The chemoselective alkynylation of dihaloquinolines by the Sonogashira-Hagihara reaction

Article abstract of DOI:10.1023/B:RUCB.0000024849.57521.49

Conditions for the regioselective Sonogashira-Hagihara alkynylation of 4-chloro-6-iodo(bromo)quinolines were found and 6-alkynyl-4-chloroquinolines were obtained in 90-100% yields.

Full text of DOI:10.1023/B:RUCB.0000024849.57521.49

Russian Chemical Bulletin, International Edition, Vol. 53, No. 1, pp. 189—193, January, 2004
189
The chemoselective alkynylation of dihaloquinolines
by the Sonogashira—Hagihara reaction
I. P. Beletskaya,ꢀ G. V. Latyshev, A. V. Tsvetkov, and N. V. Lukashev
Department of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (095) 939 3618. Eꢀmail: beletska@org.chem.msu.ru; tsvet@org.chem.msu.su
Conditions for the regioselective Sonogashira—Hagihara alkynylation of 4ꢀchloroꢀ6ꢀ
iodo(bromo)quinolines were found and 6ꢀalkynylꢀ4ꢀchloroquinolines were obtained in
90—100% yields.
Key words: alkynylation, dihaloquinolines, Sonogashira—Hagihara reaction.
The coupling reaction of terminal acetylenes with aryl
and vinyl halides catalyzed by palladium complexes in the
presence of a base and a catalytic amount of CuI (the
Sonogashira—Hagihara reaction) is a widely used tool of
fine organic synthesis.1 Owing to experimental simplicity,
high product yields, and the tolerance to a broad range of
functional groups, this reaction can be regarded as one of
the most convenient and versatile methods for the synꢀ
thesis of arylacetylenes and enynes. During the last years,
development of new catalyst systems allowed researchers
to broaden the range of substrates for this reaction and to
extend the reaction to highly hydrophilic compounds,2
baseꢀsensitive alkynes with electronꢀwithdrawing substituꢀ
ents,3 and some substrates traditionally "problematic" for
palladiumꢀcatalyzed reactions such as vinyl chlorides4 and
aryl chlorides.5
of transformations. Previously, we showed10 that one or
two halogen atoms in 4,6ꢀdihaloquinolines can be reꢀ
placed in a high yield by aryl groups using the Suzuki
reaction. The purpose of this study is to develop a facile
and convenient method for the synthesis of alkynylhaloꢀ
quinolines by selective replacement of one halogen atom
in dihaloquinolines.
Results and Discussion
For the attempt at selective monoalkynylation of
4,6ꢀdihaloquinolines, we used as the model the reacꢀ
tion of 4ꢀchloroꢀ6ꢀiodoquinoline with phenylacetylene
(Scheme 1).
Scheme 1
The use of dihalo derivatives of arenes and hetarenes
as substrates for the regioselective introduction of subꢀ
stituents via crossꢀcoupling markedly extends the scope
of the method and opens up a facile synthetic approach to
diverse classes of diꢀ and polysubstituted aromatic or
heteroaromatic compounds. This strategy was successꢀ
fully used for the synthesis of materials with liquidꢀcrystal
properties,6 natural products,7 and biologically active
compounds.8 Conduction of consecutive substitution reꢀ
actions using electronꢀdeficient heterocycles containing
two halogen atoms, one being located in a position actiꢀ
vated toward a nucleophilic attack, is of particular interꢀ
est. On the one hand, it is not always possible to predict
a priori the relative reactivities of different Csp2—Hal
bonds, for example in 4ꢀchloroꢀ6ꢀiodoquinoline, in the
crossꢀcoupling. On the other hand, a subsequent crossꢀ
coupling step or aromatic nucleophilic substitution can
be easily carried out for the resulting compound.
We found that the reaction of 4ꢀchloroꢀ6ꢀiodoquinoꢀ
line with a nearly stoichiometric amount of phenylꢀ
acetylene in THF in the presence of 2 equiv. of Et3N and
a catalytic amount of Pd(PPh3)4 and CuI gives only the
Readily accessible 4,6ꢀdihaloquinolines9 are interestꢀ
ing and convenient substrates for performing this sequence
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 181—185, January, 2004.
