L. Ai, M. Liu, X. Ji, M. Lai, M. Zhao, and T. Ren
Vol 000
flavor precursor compounds, were synthesized. The
thermal behavior analysis of NBMF and NUMF was
discussed.
carboxylic acid methyl ester pyrrole (compound 3) were
achieved according to the formerly reported procedure
[35]. Compound 2 was synthesized by the cyclization
reaction, and compound 3 was obtained by the oxidation
reaction of compound 2 with an excess of NaIO4 [22].
The synthesis of pyrrole flavor precursors 4 and 5 by
using benzyl chloride and 1-bromobutan, respectively,
was all ended within 24 h.
Pyrolysis–gas chromatography/mass spectrometry (Py-
GC/MS) is an efficient way to identify compounds
evolved during pyrolysis of organic matter [23].
Thermogravimetry (TG) and differential scanning
calorimeter (DSC) are methods for analyzing thermal
changes in samples during pyrolysis. Py-GC/MS, TG, and
DSC have been widely used in thermal studies in many
fields [24–28], such as cigarette [29], wines [30], wood
[31], French cider [32], and crude oils [33]. The thermal
behavior and decomposition mechanism of the two
compounds NBMF and NUMF have not been reported in
former literature. The aim of this study is to investigate
the pyrolysis behavior of alkylpyrroles through TG, DSC,
and Py-GC/MS analysis. The initial material compound
2-methyl-5-formyl-3-pyrrolecarboxylic acid methyl ester
was synthesized by cyclization reaction, and NBMF and
NUMF were both obtained by oxidation and alkylation
reaction. Their structures were analyzed by proton
nuclear magnetic resonance spectroscopy (1H NMR and
13C NMR), infrared (IR) spectroscopy, and high-
resolution mass spectroscopy (HRMS). Then TG, DSC,
and Py-GC/MS were used to investigate the thermal
property of NBMF and NUMF. The purpose of studying
the pyrolysis behavior of NBMF and NUMF was to
initially assess the effect of its application to tobacco
additives. According to previous literature [34], the key
controllable variables in experiments to mimic smoking
are temperature and pyrolysis atmosphere; in order to
simulate the cigarette burning process, this experiment
sets the pyrolysis temperature range at 300–900°C. The
thermal stability, pyrolysis products, and degradation
mechanism of NBMF and NUMF were discussed in
this paper.
General procedure for the synthesis of methyl N-benzyl-2-
methyl-5-formylpyrrole-3-carboxylate.
Anhydrous
K2CO3 (0.621 g, 4.5 mmol), benzyl chloride (0.189 g,
1.5 mmol), and TBAB (0.023 g, 0.07 mmol) were added
into compound 3 (0.167 g, 1 mmol) in acetonitrile
(6 mL) in sequence, and then the mixed solution was
stirred for 24 h at 55°C. The reaction was monitored by
TLC with petroleum ether : ethyl acetate (5:1, v : v).
Then, the solution was evaporated to dryness, and the
residue was treated with ethyl acetate (4 × 10 mL) and
water (4 × 10 mL); the organic phase was separated. The
mixed organic layers were dried over anhyd Na2SO4,
then filtered, and evaporated. The crude products were
finally purified by chromatography of petroleum ether
and ethyl acetate (5:1, v : v) to give NBMF as a yellow
oil. Yield: 0.218 g (85%). Spectrums of NBMF were
given in the supporting information (including NMR, IR,
and HRMS). 1H NMR (400 MHz, CDCl3) δ: 2.51
(s, 3H, CH3), 3.83 (s, 3H, CH3), 5.67 (s, 2H, CH2), 6.97
(d, 2H, J = 8.04 Hz, CH¼CH), 7.22–7.31 (m, 3H, 3CH),
7.40 (s, 1H, C¼CH), 9.51 (s, 1H, O¼CH). 13C NMR
δ: 11.13 (CH3), 48.43 (CH3), 51.26 (CH2), 114.52 (C),
126.16 (CH), 128.81 (CH), 136.43 (C), 130.7 (C),
145.11 (C), 164.57 (C), 179.36 (O¼CH). IR
(KBr) ν = 783 (C–H), 1667 (C¼C), 1670 (O¼CH), 1253
(C–O–C), 3476 (C¼O) cmꢀ1. HRMS calcd for [M+H]+
258.1130, found [M+H]+ 258.1133.
General procedure for the synthesis of methyl N-butyl-2-
methyl-5-formylpyrrole-3-carboxylate.
Compound
5
(NUMF): Replacing benzyl chloride with n-butyl
bromide (0.163 g, 1.2 mmol), other processes were the
same as above. The reaction mixture was stirred at 55°C
for 12 h, and the reaction was monitored by TLC with
petroleum ether : ethyl acetate (7:1, v : v); when reaction
completed, the mixture was also extracted and dried by
the same operations as above. The crude product was
chromatographed on a silica gel column with petroleum
ether : ethyl acetate (10:1, v : v) to give an orange-yellow
oil. Yield: 0.18 g (81%). Spectrums of NUMF were
given in the supporting information (including NMR, IR,
EXPERIMENTAL
Materials.
The reagents used in this study were
analytically pure and purchased from Tianjin Kemer
Chemical Reagent Co., Ltd. (China). Analytical thin-layer
chromatography (TLC) using glass slides pre-coated with
silica gel (GF254, Merck, China). Column chromatogra-
phy was performed on a glass column (3 cm i.
d. × 60 cm) filled with silica gel (0.03–0.06 mm, Qingdao
Ocean Chemistry Factory, China).
Synthesis. The major concern about this study is the
development of new, flavor precursor compounds of
pyrrole derivatives. As shown in Scheme 1, compounds
2, 3, 4, and 5 were synthesized. The precursors 2-methyl-
5-1′2′3′4′-tetrahydroxybutyl-3-carboxylic acid methyl
ester pyrrole (compound 2) and the 2-methyl-5-formyl-3-
1
and HRMS). H NMR δ: 0.95 (t, 3H, J = 7.28 HZ, CH3),
1.26–1.41 (m, 2H, CH2), 1.61–1.69 (m, 2H, CH2), 2.59
(s, 3H, CH3), 3.82 (s, 3H, CH3), 4.31 (d, 2H,
J = 7.64 Hz, CH2), 7.38 (s, H, CH¼C), 9.45 (s, 1H,
O¼CH). 13C NMR δ: 10.76 (CH3), 13.61 (CH3), 19.74
(CH2), 32.57 (CH2), 45.09 (CH2), 50.98 (CH3), 113.78
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet