Flash vacuum pyrolysis of N-alkenylbenzotriazoles and N-alkenylisoxazolones

Article abstract of DOI:10.1071/CH00098

The flash vacuum pyrolysis of 1-(2-ethoxycarbonylethenyl)benzotriazole has been reinvestigated, and the processes leading to the formation of ethyl indole-3-carboxylate and 2-ethoxyquinolin-4(1H)-one elucidated. Similar pathways are followed in the flash vacuum pyrolysis of the corresponding benzisoxazolone. Some N-ethenylisoxazolones have been pyrolysed to form pyrroles, but rearrangement of the intermediate carbenes may be observed. Photolysis of the isoxazolones gives carbenes that may be captured by solvent or give pyrroles. CSIRO 2000.

Full text of DOI:10.1071/CH00098

C S I R O P U B L I S H I N G
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Volume 53, 2000
© CSIRO 2000
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Aust. J. Chem., 2000, 53, 665–671
Flash Vacuum Pyrolysis of N-Alkenylbenzotriazoles
and N-Alkenylisoxazolones
Matthew Cox,A Fariba HeidarizadehA,B and Rolf H. PragerA,C
A Chemistry Department, Flinders University of South Australia, G.P.O. Box 2100, Adelaide, S.A. 5001.
B Permanent address: Chemistry Department, Chamran University, Ahwaz, I.R. Iran.
C Author to whom correspondence should be addressed.
The flash vacuum pyrolysis of 1-(2-ethoxycarbonylethenyl)benzotriazole has been reinvestigated, and the pro-
cesses leading to the formation of ethyl indole-3-carboxylate and 2-ethoxyquinolin-4(1H)-one elucidated. Similar
pathways are followed in the flash vacuum pyrolysis of the corresponding benzisoxazolone. Some N-ethenylisox-
azolones have been pyrolysed to form pyrroles, but rearrangement of the intermediate carbenes may be observed.
Photolysis of the isoxazolones gives carbenes that may be captured by solvent or give pyrroles.
Keywords. Indoles; quinolin-4-ones; pyrroles; carbenes; diradicals; dipolar cycloaddition.
N-Alkenylbenzotriazoles have now become available,8–10
and have been shown to form indoles by Schmid,11 Wender
and Cooper,10 and Edstrom12 by photochemical means, and
more recently by flash vacuum pyrolysis.13 The reaction is
presumed to occur via diradical intermediates.14 Katritzky
and coworkers,15 using a combination of mass spectrometry
and fvp, identified indoles and ketenimines as the major
products of these reactions, and suggested that the keten-
imine was the precursor of the indole (Scheme 3). Barker and
Storr13 cast doubt on this conclusion, as 2-substituted indoles
(2) were still obtained from ꢀ-substituted alkenyltriazoles (1)
which would not form an intermediate ketenimine. In this
communication we report some comparitive work on the
f.v.p. of 1-alkenylbenzotriazoles and N-alkenylbenzisoxa-
zolones, and some work on N-alkenylisoxazolones.
Introduction
We have previously reported aspects of our study of the
general reaction shown in Scheme 1, in which the carbene is
generated from an isoxazol-5(2H)-one or triazole by flash
vacuum pyrolysis (f.v.p.) or photolysis,1–3 and X is nitrogen,
oxygen or sulfur.
Examples of the reaction in Scheme 1 where X is carbon
are not numerous, although a number of N-aryltriazoles have
been shown to undergo clean cyclization. The well known
Graebe–Ullmann synthesis,4,5 (Scheme 2) is usually carried
out under acidic conditions, and may involve cationic inter-
mediates,6 although gas phase pyrolytic examples of the
reaction have also been reported.7
Discussion
Wender10 reported that addition of benzotriazole to ethyl
propiolate occurred almost quantitatively in refluxing
toluene after 14 h. We have noted that the presence of tri-
ethylamine allowed the reaction to proceed at room temper-
ature, but the 1-isomer (3) ((E)+(Z), 79%) was accompanied
by the 2-isomer (4) ((E) + (Z), 21%), which were readily sep-
Manuscript received 18 July 2000
© CSIRO 2000
10.1071/CH00098
0004-9425/00/080665

666
M. Cox, F. Heidarizadeh and R. H. Prager
arable. In the presence of a trace of trifluoroacetic acid
(cf.16), the formation of the adducts was again extremely
rapid at room temperature, but the process was not syntheti-
cally useful as (3) and (4) were now formed in equal
amounts, and each as a 1: 1 mixture of (E) and (Z) isomers.
However, in the case of addition to propiolic acid, the use of
trifluoroacetic acid catalysis led to quantitative formation of
only the 1-isomer. Unfortunately, such catalysis was not par-
ticularly useful in the additions of benzotriazole to more sub-
stituted propiolates such as ethyl phenylpropiolate, leading
to (5). The photolysis of (3) at 254 nm was reported to give
ethyl indole-3-carboxylate (6) in 74% yield,10 contaminated
only by starting material, and f.v.p. at 600° was reported to
give only polymeric material.13
The loss of ethylene from heterocyclic ethers such as (7)
on f.v.p. has been reported by McNab,19 and will be dis-
cussed in more detail by us shortly.20 On the assumption that
if the ketenimine (9) were the intermediate to the indole it
would need to undergo isomerization to the carbene (10), a
retro-Wolff transformation, we generated the carbene (10)
independently by the f.v.p. of the isoxazolone (11), and
found that the products of (10) were indeed the indole and
the quinolinone (Scheme 5). However, since the indole was
the minor product in this process, it is clear that (10) prefers
to form the ketenimine precursor of the quinolinone, rather
than vice versa, and hence the ketenimine is not likely to be
the precursor of the indole.
