Facile and efficient transformation of xanthates into thiocarbonates by anodic oxidation

Full text of DOI:10.1021/jo001206l

320
J . Org. Chem. 2001, 66, 320-322
Sch em e 1
F a cile a n d Efficien t Tr a n sfor m a tion of
Xa n th a tes in to Th ioca r bon a tes by An od ic
Oxid a tion
Bele´n Batanero, Oscar Picazo, and Fructuoso Barba*
Department of Organic Chemistry. University of Alcala´,
28871 Alcala´ de Henares, Madrid, Spain
fructuoso.barba@uah.es
Received August 7, 2000
Sch em e 2
Xanthates are among the most widely used thiocarbo-
nyl compounds in organic synthesis,1 despite the fact that
very little information is available about their oxidation.
In 1997, Metzner et al.2 reported that sulfines are
formed by reaction of xanthates with m-CPBA. These
sulfines evolve at room temperature after 4 weeks to give
dithioperoxycarbonates and thiocarbonates as main and
secondary product, respectively. The syntheses of thio-
carbonates are usually carried out by reaction of potas-
sium phenoxide with carbonyl sulfide and subsequent
alkylation using the corresponding alkyl halide3 or by
reaction of a sodium thiolate salt with alkyl chlorofor-
mates.4 Thiocarbonates show many interesting proper-
ties. They have been used as pesticides,5 insecticides,
nematocides and acaricides,6 antineoplastic7 and anti-
ulcer agents,8 as well as in the preparation of antibiotics.9
Sch em e 3
Resu lts a n d Discu ssion
We have found that the electrochemical oxidation of
O-ethyl xanthates (1a -h ) leads to the formation of the
corresponding O-ethyl thiocarbonates (2a -h ) (90-97%
yield) (Scheme 1).
A logical mechanism to explain the electrochemical
transformation is shown in Scheme 2.
electron pairs of the oxygen atom of the carbonyl group)
is lower than in the rest of xanthates due to the
withdrawing effect of the halogen. Therefore, in 1d and
1e there is some stabilization coming from the free
electron pairs of the sulfur atom, to afford the small
amount of 3, as is indicated in Scheme 3.
In the anodic oxidation of 4-chlorophenacyl xanthate
(1d ), a small amount of dithioperoxycarbonate (3d ) was
isolated, and in the oxidation of 4-bromophenacyl xan-
thate (1e) traces of the corresponding dithioperoxycar-
bonate were also found. In both cases, the stabilization
of the positive charge on the sulfur atom (by the free
The cyclic species proposed by Metzner2 in the sulfine
decomposition (Figure 1) cannot be assumed in our main
pathway because experimentally it has never been
detected and the dithioperoxycarbonate is not obtained
as the major product. However, it could be implicated in
the formation of 3 as a parallel minor pathway.
The electrochemical process was carried out under
argon atmosphere, and the consumed charge was 2 F/mol.
However, when the reaction was performed in an open
air cell the electricity consumption was lower than this
value. To demonstrate the participation of the oxygen
(atmospheric and dissolved) in the process, electrolyses
were carried out bubbling an oxygen flow through the
anodic compartment. In this case, only 1 F/mol was
consumed in the reaction. The easy oxidation of the sulfur
radical can be explained having in mind that the oxida-
tion potential of a radical is controlled largely by the
ability of a group to stabilize the developing of positive
charge. This suggests that there is considerable develop-
ment of cation character in the transition state in the
electron-transfer oxidation of radicals.10 In our case, the
free electron pairs in the oxygen atom stabilize the sulfur
cation.
(1) Kleinpeter, E.; Pilhaja, K. In Comprehensive Organic Functional
Group Transformations; Katritzky, A. R., Meth-Cohn, O., Rees C. W.,
Gilchrist, T. L., Eds.; Oxford University Press: New York, 1995; Vol.
6, p 527.
(2) Marrie`re, E.; Chevrie, D.; Metzner, P. J . Chem. Soc., Perkin
Trans. 1 1997, 2019.
(3) Chanyshev, N. T.; Kalashnikov, S. M.; Kuramshin, E. M.;
Naimushin, A. I.; Imashev, U. B. Zh. Obshch. Khim. 1990, 60, 2568.
(4) Folkmann, M.; Lund, F. J . Synthesis 1990, 1159.
