A convenient preparation of 4-methyl- and 4-phenylseleno-1,1,1-trihalo-3-alken-2-ones and their usefulness in the synthesis of 3-trihalomethylisoselenazoles

Article abstract of DOI:10.1055/s-2002-34854

A convenient synthesis of fourteen 4-methyl- and 4-phenylseleno-1,1,1-trihalo-3-alken-2-ones [CX₃C(O)CH=CR¹SeR, where X = F, Cl; R = Me, Ph; and R¹ = H, alkyl, aryl] from the reaction of the corresponding 4-methoxy-1,1,1-trihalo-3-alken-2-ones with methyl- or phenylselenols in the presence of boron trifluoride etherate is reported. The reaction of 4-methylseleno-1,1,1-trihalo-3-alken-2-ones with bromine and ammonia lead to 3-trihalomethylisoselenazoles in good yields. The usefulness of the trichloromethyl group as a carboxyl group precursor was demonstrated by the conversion of 5-ethyl-3-trichloromethylisoselenazole to 5-ethyl-3-carboxyisoselenazole acid.

Full text of DOI:10.1055/s-2002-34854

2220
PAPER
A Convenient Preparation of 4-Methyl- and 4-Phenylseleno-1,1,1-trihalo-3-
alken-2-ones and their Usefulness in the Synthesis of 3-Trihalomethyl-
isoselenazoles
S
elenoalkenones and th
a
e
Synthesi
r
s
of 3-T
r
c
ihalomethy
o
lisoselenazo
s
les A. P. Martins,* Giovani P. Bastos, Adilson P. Sinhorin, Nilo E. K. Zimmermann, Helio G. Bonacorso,
Nilo Zanatta
Núcleo de Química de Heterociclos (NUQUIMHE), Departamento de Química, Universidade Federal de Santa Maria, 97.105-900 Santa
Maria, RS, Brazil
Fax +55(55)2208031; E-mail: mmartins@base.ufsm.br
Received 28 February 2002; revised 27 May 2002
group under mild conditions14 into carboxylic groups
prompted us to devote special attention to these sub-
strates. The aim of this work is to report the synthesis of
4-methyl- and 4-phenylseleno-1,1,1-trihalo-3-alken-2-
Abstract: A convenient synthesis of fourteen 4-methyl- and 4-phe-
nylseleno-1,1,1-trihalo-3-alken-2-ones
[CX3C(O)CH=CR1SeR,
where X = F, Cl; R = Me, Ph; and R1 = H, alkyl, aryl] from the re-
action of the corresponding 4-methoxy-1,1,1-trihalo-3-alken-2-
ones with methyl- or phenylselenols in the presence of boron tri- ones 2a–n from the reaction of the corresponding 4-meth-
fluoride etherate is reported. The reaction of 4-methylseleno-1,1,1-
trihalo-3-alken-2-ones with bromine and ammonia lead to 3-triha-
oxy-1,1,1-trihalo-3-alken-2-ones 1a–n with selenium nu-
cleophiles in the presence of boron trifluoride etherate
lomethylisoselenazoles in good yields. The usefulness of the
(Scheme 1).
trichloromethyl group as a carboxyl group precursor was demon-
The advantage of our methodology if compared with the
literature methods is the facile access to several substrate
structures and the preparation of trihalomethyl containing
compounds. The reaction of 4-methylseleno-1,1,1-triha-
lo-3-alken-2-ones 2g,i,k–n with bromine and ammonia
lead to 3-trihalomethylisoselenazoles 3g,i,k–n as present-
ed in Scheme 1.
strated by the conversion of 5-ethyl-3-trichloromethylisoselenazole
to 5-ethyl-3-carboxyisoselenazole acid.
