Diels-Alder reactions: The effects of catalyst on the addition reaction

Article abstract of DOI:10.1016/j.molstruc.2015.06.012

Abstract The reaction between 2,3-dimethyl-1,3-butadiene and dimethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate is efficiently achieved with small amounts of catalyst, i.e. phenol, AcOH, nafion, and β-cyclodextrin. Exo-diastereoselective cycloa

Full text of DOI:10.1016/j.molstruc.2015.06.012

Journal of Molecular Structure 1098 (2015) 72e75
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Journal of Molecular Structure
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DielseAlder reactions: The effects of catalyst on the addition reaction
€
a
a
,
*
, Tuncay Tunç b, Ertan Sahin c
Ozgür Yilmaz , Nermin Simsek Kus
a
Department of Chemistry, Faculty of Arts and Sciences, Mersin University, 33343 Mersin, Turkey
Department of Science Education, Faculty of Education, Aksaray University, 68100 Aksaray, Turkey
Department of Chemistry, Faculty of Sciences, Atatürk University, 25240 Erzurum, Turkey
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 March 2015
Received in revised form
The reaction between 2,3-dimethyl-1,3-butadiene and dimethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-
dicarboxylate is efficiently achieved with small amounts of catalyst, i.e. phenol, AcOH, nafion, and
b
ꢀcyclodextrin. Exo-diastereoselective cycloaddition reactions were observed both without catalyst and
30 May 2015
different catalysts for 48 days. As a result, different products (tricyclicmolecule 5, retro-DielseAlder
product 6, and oxidation product 7) were obtained with different catalysts. In addition, we synthesized
DielseAlders product 8 and tricyclocyclitol 10 via DielseAlder reaction. The structures of these products
were characterized by 1H NMR, C NMR, MS and IR spectroscopy.
Accepted 2 June 2015
Available online 6 June 2015
13
Keywords:
DielseAlder addition
Cyclohexadiene
©
2015 Elsevier B.V. All rights reserved.
retro-DielseAlder reaction
Substituted benzene
1. Introduction
2.2. DielseAlder reaction of dimethyl 7-oxo-bicyclo[2.2.1]hepta-
2,5-diene-2,3-dicarboxylate (3) with 2,3-dimethyl-1,3-butadiene
The cycloaddition of alkenes and dienes known as the Diel-
seAlder reaction is a very useful method for forming substituted
cyclohexenes [1e9]. The reactions are normally synchronous and
processes are concerted. The cycloaddition products are tradition-
ally affected by modest solvents and catalysts, in accordance with
small changes in charge on going from reactants to the activated
complex [7,10].
Diene 3 (0.5 g, 2.4 mmol) and 2,3-dimethyl-1,3-butadiene 4
(0.197 g, 2.4 mmol) was dissolved in 10 mL of chloroform, and then
the reaction was stirred at room temperature for 48 days. After the
reaction, the solvent was removed to give product.
(1R,4S,4aS,8aR)-dimethyl
6,7-dimethyl-1,4,4a,5,8,8a-hexahy-
dro-1,4-epoxynaphthalene-4a,8a-dicarboxylate
(5):
M.
P.:
ꢁ
The aim of our study was to obtain the cycloaddition products of
dienophile 3 with 2,3-dimethyl-1,3-butadiene (4) using different
85e87 C (CHCl
3
),
d
H
(400 MHz, CDCl
3
): 6.52 (s, 2H), 4.65 (2H, s),
3.56 (6H, s), 2.48 A part of AB system (2H, d, J ¼ 14.0 Hz), 2.41 B part
of AB system (2H, d, J ¼ 14.0 Hz), 1.73 (6H, s, eCH (100 MHz,
CDCl ,) 173.9, 135.4, 126.9, 86.8, 63.2, 51.8, 40.2, 18.8, Anal. Calc. for
catalysts (phenol, acetic acid, nafion, and
temperature, and also without catalyst at 25 and 40 C, in addition
b
ꢀcyclodextrin) at room
3 C
) d
ꢁ
3
þ
to synthesizing the cyclitol and epoxide derivatives (9, 10).