(5) Patty, L.; Bruchet, A.; J askulke, E.; Acobas, F.; Van Hoop, F.;
Sacher, F.; Bobeldijk, I.; Ventura, F.; Marecos do Monte, M. H.
Technol., Sci., Methodes: Genie Urbain-Genie Rural 1998, 9, 24; Chem.
Abstr. 1999, 130, 100202.
(6) Fischer, R.; Bretschneider, T.; Erdelen, Ch.; Wachendorff-Neu-
mann, U.; Dollinger, M.; Turberg, A. Ger. Offen. DE Patent 19 742
492, 1999; Chem. Abstr. 1999, 130, P252239.
(7) Sartorelli, A. C.; Shyam, K.; Penketh, Ph. G.; Chen, Shu-Hui.
PCT Int. Appl. WO Patent 99 22 726, 1999; Chem. Abstr. 1999, 130,
P324930.
(8) Tomiyama, T.; Tomiyama, A.; Shirai, T.; Wakbayashi, S.; Kawai,
T.; Ueyama, N.; Sonegawa, M. Ger. Offen. DE Patent 3 943 180, 1990;
Chem. Abstr. 1990, 113, P211977.
(9) Ochiai, H.; Watanabe, Y.; Murotani, Y.; Fukuda, H.; Yoshino,
O.; Minami, S.; Hayashi, T.; Momonoi, K. Brit. UK. Pat. Appl. G. B.
Patent 2 233 330, 1991; Chem. Abstr. 1991, 115, P158827.
(10) Nonhebel, D. C.; Walton, J . C. In Free-radical Chemistry;
Cambridge University Press: New York, 1974; p 309.
10.1021/jo001206l CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/02/2000

Notes
J . Org. Chem., Vol. 66, No. 1, 2001 321
2Har), 7.84-8.06 (m, 4Har), 8.54 (s, 1Har); 13C NMR (75.4 MHz,
CDCl3) δ 13.4, 43.3, 70.4, 123.5, 126.7, 127.5, 128.4, 128.6, 129.4,
130, 132.1, 132.8, 135.5, 191.9, 213; MS m/e (relative intensity)
EI 290 (M+, 3), 258 (6), 201 (4), 155 (100), 150 (18), 127 (65), 60
(19). Anal. Calcd for C15H14O2S2: C, 62.07; H, 4.83; S, 22.07.
Found: C, 62.11; H, 4.8; S, 21.91.
O-Eth yl S-(2-oxo-cycloh exyl) d ith ioca r bon a te (1g): mp
10-11 °C; IR(KBr) ν 2940, 1715, 1219, 1112, 1051; 1H NMR (300
MHz, CDCl3) δ 1.3(t, 3H, J ) 7 Hz), 1.6-2.6(m, 8H), 4.42(m,
1H), 4.5(q, 2H, J ) 7 Hz); 13C NMR (75.4 MHz, CDCl3) δ 13.3,
24.8, 27.1, 33.9, 41.4, 59.2, 69.8, 204.4, 212.5; MS m/e (relative
intensity) EI 218 (M+, 4), 185 (100), 157 (30), 129 (20), 97 (57),
86 (21), 67 (71), 55 (39). Anal. Calcd for C9 H14 O2 S2: C, 49.54;
H, 6.42; S, 29.36. Found: C, 49.44; H, 6.21; S, 29.52.
F igu r e 1.
When the oxidation of the xanthate 1 was performed
using m-CPBA, no sulfines were detected but again
thiocarbonate was obtained in lower yield.
Our electrochemical process can only be compared
(ratio thiocarbonate/dithioperoxycarbonate) with the oxi-
dation of 1 using trans-2-(phenylsulfonyl)-3-phenylox-
aziridine2 as oxidant agent. Nevertheless, our electro-
chemical process is cheaper, cleaner, more efficient and
it does not need sophisticated reagents.
O-E t h yl S-(3,3-d im et h yl-2-oxo-b u t yl) d it h ioca r b on a t e
(1h ): mp 24-25 °C; IR(KBr) ν 2969, 1716, 1366, 1221, 1113,
1
1047, 1001; H NMR (300 MHz, CDCl3) δ 1.13 (s, 9H), 1.29 (t,
3H, J ) 7 Hz), 4.16 (s, 2H), 4.5 (q, 2H, J ) 7 Hz); 13C NMR
(75.4 MHz, CDCl3) δ 13.4, 26.3, 42.1, 43.9, 70.0, 206.7, 213.2;
MS m/e (relative intensity) EI 220 (M+,4), 187 (18), 163 (18),
135 (13), 107 (8), 85 (7), 57 (100). Anal. Calcd for C9 H16 O2 S2:
C, 49.09; H, 7.27; S, 29.09. Found: C, 48.86; H, 7.09; S, 29.39.
