The chemistry of 5-oxodihydroisoxazoles. Part 22. The synthesis of 1,3-oxazin-6-ones from N-thioacylisoxazol-5(2H)-ones

Article abstract of DOI:10.1039/a804527e

N-Thioacylisoxazol-5(2H)-ones, prepared by the reaction of thiocarbonyl chlorides with isoxazol-5(2H)-ones in the presence of base, are reduced by triphenylphosphine to afford 1,3-oxazin-6-ones and triphenylphosphine sulfide. If the thioacylation is carried out with phenyl chlorodithioformate, the thermal rearrangement of the intermediate, to again form the oxazin-6-one and sulfur, is so rapid that the use of the phosphine is not required. The presence of an ethoxycarbonyl group at C-3, or of a bromine atom at C-4 of the isoxazolone results in the formation of thiazoles.

Full text of DOI:10.1039/a804527e

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The chemistry of 5-oxodihydroisoxazoles. Part 22.1 The synthesis
of 1,3-oxazin-6-ones from N-thioacylisoxazol-5(2H)-ones
David S. Millan and Rolf H. Prager*
Department of Chemistry, Flinders University of South Australia, GPO Box 2100, Adelaide,
Australia 5001
Received (in Cambridge) 15th June 1998, Accepted 4th August 1998
N-Thioacylisoxazol-5(2H)-ones, prepared by the reaction of thiocarbonyl chlorides with isoxazol-5(2H)-ones in the
presence of base, are reduced by triphenylphosphine to afford 1,3-oxazin-6-ones and triphenylphosphine sulfide. If
the thioacylation is carried out with phenyl chlorodithioformate, the thermal rearrangement of the intermediate, to
again form the oxazin-6-one and sulfur, is so rapid that the use of the phosphine is not required. The presence of an
ethoxycarbonyl group at C-3, or of a bromine atom at C-4 of the isoxazolone results in the formation of thiazoles.
Introduction
Over the last few years isoxazol-5(2H)-ones have been used to
develop new syntheses of a variety of heterocyclic systems
including pyrimidines,2 pyrroles, furans, thiophenes, and their
benzo analogues,3 and more recently oxazoles4 and thiazoles.5
Several reports6–13 have shown that these compounds may be
used as starting materials for simple and high yielding syntheses
of 1,3-oxazin-6-ones, but by totally different pathways to
those described herein. Alkynes and other dienophiles14 add
smoothly to 1,3-oxazin-6-ones in cycloaddition reactions and
hence 1,3-oxazin-6-ones are useful intermediates in the syn-
thesis of several heterocyclic systems15–21—it would therefore be
useful to find a general procedure for their synthesis. During the
synthesis of some N-thioacylisoxazol-5-ones5 1 we noted that
several 1,3-oxazin-6-ones 4 were produced concomitantly in
moderate to high yields. It was suggested5 that thermal loss of
sulfur occurred through either of the intermediates 2 or 3
(Scheme 1).
In our earlier report5 we were unable to predict the substitu-
tion patterns in N-thioacylisoxazolones 1 which would lead to
1,3-oxazin-6-ones 4 cleanly, and hence this paper has two
aims. The first is to determine the range of 1,3-oxazin-6-ones 4
formed thermally from the N-thioacylisoxazolones 1 and sec-
ondly to find a general procedure that converts the thermally
stable isoxazolones to 1,3-oxazin-6-ones.
Koketsu22 reported the reduction of several thioketones with
triphenylphosphine, and Davis23 reported that reaction of
ethylene sulfide with triphenylphosphine resulted in the elimin-
ation of sulfur and formation of olefins. More relevant is the
Corey–Winter reaction,24 in which a cyclic thionocarbonate,
derived from a vicinal diol, is converted to the alkene and
carbon dioxide in the presence of a phosphite. To the best
of our knowledge the reaction of phosphines with thio-
carbamates has not been reported, but extrapolation of the
above observations suggested a phosphine might desulfurise
N-thioacylisoxazolones 1, leading to 1,3-oxazin-6-ones 4,
involving either an intermediate carbene 524 or a zwitterionic
intermediate 6 (Scheme 2). We herein report that N-thioacyl-
isoxazolones generally react with triphenylphosphine at room
temperature or in boiling benzene to give 1,3-oxazin-6-ones.
Scheme 1
Results and discussion
Further synthesis of N-thioacylisoxazol-5-ones, 1
We have reported5 that isoxazol-5(2H)-ones 7 react with thio-
carbonyl chlorides 8 in the presence of amines, such as pyridine,
Scheme 2
J. Chem. Soc., Perkin Trans. 1, 1998, 3245–3252
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Scheme 3
Table 1 Thioacylation of isoxazol-5-ones
Thiocarbonyl
O-Acylation
Yield (%)
N-Acylation
Yield (%)
Isoxazolone
chloride
Amine base
10
15
PhSCSCl
Me2NCSCl
PhOCSCl
Pyridine
Pyridine
Triethylamine
N,N-Diisopropylethylamine
Triethylamine
N,N-Diisopropylethylamine
Triethylamine
N,N-Diisopropylethylamine
Triethylamine
N,N-Diisopropylethylamine
Pyridine
Pyridine
(0)
11 (16); 12 (82)
14 (60)
13 (17)
16 (27)a; 17 (5)
(0)
19 (9)
(0)
23 (67)
23 (29)
25 (16)
27 (15)
(0)
(0)
(0)
18 (65)a (72)b
18 (97)
PhOCSCl
Me2NCSCl
4-ClPhOCSCl
PhOCSCl
20 (45)
21 (99)
22
24 (27)
PhOCSCl
24 (43)
Me2NCSCl
4-ClPhOCSCl
PhOCSCl
4-ClPhOCSCl
PhOCSCl
26 (42)
28 (52)
29
32
30 (95)
31 (82)
Pyridine
Pyridine
33 (100)
34 (88)
4-ClPhOSCCl
(0)
a Yield obtained prior to isomerisation. b Yield obtained after isomerisation.
to give the N-thioacylated derivatives 1 in moderate to good
yields with little competing formation of the O-thioacylated
isomer 9. This procedure has now been extended by the syn-
thesis of several new N-thioacylisoxazol-5-ones (Scheme 3,
Table 1).