C
16
H
eCH
20
O
5
: C 65.74, H 6.9. Found: C 65.66H 6.32%, MS m/z: 209 (M ,
þ
þ
3
), 194, 193, 192, 191 (M ,CH
3
), 179, 178, 177 (M , eCH
3
), 166,
þ
þ
þ
1
1
65,164,163 (M , eO),134,133, 132 (M , eC),122,121,120 (M ,-O),
þ
¡1
07, 106, 105, 104, 103 (M ,-C), IR (cm ) ¼ 1716.7 (eC]O).
2
. Experimental
Dimethyl 4,5-dimethylcyclohexa-1,4-diene-1,2-dicarboxylate (6)
ꢁ
ꢁ
[
3
1
12]: M. P: 72e72.6 C (CHCl
3
), (Lit.63e66 C),
d
H
(400 MHz, CDCl
3
):
2.1. Synthesis of dimethyl 7-oxo-bicyclo[2.2.1]hepta-2,5-diene-2,3-
.78 (s, 6H), 2.92 (s, 4H), 1.66 (s, 6H),
d
C
(100 MHz, CDCl ): 168.4,
3
dicarboxylate (3)
32.7, 121.5, 52.1, 34.1, 17.9.
ꢁ
Dimethyl 4,5-dimethylphthalate (7) [12]: M. P.: 46e48 C (CHCl
3
)
Dimethyl
7-oxo-bicyclo[2.2.1]hepta-2,5-diene-2,3-
ꢁ
(
(
Lit 56e57 C)
d
H
(400 MHz, CDCl
3
): 7.42 (2H, s), 3.81 (6H, s), 2.24
dicarboxylate (6) were prepared as described in the literature [11].
6H, s),
d
C
(100 MHz, CDCl
3
): 168.3, 140.2, 130.1, 129.4, 52.5, 19.7.
*
Corresponding author.
E-mail addresses: simner@gmail.com, simner@mersin.edu.tr (N.S. Kus).
http://dx.doi.org/10.1016/j.molstruc.2015.06.012
022-2860/© 2015 Elsevier B.V. All rights reserved.
0

€
O. Yilmaz et al. / Journal of Molecular Structure 1098 (2015) 72e75
73
C 3
d (100 MHz, CDCl ): 172.7, 126.3, 82.8, 63.7, 52.1, 49.7, 39.3, 18.8,
Anal. Calc. for C16
H
20
O
6
: C 62.33, H 6.54. Found: C 62.27, H 6.14%,
þ
þ
þ
MS m/z: 308 (M , ꢀ2 O), 276 (M , eCH
3
), 261, 262, 263 (M ,-CO),
þ
þ
221, 220, 219, 218, 217 (M , eCH
3
), 204, 203, 202, 201 (M , eCO),
þ
¡1
1
77, 176, 174, 173 (M , ꢀ2CH
3
) 147, 146, 145, 144, IR (cm ): 1716.3
(eC]O).
Scheme 1. Addition reaction of furan (1) and dimethyl acetylene dicarboxylate (2).
2.4. Synthesis of dimethyl 3,4-dimethyl-7-oxabicyclo[4.1.0]hept-3-
ene-1,6-bis(carboperoxoate) (9)
Compound 5 (1.0 g, 3.42 mmol) was dissolved in 150 mL of
chloroform, MCPBA (1.18 g, 6.84 mmol, 70%) was added, and then
the reaction was stirred at reflux temperature for 3 days. The re-
action mixture was added to 15 mL 50% NaHSO
mixture was stirred for 15 min. The organic layer was separated and
then washed with saturated aqueous NaHCO (100 mL), dried with
MgSO and concentrated to give 740 mg of 90% yield of epoxide 9.
M.P.: 64e65 C (CHCl
2H, d, J2a,b ¼ J5a,b ¼ 19.6 Hz), 2.64 (2H, d), 1,41 (s, 6H),
CDCl ): 167.9, 131.3, 60.1, 52.3, 33.3, 19.1, Anal. Calc. for C12
9.99, H 6.71, Found C 60.06, H 6.62%, MS m/z: 240, (M , eOCH
3
solution and
3
4
ꢁ
3
),
d
H
(400 MHz, CDCl
3
): 3.75 (6H, s), 2.86
(100 MHz,
: C
(
d
C
3
16 5
H O
þ
5
2
1
1
3
),
),
þ þ
11, 210, 209, 208, 207, (M , eO),194,193,192,191, 190, (M , eCH
3
þ
þ
þ
79, 178, 177, 176, 175 (M ,-CO), 151, 150, 149, 148, 147, (M , eCO),
þ
23, 122, 121, 120, 119, (M , eO), 107, 106, 105, 104, 103, (M , eCO),
¡1
Scheme 2. DielseAlder reaction of dienophile 3 and diene 4.
