Synthesis of alkyl nitrones by reaction of aldehyde and ketone oximes with α,β-unstaturated esters in the presence of Lewis acid

Article abstract of DOI:10.1134/S1070428009040204

Reactions of pyridinecarbaldehyde oximes, methyl pyridyl ketone oximes, furfural oxime, cinnamaldehyde oxime, and crotonaldehyde oxime with acrylic and methacrylic acid esters in the presence of a Lewis acid catalyst at room temperature followed the conju

Full text of DOI:10.1134/S1070428009040204

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 4, pp. 591–595. © Pleiades Publishing, Ltd., 2009.
Published in Russian in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 4, pp. 605–609.
Synthesis of Alkyl Nitrones by Reaction of Aldehyde
and Ketone Oximes with α,β-Unstaturated Esters
in the Presence of Lewis Acid*
A. Koçak, S. Malkondu, and S. Kurbanli
Department of Chemistry, Faculty of Sciences and Arts, University of Selçuk, 42075 Konya, Turkey
e-mail: skurbanli@gmail.com
Received April 16, 2007
Abstract—Reactions of pyridinecarbaldehyde oximes, methyl pyridyl ketone oximes, furfural oxime,
cinnamaldehyde oxime, and crotonaldehyde oxime with acrylic and methacrylic acid esters in the presence of
a Lewis acid catalyst at room temperature followed the conjugate addition pattern to give the corresponding
alkyl nitrones in good yield. The best yields were obtained using a 1:1 mixture of CdI2 and BF3·Bu2O as
catalyst.
DOI: 10.1134/S1070428009040204
Alkylation of aldehyde and ketone oximes was
studied in sufficient detail, and a number of physio-
logically and biologically active compounds were thus
synthesized [1–6]. It is well known that compounds
containing a pyridine ring are used as starting materials
in the preparation of many medicines [7–13]. We pre-
viously synthesized in good yields some glycol ethers
and halohydrins by alkylation of oximes containing
hydroxy groups and unsaturated bonds with mono- and
dihalohydrins in basic medium in the presence of
phase-transfer catalyst [14–16]. Reactions of oximes
with various alkylating agents provide one of the most
convenient methods for generation of nitrones [17, 18].
In the present paper we report on the alkylation of
some pyridinecarbaldehyde and methyl pyridyl ketone
oximes with α,β-unsaturated carbonyl compounds
(acrylic and methacrylic acid esters) in the presence of
Lewis acid catalyst. Pyridine-2-carbaldehyde oxime
(Ia) was allowed to react with methyl acrylate (IIa) in
methylene chloride for 7 h at room temperature in the
presence of a mixed Lewis acids catalyst, CdI2/BF3·
Bu2O (1:1). The corresponding 1:1 adduct III was
isolated in a quantitative yield and was characterized
by analytical and spectral data (Scheme 1, Table 1). As
unsaturated carbonyl component we also used methyl
methacrylate (IIb), ethyl acrylate (IIc), ethyl methac-
Scheme 1.
O
O
N
R3
R1
N
H2C
R1
OR4
CdI2/BF3 ·Bu2O, CH2Cl2
OH
OR4
+
R2
Ia–Ih
R3
IIa–IIe
R2
O
III–XVII
I, R1 = H, R2 = (Z)-pyridin-2-yl (a), (E)-pyridin-3-yl (b), (E)-pyridin-4-yl (c); R1 = Me, R2 = (E,Z)-pyridin-2-yl (d), (E,Z)-pyridin-
4-yl (e); R1 = H, R2 = 2-furyl (f), (E)-PhCH=CH (g), (E)-MeCH=CH (h); II, R3 = H, R4 = Me (a); R3 = R4 = Me (b); R3 = H, R4 =
Et (c); R3 = Me, R4 = Et (d); R3 = H, R4 = i-Pr (e); III, R1 = R3 = H, R2 = (Z)-pyridin-2-yl, R4 = Me; IV, R1 = H, R2 = (Z)-pyridin-
2-yl, R3 = R4 = Me; V, R1 = R3 = H, R2 = (Z)-pyridin-2-yl, R4 = Et; VI, R1 = R3 = H, R2 = (E)-pyridin-3-yl, R4 = Me; VII, R1 = H,
R2 = (E)-pyridin-3-yl, R3 = Me, R4 = Et; VIII, R1 = R3 = H, R2 = (E)-pyridin-4-yl, R4 = Me; IX, R1 = H, R2 = (E)-pyridin-4-yl, R3 =
R4 = Me; X, R1 = R3 = R4 = Me, R2 = (E,Z)-pyridin-2-yl; XI, R1 = Me, R2 = (E,Z)-pyridin-4-yl, R3 = H, R4 = i-Pr; XII, R1 = Me,
R2 = (E,Z)-pyridin-4-yl, R3 = H, R4 = Et; XIII, R1 = R3 = H, R2 = 2-furyl, R4 = Me; XIV, R1 = H, R2 = 2-furyl, R3 = Me, R4 = Et;
XV, R1 = R3 = H, R2 = (E)-PhCH=CH, R4 = Me; XVI, R1 = H, R2 = (E)-PhCH=CH, R3 = R4 = Me; XVII, R1 = R3 = H, R2 =
(E)-MeCH=CH, R4 = Et.
