained from amino acids by using the 90 year old Dakin–West reaction was explored. After a qualitative screening several functional proteinogenic amino acids proved good substrates for this reaction, which were successfully scaled to multigram scale synthesis of the corresponding intermediate α-acetamido ketones. Subsequently, the conditions towards pyrazine formation using δ-amino-levulinic acid were optimized, and these were employed to synthesize a relevant set of five functional dimethylpyrazines in high purity. These pyrazines can be considered a versatile toolbox of aromatic building blocks for a wide range of applications, such as in the synthesis of polymers or metal–organic frameworks.

Full text of DOI:10.1002/chem.202000475

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Title: The Forgotten Pyrazines: Exploring the Dakin-West Reaction.
Authors: Romano Orru, Martien Wurdemann, Cristina Niţu, Stefaan De
Wildemann, and Katrien Bernaerts
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The Forgotten Pyrazines: Exploring the Dakin-West Reaction.
Martien A. Würdemann[a], Cristina Niţu[b], Dr. Stefaan M.A. De Wildeman[a], Dr. Katrien V. Bernaerts*[a],
Prof. Dr. Romano V.A. Orru*[a]
Abstract: Pyrazines are an underreported class of N-heterocycles
available from nitrogen-rich biomass presenting an interesting
functional alternative for current aromatics. In this work we explore
access to pyrazines obtained from amino acids using the 90 year old
Dakin-West reaction. After a qualitative screening several functional
proteinogenic amino acids proved good substrates for this reaction,
which were successfully scaled to multigram scale synthesis of the
corresponding intermediate α-acetamido ketones. Subsequently, we
optimized the conditions towards pyrazine formation using
δ-amino-levulinic acid, these were employed to synthesize a relevant
set of five functional dimethylpyrazines in high purity. These pyrazines
can be considered a versatile toolbox of aromatic building blocks for
a wide range of applications such as in the synthesis of polymers or
metal-organic frameworks.
The 1,4-diazines (pyrazines) attracted our attention as the
most aromatic N-heterocycle known (89% aromaticity compared
to benzene).[7] We anticipate that this property combined with their
high degree of symmetry should result in similar thermal and
mechanical properties for pyrazine containing monomers
compared to those containing benzene. Furthermore, the
reactivity of both nitrogen atoms in pyrazines provides
opportunities for post-polymerization functionalization.[8-9] Despite
these properties, pyrazines have barely been reported in polymer
compositions nor as additives for polymers or advanced materials.
A
lack of precedent offering great potential for further
development. We envisioned that pyrazines should be accessible
from certain nitrogen-rich biomass streams, but to the best of our
knowledge however, no methodology for the transformation of
such biomass to pyrazines has recently been reported.
Although underreported in a polymer context, pyrazines are a
well-studied class of compounds.[8-9] The pyrazine core is found
in pharmaceuticals and dyes as well as in many flavors.[8-14] The
synthesis of para-symmetric pyrazines is most easily achieved via
the auto-condensation of α-amino-carbonyl compounds. These
are moderately stable as their salts or under air free conditions.
When the basic nitrogen in α-amino carbonyls is liberated, cyclic
Introduction
Petrochemicals are widely used, either as an energy source or as
a source of functional aromatic building blocks for the chemical
production of valued materials. However, the notable
environmental impact associated with petrochemical resources
makes their use inherently not sustainable. Current chemical
research therefore focuses on more sustainable alternatives to
access industrially important aromatic monomers. In this respect,
the use of lignocellulosic biomass has received considerable
interest.[1-5] On the other hand, nitrogen-rich biomass based on
proteins or amino acids is rather underutilized.[6] This type of
biomass is either available from waste streams or can be
generated by fermentative production and holds great promise for
sustainable use in new or existing materials. Especially since
access to a wide range of N-heterocyclic aromatic monomers
from amino acids can be achieved via numerous highly
established routes.[6]
diimines (see step
3 in Scheme 1) are formed which
spontaneously oxidize to pyrazines in the presence of oxygen (air)
or other oxidants (see step 4 in Scheme 1).[8] All synthesis
methods to form pyrazines via the α-amino carbonyl compounds
are inherently atom efficient, with only the release of water.
Synthesis of the required α-amino carbonyl compounds can
be achieved via various approaches from different starting
materials.[8-9] In our opinion the use of commercially available
proteinogenic amino acids as starting materials would be highly
interesting as they constitute a chemically pure source of
nitrogen-rich biomass, which potentially could be obtained from
waste streams. The Dakin-West reaction, reported already in
1928, is a most interesting means to obtain the desired α-amino
carbonyls directly from amino acid starting materials. In this
reaction, an α-amino acid (1) is converted to an α-amino methyl
ketone with acetic anhydride (see step 1 in Scheme 1).[15-17] The
initial product obtained is an α-acetamido ketone (2) formed after
loss of carbon dioxide. Subsequent hydrolysis of 2 to an α-amino
ketone hydrochloride (3) (see step 2 in Scheme 1) followed by
dimerization and auto-oxidation provides direct access to
pyrazines (4).
[a]
M.A.Würdemann, Dr. S.A.M. De Wildeman, Dr. K.V. Bernaerts, Dr.
R.V.A. Orru
Aachen Maastricht Institute for Biobased Materials (AMIBM)
Maastricht University
Urmonderbaan 22, 6167 RD
Geleen, the Netherlands
E-mail: r.orru@maastrichtuniversity.nl;
katrien.bernaerts@maastrichtuniversity.nl
C. Niţu
[b]
Maastricht Science Program, Maastricht University
Kapoenstraat 2, 6211 KW
Maastricht, the Netherlands
Supporting information for this article is given via a link at the end of
the document.
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of the α-acetamido ketone (2) pyrazine precursors considerably.
However, autocatalytic carbon dioxide evolution, which
accompanies the formation of the desired α-acetamido ketones
(2), is quite sudden and violent presenting a significant safety
hazard potentially hampering scale-up. In a modification, Fischer
and Misun reported the use of reduced ratios of both acetic
anhydride and TEA and they safely scaled the reaction using 0.5
kmol alanine producing the corresponding α-acetamido ketone (2,
R = Me) in excellent yields.[26]
Thus, these conditions were selected and adapted for our
screening of the amino acids (1a-m) of interest in the Dakin-West
approach towards different functionalized α-acetamido ketones
(2a-m). Since the selected amino acids contain possibly reactive
functional groups, an extra equivalent of both acetic anhydride
and TEA were added compared to the Fischer conditions. The
screening was set up at one mmol scale with the reactions being
qualitatively analyzed by LC-MS to select possible candidates for
scale-up later on. The results of this screening are summarized in
Scheme 2.
Scheme 1. Dimethylpyrazine synthesis via the Dakin-West reaction on amino
acids.
The Dakin-West reaction proceeds under relatively mild reaction
conditions, which tolerate for example carboxylic anhydrides and
functionalized α-H carboxylic acids as starting materials.[18] This
is highlighted by the fact that the reaction was already used by
Woodward et al. in his famous first total synthesis of strychnine
reported in 1963.[19] Dakin and West already showed that
proteinogenic amino acids could be transformed to pyrazines by
their procedure. However, a comprehensive overview of which
amino acids may be employed as effective substrates in the
Dakin-West reaction has never been composed.[18, 20-21] Also, in a
more recent review that discusses the mechanism in more detail,
no general substrate scope of the Dakin-West reaction is
presented.[22] In addition, analytical records for both α-acetamido
ketones (2) synthesized via the reaction as well as for the
corresponding pyrazines (4) obtained in this way are sparse or
based on derivatives, especially since most of these reactions
were performed before 1980.
Based on the above considerations we decided to explore the
potential of the Dakin-West reaction to access dimethylpyrazines
(4) from amino acids. We focused on the use of proteinogenic
amino acids with a non-aliphatic side chain, leading directly to a
dimethylpyrazine product with a reactive functionality for further
development into functionalized bio-based materials or metal
organic frameworks.
Here we discuss the results of our studies, including
optimization, scope and scale-up towards functional
dimethylpyrazines.
