Triplet lifetimes, solvent, and intramolecular capture of isoxazolones

Article abstract of DOI:10.1071/CH03140

The first step in the photochemical decarboxylation of isoxazolones is the formation of the triplet state of the isoxazolone. We present evidence for the first time from flash laser photolysis of the lifetime of such species, and examples of their capture by solvent and by intramolecular cycloaddition.

Full text of DOI:10.1071/CH03140

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2004, 57, 101–105
Triplet Lifetimes, Solvent, and Intramolecular Capture of Isoxazolones
A
A
A
A,B
A
Kiah H. Ang, Matthew Cox, Warren D. Lawrance, Rolf Prager,
Jason A. Smith, and
Warren StakerA
A
School of Chemistry, Physics and Earth Sciences, Flinders University, Adelaide SA 5001, Australia.
Author to whom correspondence should be addressed (e-mail rolf.prager@flinders.edu.au).
B
The first step in the photochemical decarboxylation of isoxazolones is the formation of the triplet state of the
isoxazolone. We present evidence for the first time from flash laser photolysis of the lifetime of such species, and
examples of their capture by solvent and by intramolecular cycloaddition.
Manuscript received: 22 May 2003.
Final version: 21 November 2003.
R1
Introduction
COPh
COPh
CO Et
2
N
N
N
O
O
For some years we have concentrated our attention on the
synthetic possibilities built into isoxazolones. These are basi-
cally of two types: base-induced reactions,
to the formation of a ketenimine that may be trapped intra-
or intermolecularly by nucleophiles, or reaction of carbenes,
O
R2
Ph
N
O
[
1–7]
O
R3
R2
which lead
CO Et
CO Et
2
2
R1
Ph
Ph
R3
1
5
6
2
3
4
7
H
H
CO Et
2
[
8–13]
generated from the isoxazolone by photochemical
or
We have reported that there is a cas-
cade of transformations that leads from the isoxazolone to
Ph
Br
[8,14,15]
thermal means.
PhCO Me
[
16]
H
CO Et Me
the key singlet carbene intermediate in the latter reactions,
and have presented evidence that the triplet isoxazolone and
carbene are important in this process.
2
Scheme 1.
[17]
However, we have
not previously had evidence for the triplet state of the initial
isoxazolone itself. In this paper we present kinetic evidence
for such species, and report the isolation of several products
derived directly from them.
Discussion
Since we had considerable evidence from product composi-
tion of isoxazolone photolyses that their reactivity and rate of
reaction is highly substituent-dependent, we initially exam-
ined the representative group of isoxazolones 1–4, Scheme 1,
by laser flash photolysis (LFP), hoping to observe evidence
fortheintermediatecarbene, forexample5. LFPof 1inaceto-
Ϫ0.25
0.00
0.25
0.50
0.75
1.00
1.25
1.50
7
−1
nitrile (Fig. 1) led to the formation (k = 4.6 ± 0.6 × 10 s at
Time (␮s)
◦
6
−1
2
5 C) and decomposition (k = 2.73 ± 0.06 × 10 s ) of the
Fig. 1. Laser flash photolysis of 2. The smooth curve is a fit to the
experimental trace.
triplet state of the isoxazolone itself. The rise time matches
that expected, given the pulse duration of the laser used
and the response time of the detector. Support for the multi-
plicity of the species was obtained by addition of the triplet
quencher, 9-methylanthracene, which resulted in immedi-
ate disappearance of the signal. While we were unable to
detect the carbenoid species 5 under these conditions, we
bromo compound 4 showed a considerably longer lifetime,
but clearly several species were now involved and it was not
possible to find detection wavelengths which would allow
deconvolution of the rise and fall of species involved. While
these measurements are clearly of a preliminary nature, they
arerelevantastheysupportourinterpretationoftheformation
and reaction of the isoxazolone in its triplet excited state.
