1,7-Electrocyclisation of non-stabilised α,β:y,o-unsaturated azomethine ylides

Article abstract of DOI:10.1039/a904571f

The generation of α,β:γ,5-unsaturated azomethine ylides 8 is described, starting from TV-substituted α-amino acids 7 and aldehydes 6,12. These azomethine ylides undergo a 1,7-electrocyclisation, followed by a [l,5]-hydrogen shift, to give the dihydrobenza

Full text of DOI:10.1039/a904571f

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1,7-Electrocyclisation of non-stabilised ꢀ,ꢁ:ꢂ,ꢃ-unsaturated
azomethine ylides
Andrea Arany,a David Bendell,b Paul W. Groundwater,*b Ian Garnettb and Miklós Nyergesc
a Department of Chemistry, University of Veterinary Science, H-1400 Budapest,
P.O.B. 2, Hungary
b Institute of Pharmacy and Chemistry, School of Sciences, University of Sunderland,
Sunderland, UK SR1 3SD
c Research Group of the Hungarian Academy of Sciences, Department of Organic Chemical
Technology, Technical University of Budapest, H-1521 Budapest, P.O.B. 91, Hungary
Received (in Cambridge, UK) 8th June 1999, Accepted 26th July 1999
The generation of α,β:γ,δ-unsaturated azomethine ylides 8 is described, starting from N-substituted α-amino acids
7 and aldehydes 6, 12. These azomethine ylides undergo a 1,7-electrocyclisation, followed by a [1,5]-hydrogen shift,
to give the dihydrobenzazepines 10a–f and 13.
The 1,7-electrocyclisation of dipolar intermediates constitutes
a general route to a variety of seven-membered heterocycles.1
Very few examples of the 1,7-electrocyclisations of azomethine
ylides have, however, been reported. Azomethine ylides have
been postulated as intermediates in some complex, multi-
step rearrangements but the only definitive examples are the
thermal ring-opening of the aziridine 1 to give the azomethine
ylide 2, which subsequently cyclises to the dihydroazepine 3
(Scheme 1),2 and the recent report by Marx and Eberbach on
Scheme 1
the 1,7-electrocyclisations of butadienyl ylides 4 to dihydro-
pyrido[2,1-a]azepines 5 (Scheme 2).3
Scheme 3 Reagents and conditions: i, p-xylene, reflux.
the condensation of N-substituted α-amino acids 7 with 3,3-
diarylpropenals 6 (Scheme 3). For example, when a mixture of
β-phenylcinnamaldehyde 6a and sarcosine 7a was refluxed
for 30 h in p-xylene, the 2,3-dihydro-2-methyl-5-phenyl-1H-
2-benzazepine 10a was obtained in 85% yield, after column
chromatography on neutral alumina. The dihydrobenzazepine
10a arises from a 1,7-electrocyclisation, to give the inter-
mediate 9, followed by a [1,5]-hydrogen shift, leading to the
re-aromatisation of the benzene ring.
Scheme 2
The intermediacy of an azomethine ylide in this process was
shown by the trapping of ylide 8a with N-phenylmaleimide to
give the two isomeric cycloadducts 11a and 11b (exo:endo ratio
1:1) (Scheme 4). The structure and stereochemistry of the exo-
We wish to report here the generation of non-stabilised
α,β:γ,δ-unsaturated azomethine ylides by the decarboxylation
method, and their subsequent cyclisation to dihydro-2-
benzazepines.4
1
cycloadduct 11a was established by 2D-COSY and H NOE
experiments. The irradiation of H-3α gave a large enhancement
of H-2a and one of the H-5 methylene protons—while the
irradiation of H-3 gave an enhancement of only the N-Me
singlet.
