Synthesis and pyrolysis of two flavor precursors of oct-1-en-3-yl methylpyrazinecarboxylates

Article abstract of DOI:10.1007/s10973-016-6083-5

To rich flavor additive species of pyrazines, two new compounds of 3,6-dimethyl-2,5-pyrazinedicarboxylic acid 1-octen-3-yl ester (DMPOE) and 3,5,6-trimethyl-2-pyrazinecarboxylic acid 1-octen-3-yl ester (TMPOE) were synthesized by KMnO₄ oxidatio

Full text of DOI:10.1007/s10973-016-6083-5

J Therm Anal Calorim (2017) 128:1627–1638
DOI 10.1007/s10973-016-6083-5
Synthesis and pyrolysis of two flavor precursors of oct-1-en-3-yl
methylpyrazinecarboxylates
Miao Lai1 Xiaoming Ji1 Tao Tao1 Yuanyuan Shan1 Pengfei Liu1
Mingqin Zhao1
•
•
•
•
•
Received: 25 August 2016 / Accepted: 19 December 2016 / Published online: 28 December 2016
´
´
Ó Akademiai Kiado, Budapest, Hungary 2016
Abstract To rich flavor additive species of pyrazines, two
new compounds of 3,6-dimethyl-2,5-pyrazinedicarboxylic
acid 1-octen-3-yl ester (DMPOE) and 3,5,6-trimethyl-2-
pyrazinecarboxylic acid 1-octen-3-yl ester (TMPOE) were
synthesized by KMnO4 oxidation, acylating chlorination
and esterification reaction, in which tetramethylpyrazine
and 1-octen-3-ol were used as initial materials. Thermo-
gravimetry (TG), differential scanning calorimeter (DSC)
and pyrolysis–gas chromatography/mass spectrometry
(Py–GC/MS) were conducted to investigate the thermal
degradation behaviors of DMPOE and TMPOE. TG–DTG
results indicated that the Tp of DMPOE and TMPOE with
the largest mass loss rate was at 310 and 250 °C, respec-
tively. The Tpeak of DMPOE and TMPOE showed by DSC
curves was 301.8 and 260.0 °C, respectively. Py–GC/MS
was performed to benefit the simulation of cigarette burn-
ing conditions, and the results indicated that DMPOE and
TMPOE could release specific flavors of 1-octen-3-ol and
diversified alkylpyrazines. Furthermore, the thermal
degradation mechanisms of the flavor precursors of
DMPOE and TMPOE were discussed. The study on the
thermal behavior of these two methylpyrazinecarboxylates
would provide theoretical basis for their application in
tobacco.
Keywords Pyrazines Á 1-Octen-3-ol Á TG Á DSC Á Py–GC/
MS Á Pyrolysis
Introduction
Pyrazines are heterocyclic aromatic organic compounds
with the underlying chemical formula C4H4N2, which are
commonly used in medicine, organic chemistry, food and
materials [1]. Pyrazines are considered to provide a rep-
resentative roasted flavor in foods and commonly exhibit
the sensory properties of toasted and nutty odor notes.
Because of their unique organoleptic properties, they have
recently been receiving increased attention in food-related
areas, and their concentrations detected in foods are in the
range from approximately 0.001 to 40 ppm. They have
been found in cocoa beans, grilled beef, potato, kohlrabi,
wheat bread, asparagus and liquors, which are toasted,
roasted or fermented in their preparation, or that involve an
extended heat treatment during the isolation procedure
[2–6]. These pyrazines are usually formed from a series of
complex chemical reactions, known as the Maillard reac-
tion or nonenzymatic browning reaction [7–11]. In the
early days, numerous pyrazines have been synthesized and
promoted as flavoring agents [12]. Pyrazine esters were
reported as aroma precursors in tobacco field. For instance,
methyl 2-pyrazinyl-2-propanyl oxalate could release
2-isopropenylpyrazine flavorant. 3,7-Dimethyl-2,6-octa-
Electronic supplementary material The online version of this
article (doi:10.1007/s10973-016-6083-5) contains supplementary
material, which is available to authorized users.
dien-1-yl
3,5,6-trimethyl-2-pyrazineacetate
produced
tetramethylpyrazine and the correspondent olefin by
cracking reaction during pyrolysis. As is well-known that
1-octen-3-ol being used for the preparation of the esters
spices is characterized by lavender smell and mushroom
fragrance [13]. Because of its inherent flavor, 1-octen-3-ol
serves as an additive to food stuffs, medicines and tobacco.
& Mingqin Zhao
zhaomingqin118@126.com
1
College of Tobacco Science, Henan Agricultural University,
Zhengzhou 450002, China
123