When the thioacylation of 3-methylisoxazol-5-one 15
was carried out in the presence of triethylamine,25 the N- and
O-thioacylated products 18 and 16 were obtained, contamin-
ated with a little 17. However, 16 was isomerised totally to the
N-acylated isoxazolone 18 in deuterochloroform overnight. The
thiocarbamate 17 arises by dealkylation of triethylamine by
phenyl chlorothionoformate,26–28 and hence N,N-diisopropyl-
ethylamine was used instead, giving the N-acylated material
18 essentially quantitatively.
N-Thioacylation of the more hindered 3-phenylisoxazolone
22 also proved difficult,25 as shown in Table 1, and only 27% of
the N-thioacylated isoxazolone 24 was obtained when triethyl-
amine was used as the base. None of the thiocarbamate 17 was
isolated from the reaction, presumably because thioacylation of
the isoxazolone 22 occurred faster than dealkylation of the
tertiary amine. However, when N,N-diisopropylethylamine
was employed, the ratio of N to O-thioacylation increased to
yield 43% of N-thioacylisoxazolone 24. O to N-acyl group
transfer could not be induced. As hoped, thioacylation on
nitrogen of the brominated isoxazolones 29 and 32 proceeded
smoothly without O-thioacylation, affording excellent yields of
N-thioacylated isoxazolones (Table 1).
Scheme 4
7, used herein, led to the formation of the corresponding 1,3-
oxazin-6-ones in varying yields (Scheme 4 and Table 2). The
yields of oxazines 36 and 37 were unexpectedly low, and it is
probable that the remaining material had undergone thioacyl-
ation at C-4, followed by decomposition during chromato-
graphic work-up. Reaction of phenyl chlorodithioformate with
the brominated isoxazolones 29 and 32 gave oxazines 38 (13%)
and 39 (36%), respectively, but also yielded the thiazoles 40
(14%) and 41 (36%),29 presumably by thermal extrusion of
carbon dioxide. This result is unprecedented as thiazoles had
previously been obtained only after photolysis of N-thioacyl-
isoxazolones.5 It is possible that the bromine atom lowers the
While the thiocarbonyl chlorides generally reacted with the
isoxazolones 7 to give mixtures of N and O-thioacylated prod-
ucts initially, as reported above, the reaction at room tempera-
ture of phenyl chlorodithioformate with all of the isoxazolones
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Table 2 Reaction of phenyl chlorodithioformate with isoxazol-5(2H)-
ones 7
Product
Yield (%)
Isoxazolone
Amine base
10
15
22
29
32
Pyridine
12 (94)
Triethylamine
Triethylamine
Pyridine
35 (32)
36 (29); 37 (7)
38 (13); 40 (14)
39 (36); 41 (36)
Pyridine
activation energy required for thermal extrusion of carbon
dioxide; the corresponding oxazole formation from 2-acyl-
isoxazolones requires flash vacuum pyrolysis at 500 ЊC.4
We previously5 reported the synthesis of ethyl 5-methyl-6-
oxo-2-phenylsulfanyl-6H-1,3-oxazine-4-carboxylate 12 after
reaction of isoxazolone 10 with phenyl chlorodithioformate at
80 ЊC, but when this reaction was repeated it was noted that the
1,3-oxazin-6-one 12 could be isolated in 94% yield, even when
the reaction was carried out at room temperature. However,
at 0 ЊC the presumed intermediate N-thioacylated isoxazolone
11 (16%), and 1,3-oxazin-6-one 12 (82%) were formed, but isox-
azolone 11 was totally converted to oxazine 12 at room tem-
perature over 2 h. This suggests that the oxazine 12, reported
from the photolysis of isoxazolone 11,5 arose not from a photo-
chemical process, but thermally. The reaction of isoxazolone 22
with phenyl chlorodithioformate gave the oxazines 36 and 37.
While the formation of oxazine 37 is unprecedented, it appears
to be consistent only with the pathway for oxazine synthesis
through the epoxythiazine intermediate 42, shown in Scheme 5.
Table 3 Synthesis of 1,3-oxazin-6-ones 4
Producta
Yield (%)
Isoxazolone
Conditions
43
14
47
18
20
21
24
26
28
30
31
33
34
58
61
16 h, r.t.
48 h, 110 ЊC, toluene
16 h, r.t.
16 h, r.t.
48 h, 110 ЊC, toluene
16 h, r.t.
16 h, r.t.
48 h, 110 ЊC, toluene
16 h, r.t.
16 h, r.t.
16 h, r.t.
44 (73)
45 (11); 46 (25)
48 (85)
49 (73)
50 (46)
51 (20) [82];b 52 (18)
53 (11) [66]b
54 (56)
55 (11) [43]b
Decomposition
Decomposition
56 (0) [71]b
57 (25)
16 h, r.t.
16 h, r.t.
16 h, r.t.
16 h, r.t.
59 (28); 60 (6)
62 (7); 63 (22); 64 (30)
a Isolated yield. b Decomposed on work-up; yield of crude material in
square brackets.
The oxazines 53, 55 and 56 were contaminated by triphenyl-
phosphine sulfide, even after chromatography. Since trifluoro-
acetic anhydride (TFAA) converts triphenylphosphine sulfide
to the more polar oxide,30 these mixtures were stirred with
TFAA which allowed removal of the phosphine oxide by
chromatography, but also led to decomposition of oxazine 56,
while oxazines 53 and 55 were isolated in low yield, due to some
decomposition. Subsequent reactions of the oxazines could
frequently be carried out in the presence of the phosphine
sulfide.
It was noted that oxazine 51 was partially converted to the
butenoate 52 during work-up (Scheme 7). The origin of the
required p-chlorophenoxide is unclear but probably arises by
decomposition of the excess thiocarbonyl chloride. When isox-
azolone 58 was reacted with triphenylphosphine no oxazine
product 65 could be detected. Instead two ring-opened com-
pounds 59 and 60 were isolated. The propenoate 60 probably
arises by attack of water on the intermediate oxazine 65 to give
Scheme 5
Synthesis of 1,3-oxazin-6-ones 4 with triphenylphosphine
The conversion of most N-thioacylisoxazolones 1 to 1,3-
oxazin-6-ones 4 proceeded smoothly by stirring the isoxazolone
with triphenylphosphine for 16 h at room temperature. The
results and conditions necessary are compiled in Table 3, and a
number of anomalies are discussed below.
