Unusual formation of tetrahydropyridazine-3,4,5,6-tetracarboxylic and pyrroletetracarboxylic esters upon decomposition of methyl diazoacetate in the presence of pyridine

Article abstract of DOI:10.1023/A:1022421020644

Thermal, photolytic, and thermocatalytic decomposition of methyl diazoacetate (MDA) in the presence of Rh₂(OAc)₄ or Cu(acac)₂ in refluxing pyridine afforded isomeric trans, cis- and cis, trans-3,4,5,6-tetra(methoxycarbonyl)-1,4,5,6-tetrahydropyridazines (～1 : 1) in a total yield of 30-70%. Decomposition of MDA in refluxing o-xylene in the presence of Rh₂(OAc)₄ and pyridine (20 mol.%) gave rise to 2,3,4,5-tetra(methoxycarbonyl)pyrrole in a yield of up to 40%. In these transformations of MDA, neither dimethyl fumarate (or dimethyl maleate) nor the corresponding 2-pyrazolines were generated as intermediates.

Full text of DOI:10.1023/A:1022421020644

Russian Chemical Bulletin, International Edition, Vol. 52, No. 1, pp. 187—191, January, 2003
187
Unusual formation of tetrahydropyridazineꢀ3,4,5,6ꢀtetracarboxylic
and pyrroletetracarboxylic esters upon decomposition
of methyl diazoacetate in the presence of pyridine*
Yu. V. Tomilov,aꢀ D. N. Platonov, B. B. Averkiev, E. V. Shulishov, and O. M. Nefedov
a
b
a
a
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 6390. Eꢀmail: tom@ioc.ac.ru
b
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
2
8 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5085
Thermal, photolytic, and thermocatalytic decomposition of methyl diazoacetate (MDA)
in the presence of Rh (OAc) or Cu(acac) in refluxing pyridine afforded isomeric trans,cisꢀ and
2
4
2
cis,transꢀ3,4,5,6ꢀtetra(methoxycarbonyl)ꢀ1,4,5,6ꢀtetrahydropyridazines (∼ 1 : 1) in a total yield
of 30—70%. Decomposition of MDA in refluxing oꢀxylene in the presence of Rh (OAc) and
2
4
pyridine (20 mol.%) gave rise to 2,3,4,5ꢀtetra(methoxycarbonyl)pyrrole in a yield of up to 40%.
In these transformations of MDA, neither dimethyl fumarate (or dimethyl maleate) nor the
corresponding 2ꢀpyrazolines were generated as intermediates.
Key words: methyl diazoacetate, tetrahydropyridazines, tetra(methoxycarbonyl)pyrrole,
catalysis, thermolysis, photolysis, dediazotization, Xꢀray diffraction analysis.
Due to high reactivity of derivatives of diazoacetic
acid and the presence of several reaction centers in their
molecules, these compounds can undergo various chemiꢀ
cal transformations and have considerable synthetic poꢀ
tential. The main characteristic feature of diazo esters,
which is common to all aliphatic diazo compounds, is
that their reactions can proceed with either retention or
elimination of the diazo fragment. The 1,3ꢀdipolar cyꢀ
cloaddition of diazo esters to compounds containing mulꢀ
tiple bonds is most typical of the first type of these transꢀ
formations.1 The second group of reactions proceeding
with elimination of the nitrogen molecule are primarily
characterized by generation of carbenes, their comꢀ
plexes with transition metals, cationoid reagents, and biꢀ
radicals.2 Subsequent transformations of these highly
reactive intermediates are rather diversified and can be
carbonylcarbenes is to use catalytic dediazotization of
diazo esters.