1066ꢀ5285/04/5301ꢀ0189 © 2004 Plenum Publishing Corporation

190
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 1, January, 2004
Beletskaya et al.
product of iodine replacement (according to 1H NMR
and elemental analysis data) in 81% yield. However, the
presence of 19% of the unchanged starting dihaloquinoline
appreciably hampers the isolation of the reaction product
in a pure state. Despite the fact that the yield of the
monoalkynylation product can be increased to nearly
100% by using an excess of phenylacetylene, this apꢀ
proach requires permanent monitoring of the reaction
progress and is hardly suitable for more expensive alkynes.
Therefore, we undertook a systematic search for the
optimal conditions of chemoselective monoalkynylation
of 4ꢀchloroꢀ6ꢀiodoquinoline.
A study of the influence of the nature of catalyst preꢀ
cursors on the yield of the monoalkynylation product
showed that Pd0 and PdII complexes with PPh3 exhibit
similar activities (Table 1, runs 1, 2). The use of pallaꢀ
dium complexes with bidentate phosphine ligands (see
Table 1, runs 4—6) results in a lower product yield and a
somewhat lower chemoselectivity of the reaction. The
low yield observed with Pd(OAc)2 as the catalyst (see
Table 1, run 3) is apparently due to its instability under
the reaction conditions.
Table 2. Effect of the solvent on the yield of the monoꢀ
alkynylation product 1a
Run
Solvent
Reaction time
/min
Yield
(%)
1
2
3
4
5
6
THF
Dioxane
MeCN
Et3N
MeCN—H2Oc
Dioxane—H2Oc
240
240
15
15
5
81
91
96 (93b)
98
98
97
5
aReaction conditions: 5 mol.% Pd(PPh3)4, 10 mol.% CuI,
1.05 equiv. of PhC≡CH, 2 equiv. of Et3N, refluxing.
b The isolated yield.
c Ratio 3 : 1.
Table 3. Effect of a base on the yield of the monoalkynylation
product 1a
Run
Baseb
Reaction time
/min
Yield
(%)
1
2
3
Et3N
K2CO3
K3PO4
5
5
120
97
98
100
The behavior of the reaction depends substantially on
the nature of the solvent. When THF is replaced by dioxꢀ
ane (Table 2, run 2), the yield slightly increases (probꢀ
ably, due to the higher boiling point of the solvent). The
use of MeCN or Et3N as the solvent (see Table 2, runs 3, 4)
resulted in an almost complete conversion of 4ꢀchloroꢀ6ꢀ
iodoquinoline over a short period of time.
a Reaction conditions: 5 mol.% Pd(PPh3)4, 10 mol.% CuI,
1.05 equiv. of PhC≡CH, dioxane—H2O (3 : 1), 100 °C.
b 2 equiv.
A significant acceleration of the reaction is attained by
using water as a coꢀsolvent (see Table 2, runs 5, 6). The
replacement of Et3N by K2CO3 does not influence the
reaction rate or the product yield (Table 3, run 2). Howꢀ
ever, the use of K3PO4 as the base (see Table 3, run 3)
substantially increases the reaction time (2 h instead
of 5 min). However, it should be noted that a decrease in
the process rate does not result in a lower product yield.
It was found that the reaction of 6ꢀbromoꢀ4ꢀchloroꢀ
quinoline with phenylacetylelene also proceeds selectively,
giving rise, under the same conditions (see Table 3, run 1),
to the substitution product of Br in a high yield (98%)
over a short period (5 min).
Table 1. Effect of the catalyst on the yield of the
monoalkynylation product of 4ꢀchloroꢀ6ꢀiodoꢀ
quinolinea
The reaction carried out without a copper coꢀcatalyst
is markedly retarded (the yield is only 33% after 4 h)
(Table 4, run 1). It is of interest that, unlike CuI, which
does not exhibit a catalytic activity, the soluble complex
Cu(PPh3)3Br (see Table 4, run 3) has an activity compaꢀ
Run
Catalyst
(mol.%)
Yield
(%)
1
2
3
4
5
6
Pd(PPh3)4 (5)
Pd(PPh3)2Cl2 (5)
Pd(OAc)2 (5)
Pd(dppf)Cl2 (5)
Pd(dppb)Cl2 (5)
Pd(OAc)2 (5)/dppe (10)
81
80
32
68
67b
59c
Table 4. Reaction of 4ꢀchloroꢀ6ꢀiodoquinoꢀ
line with phenyalacetylene in the presence
of palladium and copper catalysts*
Run
Catalyst
(mol.%)
Yield of
product 1 (%)
Note. dppf is 1,1´ꢀbis(diphenylphosphino)ferroꢀ
cene, dppb is 1,4ꢀbis(diphenylphosphino)butane,
dppe is 1,2ꢀbis(diphenylphosphino)ethane.