Results and Discussion
Qualitative screening of proteinogenic amino acids and
selection of targets.
We first set out to identify which non-aliphatic proteinogenic amino
acids (1a-m) indeed can be transformed into dimethylpyrazines
(4) using the Dakin-West reaction. By keeping currently accepted
principles of green chemistry in mind and taking potential
scalability into consideration,[23] the Steglich and Höfle reaction
conditions using triethylamine (TEA) as base and
dimethylaminopyridine (DMAP) as catalyst seemed a most
relevant starting point for optimization.[24-25] These conditions
allow for reduced reaction temperatures while increasing the yield
Scheme 2. Qualitative screening conditions and results. (‡ 1g = glutamic acid)
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Dakin and West already reported that the formation of
α-acetamido ketone 2a, from serine (1a) proceeds in poor
yields.[15] Also, under our conditions no identifiable products were
observed using serine as the starting material. On the other hand,
the reaction with threonine showed the clear formation of the side
product 5b (see frame A in Figure 1). Acetylation of the hydroxyl
group in threonine generates, under the reaction conditions, a
relatively good leaving group and elimination results in alkene 5b.
A similar elimination most likely also occurs using serine and
gives 5a, in this case. The formed alkene is not stabilized by a
methyl group resulting in further side-reactions. Dakin and West
reported that with phenylserine, indeed elimination followed by
ring closure to cinnamic acid azlactone 6 (see frame A in Figure
1) took place, an observation that further supports our findings.[15]
For the above reasons, the use of serine (1a) and threonine (1b)
in our further scale-up studies was discarded.
Under the conditions applied in this study using TEA as
base, cysteine (1c) showed the formation of the desired
α-acetamido ketone (2c) with low selectivity. This observation is
in line with earlier reports using more classical conditions that also
provided ketone 2c, in mediocre yields.[20, 27] The selectivity for
α-acetamido ketone (2c) formation using cysteine (1c) was too
low to merit further research.
Aspartic acid (2d) was not included in this study since it was
shown by others that double intramolecular ring closures of
different kinds preclude any selective formation of the desired
acetamido-ketone product.[20, 24]
To the best of our knowledge, the Dakin-West reaction has
never been performed with asparagine (1e) or glutamine (1f).
Under our conditions complex product mixtures were obtained
using these starting materials. Most notable were dehydrated,
acetylated amino acids byproducts according to the MS spectra.
The desired products (2e & 2f) were, however, not observed and
no further efforts were made with asparagine and glutamine as
starting materials.
conditions the desired ketone 2i was indeed formed selectively in
good yield and methionine was thus selected for scale-up towards
pyrazines.
The α-acetamido ketone (2j) derived from histidine (1j) was
originally reported by Dakin and West. This compound received
quite some attention as a histidine decarboxylase inhibitor.[15, 21,
40-41] The dimethylpyrazine derived from 2j has also been reported
for the same application although no synthetic details were
enclosed.[42] Under our conditions the desired product was found
to be formed with high selectivity and we thus selected 1j for our
scale-up studies discussed below.
Tyrosine (1k) was one of the first substrates for the
Dakin-West reaction, with α-acetamido ketone 2k being formed in
high yield and the corresponding pyrazine was synthesized
37]
effectively.[16-17,
The α-acetamido ketone 2k was also
synthesized in high yields under Steglich-like conditions.[39] Under
our screening conditions, tyrosine (1k) reacted smoothly and
since the pyrazine was already reported tyrosine was selected as
starting material for scale-up as well.
Similarly, the tryptophan-derived acetamido-ketone 2l has
been synthesized before in good yields.[15, 37-38, 43-44] In our study
the desired product 2l was formed together with the acetylated
indole ketone 2l’. The combined selectivity towards these two
products proved very high and tryptophan was selected for
scale-up towards the corresponding pyrazines.
Formation of α-acetamido ketone (2m) using arginine (1m) in
the Dakin-West reaction proved not succesful in earlier studies
with both the free arginine as well as N-benzoyl-arginine.[45] Under
the conditions applied in our study a range of different ketones,
different extents of acetylation on the guanidine system as well as
ring closed ornithine 8 (see frame C in Figure 1) could be
detected. Consequently, pursuit of arginine as starting material for
pyrazine synthesis was not further explored.
Glutamic acid (1g) was already reported as a substrate by
Dakin and West in 1928, however the corresponding ketone
product was not isolated by them and ultimate evidence of its
synthesis was provided by the synthesis of the corresponding
pyrazine.[15] The intermediate α-acetamido ketone product,
1,5-diacetyl-2-pyrrolidone 2g was later identified in combination
with the major side-product, acetyl-pyroglutamic acid 7 (see frame
B in Figure 1).[28] The synthesis of 2g is well reported and was
smoothly formed under our conditions. Glutamic acid (1g) was
therefore selected for scale-up towards pyrazines (see
below).[29-30]
The use of lysine (1h) in the Dakin-West reaction has only
been reported in a family of patents by Bayer.[31-35] The original
Steglich conditions were used herein to synthese the
corresponding ketone 2h. Subsequent acid hydrolysis and ring
formation affords the desired pyrazine. In the screening using our
modified conditions the α-acetamido ketone (2h) was indeed
formed smoothly. We therefore selected lysine as starting
material for scale-up towards the corresponding pyrazines.
Methionine (1i) is reported to act as a selective substrate in
the Dakin-West reaction under classical conditions[20, 36-38] as well
as under the Steglich conditions.[39] Under our screening
Figure 1. Side-products observed during qualitative screening of the
Dakin-West reaction.
Scale-up of α-acetamido ketone (2g-2l) synthesis
The qualitative screening described above identified Glu (1g),
Lys (1h), Met (1i), His (1j), Tyr (1k), and Trp (1l) as successful
amino acid starting materials in the Dakin-West synthesis of the
corresponding α-acetamido ketones (2g-2l). We now turned our
attention to explore suitable work-up conditions and therefore
performed these Dakin-West reactions at 10 mmol scale using our
initial conditions described above in Scheme 2. Further, in order
to assess safety hazards, gas formation and exotherms the
reactions were scaled further to 49-68 mmol (≈10 grams).
Except for the reaction with Glu (1g), all reactions provided the
desired ketones in good yields (73% - 84%). For glutamic acid it
was found that competition with pyroglutamic acid formation
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Table 1. Results and work-up/purification conditions for the α-acetamido ketone products
2g-2l obtained from the Dakin-West reaction using the amino acids 1g-1l.
inhibited the formation of the desired ketone (2g).
Reactions at the same scale using conditions with
more equivalents of acetic anhydride and
triethylamine provided yields of 2g similar to those
reported by Lepschy et al..[24]
#
Substrate
Glutamic acid
Lysine
Work-up
Product
Isolated yield [mol%]
1g
1h
1i
Vacuum distillation
Trituration in EtOAc
Extraction and column[b]
Continued to diHCl 3j
Trituration in Et2O
Trituration 0.7M KOH
2g
2h
2i
74[a]
77
73
x
Work-up and purification of all reaction mixtures
was performed by concentration in vacuo followed by
a distinct procedure depending on the nature of each
α-acetamido ketone (2g-2l) that was formed. Column
chromatography was considered a last resort option
for purification, as it has limited scalability and is
inherently not “green”. Concentration of the reaction
mixtures was conducted at high temperature and low
Methionine
Histidine
1j
x
1k
1l
Tyrosine
2k
2l
84
78
Tryptophan
pressure
to
minimize
product
loss.[26]
[a] Yield corrected for co-distilled ionic liquid of triethylammonium acetate. [b] See
experimental for details.
Triethylammonium acetate is formed during the
reaction and after complexation it acts as an ionic
liquid with a high boiling azeotrope of 160 °C.[46] This complex was
removed by distillation.
