Intramolecular cycloadditions of N-alkenoyl aryl azides

Article abstract of DOI:10.1039/b102686k

Depending upon the reaction conditions, intramolecular cycloadditions of variously substituted N-alkenoyl aryl azides 4 give the [1,2,3]triazolobenzodiazepine 5, bridgehead aziridines 6 or imines 7. The dipolarophilic activity of the C=N double bond of th

Full text of DOI:10.1039/b102686k

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Intramolecular cycloadditions of N-alkenoyl aryl azides
Luisa Garanti,a Giorgio Molteni*a and Gianluigi Brogginib
a Dipartimento di Chimica Organica e Industriale, Università di Milano, via Golgi 19,
20133 Milano, Italy
1
b Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Università dell’Insubria,
via Lucini 3, 22100 Como, Italy
Received (in Cambridge, UK) 22nd March 2001, Accepted 12th June 2001
First published as an Advance Article on the web 19th July 2001
Depending upon the reaction conditions, intramolecular cycloadditions of variously substituted N-alkenoyl aryl
azides 4 give the [1,2,3]triazolobenzodiazepine 5, bridgehead aziridines 6 or imines 7. The dipolarophilic activity of
the C᎐N double bond of the latter compounds is also exploited in the synthesis of [1,2,4]triazolobenzodiazepines 10
᎐
by means of nitrilimine cycloadditions.
the observed reaction paths. It is well known that several
Introduction
∆2-1,2,3-triazolines thermally decompose to give aziridines and
Organic azides occupy a prominent role in the field of 1,3-
imine derivatives.8 By occurring at room temperature, intra-
molecular cycloaddition of 4a allows the isolation of 5 with
good yields. Similar tricyclic structures, which are involved in
the thermal decomposition of substituted azides 4b,c, undergo
loss of nitrogen followed by prototropic migration to give
products 6 and/or 7. The lack of imine derivatives when starting
from azide 4c may be ascribed to the stabilising effect of the
phenyl group on the adjacent electron-deficient carbon atom.
dipolar cycloaddition chemistry.1 In the last two decades, a
copious literature has been devoted to the intramolecular
version of this methodology,2 thus providing a versatile tool
for new synthetic targets3 and mechanistic investigations.4 The
present paper describes the results which we obtained from
the intramolecular cycloaddition of the azides 4a–c bearing the
acrylamide moiety as the dipolarophile.
This picture is substantiated by thermal decomposition of
5, which was carried out in refluxing toluene and just gave a
mixture of the aziridine 6a and imine 7a (Table 1 and Scheme 2).
Results and discussion
Our synthetic sequence starts from N-benzyl-2-nitrobenzyl-
amine 1, which was readily obtained through literature pro-
cedures.5 Alkenoylation of 1 and subsequent reduction of the
aromatic nitro group of 2 gave the N-alkenoyl-2-amino-N-
benzylbenzylamines 3. Diazotisation of the latter followed by
treatment with sodium azide gave the N-alkenoyl-2-azido-N-
benzylbenzylamines 4 with yields ranging from fair to quanti-
tative (Scheme 1). Due to its lability even at room temperature,
unsubstituted 4a was not fully characterised, since it partially
undergoes spontaneous intramolecular cycloaddition to the
[1,2,3]triazolo[1,5-a][1,4]benzodiazepinone 5 during the reac-
tion work-up. The complete conversion from 4a to 5 was
achieved by stirring a 0.1 M ethereal solution of the crude azide
at room temperature. Methyl- and phenyl-substituted azides
4b,c were far more stable than 4a since, under the above reac-
tion conditions, quantitative amounts of the starting materials
were recovered. It was found that substrates 4b,c reacted
smoothly in refluxing toluene and in the presence of a little (1%)
triethylamine, giving the azirino[1,2-a][1,4]benzodiazepinones
6b,c and the 2-ethyl[1,4]benzodiazepin-3-one 7b. Products, as
well as reaction times, eluents and yields, are given in Table 1.
Basic reaction media and eluents were needed because of
the known lability of compounds such as 6 towards acidic
species,6 which can cause extensive resinification of the latter.