In the present study, we examined decomposition of
methyl diazoacetate (MDA) in the presence of pyridine
under thermal, photolytic, and thermocatalytic condiꢀ
tions and revealed the unusual formation of new methoxyꢀ
carbonyl derivatives of pyrrole and 1,4,5,6ꢀtetrahydroꢀ
pyridazine. These heterocyclic compounds are devoid of
the pyridine fragment. However, the presence of pyridine
in the reaction medium is essential for generation of these
compounds from diazoacetate molecules. Thus, the slow
addition of MDA to refluxing pyridine containing a cataꢀ
lytic amount of Cu(acac) or Rh (OAc) is accompanied
,2
2
2
4
by partial dediazotization of MDA to give a complex mixꢀ
ture of products from which two fractions were isolated by
—5
column chromatography on SiO . Each fraction was enꢀ
2
riched with one of isomeric 1,4,5,6ꢀtetrahydropyridazineꢀ
accompanied by the insertion of the
fragꢀ
3
,4,5,6ꢀtetracarboxylic esters (1). Both isomers were isoꢀ
ment into single bonds (in particular, into the C—H bond)
and the addition at the multiple bonds (including aroꢀ
matic bonds) to form threeꢀmembered rings, ylides and
their transformation products, etc. One of the main ways
of controlling the selectivity of reactions involving alkoxyꢀ
lated in individual form by repeated chromatography of
the corresponding fractions. The total yields of esters 1 in
the presence of Cu(acac) or Rh (OAc) were ∼ 50 and
2
2
4
7
0%, respectively. In both cases, the ratio between the
trans,cis and cis,trans isomers was ∼ 1 : 1. Interestingly, we
did not detect maleic and fumaric esters among the prodꢀ
ucts of decomposition of MDA, which are generally proꢀ
*
Dedicated to Academician G. A. Tolstikov on the occasion of
his 70th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 176—179, January, 2003.
066ꢀ5285/03/5201ꢀ187 $25.00 © 2003 Plenum Publishing Corporation
1

1
88
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
Tomilov et al.
3
duced upon catalytic decomposition of diazo esters. The
reactions also did not give rise to trimethyl transꢀaconitate,
which is a product of formal trimerization of methoxyꢀ
carbonylcarbene generated through dediazotization of
MDA under the action of catalytic amounts of the
completion of decomposition of MDA (after ∼ 20 h), comꢀ
pounds 1 were obtained in 30—33% yields.
It should be noted that pyridine plays a key role in the
observed transformations of MDA. The use of triethylꢀ
amine or dimethylaniline instead of pyridine did not
lead to the formation of tetrahydropyridazines 1. In
the presence of quinoline (∼ 120 °C), decomposition
of MDA afforded isomeric tetraesters 1a,b in 30—35%
yields.
Direct photolysis of MDA in pyridine at 20 °C was
also accompanied by its partial dediazotization to give
tetrahydropyridazines 1. However, their yields were at
most 30%.
6
Cu(acac) •ZnCl •C H N complex.
2
2
6
5
The structures of the resulting compounds were estabꢀ
1
13
lished by mass spectrometry and H and C NMR specꢀ
troscopy. In addition, the structure of isomer 1a with the
diaxialꢀequatorial arrangement of three methoxycarbonyl
substituents at the C(4), C(5), and C(6) atoms was conꢀ
firmed by Xꢀray diffraction analysis. The 2DꢀNOESY
spectrum of this compound has crossꢀpeaks correspondꢀ
ing to the throughꢀspace coupling of NH with H(6) as
well as of H(5) with H(4) and H(6), whereas no clear
coupling between the protons of the methoxy groups is
observed. The position of the signal for the proton at C(6)
was confirmed by the {C,H}ꢀcorrelation experiment based
To reveal the possible paths of the formation of the
tetrahydropyridazine structure in the course of the obꢀ
served transformations, we studied catalytic decomposiꢀ
tion of MDA in refluxing pyridine in the presence of halfꢀ
molar amounts of dimethyl maleate, dimethyl fumarate,
or 3,4,5ꢀtrimethoxycarbonylꢀ2ꢀpyrazoline (2), which is a
product of 1,3ꢀdipolar cycloaddition of MDA to dimethyl
fumarate. Because of the presence of the MDA fragments,
these compounds could be precursors of tetrahydroꢀ
pyridazines 1. However, it appeared that the first two
reactions proceeded primarily as the usual 1,3ꢀdipolar
cycloaddition of MDA to the corresponding unsaturated
compounds to give pyrazoline 2, which did not react with
MDA in pyridine both under the thermal and thermoꢀ
catalytic (in the presence of Rh (OAc) ) conditions. In
13
on the fact that the C NMR spectrum has signals for the
C(4) and C(5) atoms at δ 37—39, whereas a signal for
C(6) is observed at δ 52. We failed to crystallize isomer 1b.