Results and discussion
Our initial studies involved the generation of non-stabilised
The use of the 4-alkoxy substituted derivatives 6b,c, pre-
pared from the corresponding substituted benzophenone and
azomethine ylides 8 by the decarboxylation method,5 involving
J. Chem. Soc., Perkin Trans. 1, 1999, 2605–2608
This journal is © The Royal Society of Chemistry 1999
2605

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Table 1 Reaction times and % yields for 1,7-electrocyclisations of
azomethine ylides 7
Azomethine
ylide 8
Dihydrobenzazepine
10
Reaction
time/h
Yield
(%)
8a
8b
8c
8d
8e
8f
10a
10b
10c
10d
10e
10f
30
26
24
28
28
30
85
94
95
42
38
33
Scheme 6
uncorrected. Elemental analyses were performed on a Perkin-
Elmer 240C or Carlo Erba 1106 Elemental Analyser. IR spectra
were recorded on a Perkin-Elmer 1600 series FTIR or Unicam
research series FTIR spectrophotometer using sodium chloride
plates. 1H NMR spectra were acquired on a JEOL GSX 270 FT
NMR at 270 MHz. Coupling constants are given in Hz and all
chemical shifts are relative to an internal standard of tetra-
methylsilane. 13C NMR spectra were obtained on a JEOL
GFX 270 FT NMR (68 MHz) spectrometer. Low resolution
electron impact mass spectra were obtained on a Trio 2000 VG
spectrometer. High resolution spectra were obtained on a VG
ZAB-E spectrometer (EPSRC Mass Spectrometry Service
Centre, Swansea). Thin layer chromatography was performed
on Merck silica gel 60F254. All solvents were purified according
to standard procedures. Diethyl ether and tetrahydrofuran were
freshly distilled over sodium wire with a trace of benzophenone.
p-Xylene was distilled from, and stored over, sodium wire.
Fisons silica gel 60 (35–70 micron) was used for wet flash
chromatography. The samples were applied in liquid form or
were pre-adsorbed onto silica gel 60 (35–70 micron) from
dichloromethane solutions. The 4-alkoxy substituted propenals
6b,c were prepared from the corresponding substituted benzo-
phenone and diethyl 2-(cyclohexylamino)vinylphosphonate by
the method of Nagata et al.,6 and biphenyl-2-carbaldehyde 12
was prepared by the method of Cullen and Sharp.7
Scheme 4 Reagents and conditions: i, N-phenylmaleimide, PhCH3,
reflux, 3 h.
diethyl 2-(cyclohexylamino)vinylphosphonate by the method
of Nagata et al.,6 with sarcosine 7a also gave azomethine ylides,
which subsequently gave the corresponding 2,3-dihydro-2-
methyl-1H-2-benzazepines 10b,c in excellent yields (Table 1).
We next chose to form the azomethine ylides from the
propenals 6 and the cyclic secondary α-amino acids proline 7b
or pipecolinic acid 7c, allowing the formation of the pyrrolo-
[2,1-a][2]benzazepine 10d,e or pyrido[2,1-a][2]benzazepine 10f
ring systems in a single step (Scheme 3). The moderate yields
1,7-Electrocyclisations of azomethine ylides—general procedure
A solution of the N-substituted α-amino acid (1 mmol) and
β-phenylcinnamaldehyde (0.5 mmol) was refluxed in p-xylene
(5 cm3) for 24–30 h, under nitrogen. After cooling, the solvent
was evaporated under reduced pressure. The residue was
purified using column chromatography on neutral alumina,
with petrol–ethyl acetate 2:1 as the eluent, to give the
dihydrobenzazepine.
Scheme 5 Reagents and conditions: i, p-xylene, reflux.
(Table 1) for the formation of these more complex ring systems
are due to decomposition on chromatographic separation, even
on neutral alumina.
We have attempted to extend this work to the reaction of
biphenyl-2-carbaldehyde 12, prepared by the method of Cullen
and Sharp,7 with either sarcosine 7a or proline 7b (Scheme 5).
The reaction of aldehyde 12 with sarcosine 7a gave the corre-
sponding dihydrodibenz[c,e]azepine 138 in low yield—the lower
yield for this process presumably resulting from a combination
of the losses upon chromatography and the loss of aromaticity
in two aromatic rings on undergoing the initial 1,7-electro-
cyclisation. The reaction of the aldehyde 12 with proline gave a
product which was not the expected dihydrodibenz[c,e]azepine
14. Spectroscopic analysis revealed that this new product was
an oxazolidine 16 (obtained as a single isomer), resulting from
the 1,3-cycloaddition of the azomethine ylide 15 to the pre-
cursor aldehyde 12 (Scheme 6). The stereochemistry of the
oxazolidine 16 was confirmed by comparison of the 1H
chemical shifts and coupling constants with those of analogues
prepared previously.9 All attempts to obtain the dihydrodi-
benz[c,e]azepine 14, e.g. by utilising higher dilution conditions,
were unsuccessful.