Hydrolysis of α-acetamido ketones 2g-2l
Next we turned our attention to the deprotection of the
α-acetamido ketones (2g-2l) formed during the Dakin-West
reaction (step 2 in Scheme 1), which is necessary before the final
ring-closure to the desired dimethylpyrazine building blocks (4)
can occur. Except for tryptophan-based ketone 2l, the removal of
the acetyl-group was easily achieved in good yields by refluxing
in 3 equiv. of 6M hydrochloric acid (see Table 2). Reaction
progress was monitored by LC-MS and the hydrolysis was
stopped after complete consumption of the starting ketone. In
case of 2l hydrolysis was unsuccessful with numerous
side-products being formed. Based on earlier mechanistic studies,
side-reactions are likely to involve a chlorine-induced oxidation,
which could be suppressed by running the reaction under
anaerobic conditions or by using a different acid such as sulfuric
acid.[47] However, hydrolysis of ketone 2l in sulfuric acid or under
anaerobic conditions was not successful in our hands. The use of
other protective groups that could be removed under milder
conditions was outside the scope of this work and no further
efforts to synthesize a dimethylpyrazine derived from tryptophan
were pursued.
In the case of ketone 2l, derived from Trp (1l), the indole-
moiety was partially acetylated under our screening conditions.
However, easy deacetylation by stirring in a potassium hydroxide
solution at room temperature gave the desired 2l selectively after
simple filtration. Unfortunately, in case of the methionine-derived
product 2i, column chromatography proved necessary to obtain
the desired material. Finally, isolation of the ketone derived from
histidine (2j) was not possible because traces of the above
reported ionic liquid even persisted after prolonged concentration
and the reaction was therefore hydrolyzed to the dihydrochloride
salt 3j without isolation.
As mentioned before, safety and scalability aspects were
further tested performing our modified Dakin-West procedure at
10 gram scale (49-68 mmol). After stepwise addition of the solid
amino acid starting material, control over gas formation was
satisfactory. On the other hand, reversed addition, adding the
acetic anhydride to a stirred suspension of amino acid in TEA
gave no appreciable yield of the corresponding α-acetamido
ketones. During addition of the amino acid substrates in the
Dakin-West process no large exotherms were observed except
when lysine was used. Also noteworthy is that scale-up of our
Dakin-West procedure using glutamic acid at 100 gram scale
conducted in an automated setup with a Lambda solid doser,
proceeds with full control of gas formation no active cooling of the
reaction mixture was necessary affording the desired ketone
product in excellent yield. The only adjustment to the 10 grams
procedure was that we applied micro-colloidal silica as a flowing
agent.
Table 2. Results for the removal of the acetyl group of ketones 2g-2k.
#
Work-up
Product
3g
Isolated yield [mol%]
The observations during these scale-up experiments indicate
that our conditions provide a high level of control for the Dakin-
West reaction towards α-acetamido ketones (2g-2l), which is
comparable to the Fischer and Misun conditions developed for Ala.
This, together with the fact that the reaction with Ala as starting
material was scaled to 0.5 kmol (50 kg)[26], led us to conclude that
these conditions allow for safe scale-up of the Dakin-West
reaction to kilogram scale regardless of the amino acid substrate
chosen. The work-up conditions and yields for each substrate
after further scale-up to 49-68 mmol (10 g) are listed in Table 1.
2g
2h
2i
Trituration acetone, wash IPA
Trituration acetone
Trituration acetone
Trituration IPA
82
3h[a]
3i
84
69
2j
3j[a]
3k
76[b]
86
2k
Trituration acetone
[a] Isolated as double HCl salts, [b] Over two steps
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The other hydrochloride salts (3g-k) formed during hydrolysis
could be easily isolated as solids by concentration to dryness
in vacuo followed by either trituration in acetone for salts 3g-i &
3k or isopropyl alcohol (IPA) for salt 3j. After filtration and washing
with either IPA (salts 3g & 3j) or acetone (salts 3h-i & 3k), the
obtained solids were dried overnight and could be used without
further purification in the formation of pyrazines.
in aqueous solution was therefore not considered a viable
scalable synthesis method.
Several solvents tolerating higher oxygen solubility were
tested as alternatives, using triethylamine as an organic base at
100 mg scale. Methanol proved optimal because it allows
complete dissolution of the free base. In methanol the reaction
proceeds rapidly, with a suspension of the desired product
forming within several hours. The dimerization reaction towards
11 was complete upon stirring at room temperature overnight.
Two major impurities were detected in the MS-spectra of the
reaction mixture both with a mass of M+15. At first we believed
in-situ esterification after initial formation of 11 occurred. Further
scale-up was accompanied with strong discoloration of the
reaction from yellow to brown and MS-analysis showed that only
one of major impurities persisted in the reaction mixture. Since
the formation of 11 results in the formation of superoxide radical
anions according to the mechanistic studies discussed in
Scheme 3, we believed that these reactive species account for
the formation of the observed impurity and in-situ esterification is
not a major issue.[51] Especially since at increased scale limited
oxygen diffusion into the reaction gives the formed superoxide
radical anions enough time to oxidize methanol to formaldehyde,
instead of decomposing with molecular oxygen. Methanol
Pyrazine formation
The formation of pyrazines via auto-condensation of
α-amino-carbonyl compounds is well known.[8-9] The reaction
proceeds via
a
double imine formation that gives
a
dihydropyrazine intermediate which is subsequently oxidized to
the pyrazine (see Scheme 3). Various mild oxidizing agents such
as air, hydrogen peroxide, bromine water and cupric, ferric or
mercuric ions can promote the final oxidation. The pyrazine
formation from the α-amino-ketone δ-amino-levulinic acid (DALA)
9 is particularly well studied. The mechanism has been fully
elucidated including all involved intermediates and side
products.[48-51] Since 9 is commercially available and its pyrazine
formation is well studied we selected it as model compound to
establish optimal and scalable conditions to form ultimately our
pyrazines of type 4.
Scheme 3. Model reaction of pyrazine formation using DALA (top) and the side-reaction involving methanol (bottom).
The reported mechanistic studies on 9 have all been
performed in dilute aqueous solution and high conversions
towards the pyrazine end-product were observed.[49-51] During
scale-up using such conditions (water at pH 6-8) the formation of
the pyrazino dicarboxylic acid 11 proved, however, rather slow at
increasing concentrations of DALA. To the extent that, at higher
concentrations, which would allow facile isolation (> 0.1 mol/L),
the reaction was not complete after stirring for 1 week under an
air atmosphere. Although the reaction rate increases at higher pH,
an increase in pH is accompanied by discoloration of the reaction
mixture.[51] This discoloration indicates side-reactions, thus
preventing exploitation of this variable as a driver for the scale-up
of the dimerization reaction. Dimerization towards the pyrazine
scaffold is most likely rather slow in water due to the limited
solubility of oxygen in water as well as the fact that water has to
eliminate to form the intermediate non-aromatic dihydropyrazine
10. The equilibrium towards 10 is more towards the starting
material 9 due to the presence of water. Pyrazine condensation
oxidation to formaldehyde would then result in methylation of the
aromatic pyrazine ring to form 12. Aldehydes have been reported
to react with dihydropyrazines, forming alkyl substituted pyrazines.
Acetaldehyde specifically, has been reported to react with DALA
forming the ethyl substituted pyrazine, which further corroborates
our idea.[52] A cross-over experiment with formaldehyde, resulted
in the formation of a product with the same retention time and
molecular ion on LC-MS as the major impurity confirming the
hypothesis that the methylated pyrazino product 12 is the
observed impurity. Methanol oxidation by superoxide has been
reported to take place electrochemically.[53] Formation of large
amounts of the side-product could be prevented by the addition
extra oxidant to the reaction in the form of hydrogen peroxide.