We have found that the f.v.p. of (3) is quite temperature-
dependent. At 450°C we isolated the indole (6) (43%) and 2-
ethoxyquinolin-4(1H)-one (7) (57%). When the f.v.p. tem-
perature was raised to 550°C, the quinolinone underwent
loss of ethylene to give the highly insoluble 2-hydroxyquino-
linone (8), which we believe to be the ‘insoluble polymeric’
material reported by Barker and Storr.13 The origin of the
quinolinone is understandable in the light of the observation
by Katritzky15 and Storr13 of the formation of ketenimines.
The recent reports of Wentrup17,18 and McNab19 have shown
that imidoylketenes substituted by an alkoxycarbonyl group
and oxoketenimines are in rapid equilibrium by 1,3-shifts of
the alkoxy group, and hence the origin of (7) and (8) is most
likely as shown in Scheme 4.
We sought to settle this question by preparing and pyroliz-
ing a deuterated version of the ester (3). Reaction of benzo-
triazole, D2O and ethyl propiolate in boiling tetrahydrofuran
slowly delivered a mixture of (E)- and (Z) (3), but both were
totally dideuterated (3b), as established by n.m.r. and mass
spectral analysis. A similar reaction in the presence of tri-
ethylamine was more rapid, but again led only to dideuter-
ated species. When (3b) was refluxed in the presence of
either trifluoroacetic acid or triethylamine/water, (3b) was
recovered unchanged. An experiment in which the 1H n.m.r.
spectrum of a mixture of benzotriazole, triethylamine, ethyl
propiolate and D2O was followed showed that addition of
amine to the triple bond was immediate. Reactions of propi-
olate esters with tertiary amines in the presence of excess
water have been studied by Winterfeldt,21 Acheson22 and
McCulloch and McInnes.23 McCulloch and McInnes charac-
terized the formation of betaine (12) and suggested it to be
the precursor of the enol ether products observed. In addi-
tion, they reported the formation of the dideuterated betaine
(13) when D2O was used, and noted that back-exchange of
the olefinic protons was extremely slow. Thus, when com-
pound (13) was refluxed in KOH/methanol for 1 h, an
exchange of only 10% of the deuterium at C 1, and 25% of
the deuterium at C 2 occurred.23 We suggest that (3b) is
formed as shown in Scheme 6. The failure to observe back-
exchange suggests that triethylamine does not add to the
product (3) to give an intermediate analogous to (14). The
presence of tributylphosphine, which has been shown to
facilitate conjugate additions,24 failed to induce hydrogen/
deuterium exchange under basic conditions. An alternative
explanation is that the acetylenic hydrogen in ethyl propio-
late exchanges instantly with D2O in the presence of tri-

Flash Vacuum Pyrolysis of N-Alkenylbenzotriazoles
667
to essentially the same result. Because of the extremely facile
exchange of H 3 in (7), particularly under slightly acidic con-
ditions, some loss of deuterium at C 3 to the silica during the
f.v.p. was noted.
F.v.p. of the phenylpropiolate adduct (5) gave, as might be
expected from the above and the inability to form a keten-
imine, only ethyl 2-phenylindole-3-carboxylate (15) (100%).
Benzisoxazolone (16) reacted with ethyl propiolate under
basic conditions (66%), but not in the presence of trifluo-
roacetic acid, to give the (E) and (Z) adducts (17), which were
subjected to f.v.p. at 450°C to give a 3: 2 mixture of the
indole (6) (50%) and the quinolinone (7) (34%), consistent
with our impression that indole formation is favoured at
lower temperatures, presumably from a carbene intermedi-
ate. Ketenimine formation may represent a diradical pathway
at higher temperatures, or a higher activation energy pathway
for the carbene.
ethylamine, resulting in the formation of the intermediate
(13), and hence (3b). However, since (3b) is also formed
slowly in D2O in the absence of triethylamine, no definitive
pathway is yet apparent.
A logical extension to the above was to investigate the
f.v.p. of analogous N-alkenyl-1,2,3-triazoles and N-
alkenylisoxazolones. Based on the above pathways, one
might expect the products of such reactions to be pyrroles
and/or pyridinones (Scheme 8).
Flash vacuum pyrolysis of the deuterated ester (3b) gave
the indole ester (6), containing 1.0D located at C 2 (ꢁH 7.93)
and the quinolinone (7) with 1.5D located at C 3 (1D, replac-
ing H 3 at ꢁH 6.15) and C 8 (0.5D, replacing H 8 at ꢁH 8.00),
consistent only with the presence of two independent path-
ways as shown in Scheme 7, drawn with a hypothetical
carbene intermediate, although diradical intermediates lead
The 3-phenylisoxazolone (18) gave the N-alkenyl com-
pound (19) with ethyl propiolate in the presence of base
(Scheme 9). F.v.p. at 540°C gave the pyrrole (20), with none
of the expected product (21). Pyrolysis at 400°C also gave
(20). We conclude that rearrangement of the initially formed
carbene (22) to the more stable carbene (23), presumably via
the 1H-azirine, was considerably more rapid than reaction of
the carbene with the double bond. However, photolysis of
either the (E) or (Z) isomer of the N-alkenyl compound (19)
in acetone at 300 nm gave the expected pyrrole (21). The
structures of the products were assigned on the basis of their
n.m.r. and infrared spectra.

668
M. Cox, F. Heidarizadeh and R. H. Prager
Photolysis in acetonitrile gave the desired pyrrole (26)
(15%) in addition to the cycloaddition product (27) (26%)
(Scheme 11). In order to totally avoid participation by
nucleophilic solvents, photolysis was carried out in ethyl
acetate, resulting in the formation of the desired pyrrole (26)
(18%) and an unidentified compound, which was the major
product. F.v.p. of (24) at 540°C followed yet another
pathway, leading to the formation of the oxazole (28) (55%).
