Magnetite (Fe3O4) nanoparticles: An efficient and recoverable catalyst for the synthesis of alkynyl chalcogenides (selenides and tellurides) from terminal acetylenes and diorganyl dichalcogenides

Article abstract of DOI:10.1016/j.tet.2013.09.095

We present herein a new and efficient methodology for the synthesis of alkynyl chalcogenides from terminal acetylenes and diorganyl dichalcogenides, catalyzed by Fe₃O₄ nanoparticles. This new approach provided the desired products in good to excellent yields. Moreover, the catalyst was easily recoverable using an external magnet and reused for further experiments without loss of catalytic activity.

Full text of DOI:10.1016/j.tet.2013.09.095

Tetrahedron 70 (2014) 3349e3354
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Magnetite (Fe3O4) nanoparticles: an efficient and recoverable
catalyst for the synthesis of alkynyl chalcogenides (selenides and
tellurides) from terminal acetylenes and diorganyl dichalcogenides
Marcelo Godoi, Daiane G. Liz, Eduardo W. Ricardo, Manuela S.T. Rocha,
*
Juliano B. Azeredo, Antonio L. Braga
ꢀ
Departamento de Química, Universidade Federal de Santa Catarina, Florianopolis, SC 88040-970, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 August 2013
Received in revised form 18 September 2013
Accepted 24 September 2013
Available online 5 October 2013
We present herein a new and efficient methodology for the synthesis of alkynyl chalcogenides from
terminal acetylenes and diorganyl dichalcogenides, catalyzed by Fe3O4 nanoparticles. This new approach
provided the desired products in good to excellent yields. Moreover, the catalyst was easily recoverable
using an external magnet and reused for further experiments without loss of catalytic activity.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Organochalcogenides
Alkynyl selenides
Alkynyl tellurides
Nano catalysis
Magnetite nanoparticles
1. Introduction
of alkynes or strong bases, the low air stability of some organo-
metallic reagents, and the difficulties associated with the manip-
Organochalcogenides have gained special attention mainly be-
cause of their specific biological activity, e.g., antioxidant, antitu-
mor, antimicrobial, and antiviral.1 Furthermore, this class of
compounds has become an attractive synthetic target in several
transformations.2,3
Alkynyl chalcogenides have been widely used as powerful
intermediates in synthetic organic chemistry.4 These kinds of
compounds have been successfully employed, for example, as
precursors in several transformations, such as hydrohalogenation5
and hydrosulfonation,6 and for the synthesis of some
heterocycles.7
In general, alkynyl chalcogenides are prepared using hyper-
valent species of iodine or through the reaction of alkynyl bromides
with nucleophilic species of chalcogen generated from a metal.8,9
Also, the treatment of acetylene with strong bases or organome-
tallic reagents followed by reaction with a diorganyl dichalcogenide
or organochalcogenyl halide has been reported.10 However, most of
these methods have limitations, such as the use of excess amounts
ulation of some organochalcogen compounds.
Alkynyl selenides and tellurides can also be conveniently syn-
thesized through the transition metal-catalyzed reaction of
dichalcogenides with either alkynes or bromo alkynes under dif-
ferent conditions.11 Methodologies involving heterogeneous catal-
ysis have been preferred due to the easy isolation and separation of
the catalyst after the completion of the reaction.12 Nonetheless, the
development of new methods to prepare these kinds of organo-
chalcogenides employing non-toxic and easily recovered catalysts
is highly desirable.
Magnetic nano catalysts have attracted considerable atten-
tion due to their unique properties, such as large surface area
and facile separation using external magnets.13 In this
context, magnetically recoverable iron oxides have emerged as
powerful catalysts and have been recently applied in organic
synthesis.14
Although iron nano catalysts have been used for several trans-
formations, they have not yet been employed in the field of orga-
nochalcogen chemistry. Thus, in line with our ongoing research in
this area15 a new method for the synthesis of alkynyl chalcogenides
using Fe3O4 nanopowder as a recyclable catalyst is reported herein
(Scheme 1).
* Corresponding author. Tel./fax: þ55 48 3721 6427; e-mail addresses: braga.
antonio@ufsc.br, albraga1@gmail.com (A.L. Braga).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.09.095

3350
M. Godoi et al. / Tetrahedron 70 (2014) 3349e3354
Table 2
Optimization of the reaction conditionsa
Scheme 1. Synthesis of alkynyl selenides and tellurides.