The dimerization of hydrochloride salts 3g-k and the salt of 9
proceeds via a similar mechanism, therefore formaldehyde
formation by oxidation of methanol is likely to occur with these
substrates as well. Any formaldehyde formed would then frustrate
regioselective formation of the desired functional pyrazines 4. To
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test this, a condensation reaction using the keto-hydrochloride
obtained from glutamic acid 3g, which is DALA’s (9) closest
structural isomer, was performed. Formaldehyde was added to a
methanol mixture of ketone 3g followed by addition of TEA. Upon
addition a deep orange to brown reaction mixture immediately
formed, indicating that a side reaction indeed occurs.
to form the corresponding pyrazines 4g-4k (Table 3). Each
hydrochloride salt was suspended in methanol and cooled on ice
after which first hydrogen peroxide was added slowly (2 equiv.)
followed by addition of TEA (1 equiv.). In most cases this resulted
in condensation towards the desired pyrazines, however, the
relatively high pKa of the lysine-based double hydrochloride salt
3h prevented deprotonation by TEA and the desired reaction did
Attack of formaldehyde on the diimine formed from 3g,
Scheme 4. Side reactions with formaldehyde in the pyrazine formation starting from 3g.
however, would not result in an aromatic compound (13) and is
unlikely to take place (see Scheme 4). On the other hand, a
stabilized aromatic product can be formed upon formaldehyde
induced aldol reaction with the linear mono-imine, which is a
plausible intermediate. This reaction results in imidazole 14, (see
Scheme 4) and indeed an ion corresponding to this product was
detected after LC-MS analysis (see supplementary S55). This
imidazole could unfortunately not be isolated; however, since
discoloration was observed for the reaction of 3g with
formaldehyde, hydrogen peroxide was added to all further
pyrazine condensation reactions.
not proceed in this case. Deprotonation of 3h proceeds smoothly
using strong lye upon which the desired reaction took place and
4h was indeed formed in reasonable yields. For the
methionine-based hydrochloride 3i, the formation of superoxide
radical anions results in oxidation of the sulfide to the sulfoxide,
which was exacerbated by addition of peroxide. When the
dimerization reaction of 3i was performed excluding peroxide
30% mono-sulfoxide formation was observed. This sulfoxidation
could not be prevented by a radical scavenger such as butylated
hydroxytoluene (BHT).
Upon completion of the dimerization to afford the
functionalized pyrazines 4g-4k the crude reaction mixtures were
allowed to come to room temperature and left to stir overnight.
After testing for residual peroxide and concentration in vacuo, the
After we identified the optimal conditions for pyrazine
formation from DALA (9) these conditions were employed to
dimerize the hydrochloride salts of the amino acid ketones 3g-3k
Table 3. Conditions and yields for the formation of pyrazines 4g-4k.
#
Conditions[a]
Product
Isolated yield [mol%]
Overall yield[b]
(Lit. yield)[d] [mol%]
Appearance
3g
3h
3i
A
B
C
A
A
4g
4h
4i
87
52 (3)[15]
Colorless needles
Yellow wax
52
33 (87)[31]
66[c]
84
33 (n.a.)
Beige platelets
Colorless needles
Colorless powder
3j
4j
64 (n.a.)
3k
4k
92
63 (87)[17]
[a] A) 1 equiv. TEA, 2 equiv. H2O2 in MeOH from 0 °C to RT overnight; B) 10M NaOH in water; C) conditions A - H2O2. For work-up see
experimental. [b] Over three steps. [c] Corrected for oxidation. [d] Over one step
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resulting suspensions were diluted with water. The pH was
adjusted if necessary to ensure the side-groups were not present
as salts. Most of the resulting pyrazines are not soluble in water
at appropriate pH and could easily be isolated via filtration except
for lysine-based pyrazine 4h. Pyrazine 4h was therefore extracted
in portions with toluene from the lye solution in which it was
formed and subjected to vacuum distillation to obtain the product.
This vacuum distillation proved difficult due to thermal
degradation of the material. Scholl in his patents reports the use
of thin film distillation, hinting at this thermal instability, which is
not explicitly mentioned.[31] Scholl isolated the material as a yellow
oil, in our hands it solidified into a waxy low melting solid. The
other pyrazines (4g and 4i-k) could be further purified by
recrystallization in an appropriate solvent (see experimental) and
were obtained in good overall yield. The yield for the already
reported pyrazine 4g was significantly increased from 3%[15] over
3 steps to 52% following our procedures. For the other substrates,
the procedure for the pyrazine formation results in good to
excellent yields.
radical anions formed during the final dimerization/auto-oxidation
step reaction oxidize methanol, forming formaldehyde. This
formaldehyde causes side-reactions, in case of δ-amino-levulinic
acid methylation of the pyrazine occurs and in case of the other
salts (3g-k) most likely imidazole formation dominates. Addition
of hydrogen peroxide to the reaction mixture minimizes oxidation
of methanol. This enabled us to develop a scalable synthesis that
yielded the desired dimethylpyrazines in good yields. Thus, the
five successfully hydrolyzed amino-ketones were condensed into
their respective dimethyl pyrazines. The dimethylpyrazines 4g-k
are formed with our scalable and safe Dakin-West-based process
in three steps starting from renewable amino acids that can be
obtained via fermentation. Several of these functionalized
pyrazines have not been reported for more than 90 years, they
have been rediscovered and can be considered a toolbox for
application towards added functionality materials. We are
currently investigating these pyrazines as bio-based aromatic
building blocks for polyesters and polyamides or as additives for
such polymers.
All but one (4i) of these dimethylpyrazines are known
compounds (see Figure 2), however, no physical or
spectroscopic data of them have been reported before.
Experimental Section
General information
Acetic anhydride, hydrogen peroxide solution (35 wt%), formic acid,
dimethylaminopyridine, glutamic acid, lysine, tryptophan, sodium
bicarbonate, threonine, cysteine, arginine, glutamine and asparagine were
obtained at Sigma-Aldrich at >95% purity and used as received.
Triethylamine, sodium hydroxide and potassium hydroxide were obtained
from Fischer/Acros chemical at 99% purity and used as received. Glacial
acetic acid, methanol, acetone, isopropyl alcohol, acetonitrile, hydrochloric
acid, dichloromethane and chloroform were obtained from Biosolve as
analytical reagent grade or higher and used as received. Histidine,
methionine, tyrosine and serine were obtained from Bachem at >95%
purity and used as received. Δ-amino-levulinic acid hydrochloride (DALA)
was obtained from Ark-Pharm in 95% purity and used as received. Fumed
silica microcolloidal powder was obtained from Plasmachem and had an
average particle size of 7-14 nm. Deuterated solvents were obtained from
Cambridge Isotope Labs.
Preperative chromatography was performed on
a Grace Davison
Reveleris X2 system using Büchi FlashPure EcoFlex silica columns.
Liquid Chromatography – Mass Spectrometry (LC-MS) analysis was
performed on a Shimadzu Nexera 2 UHPLC system equipped with a
Shimadzu LC-30AD pump, a SPD-M30A photodiode array detector and
LCMS-2020 single quadrupole detector. The system was run on MilliQ
water and LC-MS grade acetonitrile both modified with 0.1% formic acid.
A Waters XSelect CSH C18 column (3.0 mm x 75 mm with a particle size
of 3.5 μm) was used operating at 30 °C. The method was set up with a
gradient of 5% acetonitrile in water for 2 min, an increase to 95%
acetonitrile over 6 min, 1 min at 95% followed by flushing back to 5%
acetonitrile. Samples were prepared by dissolving the reaction mixture in
methanol at ± 1 mg/mL.
Figure 2. Toolbox of amino acid derived dimethylpyrazine building blocks.
Conclusions
1H-NMR (300 MHz) and 13C-NMR (75 MHz) analysis were recorded on a
Bruker AVANCE 300 MHz apparatus at room temperature. 1H-NMR
experiments were carried out with 16 scans, using approximately 5 mg of
material in 0.5 mL deuterated solvent. Both CDCl3 and DMSO-d6 were
used as solvents. 13C-NMR attached proton test (APT) experiments were
carried out with 1024 scans, using approximately 50 mg of material. The
chemical shifts are reported in ppm referenced against either the residual
solvent peak or TMS standard (0 ppm).