2,3-Dihydro-2-methyl-5-phenyl-1H-2-benzazepine 10a
A solution of sarcosine 7a and β-phenylcinnamaldehyde 6a in
p-xylene was refluxed for 30 h. Chromatography gave the title
compound as a pale yellow oil (85%) (Found: MHϩ, 236.144.
Calc. for C17H18N: M, 236.144); νmax (liquid film)/cmϪ1 1600
(C᎐C); δ (270 MHz, CDCl ) 2.47 (3H, s, NCH ), 2.90 (2H, d,
᎐
H
3
3
J 7.3, H-3), 3.61 (2H, s, H-1), 6.47 (1H, t, J 7.3, H-4), 7.10 (1H,
m, ArH), 7.25–7.35 (7H, m, Ar-H), 7.39 (1H, m, Ar-H); δC(68
MHz, CDCl3) 43.0 (NCH3), 51.8 (CH2), 58.0 (CH2), 124.0
(CH), 127.4 (CH), 127.6 (CH), 127.8 (CH), 128.2 (2 × CH),
128.4 (2 × CH), 129.1 (CH), 129.8 (CH), 136.8 (quat.), 140.3
(quat.), 140.9 (quat.), 147.0 (quat.); m/z 236 (MHϩ, 82%), 235
(Mϩ, 100), 234 (87), 193 (30), 144 (31); picrate salt of 10a, mp
202 ЊC (lit.10 mp 209–210 ЊC) (Found: C, 59.4; H, 4.4; N, 12.1.
C23H20N4O7 requires C, 59.5; H, 4.3; N, 12.1%).
1,3-Dipolar cycloaddition between N-phenylmaleimide and
azomethine ylide 8a—derived from ꢁ-phenylcinnamaldehyde 6a
and sarcosine 7a
Experimental
Mps were determined on a Gallenkamp apparatus and are
Sarcosine 7a (44 mg, 0.49 mmol), β-phenylcinnamaldehyde 6a
2606
J. Chem. Soc., Perkin Trans. 1, 1999, 2605–2608

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(102 mg, 0.49 mmol) and N-phenylmaleimide (84 mg, 0.49
mmol) were dissolved in toluene (5 cm3) and the solution was
refluxed for 3 h. After 1 h more sarcosine (44 mg, 0.49 mmol)
was added to the reaction mixture. When the reaction was
7-Phenyl-2,3,5,11b-tetrahydro-1H-pyrrolo[2,1-a][2]benzazepine
10d
A solution of proline 7b and β-phenylcinnamaldehyde 6a was
refluxed for 28 h. Chromatography gave the title compound as a
pale yellow oil (42%) (Found: Mϩ, 261.152. Calc. for C19H19N:
M, 261.152); νmax (liquid film)/cmϪ1 1600; δH(270 MHz, CDCl3)
1.26 (2H, m, H-2), 2.11 (2H, m, H-1), 2.50 (2H, m, H-3), 2.95
(1H, br m, H-5), 3.38 (1H, dd, J 11.9, 7.2, H-11b), 4.21 (1H, br
m, H-5), 6.52 (1H, t, J 6.6, H-6), 7.08 (1H, dd, J 7.3, 1.3, H-11),
7.21–7.39 (8H, m, H-8, 9, 10 ϩ Ph); δC(68 MHz, CDCl3) 24.0
(CH2), 26.5 (CH2), 49.7 (CH2), 52.1 (CH2), 62.0 (CH), 125.7
(CH), 126.7 (CH), 127.4 (CH), 127.6 (2 × CH), 128.2 (2 × CH),
128.3 (2 × CH), 129.5 (CH), 139.0 (quat.), 140.1 (quat.), 141.6
(quat.), 146.0 (quat.); m/z 262 (50%), 261 (Mϩ, 62), 260 (49),
149 (62), 41 (38), 29 (100).
1
complete, the solvent was removed in vacuo and the H NMR
spectrum of the residue gave the syn-exo-11a:syn-endo-11b
isomer ratio as 1:1. The two isomers were separated by column
chromatography to give the syn-exo isomer 11a as a white
powder (65 mg, 32%), mp 157–158 ЊC (Found: Mϩ, 408.184.