* The text was submitted by the authors in English.
591

KOÇAK et al.
592
Table 1. Conjugate addition of oximes Ia–Ih to α,β-unsaturated esters IIa–IIe in the presence of CdI2/BF3 ·Bu2O as catalyst
(50 mol %/50 mol %)
Oxime
Ester (equiv)
Solvent
Temperature
Time, h
Product (yield, %)
Ia
Ia
Ia
Ib
Ib
Ic
Ic
Id
Ie
Ie
If
IIa (2.0)
IIb (1.5)
IIc (1.5)
IIa (1.5)
IId (1.5)
IIa (2.0)
IIb (1.5)
IIb (2.2)
IIe (2.0)
IIc (1.8)
IIa (1.3)
IId (1.2)
IIa (1.0)
IIb (1.3)
IIc (1.0)
CH2Cl2
Benzene
CH2Cl2
Benzene
CH2Cl2
Benzene
CH2Cl2
Benzene
CH2Cl2
Benzene
DMF
Room temperature
Reflux
07
24
08
07
24
06
24
24
24
24
06
24
07
24
08
III (88)
IV (100)
V (65)
Room temperature
Room temperature
Reflux
VI (78)
VII (84)
VIII (100)
IX (100)
X (100)
Room temperature
Reflux
Reflux
Room temperature
Room temperature
Room temperature
Reflux
XI (62)
XII (48)
XIII (88)
XIV (74)
XV (100)
XVI (100)
XVII (100)
If
DMF
Ig
Ig
Ih
CH2Cl2
CH2Cl2
CH2Cl2
Room temperature
Reflux
Room temperature
rylate (IId), and isopropyl acrylate (IIe), which showed
quite different reactivities (Table 1). Methyl acrylate
(IIa) turned out to be the most reactive, whereas the
reactions with methacrylic acid esters IIb and IId
required heating in boiling benzene to complete the
process. Methyl pyridyl ketone oximes, namely
(E,Z)-1-(pyridin-2-yl)ethanone oxime (Id) and (E,Z)-1-
(pyridin-4-yl)ethanone oxime (Ie), reacted with esters
IIb, IIc, and IIe at an appreciably lower rate.
Unlike the data of [19], ketone oximes showed
lower reactivity toward α,β-unsaturated esters, though
the reactions described in [19] were carried out at
elevated temperature. The formation of nitrones was
usually fairly slow at room temperature without Lewis
acid catalyst. On the other hand, (E)-cinnamaldehyde
oxime (Ig) as one of the most reactive aldehyde
oximes reacted with ester IIb in the absence of Lewis
acid. The reactions of Ig and Ih with acrylates IIa and
IIc, respectively, smoothly occurred at room tempera-
ture without a catalyst to give the corresponding
nitrones XV and XVII.