A comprehensive set of functionalized proteinogenic amino acids
proved good substrates in the Dakin-West reaction for the
synthesis of α-amino-ketones. A safe and scalable method was
developed for the transformation of six of these amino acids (1g-l),
to their corresponding α-amino-ketones (2g-l). Five of these could
be successfully hydrolyzed to the corresponding hydrochloride
salts (3g-k), which were dimerized to form the desired pyrazine
products 4g-k (see Figure 2). We found, based on a model study
with δ-amino-levulinic acid as the substrate, that the superoxide
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FTIR analysis spectra were recorded on a Shimadzu IRAffinity-1 system
fitted with a MIRacle 10 ATR at room temperature. Background corrected
measurements were conducted at 2 cm-1 resolution from 4000 to 600 cm-1
using 32 scans.
Melting points were recorded in triplicate on a Mettler-Toledo MP90
melting point system using open capillaries and are uncorrected.
Elemental analyses were recorded on an Elementar varioEL at the Institut
für Organische Chemie of the RWTH Aachen, Germany.
Work-up procedure for acetamido-ketone 2h
The crude of the reaction with lysine (9.99 g, 68.4 mmol) was quenched
by addition of 5 mL of water and concentrated in vacuo (10 mBar, 85 °C)
followed by trituration by stirring in EtOAc (120 mL). The resulting solid
was isolated by filtration, washed with EtOAc (3x 40 mL) and dried in vacuo
(40 °C). The desired acetamido-ketone 2h (12.06 g, 52.8 mmol, 77%) was
isolated as an off-white solid.
N-N’-diacetyl-3,7-diaminoheptan-2-one (2h)
General procedure for qualitative screening on amino acids (1a-1m)
(1 mmol scale)
mp 112-114 °C (lit. 111-113 °C)[31]; 1H NMR (300 MHz, CDCl3) δ (ppm) =
6.56 (d, J=7.2 Hz, 1H), 5.94 (s, 1H), 4.58 (td, J=7.8, 4.0 Hz, 1H), 3.35 –
3.11 (m, J=6.9 Hz, 2H), 2.22 (s, 3H), 2.04 (s, 3H), 1.98 (s, 3H), 1.94 – 1.80
(m, 1H), 1.68 – 1.44 (m, 3H), 1.44 – 1.22 (m, 2H); 13C NMR (75 MHz,
CDCl3) δ (ppm) = 206.9, 170.5, 170.3, 58.4, 38.7, 30.6, 28.9, 27.1, 23.2,
A 25 mL 2-necked flask fitted with a cooler and bubble-counter was loaded
with triethylamine (0.49 mL, 3.5 mmol), acetic anhydride (0.32 mL, 3.3
mmol), acetic acid (29 µL, 0.5 mmol) and dimethylaminopyridine (1.2 mg,
0.01 mmol). The resulting reaction mixture was heated to 65 °C. Upon
reaching the desired temperature, the amino acid (1 mmol) was added as
a solid and the addition neck stoppered. The reaction was sampled several
times in intervals of 1 hour to check progress and left to stir overnight
(±17h). After concentration in vacuo (10 mBar, 85 °C) the crude reaction
mixture was analyzed by LC-MS. Amino acids that showed selective
reactions were selected for scale-up.
23.1, 22.0; IR (neat) ͂ = 3296 (s), 3069 (w), 2950 (w), 1707 (vs), 1636 (vs),
1541 (vs), 1370 (s), 1289 (s), 701 (s), 610 (s) cm–1; elemental analysis
calcd (%) for C11H20N2O3: C 57.87, H 8.83, N 12.27; found: C 57.61, H
8.69, N 12.03
Work-up procedure for acetamido-ketone 2i
(adapted from Wende et al.)[39]
The crude of the reaction with methionine (9.99 g, 67.0 mmol) was
quenched by addition of 5 mL of water and concentrated in vacuo (10 mBar,
85 °C). The crude was diluted with satd. NaHCO3 (300 mL) and extracted
using CHCl3 (4x 100 mL), the combined organic layers were washed with
1M HCl (3x 100 mL) and brine (1x 100 mL). The organic phase was dried
over Na2SO4, concentrated and purified by chromatography on silica using
100% EtOAc as eluent (Rf=0.25). The desired acetamido-ketone 2i (9.26
g, 48.9 mmol, 73%) was isolated as a yellow oil.
Procedure Dakin-West reaction for acetamido-ketone 2g
A 3-necked flask fitted with a reflux condenser, bubble-counter and
thermometer was loaded with triethylamine (34 mL, 245 mmol 3.6 equiv.),
acetic anhydride (34 mL, 360 mmol, 5.3 equiv.), acetic acid (2 mL, 34 mmol,
0.5 equiv.) and dimethylaminopyridine (83 mg, 0.01 mol%). The resulting
reaction mixture was heated to 65 °C. Upon reaching the desired
temperature, glutamic acid (10 g, 68 mmol) was added step-wise. The
reaction was monitored by gas formation and amino acid was added
whenever gas formation was slow. Upon complete addition, the reaction
left to stir overnight (±19h). The crude reaction mixture was quenched by
the addition of water and concentrated in vacuo (10 mBar, 85 °C) followed
by short path vacuum distillation. The first fraction, a clear oil, consisted of
triethylammonium acetate (pass-over 30-60 °C at 0.5 mBar). The second
fraction consisting of the desired ketone 2g was collected as a yellow oil
(pass-over 100-120 °C at 0.3 mbar), which solidified in the setup and on
standing. The desired acetamido ketone 2g was isolated as a low melting
waxy solid still containing some triethylammonium acetate (8.85 g,
corrected: 50.9 mmol, 74%). Ten-fold scale-up of this reaction was easily
achieved in an automated setup using a Lambda solid doser by adding
microcolloidal silica as a flowing agent. At this scale the material was
isolated in 79.5 g yield (corrected: 459 mmol, 67%). The material can be
recrystallized from hexane to yield yellow to colorless needles that cluster
into a solid.
3-acetamido-5-(methylthio)pentan-2-one (2i)
1H NMR (300 MHz, CDCl3) δ (ppm) = 6.34 (d, J=7.3 Hz, 1H), 4.76 (td,
J=7.3, 4.4 Hz, 1H), 2.62 – 2.39 (m, 2H), 2.27 (s, 3H), 2.34 – 2.17 (m, 1H),
2.12 (s, 3H), 2.05 (s, 3H), 1.96 – 1.78 (m, 1H); 13C NMR (75 MHz, CDCl3)
δ (ppm) = 206.4, 170.2, 58.1, 30.6, 30.0, 27.2, 23.0, 15.6; IR (neat) ͂ =
3286 (br), 2917 (m), 1718 (s), 1650 (vs), 1535 (s), 1432 (m), 1372 (s), 1289
(m), 1043 (m), 604 (m) cm–1; elemental analysis calcd (%) for C8H15NO2S:
C 50.76, H 7.99, N 7.40; found: C 49.78, H 8.13, N 7.58
Work-up procedure for acetamido-ketone 2j
The crude of the reaction with histidine (10.01 g, 64.5 mmol) was
quenched by addition of 5 mL of water followed by concentration in vacuo
(10 mBar, 85 °C). The resulting crude was continued to dihydrochloride
salt 3j without further purification.
N-acetyl-5-acetylpyrrolidin-2-one (2g)
Work-up procedure for acetamido-ketone 2k
m.p. 71-72°C (lit. 68-69)[28]; 1H NMR (300 MHz, CDCl3) δ (ppm) = 4.87 (dd,
J=9.7, 2.9 Hz, 1H), 2.80 – 2.54 (m, 2H), 2.52 (s, 3H), 2.29 (s, 3H), 2.39 –
2.14 (m, 1H), 2.01 – 1.84 (m, 1H); 13C NMR (75 MHz, CDCl3) δ (ppm) =
(adapted from Wende et al.)[39]
The crude of the reaction with tyrosine (11.75 g, 64.8 mmol) was quenched
by addition of 5 mL of water and concentrated in vacuo (10 mBar, 85 °C).
The material was subsequently triturated by stirring in Et2O (30 mL). The
resulting solid was isolated by filtration, washed with Et2O (3x 10 mL) and
dried in vacuo (40 °C). The desired acetamido ketone 2k (14.47 g, 54.9
mmol, 84%) was isolated as an off-white solid.