Entry Time (h) T (ꢀC) Base
Solvent
Yield 3a (%)b Yield 4a (%)b
1
2
3
4
5
6
7
8
10
14
28
14
14
14
14
14
14
14
14
14
14
14
80
80
80
120
25
80
80
80
80
80
80
80
80
80
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3 DMF
KOH DMF
Na2CO3 DMF
DMF
DMF
DMF
DMF
DMF
66
78
69
73
d
4
4
10
6
2. Results and discussion
We began our study by screening the activity of the catalysts,
using phenylacetylene and diphenyl diselenide as standard sub-
strates, in the presence of K2CO3 and DMF (Table 1). Firstly, we
evaluated the amount of Fe3O4 nanoparticles required to obtain an
efficient reaction. Carrying out the reaction with 2 mol % of catalyst
afforded the desired product with only 30% yield (entry 1). On in-
creasing the catalyst amount to 5 mol %, the yield of the reaction
was enhanced to 67% (entry 2). Notably, when 10 mol % of nano-
Fe3O4 was used the product 3a was achieved in 78% yield (entry 3).
However, the yield value was not significantly increased when the
catalyst loading was increased to 20 mol % (entry 4).
d
58
50
Traces
d
5
20
22
6
9
d
DMF
10
11
12
13
14
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
DMSO
CH3CN
DMF/H2O
THF
65
38
40
d
8
23
14
6
[bmmim]BF4 16
83
a
Reaction conditions: phenylacetylene (0.5 mmol), diphenyl diselenide
(0.25 mmol), base (0.5 mmol), and Fe3O4 nanopowder (0.05 mmol, 10 mol %),
solvent.
b
Yields for isolated products.
Table 1
Screening of the catalystsa
Finally, we evaluated the effect of the solvents on the reaction
system (entries 9e14). However, no improvement in the chemical
yield of 3a was achieved when we changed the solvent. In-
terestingly, when the reaction was carried out using an ionic liquid
as the solvent, compound 3a was obtained in poor yield increasing
the amount of by-product (entry 14). Thus, focusing exclusively on
the synthesis of alkynyl chalcogenides, the best reaction condition
found is shown in entry 2.
After obtaining the optimized conditions we investigated the
scope and applicability of the present protocol (Table 3). Firstly, we
synthesized a series of alkynyl selenides by reacting acetylenes
with different diorganyl diselenides (entries 1e9). In terms of
electronic effects, the reaction was not sensitive, since the treat-
ment of phenylacetylene with diselenides containing either
a withdrawing or a donating electron group afforded the corre-
sponding products in very good yields (entries 2 and 3). In contrast,
a diselenide containing a releasing group at the ortho position of
the aromatic ring provided the respective product in lower yield
(entry 4). Furthermore, an aliphatic diselenide, in this case di-butyl
diselenide, also reacted well with phenylacetylene, affording the
respective product in satisfactory yield (entry 5).
Aryl-substituted alkynes also showed good activity under the
optimized conditions allowing the preparation of alkynyl selenides
in good yields (entries 6e8). However, with the employment of n-
octyne the corresponding product was synthesized in moderate
yield (entry 9).
Next we explored the generality of the reaction regarding the
preparation of alkynyl tellurides (entries 10e17). It was found that
the combination of phenylacetylene and diphenyl ditelluride, un-
der standard conditions, furnished the desired product in 91% yield
(entry10). However, when we employed a ditelluride with a with-
drawing group at the para position of the aromatic ring a decrease
in the yield was observed (entry 2).
Entry
Catalyst (mol %)
Yield 3a (%)b
Yield 4a (%)b
1
2
3
4
5
6
7
8
Nano-Fe3O4 (2)
Nano-Fe3O4 (5)
Nano-Fe3O4 (10)
Nano-Fe3O4 (20)
Nano-Fe3O4 (10)
Nano-CuFe2O3(10)
Fe3O4 (10)
Feꢀ (10)
FeCl3(10)
Fe(acac)3(10)
d
30
67
78
79
60
70
33
38
12
8
4
4
5
10
30
38
30
30
45
9
10
11
40
40
Traces
a
Reaction conditions: phenylacetylene (0.5 mmol), diphenyl diselenide
(0.25 mmol), K2CO3 (0.5 mmol), catalyst, DMF.
b
Yields for isolated products.