1
13
However, the H and C NMR spectral patterns of this
compound indicate that it has the structure of tetraꢀ
substituted 1,4,5,6ꢀtetrahydropyridazine, the NOESY and
{
C,H}ꢀcorrelation experiments for 1b also showing couꢀ
plings of NH with H(6) and of H(5) with H(4) and H(6).
However, unlike the spectrum of isomer 1a, which has
signals for the H(4) and H(6) protons at δ 4.4 and 4.2,
respectively, the spectrum of 1b is characterized by the
reverse order of these signals (at δ 4.1 and 4.5 for H(4) and
H(6), respectively). The vicinal spinꢀspin coupling conꢀ
stants in the spectra of both isomers are at most 4 Hz,
which is indicative of the absence of axialꢀaxial couplings.
It can be assumed that isomerism of these compounds
results from the different positions of the COOMe groups
at the C(4) and C(6) atoms.
2
4
the absence of pyridine (CH Cl , 20 °C), catalytic deꢀ
2
2
composition of MDA under the action of Rh (OAc) led
2
4
to the insertion of the carbene fragment into the N—H
bond to form Nꢀsubstituted pyrazoline 3. Upon refluxing
In the absence of a catalyst, decomposition of MDA
in refluxing pyridine proceeded very slowly. For example,
the conversion of MDA achieved after 6—7 h was ∼ 35%,
the percentage of tetrahydropyridazines 1 (according to
1
the H NMR spectrum of the reaction mixture) being
about 30%. However, a further increase in the reaction
time did not virtually lead to an increase in the yield of
the target product because of its thermal instability. After

Decomposition of methyl diazoacetate
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
189
in pyridine, compound 3, like compound 2, remained
unchanged. Therefore, neither maleic and fumaric esters
nor pyrazolines 2 and 3 are intermediates in the reactions
giving rise to tetrahydropyridazines 1.
firmed both by spectroscopic data and Xꢀray diffraction
analysis.
Then we attempted to perform the reaction of diꢀ
methyl fumarate as a dienophile with heterocyclic comꢀ
7
pound 4 described earlier. The latter compound is forꢀ
mally the cyclic dimer of MDA. Taking into account the
compositions of the molecules and the character of bonds,
both these compounds would be expected to be intermeꢀ
diates in the course of the transformation of MDA in
pyridine, and their [4+2] cycloaddition followed by elimiꢀ
A special experiment showed that these reaction conꢀ
ditions (refluxing in oꢀxylene in the presence of pyridine)
did not give rise to the transformation of tetrahydroꢀ
pyridazine 1 into pyrrole 5 with the formal elimination of
the ammonia molecule. Hence, the formation of pyrꢀ
role 5 is even more unexpected than the formation of
tetrahydropyridazine 1, which retains for the most part
the MDA fragments. However, it is not inconceivable
that decomposition of MDA in this case also proceeds
through the formation of pyridinium ylides and a particuꢀ
lar common intermediate. Under more drastic conditions
and at a low concentration of pyridine, aromatization of
the latter intermediate giving the pyrrole derivative in one
of the last steps becomes a preferred process. The elucidaꢀ
tion of the characteristic features of these reactions and
the mechanism of formation of heterocycles 1 and 5 will
be the subject of our further investigation.
nation of the N molecule would be expected to afford
2
tetrahydropyridazines 1. However, according to the
1
H NMR spectroscopic data, both dimethyl fumarate and
compound 4 remained unchanged upon refluxing in pyꢀ
ridine for 8 h, and tetrahydropyridazine 1 was not deꢀ
tected.
Experimental
The 1H and 13C NMR spectra were recorded on Bruker
ACꢀ200 (200.13 and 50.3 MHz) and Bruker DRXꢀ500
Consequently, compound 4 is not an intermediate of
tetramerization and partial dediazotization of MDA in
the presence of pyridine. In this connection, we assumed
that tetrahydropyridazine derivatives can be formed
through generation of pyridinium ylide and carbonꢀchain
growth by the successive addition of the carbene fragꢀ
ments (CHCOOMe) and MDA. However, this hypothꢀ
esis calls for further verification.
