Synthesis of some sulfur-containing acylferrocenes

Article abstract of DOI:10.1007/s007060170099

Acylferrocenes containing a sulfur atom within their side chain were synthesized in good yields by Friedel-Crafts acylation of ferrocene with in situ generated acyl halides from the corresponding carboxylic acids (S-protected derivatives of thioglicolic and α- and β-mercaptopropionic acids).

Full text of DOI:10.1007/s007060170099

Monatshefte fuÈr Chemie 132, 625±631 (2001)
Synthesis of some Sulfur-Containing
Acylferrocenes
Â
Rastko D. Vukicevic , Danijela Ilic , Zoran Ratkovic ,
and Mirjana Vukicevic
Â1;Ã
Â2
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1
Department of Chemistry, Faculty of Science, University of Kragujevac, YU-34000 Kragujevac,
Yugoslavia
ꢀ
University of Pristina, Faculty of Technology and Metallurgy, YU-28000 Kosovska Mitrovica,
2
Yugoslavia
Summary. Acylferrocenes containing a sulfur atom within their side chain were synthesized in good
yields by Friedel-Crafts acylation of ferrocene with in situ generated acyl halides from the
corresponding carboxylic acids (S-protected derivatives of thioglicolic and ꢀ- and ꢁ-mercaptopro-
pionic acids).
Keywords. Acylferrocenes; Friedel-Crafts acylation; ꢀ-Mercaptopropionic acid; ꢁ-Mercaptopro-
pionic acid; Thioglicolic acid.
Introduction
There is considerable interest in ferrocene derivatives containing two heteroatoms,
such as oxygen, nitrogen, sulfur, and phosphorus within the side chains. These
compounds are capable of coordinating different transition metal ions, giving
complexes that are effective catalysts in many homogenous catalytic processes.
Thus, optically active ferrocene diols, aminoalcohols, diamines, mercaptoalcohols, and
mercaptoamines are particularly interesting compounds for organic syntheses [1].
Recently, we required a ferrocene derivative containing in its structure a -CX-C-
S- or -CX-C-C-S- fragment (X ˆ OH or NR2). A Friedel-Crafts acylation of
ferrocene with a suitable carboxylic acid chloride was considered appropriate for
the syntheses of such compounds. The carbonyl group may then be easily
transformed into a hydroxyl or an amino group. The second desired functional
group may be introduced either by reaction of the acylferrocene with some sulfur
donating reagent, or by employing a sulfur-containing acyl chloride for the
acylation of ferrocene. Toma and Kaluz [2] have reported that sulfur-containing
ketones of the type C5H5FeC5H4COCH2CH2-S-Ar may be synthesized by Michael
addition of the corresponding thiophenolols to the double bond of vinyl ferrocenyl
ketone. These authors have also mentioned that the Friedel-Crafts acylation of
ferrocene with the methyl ester chloride of thiodiglycolic acid gives a sulfur
Ã
Corresponding author

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R. D. Vukicevic et al.
626
containing oxoester (C5H5FeC5H4-CO-CH2-S-CH2COOCH3) in 30% yield [3].
One more possibility described in literature for introducing a sulfur atom into a
ferrocene containing molecule is the condensation of thiols with ferrocenyl
carbinols in the presence of tri¯uoroacetic acid. This has been the method most
frequently used in the synthesis of sulfur-containing ferrocene derivatives [4±8].
The main goal of the work presented in this paper was to study possibilities of
the second approach, i.e. the direct synthesis of sulfur-containing acylferrocenes by
the Friedel-Crafts reaction. Since we have reported recently that ferrocene can be
successfully acylated by an acyl chloride generated in situ from the corresponding
carboxylic acid and phosphorus trichloride [4], we now decided to apply this
method to synthesize the above mentioned sulfur-containing acyl ferrocenes. As
the sulfur donating acylating agents, ꢀ- and ꢁ-sulfur-substituted carboxylic acids
(thioglicolic (1a), ꢀ-mercaptopropionic (1b), ꢁ-mercaptopropionic acids (1c) were
chosen.
Results and Discussion
We started the acylation reaction by stirring a mixture of ferrocene, the corre-
sponding mercaptocarboxylic acid, phosphorus trichloride, and aluminum trichlor-
ide in dichloromethane at room temperature [9, 10]. As expected, we failed to
obtain the desired ketones; evidently, the free SH group of the starting acids
prevents the reaction. Therefore, the acids 1a±c were treated with methyl iodide,
4-bromobenzylbromide, and 2,6-dichlorobenzyl bromide according to literature
[11±13] in order to protect the SH group, thus affording acids 2a±i (Scheme 1).