Key words: isoselenazoles, selenium, enones, halogens, heterocy-
cles
Organoselenium compounds are useful intermediates in
organic synthesis.1–4 The reactivity of the organoselenium
compounds is mainly associated with the ability of seleni-
um to stabilize adjacent positive and negative charges, as
well as with the ease of selenides to undergo oxidation to
selenoxides with subsequent facile selenoxide syn-elimi-
nation. The development of general and efficient methods
for large-scale preparation of important substrates such as
alkyl- and arylselenovinyl ketones (or aldehydes) has
been the subject of increasing interest. These intermedi-
ates have been used as potential precursors for a variety of
substituted five-membered heterocyclic compounds, e.g.,
isoselenazoles and 1,2-benzoselenazoles.5 The synthesis
of -methylselenovinyl ketones from the substitution of
-halovinyl ketones with methylselenol, has been report-
ed in the literature.5 Homoallyl alcohol has been trans-
formed to the corresponding methyl- or phenylselenyl
compounds from the reaction of the corresponding selenol
in the presence of boron trifluoride etherate.6 -Phenylse-
leno , -unsaturated cycloalkanones have also been ob-
tained from the reaction of -halocycloalkenones with
nucleophilic selenium reagents.7–9 As a part of our
research program, we developed a general synthesis of
4-methoxy-1,1,1-trihalo-3-alken-2-ones10,11 and their
usefulness in heterocyclic preparations has been
shown.10,12–14 The transformation of the trichloromethyl
The 4-methoxy-1,1,1-trihalo-3-alken-2-ones 1a–n were
synthesized from the reaction of the proper enol ether or
acetal with trifluoroacetic anhydride or trichloroacetyl
chloride.10,11
The reactions of compounds 1 with methyl- and phenylse-
lenol were carried out at room temperature using equimo-
lar amount of the reagents and boron trifluoride etherate in
dichloromethane. However, in the reactions of com-
pounds 1c,i–m 1.5 equivalents of boron trifluoride ether-
ate were necessary. The reaction of compounds 1b,c,f–
h,j–n with methyl- and phenylselenols led regiospecifi-
cally to the products 2 with Z-configuration and the reac-
tion of compounds 1a,d,e,i with selenols furnished a
mixture of compounds 2Z and 2E in a molar ratio of ap-
proximately 3: 1, respectively (see Table 1). In order to
show the usefulness of compounds 2 to obtain isoselena-
zoles 3, we performed cyclo-condensation of 2g,i,k–n ac-
cording with methodology developed by Weber et. al.,5
where compounds 2 react with bromine in dichlo-
romethane at –70 °C and, in a second step, at the same
temperature, an excess of ammonia is bubbled in the mix-
ture (Scheme 1).
Finally, considering the importance of isoselenazolecar-
boxylic acids15 and according with methodology devel-
oped in our laboratory14 to obtain carboxyalkylisoxazoles
and carboxyalkylpyrazoles from trichloromethylazoles,
we show the synthesis of compound 4k from the com-
pound 3k in good yield (Scheme 2). The reaction was car-
ried out in 96% sulfuric acid (acid–selenoisoxazole; 3:1)
in water at room temperature for 24 hours.
Synthesis 2002, No. 15, Print: 29 10 2002.
Art Id.1437-210X,E;2002,0,15,2220,2224,ftx,en;M00402SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

PAPER
Selenoalkenones and the Synthesis of 3-Trihalomethylisoselenazoles
2221
Scheme 1
Perkin–Elmer 599-B. Satisfactory elemental analyses (C 0.30, H
0.20) were obtained for all compounds.
4-Methyl- and 4-Phenylseleno-1,1,1-trihalo-3-alken-2-ones
2a–n; General Procedure
To a stirred solution of 4-methoxy-1,1,1-trihalomethyl-4-methoxy-
3-alken-2-ones 1a–n (10 mmol), methyl- or phenylselenol (10
mmol) in CH2Cl2 (10 mL), at 20 °C and under nitrogen atmosphere,
BF3 OEt2 (10 mmol, 15 mmol for compounds 1c,i,j–m) was added
dropwise. The mixture was stirred for 1.5 h at r.t., then the mixture
was washed with aq Na2CO3 (5%; 2 10 mL) and H2O (2 10 mL).