In the reaction of 14 with triphenylphosphine, two com-
pounds were obtained: the basic thiazole 46 (25%), and the
neutral oxazine 45 (11%). This is the only time that a thiazole
has been produced from its corresponding N-thioacylisox-
azolone on reaction with triphenylphosphine. Since this reac-
tion appears to require both the amino and the ethoxycarbonyl
groups, it is unlikely to involve carbenoid intermediates, and
we suggest the pathway shown in Scheme 6, which is clearly
dependent on the presence of the ethoxycarbonyl group.
J. Chem. Soc., Perkin Trans. 1, 1998, 3245–3252
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Scheme 8
ature values31 for C5, C4, C2 and C6, respectively. Stretching
frequencies of νmax 1602 and 1456 cmϪ1 were also consistent
with known31 1,3-thiazin-6-ones.
The mechanism postulated in Scheme 2 for the phosphine
reaction has been suggested to involve either a carbene
intermediate 5 or the zwitterionic intermediate 6. Since all
isoxazolones 1 probably follow a single pathway, the above
observations support the pathway involving intermediate 6.
The conversion of the electron rich isoxazolones 20 and 26 to
their corresponding oxazines 50 and 54 required elevated
temperatures, suggesting that the initial step in the synthesis of
the 1,3-oxazin-6-ones 4 is nucleophilic attack of phosphorous
on the sulfur of the N-thioacylisoxazolone 1, as has been
assumed in Schemes 6 and 8.
Finally, spectral data obtained for the oxazine 45 was differ-
ent to that reported in our previous paper5 and it is now
thought that the product isolated from the photolysis reaction
was actually the unreacted O-thioacylated isoxazole 13. Hence
it is clear that oxazines arise by a thermal, and not a photo-
chemical process.
Scheme 6
In conclusion, the reaction of isoxazol-5(2H)-ones with thio-
carbonyl chlorides gives N-acylisoxazolones 1 which afford
1,3-oxazin-6-ones 4 in fair to good yields on treatment with
triphenylphosphine. The use of phenyl chlorodithioformate
leads to the formation of 1,3-oxazin-6-ones directly by thermal
loss of elemental sulfur.
Experimental
General Experimental procedures have been described previ-
ously.4 All commercially available thiocarbonyl chlorides were
purchased either from the Sigma/Aldrich Chemical Company
or Merck Chemicals. Ether refers to diethyl ether, and light
petroleum refers to the fraction boiling in the range 40–60 ЊC.
Typical thioacylation: 2-phenoxythiocarbonyl-3-methylisoxazol-
5(2H)-one 18
Phenyl chlorothionoformate (0.383 g; 0.31 mL; 2.22 mmol) and
triethylamine (0.22 g; 0.31 mL; 2.22 mmol) were added to a
solution of 3-methylisoxazol-5(2H)-one 1532 (0.2 g; 2.02 mmol)
in benzene (10 mL) and the solution stirred at room temper-
ature for 16 h. The solvent was removed and the residue diluted
with dichloromethane–ether (1:4) (10 mL) and washed with
water (5 mL), dried and evaporated. The residue was subjected
to radial chromatography (dichloromethane–light petroleum,
1:4) on silica. The first fraction, obtained as a cream solid, was
a mixture of 3-methyl-5-phenoxythiocarbonyloxyisoxazole 16
(27%) and O-phenyl N,N-diethylthiocarbamate 1726 (0.125 g,
5%).
Scheme 7
the malonate derivative 66 which spontaneously decarboxylates
to give the propenoate 60. The ready hydrolysis of the oxazine-
5-carboxylate 65 suggests that these compounds are particu-
larly labile at C-6.
The reaction of isoxazolone 61 with triphenylphosphine gave
three products, two of which were the malonate 62 and the
propenoate 64. The third product was the unprecedented
1,3-thiazin-6-one 63, which is thought to have originated by
the pathway shown in Scheme 8. The structure for 63 was
confirmed by 1H NMR analysis and more specifically by obser-
vation of the H-4 proton resonating at 8.66 ppm which is con-
sistent with literature values.31 Carbon-13 atoms resonating
at δC 112.0, 157.6, 176.5 and 189.7 are consistent with liter-
Isoxazole 16: (Found: Mϩ, 235.0304. C11H9NO3S requires M,
235.0303). Spectral data is given in Table 5.
When the mixture was allowed to stand in chloroform over-
night at room temperature, the isoxazole 16 isomerised totally