IR (cm ): 1716 (eC]O).
2
.5. Synthesis of (1R,2S,3R,4S,8aS)-dimethyl 2,3-dihydroxy-6,7-
2
1
2
.3. Synthesis of (1aR,2R,2aS,7S,7aS)-dimethyl 4,5-dimethyl-
a,2,2a,3,6,6a,7,7a-octahydro-2,7-epoxynaphtho[2,3-b]oxirene-
a,6a-bis(carboperoxoate (8)
dimethyl-1,2,3,4,4a,5,8,8a-octahydro-1,4-epoxynaphthalene-4a,8a-
dicarboxylate (10)
To a stirred solution of tricyclic molecule 5 (1,0 g, 3.42 mmol) in
Diene 4 (0.500 g, 6.09 mmol) and epoxide 11 (1.38 g, 6.09 mmol)
was dissolved in 10 mL of chloroform, and then the reaction was
stirred at room temperature for 24 days. After the reaction, the
1
0 mL of acetone/H
2
O (1:1) were added NMO (0.409 g, 3.42 mmol)
ꢀ3
and 2 ml of OsO
4
(7,87.10
mmol) at room temperature. The
mixture was stirred vigorously at room temperature for 24 h. The
reaction was stopped. Evaporation of solvent gave 1.06 g of cis-diol
solvent was removed to give 0.650 mg 95% yield of product 8.
ꢁ
M. P.: 144e145 C (CHCl
3
),
d
H
(400 MHz, CDCl
3
): 4.23 (2H, s),
1
0 with 95% yield.
3
.78 (2H, s), 3.61 (6H, s), 2.43 (2H, A part of AB system d,
J ¼ 13.6 Hz), 2.28 (2H, B part of AB system d, J ¼ 13.6 Hz),1.71 (6H, s),
ꢁ
M. P.: 170e171 C (CHCl
3 H 3
d (400 MHz, CDCl ): 4.56 (2H, s) 4.12
),
(
2H, s), 3.62 (6H, s, eOCH
3
), 3.22 (2H, bs, 2eOH), 2.42 (2H, d,
Table 1
DielseAlder reaction of dienophile 3 and diene 4 (Scheme 2).
%
yield
Catalyst
Non-catalyst 25
Product
3
5
3 h
65
35
0
6 h
45
55
0
12 h
30
70
0
24 h
10
90
0
0
0
90
10
0
2
98
0
30 h
0
98
2
0
0
85
15
0
48 h
0
95
5
0
0
73
27
0
6. d
0
75
25
0
15. d
0
62
33
5
30. d
0
20
68
12
0
48. d
0
5
80
15
0
0
8
92
0
0
ꢁ
C
6
7
3
5
6
7
3
5
6
7
3
5
6
7
3
5
6
7
3
5
6
7
[12]
[12]
0
0
0
ꢁ
40
C
35
65
0
25
75
0
10
90
0
0
0
0
11
84
5
0
[12]
[12]
89
11
0
20
60
20
0
30
55
15
0
40
50
10
0
47
53
0
0
0
0
ꢁ
Acetic acid 25
C
50
50
0
30
70
0
14
86
0
0
0
0
90
10
0
0
95
5
0
0
95
5
0
80
20
0
55
45
0
0
[12]
[12]
67
33
0
30
70
0
0
0
0
0
b
¡cyclo dextrin 25 ꢁ
C
55
45
0
35
65
0
20
80
0
10
90
0
0
8
92
0
0
5
95
0
0
0
90
10
0
60
40
0
0
0
[12]
[12]
67
33
0
35
65
0
0
0
0
ꢁ
Nafion-H 25
C
60
40
0
40
60
0
27
73
0
0
0
90
10
0
65
35
0
0
0
[12]
[12]
70
30
0
0
69
31
43
57
0
0
38
62
0
0
0
ꢁ
Phenol 25
C
55
45
0
35
65
0
20
80
0
0
95
5
0
0
85
15
0
60
40
0
30
55
15
[12]
[12]
0
0
0
0
0

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74
O. Yilmaz et al. / Journal of Molecular Structure 1098 (2015) 72e75
4
Scheme 3. The reaction of tricyclic molecule 5 with MCPBA and OsO /NMO.