The organic phase was dried (Na2SO4) and the solvent was re-
moved. The products 2a,b,d were recrystallized from hexane–
EtOAc and the products 2c,e–n were purified by column chroma-
tography (silica gel 70–230 mesh; hexane–CHCl3, 1:1). Yields, se-
lected physical and spectral data are presented in Table 1.
Scheme 2
Unless otherwise indicated all common reagents and solvents were
used as obtained from commercial suppliers without further purifi-
cation. All mps were determined on a Reichert Thermovar appara-
tus. Selected physical and spectral data are presented in Tables 1
1
and 2. H and 13C NMR spectra were recorded on a Bruker DPX-
200 (1H at 200.13 MHz and 13C at 50.32 MHz), 298 K, digital reso-
lution 0.01 ppm, 0.5 M in carbon tetrachloride–TMS (with ben-
zene-d6 for lock in a capillary tube). IR spectra were obtained on a
Table 1 Selected Physical and Spectral Data of 2
Prod-
uct
MF/MW
Yielda Mp
IR (film)
(cm–1)
1H NMR
, J (Hz)
13C NMR
, J (Hz)
(%)
(°C)
2a Z
C10H7F3OSe
(279.12)
84
28–32
1770, 1680,
1661
7.14 (d,1 H, J = 8.6, H-3), 8.59 (d, 116.2 (q, 1JCF = 289.3, C-1), 179.3 (q,
1 H, J = 8.6, H-4), 7.39 (m, 5 H,
2JCF = 35.8, C-2), 118.8 (C-3),163.6 (C-
SePh)
4), 127.6, 129.6, 129.9, 135.2 (SePh)
2a E
C10H7F3OSe
(279.12)
6.90 (d, 1 H, J = 15.0, H-3), 8.76 115.9 (q, 1JCF = 279.1, C-1), 176.0 (q,
(d, 1 H, J = 15.0, H-4), 7.61 (m,
2JCF = 35.1, C-2), 113.2 (C-3), 156.2 (C-
SePh)
4), 124.6, 129.6, 129.9, 135.2 (SePh)
Synthesis 2002, No. 15, 2220–2224 ISSN 0039-7881 © Thieme Stuttgart · New York

2222
M. A. P. Martins et al.
PAPER
Table 1 Selected Physical and Spectral Data of 2 (continued)
Prod-
uct
MF/MW
Yielda Mp
IR (film)
(cm–1)
1H NMR
, J (Hz)
13C NMR
, J (Hz)
(%)
(°C)
2b
C16H10F4OSe
(373.21)
82
40 – 43 1761, 1617
7.11 (s, 1 H, H-3), (m, 4 H, Ph),
7.24 (m, 5 H, SePh)
127.1 (1JCF = 282.6, C-1), 176.6
(2JCF = 35.2, C-2), 113.6 (C-3), 173.2 (C-
4), 126.7, 127.9, 129.1, 135.0 (SePh)
2c
C11H11Cl3OSe 83
(344.52)
48– 50
Oil
1755, 1663
7.38 (s, 1 H, H-3), 2.12 (s, 3 H,
H-5), 7.20–7.72 (m, 5 H, SePh)
92.9 (C-1), 173.3 (C-2), 113.8 (C-3),
179.5 (C-4), 22.7 (C-5), 127.6, 128.9,
129.3, 137.8 (SePh)
2d Z
2d E
2e Z
2e E
2f
C5H5F3OSe
(217.05)
79
1766, 1671,
1521
7.06 (d, 1 H, J = 9.3, H-3), 8.51 (d, 116.1 (q, 1JCF = 286.6, C-1), 179.1 (q,