Structural assignments for products 5–7 are unambiguous
and rely upon analytical and spectral data. In particular,
the J-values found for the protons of the aziridine ring of 6
(3.4–3.6 Hz) agree well with those reported for similar nitro-
gen bridgehead aziridines7 and speak in favour for the trans
arrangement of such protons, thus accounting for the depicted
(1S*,1aR*) relative stereochemistry of 6b and 6c.
The latter decomposition was also carried out in CDCl3 at
60 ЊC with monitoring of the reaction progress by H NMR
1
analyses. The disappearance of the signals of 5 was accom-
panied by the simultaneous appearance of two sets of signals
easily attributable to 6a and 7a. At this point it needs to
be added that the absence of geminal coupling in the case of
6a parallels that observed for similar nitrogen bridgehead
aziridines.6,9
As a further stage of our work, we undertook the synthesis
of [1,2,4]triazolo[4,3-a][1,4]benzodiazepin-4-ones 10 by means
of nitrile imine cycloaddition onto the C᎐N double bond of
᎐
imines 7a,b (Scheme 3). Hence, treatment of the latter with the
hydrazonoyl chloride 810 in the presence of triethylamine,
which generates nitrilimine intermediates 9, gave cycloadducts
10 with good yields and full regioselectivity. The latter is con-
sistent with similar behaviour of benzodiazepines as dipolaro-
philes.11 To gain deeper insight about this point, we performed
PM3 calculations12 on the imine 7a13 (Table 2). The LUMO
atomic coefficients of C and N (C᎐N double bond of 7a) are
᎐
quite different, so accounting for the exclusive formation of
isomer 10 when the cycloaddition follows the usual HOMO-
dipole (LUMO-dipolarophile) control.14
Some conclusions can be drawn from the present work: (i)
the product output of the intramolecular cycloadditions of
the N-alkenoyl aryl azides 4a–c is strongly dependent upon
reaction conditions and the kind of substituent placed on
the alkenoyl moiety; (ii) we were able to synthesise, for the
firsttime,the3,3a,5,6-tetrahydro[1,2,3]triazolo[1,5-a][1,4]benzo-
diazepin-4-one skeleton of 5 by means of a direct intra-
molecular azide cycloaddition; and (iii) our cycloadditive
methodology provides a clean synthetic entry to compounds 5,
7 and 10 of potential pharmacological interest.
The above results deserve some comments in order to explain
1816
J. Chem. Soc., Perkin Trans. 1, 2001, 1816–1819
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102686k

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Table 1 Reaction of azides 4 and 1,2,3-triazolobenzodiazepine 5
Products and yields (%)c
Compd
Time (t/h)
5
6
7
Eluent
4aa
4bb
4cb
5b
20
1
1
74d
11
79
56
n-Hexane–CH2Cl2–Et2O–Et3N 20 : 20 : 20 : 1
n-Hexane–AcOEt–Et3N 30 : 30 : 1
90e
42
1
a In diethyl ether, rt. b In refluxing toluene. c Isolation yields. d From diisopropyl ether. e From n-hexane–benzene.
Scheme 1
Table 2 PM3 Calculations on the imine 7aa
Atomic coefficients
Frontier molecular orbitalb
Energy/eV
C
N
HOMO
LUMO
Ϫ9.177
Ϫ0.889
ϩ0.35
ϩ0.59
ϩ0.13
Ϫ0.25
a Geometry of 7a was fully optimised with the PM3 method. b FMO of
the C᎐N double bond of 7a, bond length 135.1 pm.
᎐
Scheme 2
cm3) was added dropwise at 90 ЊC. The mixture was refluxed for
9 h, then the undissolved material was filtered off. The organic
layer was washed with water (50 cm3) and dried over sodium
sulfate. The solvent was removed under reduced pressure to
afford acrylamides 2a–c as undistillable oils, not analytically
pure.
Compound 2a (4.78 g, 98%) was a colourless oil; νmax (neat)/
cmϪ1 1650; δH (DMSO-d6; 100 ЊC) 4.56 (2H, s), 4.82 (2H, s),
5.70 (1H, dd, J 10.8, 3.0), 6.22 (1H, dd, J 17.5, 3.0), 6.68 (1H,
dd, J 17.5, 10.8), 7.20–7.95 (9H, m); m/z (EI) 296 (Mϩ).