Isomerization of the isoxazolone to the nitrone26 which then
undergoes nitrone–oxaziridine–amide isomerization27 would
lead to the formation of the N-acyl ketene (29). Loss of
carbon monoxide from (29) would then give an N-acyl-
iminocarbene intermediate which can undergo cyclization in
the normal way1 to the oxazole (28) (Scheme 12).
The N-vinyl adduct (24), obtained similarly, was sub-
jected to photolysis in acetone at 300 nm, and produced
exclusively the cycloaddition product (25), isolated in 70%
yield. Reaction of the photolysis solvent with the carbenoid
in a 1,3-dipolar cycloaddition reaction gave the product (25)
(Scheme 10). This observation is consistent with that of
Regitz,25 who suggested that the presence of electron-with-
drawing groups close to the nitrogen atom of an iminocar-
bene tends to make the compound react in a 1,3-dipolar
fashion.
Conclusion
The present study has confirmed that the photolysis of
both N-alkenylbenzotriazoles and N-alkenylisoxazolones is
a more reliable method for the synthesis of indoles/pyrroles
than pyrolysis, and that such reactions probably proceed
through carbene intermediates. The pyrolysis of these com-
pounds may be complicated by concurrent side reactions,
either because of the formation of radical intermediates in
the case of benzo compounds, or because of rapid isomer-
ization of the isoxazolones or the carbene intermediates in
the case of isoxazolones.
Experimental
Proton (1H) and carbon (13C) nuclear magnetic resonance (n.m.r.)
spectra were recorded on a Varian Gemini Spectrometer at 200 or 300
MHz and 50 or 75.5 MHz respectively, in deuteriochloroform (CDCl3),
unless otherwise stated. Infrared spectra were recorded on
a
Perkin–Elmer 1600 FTIR spectrometer; solids were analysed as Nujol
mulls or KBr disks, and liquids as films . High resolution mass spectra
were recorded on a Kratos MS25RF spectrometer. Melting points were
determined on a Reichert hot-stage apparatus and remain uncorrected.
Radial chromatography was performed with silica gel 60 PF 254 coated
glass rotors by using a Chromatotron (model 7924T). Gas chroma-
tography/mass spectral analysis was performed on a Varian Saturn 4D
instrument, with a 5% phenylmethyl polysiloxane column (30 m, 0.25
mm internal diameter, 0.25 mm thickness). Flash vacuum pyrolysis was
carried out by slowly subliming the substrate through a silica tube (400
mm by 25 mm, packed with silica chips and heated to the quoted tem-
perature) under reduced pressure (c. 0.1 mmHg). The products were

Flash Vacuum Pyrolysis of N-Alkenylbenzotriazoles
669
collected in a liquid nitrogen cold trap, and analysed by gas chroma-
(25 ml) for 48 h. Separation by radial chromatography (light petroleum/
CH2Cl2) afforded the title compound (0.53 g, 50%) isolated as a pale
yellow oily mixture of (E) and (Z) isomers (Found: M+, 293.1167.
C17H15N3O2 requires M+, 293.1164). 1H n.m.r. ꢁ 8.18–8.10, m, 1H;
7.54–7.35, m, 6H; 7.15–7.10, m, 1H; 7.30–7.22, m, 2H; 6.61, s, 1H;
3.96, q, J 7 Hz, 2H; 0.92, t, J 7 Hz, 3H. 13C n.m.r. ꢁ 13.6, 60.8, 134.0,
110.4, 116.5, 120.1, 123.5, 124.2, 127.2, 128.1, 129.1, 131.4, 133.7,
134.0, 144.6, 145.6, 163.5. ꢂmax 3064, 2983, 1725, 1636, 1452, 1163,
1058, 1029, 769, 748 cm–1. Mass spectrum (m/z) 293 (13%), 265 (52),
219 (60), 193 (100), 165 (50), 147 (21), 102 (47), 77 (36).
1
tography/mass spectroscopy, H and 13C n.m.r. spectroscopy, followed
by chromatographic isolation.
Reaction of 1H-Benzotriazole with Ethyl Propiolate
A solution of benzotriazole (0.5 g, 4.2 mmol), ethyl propiolate
(0.4 g, 4.2 mmol) and triethylamine (0.42 g, 4 mmol) in ethanol (25 ml)
was stirred for 24 h at room temperature. After the solution was cooled
to 0°C, the mixture of products (0.91 g, 100%) was collected and
recrystallized from ethanol to give white needles of (E)-ethyl 3-(1H-
benzotriazol-1-yl)propenoate (3) (0.46 g, 50%), m.p. 108–109°C (lit.10
108°C) (Found: M+, 217.0850. Calc. for C11H11N3O2: M+, 217.0851).
1H n.m.r. ꢁ 8.50, d, J 4.4 Hz, 1H; 8.13, d, J 8.4 Hz, 1H; 7.9–7.35, m, 3H;
Flash Vacuum Pyrolysis of (E) and (Z) Ethyl 3-(1H-Benzotriazol-1-yl)-
3-phenylpropenoate (5)
6.74, d, J 14.4 Hz, 1H; 4.32, q, J 7.1 Hz, 2H; 1.37, t, J 7.1 Hz, 3H. 13
C
F.v.p. (450°C, 175°C, 0.1 mmHg) of the title compound (200 mg)
gave a brown solid in the exit tube, which was purified by radial chro-
matography (light petroleum/CH2Cl2) to yield off-white crystals of
ethyl 2-phenylindole-3-carboxylate (15) (185 mg, 100%), m.p.
