Synthesis of pyridothietone by flash vacuum pyrolysis of 2-mercaptonicotinic acid

Article abstract of DOI:10.1016/S0040-4039(02)01044-4

The temperature dependence of products formed from flash vacuum pyrolysis (FVP) of 2-mercaptonicotinic acid 4 has been studied. FVP of 4 at 550°C and ca. 1×10^-2 torr gave pyridothietone 3b and a trimer 5 as the major products. At higher temperature (800°C), FVP of 4 gave 2-mercaptopyridine 6 as the major product and 2,2′-dipyridyl disulphide 7 as the minor one.

Full text of DOI:10.1016/S0040-4039(02)01044-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5285–5286
Synthesis of pyridothietone by flash vacuum pyrolysis of
2-mercaptonicotinic acid
Chin-Hsing Chou,* Shao-Jung Chiu and Wei-Min Liu
Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC
Received 25 March 2002; revised 25 May 2002; accepted 31 May 2002
Abstract—The temperature dependence of products formed from flash vacuum pyrolysis (FVP) of 2-mercaptonicotinic acid 4 has
been studied. FVP of 4 at 550°C and ca. 1×10−2 torr gave pyridothietone 3b and a trimer 5 as the major products. At higher
temperature (800°C), FVP of 4 gave 2-mercaptopyridine 6 as the major product and 2,2%-dipyridyl disulphide 7 as the minor one.
© 2002 Elsevier Science Ltd. All rights reserved.
1
The short-lived a-oxo-o-quinodimethane 1a and its het-
eroanalogues 1b–d have been shown to be transient
intermediates in many reactions and have attracted
much attention as a result of their non-aromatic struc-
tural features.1 Although 1a–d have never been isolated
successfully due to their high reactivities, the closed
forms of 1a–d, benzocyclobutenone 2a,1c benzopropio-
lactone 2b,1d benzothietone 2c,1g,1h and benzoazetinone
2d,1i have been shown to be somewhat more stable and
can be isolated at low temperature. To extend the
chemistry of 2 to their pyridine analogues, we have
previously synthesized pyridocyclobutenone 3a by the
flash vacuum pyrolysis (FVP) of 3-chloroformyl-2-
methylpyridine.2 We now wish to report the synthesis
of the previously unknown pyridothietone 3b by FVP
of 2-mercaptonicotinic acid 4.
ing to room temperature, the H and 13C NMR spectra
revealed that pyridothietone 3b was present as the
major product (41%),3 along with a trimer 5 (30%)4 and
recovered 4 (19%). Unlike 2d and 3a, which can only be
isolated at low temperature, 3b is stable at room tem-
perature and can be separated from 4 and 5 by frac-
tional distillation under vacuum. When the pyrolysis
temperature was raised to 800°C, FVP of 4 gave 2-mer-
captopyridine 6 and 2,2%-dipyridyl disulphide 75 in 58%
and 11% yields, respectively, along with a small amount
of 3b (4%) and some polymers. The formation of these
products via FVP of 4 is summarized in Scheme 1.
It is noteworthy that although FVP of 4 at high tem-
peratures (>800°C) does not give 2-thiocarbonyl-2H-
pyrrole 10, such a ring-contraction reaction is not
uncommon in related systems.2,6 Our recent study on
the chemistry of 3-hydroxypyridine-2-carboxylic acid 11
indicates that FVP of 11 at 550°C and 1×10−2 torr gives
a dimer of 2-carbonyl-2H-pyrrole 12, dipyrrolo[1,2-
a;1%,2%-a]pyrazine-5,10-dione 13,7 as the major product
(38%) (Scheme 2). This result demonstrates another
example of a ring contraction reaction.
We are currently applying this approach to the prepara-
tion of other heterocyclic analogues of benzocylo-
butenone.
FVP of 4 at 550°C and ca. 1×10−2 torr produced a
pale-yellow band in the cold trap at 77 K. After warm-
Acknowledgements
We thank the National Science Council of the Republic
of China for financial support.
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01044-4

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C.-H. Chou et al. / Tetrahedron Letters 43 (2002) 5285–5286
Scheme 1.
Scheme 2.
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3. 3b: 1H NMR (300 MHz, CDCl3) l 8.82 (dd, J=4.5 and
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1H), 8.26 (dd, J=6.0 and 1.8 Hz, 1H), 7.23 (dd, J=7.8
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J=4.8, 1.2 Hz, 4H), 7.12 (m, 2H); 13C NMR (CDCl3)
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