The presence of mixed Lewis acid catalyst (CdI2/
BF3 · Bu2O) is essential to ensure high yields of
nitrones. In the reaction of Ia with IIa catalyzed by
an equimolar amount of CdI2, acceleration of not only
the nitrone formation step but also of undesired dipolar
cycloaddition step was observed, and the product was
a mixture of nitrone III and the corresponding cyclo-
adduct (Table 2). The reaction in the presence of
an equimolar amount of BF3·Bu2O was characterized
by low conversion, and both starting compounds were
partially recovered from the reaction mixture. Mixed
Lewis acid catalyst consisting of equimolar amounts of
CdI2 and BF3 ·Bu2O effectively mediated the nitrone
formation step with high to quantitative yield
(Table 1). The reason why mixed Lewis acid is par-
ticularly effective remains so far unclear. Heating
should be avoided in the reactions with acceptors other
than methyl methacrylate (IIb) and ethyl methacrylate
(IId), otherwise the nitrones formed undergo serious
decomposition to give complex mixtures of products.
Table 2. Lewis acid catalysis of the reaction of (Z)-pyridine-
2-carbaldehyde oxime (Ia) with methyl acrylate (IIa) in
methylene chloride at room temperature
Lewis acid (amount, mol %)
Time, h Yield of III, %
CdI2 (50)
42
08
42
07
07
18
a38a
19
CdI2 (100)
BF3·Bu2O (50)
BF3·Bu2O (100)
CdI2/BF3·Bu2O (50/50)
b42b
68
a
The procedure for the isolation of nitrones is also
critical: usual treatment of the reaction mixture with
water is inappropriate. Alkyl nitrones are polar sub-
The reaction was accompanied by formation of the correspond-
ing cycloadduct as by-product.
Incomplete conversion.
b
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SYNTHESIS OF ALKYL NITRONES
593
stances that are readily soluble in water; therefore,
aqueous treatment may lead to considerable loss in the
product yield. The following procedure is recom-
mended: after the reaction is complete (according to
the TLC data), the reaction mixture is filtered through
a short silica gel column. If additional purification is
necessary, repeated chromatography is performed.
Found, %: C 57.54; H 5.76; N 13.36. C10H12N2O3. Cal-
culated, %: C 57.68; H 5.81; N 13.45.
Methyl 2-methyl-3-{(Z)-oxido[1-(pyridin-2-yl)-
methylidene]amino}propanoate (IV). mp 118–
119°C. IR spectrum, ν, cm–1: 2345, 1768, 1756, 1694,
1422, 1410, 1380, 1340, 1270, 1120, 962, 820, 752,
684. 1H NMR spectrum, δ, ppm: 1.15 d (3H, CH3CH),
2.49 m (1H, CHCH3), 3.58 s (3H, OCH3), 3.93 t
(2H, NCH2), 7.91 s (1H, CH=N), 7.42–8.62 m (4H,
pyridine). Found, %: C 59.33; H 6.49; N 12.48.
C11H14N2O3. Calculated, %: C 59.45; H 6.35; N 12.60.
Thus Lewis acid-catalyzed conjugate addition of
aldehyde and ketone oximes to α,β-unsaturated esters
at room temperature provides a convenient synthetic
route to alkyl nitrones.
Ethyl 3-[(Z)-oxido(pyridin-2-ylmethylidene)-
amino]propanoate (V). mp 134–135°C. IR spectrum,
ν, cm–1: 2338, 1762, 1750, 1688, 1418, 1380, 1272,
EXPERIMENTAL
1
All chemicals were purchased from Merck and
Fluka and were used without additional purification.
The melting points were determined on a Buchi capil-
lary melting point apparatus and are uncorrected. The
IR spectra were recorded on a Pye Unicam SP 1025
1120, 962, 820, 752, 680. H NMR spectrum, δ, ppm:
1.22 t (3H, CH3), 2.42 t (2H, CH2CO), 3.94 d (2H,
NCH2), 4.09 q (2H, OCH2), 7.87 s (1H, CH=N), 7.39–
8.64 m (4H, pyridine). Found, %: C 59.27; H 6.46;
N 12.43. C11H14N2O3. Calculated, %: C 59.45; H 6.35;
N 12.60.
1
spectrometer. The H NMR spectra were recorded
from solutions in CDCl3 on a Bruker Avance DPX
instrument using tetramethylsilane as reference. The
elemental compositions were determined on a Carlo
Erba 1106 automatic CHN analyzer. Analytical TLC
was performed on Merck 60 Kieselgel F 254 silica gel
plates; spots were visualized under UV light. Prepara-
tive chromatography was performed using Merck 60
Kieselgel (0.040–0.063 mm).