154–156°C (lit.28 154–156°C) (Found: M+, 265.1102. Calc. for
C17H15NO2: M+, 265.1103). 1H n.m.r. ꢁ 8.5, bs, NH, 1H; 8.20–8.25, m,
n.m.r. ꢁ 165.9, 146.7, 135.2, 131.6, 129.4, 125.4, 120.9, 110.2, 108.3,
61.1, 14.3. ꢂmax (KBr) 1708, 1652, 1280, 1040 cm–1. Mass spectrum
(m/z) 217 (60%), 144 (40), 117 (72), 92 (100) .
The residue from the mother liquor was separated by column chro-
matography on silica by using light petroleum/CH2Cl2. Colourless
needles of ethyl (E)-3-(2H-benzotriazol-2-yl)propenoate (4) (0.127 g,
14%), m.p. 107–108°C, were obtained from the first fraction (Found:
M+, 217.0851. C11H11N3O2 requires M+, 217.0851). 1H n.m.r. ꢁ 8.40, d,
J 14.2 Hz, 1H; 7.82–7.87, m, 2H; 7.44–7.39, m, 2H; 7.00, d, J 14.2 Hz,
1H; 4.30, q, J 7 Hz, 2H; 1.35, t, J 7 Hz, 3H. 13C n.m.r. ꢁ 165.9, 135.2,
129.3, 125.4, 120.8, 110.1, 108.3, 61.0, 14.2. ꢂmax (KBr) 1708, 1652,
1280, 1040 cm–1. Mass spectrum (m/z) 217 (51%), 189 (10), 172 (14),
144 (35), 129 (21), 117 (81), 92(100).
The second fraction contained additional (3) (0.62 g in total, 68%).
The third fraction contained the (Z)-isomer of (3), which was
obtained as a yellow oil (0.1 g, 11%) (Found: M+, 217.0851. Calc. for
C11H11N3O2: M+, 217.0851). 1H n.m.r. ꢁ 8.10, d, J 9 Hz, 1H; 7.55–7.37,
m, 4H; 6.07, d, J 9.4 Hz, 1H; 4.14, q, J 7.2 Hz, 2H; 1.10, t, J 7.2 Hz, 3H.
13C n.m.r. ꢁ 164.2, 145.6, 129.2, 128.3, 127.4, 124.6, 120.4, 114.4,
110.3, 61.3, 13.8. ꢂmax 1739, 1653, 1022, 746 cm–1. Mass spectrum
(m/z) 217 (38%), 172 (12), 144 (37),117 (80), 92 (100), 63 (47).
The last fraction was identified spectroscopically as (Z)-(4) (0.064
g), and was not further characterized.
1H; 7.26–7.7, m, 9H; 4.31, q, J 7.2 Hz, 2H; 1.31, t, J 7.2 Hz, 3H. 13
C
n.m.r. ꢁ 14.3, 59.6, 110.9, 122.0, 122.2, 123.2, 127.6, 128.1, 126.0,
128.8, 129.1, 129.6, 130.2, 132.1, 135.0, 136.2, 165.2. ꢂmax (KBr) 2963,
1673, 1450, 1261, 1211, 1048, 793, 751 cm–1. Mass spectrum (m/z) 265
(100%), 237 (13), 220 (94), 193 (42), 165 (28), 95 (11), 63 (6).
Ethyl 3-(3-Oxo-1,3-dihydro-2,1-benzisoxazol-1-yl)propenoate (17)
Benzisoxazolone (0.3 g, 2.22 mmol), ethyl propiolate (0.22 g, 2.24
mmol) and triethylamine (0.224 g, 2.21 mmol) were stirred in ethanol
(20 ml) at room temperature for 24 h. The ethanol was evaporated and
the crude solid product (0.34 g, 66%) was washed with cold ethanol to
give a pure sample of the (E) isomer of (17) (0.23 g), m.p. 140–142°C.
The washings contained a mixture of (E) and (Z) products (Found: M+,
1
233.0688. C12H11NO4 requires M+, 233.0688). H n.m.r. ꢁ 7.80, d, J
13.2 Hz, 1H; 7.7–7.9, m, 2H; 7.3–7.35, m, 2H; 5.78, d, J 13.2 Hz, 1H;
4.24, q, J 7.2 Hz, 2H; 1.32, t, J 7.2 Hz, 3H. 13C n.m.r. ꢁ 167.0, 136.3,
133.6, 126.4, 124.4, 108.6, 98.2, 60.4, 14.3. ꢂmax (KBr) 1773, 1700,
1560, 1286, 1170, 755 cm–1. Mass spectrum (m/z) 233 (100%), 188
(71), 161 (58), 144 (28), 116 (55), 90 (44), 77 (69).
Flash Vacuum Pyrolysis of Ethyl 3-(1H-Benzotriazol-
1-yl)propenoate (3)
Flash Vacuum Pyrolysis of (E) Ethyl 3-(3-Oxo-1,3-dihydro-2,1-
benzisoxazol-1-yl)propenoate (17)
(i) At 450°C. F.v.p. (450°C, 135°C, 0.1 mmHg) of the title com-
pound (200 mg) gave a brown solid in the exit tube (170 mg) which was
subjected to radial chromatography on silica (light petroleum/CH2Cl2).
The first fraction was ethyl indole-3-carboxylate (43%), isolated as a
pale yellow solid, m.p. 124–125°C (lit.10 123–124oC) (Found: M+,
189.0789. Calc. for C11H11NO2: M+, 189.0790). 1H n.m.r. ꢁ 8.5, bs, 1H;
8.2–8.17, m, 1H; 7.90, d, J 8 Hz, 1H; 7.44–7.28, m, 3H; 4.40, q, J 7.2
Hz, 2H; 1.43, t, J 7.2 Hz, 3H; 13C n.m.r. ꢁ 165.2, 136.1, 130.8, 125.8,
123.1, 122.0, 121.6, 111.4, 59.7, 30.8, 14.5. ꢂmax (KBr) 3259, 1663,
1442, 1198, 1053 cm–1. Mass spectrum (m/z) 189 (50%), 161 (14), 144
(100), 116 (17), 89 (17), 63 (7).