Acylation of ferrocene with 2a was performed as described above. Column
chromatography of the reaction mixture gave ± besides some unchanged ferrocene ±
only one product. On the basis of its spectroscopic data (IR, 1H NMR, 13C NMR) it
Scheme 1

Synthesis of Sulfur-Containing Acylferrocenes
627
Scheme 2
was identi®ed as the ketone 3a. Acylation of ferrocene with the other two acids
containing the S-CH3 group (2b and 2c) under the same reaction conditions
afforded the corresponding ketones 3b and 3c. A slightly different behaviour was
observed in the case of the acids 2d±i containing 4-bromobenzyl and 2,6-
dichlorobenzyl groups instead of the methyl group. The acids 2d, 2f, 2g, and 2i
reacted in a similar manner as acids 2a±c, yielding the corresponding ketones 3d,
3f, 3g, and 3i in good to very good yields. Surprisingly, acylation of ferrocene with
the acids 2e and 2h resulted in considerable lower yields of the corresponding
ketones 3e and 3h.
In order to optimize the yields of the corresponding ketones in the acylation of
ferrocene with S-protected derivatives of ꢀ-mercaptopropionic acid (2e and 2h), this
reaction was performed at approximately 0ꢀC as well as at re¯ux temperature of
dichloromethane; however, 3e and 3h were obtained in almost the same yield as at
room temperature. Also, elongated reaction times did not increase the yield. In this
case, however, almost complete consumption of the starting ferrocene was observed
leading to the formation of considerable amounts of unknown polar products which
could not separated and identi®ed.
Other solvents were also applied at different temperatures. Thus, reaction in
carbon disul®de (ice bath, room temperature, re¯ux) gave the desired ketones in
only 5±8% yield. Reaction in benzene provided a somewhat higher yield, but still
considerably lower than that in dichloromethane. Neither nitrobenzene nor 1,2-
dichlorobenzene (ice bath, room temperature, 80ꢀC) gave any useful results. In all
solvents used, acids 2a, 2b, 2c, 2d, 2f, 2g, and 2i gave higher yields than acids 2e
and 2h.

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R. D. Vukicevic et al.
628
As our results show, in situ generated acyl chlorides from phosphorus trichloride
and ꢀ- and ꢁ-sulfur-containing carboxylic acids can be successfully used (without
isolation and puri®cation) for the acylation of ferrocene. The phosphorus species,
whatever they are (phosphorous acid or, more probably, phosphorus trioxide), do
not hinder the catalytic action, thus rendering this procedure superior to the classical
acylation with acyl chlorides or carboxylic acid anhydrides.
Experimental
All chemicals used were commercially available and were used as received. CH2Cl2 was dried over
P2O5 and distilled before use; the other solvents were puri®ed by distillation. Thioglicolic, ꢀ- and ꢁ-
mercaptopropionic acids were purchased from Merck. Methylthiocarboxylic acids 2a and 2c,
although also known and commercially available, as well as acids 2b and 2d±i, were synthesized
according to a modi®ed literature procedure as given below [11±13]. Methyl iodide, 4-bromobenzyl
bromide, and 2,6-dichlorobenzyl bromide were obtained from Fluka. IR measurements were carried
out with a Perkin-Elmer 457 grating FT instrument. NMR spectra were recorded on a Varian Gemini
200 or a Bruker AC 250 spectrometer using CDCl3 as the solvent. Chemical shifts are expressed in
ppm relative to internal TMS.
General procedure for the synthesis of acids 2a±i
The corresponding mercaptoacids 1a±c (10 mmol) and sodium hydroxide (20 mmol) were dissolved
in 50 cm3 H2O. To this solution, 11 mmol of the corresponding halide (methyl iodide in the case of
acids 2a±c and 4-bromobenzyl bromide in the case of 2d±f) dissolved in 30 cm3 methanol were
added, and the mixture was stirred for 3 h. In the case of 2g±i, 2,6-dichlorobenzyl bromide (11 mmol)
was dissolved in 30 cm3 ether and added to the aqueous solution of the disodium salt of the
corresponding mercaptocarboxylic acid. To the resulting two-phase solution methanol was added
until the mixture became homogenous (about 40 cm3); After stirring for 3 h, the organic solvents
(methanol, i.e. methanol and ether) were evaporated, and the residue (the aqueous solution) was
extracted twice with 30 cm3 portions of ether and then acidi®ed with a 10% HCl solution. The
liberated carboxylic acids were extracted with three 30 cm3 portions of ether, and the combined
organic layers were washed with H2O and brine and dried over anhydrous sodium sulfate. After
®ltration, the solvent was distilled off, and the remaining S-protected acids (60±85%) were pure
1
enough (IR, H NMR, 13C NMR) to be used in the further studies.