1 H, J = 9.3, H-4), 2,41 (s, 3 H,
SeCH3)
2JCF = 35.1, C2), 114.7 (C-3), 162.3 (C-
4), 10.8 (SeCH3)
C5H5F3OSe
(217.05)
6.54 (d, 1 H, J = 15.3, H-3), 8.69 115.6 (q, 1JCF = 280.0, C-1), 177.2 (q,
(d, 1 H, J = 15.3, H-4), 2.41 (s, 3 H JCF = 34.0, C-2), 112.8 (C-3), 160.3 (C-
SeCH3)
2
4), 6.41 (C-5) (SeCH3)
C6H7F3OSe
(231.08)
80
Oil
1688, 1577
6.65 (s, 1 H, H-3), 2.45 (s, 3 H,
H-5), 2.23 (s, 3 H, SeCH3)
115.3 (q, 1JCF = 287.4, C-1), 177.0 (q,
2JCF = 35.5, C-2), 112.4 (C-3), 160.5 (C-
4), 26.2 (C-5), 5.97 (SeCH3)
C6H7F3OSe
(231.08)
6.88 (s, 1 H, H-3), 2.57 (s, 3 H,
H-5), 2.27 (s, 3 H, SeCH3)
118.9 (q, 1JCF = 278.2, C-1), 179.1 (q,
2JCF = 34.7, C-2), 115.7 (C-3), 175.3 (C-
4), 29.4 (C-5), 6.99 (SeCH3)
C11H9F3OSe
(293.14)
88
87
Oil
Oil
1698, 1597,
1471
7.46 (s, 1 H, H-3), 7.52 (m, 5 H,
Ph), 1.97 (s, 3 H, SeCH3)
129.5 (q, 1JCF = 288.4, C-1), 177.8 (q,
2JCF = 36.1, C-2), 117.3 (C-3), 186.4 (C-
4), 128.6, 127.7, 139.8 (Ph), 9.02
(SeCH3)
2g
C11H8F4OSe
(311.14)
1601, 1533,
1501
6.92 (s,1 H, H-3), 7.17 (m, 4 H,
Ph), 1.78 (s, 3 H, SeCH3)
116.8 (q, 1JCF = 289.1, C-1), 177.1 (q,
2JCF = 35.4, C-2), 117.5(C-3), 185.5 (C-
4), 8.82 (SeCH3), 127.5, 128.5, 130.5,
135.6 (Ph)
2h
C11H8F3ClOSe 82
(327.59)
Oil
Oil
1650, 1594,
1481
6.92 (s, 1 H, H-3), 7.23–7.58 (m, 127.1 (q, 1JCF = 289.4, C-1), 178.5 (q,
4 H, Ph), 1.79 (s, 3 H, SeCH3)
2JCF = 34.9, C-2), 117.3 (C-3), 185.1 (C-
4), 127.5, 128.4, 129.7, 137.9 (Ph)
2i Z
2i E
C5H5Cl3OSe
(266.41)
84
1699, 1677,
1518
7.26 (d, 1 H, J = 9.2, H-3), 8.31 (d, 96.4 (C-1), 179.9 (C-2), 115.7 (C-3),
1 H, J = 9.2, H-4), 2.29 (s, 3 H,
160.9 (C-4), 10.5 (SeCH3)
SeCH3)
C5H5Cl3OSe
(266.41)
6.76 (d, 1 H, J = 5.0, H-3), 8.54 (d, 97.5 (C-1), 175.7 (C-2), 116.9 (C-3),
1 H, J = 15.0, H-5), 2.27 (s, 3 H,
158.3 (C-4), 6.93 (SeCH3)
SeCH3)
2j
C6H7Cl3OSe
(280.44)
85
81
Oil
Oil
1697, 1612
1690, 1622
6.76 (s, 1 H, H-3), 1.80 (s, 3 H,
H-5), 1.71 (s, 3 H, SeCH3)
93.4 (C-1), 159.4 (C-2), 114.5 (C-3),
179.6 (C-4), 26.4 (C-5), 6.04 (SeCH3)
2k
C7H9Cl3OSe
(294.46)
7.18 (s, 1 H, H3), 2.72 (q, 2 H,
J = 7, H-5), 2.25 (s, 3 H, SeCH3), 191.5 (C-4), 31.6 (C-5), 14.2 (C-6), 5.43
1.21 (t, 3 H, J = 7, H-6) (SeCH3)
91.4 (C-1), 175.2 (C-2), 112.5 (C-3),
2l
C8H11Cl3OSe
(308.49)
82
79