Pyrolysis (450°C, 175°C, 0.1 mmHg) of the title compound (100
mg) gave a light brown solid (75 mg) in the exit tube, which was iden-
tified as a 3:2 mixture of the indole (6) and the quinolinone (7) by a
combination of gas chromatography/mass spectroscopy and H n.m.r.
spectroscopy.
1
FlashVacuum Pyrolysis of Ethyl 5-Oxo-2-phenyl-2,5-dihydroisoxazole-
4-carboxylate (11)
Pyrolysis (450°C, 175°C, 0.1 mmHg) of the title compound (300
mg) gave a brown solid (140 mg) which was found to be a 2 :1 mixture
of the quinolinone (7) and the indole (6) by 1H n.m.r. spectroscopy and
gas chromatography/mass spectroscopy.
The second fraction, isolated as a pale yellow solid, and recrystal-
lized from ethanol, was 2-ethoxyquinolin-4(1H)-one (57%), m.p.
184–185°C (lit.28 184–184.5°C) (Found: M+, 189.0789. Calc. for
1
C11H11NO2: M+, 189.0790). H n.m.r. ꢁ ((CD3)2SO) 8.00, d, J 8.4 Hz,
Dideuterated Ethyl 3-(1H-Benzotriazol-1-yl)propenoate (3b)
1H; 7.6–7.2, m, 3H; 6.15, s, 1H; 4.34, q, J 7.2 Hz, 2H; 1.33, t, J 7.2 Hz,
3H. 13C n.m.r. ꢁ (CDCl3/trifluoroacetic acid) 172.0, 131.5, 135.7, 127.0,
124.7, 117.5, 90.0, 68.9, 13.6. ꢂmax (KBr) 2902, 1633, 1592, 1221, 1031
cm–1. Mass spectrum (m/z) 190 (6%), 189 (52), 161 (15), 144 (100), 116
(18), 91 (14), 77 (18). The above properties were identical with those of
an authentic sample, synthesized by the method of Harris.28
(ii) At 550°C. F.v.p. (550°C, 135°C, 0.1 mmHg) of the title com-
pound (300 mg) gave a brown solid (220 mg) which was analysed by 1H
n.m.r. spectroscopy, and was shown to contain the ethoxyquinolinone
(7) (6%), the indole (6) (32%), and the hydroxyquinolinone (8) (61%).
1H-Benzotriazole (0.5 g, 4.2 mmol), ethyl propiolate (0.4 g, 4.2
mmol) triethylamine (0.42 g, 0.4 mmol) and D2O (2 ml) were stirred for
24 h at room temperature in tetrahydrofuran (15 ml). The tetrahydro-
furan was removed under vacuum and the white crystalline product was
freed from the 2-isomer by recrystallization from ethanol to produce
colourless crystals, m.p. 109–110°C. 1H n.m.r. ꢁ 8.14, d, J 8.4 Hz,1H;
7.60–7.73, m, 2H; 7.45–7.53, m, 1H; 4.63, q, J 7.2 Hz, 2H; 1.38, t, J 7.2
Hz, 3H. Mass spectrum (m/z) 219 (34%), 146 (23), 119 (57), 92 (100).
Flash Vacuum Pyrolysis of Dideuterated Ethyl 3-(1H-Benzotriazol-1-
Ethyl 3-(1H-Benzotriazol-1-yl)-3-phenylpropenoate (5)
yl)propenoate (3b)
1H-Benzotriazole (0.5 g, 4.2 mmol ), ethyl phenylpropiolate (0.73 g,
4.2 mmol) and triethylamine (0.42 g, 4 mmol) were refluxed in ethanol
Pyrolysis (450°C, 135°C, 0.1 mmHg) of the title compound gave the
indole (6) and quinolinone (7) as above. The 1H n.m.r. and mass spectra

670
M. Cox, F. Heidarizadeh and R. H. Prager
Hz, 1H; 4.33, q, J 7.1 Hz, 2H; 1.37, t, J 7.1 Hz, 3H. 13C n.m.r. ꢁ 161.3,
136.8, 131.4, 128.9, 127.7, 124.8, 123.3, 116.7, 107.9, 60.4, 14.5. ꢂmax
1694, 1511, 1466 cm–1. Mass spectrum (m/z) 215 (83%), 169 (100), 141
(35), 115 (27), 105 (38), 77 (21).
of each compound were carefully analysed to determine the deuterium
content and distribution in each compound.
Flash Vacuum Pyrolysis of 2-Ethoxyquinolin-4(1H)-one (7)
Photolysis of (Z)-(19) gave a product which was identical to that
obtained above from the (E)-isomer.
Pyrolysis (450°C, 150°C, 0.1 mmHg) of the title compound (120
mg) gave 2-hydroxyquinolin-4(1H)-one (8) as the only product, the
identity of which was confirmed by comparison with an authentic
sample,29 m.p. 355°C, its 1H n.m.r. spectrum and gas chromatogra-
phy/mass spectral analysis).