a Reaction conditions: 1.05 equiv. of PhC≡CH,
2 equiv. of Et3N, 10 mol.% CuI, THF, 65 °C, 4 h.
b The yield of the dialkynylation product was 3%.
c The yield of the dialkynylation product was 4%.
1
2
3
Pd(PPh3)4 (5)
CuI (10)
Cu(PPh3)3Br (10)
33
1
38
* Reaction conditions: 1.05 equiv. of
PhC≡CH, MeCN—H2O (3 : 1), 80 °C, 4 h.

Alkynylation of dihaloquinolines
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 1, January, 2004
191
Scheme 3
rable to that of Pd(PPh3)4 (in the absence of a copper coꢀ
catalyst).
The selective replacement of a halogen atom in 6ꢀhaloꢀ
4ꢀchloroquinolines can be extended to other alkynes; the
reactions afford 6ꢀalkynylꢀ4ꢀchloroquinolines 2 and 3 in
high preparative yields (Scheme 2).
Scheme 2
of a fresh portion of phenylacetylene results in the formaꢀ
tion of compound 4 in a quantitative yield. The reaction
can also be brought to completion (conversion 98%) when
4 equiv. of phenylacetylene are taken from the very beꢀ
ginning.
The method for the synthesis of alkynylquinolines that
we developed in combination with the selective arylation
of dihaloquinolines described in our previous publicaꢀ
Table 5. Effect of the solvent on the yield of the product of
dialkynylation of 4ꢀchloroꢀ6ꢀiodoquinoline*
Run
Solvent
(ratio)
Reaction
time/h
Yield of
product 4 (%)
1
2
3
4
Dioxane—H2O (3 : 1)
Dioxane—H2O (19 : 1)
THF—H2O (3 : 1)
4
4
4
7
83
80
70
70
MeCN—H2O (3 : 1)
We attempted to perform a successive dialkynylation
of 4ꢀchloroꢀ6ꢀiodoquinoline by introducing it into the
reaction with a twofold amount of phenylacetylene
(2.2 equiv.) (Scheme 3).
* Reaction conditions: 2.2 equiv. of PhC≡CH, 5 mol.%
Pd(PPh3)4, 10 mol.% CuI, 4 equiv. of Et3N, 4 h, refluxing.
The reaction carried out in an aqueous dioxane in the
presence of 4 equiv. of Et3N, 5 mol.% Pd(PPh3)4, and
10 mol.% CuI affords a mixture of 4,6ꢀbis(phenylꢀ
ethynyl)quinoline 4 (83%) and 6ꢀmonoalkynylated comꢀ
pound 1 (15%). The use of other water—organic mixtures
does not increase the yield of the disubstitution product
(Table 5).
The use of other bases, instead of Et3N, also does not
increase the degree of conversion of compound 1 into
product 4 (Table 6).
The incomplete conversion of alkynylchloroquinoline
is apparently caused by side processes that lead to conꢀ
sumption of phenylacetylene. Indeed, the repeated addiꢀ
tion of the catalyst to the reaction mixture does not inꢀ
duce an increase in the product yield, while the addition
Table 6. Effect of the base on the yield
of the product of dialkynylation of
4ꢀchloroꢀ6ꢀiodoquinolinea
Run
Base
Yield (%)
1
2
3
Et3N
83
65
52
DABCOb
c
K2CO3
a
Reaction conditions: 2.2 equiv. of
PhC≡CH, 5 mol.% Pd(PPh3)4, 10 mol.%
CuI, dioxane—H2O (3 : 1), 4 h, 100 °C.
b 3 equiv.
c
3 equiv. (reaction with 6ꢀbromoꢀ4ꢀ
chloroquinoline).