1628
M. Lai et al.
[14, 15]. However, the volatility and low odor threshold
limits its application in the high-temperature processing
[16]. Over recent years, increasing attention has been paid
to pyrazine ester precursors to achieve the purpose of flavor
stable release, especially for high-temperature processing
production. Therefore, in order to get the two characteristic
flavor of pyrazines and 1-octen-3-ol, using tetram-
ethylpyrazine and 1-octen-3-ol as raw materials, new flavor
precursors of 3,6-dimethyl-2,5-pyrazinedicarboxylic acid
1-octen-3-yl ester (DMPOE) and 3,5,6-trimethyl-2-
pyrazinecarboxylic acid 1-octen-3-yl ester (TMPOE) were
synthesized by chemical methods. Although the two pyr-
azine esters are nonvolatile and flavorless, pyrolysis of
them could release compounds that may contribute to fla-
vor either directly or by subsequent rearrangement.
Experimental
Materials
DMPOE and TMPOE were prepared by oxidation, acy-
lating chlorination, and esterification reaction (Scheme 1)
using tetramethylpyrazine (99%, Sigma-Aldrich) and
1-octen-3-ol (C98%, AR) as initial materials. 1-Octen-3-ol
was purchased from Henan Xinzheng Gold Leaf Spice Co.,
Ltd. (China). Other reagents were of analytical grade
quality and purchased from Tianjin Kermel Chemical
Reagent Co., Ltd (China) and, when necessary, were
purified and dried by standard methods.
Thin-layer chromatography (TLC) was performed on
silica gel GF254 (Merck), and spots were visualized by
irradiation with UV light (254 nm). Silica gel column
chromatography was carried out using a glass column
(3 cm i.d. 9 60 cm) filled with silica gel (0.03-0.06 mm,
Qingdao ocean chemistry factory, China).
Thermogravimetry (TG), differential scanning calorimeter
(DSC) and pyrolysis–gas chromatography/mass spectrometry
(Py–GC/MS) are useful tools for studying the pyrolysis
behavior of isolated tobacco additives and flavor precursors
[17–20]. The researches on the thermal behavior and
decomposition mechanism of compounds were reported a lot,
such as materials [21], polymers [22–24] and proteins [25].
The two new compounds of DMPOE and TMPOE were
rarely reported in the literature before, let alone the thermal
behavior of them. Generally, the study on pyrolytic behavior
of the sample is a first step in its total toxicological assess-
ment. The aim of this research is to present the preliminary
results obtained by applying TG, DSC and Py–GC/MS for the
pyrolysis behavior of alkylpyrazine esters (DMPOE and
TMPOE). This will provide more important basic theory for
the possibility of DMPOE and TMPOE being used as tobacco
flavor additives in burning processing.
Preparation of DMPOE
General procedures for the synthesis of DMPOE were as
follows. It was prepared according to our previous work
[29], but with some modifications in the last process. 3,6-
Dimethyl-2,5-pyrazinedicarbonyl chloride (compound 2)
was prepared from oxidation with KMnO4 at 100 °C and
from acylating chlorination with SOCl2, which was used
directly in the next step without any purification. Com-
pound 2 (4.2 mmol) was dissolved in 25 mL of ether. Then
0.897 g 1-octen-3-ol (7.0 mmol) and 1.0 mL (about
7.0 mmol) of dried triethylamine (Et3N) were added to the
solution of compound 2. The reaction mixture was stirred
overnight at room temperature. Progress of the reaction
was monitored by TLC with petroleum ether–ethyl acetate
(15:1, v:v). Upon completion of the reaction, the mixture
was washed in sequence with 3 9 25 mL of saturated
sodium bicarbonate, 3 9 25 mL of H2O, and 3 9 25 mL
of saturated sodium chloride. The organic layer was dried
(Na2SO4), filtered and concentrated. The residue was sep-
arated on silica gel column chromatography with petro-
leum ether–ethyl acetate (45:1, v:v) and recrystallized from
ethyl acetate to give compound 4 (DMPOE) as white solid
with purity of C98% (0.978 g, yield 67%). The yield was
calculated by the last step reaction.
Firstly, DMPOE and TMPOE were synthesized and they
were characterized by proton nuclear magnetic resonance
spectroscopy (1H NMR, 13C NMR), infrared spectroscopy
(IR) and high-resolution mass spectroscopy (HRMS). Then
the thermal changes of DMPOE and TMPOE in pyrolysis
process were investigated by TG and DSC. According to
the studies of Stotesbury et al. [26] and Baker et al. [27, 28]
on the pyrolytic behavior of tobacco ingredients, we know
that the temperature and pyrolysis atmosphere are the key
controllable variables in experiments to mimic smoking.
Inside the cigarette burning zone, there are about 9%
oxygen concentration distributing at 300–700 °C and
almost no oxygen at above 700 °C. Therefore, pyrolysis
conditions were chosen to simulate the cigarette burning
process, which were set at 300, 450 and 600 °C in atmo-
sphere of nitrogen and 9% oxygen in nitrogen, and at 750
and 900 °C in nitrogen only. Py–GC/MS was extensively
carried out for the qualitative and quantitative analysis of
pyrolysis products. The thermal stability, pyrolysis prod-
ucts and degradation mechanism of DMPOE and TMPOE
were discussed in this paper.
Preparation of TMPOE
TMPOE was prepared with some modifications according
to the literature [30]. 3,5,6-Trimethylpyrazine-2-carboxylic
acid was prepared from oxidation with KMnO4 at 65 °C for
24 h and then underwent acylating chlorination with
123