to the isoxazolone 18.
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Table 4 2-Thioacylisoxazol-5(2H)-ones,a 1
νmax/cmϪ1
(CO)
Compound
R1
R2
R3
Mp (ЊC)
Yield (%)
11
δH
δC
11
CO2Et
Me
PhS
Oil
1.31 (3H, t, J 7.0), 1.98 (3H, s), 6.9, 13.5, 63.5, 110.4, 128.0, NA
4.32 (2H, q, J 7.0), 7.40–7.63
(5H, m)
129.8, 131.2, 136.6, 145.9,
158.8, 165.9, 190.7
14
18
CO2Et
Me
Me
H
Me2N
PhO
Oil
60
97
1.36 (3H, t, J 7.2), 2.13 (3H, s), 7.8, 13.4, 43.0, 62.2, 114.4, 1766,
3.48 (3H, br s), 4.37 (2H, q,
151.2, 158.3, 169.7, 180.7
1739
J 7.2)
92–94
2.72 (3H, d, J 1.2), 5.50 (1H, q, 17.7, 97.6, 122.0, 127.0,
J 0.9), 7.09–7.12 (2H, m), 7.30– 129,7, 151.9, 158.7, 164.6,
7.35 (1H, m), 7.40–7.48 (2H, m) 176.6
1794
20
21
Me
Me
H
H
Me2N
66–68
45
99
2.45 (3H, s), 3.43 (6H, s), 5.36
(1H, s)
2.74 (3H, s), 5.53 (1H, d, J 0.9), 17.8, 98.0, 123.6, 129.8,
7.04–7.09 (2H, m), 7.39–7.44
(2H, m)
16.1, 43.1, 96.2, 165.0,
168.5, 177.0
1744
1773
4-ClPhO
98–100
132.7, 150.3, 158.9, 164.4,
176.2
24
26
28
30
31
33
34
Ph
Ph
Ph
Me
Me
Ph
Ph
H
PhO
141–143
148–150
146–148
124–126
154–156
154–156
Oil
43
42
52
95
82
99
88
5.71 (1H, s), 6.92–7.00 (2H, m), 98.7, 121.6, 126.9, 128.1,
1771
1745
1774
1780
1780
1776
1774
7.22–7.64 (8H, m)
128.5, 128.6, 129.6, 131.2,
152.0, 160.8, 165.3, 178.3
42.3, 43.6, 96.5, 127.2,
129.0, 129.3, 131.5, 168.2,
169.1, 180.3
H
Me2N
4-ClPhO
PhO
3.39 (3H, s), 3.62 (3H, s), 5.80
(1H, s), 7.41–7.52 (5H, m)
H
5.73 (1H, s), 6.84–6.92 (2H, m), 99.0, 123.2, 128.1, 128.6,
7.24–7.36 (2H, m), 7.38–7.58
(5H, m)
129.8, 131.3, 132.6, 150.4,
160.8, 165.2, 177.8
Br
Br
Br
Br
2.83 (3H, s), 7.09–7.16 (2H, m), 17.6, 90.9, 122.1, 127.4,
7.32–7.42 (1H, m), 7.43–7.51
(2H, m)
129.9, 152.0, 155.5, 161.7,
176.4
4-ClPhO
PhO
2.83 (3H, s), 7.03–7.11 (2H, m), 17.7, 91.2, 123.5, 130.0,
7.39–7.47 (2H, m)
133.0, 150.3, 155.5, 161.5,
175.8
6.86–6.94 (2H, m), 7.22–7.30
(1H, m), 7.30–7.40 (2H, m),
7.48–7.62 (5H, m)
6.8–6.9 (2H, m), 7.24–7.34
(2H, m), 7.44–7.60 (5H, m)
91.9, 121.6, 127.2, 127.9,
128.6, 128.8, 129.7, 131.5,
152.1, 156.3, 162.8, 177.6
92.0, 123.0, 128.5, 128.5,
128.7, 129.8, 131.5, 132.6,
150.2, 154.3, 162.0, 177.4
4-ClPhO
a All compounds gave satisfactory CHN analyses (solids) or high resolution mass spectral data (liquids).
Table 5 Thiocarbonyloxyisoxazoles, 9a
Compound
R1
R2
R3
Mp (ЊC)
Yield (%)
17
δH
δC
13
CO2Et
Me
Me2N
Oil
1.42 (3H, t, J 7.2), 2.07 (3H, s), 3.39
(3H), 3.44 (3H, s), 4.44 (2H, q, J 7.2)
6.3, 13.7, 39.0, 43.5, 61.6,
102.2, 156.3, 159.9, 165.1,
182.1
16
Me
H
PhO
Oil
27
2.31 (3H, s), 5.94 (1H, s), 7.15–7.20
(2H, m), 7.30–7.36 (1H, m), 7.42–7.48
(2H, m)
12.4, 90.8, 121.3, 127.3, 129.9,
153.3, 162.3, 165.7, 188.9
19
23
Me
Ph
H
H
Me2N
PhO
Oil
Oil
9
2.30 (3H, s), 3.34 (3H, s), 3.43 (3H, s),
5.80 (1H, s)
6.41 (1H, s), 7.16–7.22 (2H, m), 7.28–
7.36 (1H, m), 7.40–7.48 (5H, m), 7.78–
7.84 (2H, m)
12.4, 39.2, 43.6, 91.4, 162.2,
166.8, 182.7
88.5, 121.2, 126.5, 127.3, 128.7,
128.9, 129.9, 130.5, 153.3,
164.3, 166.2, 188.7
67
25
27
Ph
Ph
H
H
Me2N
91–92
97–98
16
27
3.53 (3H, s), 3.44 (3H, s), 6.29 (1H, s),
7.42–7.48 (3H, m), 7.76–7.84 (2H, m)
39.3, 43.6, 89.1, 126.5, 128.9,
129.2, 130.3, 164.1, 167.3,
182.6
88.5, 122.9, 126.6, 128.7, 129.0,
130.1, 130.6, 133.1, 151.7,
164.4, 166.1, 188.5
4-ClPhO
6.42 (1H, s), 7.12–7.20 (2H, m), 7.40–
7.52 (5H, m), 7.78–7.84 (2H, m)
a All compounds gave satisfactory CHN analyses (solids) or high resolution mass spectral data (liquids).
The second fraction was recrystallised from dichloro-
methane–ether–light petroleum as tan cubic crystals, identified
as the title compound 18 (0.31 g, 72%), mp 92–94 ЊC (Found: C,
56.4; H, 4.0; N, 6.1%; Mϩ, 235.0304. C11H9NO3S requires C,
56.2; H, 3.8; N, 6.0%; M, 235.0303); m/z 235 (M, <1%), 203 (5),
191 (23), 137 (40), 118 (25), 110 (100), 109 (22), 94 (25), 77 (64).
The spectral data is given in Table 4.
When the reaction was repeated using N,N-diisopropyl-
ethylamine (0.29 g; 0.357 mL; 2.22 mmol) in place of triethyl-
amine, the title compound 18 was obtained as tan cubic crystals
(0.46 g; 97%).
Further examples are given in Tables 4 and 5, and the
relevant base used may be seen from Table 1.