On the other hand, the catalytic activity of the bulk Fe3O4 was
very low when compared to its nanoparticle analogue, affording
the desired product in only 33% yield (entry 7 vs 3). Similarly, the
other bulk iron catalysts employed furnished the product 3a in
lower yields (entries 8e10). Moreover, we also detected the for-
mation of vinylic selenide as a by-product, which was not sig-
nificant in the presence of 10 mol % of nano-Fe3O4, as shown in
Table 1.
Having defined the best catalyst we investigated several pa-
rameters including time, temperature, base, and solvent (Table 2).
First of all, the time was screened and 14 h was the most appro-
priate choice (entries 1e3). Next, we investigated the effect of
raising the reaction temperature from 80 to 120 ꢀC and a slight
decreased in the yield was observed (entry 4). In addition, the
product 3a was not obtained when the reaction was performed at
room temperature (entry 5).
The influence of the base was also evaluated in detail (entries
6e9). The results showed that using Cs2CO3 and KOH as the base
afforded the desired alkynyl selenide in 58 and 50% yields, re-
spectively (entries 6 and 7). However, on employing Na2CO3 only an
insignificant amount of product was obtained, and in the absence of
base the formation of compound 3a was not observed (entries 8
and 9).
It can be noted that the presence of a methyl group at the same
position on the aromatic ring provided the respective alkynyl tel-
luride in 92% yield (entry 12). In addition, the reaction proceeded
well when an alkyl ditelluride was employed, affording the product
3m in reasonable yield (entry 13).
Subsequently, the combination of structurally diverse alkynes
with diphenyl ditelluride was also evaluated (entries 14e16). A
series of aromatic alkynes with different substituents provided the
expected products 3nep with 60e70% yield (entries 14e16).

M. Godoi et al. / Tetrahedron 70 (2014) 3349e3354
3351
Table 3
Synthesis of several alkynyl chalcogenidesa
Entry
1
Alkyne
R2
Y
Product
Yieldb
Ph
Se
78
76
77
3a
2
3
p-MeOPh
p-ClPh
Se
Se
3b
3c
4
o-MePh
Se
45
3d
5
6
7
n-C4H9
Se
Se
Se
51
60
65
3e
3f
Ph
Ph
3g
8
Ph
Se
65
3h
9
Ph
Ph
Se
Te
50
91
3i
3j
10
11
12
p-ClPh
Te
Te
70
92
3k
3l
p-MePh
13
14
n-C4H9
Te
Te
66
60
3m
3n
Ph
15
Ph
Te
65
3o
(continued on next page)

3352
M. Godoi et al. / Tetrahedron 70 (2014) 3349e3354
Table 3 (continued )
Entry
Alkyne
R2
Y
Product
Yieldb
70
16
Ph
Te
3p
3q
17
Ph
Te
52
a
Reaction conditions: acetylene (0.5 mmol), diorganyl dichalcogenide (0.25 mmol), base (0.5 mmol), and Fe3O4 nanopowder (0.05 mmol, 10 mol %), DMF.
Isolated yields.
b
Moreover, the methodology was also applicable to the preparation
of an aliphatic alkynyl telluride, which was obtained in reasonable
yield (entry 17).
We also tested the recyclability of the Fe3O4 nanoparticles in
order to evaluate the sustainability of the present methodology.
Therefore, the catalyst was recovered from the reaction medium
using an external magnet and reused for further reactions. As
a noteworthy result, the catalyst maintained its activity and ac-
ceptable efficiency for up to four recycling experiments (Fig. 1).
Based on reports in the literature,16 we can outline a plausible
reaction pathway for the preparation of alkynyl chalcogenides
(Scheme 2). It can be postulated that in the first step the [alkenyl-
iron] cluster 4 could be formed from the terminal alkyne and the
catalyst. Instantly, this cluster could react with diorganyl dichal-
cogenide leading to 5. Next, the reductive elimination could take
place, giving the desired alkynyl chalcogenides and species 6.