Compound 2b (5.01 g, 98%) was a colourless oil; νmax (neat)/
cmϪ1 1660; δH (CDCl3; 60 ЊC) 1.94 (3H, d, J 8.0), 4.70 (2H, s),
4.94 (2H, s), 6.25 (1H, br s), 7.10–7.60 (9H, m), 8.05 (1H, br s);
m/z (EI) 310 (Mϩ).
Experimental
Mps were measured with a Büchi apparatus in open capillary
tubes and are uncorrected. IR spectra were recorded with a
Perkin-Elmer 1725X spectrophotometer. Mass spectra were
determined with a VG-70EQ apparatus. 1H NMR and 13C
NMR spectra were taken with a Bruker AC 300 or AMX 300
instrument for samples in CDCl3 solutions at room temperature
unless otherwise stated. Chemical shifts are given as ppm from
tetramethylsilane; J-values are given in Hz. Because of severe
overlapping of the signals, 1H NMR spectra of compounds 2a,
3a and 7a were taken in DMSO-d6 solutions at 100 ЊC.
General procedure for the preparation of N-benzyl-2-nitro-N-
(1-oxoalk-2-enyl)benzylamines 2
A solution of 1 (4.00 g, 16.5 mmol) in dry toluene (110 cm3) was
treated with K2CO3 (4.30 g, 31.2 mmol). The appropriate
alkenoyl chloride (16.5 mmol) as a solution in dry toluene (4.0
Compound 2c (4.42 g, 72%) was a pale yellow oil; νmax (neat)/
cmϪ1 1650; δH (CDCl3) 4.72 (2H, s), 4.91 (1H, d, J 14.7), 4.98
(1H, d, J 14.7), 6.68 (1H, d, J 16.2), 7.10–7.70 (14H, m), 7.82
(1H, d, J 16.2); m/z (EI) 372 (Mϩ).
J. Chem. Soc., Perkin Trans. 1, 2001, 1816–1819
1817

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4.67 (2H, s), 6.73 (1H, d, J 15.5), 7.10–7.40 (14H, m), 7.82 (1H,
d, J 15.5); m/z (EI) 280 (Mϩ).
5-Benzyl-3,3a,5,6-tetrahydro[1,2,3]triazolo[1,5-a][1,4]benzo-
diazepin-4-one 5
A solution of 4a (2.10 g, 7.2 mmol) in dry diethyl ether (72 cm3)
was stirred at room temperature for 20 h. The solvent was
removed under reduced pressure and the residue was crystal-
lised from diisopropyl ether to give tricycle 5 (1.56 g, 74%) as a
colourless solid, mp 85 ЊC (Found: C, 69.90; H, 5.55; N, 19.10,
C17H16N4O requires C, 69.85; H, 5.52; N, 19.16%); νmax (Nujol)/
cmϪ1 1675; δH (CDCl3) 3.82 (1H, d, J 16.7), 4.37 (1H, d, J 14.9),
4.58 (1H, dd, J 16.6, 12.1), 5.00 (1H, d, J 14.9), 5.44–5.63 (3H,
m), 6.85–7.30 (8H, m), 7.83 (1H, dd, J 7.8, 1.8); δC (CDCl3)
50.16 (t), 50.56 (t), 53.93 (d), 68.52 (t), 116.65 (d), 122.04 (d),
127.7–129.8, 136.23 (s), 165.46 (s); m/z (EI) 292 (Mϩ).
Scheme 3
Thermal behaviour of (E)-2-azido-N-benzyl-N-[1-oxobut-2-enyl]-
benzylamine 4b
General procedure for the preparation of 2-amino-N-benzyl-N-
(1-oxoalk-2-enyl)benzylamines 3
A solution of azide 4b (1.56 g, 5.0 mmol) and triethylamine
(0.36 g, 3.6 mmol) in dry toluene (50 cm3) was refluxed for 1 h.