Synthesis and pyrolysis of two flavor precursors of oct-1-en-3-yl methylpyrazinecarboxylates
1629
O
N
ClOC
N
OH
Et3N
O
(a)
(b)
(c)
O
COCl
N
2
N
N
N
4
O
N
N
1
(d)
(e)
(f)
N
OH
Et3N
O
O
COCl
N
3
5
Scheme 1 Synthesis of the flavor precursors of oct-1-en-3-yl methylpyrazinecarboxylates 4 (DMPOE) and 5 (TMPOE). (a) KMnO4, H2O,
100 °C; (b) H2SO4, pH 2–3; (c) SOCl2, reflux; (d) KMnO4, H2O, 65 °C; (e) HCl, pH 2–3; (f) C2Cl2O2, rt
C2Cl2O2 to give 3,5,6-trimethylpyrazine-2-carbonyl chlo-
ride (compound 3). Then 0.448 g 1-octen-3-ol (3.5 mmol)
and 0.5 mL (about 3.5 mmol) of dried triethylamine (Et3N)
were added to the solution of compound 3 (4.2 mmol) in
25 mL of dried CH2Cl2. Progress of the reaction was
monitored by TLC with petroleum ether–ethyl acetate (5:1,
v:v). The reaction mixture was stirred at room temperature
for 3 h. On completion of the reaction, the mixture was
also washed, dried (Na2SO4), filtered and concentrated by
the same operations as above. The residue was purified
twice by silica gel column chromatography with petroleum
ether–ethyl acetate (20:1, v:v) to afford compound 5
(TMPOE) as colorless liquid with purity of C98%
(0.537 g, yield 56%). The yield was calculated by the last
step reaction.
Germany). The mass of each sample loaded into the alu-
mina crucibles for each run was about 5 mg. The tests were
carried out under argon atmosphere with a flow rate of
60 mL min-1. Thermal degradation temperature ranged
from 30 to 900 °C at a constant heating rate of
10 °C min-1. The TG curve, DTG curve and DSC curve of
samples were simultaneously performed.
Py–GC/MS analysis
Py–GC/MS analysis used in this study has a Pyroprobe
5250T (CDS, Analytical Inc.,) connected directly to a GC/
MS (Agilent, 7890A/5975C). About 0.20 mg of each
sample was centered in a 25-mm quartz tube and pyrolyzed
isothermally at the designed temperatures for 10 s. The
pyrolysis temperatures were set up at 300, 450, 600, 750
and 900 °C, respectively. The temperature of the reactor
was initially set at 50 °C and heated nominally at the rate
of 10 °C ms-1. The pyrolysis atmospheres were both in
nitrogen (0% oxygen) and in the mixture gases of oxygen
(9%) and nitrogen (91%) at 300, 450, 600 °C, and in
nitrogen at 750 and 900 °C only. The final pyrolysis vapor
was directly transferred to the GC/MS and analyzed.
The chromatographic separation was performed using a
DB-5MS fused silica capillary column (60 m 9 250 lm
id 9 0.25 lm df, Agilent). The injector temperature was
kept at 300 °C. Initial oven temperature was set at 50 °C,
then heated to 80 °C at the rate of 6 °C min-1, followed by
a heating rate of 4 °C min-1 to 110 °C, held there for
Structure identification techniques
IR spectra were recorded with Thermo/Nicolet Nexus 470
FT-IR ESP spectrometer (USA). The samples were ana-
lyzed as KBr micropellets. 1H NMR and 13C NMR spectra
data were recorded by a BRUKER AVANCE III 400 MHz
spectrometer (Switzerland), and each sample was dissolved
in CDCl3 with tetramethylsilane (TMS) as the internal
reference standard. HRMS data were obtained using the
Waters Micromass Q-TOF Micro TM high resolution mass
spectrometer (USA). Purity values were detected by
Waters 2695 HPLC with 2996 diode array detector and
chromatographic column of Agilent ZORBAX SB-C18
(4.6 mm 9 250 mm, 5 lm).
2 min and finished at 250 °C with a rate of 5 °C min-1
.
Helium at a constant flow rate of 1 mL min-1 was used as
the carrier gas, and the split ratio was 100:1. The separated
compounds were analyzed by the mass spectrometer. The
EI ionization energy was 70 eV, and the transfer line
temperature was 300 °C. Ion source temperature was
230 °C, and quadrupole temperature was 150 °C. The mass
Thermal analysis
Thermal decomposition of DMPOE and TMPOE was
compared by TG-DTG and DSC analysis using a simul-
taneous thermal analyzer (NETZSCH STA 449 F3,
123

1630
M. Lai et al.
spectra were obtained from m/z 30 to 500, and solvent
delay time was 3.9 min. The pyrolysis products were
identified by comparison between the experimental mass
spectrum and mass spectrum library (NIST 11) attached to
the Py–GC/MS apparatus. All quantitative data were
expressed by average values of the duplicate pyrolysis
runs. For each product, its peak area% value obtained
under different pyrolysis conditions can be compared to
reveal the changing of its relative content among the
detected products.
samples and the peak temperature of DSC curves were
recorded by apparatus. It presented that the Tpeak of
DMPOE and TMPOE in DSC curves was 301.8 and
260.0 °C, respectively. The onset temperature of thermal
decomposition was measured to help us determining the
thermal stability of samples from another point. In Fig. 3,
melting point of DMPOE measured by DSC and TG was
87.6 °C. Tonset, Tend and DH of the samples in DSC curves
could be detected by apparatus. In main mass loss regions
of samples, DSC analysis of DMPOE and TMPOE both
showed the endothermic decomposition. In general, the
results illustrated in TG–DTG and DSC curves were cor-
responding to each other. The data concerning the
decomposition temperatures are summarized in Table 2.
Results and discussion
Structure identification
Py–GC/MS analysis of DMPOE and TMPOE
samples
The synthetic route of DMPOE and TMPOE is shown in
Scheme 1. The chemical structures of the two samples
1
were new and identified by techniques of H NMR, 13C
Although TG and DSC seemed to be significant means to
study thermal properties of organic compounds, the
pyrolysis products were not identified. From results of the
TG and DSC analysis, the samples were pyrolyzed at
300 °C. These TG and DSC results were the verification
for the pyrolysis temperatures. Py–GC/MS was the useful
means to indicate the products distribution under different
temperatures. In order to further understand the influence
of chemical structure and pyrolysis conditions on the
samples decomposition process, Py–GC/MS experiments
were conducted based on the composition detection of
pyrolysis products from DMPOE and TMPOE.
NMR, IR and HRMS (see Table 1). These spectra dia-
grams were included in electronic supplementary material.
Thermal analysis of DMPOE and TMPOE samples
The TG curves and DTG curves of DMPOE and TMPOE
during degradation at the heating rate of 10 °C min-1 are
presented in Figs. 1 and 2, respectively. In Fig. 1, it was
observed that the main stage was between 170 and 420 °C,
and there was a sharp decrease in mass loss of 95.85%. The
maximum decomposition rate of DMPOE was achieved at
Tp of 310 °C with 60.70% mass loss from the beginning.
From 420 to 480 °C, the sample exhibited a slow mass loss
process with mass loss of 2.17%. Finally, DMPOE sample
remained a solid residue and equaled to 1.34% of its
original mass.
The research on the pyrolysis products was important
for the application of DMPOE and TMPOE in tobacco. The
pyrolysis conditions were in atmosphere of nitrogen and
9% oxygen in nitrogen at 300, 450 and 600 °C, and just in
nitrogen at 750 and 900 °C.
Figure 2 shows that the mass of TMPOE sample
decreased with the rise of temperature. It revealed that the
main degradation stage was lost about 98.37% of the total
mass between 110 and 270 °C. In this stage, mass loss rate
was largest appeared at Tp of 250 °C and mass loss was
about 81.08% from the beginning. The last stage was high
temperature charring of the residue, and the mass loss was
about 0.64% of original mass. Finally, the mass of TMPOE
remained 0.10% of its original mass.
Figure 5 shows Py–GC/MS detection of products
evolved from pyrolysis of DMPOE and TMPOE under the
pyrolysis conditions. Tables 3 and 4 list the main compo-
nents released from the two samples. As shown in Table 3,
the main compound types evolved out from the pyrolysis of
DMPOE were diverse and the relative contents also
showed great differences. As far as the pyrolysis of
TMPOE was concerned, shown in Table 4, no obvious
difference was observed for the pyrolysis product type in
atmosphere of N2 and 9% O2 in nitrogen at 300, 450 and
600 °C, whereas more pyrolysis compound types were
detected at 750 and 900 °C. By analysis on the volatile
pyrolysis compounds of DMPOE and TMPOE, it could be
found that 1-octen-3-ol and 2,5-dimethyl-3-propylpyrazine
were the common products. 1,3-Octadiene, 2,4-octadiene
and (E)-2-octen-1-ol were the conversion products of
1-octen-3-ol, the relative yields of them varied greatly. 1,3-
Octadiene and 2,4-octadiene were isomer formed from the
TG analysis indicated that the DMPOE and TMPOE
sample had good stability at room temperature, and the
breaking temperature of DMPOE was higher than that of
TMPOE. These phenomena might be attributed to the
difference on physical form and chemical structure of
DMPOE and TMPOE, which resulted in different pyrolysis
reactions.
The DSC curves of DMPOE and TMPOE are shown in
Figs. 3 and 4, respectively. The enthalpy change of the
123