(
500.13 MHz) spectrometers in solutions in CDCl containing
3
0
.05% of Me Si as the internal standard. The mass spectra were
4
measured on a Finnigan MAT INCOSꢀ50 instrument (EI,
0 eV). The reactions were carried out with the use of freshly
distilled pyridine. The catalysts Rh (OAc) and Cu(acac) were
7
2
4
2
purchased from Acros Organics. 3,4,5ꢀTrimethoxycarbonylꢀ2ꢀ
9
pyrazoline (2) and dihydroꢀ3,6ꢀbismethoxycarbonylꢀ1,2,4,5ꢀ
tetrazine (4)7 were prepared according to known procedures.
Column chromatography was carried out with the use of silica
gel 60 (0.063—0.200 mm; Merck).
A decreases in the amount of pyridine used as the
solvent to ∼ 20 mol.% and the rise of the temperature by
performing the reaction in refluxing oꢀxylene resulted in a
change in the pathway of the transformations of MDA.
Thermal decomposition of MDA (10 h) facilitated preꢀ
dominantly the reaction of methoxycarbonylcarbene that
Singleꢀcrystal Xꢀray diffraction study of compounds 1a and
5
was carried out on an automated Bruker 1K SMART CCD
diffractometer (MoꢀKα radiation). The crystals are monoclinic,
at 293 K a = 10.379(3) Å, b = 14.408(5) Å, c = 9.772(4) Å, V =
3
–3
1455.3(8) Å , Z = 4, d
= 1.443 g cm , space group P2(1)/c
calc
8
formed with oꢀxylene. By contrast, the reaction perꢀ
for compound 1a and a = 17.428(4) Å, b = 10.358(2) Å, c =
3
–3
8
.013(2) Å, V = 1319.0(5) Å , Z = 4, d
= 1.507 g cm , space
formed under the thermocatalytic conditions (Rh (OAc) ,
calc
2
4
group C2/c for compound 5. All calculations were carried out
2
h) afforded a new compound, viz., 2,3,4,5ꢀtetramethoxyꢀ
1
0
with the use of the SHELXTL PLUS program package. The
atomic coordinates and complete data for compounds 1a and 5
were deposited with the Cambridge Structural Database. The
selected geometric parameters of the compounds are given in
Tables 1 and 2.
carbonylpyrrole (5), along with the adducts of CHCOOMe
with xylene, which were detected by GLCꢀmass specꢀ
trometry. The yield of compound 5 reached 40%. It should
be noted that this reaction did not give rise to sixꢀmemꢀ
bered heterocycle 1. Product 5 was isolated by column
chromatography on SiO2 and its structure was conꢀ
3,4,5,6ꢀTetra(methoxycarbonyl)ꢀ1,4,5,6ꢀtetrahydropyridꢀ
azine (1). Method A. A solution of methyl diazoacetate (MDA)

1
90
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
Tomilov et al.
Table 1. Selected geometric parameters of molecule 1a
(0.10 g, 0.38 mmol) in refluxing pyridine (20 mL) in a yield of
0
.79 g (50%) in a ratio of ∼ 1 : 1.
Method C. A solution of MDA (0.2 g) in pyridine (2 mL) was
Bond
d/Å
Angle
ω/deg
irradiated using a quartz lamp for 8 h, the solvent was removed
1
N(1)—N(2)
N(1)—C(6)
N(2)—C(3)
C(3)—C(4)
C(4)—C(5)
C(5)—C(6)
1.338(2)
1.442(3)
1.289(2)
1.504(3)
1.534(3)
1.539(3)
N(2)—N(1)—C(6) 121.2(2)
N(1)—N(2)—C(3) 119.1(2)
N(2)—C(3)—C(4)
C(3)—C(4)—C(5)
C(4)—C(5)—C(6)
C(5)—C(6)—N(1)
in vacuo, and the residue was analyzed by H NMR spectrosꢀ
copy. The total yield of isomers 1a and 1b was ∼ 30%, the isomer
ratio was ∼ 1 : 1.