204.4, 174.5, 171.0, 63.1, 31.7, 26.9, 24.5, 19.8; IR (neat) ͂ = 2970 (w),
1734 (vs), 1714 (s), 1691 (vs), 1367 (vs), 1281 (vs), 1174 (vs), 1030 (m),
964 (m), 854 (m) cm–1; elemental analysis calcd (%) for C8H11NO3: C 56.80,
H 6.55, N 8.28; found: C 56.50, H 6.31, N 8.13.
3-acetamido-4-(4-acetoxyphenyl)butan-2-one (2k)
General procedure Dakin-West reaction, formation of ketones 2h-2l
A 3-necked flask fitted with a reflux condenser, bubble-counter and
thermometer was loaded with triethylamine (3.5 equiv.), acetic anhydride
(3.3 equiv.), acetic acid (0.5 equiv.) and dimethylaminopyridine (0.01
mol%). The resulting reaction mixture was heated to 65 °C. Upon reaching
the desired temperature, 10 grams of the amino acid (between 49-68
mmol) was added step-wise. The reaction was monitored by gas formation
and amino acid was added whenever gas formation was slow. Upon
complete addition, the reaction left to stir until gas formation had seized
completely or overnight (between 5 and 19 hours). Each substrate required
the following work-up.
129-130 °C (lit. 123-123.5 °C)[39]; 1H NMR (300 MHz, CDCl3) δ (ppm) =
7.19 – 7.10 (m, 2H), 7.09 – 7.00 (m, 2H), 6.09 (d, J=7.2 Hz, 1H), 4.88 (q,
J=6.8 Hz, 1H), 3.23 – 3.00 (m, 2H), 2.31 (s, 3H), 2.19 (s, 3H), 2.01 (s, 3H);
13C NMR (75 MHz, CDCl3) δ (ppm) = 206.1, 169.7, 169.4, 149.8, 133.4,
130.2, 121.8, 59.4, 36.4, 28.1, 23.1, 21.1; IR (neat) ͂ = 3373 (s), 1746 (s),
1708 (s), 1668 (vs), 1525 (s), 1373 (m), 1278 (m), 1190 (vs), 1013 (m),
911 (s) cm–1; elemental analysis calcd (%) for C14H17NO4: C 63.87, H 6.51,
N 5.32; found: C 63.61, H 6.32, N 5.22
Work-up procedure for acetamido-ketone 2l
(adapted from Engel et al.)[44]
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The crude of the reaction with tryptophan (10.33 g, 50.5 mmol) was
quenched by addition of 30 mL of methanol while cooling on ice. The
quenched crude was poured into 650 mL of ice water followed by addition
of potassium hydroxide (24 g, 0.43 mol) and left to stir overnight. The
resulting brown suspension was filtered, washed with water and dried in
vacuo (40 °C). The desired acetamido ketone 2l (9.67 g, 39.6 mmol, 78 %)
was isolated as a brownish solid. The material could be recrystallized from
methanol/water to give light brown needles.
and washed with acetone (2x 5 mL) resulting in hydrochloride salt 3i (5.41
g, 29.4 mmol, 69% yield) as an off-white solid.
3-amino-5-(methylthio)pentan-2-one hydrochloride (3i)
mp 141-143 °C (decomp.)(lit. 133-135 °C)[36] 1H NMR (300 MHz,
;
DMSO-d6) δ (ppm) = 8.53 (s, 3H), 4.16 (s, 1H), 2.70 – 2.53 (m, 2H), 2.27
(s, 3H), 2.24 – 2.11 (m, 1H), 2.08 (s, 3H), 2.11 – 1.94 (m, 1H); 13C NMR
(75 MHz, DMSO-d6) δ (ppm) = 204.4, 58.0, 29.0, 28.8, 27.3, 14.8; IR (ATR)
͂ = 2980 (br), 2879 (br), 1980 (w), 1718 (vs), 1473 (vs), 1362 (m), 1174
(s), 1146 (m), 1019 (m), 779 (w) cm-1; elemental analysis calcd (%) for
C6H14ClNOS: C 39.23, H 7.68, N 7.63; found: C 39.57, H 7.83, N 7.65
3-acetylamino-4-(3-indolyl)butan-2-one (2l)
mp 138-140 °C (lit. 134-135 °C)[44]; 1H NMR (300 MHz, CDCl3) δ (ppm) =
8.36 (s, 1H), 7.59 (d, J=7.8 Hz, 1H), 7.35 (d, J=7.9 Hz, 1H), 7.16 (dt, J=22.4,
7.3 Hz, 2H), 6.96 (d, J=2.4 Hz, 1H), 6.24 (d, J=7.3 Hz, 1H), 4.96 (q, J=6.4
Hz, 1H), 3.39 – 3.17 (m, 2H), 2.15 (s, 3H), 1.95 (s, 3H); 13C NMR (75 MHz,
CDCl3) δ (ppm) = 207.0, 170.0, 136.1, 127.6, 122.6, 122.3, 119.8, 118.6,
Dihydrochloride salt 3j
The crude of the Dakin-West reaction with histidine (1j) (10.01 g, 64.5
mmol) was hydrolyzed (6M HCl, 32 mL, 194 mmol). The resulting crude
was concentrated to dryness. The obtained brown solid was triturated in
IPA (60 mL), isolated by filtration and washed with IPA (2x 30 mL) resulting
in dihydrochloride salt 3j (11.16 g, 49.3 mmol, 76%) as a yellow solid.
4-(4-imidazolyl)-3-amino-butan-2-one dihydrochloride (3j)
mp 207-208 °C (decomp.)(recryst. MeOH/IPA)(lit. 212-215 °C)[21]; 1H NMR
(300 MHz, DMSO-d6) δ (ppm) = 14.74 (s, 2H), 9.09 (s, 1H), 8.57 (s, 3H),
7.50 (s, 1H), 4.56 (dd, J=8.7, 4.8 Hz, 1H), 3.47 (dd, J=15.5, 4.8 Hz, 1H),
3.17 (dd, J=15.5, 9.1 Hz, 1H), 2.35 (s, 3H); 13C NMR (75 MHz, DMSO-d6)
111.4, 110.0, 59.2, 28.0, 27.0, 23.2; IR (neat) ͂ = 3345 (s), 3246 (br), 1704
(vs), 1654 (vs), 1539 (vs), 1356 (s), 1221 (m), 1104 (m), 739 (vs), 657 (m)
cm–1; elemental analysis calcd (%) for C14H16N2O2: C 68.83, H 6.60, N
11.47; found: C 68.43, H 6.41, N 11.31
General procedure hydrolysis of ketones to synthesize 3g-3k
A round-bottom flask was filled with the respective acetamido ketone and
taken up in 3 equiv. of 6M hydrochloric acid. The resulting solution was
heated to reflux until complete hydrolysis was achieved according to
LC-MS. The crude was concentrated to dryness in vacuo and the obtained
solid triturated with either acetone (ketones 3g-I & 3k) or IPA (ketone 3j).
The resulting solids were isolated by filtration, washed on the filter with an
equal volume of either acetone (ketones 3h, 3i & 3k) or IPA (ketones 3g
& 3j) and subsequently dried in vacuo (40 °C). The obtained material was
pure enough for the next reaction step.
δ (ppm) = 203.4, 127.1, 118.7, 57.9, 27.6, 24.5; IR (ATR) ͂ = 3319 (br),
2859 (br), 2597 (br), 1723 (vs), 1521 (s), 1419 (s), 1068 (vs), 810 (vs), 719
(s), 624 (s) cm-1; elemental analysis calcd (%) for C7H13Cl2N3O: C 37.18,
H 5.80, N 18.59; found: C 37.20, H 5.88, N 18.79
Hydrochloride salt 3k
Acetamido ketone 2k (11.50 g, 43.7 mmol) was hydrolyzed (6M HCl, 22
mL, 132 mmol). The resulting crude was concentrated to dryness. The
obtained black solid was triturated in acetone (20 mL), isolated by filtration
and washed with acetone (4x 10 mL) resulting in hydrochloride salt 3k
(8.12 g, 37.7 mmol, 86% yield) as an off-white solid.