Synthesis and pyrolysis of two flavor precursors of oct-1-en-3-yl methylpyrazinecarboxylates
1631
Table 1 Characterization of the two synthesized compounds
Compounds
1H NMR d/ppm
13C NMR d/ppm
IR/cm-1
HRMS
13.96(C-80, C-800),
22.21(C-9, C-10),
22.49(C-70, C-700),
24.75(C-50, C-500),
31.48(C-60, C-600, C-40,
C-400), 76.70(C-10,
C-100), 118.07(C-30,
C-300), 135.78(C-20,
C-200), 144.49(C-5, C-2),
150.25(C-3, C-6),
164.73(C-7, C-8)
2935(vC–H),
2872(vC–H),
1720(vO=C),
1262(vC–O–C),
1127(vC–O–C),
987(d=C–H),
[M ? H]?
417.2740
(calcd.
417.2753),
[M ? Na]?
439.2578
(calcd.
3
0.89(t, J = 6.88 Hz, 6H,
3H-80, 3H-800),
1.26–1.37(m, 8H, 2H-
500, 2H-600, 2H-60, 2H-
50), 1.38–1.47(m, 4H,
2H-70, 2H-700),
1.69–1.78(m, 2H, H-40,
H-400), 1.80–1.89(m, 2H,
H-40, H-400), 2.75(s, 6H,
3H-9, 3H-10), 5.27(d,
J = 10.44 Hz, 2H, H-30,
H-300), 5.40(d,
2
1
O
7
5
1
O
7
9
N
2
6
8
6
5
4
4
6
8
8
O
2
10
3
N
1
4
5
O
7
940(d=C–H)
3
439.2573)
DMPOE
J = 17.24 Hz, 2H, H-30,
H-300), 5.54(q,
J = 6.72 Hz, 2H, H-10,
H-100), 5.87–5.95(m, 2H,
H-20, H-200)
0.89(t, J = 6.96 Hz, 3H,
3H-80), 1.26–1.37(m,
4H, 2H-50, 2H-60),
1.38–1.46(m, 2H, 2H-
70), 1.69–1.78(m, 1H,
H-40), 1.80–1.88(m, 1H,
H-40), 2.55(s, 6H, 3H-8,
3H-10), 2.71(s, 3H, 3H-
9), 5.25(dt, J = 10.48,
1.0 Hz, 1H, H-30),
13.95(C-80), 21.57(C-8,
C-10), 22.10(C-9),
22.47(C-70), 24.77(C-
50), 31.50(C-60),
34.09(C-40), 76.73(C-
10), 117.67(C-30),
136.12(C-20), 140.36(C-
2), 149.36(C-6),
150.39(C-5), 153.96(C-
3), 165.55(C-7)
2932(vC–H),
[M ? H]?
277.1934
(calcd.
277.1916),
[M ? Na]?
299.1732
(calcd.
4
8
N
9
2861(vC–H),
1722(vO=C),
1299(vC–O–C),
1174(vC–O–C),
987(d=C–H),
931(d=C–H).
3
5
4′
6′
7
O
2
6
8′
N
1
10
1′
O
5′
7′
2′
3′
299.1735)
TMPOE
5.39(dt, J = 17.24,
1.16 Hz, 1H, H-30),
5.53(q, J = 6.76 Hz,
1H, H-10), 5.88–5.96(m,
1H, H-20)
2
2
100
80
60
40
20
0
100
0
0
–2
–2
80
–4
–4
–6
60
40
20
0
–8
–6
DMPOE
TG
TMPOE
TG
–10
–12
–14
–16
–18
–20
–8
DTG
–10
–12
–14
DTG
310 °C
250 °C
0
100 200 300 400 500 600 700 800 900
Temperature/°C
0
100 200 300 400 500 600 700 800 900
Temperature/°C
Fig. 1 TG and DTG curves of DMPOE
Fig. 2 TG and DTG curves of TMPOE
elimination reaction of 1-octen-3-ol, while 2,4-octadiene
could be generated from 450 to 900 °C with little amount.
The result of rearrangement of 1-octen-3-ol under high
heating temperature gave rise to formation of (E)-2-octen-
1-ol, which could continuously underwent the loss of water
to generate 1,3-octadiene. The other main alkylpyrazines
resulting from DMPOE pyrolysis in Table 3 are 2,5-
dimethylpyrazine, 2-ethyl-5-methylpyrazine, 2-isobutyl-3-
methylpyrazine and 2-methyl-5-propylpyrazine. As shown
in Table 4, the other main alkylpyrazines resulting from
123