192
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 1, January, 2004
Beletskaya et al.
tion10 permits easy synthesis of alkynylarylquinolines in
high yields using an arylation—alkynylation reaction seꢀ
quence (Scheme 4).
0.158 mmol), Et3N (42 µL, 0.3 mmol), CuI (2.9 mg, 15 µmol,
10 mol.%), and Pd(PPh3)4 (8.7 mg, 7.5 µmol, 5 mol.%) in 2 mL
of MeCN were mixed under argon in a flask equipped with a
reflux condenser. The reaction mixture was refluxed until
4ꢀchloroꢀ6ꢀiodoquinoline disappeared (according to TLC),
cooled, diluted with CH2Cl2, and filtered through a silica gel
layer (3 cm), and the solvents were evaporated under reduced
pressure. The product was isolated by column chromatography
on silica gel using a light petroleum—Et2O mixture for elution.
Yield 36.8 mg (93%), m.p. 75—76 °C. Found (%): C, 76.98;
H, 4.09; N, 5.00. C17H10ClN. Calculated (%): C, 77.42; H,
3.82; N, 5.31. 1H NMR, δ: 7.36 (m, 3 H); 7.49 (d, 1 H, J =
4.7 Hz); 7.58 (m, 2 H); 7.84 (dd, 1 H, J = 8.7 Hz, J = 1.8 Hz);
8.07 (d, 1 H, J = 8.7 Hz); 8.39 (d, 1 H, J = 1.8 Hz); 8.75 (d, 1 H,
J = 4.7 Hz).
Scheme 4
4ꢀChloroꢀ6ꢀ(4ꢀdimethylaminophenylethynyl)quinoline (2) was
synthesized in a similar way from 4ꢀchloroꢀ6ꢀiodoquinoline9
(43.4 mg, 0.15 mmol) and 4ꢀdimethylaminophenylacetylene
(22.9 mg, 0.158 mmol) in 2 mL of a dioxane—water (3 : 1)
mixture in a yield of 42.3 mg (92%), m.p. 108—109 °C.
Found (%): C, 74.36; H, 4.99; N, 9.08. C19H15ClN2. Calcuꢀ
lated (%): C, 74.38; H, 4.93; N, 9.13. 1H NMR, δ: 3.00 (s, 6 H);
6.67 (m, 2 H); 7.46 (m, 3 H); 7.82 (dd, 1 H, J = 8.7 Hz, J =
1.8 Hz); 8.04 (d, 1 H, J = 8.7 Hz); 8.34 (d, 1 H, J = 1.8 Hz);
8.73 (d, 1 H, J = 4.7 Hz).
4ꢀChloroꢀ6ꢀ(3ꢀhydroxypropꢀ1ꢀynyl)quinoline (3) was synꢀ
thesized in a similar way from 6ꢀbromoꢀ4ꢀchloroquinoline9
(60.6 mg, 0.25 mmol) and propꢀ2ꢀynol (14.7 mg, 0.262 mmol)
in a yield of 54.3 mg (100%), m.p. 136—138 °C. Found (%):
C, 66.61; H, 3.69; N, 6.61. C12H8ClNO. Calculated (%):
C, 66.22; H, 3.70; N, 6.44. 1H NMR, δ: 3.49 (br.s, 1 H); 4.56 (d,
2 H, J = 6.1 Hz); 7.50 (d, 1 H, J = 4.7 Hz); 7.74 (dd, 1 H, J =
8.7 Hz, J = 1.8 Hz); 8.05 (d, 1 H, J = 8.7 Hz); 8.32 (d, 1 H, J =
1.8 Hz); 8.76 (d, 1 H, J = 4.7 Hz).
4,6ꢀBis(phenylethynyl)quinoline (4) was synthesized in a simiꢀ
lar way from 4ꢀchloroꢀ6ꢀiodoquinoline9 (43.4 mg, 0.15 mmol),
phenylacetylene (61.2 mg, 0.6 mmol), Et3N (84 µL, 0.6 mmol),
CuI (2.9 mg, 15 µmol, 10 mol.%), and Pd(PPh3)4 (8.7 mg,
7.5 µmol, 5 mol.%) in 1.5 mL of dioxane and 0.5 mL of H2O.