[
13]
nonetheless isolated the derived oxazole product 6
from
the photolysis mixture. LFP of the isoxazolones 2 and 3
gave species with similar lifetimes to that of 1; that of the
©
CSIRO 2004
10.1071/CH03140
0004-9425/04/010101

1
02
K. H. Ang et al.
We had noticed that the product composition from
isoxazolones derived from the 3-ester 7, as from the cor-
arises from an alternative thermal pathway involving isomer-
ization of the isoxazolone to the imidoylketene, cyclization
and deoxygenation of which (Scheme 3) gives 11. Such
[18]
responding 1,2,3-triazoles,
was routinely the most com-
[11]
pathways have been observed previously.
plex, which suggested that with this substitution pattern,
intermediate lifetimes were longer and more comparable,
so several derivatives have been studied in more detail.
Treatment of 1-chloroisoquinoline with ethyl 4-methyl-2,5-
dihydroisoxazole-3-carboxylate 7, gave the isoxazolone 8,
Scheme 2, photolysis of which in acetonitrile at 300 nm gave
in low yield the carbene cyclization product 9 (13%), together
with the solvent capture product 10 (75%). While such
[
19]
We have previously reported
that the N-vinyl com-
pound 12, Scheme 4, when photolyzed in acetone or aceto-
nitrile at 300 nm gave the carbene adducts 13 and 14. We
have re-examined these products, and find they are actually
the [2 + 2]-cycloaddition products 15 and 16. Their com-
position was confirmed by mass spectrometry and infrared
spectroscopy (presence of the saturated lactone stretch at
−1
[
2 + 2]-cyclizations might be expected, we have not observed
1798 cm ). The regiochemistry of the addition in 15 was
supported by the chemical shift of C-3 at 111.3 ppm; the rele-
vant shifts in 16 were similar to those in 10. The acetone
acts as a sensitizer for the formation of triplet states in the
photolysis of 12, leading to higher yields of 15 than of 16.
Since the lifetime of the isoxazolone triplet state in these
systems appeared sufficient to allow reactions in competition
with decarboxlation to the carbene, we sought an intramole-
cular example to test this hypothesis.The conjugated ester 17,
them with other substitution patterns. The regioselectivity
1
3
was attested to by the C NMR spectrum of 10, in which C-3
oftheisoxazoloneresonatedat99.6 ppmandC-4at69.6 ppm.
The methylene group of the ester resonated as two doublets
of quartets, as expected for their diastereotopic nature.
In order to obtain a better source of the imidazoiso-
quinoline 9 for confirmation of structure, the isoxazolone
8
was subjected to pyrolysis under varying conditions.
[
20]
Attempted flash vacuum pyrolysis (FVP) was unsatisfactory,
Scheme 5, appeared to be the ideal model, and was synthe-
sized in two steps by treating the isoxazolone 7 with acrolein,
which yielded the N-alkylated product 18 (29%) and the
C-alkylated product 19 (41%). The aldehyde 18 was
treated with ethoxycarbonylmethylenetriphenylphosphorane
as 8 underwent considerable decomposition in the sublima-
◦
tion oven at only 130 C. While the small amount of material
that did sublime through to the pyrolysis furnace provided
us with the authentic sample of 9 for comparison, the partly
decomposed material in the sublimation furnace contained
the pyrimidine 11 and not the imidazole 9. Pyrimidine 11
◦
at 25 C to yield 17. While 17 decomposed under pyrolytic
conditions, photolysis at 300 nm in acetone gave the intra-
molecular [2 + 2]-cyclization product 20 as the sole isolable
material. NMR spectroscopy showed the presence of only a
single diastereomer, the relative stereochemistry and connec-
tivity of which was deduced by two-dimensional NMR stud-
ies (Table 1). In particular, the observation of nuclear Over-
hauser effect interactions between H and H and between
F D
E C
H and H confirms the stereochemistry of the cyclobutane
ring. It is interesting that the intramolecular cycloaddition
occurs to the total exclusion of reaction with the solvent.