1632
M. Lai et al.
2
0
3272. These alkylpyrazines were thermal degradation
compounds or rearrangement compounds during high-
temperature conditions.
87.6 °C
Exo
301.8 °C
Table 3 shows that 1,3-octadiene, 2,5-dimethylpyrazine,
1-octen-3-ol and 2-isobutyl-3-methylpyrazine were gener-
ated by DMPOE degradation under every pyrolysis con-
dition. 1,3-Octadiene, 2,4-octadiene and (E)-2-octen-1-ol
were detected in the volatiles, among which 1,3-octadiene
accounted for a large proportion. In Table 3, the relative
amount of the characteristic flavor compound of 2,5-
dimethylpyrazine was increased with the rising of tem-
perature. The maximum amount of 1-octen-3-ol was
12.30% at 600 °C in N2 atmosphere. 2-Isobutyl-3-
methylpyrazine existed in coffee [12], amount of which
achieved maximum amount of 20.35% at 750 °C. Flavor
compound of 2,5-dimethyl-3-propylpyrazine was produced
only at 300 °C with relative content of 44.61% and 57.35%
in two atmosphere and might transform into 2-methyl-5-
propyl pyrazine at the higher temperature.
–2
–4
–6
DMPOE
DSC
–8
–10
–12
0
100 200 300
400 500 600 700 800 900
Temperature/°C
Fig. 3 DSC curve of DMPOE
0
Exo
–1
From Table 4, it was observed that 1,3-octadiene, 2,3,5-
trimethylpyrazine, 2,5-dimethyl-3-propylpyrazine and 2,5-
dimethyl-3-pentylpyrazine were produced from TMPOE
under every pyrolysis condition. Although 1,3-octadiene,
2,4-octadiene and (E)-2-octen-1-ol were also detected,
their relative contents were comparatively lower than that
of DMPOE pyrolysis. This was due to the proportion of
1-octen-3-ol existed in their original structure of the sam-
ples. The amount of 2,3,5-trimethylpyrazine attained the
maximum amount of 23.03% at 750 °C. 2,5-Dimethyl-3-
propylpyrazine accounted for the most proportion com-
pared to other products on every pyrolysis condition;
nevertheless, the relative content of 2,5-dimethyl-3-pen-
tylpyrazine was lower than that of the former. It was
considered that 2,5-dimethyl-3-propylpyrazine was the
main pyrolysis product and formed easily in all pyrolysis
products.
260.0 °C
–2
–3
–4
TMPOE
DSC
0
100 200 300
400
500 600 700
800 900
Temperature/°C
Fig. 4 DSC curve of TMPOE
TMPOE pyrolysis are 2,6-dimethylpyrazine, 2,3,5-tri-
methylpyrazine, 3-ethyl-2,5-dimethylpyrazine, 2-ethyl-3,5-
dimethylpyrazine, 2,3-dimethyl-5-ethylpyrazine and 2,5-
dimethyl-3-pentylpyrazine. Among all pyrolysis products
of the two samples, 1-octen-3-ol, 2,3,5-trimethylpyrazine,
It was observed that DMPOE and TMPOE could be
used as flavor precursors to release characteristic flavors
under similar cigarette combustion conditions. Meanwhile,
the Py–GC/MS analysis indicated that the sample chemical
structure had obvious significance on the type and relative
amount of pyrolysis products. Combining TG with DSC
analysis, it was the cleavage of ester bond of samples to
release the pyrolysis products and rearranged to produce
other useful flavor compounds.
2,5-dimethylpyrazine,
2-ethyl-3,5-dimethylpyrazine,
2-isobutyl-3-methylpyrazine, 2,3-dimethyl-5-ethylpyrazine
and 2,5-dimethyl-3-propylpyrazine were the characteristic
flavor compounds we expected. Some of them could be
detected in Cocoa products [12]. Meanwhile, Adams et al.
[31] reported that the FEMA number of 2,3,5-tri-
methylpyrazine was 3244 and 2,5-dimethylpyrazine was
Table 2 Thermal analysis data for DMPOE and TMPOE
Sample name
DSC
TG–DTG
Tonset/°C
Tpeak/°C
Tend/°C
DH/kJ mol-1
Tp/°C
Trange/°C
DMPOE
TMPOE
240.4
247.9
301.8
260.0
359.4
272.1
62.44
25.84
310
250
170–420
110–270
123

Synthesis and pyrolysis of two flavor precursors of oct-1-en-3-yl methylpyrazinecarboxylates
1633
Fig. 5 Typical ion
DMPOE
TMPOE
Abundance
Abundance
chromatograms from pyrolysis
of the DMPOE and TMPOE at
different temperatures and
atmospheres (peak identification
referred in Tables 3 and 4,
respectively). Pyrolysis
300 °C/N2
300 °C/N2
conditions were set at 300, 450
and 600 °C in atmosphere of N2
and mixed gases of 9% oxygen
and 91% nitrogen, as well as at
750 and 900 °C in N2 only
10.00 15.00 20.00 25.00 30.00 35.00 40.00 Time/min
Time/min
5.00
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Abundance
Abundance
300 °C/9% O2
300 °C/9% O2
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Time/min
10.00 15.00 20.00 25.00 30.00 35.00 40.00
Time/min
5.00
Abundance
Abundance
450 °C/N2
450 °C/N2
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Time/min
Time/min
Abundance
Abundance
450 °C/9% O2
450 °C/9% O2
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Time/min
Time/min
Abundance
Abundance
600 °C/N2
600 °C/N2
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Time/min
Time/min
Abundance
Abundance
600 °C/9% O2
600 °C/9% O2
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Time/min
Time/min
Abundance
Abundance
750 °C/N2
750 °C/N2
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Time/min
Time/min
Abundance
Abundance
900 °C/N2
900 °C/N2
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
10.00 15.00 20.00 25.00 30.00 35.00 40.00
5.00
Time/min
Time/min
123