Methyl 3-[(E)-oxido(pyridin-3-ylmethylidene)-
amino]propanoate (VI). mp 128–129°C. IR spec-
trum, ν, cm–1: 2325, 1760, 1745, 1680, 1410, 1384,
1
1270, 1120, 964, 820, 750, 680. H NMR spectrum, δ,
ppm: 2.43 t (2H, CH2CO), 3.58 s (3H, OCH3), 3.94 t
(2H, NCH2), 7.62 s (1H, CH=N), 7.22–8.74 m (4H,
pyridine). Found, %: C 57.52; H 5.66; N 13.31.
C10H12N2O3. Calculated, %: C 57.68; H 5.81; N 13.45.
General procedure for the alkylation of oximes.
Acrylate IIa–IId, 1.5 or 2 equiv, was added to a solu-
tion of pyridinecarbaldehyde oxime Ia–Ic, 25 mmol,
and Lewis acid catalyst (CdI2/BF3·OBu2), 25 mmol, in
25 ml of methylene chloride unless otherwise stated
(see Table 1). The mixture was stirred under the con-
ditions indicated in Table 1, and the progress of the
reaction was monitored by TLC. When the reaction
was complete, the mixture was treated with 10 ml of
methanol and evaporated, the residue was dissolved in
methylene chloride, and the solution was subjected to
column chromatography on silica gel or aluminum
oxide using ethyl acetate–acetone (3:1) as eluent.
Compounds III–IX were additionally recrystallized
from petroleum ether or chloroform.
Ethyl 2-methyl-3-[(E)-oxido(pyridin-3-ylmethyli-
dene)amino]propanoate (VII). mp 126–127°C. IR
spectrum, ν, cm–1: 2316, 1758, 1740, 1665, 1402,
1
1380, 1276, 1120, 960, 815, 750, 675. H NMR spec-
trum, δ, ppm: 1.12 d (3H, CHCH3), 1.24 t (3H,
CH3CH2), 2.42 m (1H, CHCH3), 3.92 d (2H, NCH2),
4.08 q (2H, OCH2), 7.87 s (1H, CH=N), 7.76–8.82 m
(4H, pyridine). Found, %: C 61.14; H 6.98; N 11.72.
C12H16N2O3. Calculated, %: C 61.00; H 6.84; N 11.85.
Methyl 3-[(E)-oxido(pyridin-4-ylmethylidene)-
amino]propanoate (VIII). mp 151–152°C. IR spec-
trum, ν, cm–1: 2312, 1780, 1742, 1660, 1400, 1384,
1
1270, 1120, 965, 812, 755, 670. H NMR spectrum, δ,
ppm: 2.43 t (2H, CH2CO), 3.58 s (3H, OCH3), 3.94 t
(2H, NCH2), 8.02 d (1H, CH=N), 7.52–8.76 m
(4H, pyridine). Found, %: C 57.48; H 5.96; N 13.55.
C10H12N2O3. Calculated, %: C 57.67; H 5.81; N 13.44.
Methyl 3-[(Z)-oxido(pyridin-2-ylmethylidene)-
amino]propanoate (III). mp 130–131°C. IR spec-
trum, ν, cm–1: 2355, 1780, 1762, 1694, 1422, 1400,
1380, 1362, 1344, 1275, 1191, 1120, 1043, 962, 940,
Methyl 2-methyl-3-[(E)-oxido(pyridin-4-ylmeth-
ylidene)amino]propanoate (IX). mp 146–147°C. IR
spectrum, ν, cm–1: 2325, 1745, 1720, 1665, 1400,
1
825, 758, 692. H NMR spectrum, δ, ppm: 2.44 t
1
(2H, CH2CO), 3.58 s (3H, OCH3), 3.94 t (2H, NCH2),
7.84 d (1H, CH=N), 7.02–8.63 m (4H, pyridine).
1375, 1275, 1120, 962, 810, 750, 665. H NMR spec-
trum, δ, ppm: 1.18 d (3H, CHCH3), 2.48 m (1H,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 4 2009

KOÇAK et al.
594
CHCH3), 3.58 s (3H, OCH3), 3.93 t (2H, NCH2),
7.81 s (1H, CH=N), 7.48–8.76 m (pyridine). Found, %:
C 59.58; H 6.54; N 12.47. C12H14N2O3. Calculated, %:
C 59.45; H 6.36; N 12.61.
chromatography. The product was additionally purified
by recrystallization from petroleum ether or DMSO.