Reaction of 3-methylisoxazol-5(2H)-one 15 with phenyl
chlorodithioformate
The major fraction, identified as 4-methyl-2-phenylsulfanyl-6H-
1,3-oxazin-6-one 35, was obtained as a yellow solid which was
recrystallised from ether–light petroleum as yellow cubic
crystals (0.14 g; 32%), mp 89–90 ЊC (Found: C, 60.1; H, 3.9; N,
6.4. C11H9NO2S requires C, 60.3; H, 4.1; N, 6.4%); m/z 219
J. Chem. Soc., Perkin Trans. 1, 1998, 3245–3252
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Table 6 1,3-Oxazin-6-ones, 4a
νmax/cmϪ1
(CO)
Compound R1
R2
R3
Mp (ЊC)
Yield (%) δH
δC
12
38
CO2Et
Me
PhS
61–63
94
1.33 (3H, t, J 6.9), 2.18 (3H, s),
11.8, 13.9, 62.2, 117.8,
1751
4.32 (2H, q, J 6.9), 7.40–7.52
(3H, m), 7.58–7.66 (2H, m)
2.26 (3H, s), 2.29 (3H, s), 7.39–
7.52 (3H, m), 7.53–7.63 (2H, m)
125.4, 129.6, 130.5, 135.4,
148.5, 160.4, 163.8, 167.4
21.6, 24.2, 102.4, 125.3,
129.7, 130.8, 135.4, 155.4,
155.4, 161.5, 164.0, 167.0,
168.1
Me
Ph
Br
Br
PhS
PhS
140–144
Oil
13
1766
39
36
7.31–7.52 (6H, m), 7.57–7.65 (2H, 100.6, 110.8, 125.4, 125.5, 1759
m), 7.69–7.81 (2H, m)
128.0, 128.2, 129.6, 129.7,
129.7, 130.7, 131.2, 131.3,
133.8, 135.2, 135.4, 135.5,
156.5, 156.8, 157.1, 160.2,
166.6, 167.9
35
36
37
Me
Ph
Ph
H
PhS
PhS
PhS
89–90
32
29
7
2.12 (3H, d, J 1.2), 5.78 (1H, q,
J 0.9), 7.41–7.53 (3H, m), 7.58–
7.61 (2H, m)
6.35 (1H, s), 7.34–7.56 (6H, m),
7.62–7.78 (4H, m)
23.4, 102.9, 125.5, 129.6,
130.5, 135.5, 158.5, 166.5,
170.3
98.5, 126.0, 127.4, 128.9,
129.6, 130.5, 132.0, 133.9,
135.7, 159.4, 161.5, 170.9
112.6, 128.3, 128.4, 128.5, 1743
129.3, 129.4, 129.5, 129.8,
129.9, 130.5, 132.2, 134.1,
161.3, 167.9, 174.9
1762
1745
H
117–118
172–174
PhS
6.64–6.70 (2H, m), 7.00–7.07
(2H, m), 7.12–7.42 (11H, m)
44
45
48
49
CO2Et
CO2Et
CO2Et
Me
Me
Me
Me
H
PhO
90–92
39–40
73
11
85
73
1.32 (3H, t, J 6.9), 2.19 (3H, s),
4.31 (2H, q, J 7.2), 7.25–7.42
(5H, m)
1.39 (3H, t, J 7.2), 2.02 (3H, s),
3.14 (6H, br s), 4.38 (2H, q, J 7.2) 103.8, 154.5, 157.9, 161.4, 1722
11.6, 13.8, 62.2, 115.1,
120.9, 126.7, 129.7, 150.6, 1724
151.0, 156.8, 160.4, 164.0
1759,
Me2N
4-ClPhO
PhO
10.8, 13.9, 36.1, 37.1, 61.8, 1743,
165.5
88–90
1.34 (3H, t, J 6.9), 2.19 (3H, s),
4.33 (2H, q, J 6.9), 7.20–7.26
(2H, m), 7.34–7.42 (2H, m)
2.11 (3H, d, J 0.9), 5.83 (1H, q,
J 0.9), 7.20–7.26 (2H, m), 7.27–
7.33 (1H, m), 7.40–7.48 (2H, m)
11.7, 13.9, 62.3, 115.5,
122.4, 129.8, 132.2, 149.5, 1721
150.3, 156.6, 160.2, 163.9
23.6, 100.9, 121.1, 126.7,
129.8, 151.1, 158.7, 159.0,
169.4
1772,
190–192
1705
50
51
Me
Me
H
H
Me2N
82–86
Oil
46
2.07 (3H, d, J 0.6), 3.11 (6H, br s), 24.1, 35.9, 37.8, 92.8,
1749
1778
5.33 (1H, q, J 0.6)
159.2, 160.1, 170.9
4-ClPhO
20 (82)b
2.12 (3H, d, J 0.9), 5.84 (1H, q,
J 0.9), 7.15–7.22 (2H, m), 7.38–
7.43 (2H, m)
23.7, 101.2, 122.7, 129.9,
132.3, 149.5, 158.5, 158.8,
169.2
53
54
55
56
57
Ph
Ph
Ph
Ph
Ph
H
PhO
72–76
114–118
120–124
NA
11 (66)b
56
6.43 (1H, s), 7.24–7.58 (8H, m),
7.76–7.86 (2H, m)
97.0, 121.2, 126.7, 127.4,
128.9, 129.8, 132.2, 134.1,
151.3, 159.5, 159.5, 164.4
36.1, 37.3, 89.6, 127.2,
128.6, 131.3, 136.1, 159.3,
161.0, 166.3
1762
1740
1770
H
Me2N
4-ClPhO
PhO
3.19 (3H, s), 3.28 (3H, s), 5.98
(1H, s), 7.40–7.52 (3H, m), 7.92–
7.82 (2H, m)
H
11 (43)b
0 (71)b
25
6.44 (1H, s), 7.24–7.32 (2H, m),
7.38–7.58 (6H, m), 7.76–7.82
(2H, m)
7.24–7.31 (2H, m), 7.33–7.52
(8H, m)
97.2, 122.7, 127.3, 129.0,
129.9, 132.2, 132.4, 133.9,
149.7, 159.0, 159.2, 164.2
97.9, 120.8, 126.8, 128.0,
129.4, 129.8, 131.2, 135.3,
151.1, 156.4, 156.9, 162.2
Br
Br
4-ClPhO
140–141
7.21–7.29 (2H, m), 7.38–7.53 (5H, 109.2, 122.4, 128.3, 129.6, 1777
m), 7.78–7.84 (2H, m)
130.0, 131.5, 132.4, 133.9,
149.6, 155.6, 156.4, 159.0
a All compounds gave satisfactory CHN analyses (solids) or high resolution mass spectral data (liquids). b Yields of crude material in parentheses.
(M, 5%), 110 (100), 70 (13). Further spectral data is given in
Table 6.
The remaining material could not be identified.
N, 5.0%; Mϩ, 275.0797. C14H13NO5 requires C, 61.0; H, 4.7; N,
5.0%; M, 275.0794); m/z 275 (Mϩ, 19%), 182 (100), 154 (32), 94
(21), 83 (11), 82 (30), 77 (28). Further spectral data is given in
Table 6.
Typical reaction of 2-thiocarbonylisoxazolones with triphenyl-
phosphine: ethyl 5-methyl-6-oxo-2-phenoxy-6H-1,3-oxazine-4-
carboxylate 44
The compounds described in Table 6 were prepared by the
above method.