1634
M. Lai et al.
Table 3 Pyrolysis products of DMPOE
300 °C
450 °C
600 °C
750 °C
N2
900 °C
N2
Peak
no.
RT
/min
Compounds
N2
9% O2
N2
9% O2
N2
9% O2
1
2
5.43
5.98
47.06
–
25.33
20.89
3.65
36.83
30.83
15.65
26.74
23.26
10.79
23.77
9.78
1,3-Octadiene
–
2.82
9.70
2,4-Octadiene
N
3
4
5
6
7
6.92
8.47
0.46
0.98
–
0.54
1.71
–
0.92
2.79
–
1.60
10.04
–
4.69
12.30
–
7.52
6.75
–
11.80
3.00
18.35
3.56
1.73
1.66
6.11
N
2,5-Dimethylpyrazine
HO
1-Octen-3-ol
N
9.05
0.59
N
2-Ethyl-5-methylpyrazine
HO
10.89
31.92
–
–
3.39
13.65
3.73
18.83
7.76
13.88
3.05
13.16
1.46
(E)-2-Octen-1-ol
N
2.26
2.90
20.35
N
2-Isobutyl-3-methylpyrazine
N
8
9
33.99
34.73
44.61
–
57.30
1.02
–
0.82
–
8.43
–
–
N
2,5-Dimethyl-3-propylpyrazine
N
44.81
13.87
5.34
16.06
17.66
22.96
N
2-Methyl-5-propylpyrazine
RT retention time, – compound was undetected
N2 means the pyrolysis atmosphere was only in nitrogren. 9% O2 means that the atmosphere was in the mixed gases of 9% oxygen and 91%
nitrogen
Pyrolysis mechanism analysis
pathways of the two samples were proposed [32–34]. The
pyrolysis products were generated ultimately resulting
from the interaction among pyrolysis temperature and
atmosphere. From Tables 3 and 4, the pyrolysis tempera-
tures were influential, while the influence of pyrolysis
atmospheres on the types and contents of pyrolysis
The study on the formation of characteristic pyrolysis
products was considerable for potential application of
DMPOE and TMPOE as flavor additives in tobacco. Under
the cigarette burning conditions, the possible pyrolysis
123

Synthesis and pyrolysis of two flavor precursors of oct-1-en-3-yl methylpyrazinecarboxylates
1635
Table 4 Pyrolysis products of TMPOE
300 °C
9% O2
450 °C
9% O2
600 °C
750 °C
N2
900 °C
Peak
no.
RT
/min
Compounds
N2
N2
N2
9% O2
N2
1
5.14
1.78
4.64
3.69
5.19
10.93
11.63
15.49
3.26
1,3-Octadiene
2
3
4
5.98
7.01
8.33
–
–
–
–
–
0.27
–
0.08
–
5.22
–
5.19
–
10.25
0.73
1.21
1.20
0.54
0.13
2,4-Octadiene
N
N
2,6-Dimethylpyrazine
HO
0.27
0.38
0.19
0.51
0.28
1-Octen-3-ol
N
5
6
7
8.94
10.95
11.27
0.57
–
2.36
–
2.90
–
2.05
–
11.51
–
12.72
–
23.03
0.33
10.98
0.20
2,3,5-TrimeNthylpyrazine
HO
(E)-2-Octen-1-ol
N
–
–
–
–
–
–
0.70
0.25
N
3-Ethyl-2,5-dimethylpyrazine
N
8
9
11.43
11.50
34.30
37.06
–
–
–
–
–
–
–
–
–
–
–
–
0.71
0.82
0.24
0.29
N
2-Ethyl-3,5-dimethylpyrazine
N
N
2,3-Dimethyl-5-ethylpyrazine
N
10
11
95.61
1.02
86.69
3.43
89.06
0.87
86.87
2.35
60.53
7.67
67.13
2.45
42.32
0.55
78.18
2.32
N
2,5-Dimethyl-3-propylpyrazine
N
N
2,5-Dimethyl-3-pentylpyrazine
RT retention time, – compound was undetected
N2 means the pyrolysis atmosphere was only in nitrogren. 9% O2 means that the atmosphere was in the mixed gases of 9% oxygen and 91%
nitrogen
123