While [2 + 2]-cycloadditions are generally believed to
proceed by a stepwise pathway involving triplet biradical
N
N
N
99.6 ppm
Me
N
N
CO Et
2
O
O
CO Et
N
2
N
CO Et
2
O
O
Me
Me
6
9.6 ppm
Me
8
9
10
Scheme 2.
O
N
O
N
N
O
N
C
∆
N
O
N
N
N
CO Et
Me
Me
Me
2
O
O
CO Et
CO Et
CO Et
2
2
2
O
Me
8
11
Scheme 3.
CO Et
CO Et
CO Et
CO Et
CO Et
2
2
2
2
2
1
11.3 ppm
100.9 ppm
Me
Me
N
N
N
N
N
CO Et
CO Et
2
2
O
CO Et
O
O
CO Et
CO Et
2
2
2
O
N
O
N
O
O
O
Me
Me
Me
Me
Me
Me
Me
Me
71.9 ppm
12
13
14
Scheme 4.
15
16

Triplet Lifetimes, Solvent, and Intramolecular Capture of Isoxazolones
103
HA
HB
HC
R
1
1
N
N
N
10
9
O
O
O
CO Et
CO Et
2
N
N
HD
H
2
O
O
CHCO Et
H
2
O
O
1
CO Et
2
O
CH2CH2CHO
CO Et
HE
HF
Me
O
2
4
5
Me
Me
CO Et
2
2
Me
12
OC
O
6
8
CO CH CH
1
1
7
8
Rϭ(E )-(CH CH CHϭCHCO Et)
2 2 3
2
2
2
3
Me
O
RϭCH CH CHO
19
20
21
2
2
H C 14
2
CH3
20a
Scheme 5.
Table 1. NMR correlations for compound 20a
AtomA
δC [ppm]
δH [ppm]
HMBC
COSY
ROESY
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
80.0
48.6
14.1
177.1
47.5
170.0
61.5
14.2
39.2
31.3
31.3
61.5
61.5
167.1
62.1
14.2
C-1 → HA, HE, HF, HD, H-3
C-2 → HE, HF, H-3
C-3 → HF
1.38
C-4 → HF, H-3
(HF) 2.85
C-5 → HE, HC, HF, HD, H-3
C-6 → HE, HF, HD, H-7
C-7 → H-8
HF → HE
HF → HD, H-8
4.26
1.29
(HE) 3.60
(HC) 2.40
(HD) 2.04
H-7 → H-8
H-8 → H-7
H-7 → H-8
H-8 → H-7
HE → HC
C-8 → H-7
C-9
C-9 → HA, HB, HE, HC, HD, H-3
C-10 → HE, HB, HF
C-10 → HE, HB, HF
C-11 → HE, HD
HE → HC, HD, HF
C-10
C-10
C-11
C-11
C-12
C-13
C-14
HC → HA, HB, HE, HD
HD → HA, HB, HE, HC
HA → HB, HC, HD
HB → HA, HC, HD
HC → HB, HE, HD
HD → HA, HF, HC
HA → HB, HD
(HA) 3.75
(HB) 3.30
C-11 → HE, HD
HB → HA, HC
C-12 → HE, H-13
C-13 → H-14
4.22
1.30
H-13 → H-14
H-14 → H-13
H-13 → H-14
H-14 → H-13
C-14 → H-13
A
Arbitrary numbering; see 20a.
21]
[
intermediates,
the stereospecificity in this case would
and high resolution mass spectra were recorded on a Kratos MS25RF
spectrometer. Microanalyses were performed by Chemical & Micro
Analytical Services, Melbourne. FVP studies were carried out by slowly
subliming the substrate through a silica tube (400 mm × 25 mm), packed
with silica chips, and heated to the quoted temperature under reduced
pressure. The products were collected in a liquid nitrogen cold trap.