125.5(2)
110.6(2)
107.5(2)
107.9(2)
Method D. A solution of MDA (0.2 g) in pyridine (2 mL) was
refluxed for 20 h, the solvent was removed in vacuo, and the
1
residue was analyzed by H NMR spectroscopy. The total
yield of isomers 1a and 1b was 30—33%, the isomer ratio
was ∼ 1 : 1.
3
,4,5ꢀTri(methoxycarbonyl)ꢀ1ꢀ(methoxycarbonylmethyl)ꢀ2ꢀ
Table 2. Selected geometric parameters of molecule 5
pyrazoline (3). Methyl diazoacetate (0.33 g, 3.3 mmol) was added
to a solution of 3,4,5ꢀtri(methoxycarbonyl)ꢀ2ꢀpyrazoline (2)
0.73 g, 3 mmol) and Rh (OAc)4 (7.5 mg, 0.017 mmol) in
Bond
d/Å
Angle
ω/deg
(
2
CH Cl (4 mL) at 20 °C for 2 h. Then the reaction mixture was
stirred for 30 min and the solvent was removed in vacuo. The
N(1)—C(2)
C(2)—C(3)
C(3)—C(4)
C(2)—C(O) 1.475(2)
C(3)—C(O) 1.485(2)
1.359(2)
1.388(2)
1.407(3)
C(2)—N(1)—C(5) 109.9(2)
N(1)—C(2)—C(3) 107.9(2)
C(2)—C(3)—C(4) 107.1(1)
N(1)—C(2)—C(O) 123.3(1)
C(2)—C(3)—C(O) 127.1(1)
2
2
residue was chromatographed on a column with SiO to obtain a
2
1
fraction (0.61 g). According to the H NMR spectroscopic data,
this fraction contained ∼ 85% of pyrazoline 3 (58% yield), 10% of
1
the starting pyrazoline 2, and 5% of dimethyl fumarate. H NMR
of compound 3 (200 MHz, CDCl ), δ: 3.72, 3.77, 3.78, and 3.81
3
(
all s, 3 H each, OMe); 4.29 and 4.46 (both d, 1 H each, CH2,
J = 18.2 Hz); 4.49 (d, 1 H, H(4), J4,5 = 10.0 Hz); 4.92 (d, 1 H,
(
5.0 g, 50 mmol) in pyridine (40 mL) was added with stirring and
2
refluxing to a solution of Rh (OAc)4 (0.11 g, 0.25 mmol) in
2
H(5), J4,5 = 10.0 Hz). The sample thus obtained was refluxed in
pyridine for 7 h. Then the pyridine was completely removed in
vacuo. According to the H NMR spectrum of the residue, the
composition of the reaction mixture remained unchanged and
the formation of tetrahydropyridazine 1 was not observed.
pyridine (80 mL) during 2 h. The reaction mixture was refluxed
with stirring for 30 min and the pyridine was removed in vacuo.
The black resinous precipitate that formed was dissolved in a
minimum volume of AcOEt and passed through a layer of silica
gel (∼ 8 cm); elution was performed with AcOEt (20 mL). The
solvent was removed in vacuo and the residue was chromatoꢀ
graphed on a column with SiO2 (benzene—AcOEt, 1 : 1, as
the eluent). Fraction of approximately the same mass were obꢀ
tained. The fractions were enriched with isomers 1a and 1b,
respectively, by 80—85%; the total yield was 2.68 g (68%). The
isomers were separated by repeated chromatography of each
fraction.
1
2
,3,4,5ꢀTetra(methoxycarbonyl)pyrrole (5). A solution of
MDA (1.0 g, 10 mmol) in oꢀxylene (4 mL) was added with
stirring and refluxing to a solution of Rh (OAc)4 (22 mg,
0
2
stirring for 30 min and the solvents were removed in vacuo. The
black oily residue was supported onto silica gel (∼ 8 mL) and
eluted first with benzene (100 mL) to remove the adducts of
methoxycarbonylcarbene with oꢀxylene8 and then with ethyl
acetate (100 mL). A solution in AcOEt was concentrated in
vacuo and the resulting red oil was purified by column chromaꢀ
2
.05 mmol) in a mixture of oꢀxylene (8 mL) and pyridine (0.16 g,
mmol) for 2.5 h. The reaction mixture was refluxed with
Compound 1a, weakly colored crystals, m.p. 149—150 °C.