Yield 44.3 mg (90%), m.p. 143 °C. 1H NMR, δ: 7.36, 7.42
(both m, each 3 H); 7.55 (d, 1 H, J = 4.5 Hz); 7.60, 7.68
(both m, each 2 H); 7.83 (dd, 1 H, J = 8.7 Hz, J = 1.8 Hz); 8.07
(d, 1 H, J = 8.7 Hz); 8.49 (d, 1 H, J = 1.8 Hz); 8.86 (d, 1 H, J =
4.5 Hz). The substance is partially resinified over a period of
1—2 days at ∼20 °C, which precluded the preparation of an
analytical grade sample.
6ꢀ(4ꢀAnisyl)ꢀ4ꢀphenylethynylquinoline (6) was synthesized in
a similar way from 6ꢀ(4ꢀanisyl)ꢀ4ꢀchloroquinoline (50 mg,
0.186 mmol) and phenylacetylene (38 mg, 0.372 mmol) in a
yield of 61 mg (95%), m.p. 141 °C. Found (%): C, 85.64; H, 4.98;
N, 4.31. C24H17NO. Calculated (%): C, 85.94; H, 5.11; N, 4.18.
1H NMR, δ: 3.86 (s, 3 H); 7.04 (m, 2 H); 7.41 (m, 3 H); 7.55 (d,
1 H, J = 4.4 Hz); 7.64, 7.69 (both m, each 2 H); 7.97 (dd, 1 H,
J = 8.9 Hz, J = 2.0 Hz); 8.14 (d, 1 H, J = 8.9 Hz); 8.46 (d, 1 H,
J = 2.0 Hz); 8.84 (d, 1 H, J = 4.4 Hz).
Reagents and conditions: i. 2 mol.% Pd(PPh3)4, K2CO3,
dioxane—H2O (3 : 1), ∆. ii. 5 mol.% Pd(PPh3)4,
10 mol.% CuI, Et3N, dioxane—H2O (3 : 1), ∆.
Experimental
The reactions were monitored by TLC on Silufol UVꢀ254
plates and by 1H NMR spectroscopy. The yields of the products
of alkynylation of 4,6ꢀdihaloquinolines were determined by
1H NMR spectroscopy (except for the runs where the products
1
were isolated). H NMR spectra were recorded in CDCl3 on a
Varian VXR 400 spectrometer operating at 400 МHz. The chemiꢀ
cal shifts are given in the δ scale and referred to HMDS. Comꢀ
mercial chemicals (Aldrich) were used. 6ꢀBromoꢀ and 6ꢀiodoꢀ
4ꢀchloroquinolines were prepared by a known procedure.9
4ꢀChloroꢀ6ꢀphenylethynylquinoline (1). 4ꢀChloroꢀ6ꢀiodoꢀ
quinoline9 (43.4 mg, 0.15 mmol), phenylacetylene (16.1 mg,
4ꢀ(4ꢀAnisyl)ꢀ6ꢀphenylethynylquinoline (5). 4ꢀChloroꢀ6ꢀ
phenylethynylquinoline (50 mg, 0.19 mmol), 4ꢀanisylboronic
acid (38 mg, 0.25 mmol), K2CO3 (78.7 mg, 0.57 mmol), and
Pd(PPh3)4 (4.4 mg, 3.8 µmol) in 1.5 mL of dioxane and 0.5 mL
of H2O were mixed under argon in a flask equipped with a reflux
condenser. The reaction mixture was refluxed for 2 h, cooled,

Alkynylation of dihaloquinolines
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 1, January, 2004
193
diluted with CH2Cl2, and filtered through a silica gel layer
(3 cm), and the solvents were evaporated under reduced presꢀ
sure. The product was purified by column chromatography on
silica gel in a light petroleum—Et2O mixture (1 : 1). Yield 59.4 mg
(94%), m.p. 139 °C. Found (%): C, 86.02; H, 5.15; N, 4.11.
C24H17NO. Calculated (%): C, 85.94; H, 5.11; N, 4.18.