Despite the fact that the obtained voltammetric Epa values
are around +2.0 V (vs SCE), the electrochemical oxidations were
carried out at +1.8 V (vs SCE) (the initial current intensity was
not over 150 mA to avoid danger due to the use of LiClO4 as
electrolyte) using a concentric cell with two compartments
separated by a porous (D3) glass tubing diaphragm and equipped
with a magnetic stirrer. The solvent supporting electrolyte (SSE)
was CH3CN-H2O (1%)/LiClO4 (0.1 M). Other details: Anode:
platinum; anolyte: O-ethyl dithiocarbonate 1 (2.0 mmol) in SSE
(40 mL). Cathode: platinum; catholyte: SSE (15 mL). At the
end of the electrolysis the solvent in the anodic solution was
removed under reduced pressure. The residue was extracted
with ether/water and the organic phase was dried over MgSO4
and concentrated by evaporation. The resulting solid or oil was
chromatographed on a silica gel (18 × 3 cm) column, using
hexane/EtOAc (6/1) as eluent. Boiling points are given at
atmospheric pressure.
Exp er im en ta l Section
â-Haloketones are commercially available and have been used
without purification. Dithiocarbonates (1a -h ) were prepared in
almost quantitative yield according to Whitham.11
O-Eth yl S-p h en a cyl d ith ioca r bon a te (1a ): mp 31-32 °C
(lit.12 mp 32 °C); 13C NMR (75.4 MHz, CDCl3) δ 13.7, 43.5, 70.7,
128.4, 128.8, 133.7, 135.8, 192, 213.8; MS m/e (relative intensity)
EI 240 (M+, 5), 207 (22), 180 (20), 151 (11), 105 (100), 91 (8), 77
(40), 51 (10).
O-E t h yl S-(4-m et h oxyp h en a cyl) d it h ioca r b on a t e (1b ):
mp 68-69 °C; IR (KBr) ν 2909, 1665, 1600, 1575, 1256, 1216,
1109, 1056, 832; H NMR (300 MHz, CDCl3) δ 1.38 (t, 3H, J )
1
5 Hz), 3.88 (s, 3H), 4.62 (q, 2H, J ) 5 Hz), 4.62 (s, 2H), 6.96 (d,
2Har, J ) 5.8 Hz), 8.0 (d, 2Har, J ) 5.8 Hz); 13C NMR (75.4
MHz, CDCl3) δ: 13.4, 43.0, 55.2, 70.3, 113.6, 128.5, 130.5, 163.7,
190.4, 213.1; MS m/e (relative intensity) EI 270 (M+, 3), 237 (5),
210 (3), 181 (5), 135 (100), 121 (8), 107 (8), 92 (10), 77 (16), 64
(6), 51 (2). Anal. Calcd for C12H14O3S2: C, 53.33; H, 5.18; S, 23.7.
Found: C, 53.3; H, 5.08; S, 23.9.
O-Eth yl S-(4-m eth ylp h en a cyl) d ith ioca r bon a te (1c): mp
88-89 °C; IR (KBr) ν 2971, 1690, 1604, 1226, 1051, 811; 1H NMR
(300 MHz, CDCl3) δ 1.39 (t, 3H, J ) 7 Hz), 2.43 (s, 3H), 4.62 (q,
2H, J ) 7 Hz), 4.64 (s, 2H), 7.3 (d, 2Har, J ) 8 Hz), 7.9 (d, 2Har,
J ) 8 Hz); 13C NMR (75.4 MHz, CDCl3) δ 13.4, 21.4, 43.2, 70.3,
128.2, 129.2, 133, 144.4, 191.5, 213.0; MS m/e (relative intensity)
EI 254 (M+, 4), 237 (5), 221 (11), 194 (9), 165 (7), 119 (100), 105
(6), 91 (30), 65 (14), 51 (3). Anal. Calcd for C12H14O2S2: C, 56.69;
H, 5.51; S, 25.2. Found: C, 56.59; H, 5.7; S, 25.5.