Oil
Oil
1632, 1512,
1476
7.10 (s, 1 H, H-3), 3.00 (sep, 1 H, 90.3 (C-1), 181.0 (C-2), 110.9 (C-3),
J = 7, H-5), 1.26 (d, 6 H, J = 7,
H-6), 2.15 (s, 3 H, SeCH3)
178.4 (C-4), 34.9 (C-5), 23.6 (C-6), 5.76
(SeCH3)
2m
C9H13Cl3OSe
(322.52)
1682, 1572,
1426
7.12 (s, 1 H, H-3), 2.64 (d, 2 H,
J = 7, H-5), 2.01 (m, 1 H, H-6),
93.3 (C-1), 188.7 (C-2), 115.1 (C-3),
173.4 (C-4), 29.5 (C-5), 46.1 (C-6), 23.0
1.13 (d, 6 H, J = 7, H-7), 2.33 (s, 3 (C-7), 5.83 (SeCH3)
H, SeCH3)
2n
C11H9Cl3OSe
(342.51)
81
71– 75
1702, 1623
7.35 (s, 1 H, H-3), 7.47 (m, 5 H,
Ph), 1.90 (s, 3 H, SeCH3)
89.3 (C-1), 160.5 (C-2), 116.1 (C-3),
179.9 (C-4), 128.5, 127.7, 128.2, 139.0
(Ph), 8.32 (SeCH3)
a Yields are of isolated compounds and refers to a mixture of Z and E isomers.
Synthesis 2002, No. 15, 2220–2224 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Selenoalkenones and the Synthesis of 3-Trihalomethylisoselenazoles
2223
Table 2 Selected Physical and Spectral Data of 3, 4
Prod- MF/MW
uct
Mp
(°C)
Yielda
(%)
CG–MS
m/z (%)
1H NMR ,
J (Hz)
13C NMR ,
J (Hz)
3g
C10H5F4NSe
48– 51
66
295 (M+, 100),
7.63 (s, 1 H, H-4), 7.07 (d, 4 H, 161.3 (q, 2J = 35.7, C3),122.7
294.11
276 (10), 226 (5)
Ph), 7.39 (m, 5 H, SePh)
(C4), 176.2 (C5), 116.3, 116.9,
128.7, 132.5 (Ph), 120.0 (q,
1J = 275.2, CF3)
3i
C4H2Cl3NSe
249.38
40– 43
Oil
72
81
249 (M+, 40), 214 7.33 (d, 1 H, J = 6.8, H-5), 7.90 153.1 (C3), 115.4 (C4), 161.5
(100)
(d, 1 H, J = 6.8, H4)
(C5), 96.9 (CCl3)
3k
C6H6Cl3NSe
277.44
277 (M+, 33), 242 7.61 (s, 1 H, H4), 3.00 (q, 2 H,
167.1 (C3), 121.1 (C4), 183.1
(100), 158 (40)
H6), 1.43 (t, 3 H, H7)
(C5), 25.0 (C6), 15.9 (C7), 91.4
(CCl3)
3l
C7H8Cl3NSe
291.46
Oil
68
74
69
70
291 (M+,10), 256
(100)
7.52 (s, 1 H, H4), 2.19 (sep, 1 H, 168.3 (C3), 120.5 (C4), 187.2
H6), 1.41 (d, 6 H, H7)
(C5), 35.1 (C6), 25.8 (C7), 90.3
(CCl3)
3m
3n
4k
C8H10Cl3NSe Oil
305.49
305 (M+,15), 270
(100), 227 (25),
119 (15)
7.34 (s, 1 H, H4), 2.70 (d, 2 H,
168.2 (C3), 122.3 (C4), 183.7
H6), 2.16 (m, 1 H, H7), 1.03 (d, (C5), 27.5 (C6), 42.8 (C7), 23.1
6 H, H8) (C8), 94.0 (CCl3)
C10H6Cl3NSe 82– 75
325.48
325 (M+, 20), 290 8.00 (s, 1 H, H4), 7.37 (m, 5 H, 153.1 (C3), 120.4 (C4), 176.5
(100), 207 (5), 140 Ph)
(30)
(C5), 127.4, 127.9, 130.5, 139.4
(Ph), 90.0 (CCl3)
C6H7NO2Se