Photolyses were performed under nitrogen using a Hanovia mercury
medium pressure arc source filtered through pyrex, to give an irradia-
tion source considered to be nominally 300 nm and above. Compounds
appear to preclude such a pathway. Nonetheless, the regio-
chemistry of the addition leading to 20 is that expected from
the observation that the major product in such reactions arises
from initial addition of the biradical in a manner that leads
to the formation of a five-membered ring, if possible,
shown in 21.
[
22]
as
[
24] [25] [26] [13]
[27]
and 7 were available from previous work.
1
,
2,
3,
4,
Conclusions
Laser Flash Photolysis
The presence of an ethoxycarbonyl group at C-3 of an isox-
azolone ring increases the lifetime of its triplet state to allow
A solution of the isoxazolone in distilled acetonitrile was flowed contin-
uously through a quartz cuvette which was irradiated with the output of a
XeCl excimer laser (308 nm; pulse duration approx. 20 ns; pulse energy
approx. 50 mJ). The excimer pulse initiates the photophysical or photo-
chemical process of interest. A continuous-wave argon ion laser operat-
ing at 488 nm provided the probe beam. The probe beam was split into
two beams before the sample cell to create a signal and reference beam.
The signal beam entered the solution orthogonal to the XeCl pulse. The
signal and reference beam intensities were monitored by photodiodes,
and the signals subtracted in a differential amplifier to yield the transient
absorption signal. This signal was observed on a digital oscilloscope
(HP54510), averaged, and downloaded to a computer for analysis and
curve fitting.The curves were fitted to a bi-exponential to determine rise
and decay times. Oscillations in the experimental traces are an artifact of
the differential amplifier, and do not affect the measured rate constants.
[
2 + 2]-cycloaddition reactions either intermolecularly with
molecules containing polar multiple bonds, or intramole-
cularly with carbon–carbon double bonds. The presence of
the easily reducible N O bond suggests these products may
have some synthetic interest.
Experimental
All solvents used were freshly distilled and dried according to the
[
23]
Melting points were determined on
methods of Perrin and Amarego.
a Reichert hot stage microscope and are uncorrected. H (300 MHz) and
1
13
C (75.5 MHz) NMR measurements were recorded on a GeminiVarian
00 spectrometer in deuteriochloroform using tetramethylsilane for cal-
3
ibration, unless otherwise stated. 2D NMR spectra were determined
on a Varian Unity 600 MHz spectrometer, using the pulse sequences
gHMBC, gHMQC, gCOSY, ROESY, andTOCSY. Infrared spectra were
recorded on a Perkin–Elmer 1600 FT-IR spectrophotometer, using fused
sodium chloride cells, measured as Nujol mulls or films. Mass spectra
Ethyl 2-(Isoquinolin-1-yl)-4-methyl-5-oxo-2,5-dihydroisoxazole-3-
carboxylate 8
A
mixture of the ethyl 4-methyl-5-oxo-1,5-dihydroisoxazole-3-
carboxylate7 (960 mg, 5.6 mmol)and1-chloroquinoline(1 g, 5.6 mmol)

1
04
K. H. Ang et al.
was refluxed in dichloroethane (20 mL) for 7 h. The solvent was
removed, and the product passed through a plug of silica with
dichloromethane to give a cream solid which was recrystallized from
2.16–2.09 (1H, m), 1.54 (3H, s), 1.39 (3H, t, J 7.1). δC 198.8, 178.7,
−
1
162.6, 157.9, 62.9, 48.3, 38.5, 27.5, 20.9, 13.9. vmax/cm 1809, 1728,
+
1584, 1451, 1403, 1376. m/z 228 ([M + H] , 8%), 208 (10), 182 (48),
◦
methyl t-butyl ether to give 8 as fine colourless needles, mp 125–127 C
170 (100), 156 (54), 143 (27), 110 (56), 95 (29), 83 (53).