O-Eth yl S-p h en a cyl th ioca r bon a te (2a ): 94% yield; bp
171-173 °C; IR (KBr) ν 3060, 2981, 1718, 1697, 1597, 1148, 750,
689; 1H NMR (300 MHz, CDCl3) δ 1.22 (t, 3H, J ) 7.1 Hz), 4.21
(q, 2H, J ) 7.1 Hz), 4.31 (s, 2H), 7.3-7.6 (m, 3Har), 7.8-8.0 (m,
2Har); 13C NMR (75.4 MHz, CDCl3) δ 14.2, 38.5, 64.1, 128.4,
128.7, 133.7, 135.5, 169.8, 193.0; MS m/e (relative intensity) EI
224 (M+, 1), 105 (100), 77 (38), 51 (13). Anal. Calcd for
C
11H12O3S: C, 58.93; H, 5.36; S, 14.28. Found: C, 59.11; H, 5.44;
S, 14.01.
O-Eth yl S-(4-m eth oxyp h en a cyl) th ioca r bon a te (2b): 95%
O-Eth yl S-(4-ch lor op h en a cyl) d ith ioca r bon a te (1d ): mp
63-65 °C; IR (KBr) ν 2979, 1696, 1589, 1256, 1112, 1052, 990,
814; 1H NMR (300 MHz, CDCl3) δ 1.31 (t, 3H, J ) 7.3 Hz), 4.53
(q, 2H, J ) 7.3 Hz), 4.57 (s, 2H), 7.39 (d, 2Har, J ) 8.4 Hz), 7.9
(d, 2Har, J ) 8.4 Hz); 13C NMR (75.4 MHz, CDCl3) δ 13.4, 43.0,
70.5, 128.7, 129.5, 133.8, 139.8, 190.8, 212.6; MS m/e (relative
intensity) EI 276 (M+ + 2, 2), 274 (M+, 4), 243 (4), 241 (12), 216
(6), 214 (16), 185 (10), 141 (32), 139 (100), 111 (27), 75 (16). Anal.
Calcd for C11H11O2S2Cl: C, 48.09; H, 4.01; S, 23.32. Found: C,
48.27; H, 3.92; S, 23.11.
O-Eth yl S-(4-br om op h en a cyl) d ith ioca r bon a te (1e): mp
81-83 °C; IR (KBr) ν 2979, 1694, 1584, 1256, 1113, 1052, 988,
810; 1H NMR (300 MHz, CDCl3) δ 1.4 (t, 3H, J ) 7 Hz), 4.61 (s,
2H), 4.63 (q, 2H, J ) 7 Hz), 7.64 (d, 2Har, J ) 8.4 Hz), 7.87 (d,
2Har, J ) 8.4 Hz); 13C NMR (75.4 MHz, CDCl3) δ 13.4, 43.0,
70.5, 128.7, 129.6, 131.8, 134.2, 191.2, 212.8; MS m/e (relative
intensity) EI 320 (M+ + 2, 3), 318 (M+, 3), 287 (10), 285 (10),
260 (13), 258 (13), 232 (6), 230 (6), 185 (100), 183 (100), 157 (32),
155 (32), 89 (17), 76 (32). Anal. Calcd for C11H11O2S2Br: C, 41.38;
H, 3.45; S, 20.06. Found: C, 41.37; H, 3.65; S, 20.17.
yield; mp 40-41 °C; IR (KBr) ν 2979, 1704, 1670, 1599, 1261,
1146, 826; 1H NMR (300 MHz, CDCl3) δ 1.28 (t, 3H, J ) 7 Hz),
3.85 (s, 3H), 4.26 (q, 2H, J ) 7 Hz), 4.32 (s, 2H), 6.93 (d, 2Har,
J ) 8.8 Hz), 7.96 (d, 2Har, J ) 8.8 Hz); 13C NMR (75.4 MHz,
CDCl3) δ 13.9, 37.9, 55.2, 63.7, 113.6, 128.2, 130.5, 163.6, 169.7,
191.2; MS m/e (relative intensity) EI 254 (M+, 2), 136 (9), 135
(100), 107 (6), 92 (7), 77 (9). Anal. Calcd for C12H14O4S: C, 56.69;
H, 5.51; S, 12.60. Found: C, 56.61; H, 5.72; S, 12.41.