Methyl 3-{[(furan-2-yl)methylidene](oxido)-
amino}propanoate (XIII). mp 112–113°C. IR spec-
trum, ν, cm–1: 2310, 1755, 1710, 1665, 1410, 1362,
Nitrones X–XII (general procedure). Ester IIb,
IIc, or IIe, 25 mmol, was added to a solution of methyl
pyridyl ketone oxime Id or Ie, 10 mmol, and an equiv-
alent amount of Lewis acid catalyst (CdI2/BF3·OBu2)
in 20 ml of benzene or CH2Cl2. The mixture was
stirred under the conditions indicated in Table 1 and
treated with 15 ml of methanol, the solvent was re-
moved on a rotary evaporator, the residue was dis-
solved in acetone or ethyl acetate, and the product was
isolated by column chromatography and additionally
recrystallized from petroleum ether or chloroform.
1
1280, 1120, 962, 812, 750, 660. H NMR spectrum, δ,
ppm: 2.42 m (2H, CH2CO), 3.58 s (3H, OCH3), 3.92 t
(2H, NCH2), 6.63–6.66 m (2H, furan), 7.42 d (1H,
CH=N), 7.62 d (1H, furan). Found, %: C 54.65;
H 5.88; N 7.23. C9H11NO4. Calculated, %: C 54.83;
H 5.60; N 7.11.
Ethyl 2-methyl-3-{[(furan-2-yl)methylidene]-
(oxido)amino}propanoate (XIV). mp 119–120°C. IR
spectrum, ν, cm–1: 2314, 1750, 1714, 1662, 1408,
1
1350, 1285, 1125, 965, 814, 755, 665. H NMR spec-
trum, δ, ppm: 1.13 d (3H, CHCH3), 1.32 t (3H,
CH2CH3), 2.45 m (1H, CHCH3), 3.92 m (2H, NCH2),
4.12 q (2H, OCH2), 6.56–6.59 m (2H, furan), 7.54 d
(1H, furan). Found, %: C 54.65; H 5.88; N 7.23.
C11H15NO4. Calculated, %: C 58.65; H 6.75; N 6.21.
Methyl 2-methyl-3-{(E,Z)-oxido[1-(pyridin-2-yl)-
ethylidene]amino}propanoate (X). mp 138°C. IR
spectrum, ν, cm–1: 2318, 1748, 1716, 1660, 1415,
1
1372, 1270, 1122, 960, 812, 755, 662. H NMR spec-
trum, δ, ppm: 1.14 d (3H, CH3CH), 2.10 s (3H,
CH3C=N), 2.52 m (1H, CH3CH), 3.88 m (2H, NCH2),
4.05 s (3H, OCH3), 6.86–8.54 m (4H, pyridine).
Found, %: C 62.18; H 7.42; N 11.04. C13H18N2O3. Cal-
culated, %: C 62.37; H 7.23; N 11.19.
Nitrones XV–XVII (general procedure). Ester
IIa–IIc, 20 mmol, was added to a solution of 20 mmol
of (E)-cinnamaldehyde oxime (Ig) or (E)-crotonalde-
hyde oxime (Ih) and an equivalent amount of CdI2/
BF3·Bu2O in 25 ml of CH2Cl2. The mixture was stirred
under the conditions indicated in Table 1 and was then
treated as described above for nitrones III–IX.
Isopropyl 3-{(E,Z)-oxido[1-(pyridin-4-yl)ethyli-
dene]amino}propanoate (XI). mp 158–159°C. IR
spectrum, ν, cm–1: 2314, 1745, 1710, 1662, 1410,
1
1370, 1280, 1120, 966, 814, 750, 660. H NMR spec-
Methyl 3-{oxido[(2E)-3-phenylprop-2-en-1-
ylidene]amino}propanoate (XV). mp 150–151°C. IR
spectrum, ν, cm–1: 2295, 1755, 1710, 1660, 1410,
trum, δ, ppm: 1.14 d [6H, CH(CH3)2], 2.42 t (2H,
CH2CO), 3.43 s (3H, CH3C=N), 3.86 t (2H, NCH2),
5.02 m [1H, CH(CH3)2], 6.70–8.42 m (4H, pyridine).