Ethyl 2-dimethylamino-5-methyl-6-oxo-6H-1,3-oxazine-4-carb-
oxylate 45
Triphenylphosphine (0.094 g; 0.36 mmol) was added to a solu-
tion of isoxazolone 435 (0.1 g; 0.33 mmol) in benzene (10 mL)
and the mixture was stirred in the dark under an atmosphere of
nitrogen for 16 h. The solvent was evaporated under reduced
pressure. The residue was subjected to radial chromatography
(dichloromethane–ether–light petroleum, 1:3:6) on silica,
giving the title compound 44 as white needles (0.065 g; 73%),
mp 90–92 ЊC (ether–light petroleum) (Found: C, 61.3; H, 4.8;
Triphenylphosphine (0.20 g; 0.77 mmol) was added to the mix-
ture of thioacylated compounds 13 and 14 (0.18 g; 0.70 mmol)
in toluene (5 mL) and the solution was refluxed under nitrogen
in the dark. After 48 h the solvent was evaporated and the
oil was subjected to radial chromatography on silica (ether–
dichloromethane–light petroleum, 1:1:8) to give a major frac-
3250
J. Chem. Soc., Perkin Trans. 1, 1998, 3245–3252

View Article Online
tion (0.07 g), which still contained two compounds. The oil was
redissolved in ether (10 mL) and washed with 2 M HCl (5 mL),
and the ethereal layer dried and the solvent evaporated. The
pale green solid was recrystallised from ether–light petroleum
as white needles (0.038 g; 11%), mp 39–40 ЊC, identified as the
title compound 45 (Found: C, 53.1; H, 6.0; N, 12.3%; Mϩ,
226.0953. C10H14N2O4 requires C, 53.1; H, 6.2; N, 12.4%; M
226.0955). Further spectral data is collected in Table 6.
The aqueous layer was basified (NaHCO3) and extracted
with ether (2 × 10 mL) and the extract dried and evaporated
affording a colourless oil identified as ethyl 2-dimethylamino-5-
methylthiazole-4-carboxylate 46 (0.036 g; 25%) by direct com-
parison with an authentic sample.5 (Found: Mϩ, 214.0777. Calc.
for C9H14N2O2: M, 214.0776).
m/z 299, 297 (Mϩ, 2, 3%), 218 (90), 190 (35), 188 (30), 144 (25),
110 (85), 109 (100). Other spectral data is collected in Table 6.
Reaction of 4-bromo-3-phenylisoxazol-5(2H)-one 32 with phenyl
chlorodithioformate
Phenyl chlorodithioformate (0.16 g; 0.12 mL; 0.83 mmol) and
pyridine (0.066 g; 0.067 mL; 0.83 mmol) were added to a solu-
tion of isoxazolone 3233 (0.2 g; 0.83 mmol) in benzene (10 mL)
and the solution was stirred in the dark under nitrogen. After
16 h at room temperature the residue was subjected to radial
chromatography, on silica (dichloromethane–light petroleum,
1:4). The first fraction was 5-bromo-4-phenyl-2-phenylsulfanyl-
1,3-thiazole 41, white needles (105 mg; 36%), mp 72–75 ЊC
(ether–light petroleum) (Found: Mϩ, 346.9437. C15H1081BrNS2
requires M, 346.9439); δH 7.80–7.48 (6H, m), 7.60–7.70 (2H, m),
7.84–7.96 (2H, m); δC 103.0, 128.3, 128.4, 128.4, 128.6, 128.6,
128.7, 130.1, 130.1, 130.2, 130.3, 130.6, 130.7, 132.5, 133.0,
134.5, 134.6, 150.5, 153.2, 163.6, 166.7; νmax/cmϪ1 1560, 1424,
1150; m/z 348, 346 (Mϩ, 21, 19%), 305 (45), 304 (33), 303 (100),
302 (44), 268 (12), 218 (20), 168 (47), 132 (91), 121 (17), 110
(25), 109 (56), 89 (52). The 13C NMR spectrum at 40 ЊC
(CDCl3) reduced the six resonances seen at 22 ЊC i.e. 128.3,
128.4, 128.4, 128.6, 128.6, 128.7 ppm to four resonances at
128.4, 128.5, 128.7, 128.8 ppm. In addition, the remaining
resonances approached coalescence at 40 ЊC.
Reaction of isoxazolone 21 with triphenylphosphine
Isoxazolone 21 (0.1 g; 0.37 mmol) and triphenylphosphine (0.11
g; 0.41 mmol) were reacted in the usual way. Radial chrom-
atography (dichloromethane–light petroleum, 1:4) on silica
gave two fractions: the first was 4-chlorophenyl (Z)-3-[(4-
chlorophenoxy)carbonylamino]but-2-enoate 52 as white needles
(0.024 g; 18%), mp 141–142 ЊC (ether–light petroleum)
(Found: Mϩ Ϫ C6H4ClO, 238.0262. C11H935ClNO3 requires M,
238.0271); δH 2.12 (3H, d, J 0.9), 5.35 (1H, s), 7.06–7.16 (2H,
m), 7.30–7.40 (2H, m), 11.27 (1H, br s); δC 21.3, 95.6, 122.9,
123.2, 129.6, 131.4, 131.5, 148.79, 148.9, 150.8, 157.1, 167.5;
νmax/cmϪ1 3320, 1762, 1698, 1490, 1269, 1198, 1152; m/z 238
(M Ϫ C6H4ClO, 11%), 139 (14), 128 (54), 116 (13), 110 (100),
100 (32), 88 (21).
The structure was confirmed by single crystal X-ray analysis.
The second fraction was 5-bromo-4-phenyl-2-phenylsulfanyl-
6H-1,3-oxazin-6-one 39 (107 mg; 36%), obtained as a thick
yellow–green oil (Found: Mϩ, 358.9615, 360.9563; C16H10-
79BrNO2S, C16H1081BrNO2S require M, 358.9616, 360.9596);
m/z 361, 359 (Mϩ, 1, 1%), 252 (7), 250 (10), 218 (27), 206 (13),
110 (77), 109 (34), 105 (100). Additional spectral data is found
in Table 6.
The second fraction was a colourless oil, identified as 2-(4-
chlorophenoxy)-4-methyl-6H-1,3-oxazin-6-one 51 (0.018 g;
20%) (Found: Mϩ, 237.0191. C11H835ClNO3 requires M,
237.0193); m/z 237 (M, 3%), 128 (23), 110 (100). Further spec-
tral data is given in Table 6.