Calc. for C27H24N2O2: M, 408.184); νmax (liquid film)/cmϪ1 1704
(C᎐O); δ (270 MHz, CDCl ) 2.26 (3H, s, NCH ), 2.35 (1H, m,
᎐
H
3
3
H-5), 2.99 (1H, m, H-5), 3.22 (2H, m, H-2a and H-5a), 3.49
(1H, d, J 9.5, H-3), 5.93 (1H, d, J 9.5, H-3α), 7.21–7.50 (15H,
m, 3 × Ph); δC(68 MHz, CDCl3) 39.6 (NCH3), 44.1 (CH2), 49.5
(CH), 58.5 (CH), 67.5 (CH), 125.8 (C᎐CH), 126.5 (CH), 127.5
᎐
(CH), 128.2 (CH), 128.5 (CH), 129.1 (CH), 129.5 (CH), 132.2
(quat.), 139.9 (quat.), 141.5 (quat.), 146.4 (quat.), 178.3
(2 × C᎐O); m/z 408 (Mϩ, 30%), 393 (10), 331 (20), 317 (20), 241
᎐
10-Benzyloxy-7-(4Ј-benzyloxyphenyl)-2,3,5,11b-tetrahydro-1H-
pyrrolo[2,1-a][2]benzazepine 10e
(12), 235 (43), 220 (35), 158 (100).
The syn-endo isomer 11b was also obtained as a white powder
(54 mg, 27%), mp 117–118 ЊC (Found: Mϩ, 408.184. Calc. for
A solution of proline 7b and 3,3-bis(4Ј-benzyloxyphenyl)-
propenal 6c was refluxed for 28 h. Chromatography gave the
title compound as a pale brown oil (38%) (Found: (M Ϫ H)ϩ,
472.228. Calc. for C33H30NO2: M, 472.226); νmax (liquid film)/
cmϪ1 1604; δH(270 MHz, CDCl3) 1.20 (2H, m, H-2), 1.95 (2H,
m, H-1), 2.32 (2H, m, H-3), 2.68 (1H, br m, H-5), 4.22 (2H, m,
H-5 and H-11b), 5.08 (2H, s, CH2), 5.12 (2H, s, CH2), 6.57 (1H,
t, J 7.0, H-6), 6.87 (1H, d, J 7.9, H-9), 7.01 (2H, d, J 7.9, H-
3Ј,5Ј), 7.16 (2H, d, J 7.9, H-2Ј,6Ј), 7.22 (1H, d, J 7.9, H-8), 7.24
(1H, s, H-11), 7.41 (10H, m, ArH); δC(68 MHz, CDCl3) 29.7
(CH2), 30.0 (CH2), 51.2 (CH2), 63.6 (CH2), 68.5 (CH), 70.0
(2 × CH2), 114.6 (2 × CH), 114.7 (2 × CH), 123.5 (CH), 124.3
(CH), 127.5 (2 × CH), 127.6 (2 × CH), 128.0 (2 × CH), 128.2
(CH), 128.6 (4 × CH), 129.0 (quat.), 129.2 (CH), 129.4 (quat.),
131.8 (quat.), 132.0 (quat.), 136.9 (quat.), 156.6 (quat.), 158.9
(quat.); m/z (rel. intensity, %): 473 (6%), 472 (23), 471 (25), 288
(10), 92 (11), 91 (100).
C27H24N2O2: M, 408.184); νmax (liquid film)/cmϪ1 1708 (C᎐O);
᎐
δH(270 MHz, CDCl3) 2.25 (3H, s, NCH3), 2.75 (1H, dd, J 9.9,
4.6, H-3), 3.36 (2H, m), 3.55 (2H, m), 6.08 (1H, d, J 9.9, H-3α),
7.19–7.49 (15H, m, 3 × Ph); δC(68 MHz, CDCl3) 38.0 (NCH3),
44.4 (CH2), 51.6 (CH), 56.5 (CH), 66.3 (CH), 125.0 (CH), 126.5
(CH), 127.4 (CH), 127.6 (CH), 127.9 (CH), 128.3 (CH), 128.6
(CH), 129.1 (CH), 131.9 (quat.), 138.7 (quat.), 141.6 (quat.),
147.3 (quat.), 175.8 (2 × C᎐O); m/z 408 (Mϩ, 65%), 393 (24),
᎐
331 (48), 317 (25), 241 (48), 235 (88), 220 (62), 158 (100), 115
(25), 82 (31).