3-amino-4-(4-hydroxyphenyl)butan-2-one hydrochloride (3k)
mp 170-172 °C (decomp.)(lit. 165-166 °C)[17] 1H NMR (300 MHz,
;
DMSO-d6) δ (ppm) = 9.56 (s, 1H), 8.40 (s, 3H), 7.09 (d, J=8.3 Hz, 2H),
6.75 (d, J=8.2 Hz, 2H), 4.29 (d, J=7.9 Hz, 1H), 3.14 – 2.91 (m, 2H), 2.12
(s, 3H); 13C NMR (75 MHz, DMSO-d6) δ (ppm) = 204.7, 157.2, 130.9, 125.1,
Hydrochloride salt 3g
Acetamido ketone 2g (8.00 g, 47.3 mmol) was hydrolyzed (6M HCl, 24 mL,
144 mmol). The resulting crude was concentrated to dryness. The
obtained orange solid crushed, triturated in acetone (10 mL) isolated by
filtration and washed with IPA (3x 5 mL) resulting in hydrochloride salt 3g
(7.14 g, 39.3 mmol, 82% yield) as colorless cubic crystals.
4-amino-5-oxohexanoic acid hydrochloride (3g)
mp 166-168 °C (decomp.)(lit. 162 °C)[54]; 1H NMR (300 MHz, DMSO-d6) δ
(ppm) = 12.36 (s, 1H), 8.45 (s, 3H), 4.13 (t, J=6 Hz, 1H), 2.57 – 2.27 (m,
2H), 2.26 (s, 3H), 2.24 – 2.06 (m, 1H), 2.03 – 1.85 (m, 1H); 13C NMR (75
MHz, DMSO-d6) δ (ppm) = 204.6, 173.8, 58.0, 29.4, 27.3, 24.6; IR (ATR)
116.0, 59.9, 34.7, 28.4; IR (ATR) ͂ = 3145 (br), 2859 (br), 1725 (vs), 1614
(m), 1513 (vs), 1495 (m), 1362 (s), 1245 (vs), 836 (m), 687 (w) cm-1;
elemental analysis calcd (%) for C10H14ClNO2: C 55.69, H 6.54, N 6.49;
found: C 55.73, H 6.81, N 6.48
͂ = 2976 (br), 2479 (w), 1719 (s), 1606 (m), 1475 (m), 1409 (m), 1256 (m),
1172 (s), 1084 (s), 812 (m) cm-1; elemental analysis calcd (%) for
C6H12ClNO3: C 39.68, H 6.66, N 7.71; found: C 39.65, H 6.77, N 7.78
General condensation procedure of hydrochloride salts (DALA, 3g, 3i-k)
to form pyrazines 11, 4g, 4i-4k
Dihydrochloride salt 3h
Each hydrochloride salt was dissolved in methanol in a round-bottom flask
to form a 0.2 M solution. The solution was cooled on ice. Once reaching
0 °C, a solution of hydrogen peroxide (2 equiv.) was added followed by
slow addition of TEA (1 equiv.). The resulting solution was allowed to come
to room temperature and left to stir overnight followed by further work-up.
Acetamido ketone 2h (12.00 g, 52.6 mmol) was hydrolyzed (6M HCl, 27
mL, 162 mmol). The resulting crude was concentrated to dryness. The
obtained brown solid was triturated in acetone (10 mL), isolated by filtration
and washed with acetone (3x 10 mL) resulting in dihydrochloride salt 3h
(9.59 g, 44.2 mmol, 84% yield) as a grey solid.
3,7-diaminoheptan-2-one dihydrochloride (3h)
2,5-bis(2-carboxyethyl)-pyrazine (11)
mp 166-170 °C (decomp.)(lit. 160-166 °C)[55]
;
1H NMR (300 MHz,
Δ-aminolevulic acid hydrochloride (DALA)(5.03 g, 30.0 mmol) was
dissolved in methanol (150 mL) and cooled on ice. Upon reaching 0 °C, a
35wt% solution of hydrogen peroxide was added slowly (5.2 mL, 59.7
mmol) followed by slow addition of TEA (4.1 mL, 30.0 mmol). The formed
yellow solution was allowed to come to room temperature and left to stir
overnight (±18 h). The resulting suspension was concentrated in vacuo,
resulting in a yellow solid. The material was dissolved in 10 mL of water
and the pH adjusted to 4 using 1M HCl. A yellow solid precipitated which
was isolated by filtration, washed with 3x 10 mL water and dried in vacuo
(40 °C) overnight to yield the desired pyrazine (2.14 g, 9.56 mmol, 63%).
The material could be recrystallized from water to give yellow crystals.
DMSO-d6) δ (ppm) = 8.38 (s, 3H), 8.10 (s, 3H), 4.11 (s, 1H), 2.76 (s, 2H),
2.26 (s, 3H), 1.99 – 1.85 (m, 1H), 1.85 – 1.70 (m, 1H), 1.67 – 1.53 (m, 2H),
1.53 – 1.28 (m, 2H); 13C NMR (75 MHz, DMSO-d6) δ (ppm) = 204.8, 58.5,
38.5, 28.5, 27.4, 26.7, 21.4; IR (ATR) ͂ = 2869 (br), 1998 (br), 1714 (vs),
1586 (m), 1527 (s), 1370 (m), 1207 (m), 1148 (m), 949 (w), 739 (w) cm-1,
elemental analysis calcd (%) for C7H18Cl2N2O: C 38.72, H 8.36, N 12.90;
found: C 37.90, H 8.26, N 13.05
Hydrochloride salt 3i
Acetamido ketone 2i (8.00 g, 42.3 mmol) was hydrolyzed (6M HCl, 21 mL,
127 mmol). The resulting crude was concentrated to dryness. The
obtained brown solid was triturated in acetone (20 mL), isolated by filtration
mp 221-223 °C (decomp.)(lit. 219-221 °C)[48]
;
1H NMR (300 MHz,
DMSO-d6) δ (ppm) = 12.16 (s, 2H), 8.46 (s, 2H), 2.97 (t, J=7.2 Hz, 4H),
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2.68 (t, J=7.2 Hz, 4H); 13C NMR (75 MHz, DMSO-d6) δ (ppm) = 174.1,
Keto hydrochloride 3k (5.50 g, 25.50 mmol) was dissolved in methanol
(125 mL) and cooled on ice. Upon reaching 0 °C, a 35wt% solution of
hydrogen peroxide was added slowly (4.5 mL, 51.00 mmol) followed by
slow addition of TEA (3.5 mL, 25.50 mmol). The formed yellow solution
was allowed to come to room temperature and left to stir overnight (±18 h).
The material was isolated by filtration, washed with methanol (3x 20 mL)
and dried in vacuo (40 °C) to yield the desired pyrazine (3.76 g, 11.7 mmol,
92%) as a colorless solid.