2-Methylthio-propanoic acid (2b; C4H8O2S)
Yield: 60%; IR (®lm): ꢂmax ˆ 2986, 2928, 1708, 1455, 1421, 1379, 1323, 1287, 1241, 1197, 1072,
1
1000, 957 cm 1; H NMR (200 MHz, CDCl3): ꢃ ˆ 1:46 (d, J ˆ 7:1 Hz, 3H), 2.21 (s, 3H), 3.36 (q,
J ˆ 7:1, 1H), 10.33 (br s, 1H) ppm; 13C NMR (50 MHz, CDCl3): ꢃ ˆ 14:0, 18.1, 41.7, 179.0 ppm.
4-Bromobenzylthio-acetic acid (2d; C9H9BrO2S)
Yield: 79%; IR (KBr): ꢂmax ˆ 3445 (br), 3076, 2998, 2899, 2657, 2543, 1694, 1484, 1432, 1408,
1
1292, 1160, 1126, 1099, 1069, 1009, 917, 835, 727, 671, 611, 586, 495, 471 cm
;
1H NMR
(250 MHz, CDCl3): ꢃ ˆ 3:08 (s, 2H), 3.81 (s, 2H), 7.21±7.26 (m, 2H), 7.43±7.47 (m, 2H), 10.20 (br
s, 1H) ppm; 13C NMR (62.5 MHz, CDCl3): ꢃ ˆ 31:9, 35.7, 121.4, 130.9, 131.7, 135.9, 176.7 ppm.
2-(4-Bromobenzylthio)-propanoic acid (2e; C10H11BrO2S)
Yield: 82%; IR (KBr): ꢂmax ˆ 3026, 2982, 2931, 2721, 2636, 2549, 2363, 1902, 1707, 1591, 11487,
1
1453, 1416, 1379, 1323, 1286, 1240, 1196, 1071, 1011, 935, 882, 827, 729 cm
;
1H NMR

Synthesis of Sulfur-Containing Acylferrocenes
629
(250 MHz, CDCl3): ꢃ ˆ 1:40 (d, J ˆ 7:1, 3H), 3.25 (q, J ˆ 7:1, 1H), 3.77 and 3.88 (AB system,
JAB ˆ 10; 9 Hz, 2H), 7.21±7.26 (m, 2H), 7.42±7.47 (m, 2H), 10.50 (br s, 1H) ppm; 13C NMR
(62.5 MHz, CDCl3): ꢃ ˆ 16:3, 35.4, 39.6, 121.1, 130.8, 131.6, 136.3, 179.6 ppm.
3-(4-Bromobenzylthio)-propanoic acid (2f; C10H11BrO2S)
Yield: 85%; IR (KBr): ꢂmax ˆ 3444 (br), 3064, 2961, 2922, 2746, 2669, 2585, 1907, 1696, 1485,
1424, 1400, 1344, 1267, 1244, 1199, 1157, 1099, 1068, 1007, 948, 865, 823, 772, 743, 721, 683,
1
655, 609, 494 cm 1; H NMR (200 MHz, CDCl3): ꢃ ˆ 2:62 (m, 4H), 3.67 (s, 2H), 7.16±7.20 (m,
2H), 7.40±7.45 (m, 2H), 11.50 (br s, 1H) ppm; 13C NMR (50 MHz, CDCl3): ꢃ ˆ 25:7, 43.2, 35.6,
120.9, 130.4, 131.6, 136.9, 178.2 ppm.