Found, %: C 62.21; H 7.34; N 11.32. C13H18N2O3. Cal-
culated, %: C 62.37; H 7.23; N 11.19.
1
1382, 1260, 1120, 960, 812, 752, 665. H NMR spec-
trum, δ, ppm: 2.44 t (2H, CH2CO), 3.58 s (3H, OCH3),
3.90 d (2H, NCH2), 7.10–7.12 m (2H, CH=CH),
7.50 m (1H, CH=N), 7.24–7.53 m (5H, Harom). Found,
%: C 66.76; H 6.63; N 6.13. C13H15NO3. Calculated,
%: C 66.93; H 6.48; N 6.02.
Ethyl 3-{(E,Z)-oxido[1-(pyridin-4-yl)ethylidene]-
amino}propanoate (XII). mp 162°C. IR spectrum, ν,
cm–1: 2312, 1746, 1712, 1660, 1414, 1368, 1282, 1120,
1
965, 812, 755, 665. H NMR spectrum, δ, ppm: 1.28 t
Methyl 2-methyl-3-{oxido[(2E)-3-phenylprop-
2-en-1-ylidene]amino}propanoate (XVI). mp 142–
143°C. IR spectrum, ν, cm–1: 2285, 1750, 1720, 1665,
(3H, CH3), 2.42 t (2H, CH2CO), 3.36 s (2H, CH2C=N),
3.86 t (2H, NCH2), 4.08 q (2H, OCH2), 6.70–8.42 m
(4H, pyridine). Found, %: C 60.86; H 6.74; N 11.97.
C12H16N2O3. Calculated, %: C 61.01; H 6.83; N 11.86.
1
1492, 1380, 1265, 1125, 965, 812, 750, 660. H NMR
spectrum, δ, ppm: 1.19 d (3H, CH3CH), 2.46 m (1H,
CH3CH), 3.56 s (3H, OCH3), 3.90 d (2H, NCH2),
6.92–7.20 d (2H, CH=CH), 7.30–7.50 m (5H, Harom),
7.62 m (1H, CH=N). Found, %: C 68.11; H 6.85;
N 5.56. C14H17NO3. Calculated, %: C 68.01; H 6.94;
N 5.66.
Nitrones XIII and XIV (general procedure). Ester
IIa or IId, 45 mmol, was added to a solution of
20 mmol of furfural oxime (If) and an equivalent
amount of Lewis acid catalyst (CdI2/BF3 ·Bu2O) in
20 ml of anhydrous dimethylformamide. The mixture
was stirred under the conditions indicated in Table 1
and treated with 15 ml of methanol. The solvent was
evaporated, the residue was dissolved in methylene
chloride, and the solution was subjected to column
Ethyl 3-[(2E)-but-2-en-1-ylidene(oxido)amino]-
propanoate (XVII). mp 165–167°C. IR spectrum, ν,
cm–1: 2270, 1760, 1715, 1662, 1495, 1384, 1260, 1120,
1
960, 815, 755, 668. H NMR spectrum, δ, ppm: 1.22 t
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 4 2009

SYNTHESIS OF ALKYL NITRONES
595
9. Manna, F., Bolasco, A., Bizzari, B., and Lena, R.,
(3H, CH3), 1.81 d (3H, CH3C=), 2.36 t (2H, CH2CO),
3.77 t (2H, NCH2), 4.07 m (2H, OCH2), 6.17–6.36 m
(2H, CH=CH), 7.52 d (1H, CH=N). Found, %:
C 58.12; H 8.31; N 7.40. C9H15NO3. Calculated, %:
C 58.37; H 8.15; N 7.55.
Il Farmaco, 1996, vol. 51, p. 579.
10. Hodgson, D.M. and Lee, G.P., Il Farmaco, 1996,
vol. 51, p. 254; Major, R.T. and Only, K.W., J. Med.
Pharm. Chem., 1961, vol. 4, no. 1, p 51; Chem. Abstr.,
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Financial support for this research from the Selçuk
University Scientific Research Projects is gratefully
acknowledged.
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laus, B.J.R., Pagani, G., and Testa, E., Helv. Chim. Acta,
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