(
90%). (Found: C 64.5, H 4.8, N 9.4%. C16H14N2O4 requires C 64.4,
H 4.7, N 9.4%). δH 8.53 (1H, dd, J 8, 1), 8.19 (1H, d, J 5.6), 7.84
1H, dd, J 8, 1), 7.71 (2H, dt, J 10, 1), 7.63 (1H, d, J 5.6), 4.26 (2H, q,
J 7), 2.26 (3H, s), 1.11 (3H, t, J 7). δC 170.8, 159.0, 151.6, 151.4, 140.1,
37.9, 131.1, 128.3, 126.5, 125.1, 123.5, 122.2, 113.3, 61.9, 13.6, 8.1.
The second fraction contained ethyl 2-(2-formylethyl)-4-methyl-5-
oxo-2,5-dihydroisoxazole-3-carboxylate 18 (191 mg, 29%), isolated as
a pale yellow oil. (Found: M 227.0798. C10H13NO5 requires M
+
+
(
227.0794). δH 9.74 (1H, t, J 0.96), 4.41 (2H, q, J 7.1), 4.08 (2H, t,
J 6.4), 2.8 (2H, dt, J 0.96, 6.4), 2.03 (3H, s), 1.39 (3H, t, J 7.1). δC
1
−1
+
−1
vmax/cm 1770, 1740, 1252, 1240, 1076, 846, 757, 738. m/z 298 (M ,
3%), 254 (27), 209 (16), 180 (37), 128 (100), 44 (36).
198.9, 170.9, 158.6, 151.4, 111.2, 62.6, 47.1, 39.9, 13.9, 8.4. vmax/cm
+
4
1808, 1730, 1624, 1444, 1411. m/z 227 (M , 22%), 184 (100), 171 (98),
1
56 (83), 143 (98), 125 (81), 110 (34), 95 (31), 82 (30), 67 (52).
Photolysis of 8
Ethyl 2-(4-Ethoxycarbonylbut-3-enyl)-4-methyl-5-oxo-2,5-
dihydroisoxazole-3-carboxylate 17
Isoxazolone 8 (200 mg, 0.67 mmol) was dissolved in acetonitrile
(
200 mL) and irradiated (300 nm) for 2 h. The solvent was removed
and the crude product purified by radial chromatography (ethyl acetate/
dichloromethane 3 : 97) to afford two products.
Isoxazolone 18 (104 mg, 0.45 mmol) and ethoxycarbonylmethylene-
triphenylphosphorane (159 mg, 0.45 mmol) were stirred at room tem-
perature in anhydrous tetrahydrofuran (3 mL) under N2 for 24 h. The
solvent was evaporated to give a deep orange red oil (260 mg), which was
purified by radial chromatography (20% ethyl acetate/light petroleum)
to give the title compound as a colourless oil (71 mg, 52%). (Found:
The first fraction was isolated as a gummy solid and iden-
tified as ethyl 2-(isoquinolin-1-yl)-5,6-dimethyl-4-oxo-2,7-diaza-3-
oxa[3.2.0]bicyclohept-6-ene-1-carboxylate 10 (170 mg, 75%). (Found:
+
+
M
339.1211. C18H17N3O4 requires M 339.1219). δH 8.45 (1H, d,
+
J 7.5), 7.69 (1H, d, J 8), 7.56 (1H, t, J 7.5), 7.39 (1H, t, J 7.5), 7.31 (1H,
d, J 7.5), 6.36 (1H, d, J 8), 4.30 (2H, m), 2.13 (3H, s), 1.53 (3H, s), 1.28
M 297.1215. C14H19NO requires 297.1212). δH 6.84 (1H, dt, J 7.0,
6
15.8), 5.85 (1H, dt, J 1.5, 15.7), 4.38 (2H, q, J 7.1), 4.15 (2H, q, J 7.1),
3.89 (2H, t, J 6.9), 2.53 (2H, dq, J 1.5, 6.9), 2.04 (3H, s), 1.38 (3H, t,
J 7.1), 1.25 (3H, t, J 7.1). δC 171.0, 165.8, 158.6, 150.9, 143.4, 124.2,
(
3H, t, J 7). δC 167.7, 165.7, 165.7, 148.5, 133.4, 133.0, 128.2, 128.1,
−
26.3, 125.5, 121.7, 109.7, 99.5, 69.5, 62.5, 30.4, 14.5, 14.1. vmax/cm
1
1
1
2
+
−1
765, 1732, 1659, 1601, 1735, 1303, 1266. m/z 339 (M , 29%), 297 (8),
66 (100), 225 (11), 197 (23), 169 (22), 155 (28), 128 (80), 96 (69).