1H NMR, δ: 3.87 (s, 3 H); 7.05 (m, 2 H); 7.28 (d, 1 H, J =
4.4 Hz); 7.31 (m, 3 H); 7.43, 7.51 (both m, each 2 H); 7.79 (dd,
1 H, J = 8.8 Hz, J = 1.7 Hz); 8.10 (d, 1 H, J = 8.8 Hz); 8.12 (d,
1 H, J = 1.7 Hz); 8.67 (d, 1 H, J = 4.4 Hz).
and L. G. Fedenok, Mendeleev Commun., 1997, 98;
L. Anastasia and E.ꢀI. Negishi, Org. Lett., 2001, 3, 3111;
U. Radhakrishnan and P. J. Stang, Org. Lett., 2001, 3, 859.
4. M. Alami, F. Ferri, and G. Linstrumelle, Tetrahedron Lett.,
1993, 34, 6403; M. Alami, B. Crousse, and F. Ferri,
J. Organomet. Chem., 2001, 624, 114; D. Chemin and
G. Linstrumelle, Tetrahedron, 1994, 50, 5335.
5. R. Menicagli, S. Samaritani, and S. Gori, Tetrahedron Lett.,
1999, 40, 8419; M. R. Buchmeiser, T. Schareina, R. Kempe,
and K. Wurst, J. Organomet. Chem., 2001, 634, 39.
6. J. W. Goodby, M. Hird, R. A. Lewis, and K. J. Toyne, Chem.
Commun., 1996, 2719; K.ꢀT. Wong, T. S. Hung, Y. Lin,
C.ꢀC. Wu, G.ꢀH. Lee, S.ꢀM. Peng, C. H. Chou, and
Y. O. Su, Org. Lett., 2002, 4, 513.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 01ꢀ03ꢀ
33144).
7. H. Nakamura, M. Aizawa, D. Takeuchi, A. Murai, and
O. Shimoura, Tetrahedron Lett., 2000, 41, 2185; T. Bach
and L. Kruger, Tetrahedron Lett., 1998, 39, 1729; T. Bach
and L. Kruger, Synlett, 1998, 1185; T. Bach and L. Kruger,
Eur. J. Org. Chem., 1999, 2045.
References
1. K. Sonogashira, J. Tohda, and N. Hagihara, Tetrahedron
Lett., 1975, 16, 4467; K. Sonogashira, Comprehensive Orꢀ
ganic Synthesis, Eds. B. M. Trost and I. Fleming, Pergamon
Press, Oxford, 1991, 3, 521; K. Sonogashira, MetalꢀCataꢀ
lyzed CrossꢀCoupling Reactions, Eds. F. Diederich and P. J.
Stang, Wiley—VCH, Weinheim, 1998, 203; K. Sonogashira,
Handbook of Organopalladium Chemistry for Organic Syntheꢀ
sis, Ed. E. Negishi, Wiley—VCH, New York, 2002, 493.
2. M. P. LopezꢀDeber, L. Castedo, and J. R. Granja, Org.
Lett., 2001, 3, 2823; D. T. Bong and M. R. Ghadiri, Org.
Lett., 2001, 3, 2509.
8. L.ꢀL. Gundersen, G. Langli, and F. Rise, Tetrahedron Lett.,
1995, 36, 1945; G. Langli, L.ꢀL. Gundersen, and F. Rise,
Tetrahedron, 1996, 52, 5625; P. Dasa, C. P. Spearsb, A. H.
Shahinianb, S. K. Dasguptaa, and N. G. Kundu, Bioorg.
Med. Chem. Lett., 1996, 6, 2477.
9. J. Ellis, E. Gellert, and J. Robson, Aust. J. Chem., 1973, 26,
907; N. J. Leonard and S. N. Boyd, Jr., J. Org. Chem., 1946,
11, 419.
10. A. V. Tsvetkov, G. V. Latyshev, N. V. Lukashev, and I. P.
Beletskaya, Tetrahedron Lett., 2002, 43, 7267.
3. I. I. Barabanov, L. G. Fedenok, and M. S. Shvartsberg, Izv.
Akad. Nauk, Ser. Khim., 1998, 2327 [Russ. Chem. Bull., 1998,
47, 2256 (Engl. Transl.)]; M. S. Shvartsberg, I. I. Barabanov,
Received July 31, 2003;
in revised form December 2, 2003