1
H NMR (CDCl ), δ: 3.68, 3.76, 3.84, and 3.85 (all s, 4 3 H each,
3
OMe); 3.74 (dd, 1 H, H(5), J4,5 = 1.8 Hz, J5,6 = 3.6 Hz); 4.12
(
dt, 1 H, H(6), J5,6 = 3.6 Hz, J1,6 = J4,6 = 1.8 Hz); 4.36 (t, 1 H,
tography (SiO , benzene—AcOEt, 1 : 1, as the eluent). Pyrꢀ
role 5 was obtained in a yield of 0.28 g (38%) as colorless crysꢀ
tals, m.p. 127—128 °C, R = 0.6. H NMR (CDCl ), δ: 3.88 and
3
13
2
H(4), J4,5 = J4,6 = 1.8 Hz); 7.21 (br.s, 1 H, NH). C NMR
CDCl ), δ: 37.8 (C(5)); 39.4 (C(4)); 51.5 (C(6)); 52.3, 52.8,
(
3
1
f
3
5
1
2
2.9, and 53.0 (4 OMe); 126.3 (C(3)); 164.5, 168.6, 168.9, and
13
.91 (both s, 6 H each, 4 OMe); 10.4 (br. s, 1 H, NH). C NMR
+
70.9 (4 CO). Partial MS, m/z (Irel (%)): 316 (6) [M] , 284 (12),
(
CDCl ), δ: 52.6 and 52.8 (2 OMe each); 121.6 (C(3) and
3,
57 (8), 225 (24), 197 (100). Selected geometric parameters of
C(4)); 123.4 (C(2) and C(5)); 159.2 and 163.3 (2 CO each).
Partial MS, m/z (Irel (%)): 299 (53) [M] , 268 (67), 236 (100).
Found (%): C, 47.91; H, 4.49; N, 4.51. C H NO . Calcuꢀ
lated (%): C, 48.17; H, 4.38; N, 4.68. Selected geometric paꢀ
rameters of molecule 5 determined by Xꢀray diffraction analysis
are given in Table 2.
molecule 1a are given in Table 1.
+
Compound 1b (with an impurity of ∼ 6% of isomer 1a), visꢀ
1
12 13
8
cous weakly colored liquid. H NMR (CDCl ), δ: 3.69, 3.72,
3
J4,5 = 2.6 Hz, J5,6 = 1.3 Hz); 4.18 (dd, 1 H, H(4), J4,5 = 2.6 Hz,
J4,6 = 1.7 Hz); 4.50 (ddd, 1 H, H(6), J = 3.1 Hz, J4,6 = 1.7 Hz,
J5,6 = 1.3 Hz); 7.18 (br.s, 1 H, NH). 13C NMR (CDCl ), δ: 37.3
(
(
Partial MS, m/z (Irel (%)): 316 (7) [M] , 284 (12), 257 (15), 225
(
C H N O . Calculated (%): C, 45.57; H, 5.10; N, 8,86.
Method B. A mixture of tetrahydropyridazines 1a and 1b was
prepared analogously from MDA (2.0 g, 20 mmol) and Cu(acac)2
3
.76, and 3.83 (all s, 4 3 H each, OMe); 3.86 (dd, 1 H, H(5),
1,6
3,
This study was financially supported by the Russian
Foundation for Basic Research (Project Nos. 02ꢀ03ꢀ33365
and 00ꢀ15ꢀ97387) and by the Federal Target Scientific
and Technological Program of the Ministry of Industry,
Science, and Technology of the Russian Federation ("New
Materials and Chemical Products," State Contract No.
41.002.1.1.1401).
C(4)); 38.7 (C(5)); 52.3, 52.8, 53.0, and 53.2 (4 OMe); 52.9
C(6)); 128.1 (C(3)); 164.5, 169.5, 170.2, and 170.4 (4 CO).
+
25), 197 (100). Found (%): C, 45.25; H, 5.49; N, 8,47.
12
16
2
8

Decomposition of methyl diazoacetate
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 1, January, 2003
191
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Received October 28, 2002