Scheme 4. Synthesis of DielseAlders product 8 from dienophile 3.
J ¼ 13.6 Hz), 2.28 (2H, d, J ¼ 13.6 Hz), 1.69 (6H, s, eCH
3
),
d
C
reaction mixture was worked up as described in the general pro-
ꢁ
(
100 MHz, CDCl ): 172.8, 126.4, 91.5, 72.0, 60.1, 52,1, 39.3,18.8, Anal.
3
cedure to give 1.02 g of epoxide 11 (95%). M.P. 82e83 C (CHCl
3
),
Calc. for C16
H
22
O
7
: C 59.89, H 5.79, Found C 59.22, H 5.33%, MS m/z:
(Lit. not found),
d
H
(400 MHz, CDCl
3
): 5.05 (2H, s), 3.76 (6H, s) 3.70
þ
þ
3
2
2
26, (M , ꢀ2 CH
3
), 297, 296, 295, 294, 293, (M , eCO), 268, 267,
(2H, s), (100 MHz, CDCl
d
C
3
): 161.4,146.9, 78.7, 54.3, 51.6, Anal. Calc.
þ
þ
þ
66, 265, (M , eOH), 250, 249, 248, 247, (M , eOH), 233, 232, 231,
for C10
(M , eOCH
H
10
O
6
: C 53.10, H 4.46, Found: C 53.25, H 4.34%, MS m/z: 226,
), 195, 194, (M , eOCH
þ
þ
þ
þ
30, 229, (M , eCO), 202, 201, 200, 199, 198, (M , eCH
3
) 187, 186,
) 171, 170, 169, 168, 167, (M ,-O) 155, 154,
53, 152,151, IR (cm ): 3480.2, 3405.8 (eOH),1725.6,1697.4 (eC]
O).
3
3
), 167, 166, 165, 162.9, (M , eC]
þ
þ
þ
þ
185, 184, 183, (M , eCH
3
O), 139, 137, 136, 135, 134 (M , eC]O), 109, 108, 107, 106 (M , eC,
eO), 81, 80, 79, 78, 77 (M , eC, eO), 55, 53, 52, 51, 50, IR (cm ):
1720 (eC]O).
¡1
þ
¡1
1
2
.6. Synthesis of (1R, 2R, 4S, 5S)-dimethyl 3,8-dioxatricyclo
3. Results and discussion
2
,4
[3.2.1.0 ]oct-6-ene-6,7-dicarboxylate (11) [13,14]
In a preliminary study, dimethyl 7-oxa-bicyclo[2.2.1]hepta-2,5-
1
.0 g (4.76 mmol) of bicyclicdiene 3 was dissolved in 150 mL of
diene-2,3-dicarboxylate (3) [11] was prepared by the cycloaddi-
tion reaction of dimethyl acetylene dicarboxylate (2) to furan (1) in
high yield (Scheme 1). Then, 1 M eq. dienophile 3 was reacted with
chloroform, 1.66 g of MCPBA (9.62 mmol, 70%) was added, and then
the reaction was stirred at room temperature for 3 days. The
1
M eq. diene 4 in CHCl
3
without catalyst at room temperature
(
Scheme 2). After 24 h, the 90% conversion of tricyclic molecule 5
1
and 10% conversion of dienophile 3 were achieved according to H
1
13
NMR spectra of the mixture. H NMR and C NMR spectra of tri-
cyclic molecule 5 were highly symmetrical due to the symmetry
molecule. When the reaction was continued, after 30 h, retro-
DielseAlder product 6 [12] was observed with 2% conversion. The
1
13
H and C NMR spectra were in good agreement with the structure
of retro-DielseAlder product 6. Retro-DielseAlder product 6 was
synthesized by cycloaddition reaction of dimethyl acetylene
dicarboxylate (2) to diene 4 in the literature [12]. After 6 days, 62%
conversion of tricyclic molecule 5 and 33% conversion of retro-
1
DielseAlder product 6 were observed in H NMR spectra of the
mixture. When the reaction was continued, after 48 days, it was
observed that the reaction was completed and 80% conversion of
retro-DielseAlder product 6 [12], 15% conversion of oxidation
product 7 [12] and 5% conversion of tricyclic molecule 5 were
achieved. The structural assignments of the oxidation product 7 are
based on their spectroscopic data. When the reaction temperature
ꢁ
was 40 C, it was observed that the reaction speed was in parallel
with an increase in the amount of retro-DielseAlder product 6 and
oxidation product 7 (Table 1).