The second fraction was identified as ethyl 3-methylimidazo[2,1-
110.7, 62.6, 60.3, 52.1, 29.2, 14.1, 13.9, 8.5. vmax/cm 1753, 1734,
+
1703, 1655, 1275, 1227, 1178. m/z 297 (M , 7%), 252 (23), 224 (21),
206 (8), 184 (100), 156 (28), 127 (4), 99 (3), 83 (12).
a]isoquinoline-2-carboxylate 9 (22 mg, 13%) by direct comparison with
the sample obtained below.
Photolysis of 17
Isoxazolone 17 (71 mg, 0.24 mmol) was photolyzed through pyrex at
Pyrolysis of 8
3
00 nm in dry acetone (150 mL) under N2 at room temperature for 3 h.
The isoxazolone 8 (100 mg, 0.33 mmol) was pyrolyzed under FVP con-
The solvent was evaporated and the residue purified by radial chro-
matography (10% ethyl acetate/light petroleum) to give diethyl 1-aza-
8-oxa-7-oxo-6-methyltricyclo[4.2.1.04 ]nonan-5,9-dicarboxylate 2 as
◦
◦
ditions (540 C, 130 C, 0.05 mmHg, 60 min). Only a small amount of
the isoxazolone had sublimed when the pyrolysis was stopped, as the
sample melted and appeared to decompose. The product was washed
,9
+
a colourless oil (47 mg, 66%). (Found: M 297.1213. C14H19NO6
from the pyrolysis tube, and eluted through a short plug of silica
requires 297.1212). δH (600 MHz) 4.26 (2H, dq, J 1.8, 7.14), 4.22 (2H,
dq, J 1.8, 7.4), 3.75 (1H, ddd, J 3.0, 6.6, 13.8), 3.60 (1H, ddd, J 4.2,
4.8, 9.0), 3.30 (1H, ddd, J 7.2, 9.6, 13.8), 2.85 (1H, d, J 4.0), 2.40 (1H,
dddd, J 3.6, 7.8, 9.0, 13.8), 2.04 (1H, dddd, J 5.4, 6.6, 9.6, 13.8), 1.38
C
◦
(
dichloromethane) to give 9 (17 mg, 20%), mp 154–155 C, identical to
[
11]
the sample obtained previously.
(Found: C 71.1, H 5.4, N 11.1%,
+
+
M
254.1060. C15H14N2O2 requires C 70.9, H 5.6, N 11.0%, M
2
4
1
9
6
54.1055). δH 8.78 (1H, d, J 7.2), 7.75–7.58 (4H, m), 7.15 (1H, d, J 7.4),
.50 (2H, q, J 7), 2.84 (3H, s), 1.48 (3H, t, J 7). δC 163.8, 141.8, 132.6,
29.6, 129.2, 128.7, 127.0, 124.4, 121.6, 120.1, 116.0, 114.9, 61.0, 14.5,
.5. vmax/cm 1700, 1656, 1570, 1311, 1283, 1218, 1153, 1069, 786,
97. m/z 254 (M , 42%), 210 (26), 180 (89), 144 (33), 128 (100).
(3H, s), 1.30 (3H, t, J 7.14), 1.29 (3H, t, J 7.14). δ 177.1, 170.0, 167.1,
−1
80.0, 62.1, 61.5, 61.5, 48.6, 47.5, 39.2, 31.3, 14.2, 14.2, 14.1. vmax/cm
1777, 1735, 1720, 1642, 1449, 1371, 1309, 1238. m/z 297 (M, 6%), 280
(3), 252 (5), 224, (18), 206 (4), 196 (7), 184 (28), 168 (5), 156 (14), 141
(100), 129 (7), 113 (73), 85 (20), 67 (5), 43 (29).