2,3-Dihydro-8-methoxy-5-(4Ј-methoxyphenyl)-2-methyl-1H-2-
benzazepine 10b
A solution of sarcosine 7a and 3,3-bis(4Ј-methoxyphenyl)-
propenal 6b was refluxed for 26 h. Chromatography gave the
title compound as a pale yellow oil (94%) (Found: MHϩ,
296.165. Calc. for C19H22NO2: M, 296.165); νmax (liquid film)/
8-Phenyl-1,2,3,4,6,12b-hexahydropyrido[2,1-a][2]benzazepine
10f
cmϪ1 1604 (C᎐C); δ (270 MHz, CDCl ) 2.48 (3H, s, NCH ),
᎐
H
3
3
2.91 (2H, d, J 7.3, H-3), 3.59 (2H, s, H-1), 3.82 (3H, s, OCH3),
3.85 (3H, s, OCH3), 6.32 (1H, t, J 7.3, H-4), 6.86 (2H, d, J 8.6,
H-3Ј,5Ј), 6.85 (1H, dd, J 8.6, 2.6, H-7), 6.94 (1H, d, J 2.6, H-9),
7.06 (1H, d, J 8.6, H-6), 7.23 (2H, d, J 8.6, H-2Ј,6Ј); δC(68 MHz,
CDCl3) 42.9 (NCH3), 51.8 (CH2), 55.3 (2 × MeO), 58.0 (CH2),
113.25 (CH), 113.6 (2 × CH), 115.0 (CH), 120.7 (CH), 129.5
(2 × CH), 130.4 (CH), 132.9 (quat.), 133.6 (quat.), 138.0
(quat.), 146.5 (quat.), 159.0 (quat.), 159.5 (quat.); m/z 296
(MHϩ, 55%), 295 (87), 294 (55), 188 (25), 174 (33), 41 (55), 29
(100).
A solution of pipecolinic acid 7c and β-phenylcinnamaldehyde
6c was refluxed for 30 h. Chromatography gave the title com-
pound as a pale brown oil (33%) (Found: MHϩ, 276.175. Calc.
for C20H22N: M, 276.175); νmax (liquid film)/cmϪ1 1446; δH(270
MHz, CDCl3) 0.86 (2H, m, CH2), 1.35 (2H, m, CH2), 1.61 (1H,
m, CH2), 1.72 (2H, m, CH2), 1.96 (1H, m, CH2), 3.05 (3H, m,
H-6 and H-12b), 6.49 (1H, t, J 7.3, H-7), 7.03 (1H, d, J 7.9,
ArH), 7.21–7.39 (7H, m, ArH), 7.53 (1H, d, J 7.9, ArH); δC(68
MHz, CDCl3) 25.0 (CH2), 26.5 (CH2), 29.0 (CH2), 30.8 (CH2),
52.6 (CH2), 61.5 (CH), 125.6 (CH), 125.8 (quat.), 126.8 (CH),
127.2 (CH), 127.7 (CH), 127.9 (CH), 128.2 (3 × CH), 128.3
(2 × CH), 128.6 (quat.), 131.6 (quat.); m/z 276 (86%), 275 (Mϩ,
100), 274 (55), 247 (43), 218 (18), 193 (31), 184 (20), 110 (23),
41 (22).
2,3-Dihydro-8-benzyloxy-5-(4Ј-benzyloxyphenyl)-2-methyl-1H-
2-benzazepine 10c
A solution of sarcosine 7a and 3,3-bis(4Ј-benzyloxyphenyl)-
propenal 6c was refluxed for 24 h. Chromatography gave the
title compound as a pale yellow oil (95%) (Found: MHϩ,
448.228. Calc. for C31H30NO2: M, 448.228); νmax (liquid film)/
6,7-Dihydro-6-methyl-5H-dibenz[c,e]azepine 13
cmϪ1 1604 (C᎐C); δ (270 MHz, CDCl ) 2.43 (3H, s, NCH ),
A solution of 2-phenylbenzaldehyde 12 (0.915 g, 5 mmol) and
sarcosine 7a (0.890 g, 10 mmol) in p-xylene (20 cm3) was
refluxed for 24 h. Chromatography (eluent EtOAc to 90%
EtOAc–10% MeOH) gave the title compound 13 as a yellow oil
(0.088 g, 9%) (Found: Mϩ, 209.123. Calc. for C15H15N: M,
209.120. Found: (M Ϫ H)ϩ, 208.114. Calc. for C15H14N: M,
208.113); νmax (liquid film)/cmϪ1 1450; δH(270 MHz, CDCl3),
2.51 (3H, s, NCH3), 3.44 (4H, s, 2 × CH2), 7.30–7.50 (8H, m,
Ar-H); δC(68 MHz, CDCl3) 42.1 (NCH3), 57.1 (2 × CH2), 127.6
(4 × CH), 128.1 (2 × CH), 129.7 (2 × CH), 134.2 (2 × quat.),
141.0 (2 × quat.); m/z 209 (Mϩ, 89%), 208 (100), 166 (89), 165
(93).