(2,6-Dichlorobenzylthio)-acetic acid (2g; C9H8Cl2O2S)
Yield: 76%; IR (KBr): ꢂmax ˆ 3470 (br), 2983, 2926, 2706, 2580, 2362, 1950, 1804, 1699, 1576,
1557, 1432, 1377, 1305, 1239, 1209, 1175, 1124, 1086, 977, 919, 897, 841, 787, 761, 670, 537,
1
509 cm 1; H NMR (250 MHz, CDCl3): ꢃ ˆ 3:40 (s, 2H), 4.19 (s, 2H), 7.10±7.17 (m, 2H), 7.27±
7.33 (m, 2H); 11.10 (br s, 1H) ppm; 13C NMR (62.5 MHz, CDCl3): ꢃ ˆ 32:1, 34.0, 128.5, 129.0,
134.2, 135.6, 176.6 ppm.
2-(2,6-Dichlorobenzylthio)-propanoic acid (2h; C10H10Cl2O2S)
Yield: 77%; IR (KBr): ꢂmax ˆ 3443 (br), 2977, 2930, 2726, 2632, 2554, 1697, 1580, 1560, 1438,
1
1419, 1379, 1324, 1292, 1244, 1194, 1121, 1073, 1001, 977, 925, 897, 826, 767, 681, 606, 516 cm
;
1H NMR (250 MHz, CDCl3): ꢃ ˆ 1:51 (d, J ˆ 7:2 Hz, 3H), 3.65 (q, J ˆ 7:2 Hz, 1H), 4.23 (s, 2H),
7.09±7.16 (m, 2H), 7.26±7.32 (m, 2H), 10.57 (br s, 1H) ppm; 13C NMR (62.5 MHz, CDCl3):
ꢃ ˆ 17:0, 31.6, 42.0, 128.4, 128.9, 134.0, 135.7, 179.3 ppm.
3-(2,6-Dichlorobenzylthio)-propanoic acid (2i; C10H10Cl2O2S)
Yield: 75%; IR (KBr): ꢂmax ˆ 3462 (br), 3045, 2941, 2753, 2658, 1692, 1579, 1560, 1436, 1419,
1
1332, 1263, 1197, 1088, 987, 913, 893, 870, 782, 760, 684, 655, 489 cm 1; H NMR (250 MHz,
CDCl3): ꢃ ˆ 2:79 (m, 4H), 4.05 (s, 2H), 7.09±7.15 (m, 1H), 7.29±7.32 (m, 2H), 1060 (br s, 1H) ppm;
13C NMR (62.5 MHz, CDCl3): 26.8, 31.5, 34.9, 128.5, 128.7, 135.0, 135.4, 178.1 ppm.
General procedure for the acylation of ferrocene
To a solution of 1.2 mmol of the corresponding carboxylic acid in 20 cm3 of the appropriate solvent
placed in a two-necked round bottom ¯ask supplied with a magnetic stirrer and a re¯ux condenser,
0.1 cm3 (ꢁ1 mmol) PCl3 were added under an argon atmosphere and the resulting mixture was
stirred overnight. 186 mg of ferrocene (1 mmol) and 200 mg of AlCl3 were added, and the mixture
was stirred for 3 h under cooling in an ice-water bath, at room temperature, under re¯ux, or at 80ꢀC
(cf. Results and Discussion). Subsequently, the reaction mixture was poured into 40 cm3 of a cold
solution of KOH (1 mol/dm3) in order to remove the remaining carboxylic acid as well as aluminum
and phosphorus species. The organic layer was separated, and the aqueous phase was extracted with
30 cm3 of diethyl ether. The organic layers were collected and washed with H2O, brine, and again
H2O and dried over anhydrous Na2SO4. After evaporation of the solvents, the crude reaction mixture
was separated by column chromatography (SiO2/toluene, then toluene: ethyl acetate ˆ 9:1). In all
experiments, the toluene fractions contained unchanged ferrocene, whereas the second fractions
contained the acylated products 3a±i. When nitrobenzene and 2,6-dichlorobenzene were used as
solvents, they were removed from the reaction mixture prior to work-up by steam distillation.

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R. D. Vukicevic et al.
630
1-Ferrocenyl-3-thiabutan-1-one (3a; C13H14FeOS)
Yield: 60%; IR (KBr): ꢂmax ˆ 1635, 1450, 1410, 1370, 1290, 1170, 1105, 1070, 995, 820 cm 1; 1H
NMR (250 MHz, CDCl3): ꢃ ˆ 2:17 (s, 3H), 3.52 (s, 2H), 4.22 (s, 5H), 4.53 (t, J ˆ 1:6 Hz, 2H), 4.79
(t, J ˆ 1:6 Hz, 2H) ppm; 13C NMR (62.5 MHz, CDCl3): ꢃ ˆ 16:1, 40.2, 69.7, 69.9, 72.5, 77.2,
198.4 ppm.