204.08
136–138
205 (M+, 25), 133 7.66 (s, 1 H, H4), 3.10 (q, 2 H,
(22), 80 (100)
173,1 (C3), 119,5 (C4), 160.2
(C5), 32.8 (C6), 19.0 (C7), 173.0
(C=O)
H6), 1.25 (t, 3 H, H7)
a Yields are of isolated compounds.
3-Trihalomethylisoselenazoles 3g,i,k–n; General Procedure
A three-necked flask containing a solution of 4-methylseleno-1,1,1-
trihalo-3-alken-2-ones 2g,i,k–n (5 mmol) in CH2Cl2 (20 mL) was
cooled to –70 °C. Bromine (5 mmol) in CH2Cl2 (10 mL) was added
dropwise to this stirred solution. After a few minutes, an excess of
ammonia was bubbled in the mixture, which was then allowed to
reach r.t.. H2O (50 ml) was added, and the organic phase separated
and dried (MgSO4). Products 3g,i,k–n were obtained after distilling
off the solvent and separation by chromatographic column (silica
gel; cyclohexane–EtOAc). Yields, selected physical and spectral
data are presented in Table 2.
References
(1) Patai, S.; Rappoport, Z. The Chemistry of Organic Selenium
and Tellurium Compounds; Wiley: New York, 1986.
(2) Paulmier, C. Selenium Reagents and Intermediates in
Organic Synthesis; Pergamon: Oxford, 1986.
(3) Liotta, D. Organoselenium Chemistry; Wiley: New York,
1987.
(4) Krief, A.; Hevesi, L. Organoselenium Chemistry; Springer:
Berlin, 1988.
(5) Weber, R.; Renson, M. J. Heterocycl. Chem. 1973, 10, 267.
(6) Krief, A.; Hobe, M.; Badaoui, E.; Bousbaa, J.; Dumont, W.;
Nazih, A. Synlett 1993, 707.
5-Ethyl-3-carboxyisoselenazole Acid (4k)
To a stirred solution of 5-ethyl-3-trichloromethylisoselenazole (3k)
(5 mmol), a solution of H2SO4 (96%; 1.47 g, 15 mmol) and H2O (0.8
mL) (1:1 v/v) at r.t. was added. The mixture was refluxed for 24 h
and then extracted with CHCl3. The organic phase was then washed
with H2O (3 10 mL) and aq Na2CO3 (5%; 1 10 mL). The organic
phase was dried (Na2SO4) and the solvent was removed on a rotary
evaporator. After evaporation of the solvent, the product 4k was re-
crystallized from cyclohexane. Yields, selected physical and spec-
tral data are presented in Table 2.
(7) Shimizu, M.; Ando, R.; Kuwajima, I. J. Org. Chem. 1984,
49, 1230.
(8) Hamaguchi, M.; Nagai, T. J. Org. Chem. 1989, 54, 3957.
(9) Sakakibara, M.; Katsumita, K.; Watanabe, Y.; Toru, T.;
Ueno, Y. Synthesis 1992, 377.