Ethyl 2-(2-Ethoxycarbonylethenyl)-4-methyl-2,5-dihydroisoxazole-3-
carboxylate (24)
Ethyl 4-methyl-5-oxo-2,5-dihydroisoxazole-3-carboxylate (0.5 g,
2.9 mmol), ethyl propiolate (0.29 g, 2.9 mmol) and triethylamine (0.3
g, 2.9 mmol) were refluxed in dichloromethane (15 ml) for 2 h. The
solvent was evaporated and the residue was then redissolved in diethyl
ether (30 ml) and washed with 1 M HCl (2 × 20 ml). The organic layer
was separated, dried (Na2SO4), filtered and evaporated to give the pure
product as a dark orange oil which crystallized on standing (0.71 g,
90%), m.p. 65–67oC (Found: M+, 269.0900. C12H15NO6 requires M+,
269.0899). 1H n.m.r. ꢁ 8.28, d, J 13.2 Hz, 1H; 5.76, d, J 13.2 Hz, 1H;
4.45, q, J 7.1 Hz, 2H; 4.19, q, J 7.1 Hz, 2H; 2.12, s, 3H; 1.42, t, J 7.1
Hz, 3H; 1.27, t, J 7.1 Hz, 3H. 13C n.m.r. ꢁ 167.4, 166.3, 157.4, 141.7,
134. 1, 108.5, 101.0, 63.4, 60.3, 14.1, 13.8, 8.3. ꢂmax 1765, 1729, 1707,
1625, 1604 cm–1. Mass spectrum (m/z) 269 (76%), 224 (50), 196 (23),
154 (25), 125 (52), 107 (38), 85 (55), 44 (100).
(E) and (Z) Ethyl 3-(5-Oxo-3-phenyl-2,5-dihydroisoxazol-2-
yl)propenoate (19)
3-Phenylisoxazol-5(2H)-one (0.5 g, 3.1 mmol), ethyl propiolate
(0.304 g, 3.1 mmol) and triethylamine (0.310 g, 3.1 mmol) were strirred
in dichloromethane (15 ml) at reflux for 6 h. The solvent was evapo-
rated and the residue was redissolved in diethyl ether (30 ml) and then
washed with 1 M HCl (2 × 20 ml). The organic layer was separated, dried
(Na2SO4), filtered and evaporated to give the crude product as an
orange gum (0.8 g, 99%), which was further purified by radial chro-
matography (10% ethyl acetate/light petroleum), to give the (E)
isomer* of compound (19) as a pale yellow powder (0.184 g, 23%),
m.p. 114–116oC (Found: M+, 259.0844. C14H13NO4 requires M+,
259.0845). 1H n.m.r. ꢁ 7.48–7.66, m, 6H; 5.9, d, J 13.3 Hz, 1H; 5.51, s,
1H; 4.18, q, J 7.1 Hz, 2H; 1.27, t, J 7.1 Hz, 3H. 13C n.m.r. ꢁ 166.8,
166.4, 160.5, 152.1, 133.0, 132.2, 129.7, 128.4, 125.1, 102.2, 91.3,
60.6, 14.2. ꢂmax (NaCl) 1761, 1711, 1634, 1569, 1490 cm–1. Mass spec-
trum (m/z) 259 (67%), 214 (100), 186 (46), 169 (52), 142 (41), 129 (38),
115 (35), 102 (65), 85 (36).
Ethyl (Z)-3-(5-oxo-2,5-dihydroisoxazol-2-yl)propenoate (19) (0.04
g, 5%) was isolated after chromatography (10% ethyl acetate/light
petroleum) as a pale yellow solid, m.p. 50–52oC (Found: M+, 259.0844.
C14H13NO4 requires M+, 259.0845). 1H n.m.r. ꢁ 7.50–7.64, m, 5H; 6.48,
d, J 9.9 Hz, 1H; 5.47, s, 1H; 5.32, d, J 9.9 Hz, 1H; 4.29, q, J 7.1 Hz, 2H;
1.35, t, J 7.1 Hz, 3H. 13C n.m.r. ꢁ 167.7, 164.5, 161.8, 152.1, 132.1,
129.5, 128.5, 126.9, 125.9, 104.8, 90.2, 61.1, 14.0. ꢂmax 1768, 1715,
1635, 1611, 1586, 1568, 1491 cm–1. Mass spectrum (m/z) 259 (47%),
214 (100), 186 (53), 169 (58), 142 (48), 129 (45), 102 (73), 89 (23), 77
(37).
Photolysis of Ethyl 2-(2-Ethoxycarbonylethenyl)-4-methyl-2,5-
dihydroisoxazole-3-carboxylate (24) in Acetone
The title compound (95 mg, 0.35 mmol) was photolysed through
Pyrex at 300 nm in anhydrous acetone (150 ml) under N2 at room tem-
perature for 3 h. The solvent was evaporated and the residue was puri-
fied by radial chromatography (50% ethyl acetate/light petroleum), to
give the carboxylate† (25) as a yellow oil (70 mg, 70%) (Found: M+,
1
283.1419. C14H21NO5 requires M+, 283.1420). H n.m.r. ꢁ 7.89, d, J
13.6 Hz, 1H; 5.88, d, J 13.6 Hz, 1H; 4.34, q, J 7.3 Hz, 2H; 4.19, q, J 7.0
Hz, 2H; 1.75, s, 3H; 1.64, s, 3H; 1.56, s, 3H; 1.33, t, J 7 Hz, 3H; 1.27,
t, J 7.3 Hz, 3H. 13C n.m.r. ꢁ 170.0, 165.9, 161.5, 147.9, 117.1, 111.3,
81.3, 62.5, 60.6, 28.6, 26.8, 22.9, 14.2, 14.0. ꢂmax 1798m, 1732, 1705,
1622 cm–1. Mass spectrum (m/z) 283 (0.5%), 198 (26), 126 (19), 101
(13), 98 (14), 70 (7), 59 (29), 43 (100).