153.4, 143.7, 32.6, 29.5; IR (ATR) ͂ = 2863 (br), 2498 (br), 1702 (vs), 1493
(m), 1382 (m), 1229 (s), 1189 (vs), 1060 (s), 894 (m), 820 (m) cm-1;
elemental analysis calcd (%) for C10H12N2O4: C 57.57, H 5.39, N 12.49;
found: C 53.37, H 5.24, N 12.48
2,5-bis(2-carboxyethyl)-3,6-dimethylpyrazine (4g)
Keto hydrochloride 3g (4.99 g, 27.5 mmol) was dissolved in methanol (135
mL) and cooled on ice. Upon reaching 0 °C, a 35wt% solution of hydrogen
peroxide was added slowly (4.8 mL, 55.5 mmol) followed by slow addition
of TEA (3.8 mL, 27.5 mmol). The formed yellow solution was allowed to
come to room temperature and left to stir overnight (±18 h). The resulting
suspension was concentrated in vacuo, resulting in a colorless solid. The
material was suspended in 10 mL of water. The precipitate was isolated
by filtration, washed with 3x 10 mL water and dried in vacuo (40 °C)
overnight to yield the desired pyrazine (3.03 g, 12.0 mmol, 87%). The
material could be recrystallized from water giving colorless needles or
methanol giving colorless stout prisms.
no mp (decomp.); 1H NMR (300 MHz, DMSO-d6) δ (ppm) = 9.22 (s, 2H),
6.96 (d, J=8.2 Hz, 4H), 6.72 – 6.58 (m, 4H), 3.96 (s, 4H), 2.38 (s, 6H); 13
C
NMR (75 MHz, DMSO-d6) δ (ppm) = 156.1, 151.3, 148.5, 129.9, 129.0,
= 3100 (br), 1612 (m), 1595 (m), 1517 (vs), 1417
115.7, 21.4; IR (ATR) 
͂
(s), 1237 (s), 1174 (s), 1124 (s), 993 (m), 808 (vs) cm-1; elemental analysis
calcd (%) for C20H20N2O2: C 74.98, H 6.29, N 8.74; found: C 74.09, H 6.12,
N 8.74
2,5-bis(4-aminobutyl)-3,6-dimethylpyrazine (4h)
(adapted from J. Scholl)[31]
mp 211-213 °C (decomp.)(lit. 211-213 °C)[15]
;
1H NMR (300 MHz,
Dihydrochloride salt 4h (8.08 g, 37.2 mmol) was dissolved in water (8 mL)
and cooled on ice. A 45% solution of NaOH (29 mL) was added slowly
resulting in the formation of a yellow to brown solution. The reaction was
allowed to warm to room temperature resulting in a biphasic system. After
stirring at room temperature for 2h the mixture was extracted with 5x 50
mL toluene and the organic phase dried over K2CO3. After removal of the
solvent in vacuo, the resulting yellow oil was subjected to vacuum
distillation in a Kugelrohr at 205 °C and 1 mBar. The desired material was
collected as a yellow oil boiling at 170-175 °C. If sufficiently pure, the oil
will crystalize into a yellow waxy solid (2.45 g, 9.78 mmol, 52%). Thermal
degradation takes place during distillation lowering yield and purity.
mp 54-55 °C; 1H NMR (300 MHz, DMSO-d6) δ (ppm) = 2.71 – 2.62 (t, J=6.9
Hz, 4H), 2.55 (t, J=6.9 Hz, 4H), 2.42 (s, 6H), 1.71 – 1.51 (m, 4H), 1.49 –
1.28 (m, 4H); 13C NMR (75 MHz, DMSO-d6) δ (ppm) = 151.6, 147.8, 42.0,
DMSO-d6) δ (ppm) = 12.09 (s, 2H), 2.91 (t, J=7.1 Hz, 4H), 2.66 (t, J=7.1
Hz, 4H), 2.43 (s, 6H); 13C NMR (75 MHz, DMSO-d6) δ (ppm) = 174.5, 149.9,
148.0, 31.6, 28.5, 21.2; IR (ATR) ͂ = 2901 (br), 2513 (br), 1928 (w), 1706
(s), 1419 (m), 1300 (m), 1189 (vs), 1128 (s), 1035 (m), 647 (m) cm-1;
elemental analysis calcd (%) for C12H16N2O4: C 57.13, H 6.39, N 11.10;
found: C 56.90, H 6.12, N 11.06
2,5-dimethyl-3,6-bis(2-(methylthio)ethyl)pyrazine (4i)
Keto hydrochloride 3i (4.50 g, 24.50 mmol) was dissolved in methanol (120
mL) and cooled on ice. Upon reaching 0 °C, TEA (3.4 mL, 24.5 mmol) was
added. The formed yellow solution was allowed to come to room
temperature and left to stir overnight (±18 h). The resulting suspension
was concentrated in vacuo, resulting in a brown solid. The material was
suspended in 50 mL EtOAc, isolated by filtration over a silica plug and
washed with 50 mL of EtOAc. The filtrate was concentrated and the
resulting brown solid was recrystallized twice from EtOAc, to yield the
desired pyrazine (1.46 g, 5.71 mmol, 66% corrected for 30% oxidation) as
beige platelets.
34.0, 33.8, 25.9, 21.2; IR (ATR) ͂ = 3353 (m), 3256 (br), 3180 (br), 2917
(vs), 2860 (s), 1610 (m), 1414 (vs), 1396 (s), 1027 (vs), 879 (s) cm-1;
elemental analysis calcd (%) for C14H26N4: C 67.16, H 10.47, N 22.38;
found: C 64.94, H 9.75, N 20.94
mp 87-88 °C; 1H NMR (300 MHz, DMSO-d6) δ (ppm) = 3.01 – 2.93 (m, 4H),
2.85 – 2.76 (m, 4H), 2.46 (s, 5H), 2.10 (s, 6H); 13C NMR (75 MHz, DMSO-
d6) δ (ppm) = 150.3, 148.4, 33.8, 32.2, 21.2, 15.3; IR (ATR) ͂ = 2920 (m),
Acknowledgements
2909 (m), 1416 (vs), 1390 (s), 1221 (m), 1104 (s), 981 (m), 961 (m), 711
(m), 683 (m) cm-1; elemental analysis calcd (%) for C12H20N2S2: C 56.20,
H 7.86, N 10.92; found: C 56.39, H 7.94, N 11.02
The authors would like to thank Prof. Dr. Andrij Pich for providing
feedback on the manuscript, Ayse Yalcin for conducting the
elemental analyses and the Dutch Province of Limburg for
providing funding.
2,5-bis(imidazol-5-yl)methyl)-3,6-dimethylpyrazine (4j)
Keto dihydrochloride 3j (5.65 g, 25.00 mmol) was dissolved in methanol
(125 mL) and cooled on ice. Upon reaching 0 °C, a 35wt% solution of
hydrogen peroxide was added slowly (4.4 mL, 50.0 mmol) followed by slow
addition of TEA (3.5 mL, 25.0 mmol). The resulting suspension was
concentrated in vacuo, resulting in an orange solid. The material was
suspended in 50 mL of 2M NaOH accompanied by a color change from
orange to colorless. The suspension was left overnight to release
triethylamine, the remainder of which was liberated in vacuo. The resulting
colorless solid was isolated by filtration and recrystallized from 250 mL of
boiling water. The crystals were isolated by filtration, washed with 3x 50
mL water and dried in vacuo (60 °C) overnight to yield the desired pyrazine
(2.81 g, 10.5 mmol, 84%) as very fine colorless needles.
no mp (decomp.); 1H NMR (300 MHz, DMSO-d6) δ = 7.31 (s, 2H), 6.54 (s,
2H), 3.93 (s, 4H), 2.43 (s, 6H); 13C NMR (75 MHz, DMSO-d6) δ (ppm) =
150.6, 148.4, 135.9, 135.2, 117.1, 33.7, 21.3; IR (ATR) ͂ = 2800 (br), 2619
(br), 1415 (m), 1232 (m), 1110 (m), 999 (vs), 839 (vs), 745 (vs), 696 (s),
626 (vs) cm-1; elemental analysis calcd (%) for C14H16N6: C 62.67, H 6.01,
N 31.32; found: C 62.51, H 5.91, N 31.86)
Keywords: Amino acids • Nitrogen heterocycles • Biobased
building blocks • Synthetic methods • Radical ions
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Entry for the Table of Contents (Please choose one layout)
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Martien A. Würdemann, Cristina Niţu,
Dr. Stefaan M.A. De Wildeman,
Dr. Katrien V. Bernaerts, Prof. Dr.
Romano V.A. Orru*
Page No. – Page No.
The Forgotten Pyrazines: Exploring
the Dakin-West Reaction.
Forgotten pyrazines: Pyrazines are an underreported class of aromatics available
from nitrogen-rich biomass. Functional proteinogenic amino acids were screened in
the Dakin-West reaction and a three-step procedure to synthesize dimethylpyrazines
was developed. The resulting five functional pyrazines can be considered a toolbox
for new metal-organic frameworks, polymers and materials.
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