1-Ferrocenyl-2-methyl-3-thiabutan-1-one (3b; C14H16FeOS)
Yield: 70%; IR (KBr): ꢂmax ˆ 3080, 2940, 2915, 1650, 1450, 1375, 1255, 1200, 1110, 1050, 1025,
1
100, 825, 490 cm 1; H NMR (250 MHz, CDCl3): ꢃ ˆ 1:53 (d, J ˆ 6:9 Hz, 3H), 1.99 (s, 3H), 3.87
(q, J ˆ 6:9 Hz, 1H), 4.20 (s, 5H), 4.49 (t, J ˆ 1:4, 1H), 4.54 (t, J ˆ 1:4, 1H), 4.73 (t, J ˆ 1:4, 1H),
4.88 (t, J ˆ 1:4, 1H) ppm; 13C NMR (62.5 MHz, CDCl3): ꢃ ˆ 11:1, 15.2, 42.5, 69.2, 69.7, 70.1, 72.3,
77.3, 199.3 ppm.
1-Ferrocenyl-4-thiapentan-1-one (3c; C14H16FeOS)
Yield: 66%; IR (KBr): ꢂmax ˆ 1655, 1450, 1405, 1370, 1330, 1260, 1210, 1105, 1080, 1025, 1000,
890, 820 cm 1; 1H NMR (200 MHz, CDCl3): ꢃ ˆ 2:19 (s, 3H), 2.86±3.02 (m, 4H), 4.23 (s, 5H), 4.52
(t, J ˆ 2:0 Hz, 2H), 4.80 (t, J ˆ 2:0 Hz, 2H) ppm; 13C NMR (50 MHz, CDCl3): ꢃ ˆ 16:0, 28.6, 38.4,
69.2, 69.8, 72.3, 78.6, 202.3 ppm.
4-(4-Bromophenyl)-1-ferrocenyl-3-thiabutan-1-one (3d; C19H17BrFeOS)
Yield: 60%; IR (KBr): ꢂmax ˆ 3094, 2921, 1730, 1657, 1590, 1486, 1452, 1406, 1377, 1289, 1105,
1069, 1030, 1009, 893, 826, 730 cm 1; 1H NMR (250 MHz, CDCl3): ꢃ ˆ 3:38 (s, 2H), 3.74 (s, 2H),
4.19 (s, 5H), 4.53 (t, J ˆ 1:8 Hz, 2H), 4.75 (t, J ˆ 1:8 Hz, 2H), 7.24±7.28 (m, 2H), 7.43±7.46 (m,
2H) ppm; 13C NMR (62.5 MHz, CDCl3): ꢃ ˆ 35:4, 36.4, 69.7, 69.9, 72.7, 77.3, 121.0, 131.0, 131.5,
136.5, 198.7 ppm.
4-(4-Bromophenyl)-1-ferrocenyl-2-methyl-3-thiabutan-1-one (3e; C20H19BrFeOS)
Yield: 50%; IR (KBr): ꢂmax ˆ 3096, 2972, 2925, 1728, 1659, 1589, 1486, 1450, 1406, 1376, 1320,
1
1257, 1195, 1104, 1069, 1052, 1009, 964, 879, 826, 778, 723 cm 1; H NMR (250 MHz, CDCl3):
ꢃ ˆ 1:58 (d, J ˆ 6:9 Hz, 3H), 3.64 and 3.73 (AB system, JAB ˆ 9:5 Hz, 2H), 3.87 (q, J ˆ 6:9 Hz,
1H), 4.18 (s, 5H), 4.50 (m, 1H), 4.56 (m, 1H), 4.64 (m, 1H), 4.83 (m, 1H), 7.17±7.20 (m, 2H), 7.41±
7.44 (m, 2H) ppm; 13C NMR (62.5 MHz, CDCl3): ꢃ ˆ 16:9, 33.1, 43.8, 69.4, 69.8, 70.0, 72.5, 77.1,
120.9, 130.8, 131.5, 136.7, 200.3 ppm.