Reaction of ethyl 5-oxo-2-phenoxythiocarbonyl-2,5-dihydro-
isoxazole-4-carboxylate 58 with triphenylphosphine
2-Phenoxy-4-phenyl-6H-1,3-oxazin-6-one 53
Isoxazolone 24 (0.1 g; 0.34 mmol) was reacted with triphenyl-
phosphine (0.097 g; 0.37 mmol) in the usual way. Trifluoroacetic
anhydride (2 mL) was added to the solid residue (0.2 g) in
dichloromethane (10 mL) and the solution stirred for 6 h. The
solvent was evaporated under reduced pressure and the oil was
subjected to radial chromatography (dichloromethane–light
petroleum, 1:4) on silica. The title compound was isolated as a
pale green oil which later solidified (10 mg, 11%), mp 72–76 ЊC
(Found: Mϩ, 265.0742. C16H11NO3 requires M, 265.0739); m/z
265 (M, 2%), 204 (10), 172 (52), 146 (46), 105 (39), 94 (100).
Further spectral data is collected in Table 6.
Isoxazolone 585 (0.1 g; 0.34 mmol) was reacted with triphenyl-
phosphine (0.098 g; 0.38 mmol) in the usual way. The product
was subjected to radial chromatography (dichloromethane–
light petroleum, 1:9), on silica. The first fraction was ethyl
phenyl (Z)-2-(phenoxycarbonylaminomethylene)malonate 59
(0.034 g, 28%), mp 110–112 ЊC (dichloromethane–light petrol-
eum) (Found: Mϩ Ϫ PhO, 262.0717. C13H12NO5 requires M,
262.0715); δH 1.39 (3H, t, J 7.2), 4.37 (2H, q, J 7.2), 7.13–7.35
(3H, m), 7.35–7.50 (2H, m), 8.70 (1H, d, J 12.3), 10.80 (1H, d,
J 12.3); δC 14.0, 61.6, 101.5, 121.1, 121.9, 125.9, 126.6, 129.5,
129.8, 149.7, 150.2, 150.2, 151.1, 162.7, 167.3; νmax/cmϪ1 3350,
1769, 1741, 1725, 1683, 1609, 1458, 1377, 1243, 1179; m/z 355
(Mϩ, 1%), 262 (31), 168 (51), 142 (11), 140 (10), 96 (13), 94
(100).
Reaction of 4-bromo-3-methylisoxazol-5(2H)-one 29 with phenyl
chlorodithioformate
The second fraction was ethyl (E)-3-(phenoxycarbonyl-
amino)prop-2-enoate 60, white needles (5 mg; 6%), mp 136–
138 ЊC (dichloromethane–light petroleum) (Found: Mϩ,
235.0844. C12H13NO4 requires, M 235.0845); δH 1.31 (3H, t,
J 7.2), 4.28 (2H, q, J 7.2), 6.13 (1H, br s), 7.08–7.28 (3H,
m), 7.32–7.42 (2H, m), 8.27 (1H, dd, J 8.7, 8.7), 8.86 (1H, br s);
δC 14.2, 60.1, 91.3, 122.2, 125.4, 129.4, 151.2, 159.0, 164.6,
168.8; νmax/cmϪ1 3373, 3287, 3229, 1699, 1670, 1513, 1338, 1284;
m/z 235 (Mϩ, 2%), 190 (5), 142 (100), 114 (21), 98 (11), 94 (44).
Phenyl chlorodithioformate (0.21 g; 0.16 mL; 1.12 mmol) and
pyridine (0.089 g; 0.091 mL; 1.12 mmol) were added to a solu-
tion of isoxazolone 2925 (0.2 g; 1.12 mmol) in benzene (10 mL)
and the solution was stirred in the dark under nitrogen. After
16 h at room temperature the residue was subjected to radial
chromatography, on silica, (dichloromethane–light petroleum,
1:4). The first fraction was a yellow oil (45 mg; 14%) identified
as 5-bromo-4-methyl-2-phenylsulfanyl-1,3-thiazole 40 (Found:
Mϩ, 284.9281. C10H879BrNS2 requires M, 284.9282); δH 2.34
(3H, s), 2.35 (3H, s), 7.38–7.48 (3H, m), 7.58–7.66 (2H, m); δC
14.4, 15.6, 104.1, 130.0, 130.0, 131.1, 131.3, 134.0, 134.1, 150.0,
152.7, 162.3, 165.3; νmax/cmϪ1 1476, 1440, 1409, 1374, 748, 691;
m/z 287, 285 (Mϩ, 72, 74%), 241 (75), 206 (60), 121 (36), 109
(51), 77 (43), 69 (100).
Reaction of ethyl 5-oxo-2-phenylsulfanylthiocarbonyl-2,5-
dihydroisoxazole-4-carboxylate 61 with triphenylphosphine
Isoxazolone 615 (0.1 g) was reacted with triphenylphosphine
(0.093 g) in the usual way. The product was subjected to radial
chromatography (dichloromethane–ether–light petroleum,
1:3:7), on silica. The first fraction was ethyl hydrogen (E)-2-
(phenylsulfanylcarbonylaminomethylene)malonate 62, white
needles (0.007 g; 7%), mp 90–92 ЊC (ether–light petroleum)
The second fraction, a light brown solid (42 mg; 13%), mp
140–144 ЊC was 5-bromo-4-methyl-2-phenylsulfanyl-6H-1,3-
oxazin-6-one 38 (Found: Mϩ, 296.9473, 298.9431; C11H8-
79BrNO2S, C11H881BrNO2S requires M, 296.9460, 298.9440);
J. Chem. Soc., Perkin Trans. 1, 1998, 3245–3252
3251

View Article Online
(Found: Mϩ Ϫ Ph, 218.0123. C7H8NO5S requires M, 218.0125);
δH 1.38 (3H, t, J 7.2), 4.37 (2H, q, J 7.2), 7.48–7.62 (5H, m), 9.07
(1H, d, J 11.7), 11.66 (1H, d, J 11.7); δC 14.0, 62.5, 96.7, 125.2,
130.1, 131.1, 135.5, 147.6, 167.9, 167.9, 170.0; νmax/cmϪ1 3210,
1719, 1693, 1594, 1459, 1377, 1264; m/z 218 (Mϩ Ϫ Ph, 30%),
168 (10), 142 (19), 141 (12), 110 (100), 109 (70), 96 (11).