Fig. 1. ORTEP view of DielseAlders product 8 showing the atom-labeling scheme.
Displacement ellipsoids are drawn at the 40% probability level.

€
O. Yilmaz et al. / Journal of Molecular Structure 1098 (2015) 72e75
75
ꢁ
In a secondary study, 1 M eq. dienophile 3 and 1 M eq. diene 4
0.103 mmꢀ1; F(000): 328;
q
-range for data collection 1.5e26.4 ;
ꢁ
2
were dissolved in CHCl
3
with acetic acid at 25 C and it was
refinement method: full matrix least-square on F ; data/parame-
ters: 2439/201; goodness-of-fit on F : 1.183; final R-indices
2
observed that dienophile 3 ran out after 30 h. At the end of the
reaction, 30% of retro-DielseAlder product 6 remained in the re-
action medium with 70% converted to oxidation product 7 under
acetic acid catalyzed conditions; this is higher than conversion
achieved by the reaction in which no catalyst was used. In contrast,
[I > 2
and ꢀ0.387 e Å
s
(I)]: R
1
¼0.116, wR ¼ 0.215; largest diff. peak and hole: 0.863
2
ꢀ
3
.
4. Conclusion
7
0% conversion of oxidation product 7 was achieved in the reaction.
When we used
ꢀcyclodextrin and phenol as catalyst, similar
outcomes were observed. When phenol was used as catalyst, about
0% conversion of oxidation product 7 and 40% conversion retro-
b
In summary, we achieved the chemoselective and exo-diaster-
eoselective reaction between 2,3-dimethyl-1,3-butadiene and
6
dimethyl
without catalyst and with small amounts of catalyst (phenol, acetic
acid, nafion, and -cyclodextrin). In addition, we described a useful
7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate
DielseAlder product 6 were seen. The reaction with nafion as
catalyst gave 57% conversion of oxidation product 7 and 43% con-
version of retro-DielseAlder product 6 after 48 days. Finally, in
terms of reaction yield of tricyclic molecule 5 with acetic acid as
b
method for the synthesis of important tricylocyclitol and dioxa-
tricyclo derivatives useful molecules with anticancer, antibiotic,
anti-feedant, and anti-leukemic activity and versatility as synthetic
intermediates.
catalyst, retro-DielseAlder product 6 and oxidation product 7 at
ꢁ
4
0 C were synthesized in high yield (Table 1).
After the successful isolation and characterization of all mole-
cules, we turned our attention to the fragmentation of tricyclic
molecule 5. First, we tried the oxidation reaction of tricyclic
molecule 5 with m-CPBA. At the end of the reaction, tricyclic
molecule 5 was fragmented into diene 6 and diene 6 gave the
epoxidation reaction. At the end of the epoxidation reaction,
epoxide 9 was obtained in 85% yield. Although the reaction of tri-
cyclic molecule 5 with OsO /NMO (N-methyl morpholine N-oxide)
4
gave tricyclic molecule 10 in 95% yield, degradation was not
observed in this reaction (Scheme 3).
Acknowledgements
The authors are indebted to Mersin University (BAP-FBE KA
OY) 2010-5 YL) for its financial support of this work and Prof. Dr.
€
(
Hasan Seçen, Prof Dr. Arif Da s¸ tan for their help during this work.
The authors acknowledge Aksaray University, Science and Tech-
nology Application and Research Center, Aksaray, Turkey, for the
use of the Bruker SMART BREEZE CCD diffractometer (purchased
under grant No. 2010K120480 of the State of Planning
Organization).