The solvent was removed under reduced pressure and the resi-
due was chromatographed on a silica gel column with n-hexane–
dichloromethane–diethyl ether–triethylamine 20 : 20 : 20 : 1 as
eluent. First fractions contained 4-benzyl 2-ethyl-4,5-dihydro-
[1,4]benzodiazepin-3-one 7b (1.10 g, 79%) as a colourless oil
(Found: C, 77.72; H, 6.44; N, 9.57. C18H18N2O requires C,
77.67; H, 6.52; N, 10.06%); νmax (Nujol)/cmϪ1 1660; δH (CDCl3)
1.32 (3H, t, J 7.4), 2.98 (2H, q, J 7.4), 3.99 (2H, s), 4.66 (2H, s),
6.80 (1H, dd, J 7.8, 1.2), 7.06 (1H, dt, J 8.0, 1.4), 7.20–7.35 (7H,
m); δC (CDCl3) 10.44 (q), 30.69 (t), 47.55 (t), 48.97 (t), 125.6–
128.66, 136.00 (s), 146.07 (s), 162.37 (s), 168.52 (s); m/z (EI) 278
(Mϩ).
Further elution gave (1S*,1aR*)-3-benzyl-1-methyl-1,1a,3,4-
tetrahydroazirino[1,2-a][1,4]benzodiazepin-2-one 6b (0.18 g,
11%) as a colourless oil (Found: C, 77.75; H, 6.60; N, 9.55.
C18H18N2O requires C, 77.67; H, 6.52; N, 10.06%); νmax (Nujol)/
cmϪ1 1640; δH (CDCl3) 1.53 (3H, d, J 5.5), 2.33 (1H, dq, J 5.5,
3.6), 3.06 (1H, d, J 3.6), 3.58 (1H, d, J 14.8), 4.13 (1H, d,
J 14.9), 4.96 (1H, d, J 14.9), 5.01 (1H, d, J 14.8),† 6.70–7.30
(9H, m); δC (CDCl3) 27.35 (q), 53.33 (t), 62.83 (t), 69.03 (d),
116.50–132.20, 135.74 (s), 143.67 (s), 157.10 (s), 162.12 (s);
m/z (EI) 278 (Mϩ).
A solution of a nitro compound 2 (11.0 mmol) in ethanol (15
cm3) was treated with iron dust (4.92 g, 0.088 mol) and 20% aq.
acetic acid (6.0 cm3), and then refluxed for 4 h under vigorous
stirring. The mixture was taken up with ethyl acetate (80 cm3)
and filtered over Celite. The organic layer was washed succes-
sively with 5% aq. sodium hydrogen carbonate (40 cm3) and
water (2 × 50 cm3), and dried over sodium sulfate. Evapor-
ation of the solvent gave amines 3a,b as undistillable oils, not
analytically pure, and solid amine 3c.
Amine 3a (2.19 g, 75%) was a pale yellow oil; νmax (neat)/cmϪ1
3420, 3350, 3235, 1640; δH (DMSO-d6; 100 ЊC) 4.46 (2H, s), 4.60
(2H, s), 4.74 (2H, br s), 5.62 (1H, dd, J 10.7, 3.5), 6.20 (1H, dd,
J 16.7, 3.5), 6.53 (1H, dt, J 7.2, 2.0), 6.68 (1H, dd, J 16.7, 10.7),
6.85–7.35 (8H, m); m/z (EI) 266 (Mϩ).
Amine 3b (2.46 g, 80%) was a pale yellow oil; νmax (neat)/cmϪ1
3450, 3350, 3230, 1660; δH (CDCl3) 1.84 (3H, dd, J 6.9, 1.7),
4.50 (2H, s), 4.54 (2H, s), 4.75 (2H, br s), 6.24 (1H, dq, J 14.8,
1.5), 6.60–7.45 (10H, m); m/z (EI) 280 (Mϩ).
Amine 3c (2.84 g, 76%) mp 79 ЊC (from diisopropyl ether)
(Found: C, 81.21; H, 5.94; N, 8.28. C23H22N2O requires C,
80.70; H, 6.43; N, 8.19%); νmax (Nujol)/cmϪ1 3440, 3340, 3230,
1660; δH (CDCl3) 4.60 (2H, s), 4.65 (2H, s), 4.72 (2H, br s), 6.73
(1H, dt, J 7.8, 1.2), 6.83 (1H, d, J 15.4), 6.92 (1H, dd, J 8.0, 1.7),
7.11 (1H, dt, J 7.8, 1.7), 7.20–7.40 (11H, m), 7.82 (1H, d,
J 15.4); m/z (EI) 342 (Mϩ).