The second fraction was a yellow oil (0.021 g; 22%), ethyl 2-
phenylsulfanyl-6-oxo-6H-1,3-thiazine-5-carboxylate 63 (Found:
Mϩ Ϫ EtO, 247.9847. C11H6NO2S2 requires M, 247.9840);
δH 1.34 (3H, t, J 7.2), 4.33 (2H, q, J 7.2), 7.50–7.66 (5H, m), 8.66
(1H, s); δC 14.1, 61.8, 112.0, 124.5, 130.5, 132.0, 136.5, 157.6,
163.5, 176.5, 189.7; νmax/cmϪ1 1735, 1713, 1684, 1602, 1456,
1275, 1111, 983; m/z 293 (Mϩ, 3%), 248 (5), 218 (12), 184 (100),
110 (99).
The third fraction was a white solid (0.024 g; 30%), identified
as ethyl (E)-3-(phenylsulfanylcarbonylamino)prop-2-enoate 64,
mp 94–96 ЊC (ether–light petroleum) (Found: Mϩ, 251.0610.
C12H13NO3S requires M, 251.0616); δH 1.43 (3H, t, J 7.2), 4.36
(2H, q, J 7.2), 7.26–7.50 (5H, m), 8.13 (1H, dd, J 8.4, 8.4), 9.40
(1H, br s); δC 14.4, 60.1, 99.9, 129.0, 129.2, 129.9, 135.8, 156.6,
166.5, 190.7; νmax/cmϪ1 3372, 3235, 1664, 1616, 1500, 1379,
1325, 1287, 1151, 923; m/z 251 (Mϩ, 4%), 206 (4), 142 (100), 114
(34), 110 (40), 109 (30), 98 (14).
7 E. M. Beccalli, T. Benincori and A. Marchesini, Synthesis, 1988, 8,
630.
8 E. M. Beccalli and A. Marchesini, J. Org. Chem., 1987, 52, 3426.
9 E. M. Beccalli, M. L. Gelmi, T. Pilati and A. Marchesini, J. Org.
Chem., 1987, 52, 1666.
10 E. M. Beccalli, A. Marchesini and H. Molinari, Tetrahedron Lett.,
1986, 27, 3426.
11 E. M. Beccalli, C. La Rosa and A. Marchesini, J. Org. Chem., 1984,
49, 4287.
12 E. M. Beccalli, B. Gioia and A. Marchesini, J. Heterocycl. Chem.,
1980, 17, 763.
13 F. Caruso, F. Foti, G. Grassi, G. Lo Vecchio and F. Risitano,
J. Chem. Soc., Perkin Trans. 1, 1979, 1522.
14 E. Buschmann, E. Jeschke and W. Steglich, Gazz. Chim. Ital., 1986,
116, 361.
15 T. Aoyama, T. Shioiri and K. Takaoka, Tetrahedron Lett., 1996, 37,
4973.
16 P. Meffert, R. Neidlein and Z. Sui, Synthesis, 1992, 6, 443.
17 R. Neidlein and Z. Sui, Synthesis, 1991, 8, 658.
18 D. L. Boger and R. J. Wysocki, Jr., J. Org. Chem., 1989, 54, 714.
19 E. Buschmann and W. Steglich, Angew. Chem., Int. Ed. Engl., 1974,
13, 484.
20 E. Buschmann, O. Hollitzer and W. Steglich, Angew. Chem., Int. Ed.
Engl., 1974, 13, 533.
21 G. Hofle, O. Hollitzer and W. Steglich, Angew. Chem., Int. Ed. Engl.,
1972, 11, 720.
22 M. Tamano and J. Koketsu, Bull. Chem. Soc. Jpn., 1985, 58, 2577.
23 R. R. Davis, J. Org. Chem., 1958, 23, 1767.
24 E. J. Corey and R. A. E. Winter, J. Am. Chem. Soc., 1963, 85, 2677;
E. J. Corey, F. A. Carey and R. A. E. Winter, J. Am. Chem. Soc.,
1965, 87, 934.
Acknowledgements
The authors are grateful for support from the Australian
Research Council. D. S. M. acknowledges an Australian Post-
graduate Award. The authors are also grateful for the single
crystal X-ray analysis carried out by Dr M. R. Taylor and
Shona-Marie Norman.
25 R. H. Prager, J. A. Smith, B. Weber and C. M. Williams, J. Chem.
Soc., Perkin Trans. 1, 1997, 2659.
26 D. S. Millan and R. H. Prager, Tetrahedron Lett., 1998, 39,
4387.
27 J. D. Hobson and J. G. McCluskey, J. Chem. Soc. (C), 1967,
2015.
28 P. Pfaffli and H. Hauth, Helv. Chim. Acta, 1973, 56, 347.
29 The connectivity of the molecule 41 was confirmed by a single
crystal X-ray analysis, but the accuracy of the atomic coordinates
does not justify their publication.
30 J. Helinski, Z. Skrzypczynski, J. Wasiak and J. Michalski,
Tetrahedron Lett., 1990, 28, 4081.
References
1 Part 21, M. M. Baradarani, A. Clark and R. H. Prager, Aust. J.
Chem., 1998, 51, 491.
2 R. H. Prager and Y. Singh, Aust. J. Chem., 1994, 47, 1263.
3 R. H. Prager and C. M. Williams, Aust. J. Chem., 1996, 49, 1315.
4 K. H. Ang, R. H. Prager, J. A. Smith, B. Weber and C. M. Williams,
Tetrahedron Lett., 1996, 37, 675; R. H. Prager, J. A. Smith, B. Weber
and C. M. Williams, J. Chem. Soc., Perkin Trans. 1, 1997, 2665.
5 R. H. Prager, M. R. Taylor and C. M. Williams, J. Chem. Soc.,
Perkin Trans. 1, 1997, 2673.
31 H. Quiniou and O. Guilloton, Adv. Heterocycl. Chem., 1990, 50, 85.
32 R. Jacquier, C. Petrus, F. Petrus and J. Verducci, Bull. Soc. Chim. Fr.,
1970, 7, 2685.
33 T. Posner, Ber. Dtsch. Chem. Ges., 1906, 39, 3515.
6 N. Almirante, M. L. Gelmi and C. Scarpellini, Heterocycles, 1991,
32, 1181.
Paper 8/04527E
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J. Chem. Soc., Perkin Trans. 1, 1998, 3245–3252