O-Eth yl S-(4-m eth ylp h en a cyl) th ioca r bon a te (2c): 97%
yield; mp 20-21 °C; IR (KBr) ν 2982, 1714, 1692, 1604, 1146,
1
806; H NMR (300 MHz, CDCl3) δ 1.25 (t, 3H, J ) 7 Hz), 2.37
(s, 3H), 4.24 (q, 2H, J ) 7 Hz), 4.32 (s, 2H), 7.23 (d, 2Har, J )
8.1 Hz), 7.85 (d, 2Har, J ) 8.1 Hz); 13C NMR (75.4 MHz, CDCl3)
δ 13.9, 21.4, 38.1, 63.8, 128.2, 129.1, 132.6, 144.4, 169.6, 192.4;
MS m/e (relative intensity) EI 238 (M+, 2), 193 (1), 165 (2), 120
(9), 119 (100), 105 (4), 91 (26), 77 (3), 65 (11). Anal. Calcd for
C12H14O3S: C, 60.50; H, 5.88; S, 13.45. Found: C, 60.60; H, 5.99;
S, 13.23.
O-Eth yl S-(4-ch lor op h en a cyl) th ioca r bon a te (2d ): 90%
yield; mp 68-69 °C; IR (KBr) ν 3091, 2973, 1699, 1674, 1586,
1
O-Eth yl S-(2-n a p h th a len -2-yl-2-oxoeth yl) d ith ioca r bon -
a te (1f): mp 93-94 °C; IR (KBr) ν 3051, 2981, 1691, 1624, 1591,
1347, 1212, 1054, 823; 1H NMR (300 MHz, CDCl3) δ 1.38 (t, 3H,
J ) 7 Hz), 4.63 (q, 2H, J ) 7 Hz), 4.8 (s, 2H), 7.52-7.66 (m,
1402, 1143, 831; H NMR (300 MHz, CDCl3) δ 1.29 (t, 3H, J )
7 Hz), 4.27 (q, 2H, J ) 7 Hz), 4.32 (s, 2H), 7.4 (d, 2Har, J ) 8.4
Hz), 7.9 (d, 2Har, J ) 8.4 Hz); 13C NMR (75.4 MHz, CDCl3) δ
13.9, 37.9, 63.9, 128.8, 129.5, 133.5, 139.9, 169.4, 191.7; MS m/e
(relative intensity) EI 260 (M+ + 2, 3), 258 (M+, 2), 213 (1), 185
(3), 141 (31), 139 (100), 113 (6), 111 (18), 75 (10), 50 (3). Anal.
Calcd for C11H11O3SCl: C, 51.06; H, 4.26; S, 12.38. Found: C,
50.89; H, 4.37; S, 12.42.
(11) Whitham, G. H.; Bridges A. J . J . Chem. Soc., Perkin Trans. 1
1975, 1603.
(12) Groth, B. Arkiv Kemi 1924, 9, 63.

322 J . Org. Chem., Vol. 66, No. 1, 2001
Notes
O-Eth yl S-(4-br om op h en a cyl) th ioca r bon a te (2e): 92%
yield; mp 79-80 °C; IR (KBr) ν 3091, 2973, 1699, 1677, 1580,
(72), 97 (82), 83 (66), 67 (100), 55 (71). Anal. Calcd for C9 H14 O3
S: C, 53.47; H, 6.93; S, 15.85. Found: C, 53.67; H, 7.11; S, 16.07.
1
1174, 1142, 814; H NMR (300 MHz, CDCl3) δ 1.26 (t, 3H, J )
O-Eth yl S-(3,3-d im eth yl-2-oxobu tyl) th ioca r bon a te (2h ):
92% yield; bp 133-135 °C; IR (KBr) ν 2972, 1728, 1705, 1367,
7 Hz), 4.25 (q, 2H, J ) 7 Hz), 4.3 (s, 2H), 7.58 (d, 2Har, J ) 8.1
Hz), 7.82 (d, 2Har, J ) 8.1 Hz); 13C NMR (75.4 MHz, CDCl3) δ
13.9, 37.9, 64, 128.6, 129.6, 131.7, 133.9, 169.4, 191.9, 212.8; MS
m/e (relative intensity) EI 304 (M+ + 2, 3), 302 (M+, 3), 259 (1),
257 (1), 231 (3), 229 (3), 200 (2), 198 (2), 185 (100), 183 (100),
157 (22), 155 (22), 104 (5), 90 (7), 76 (18), 50 (12). Anal. Calcd
for C11H11O3SBr: C, 43.56; H, 3.63; S, 10.56. Found: C, 43.26;
H, 3.81; S, 10.37.