Thermal behaviour of (E )-2-azido-N-benzyl-N-[1-oxo-3-
phenylprop-2-enyl]benzylamine 4c
General procedure for the preparation of 2-azido-N-benzyl-
N-(1-oxoalk-2-enyl)benzylamines 4
A solution of azide 4c (1.84 g, 5.0 mmol) and triethylamine
(0.36 g, 3.6 mmol) in dry toluene (50 cm3) was refluxed for
1 h. The solvent was removed under reduced pressure and
the residue was crystallised from n-hexane–benzene to give
(1S*,1aR*)-3-benzyl-1-phenyl-1,1a,3,4-tetrahydroazirino[1,2-
a][1,4]benzodiazepine-2-one 6c (1.53 g, 90%) as a colourless
solid, mp 90 ЊC (Found: C, 81.22; H, 5.90; N, 8.30. C23H20N2O
requires C, 81.15; H, 5.92; N, 8.23%); νmax (Nujol)/cmϪ1 1640;
δH (CDCl3) 3.32 (1H, d, J 3.4), 3.42 (1H, d, J 3.4), 3.67 (1H, d,
J 14.9), 4.22 (1H, d, J 14.8), 5.00 (1H, d, J 14.8),‡ 5.09 (1H, d,
J 14.9), 6.70–7.40 (14H, m); δC (CDCl3) 45.41 (t), 47.88 (d),
48.50 (t), 49.33 (d), 121.9–129.7, 136.45 (s), 137.78 (s), 149.25
(s), 165.89 (s); m/z (EI) 340 (Mϩ).
A solution of an amine 3 (8.0 mmol) in 2 M hydrochloric acid
(20 cm3) was stirred and cooled to 0 ЊC. Sodium nitrite (1.10 g,
16.0 mmol) was added portionwise over a period of 30 min, and
cold diethyl ether (50 cm3) was added to the reaction mixture.
Sodium azide (2.60 g, 40.0 mmol) was then added portion-
wise under vigorous stirring and ice-cooling. After 1 h, the
organic layer was separated, washed successively with 5% aq.
sodium hydrogen carbonate (50 cm3) and water (50 cm3), and
dried over sodium sulfate. The solvent was removed under
reduced pressure to give brown oily residues.
Acryloyl azide 4a (2.29 g, 98%), νmax (neat)/cmϪ1 2130, 1650,
was used without further purification.
In the remaining cases, the residue was chromatographed on
a silica gel column with diethyl ether as eluent to give azides
4b,c as undistillable oils, not analytically pure.
Thermal behaviour of the 1,2,3-triazolobenzodiazepine 5
A solution of compound 5 (1.47 g, 5.0 mmol) and triethylamine
(0.36 g, 3.6 mmol) in dry toluene (50 cm3) was refluxed for
1 h. The solvent was removed under reduced pressure and
the residue was chromatographed on a silica gel column with
Azide 4b (1.71 g, 70%) was a pale yellow oil; νmax (neat)/cmϪ1
2130, 1660; δH (CDCl3) 1.84 (3H, d, J 6.9), 4.43 (2H, s), 4.61
(2H, s), 6.22 (1H, dq, J 15.0, 1.5), 7.00–7.40 (10H, m); m/z (EI)
306 (Mϩ).
Azide 4c (1.47 g, 50%) was a pale yellow oil; νmax (neat)/cmϪ1
2125, 1650; δH (CDCl3) 4.56 (1H, d, J 15.6), 4.62 (1H, d, J 15.6),
† After irradiation at δ 3.58: δ 5.01 (1H, s).
‡ After irradiation at δ 4.22: δ 5.00 (1H, s).
1818
J. Chem. Soc., Perkin Trans. 1, 2001, 1816–1819

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n-hexane–ethyl acetate–triethylamine 30 : 30 : 1 as eluent. First
fractions contained 4-benzyl-2-methyl-4,5-dihydro-[1,4]benzo-
diazepin-3-one 7a (0.74 g, 56%) as a colourless oil (Found: C,
77.16; H, 6.14; N, 10.51. C17H16N2O requires C, 77.25; H, 6.10;
N, 10.60%); νmax (Nujol)/cmϪ1 1650; δH (DMSO-d6; 100 ЊC) 2.48
(3H, s), 4.17 (2H, s), 4.58 (2H, s), 7.00–7.40 (9H, m); δC (CDCl3)
25.17 (q), 47.84 (t), 49.40 (t), 124.80–130.3, 136.08 (s), 164.35
(s); m/z (EI) 264 (Mϩ).