5-(4-Bromophenyl)-1-ferrocenyl-4-thiapentan-1-one (3f; C20H19BrFeOS)
Yield: 70%; IR (KBr): ꢂmax ˆ 3093, 2922, 2853, 1732, 1665, 1590, 1486, 1454, 1406, 1379, 1343,
1
1258, 1209, 1105, 1075, 1030, 1008, 882, 826, 734, 698 cm
;
1H NMR (200 MHz, CDCl3):
ꢃ ˆ 2:83 (m, 4H), 3.70 (s, 2H), 4.17 (s, 5H), 4.47 (t, J ˆ 1:9 Hz, 2H 1.9 Hz, 2H), 4.71 (t, J ˆ 1:9 Hz,
2H), 7.19±7.24 (m, 2H), 7.41±7.45 (m, 2H) ppm; 13C NMR (50 MHz, CDCl3): ꢃ ˆ 25:6, 36.1, 39.3,
69.0, 69.6, 72.1, 78.2, 120.6, 130.4, 131.4, 137.4, 201.7 ppm.
4-(2,6-Dichlorophenyl)-1-ferrocenyl-3-thiabutan-1-one (3g; C19H16Cl2FeOS)
Yield: 67%; IR (KBr): ꢂmax ˆ 3092, 2926, 1737, 1661, 1580, 1562, 1514, 1440, 1413, 1377, 1347,
1289, 1220, 1153, 1126, 1109, 1088, 1067, 1028, 1001, 893, 824, 781, 761, 721, 689 cm 1; 1H NMR

Synthesis of Sulfur-Containing Acylferrocenes
631
(200 MHz, CDCl3): ꢃ ˆ 3:77 (s, 2H), 4.16 (s, 2H), 4.19 (s, 5H), 4.52 (s, 2H), 4.76 (s, 2H), 7.08±7.15
(m, 1H), 7.28±7.31 (m, 2H) ppm; 13C NMR (50 MHz, CDCl3): ꢃ ˆ 32:0, 39.2, 69.5, 69.8, 72.5, 77.4,
128.3, 128.7, 134.5, 135.5, 198.6 ppm.
4-(2,6-Dichlorophenyl)-1-ferrocenyl-2-methyl-3-thiabutan-1-one (3h; C20H18Cl2FeOS)
Yield: 51%; IR (KBr): ꢂmax ˆ 3098, 2972, 2925, 1786, 1655, 1579, 1559, 1438, 1376, 1321, 1256,
1213, 1192, 1156, 1104, 1053, 1001, 961, 887, 830, 782, 759, 721, 687, 527, 497, 460, 425 cm 1; 1H
NMR (200 MHz, CDCl3): ꢃ ˆ 1:7 (d, J ˆ 6:9 Hz, 3H), 3.94 and 4.17 (AB system, JAB ˆ 12:3 Hz,
2H), 4.06 (q, J ˆ 6:8 Hz, 1H), 4.15 (s, 5H), 4.46 (s, 1H), 4.51 (s, 1H), 4.66 (s, 1H), 4.78 (s, 1H),
7.06±7.13 (m, 1H), 7.25±7.29 (m, 2H) ppm; 13C NMR (50 MHz, CDCl3): ꢃ ˆ 17:5, 29.5, 44.4, 69.2,
69.6, 72.3, 77.0, 128.3, 128.7, 134.0, 135.5, 200.2 ppm.
5-(2,6-Dichlorophenyl)-1-ferrocenyl-4-thiapentan-1-one (3i; C20H18Cl2FeOS)
Yield: 83%; IR (KBr): ꢂmax ˆ 3332, 3089, 1733, 1667, 1580, 1561, 1454, 1436, 1413, 1379, 1340,
1256, 1213, 1159, 1107, 1083, 1029, 1001, 977, 877, 825, 781, 762, 695 cm 1; 1H NMR (200 MHz,
CDCl3): ꢃ ˆ 3:01 (s, 4H), 4.10 (s, 2H), 4.20 (s, 5H), 4.49 (s, 2H), 4.76 (s, 2H) ppm; 13C NMR
(50 MHz, CDCl3): ꢃ ˆ 26:7, 32.0, 39.9, 69.1, 59.7, 72.2, 78.4, 128.4, 128.5, 135.3, 202.0 ppm.
Acknowledgements
The authors are grateful to Professor Radomir N. Saiꢀciꢁc from the Faculty of Chemistry, University of
Belgrade, for stimulating discussions.
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Received September 17, 2000. Accepted (revised) October 16, 2000