Lastly, in the presence of base, the catalyst would be regenerated,
completing the catalytic cycle.
Fig. 1. Recyclability of the catalyst.
Scheme 2. Plausible reaction pathway.
Additionally, transmission electron microscopy (TEM) analysis
of the Fe3O4 nanopowder was performed before use and after an
additional four reaction runs (Fig. 2). These results might suggest
that the magnetite nanoparticles were present in the reaction even
in the fourth run.
3. Conclusion
In summary, we have developed a mild and efficient methodol-
ogy for the synthesis of alkynyl chalcogenides, using Fe3O4 nano-
particles as a reusable catalyst. This new protocol generally afforded
the corresponding products in good to excellent yields. In addition,
the nanocatalyst was easily recovered from the reaction medium by
simple magnetic force, avoiding tedious work-up separation, and
reused for up to four cycles without significant loss of activity.
4. Experimental section
4.1. General remarks
1H and 13C NMR spectra were recorded in CDCl3 at 400 MHz and
200 MHz or at 100 MHz and 50 MHz, respectively. Chemical shifts
Fig. 2. TEM images of Fe3O4 nanoparticles. (a) Fresh Fe3O4 nanopowder (b) Fe3O4
nanopowder after fourth run.

M. Godoi et al. / Tetrahedron 70 (2014) 3349e3354
3353
(
d
) are reported (in parts per million) relative to the TMS (1H NMR)
1.8 Hz); 7.36e7.19 (m, 4H); 6.95e6.84 (m, 2H); 3.92 (s, 3H) ppm. 13
C
NMR (100 MHz, CDCl3): 160.1; 133.3; 129.9; 129.4; 128.6; 126.8;
120.3; 112.3; 110.5; 99.6; 72.7; 55.7 ppm.
and to the solvent (13C NMR). The solvents were used without
further purification. Thin layer chromatography (TLC) was per-
formed using Merck Silica Gel GF254, 0.25 mm thickness. For visu-
alization, TLC plates were either placed under ultraviolet light, or
stained with iodine vapor, or acidic vanillin. The Fe3O4 nanopowder
was purchased from SigmaeAldrich (particle size: 50e100 nm;
surface area: 60 m2/g).
4.3.8. ((2-Methoxyphenyl)ethynyl)(phenyl)selane7b (3h). 1H NMR
(200 MHz, CDCl3): 7.65e7.59 (m, 2H); 7.44 (dd, 1H, J¼7.7 and
1.8 Hz); 7.36e7.19 (m, 4H); 6.95e6.84 (m, 2H); 3.92 (s, 3H) ppm. 13
C
NMR (100 MHz, CDCl3): 160.1; 133.3; 129.9; 129.4; 128.6; 126.8;
120.3; 112.3; 110.5; 99.6; 72.7; 55.7 ppm.
4.2. General procedure for the preparation of alkynyl
chalcogenides
4.3.9. But-1-ynyl(phenyl)selane11c (3i). 1H NMR (400 MHz, CDCl3):
7.53e7.46 (m, 2H); 7.32e7.22 (m, 3H); 2.46 (t, 2H, J¼8 Hz); 1.44 (m,
6H);0.90(t, 3H, J¼8Hz)ppm.13CNMR(100MHz,CDCl3):133.2;129.6;
128.7; 126.9; 105.1; 57.6; 31.6; 28.9; 28.8; 22.81; 20.8; 14.3 ppm.
The terminal acetylene (0.5 mmol) and Fe3O4 nanopowder
(0.05 mmol, 10 mol %) were placed into a round-bottom flask, fol-
lowed by diorganyl dichalcogenide (0.25 mmol) and DMF. The re-
action was stirred at 80 ꢀC in an oil bath for 14 h. After this time, the
reaction was cooled to room temperature, quenched with water,
and the aqueous layer was extracted with ethyl acetate. The organic
phase was dried over MgSO4, filtered, and the volatiles were
completely removed under vacuum to give the crude residue. Pu-
rification by flash chromatography with a mixture of hexane/ethyl
acetate (99:1) afforded the desired alkynyl chalcogenide.