᎐
H
3
3
2.85 (2H, d, J 7.3, H-3), 3.54 (2H, s, H-1), 5.07 (2H, s, OCH2),
5.10 (2H, s, OCH2), 6.32 (1H, t, J 7.3, H-4), 6.90 (2H, m, Ar-H),
7.05 (2H, m, Ar-H), 7.23 (2H, d, J 7.9, Ar-H), 7.30–7.46 (10H,
m, Ar-H), 7.78 (1H, d, J 9.2, Ar-H); δC(68 MHz, CDCl3) 43.2
(NCH3), 52.1 (CH2), 58.3 (CH2), 70.1 (2 × OCH2), 113.9 (CH),
114.4 (2 × CH), 114.6 (2 × CH), 115.9 (CH), 121.6 (CH), 127.5
(CH), 128.0 (CH), 128.2 (2 × CH), 128.6 (3 × CH), 129.5
(2 × CH), 130.4 (CH), 132.2 (CH), 133.3 (quat.), 134.1 (quat.),
136.9 (quat.), 137.0 (quat.), 138.6 (quat.), 146.0 (quat.), 158.1
(quat.), 158.7 (quat.); m/z 448 (MHϩ, 30%), 447 (40), 395 (18),
356 (80), 264 (18), 250 (18), 211 (12), 91 (100).
J. Chem. Soc., Perkin Trans. 1, 1999, 2605–2608
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2,3-Bis(biphenyl-2Ј-yl)-2,3,5,6,7,8-hexahydropyrrolo[2,1-b]-
oxazole 16
References
1 P. W. Groundwater and M. Nyerges, Adv. Heterocycl. Chem., 1999,
73, 97.
A solution of 2-phenylbenzaldehyde 12 (0.915 g, 5 mmol) and
proline 7b (1.150 g, 10 mmol) in p-xylene (20 cm3), was heated
to reflux under an atmosphere of argon for 24 h. Chrom-
atography (eluent EtOAc to 90% EtOAc–10% MeOH) gave the
title compound 16 as a yellow oil (0.409 g, 35%) (Found: MHϩ,
418.217. Calc. for C30H28NO: M, 418.217); νmax (liquid film)/
cmϪ1 1477; δH(270 MHz, CDCl3) 1.90 (2H, m, H-7), 2.67 (2H,
dt, J 10.55, 6.3, H-6), 3.05 (2H, dt, J 10.55, 6.3, H-5), 4.03 (1H,
d, J 8.4, H-3), 4.84 (1H, d, J 8.4, H-2), 5.15 (1H, dd, J 4.2, 2.6,
H-8), 7.0–7.7 (18H, m, Ar-H); δC(68 MHz, CDCl3) 24.0 (C-6),
31.7 (C-7), 55.3 (C-5), 74.8 (C-3), 85.4 (C-2), 98.6 (C-8), 126.3
(CH), 126.4 (CH), 126.45 (CH), 127.15 (CH), 127.25 (CH),
127.3 (CH), 127.6 (CH), 128.0 (CH), 128.4 (CH), 128.7 (quat.),
129.1 (CH), 129.2 (CH), 129.35 (CH), 129.7 (quat.), 135.1
(quat.), 140.6 (quat.), 141.0 (quat.), 142.25 (quat.); m/z 419
(35%), 418 (MHϩ, 100%), 282 (35), 264 (97), 236 (87), 235 (74),
183 (100), 70 (85), 43 (86).
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J. Chem. Soc., Perkin Trans. 1, 1999, 2605–2608