(10) Colla, A.; Martins, M. A. P.; Clar, G.; Krimmer, S.; Fischer,
P. Synthesis 1991, 6, 483.
(11) (a) Martins, M. A. P.; Bastos, G. P.; Bonacorso, H. G.;
Zanatta, N.; Flores, A. F. C.; Siqueira, G. M. Tetrahedron
Lett. 1999, 40, 4309. (b) Bonacorso, H. G.; Martins, M. A.
P.; Bittencourt, S. R. T.; Lourega, R. V.; Zanatta, N.; Flores,
A. F. C. J. Fluorine Chem. 1999, 99, 177. (c) Martins, M.
A. P.; Siqueira, G. M.; Flores, A. F. C.; Clar, G.; Zanatta, N.
Química Nova 1994, 17, 24.
(12) (a) Martins, M. A. P.; Zoch, A. N.; Bonacorso, H. G.;
Zanatta, N.; Clar, G. J. Heterocycl. Chem. 1995, 32, 739.
(b) Martins, M. A. P.; Flores, A. F. C.; Freitag, R.; Zanatta,
N. J. Heterocycl. Chem. 1995, 32, 731. (c) Martins, M. A.
P.; Flores, A. F. C.; Freitag, R.; Zanatta, N. J. Heterocycl.
Chem. 1996, 33, 1223. (d) Martins, M. A. P.; Siqueira, G.
Acknowledgments
The authors thank the Conselho Nacional de Desenvolvimento
Científico e Tecnológico (CNPq/PADCT III, contract No. 62.0228/
97-0; and CNPq Contract No. 46.0804/00-6), Fundação de Amparo
à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) for finan-
cial support and fellowships.
Synthesis 2002, No. 15, 2220–2224 ISSN 0039-7881 © Thieme Stuttgart · New York

2224
M. A. P. Martins et al.
PAPER
(14) (a) Martins, M. A. P.; Flores, A. F. C.; Bastos, G. P.;
M.; Bastos, G. P.; Zanatta, N. J. Heterocycl. Chem. 1996, 33,
1619. (e) Braibante, M. E. F.; Martins, M. A. P.; Clar, G. J.
Heterocycl. Chem. 1993, 30, 1159.
Sinhorin, A.; Bonacorso, H. G.; Zanatta, N. Tetrahedron
Lett. 2000, 41, 293. (b) Martins, M. A. P.; Freitag, R. A.;
Rosa, A.; Flores, A. F. C.; Zanatta, N.; Bonacorso, H. G. J.
Heterocycl. Chem. 1995, 32, 731. (c) Martins, M. A. P.;
Freitag, R. A.; Flores, A. F. C.; Zanatta, N. Synthesis 1995,
1491.
(13) (a) Zanatta, N.; Pacholski, I. L.; Blanco, I.; Martins, M. A. P.
J. Braz. Chem. Soc. 1991, 1, 118. (b) Zanatta, N.; Madruga,
C. C.; Clerici, E.; Martins, M. A. P. J. Heterocycl. Chem.
1995, 32, 735. (c) Zanatta, N.; Cortelini, M. F. M.; Carpes,
M. J. S.; Bonacorso, H. G.; Martins, M. A. P. J. Heterocycl.
Chem. 1997, 34, 509. (d) Bonacorso, H. G.; Bittencourt, S.
T.; Wastowski, A. D.; Wentz, A. P.; Zanatta, N.; Martins, M.
A. P. Tetrahedron Lett. 1996, 37, 9155. (e) Zanatta, N.;
Fagundes, M. B.; Ellenshon, R.; Marques, M.; Bonacorso,
H. G.; Martins, M. A. P. J. Heterocycl. Chem. 1998, 35, 451.
(15) Lucchesini, F.; Bertini, V.; De Munno, A. Heterocycles
1985, 23, 127.
Synthesis 2002, No. 15, 2220–2224 ISSN 0039-7881 © Thieme Stuttgart · New York