1
1149, 847, 675; H NMR (300 MHz, CDCl3) δ 1.18 (s, 9H), 1.25
(t, 3H, J ) 7 Hz), 3.91 (s, 2H), 4.2 (q, 2H, J ) 7 Hz); 13C NMR
(75.4 MHz, CDCl3) δ 13.8, 26.1, 36.9, 43.9, 63.4, 169.5, 207.6;
MS m/e (relative intensity) EI 204 (M+, 3), 187 (18), 164 (1), 147
(19), 119 (8), 85 (23), 57 (100). Anal. Calcd for C9 H16 O3 S: C,
52.94; H, 7.84; S, 15.69. Found: C, 53.03; H, 8.01; S, 15.47.
O-E t h yl S-(4-ch lor op h en a cyl) d it h iop er oxyca r b on a t e
(3d ): 5% yield; mp 47-49 °C; IR (KBr) ν 2956, 1730, 1673, 1587,
1147, 1091, 999, 813; 1H NMR (300 MHz, CDCl3) δ 1.31 (t, 3H,
J ) 7 Hz), 4.17 (s, 2H), 4.31 (q, 2H, J ) 7 Hz), 7.4 (d, 2Har, J )
8.4 Hz), 7.9 (d, 2Har, J ) 8.4 Hz); 13C NMR (75.4 MHz, CDCl3)
δ 13.9, 44.2, 65.2, 128.8, 129.7, 133.2, 139.9,168.1, 191.6; MS
m/e (relative intensity) EI 220 (4-ClC6H4COCH2SSH, 3), 218 (7),
186 (3), 154 (27), 141 (32), 139 (100), 113 (11), 111 (35), 75 (22),
61 (14), 50 (8). Anal. Calcd for C11H11O3S2Cl: C, 45.44; H, 3.79;
S, 22.03. Found: C, 45.75; H, 4.01; S, 21.83.
O-Eth yl S-(2-n a p h th a len -2-yl-2-oxoeth yl) th ioca r bon a te
(2f): 95% yield; mp 50-52 °C; IR (KBr) ν 3062, 2976, 1708, 1690,
1623, 1358, 1174, 1152, 1020, 816, 746; 1H NMR (300 MHz,
CDCl3) δ 1.28 (t, 3H, J ) 7 Hz), 4.28 (q, 2H, J ) 7 Hz), 4.3 (s,
2H), 7.5-7.64 (m, 2Har), 7.82-8.2 (m, 4Har), 8.5 (s, 1Har); 13
C
NMR (75.4 MHz, CDCl3) δ 13.9, 38.3, 63.9, 123.5, 126.7, 127.5,
128.4, 128.6, 129.3, 130.2, 132.0, 132.4, 135.5, 169.7, 193; MS
m/e (relative intensity) EI 274 (M+, 7), 173 (3), 156 (12), 155
(100), 127 (47), 101 (4), 77 (6). Anal. Calcd for C15H14O3S: C,
65.69; H, 5.11; S, 11.68. Found: C, 65.49; H, 5.16; S, 11.88.
O-Eth yl S-(2-oxocycloh exyl) th ioca r bon a te (2g): 97%
yield; bp 150-151 °C; IR(KBr) ν 2928, 1720, 1702, 1147, 848;
1H NMR (300 MHz, CDCl3) δ 1.28 (t, 3H, J ) 7 Hz), 1.6-2.7 (m,
8H), 4.1-4.2 (m, 1H), 4.24 (q, 2H, J ) 7 Hz); 13C NMR (75.4
MHz, CDCl3) δ 13.9, 24.8, 27.0, 34.8, 41.2, 54.8, 63.5, 169.3,
205.1; MS m/e (relative intensity) EI 202 (M+, 22), 171 (33), 130
Ack n ow led gm en t. This research was supported by
the Spanish Ministry of Education and Culture.
PB97-0753.
J O001206L