4.3.10. Phenyl(phenylethynyl)tellane11c (3j). 1H NMR (400 MHz,
CDCl3): 7.79e7.77 (m, 2H); 7.52e7.50 (m, 2H); 7.38e7.33 (m, 3H);
7.32e7.29 (m, 3H) ppm. 13C NMR (100 MHz, CDCl3): 135.0; 131.8;
129.7; 128.6; 128.2; 127.9; 123.3; 114.2; 113.1; 47.5 ppm.
4.3.11. (4-Chlorophenyl)(phenylethynyl)tellane
(3k). 1H
NMR
(200 MHz CDCl3): 7.55e7.49 (m, 4H); 7.38e7.27 (m, 5H) ppm. 13
C
NMR (100 MHz, CDCl3): 133.3; 131.8; 130.3; 129.6; 128.7; 128.4;
4.3. Recyclability of the catalyst
127.1; 122.9; 103.3; 68.6 ppm. HRMS (ESIþ) m/z: calculated for
C
14H9TeCl [MþCH3OH]þ: 372.9623; found, 372.9629.
After completion of the experiment, the reaction system was
cooled to room temperature and the Fe3O4 nanoparticles were
collected on an external magnet, washed with a 1:1 mixture of
ethyl acetate and water (3 mL), and dried under vacuum. The re-
covered catalyst was reused directly for the next run.
4.3.12. (Phenylethynyl)(p-tolyl)tellane10e (3l). 1H NMR (400 MHz,
CDCl3): 7.66 (d, 2H, J¼8.0 Hz); 7.47e7.42 (m, 2H); 7.33e7.28 (m,
3H); 7.09 (d, 2H, J¼8.0 Hz); 2.34 (s, 3H) ppm. 13C NMR (100 MHz,
CDCl3): 138.1; 135.7; 131.8; 130.6; 128.5; 128.2; 123.5; 113.7; 108.6;
47.5; 21.1 ppm.
4.3.1. Phenyl(phenylethynyl)selane9 (3a). 1H NMR (200 MHz,
CDCl3): 7.65e7.55 (m, 2H); 7.45e7.54 (m, 2H); 7.32e7.17 (m, 6H)
ppm. 13C NMR (100 MHz, CDCl3): 131.7; 129.5; 128.9; 128.3; 127.1;
123.1; 102.9; 69.2 ppm.
4.3.13. Butyl(phenylethynyl)tellane12b (3m). 1H NMR (200 MHz,
CDCl3): 7.44e7.36 (m, 2H); 7.34e7.25 (m, 3H); 2.90 (t, 2H, J¼7.5 Hz);
1.99e1.85 (m, 2H); 1.55e1.37 (m, 2H); 0.96 (t, 3H, J¼7.3 Hz) ppm.
13C NMR (50 MHz, CDCl3): 131.7; 128.1; 123.7; 111.3; 44.4; 33.6;
24.7; 13.4; 9.94 ppm.
4.3.2. (4-Methoxyphenyl)(phenylethynyl)selane12b (3b). 1H NMR
(200 MHz, CDCl3): 7.53 (d, 2H, J¼8.9 Hz); 7.50e7.41 (m, 2H);
7.34e7.27 (m, 3H); 6.88 (d, 2H, J¼8.9 Hz); 3.79 (s, 3H) ppm. 13C NMR
(100 MHz, CDCl3): 159.4; 131.8; 131.7; 128.4; 128.3; 123.3; 118.3;
115.3; 101.5; 70.4; 55.4 ppm.
4.3.14. Phenyl(p-tolylethynyl)tellane10e (3n). 1H NMR (400 MHz,
CDCl3): 7.66 (d, 2H, J¼8.0 Hz); 7.47e7.42 (m, 2H); 7.33e7.28 (m,
3H); 7.09 (d, 2H, J¼8.0 Hz); 2.34 (s, 3H) ppm. 13C NMR (100 MHz,
CDCl3): 138.1; 135.7; 131.8; 130.6; 128.5; 128.2; 123.5; 113.7; 108.6;
47.5; 21.1 ppm.
4.3.3. (4-Chlorophenyl)(phenylethynyl)selane9
(3c). 1H
NMR
(200 MHz, CDCl3): 7.85e7.76 (m, 1H); 7.55e7.47 (m, 2H); 7.37e7.30
(m, 3H); 7.23e7.15 (m, 3H); 2.37 (s, 3H) ppm. 13C NMR (50 MHz,
CDCl3): 136.5; 131.7; 130.22; 129.5; 129.3; 128.5; 128.3; 127.1;
123.2; 103.0; 69.1; 20.9 ppm.