Next, dienophile 3 was converted to the desired epoxide 11
ꢁ
[13,14] in the presence of MCPBA in CH
2
Cl
2
at 25 C in 90% yields.
When the epoxide 11 was reacted with 2,3-dimethy-1,3-butadiene
4) in CHCl , the desired DielseAlders product 8 was obtained in
Appendix A. Supplementary data
(
3
good yield (Scheme 4). In addition, the exact structure of Diel-
seAlders product 8 was determined by X-ray diffraction analysis
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2015.06.012.
(Fig. 1).
References
3.1. Crystallography
[1] L.L. Butz, A.W. Rytina, Org. React. 5 (1949) 136e192.
Single crystals of DielseAlders product 8 were obtained by slow
[2] M.C. Kloetzel, Org. React. 4 (1948) 1e59.
[
[
3] A. Wassermann, DielseAlder Reactions, Elsevier, New York, 1965.
4] R. Huisgen, R. Grashey, J. Sauer, in: S. Patai (Ed.), Chemistry of Alkenes, Wiley
Interscience, New York, 1964, pp. 878e928.
evaporation of the compounds in acetonitrile at room temperature.
Selected single-crystal of DielseAlders product 8 was used for data
collection on a Bruker SMART BREEZE CCD diffractometer. The
[5] J.G. Martin, R.K. Hill, Chem. Rev. 61 (1961) 537e562.
[6] J. Hamer, 1,4 Cycloaddition Reactions: the Diels-alder Reaction in Heterocyclic
Synthesis, Academic Press, New York, London, 1967, p. 143.
graphite-monochromatized MoK
a
radiation (
l
¼ 0.71073 Å) and
ꢁ
oscillation scans technique with Du ¼ 5 for one image were used
[
7] J. Sauer, R. Sustmann, Angew. Chem. Int. Ed. Engl. 19 (1980) 779e807.
for data collection. The lattice parameters were determined by the
[8] R. Gleiter, M.C. B o€ hm, in: W.H. Watson (Ed.), Stereochemistry and Reactivity
2
of Systems Containing
Beach, Florida, 1983, pp. 105e146.
P
ꢀElectrons, Verlag Chemie International Deerfield
least-squares methods on the basis of all reflections with F > 2
s
2
(
F ). Integration of the intensities, correction for Lorentz and po-
[9] F. Fringuelli, A. Taticchi, The Dielsealder Reaction: Selected Practical Methods,
larization effects and cell refinement was performed using Bruker
SAINT (Bruker AXS Inc, 2012) software [15]. The structures were
solved by direct methods using SHELXS-97 [16] and refined by a
full-matrix least-squares procedure using the program SHELXL-97
Wiley, Chichester, UK, 2002.
10] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Cam-
[
[
bridge, 1990.
11] N. Simsek, C. Arici, M.L. McKee, D. Ülkü, M. Balci, Struct. Chem. 12 (2001)
305e311.
[
16]. H atoms were positioned geometrically and refined using a
riding model. The final difference Fourier maps showed no peaks of
chemical significance. Crystal data for 8: C16 , crystal system,
space group: triclinic, P-1; (no:2); unit cell dimensions:
a ¼ 7.0375(5), b ¼ 8.2899(5), c ¼ 13.6981(9) Å, ¼ 93.342(4),
¼ 101.770(5), ¼ 101.610(4) Å; volume: 762.22(9) Å ; Z ¼ 2;
calculated density: 1.343 g/cm ; absorption coefficient:
[12] K.K. Gnanasekaran, R.A. Bunce, K.D. Berlin, Molbank M835 (2014), http://
dx.doi.org/10.3390/M835.
13] W.K. Anderson, R.H. Dewey, J. Med. Chem. 20 (2) (1977) 306e308.
14] S. Niwayama, S. Kobayashi, M. Ohno, J. Am. Chem. Soc. 116 (1994)
3290e3295.
[
[
20 6
H O
[
[
15] Bruker, APEX2, SAINT and SADABS, Bruker AXS Inc, Madison, Wisconsin, USA,
a
2012.
3
b
g
16] G.M. Sheldrick, SHELX-97 and SHELXL-97, University of G o€ ttingen, Germany,
3
1997.