1636
M. Lai et al.
products was not obvious. As shown in Scheme 2, the 1,3-
octadiene (peak no. 1) was formed from dehydration of
1-octen-3-ol (peak no. 4) in both atmosphere of N2 and 9%
O2 in nitrogen. High heating temperature was beneficial to
the formation of various radicals, which could promote the
formation of pyrolysis products.
2-methyl-5-propylpyrazine (peak no. 9) with the capture
of three H radicals and one electron. Meanwhile, allyl
carbocation was captured in ortho-position of 2-methyl-
pyrazine and then underwent addition reaction with CH3
and H radicals to afford 2-isobutyl-3-methylpyrazine
(peak no. 7) with the capture of an electron. With
breakage of the bond a and elimination of CO, 2,5-
dimethyl-3-propylpyrazine (peak no. 8) was produced
through the capture of allyl carbocation, three H radicals
and one electron.
From the pyrolysis products of DMPOE and TMPOE,
it was believed that the cleavage of ester bond of C-O
could be carried out in the decomposition process. As
shown in Scheme 2, for DMPOE and TMPOE, the
chemical bond a and c ruptured and then left CO to
produce the aroma compounds of 2,5-dimethylpyrazine
with peak no. 3 and 2,3,5-trimethylpyrazine with peak no.
5, respectively. At the moment, 1-octen-3-ol (peak no. 4)
was obtained and could easily undergo elimination reac-
tion to form 1,3-octadiene (peak no. 1). The bonds e and
f of 1-octen-3-ol broke simultaneously, and H2O molecule
was eliminated to generate allyl carbocation and pentyl
radical. For DMPOE, the chemical bond a and b broke
and then eliminated CO. After that, allyl carbocation was
captured in para-position of 2-methylpyrazine to produce
For TMPOE, the pyrolysis process was similar to that
of DMPOE. The bond c and d broke and then eliminated
CO. Afterward, 2,5-dimethyl-3-pentylpyrazine (peak
no. 11) was formed by capturing one pentyl radical.
Meanwhile, allyl carbocation was captured and then
three H radicals were added. These reactions resulted in
the generation of 2,5-dimethyl-3-propylpyrazine (peak
no. 10) with the capture of one electron. In addition to
the above pyrolysis procedures of the samples, some
radicals reacted or rearranged to produce other com-
pounds [35].
N
N
N
N
N
H
e
N
N
3
O
+
C
9
, –
a
O
C
–
,
b
,
a
N
N
CH3
e,f,–H2O
a
O
a
H
N
N
N
O
e
+
7
O
,
–
C
O
N
N
N
a
b
O
H
e
e,f,–H2O
N
8
DMPOE
+
1
HO
f
–H2O
e
4
e
N
N
N
N
CO
–
d,
c,
N
d
O
c
11
N
e,f, –H2O
O
N
N
H
c
,
d
,
–
CO
TMPOE
e
c
,
–
C
O
+
10
N
N
5
Scheme 2 Proposed pyrolysis pathways for DMPOE and TMPOE. The numbers represented the peak number of identified compounds
123

Synthesis and pyrolysis of two flavor precursors of oct-1-en-3-yl methylpyrazinecarboxylates
1637
6. Fan W, Xu Y, Zhang Y. Characterization of pyrazines in some
Chinese liquors and their approximate concentrations. J Agric
Food Chem. 2007;55:9956–62.
Conclusions
The flavor precursors of DMPOE and TMPOE were syn-
1
thesized, and their structures were confirmed by H NMR,
7. Lancker FV, Adams A, Kimpe ND. Impact of the N-terminal
amino acid on the formation of pyrazines from peptides in
maillard model systems. J Agric Food Chem. 2012;60:4697–708.
8. Yu AN, Tan ZW, Wang FS. Mechanistic studies on the formation
of pyrazines by Maillard reaction between L-ascorbic acid and L-
glutamic acid. LWT-Food Sci Technol. 2013;50:64–71.
9. Nicolau LP, Revel GD, Bertrand A, Maujean A. Formation of
flavor components by the reaction of amino acid and carbonyl
compounds in mild conditions. J Agric Food Chem. 2000;48:
3761–6.
10. Hwang HI, Hartman TG, Rosen RT, Chi TH. Formation of
pyrazines from the Maillard reaction of glucose and glutamine-
amide-15N. J Agric Food Chem. 1993;41:2112–5.
11. Adams A, Polizzi V, Boekel MV, Kimpe ND. Formation of
pyrazines and a novel pyrrole in Maillard model systems of 1,3-
dihydroxyacetone and 2-oxopropanal. J Agric Food Chem.
2008;56:2147–53.
12. Maga JA, Sizer CE, Myhre DV. Pyrazines in foods. Crit Rev
Food Sci. 1973;4:39–115.
13. Niebler J, Buettner A. Pyrolysis–GC–MS–olfactometry: a new
approach to identify thermally generated odorants in frankin-
cense. J Anal Appl Pyrolysis. 2015;113:690–700.
14. Takken W, Kline DL. Carbon dioxide and 1-octen-3-ol as mos-
quito attractants. J Am Mosq Control. 1989;5:311–6.
15. Leduc F, Tournayre P, Kondjoyan N, Mercier F, Malle P, Kol O,
13C NMR, IR and HRMS. The thermal behavior and
decomposition compounds of DMPOE and TMPOE were
discussed by TG, DSC and Py–GC/MS analysis. TG
analysis indicated that the DMPOE and TMPOE had good
stability at room temperature, whereas the breaking tem-
perature of DMPOE was higher than that of TMPOE. Tp of
DMPOE and TMPOE was 310 and 250 °C, respectively.
DSC curves demonstrated that Tpeak was 301.8 °C for
DMPAOE and 260.0 °C for TMPAOE with endothermic
decomposition. Py–GC/MS analysis showed that there
were some characteristic aroma compounds produced such
as 1-octen-3-ol, 2,3,5-trimethylpyrazine, 2,5-dimethylpyr-
azine, 2-isobutyl-3-methylpyrazine and 2,5-dimethyl-3-
propylpyrazine. The sample chemical structure had obvi-
ous significance on the type and relative amount of
pyrolysis products. The influence of pyrolysis atmospheres
was not obvious. The pyrolysis mechanisms of DMPOE
and TMPOE were proposed. Under pyrolysis condition, the
primary decomposition reaction was that DMPOE and
TMPOE broke down to regenerate the corresponding
characteristic flavor products.
´
Berdague JL, Duflos G. Evolution of volatile odorous compounds
during the storage of European seabass (Dicentrarchus labrax).
Food Chem. 2012;131:1304–11.
16. Song S, Zhang X, Hayat K, Liu P, Jia C, Xia S, Xiao Z, Tian H,
Niu Y. Formation of the beef flavour precursors and their cor-
relation with chemical parameters during the controlled thermal
oxidation of tallow. Food Chem. 2011;124:203–9.
Supporting information
17. Baker RR. A review of pyrolysis studies to unravel reaction steps
in burning tobacco. J Anal Appl Pyrolysis. 1987;11:555–73.
18. Xie WC, Gu XH, Tan ZC, Tang J, Wang GY, Luo CR, Sun LX.
Thermal decomposition of two synthetic glycosides by TG, DSC
and simultaneous Py–GC–MS analysis. J Therm Anal Calorim.
2006;87:505–10.
1
The H NMR, 13C NMR, IR and HRMS spectra diagrams
of oct-1-en-3-yl methylpyrazinecarboxylates are available
to authorized users.
19. Xie W, Tang J, Gu X, Luo C, Wang G. Thermal decomposition
study of menthyl-glycoside by TGA/SDTA, DSC and simulta-
neous Py–GC–MS analysis. J Anal Appl Pyrolysis. 2007;78:
180–4.
Acknowledgements This work was supported by Key Science and
Technology Program of Science and Technology Department of
Henan Province (Nos. 132102210042, 152102210058).
20. Zeng S, Sun S, Liu S, Hu J, He B. Synthesis of a-ionyl-b-D-
glucoside and its property of flavor release. J Therm Anal
Calorim. 2014;115:1049–56.
References
21. Zhu P, Sui S, Wang B, Sun K, Sun G. A study of pyrolysis and
pyrolysis products of flame-retardant cotton fabrics by DSC,
TGA, and PY–GC–MS. J Anal Appl Pyrolysis. 2004;71:645–55.
1. Pagoria PF, Lee GS, Mitchell AR, Schmidt RD. A review of
energetic materials synthesis. Thermochim Acta. 2002;384:
187–204.
¨
22. Werner K, Pommer L, Brostrom M. Thermal decomposition of
2. Hawksworth G, Scheline RR. Metabolism in the rat of some
pyrazine derivatives having flavour importance in foods. Xeno-
biotica. 1975;5:389–99.
3. Ramli N, Hassan O, Said M, Samsudin W, Idris NA. Influence of
roasting conditions on volatile flavor of roasted Malaysian cocoa
beans. J Food Process Preserv. 2006;30:280–98.
hemicelluloses. J Anal Appl Pyrolysis. 2014;110:130–7.
23. Bikiaris DN, Chrissafis K, Paraskevopoulos KM, Triantafyllidis KS,
Antonakou EV. Investigation of thermal degradation mechanism of
an aliphatic polyester using pyrolysis–gas chromatography–mass
spectrometry and a kinetic study of the effect of the amount of
polymerisation catalyst. Polym Degrad Stab. 2007;92:525–36.
24. Price D, Horrocks AR, Akalin M, Faroq AA. Influence of flame
retardants on the mechanism of pyrolysis of cotton (cellulose)
fabrics in air. J Anal Appl Pyrolysis. 1997;40:511–24.
25. Chen T, Oakley DM. Thermal analysis of proteins of pharma-
ceutical interest. Thermochim Acta. 1995;248:229–44.
26. Stotesbury S, Digard H, Willoughby L, Couch A. The pyrolysis
of tobacco additives as a means of predicting their behaviour in a
burning cigarette. Beitr Tab Int. 1999;18:147–63.
´
4. Ruan ED, Aalhus JL, Juarez M, Sabik H. Analysis of volatile and
flavor compounds in grilled lean beef by stir bar sorptive
extraction and thermal desorption–gas chromatography mass
spectrometry. Food Anal Methods. 2015;8:363–70.
5. Lee SE, Chung H, Kim YS. Effects of enzymatic modification of
wheat protein on the formation of pyrazines and other volatile
components in the Maillard reaction. Food Chem. 2012;
131:1248–54.
123