−
1
+
The material from the sublimation flask was passed through a
short plug of silica (dichloromethane) to give a solid (13 mg, 15%),
◦
mp 87–89 C, identified as ethyl 3-methyl-4-oxo-4H-pyrimido[2,1-
Acknowledgments
+
a]isoquinoline-3-carboxylate 11. (Found: M 282.1000. C16H14N2O3
+
requires M 282.1004). δH 9.03 (1H, dd, J 8.2, 1.6), 8.72 (1H, d,
The authors are grateful to the Australian Research Grants
Committee for support of this work.
J 7.7), 7.82–7.66 (3H, m), 7.28 (1H, d, J 7.7), 4.51 (2H, q, J 7), 2.39
3H, s), 1.47 (3H, t, J 7). δC 160.4 (vwk), 160.0, 134.4 (vwk), 132.9,
32.6, 128.9, 127.3, 126.5, 121.6, 116.8, 116.0, 62.0, 14.2, 12.4, two
resonances unsighted. vmax/cm 1732, 1682, 1519, 1248, 1215, 1062,
07, 763. m/z 282 (M , 49%), 253 (5), 236 (9), 208 (76), 182 (20),
(
1
−
1
References
+
8
1
[
[
1] R. H. Prager, C. M. Williams, Heterocycles 1999, 51, 3013.
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5
1, 491.
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[
3] M. Baradarani, R. H. Prager, K. Schafer, Aust. J. Chem. 1996,
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[4] D. Caiazza, R. H. Prager, K. Schafer, Aust. J. Chem. 1995, 48,
Isoxazolone 7 (500 mg, 2.9 mmol), acrolein (164 mg, 2.9 mmol), and tri-
ethylamine (30 mg, 0.3 mmol) were stirred in dichloromethane (10 mL)
at room temperature for 30 min. The solvent was evaporated to give an
orange oil (536 mg), which was purified by radial chromatography (50%
ethyl acetate/light petroleum) to give two fractions.
1861.
[5] K. H. Ang, R. H. Prager, C. M. Williams, Aust. J. Chem. 1995,
48, 55.
The first fraction contained ethyl 4-(2-formylethyl)-4-methyl-5-oxo-
[6] C. Donati, W. K. Janowski, R. H. Prager, M. R. Taylor,
L. M. Vilkins, Aust. J. Chem. 1989, 42, 2161.
[7] R. H. Prager, T. K. Rosenzweig, Y. Singh, Aust. J. Chem. 1992,
45, 1825.
4
(
2
,5-dihydroisoxazole-3-carboxylate 19, isolated as a pale orange oil
+
275 mg, 41%). (Found: [M + H] 228.0873. C10H14NO5 requires
28.0875). δH 9.66 (1H, s), 4.4 (2H, q, J 7.1), 2.48–2.30 (3H, m),

Triplet Lifetimes, Solvent, and Intramolecular Capture of Isoxazolones
105
[
[
8] R. H. Prager, D. S. Millan, Adv. Nitrogen Heterocycl. 2000, 4, 1.
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53, 665.
4
8, 567.
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Soc., Perkin Trans. 1 1997, 1, 2659. doi:10.1039/A700133I
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[
10] K. H. Ang, R. H. Prager, Tetrahedron Lett. 1992, 33, 2845.
doi:10.1016/S0040-4039(00)78875-7
[
[
[
11] Y. Singh, R. H. Prager, Aust. J. Chem. 1992, 45, 1811.
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16] D. W. Jeffery, T. Lister, R. H. Prager, Aust. J. Chem. 2003,
submitted.
[
[
[
[
[
17] K. H. Ang, R. H. Prager, Aust. J. Chem. 1998, 51, 483.
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