Flash Vacuum Pyrolysis of (E) and (Z) Ethyl 3-(5-Oxo-3-phenyl-2,5-
dihydroisoxazol-2-yl)propenoate (19)
Photolysis of Ethyl 2-(2-Ethoxycarbonylethenyl)-4-methyl-2,5-
dihydroisoxazole-3-carboxylate (24) in Acetonitrile
Pyrolysis (540oC, 150oC, 0.1 mmHg) of the title compound (93 mg,
0.36 mmol) gave a dark red oil (90 mg, 97%), which was further puri-
fied by radial chromatography (10% ethyl acetate/light petroleum) to
give ethyl 4-phenylpyrrole-3-carboxylate (20) as a pale yellow solid (9
mg, 12%), m.p. 123–125oC (lit.30 121–124oC) (Found: M+, 215.0946.
Calc. for C13H13NO2: M+, 215.0946). 1H n.m.r. ꢁ 8.8, bs, 1H; 7.28–7.52,
m, 6H; 6.92, dd, J 2.5 and 1.7 Hz, 1H; 4.31, q, J 7 Hz, 2H; 1.36, t, J 7
Hz, 3H. 13C n.m.r. ꢁ 161.7, 142.9, 131.7, 129.0, 127.0, 124.7, 124.0,
106.6, 96.9, 59.9, 14.5. ꢂmax 1706, 1515, 1461 cm–1. Mass spectrum
(m/z) 215 (72%), 187 (18), 170 (83), 153 (100), 115 (21), 107 (34), 77
(31).
The title compound (120 mg, 0.45 mmol) was photolysed through
Pyrex at 300 nm in anhydrous acetonitrile (180 ml) under N2 at room
temperature for 3 h. The solvent was evaporated and the residue was
purified by radial chromatography (30% ethyl acetate/light petroleum)
to give two main fractions, the first of which contained diethyl 3-
methylpyrrole-2,4-dicarboxylate (26) as a pale yellow solid (15 mg,
15%), m.p. 72–74oC (lit.32 79–80oC) (Found: M+, 225.1001. Calc. for
C11H15NO4: M+, 225.1001). 1H n.m.r. ꢁ 9.22, bs, 1H; 7.47, d, J 3.7 Hz,
1H; 4.33, q, J 7 Hz, 2H; 4.28, q, J 7 Hz, 2H; 2.59, s, 3H; 1.37, t, J 7 Hz,
3H; 1.34, t, J 7 Hz, 3H. 13C n.m.r. ꢁ 164.6, 161.5, 129.7, 126.8, 120.8,
117.0, 60.5, 60.0, 14.4, 13.9, 11.2. ꢂmax 1708, 1694, 1564, 1509, 1269
cm–1. Mass spectrum (m/z) 225 (100%), 215 (8), 196 (29), 180 (88), 168
(46), 150 (33), 142 (51), 134 (84), 125 (52), 114 (34), 107 (57), 96 (45),
70 (17), 43 (62).
Pyrolysis of the title compound at 400oC gave the same product.
Photolysis of (E) and (Z) Ethyl 3-(5-Oxo-3-phenyl-2,5-
dihydroisoxazol-2-yl)propenoate (19)
The (E)-isomer of (19) (93 mg, 0.36 mmol) was photolysed through
Pyrex at 300 nm in anhydrous acetone (150 ml) under N2 at room tem-
perature for 3 h. The solvent was evaporated and the residue was puri-
fied by radial chromatography (10% ethyl acetate/light petroleum) to
give ethyl 5-phenylpyrrole-3-carboxylate (21) as a pale yellow powder
(47 mg, 61%), m.p. 138–145oC (lit.31 149–151oC) (Found: M+,
215.0945. Calc. for C13H13NO2: M+, 215.0946). 1H n.m.r. ꢁ 9.6, bs, 1H;
7.27–7.61, m, 5H; 6.96, dd, J 2.5 and 3.9 Hz, 1H; 6.54, dd, J 2.5 and 3.9
Fraction 2 contained the carboxylate‡ (27) which was isolated as a
yellow oil (31 mg, 26%) (Found: M+, 266.1266. C13H18N2O4 requires
M+, 266.1267). 1H n.m.r. ꢁ 8.11, d, J 13.2 Hz, 1H; 5.83, d, J 13.2 Hz,
1H; 4.21, q, J 6.9 Hz, 4H; 2.25, s, 3H; 1.63, s, 3H; 1.30, t, J 6.9 Hz, 3H;
1.28, t, J 6.9 Hz, 3H. 13C n.m.r. ꢁ 165.9, 165.4, 159.4, 156.9, 148.3,
115.6, 100.9, 62.7, 60.5, 21.3, 15.4, 14.2, 13.9. ꢂmax 1722, 1682, 1638
cm–1. Mass spectrum (m/z) 266 (1%), 241 (4), 215 (17), 170 (14), 142
(100), 114 (72), 96 (92), 70 (18), 43 (51).
* Ethyl (E)-3-(5-oxo-3-phenyl-2,5-dihydroisoxazol-2-yl)propenoate.
† Ethyl 3-(2-ethoxycarbonylethenyl)-2,2,5-trimethyl-2.3-dihydrooxazole-4-carboxylate.
‡ Ethyl 1-(2-ethoxycarbonylethenyl)-2,4-dimethylimidazole-5-carboxylate.

Flash Vacuum Pyrolysis of N-Alkenylbenzotriazoles
671
10
11
Flash Vacuum Pyrolysis of (24)
Wender, P. A., and Cooper, C. B., Tetrahedron, 1986, 42, 2985.
Marky, M., Schmid, H., and Hauser, H.-J., Helv. Chim. Acta, 1979,
62, 2129.
Edstrom, E. D., and Yuan, W., Tetrahedron Lett., 1991, 32, 323.