4.3.15. ((4-Methoxyphenyl)ethynyl)(phenyl)tellane12b (3o). 1H NMR
(400 MHz, CDCl3): 7.76e7.73 (m, 2H); 7.44 (d, 2H, J¼8.5 Hz);
7.29e7.26 (m, 3H); 6.86 (d, 2H, J¼8.5 Hz); 3.82 (s, 3H) ppm. 13C NMR
(100 MHz, CDCl3): 159.9; 134.9; 133.7; 129.7; 127.8; 115.5; 114.2;
113.8; 113.4; 55.3; 45.1 ppm.
4.3.4. (Phenylethynyl)(o-tolyl)selane4c (3d). 1H NMR (200 MHz,
CDCl3): 7.65e7.55 (m, 2H); 7.45e7.54 (m, 2H); 7.32e7.17 (m, 6H)
ppm. 13C NMR (100 MHz, CDCl3): 131.7; 129.5; 128.9; 128.3; 127.1;
123.1; 102.9; 69.2 ppm.
4.3.16. ((2-Methoxyphenyl)ethynyl)(phenyl)tellane7b (3p). 1H NMR
(400 MHz, CDCl3): 7.77e7.74 (m, 2H); 7.42 (dd, 1H, J¼7.6 and
1.7 Hz); 7.29e7.24 (m, 4H); 6.93e6.87 (m, 2H); 3.88 (s, 3H) ppm. 13
C
4.3.5. Butyl(phenylethynyl)selane11c (3e). 1H NMR (200 MHz,
CDCl3): 7.44e7.38 (m, 2H); 7.33e7.27 (m, 3H); 2.88 (t, 2H, J¼6 Hz);
1.93e1.78 (m, 2H); 1.57e1.43 (m, 2H); 0.96 (t, 3H, J¼14.67 Hz) ppm.
13C NMR (100 MHz, CDCl3): 131.4; 128.2; 128.0; 123.7; 99.3; 70.5;
32.2; 29.3; 22.5; 13.5 ppm.
NMR (100 MHz, CDCl3): 160.3; 134.6; 133.7; 129.9; 129.6; 127.6;
120.3; 113.7; 112.5; 110.7; 55.7; 51.1 ppm.
4.3.17. But-1-ynyl(phenyl)tellane10e (3q). 1H NMR (400 MHz,
CDCl3): 7.70e7.64 (m, 2H); 7.28e7.19 (m, 3H); 2.57 (t, 2H, J¼8 Hz);
1.60e1.52 (m, 2H); 1.45e1.38 (m, 2H); 1.32e1.25 (m, 4H); 0.89 (t,
3H, J¼8 Hz) ppm. 13C NMR (100 MHz, CDCl3): 137.9; 134.7; 129.6;
127.6; 116.2; 34.7; 31.3; 28.9; 28.5; 22.6; 21.1; 14.1 ppm.
4.3.6. Phenyl(p-tolylethynyl)selane11b (3f). 1H NMR (200 MHz,
CDCl3): 7.64e7.58 (m, 2H); 7.48e7.28 (m, 5H); 7.21e7.14 (m, 2H);
2.40 (s, 3H) ppm. 13C NMR (50 MHz, CDCl3): 138.9; 132.4; 131.8;
129.6; 129.2; 128.9; 127.0; 120.1; 103.2; 68.2; 21.6 ppm.
Acknowledgements
4.3.7. ((4-Methoxyphenyl)ethynyl)(phenyl)selane12b (3g). 1H NMR
(200 MHz, CDCl3): 7.65e7.59 (m, 2H); 7.44 (dd, 1H, J¼7.7 and
We are grateful to CNPq, INCT-Catalise, CAPES, and FAPESC for
ꢀ
financial support. CAPES is acknowledged for the post-doctorate

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M. Godoi et al. / Tetrahedron 70 (2014) 3349e3354
10. (a) Braga, A. L.; Silveira, C.; Reckziegel, A.; Menezes, P. Tetrahedron Lett. 1993,
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