1638
M. Lai et al.
27. Baker RR, Bishop LJ. The pyrolysis of non-volatile tobacco
ingredients using a system that simulates cigarette combustion
conditions. J Anal Appl Pyrolysis. 2005;74:145–70.
28. Baker RR, Bishop LJ. The pyrolysis of tobacco ingredients.
J Anal Appl Pyrolysis. 2004;71:223–311.
32. Holmelid B, Kleinert M, Barth T. Reactivity and reaction path-
ways in thermochemical treatment of selected lignin-like model
compounds under hydrogen rich conditions. J Anal Appl Pyrol-
ysis. 2012;98:37–44.
33. Lai M, Zhao B, Ji X, Fu P, Wang P, Bao X, Zhao M. Thermal
behavior of two synthesized flavor precursors of pyrazine esters.
J Therm Anal Calorim. 2016;123:479–87.
29. Lai M, Zhao B, Bao X, Zhao M, Ji X, Fu P, Zhang Y. Pyrolysates
of novel latent fragrant compound 3,6-dimethyl-2,5-pyrazinedi-
carboxylic acid menthol ester. Chin
2015;33:46–51.
J
Chromatogr.
34. Paine JB, Pithawalla YB, Naworal JD. Carbohydrate pyrolysis
mechanisms from isotopic labeling: part 4. The pyrolysis of D-
30. Chen H, Li G, Zhan P, Li H, Wang S, Liu X. Design, synthesis
and biological evaluation of novel trimethylpyrazine-2-carbony-
loxy-cinnamic acids as potent cardiovascular agents. Med-
ChemComm. 2014;5:711–8.
31. Adams TB, Doull J, Feron VJ, Goodman JI, Marnet LJ, Munro
IC, Newberne PM, Portoghese PS, Smith RL, Waddell WJ,
Wagner BM. The FEMA GRAS assessment of pyrazine deriva-
tives used as flavor ingredients. Food Chem Toxicol.
2002;40:429–51.
glucose: the formation of furans.
2008;83:37–63.
35. Wang S, Liu B, Su Q. Pyrolysis–gas chromatography/mass spec-
trometry as a useful technique to evaluate the pyrolysis pathways of
phenylalanine. J Anal Appl Pyrolysis. 2004;71:393–403.
J Anal Appl Pyrolysis.
123