Further elution gave 3-benzyl-1,1a,3,4-tetrahydroazirino[1,2-
a][1,4]benzodiazepine-2-one 6a (0.56 g, 42%) as a colourless oil
(Found: C, 77.33; H, 6.11; N, 10.51. C17H16N2O requires C,
77.25; H, 6.10; N, 10.60%); νmax (Nujol)/cmϪ1 1650; δH (CDCl3)
2.16 (1H, d, J 4.0), 2.74 (1H, d, J 5.8), 3.32 (1H, dd, J 5.8, 4.0),
3.61 (1H, d, J 14.8), 4.18 (1H, d, J 14.9), 4.95 (1H, d, J 14.9),§
5.03 (1H, d, J 14.8), 6.70–7.30 (9H, m); δC (CDCl3) 31.90 (t),
38.81 (d), 47.89 (t), 49.16 (t), 122.00–129.70, 136.39 (s), 165.16
(s); m/z (EI) 264 (Mϩ).
(s), 132.53 (d), 136.16 (s), 137.82 (s), 137.95 (s), 139.19 (s),
139.86 (s), 167.40 (s); m/z (EI) 468 (Mϩ).
Acknowledgements
We thank MURST and CNR for financial support.
References
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3a-Alkyl-5-benzyl-1-methoxycarbonyl-3-(4-methylphenyl)-
3,3a,5,6-tetrahydro[1,2,4]triazolo[4,3-a][1,4]benzodiazepin-
4-ones 10
A solution of a benzodiazepinone 7a or 7b (4.0 mmol) and
8 (0.76 g, 4.0 mmol) in dry chloroform (40 cm3) was treated
with triethylamine (2.02 g, 20.0 mmol) and refluxed for
24 h. The solvent was removed under reduced pressure and
the residue was chromatographed on a silica gel column
with dichloromethane–n-hexane 3 : 1. Crystallisation from
diisopropyl ether gave a pure tricycle 10.
4 (a) L. Bruché, L. Garanti and G. Zecchi, J. Chem. Res. (S), 1983,
202; (b) L. Garanti and G. Zecchi, J. Chem. Soc., Perkin Trans. 2,
1979, 1176.
Compound 10a (1.31 g, 72%) was a yellow solid, mp 188 ЊC
(Found: C, 71.29; H, 5.80; N, 12.39. C27H26N4O3 requires C,
71.35; H, 5.77; N, 12.33%); νmax (Nujol)/cmϪ1 1730, 1660; δH
(CDCl3) 1.65 (3H, s), 2.33 (3H, s), 3.71 (1H, d, J 14.7), 3.74
(3H, s), 4.37 (1H, d, J 15.1), 4.77 (1H, d, J 14.7), 4.92 (1H, d, J
15.1), 6.90–7.60 (13H, m); δC (CDCl3) 20.76 (q), 24.02 (q), 48.90
(t), 52.02 (t), 52.45 (q), 119.94 (d), 127.40–129.30, 131.57 (d),
133.06 (s), 136.27 (s), 137.42 (s), 137.50 (s), 137.93 (s), 139.88
(s), 168.53 (s); m/z (EI) 454 (Mϩ).
Compound 10b (1.50 g, 80%) was a yellow solid, mp 163 ЊC
(Found: C, 71.84; H, 5.98; N, 12.03. C28H28N4O3 requires C,
71.78; H, 6.02; N, 11.96%); νmax (Nujol)/cmϪ1 1730, 1655; δH
(CDCl3) 1.06 (3H, t, J 7.3), 1.95 (1H, dq, J 16.7, 7.3), 2.33 (3H,
s), 2.62 (1H, dq, J 16.7, 7.3), 3.61 (1H, d, J 14.9), 3.73 (3H, s),
4.14 (1H, d, J 15.1), 4.64 (1H, d, J 14.9), 4.96 (1H, d, J 15.1),
6.80–7.60 (13H, m); δC (CDCl3) 20.55 (q), 22.71 (q), 48.16 (t),
51.73 (t), 52.20 (q), 117.66 (d), 127.20–129.30, 131.75 (s), 131.88
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§ After irradiation at δ 4.18: δ 4.95 (1H, s).
J. Chem. Soc., Perkin Trans. 1, 2001, 1816–1819
1819