Barker, S. J., and Storr, R. C., J. Chem. Soc., Perkin Trans. 1, 1990,
485.
Tsujimoto, K., Ohashi, M., andYonezawa, T., Bull. Chem. Soc. Jpn,
1973, 46, 305.
Maquestiau, A., Beugnies, D., Flammang, R., Katritzky, A. R.,
Soleiman, M., Davis, T., and Lam, J. N., J. Chem. Soc., Perkin Trans.
2, 1988, 1071.
Truce, W. E., and Brady, D. G., J. Org. Chem., 1964, 29, 385.
Kappe, C. O., Kollenz, G., Leung-Toung, R., and Wentrup, C., J.
Chem. Soc., Chem. Commun., 1992, 487.
Fulloon, B., El-Nabi, H. A. A., Kollenz, G., and Wentrup, C.,
Tetrahedron Lett., 1995, 36, 6547.
Clark, D., Mares, R. W., and McNab, H., J. Chem. Soc., Chem.
Commun., 1993, 1026.
Prager, R. H., Tsaconas, M., and Wilkinson, K., unpublished data.
Winterfeldt, I. E., and Preuss, H., Chem. Ber., 1966, 99, 450.
Acheson, R. M., and Plunkett, A. O., J. Chem. Soc., 1964, 2676.
McCulloch, A. W., and McInnes, A. G., Can. J. Chem., 1974, 52,
3579.
Wiley, R. H., Smith, N. R., Johnson, D. M., and Moffat, J., J. Am.
Chem. Soc., 1954, 76, 4933.
Regitz, M., Arnold, B., Danion, D., Schubert, H., and Fusser, G.,
Bull. Soc. Chim. Belg., 1981, 90, 615.
Singh, Y., and Prager, R. H., Aust. J. Chem., 1992, 45, 1811; Prager,
R. H., Singh, Y., and Weber, B., Aust. J. Chem., 1994, 47, 1249.
Emmons, W. D., J. Am. Chem. Soc., 1957, 79, 5739; Emmons, W.
D., Chem. Heterocycl. Compd., 1964, 19, 624; Ang, K. H., and
Prager, R. H., Tetrahedron Lett., 1992, 33, 2845.
Harris, W. D., Synthesis, 1976, 826.
Baeyer, A., and Bloem, F., Ber. Dtsch. Chem. Ges., 1882, 15, 2147.
Van Leusen, D., van Echten, E., and van Leusen, A. M., J. Org.
Chem., 1992, 57, 2245.
Pyrolysis (540oC, 150oC, 0.1 mmHg) of the title compound (89 mg,
0.33 mmol) gave a dark brown oil (44 mg, 55%), which was further
purified by radial chromatography (20% ethyl acetate/light petroleum)
to give the carboxylate* (28) as a yellow oil (14 mg, 18%) (Found: M+,
241.0942. C11H15NO5 requires M+, 241.0949). 1H n.m.r. ꢁ 4.37, q, J 7.3
Hz, 2H; 4.19, q, J 7.0 Hz, 2H; 3.83, s, 2H; 2.62, s, 3H; 1.38, t, J 7.3 Hz,
3H; 1.26, t, J 7.0 Hz, 3H. 13C n.m.r. ꢁ 167.2, 162.1, 157.1, 155.4, 127.8,
61.8, 60.9, 34.4, 14.3, 14.0, 12.0. ꢂmax 1741, 1710, 1620, 1371 cm–1.
Mass spectrum (m/z) 241 (27%), 214 (30), 184 (48), 168 (100), 138
(79), 113 (47), 69 (30), 43 (70).
12
13
14
15
16
17
Acknowledgments
18
19
The authors are grateful for support of this project by the
Australian Research Council. M.C. acknowledges the award
of a Flinders University Research Scholarship, and F.H.
acknowledges a travelling scholarship from the Ministry of
Higher Education, IR of Iran.
20
21
22
23
References
1
Ang, K. H., Prager, R. H., Smith, J. A., Weber, B., and Williams, C.
24
25
26
27
M., Tetrahedron Lett., 1996, 37, 675; Prager, R. H., Smith, J. A.,
Weber, B., and Williams, C. M., J. Chem. Soc., PerkinTrans. 1, 1997,
2665.
Prager, R. H., and Singh, Y., Tetrahedron, 1993, 49, 8147.
Prager, R. H., Taylor, M. R., and Williams, C. M., J. Chem. Soc.,
2
3
Perkin Trans. 1, 1997, 2673.
Graebe, C., and Ullmann, F., Justus Liebigs Ann. Chem., 1896, 291,
4
16.
5
Freudenburg, W., Heterocycl. Compd., 1952, 3, 298.
Hubert, A. J., J. Chem. Soc. C, 1969, 1334.
Wentrup, C., Helv. Chim. Acta, 1972, 55, 1613.
Rees, C. W., and Storr, R. C., J. Chem. Soc. C, 1969, 1478.
28
29
30
6
7
8
9
Katritzky, A. R., Rachwal, S., Caster, K. C., Mahni, F., Law, K. W.,
31
32
Di Santo, R., Costi, R., Massa, S., and Artico, M., Synth. Commun.,
1995, 25, 795.
Selic, L., and Stanovnik, B., Synthesis, 1999, 3, 479.
and Rubio, O., J. Chem. Soc., Perkin Trans. 1, 1987, 781; Katritzky,
A. R., Kuzmierkiewicz, W., Rachwal, B., Rachwal, S., and
Thomson, J., J. Chem. Soc., Perkin Trans. 1, 1987, 811.
* Ethyl 2-ethoxycarbonylmethyl-5-methyloxazole-4-carboxylate.

672
M. Cox, F. Heidarizadeh and R. H. Prager