Isoxazoles as Latent Siloxybutadienes: An Easy Entry to Polyfunctionalized Benzene Systems via Diels-Alder Reaction with Acetylenes

Article abstract of DOI:10.1055/s-2004-815929

Base-induced or reductive cleavage of isoxazole rings followed by silylation of the resulting open-chain products affords bis(siloxy)butadienes in high yields. Their synthetic usefulness in the construction of multifunctionalized aromatic systems, via Diels-Alder reactions, is shown. The ability of bis(siloxy)butadienes to undergo deprotonation and reaction with electrophiles is also studied.

Full text of DOI:10.1055/s-2004-815929

PAPER
401
Isoxazoles as Latent Siloxybutadienes: An Easy Entry to Polyfunctionalized
Benzene Systems via Diels–Alder Reaction with Acetylenes
B
is(siloxy)butadie
s
nes from
I
u
soxazoles nción Barbero, Francisco J. Pulido*
Departamento de Química Orgánica, Universidad de Valladolid, Valladolid 47005, Spain
Fax +34(983)423210; E-mail: pulido@qo.uva.es
Received 22 November 2003; revised 8 December 2003
several hours, followed by slow addition of an excess of
Abstract: Base-induced or reductive cleavage of isoxazole rings
followed by silylation of the resulting open-chain products affords
bis(siloxy)butadienes in high yields. Their synthetic usefulness in
the construction of multifunctionalized aromatic systems, via
triethylamine/trimethylsilyl triflate at 0 °C and warming
to room temperature gives the bis(siloxy)butadienes 2b,c
(Scheme 1). Compounds 2a–c are obtained as mixtures of
Diels–Alder reactions, is shown. The ability of bis(siloxy)buta- Z/E-isomers, which were used directly without separa-
dienes to undergo deprotonation and reaction with electrophiles is
also studied.
tion. As shown below, this is not a limitation for the re-
ported procedure since both stereoisomers undergo Diels–
Key words: isoxazoles, siloxybutadienes, silyl dienol ethers,
Diels–Alder cycloaddition -diketones
Alder cycloaddition with acetylenes leading to the same
product.
R3
R
R2
Some time ago we reported a general method for the syn-
thesis of -cyano silyl enol ethers from 3-unsubstituted
isoxazoles.1 They are masked -enaminoketones2 of wide
application as intermediates in the synthesis of nitrogen-
containing natural products.3 We also noted their ability to
undergo inverse electron demand hetero-Diels–Alder re-
actions when treated with heterodienes (enones, azabuta-
dienes),1,4 thus providing new pathways for heterocyclic
synthesis. Isoxazoles5 bearing no substituents at C-3 are
readily attacked by bases leading to (Z)- -cyanoenolates,
which are trapped by chlorosilanes giving -cyano silyl
enol ethers in high yield.1–3
LDA/THF/TMSCl/–78 °C or
Li/THF/TMSOTf/0 °C
H3C
N
O
R1
TMSO
OTMS
1a: R1 = Me, R2 = Ac, R3 = H
1b: R1 = R3 = Me, R2 = H
1c: R1 = R2 = R3 = Me
2a: R = CN (93%)
2b: R = H (91%)
2c: R = Me (87%)
Scheme 1
Siloxybutadienes 2a–c react with methyl propiolate and
diethyl acetylenedicarboxylate giving the phenols 3a–c
and 4a–c in good yields (Scheme 2). The reaction takes
place in a closed flask, without solvent, at temperatures
between 120–140 °C. After stirring the mixture for three
hours, the crude is poured into saturated ammonium chlo-
ride and extracted with diethyl ether. Apparently, the in-
termediate Diels–Alder adduct undergoes elimination of
silanol leading to the aromatic nucleus (Scheme 3). NMR
analysis of the crude showed that the benzene ring was
present before the final hydrolysis. Protodesilylation of
the resulting silyl phenol ether affords 3 and 4
(Scheme 3).
In the past years, our effort has been directed to the devel-
opment of new synthetic methodologies using silicon
compounds.6 As part of this work, we now report an effi-
cient synthesis of bis(siloxy)butadienes from isoxazole
precursors and their usefulness in the construction of
polyfunctionalized benzene systems. Siloxybutadienes7
are masked -diketones and useful intermediates in
Diels–Alder reactions.8
Treatment of 4-acetyl-5-methylisoxazole (1a) with LDA
at –78 °C followed by reaction with an excess of TMSCl
and gentle warming to room temperature, in the presence
of a catalytic amount of zinc chloride, gives the cyano-
bis(siloxy)butadiene 2a. The cleavage of the ring by at-
tack of LDA to H-3 and the capture of the resulting inter-
mediate enolate with TMSCl are accompanied with
further silyl enol ether formation from the remaining 4-
acetyl group, leading to the silyl dienol ether 2a as the
temperature increases (Scheme 1). On the other hand,
metal-acid reductive cleavage of isoxazoles 1b,c with lith-
ium granulated in wet THF (1%) at room temperature for
The high regioselectivity of the cycloaddition is to be not-
ed. Reactions with methyl propiolate lead to 3a–c as the
only products; we were not able to detect other isomeric
compounds. Thus, the procedure provides a facile and re-
gioselective entry to polyfunctionalized benzene rings.
The synthetic versatility of siloxybutadienes of type 2 is
also shown in their ability to generate carbanionic species,
which are synthetically equivalent to -diketone anions,
by deprotonation of the 5-methyl group (Figure 1). Allylic
deprotonation in electron rich systems as the silyl dienol
ethers 2a–c is known to proceed with difficulty and little
work has been reported in this area.9 We wondered if this
possibility could be taken into account. Effectively, meta-
lation of 2a,c with BuLi in THF at low temperature for
half an hour, followed by reaction with simple electro-
SYNTHESIS 2004, No. 3, pp 0401–0404
x
x
.
x
x
.2
0
0
4
Advanced online publication: 26.01.2004
DOI: 10.1055/s-2004-815929; Art ID: Z16503SS
© Georg Thieme Verlag Stuttgart · New York

402
A. Barbero, F. J. Pulido
PAPER
R
Me
R
R
1. BuLi/THF/–78 °C
H3C
R
CO2Me
H3C
CO2Me or
E
2. Br2 or MeI or EtBr, –78 °C
3. H3O+
EtO2C
CO2Et
TMSO
OTMS
TMSO
OTMS
O
O
HO
120–140 °C
2a: R = CN
2b: R = H
2c: R = Me
3a (75%)
3b (83%)
3c (87%)
5a: R = CN, E = Br (77%)
6a: R = CN, E = Me (75%)
7a: R = CN, E = Et (81%)
5c: R = Me, E = Br (70%)
6c: R = Me, E = Me (76%)
2a: R = CN
2c: R = Me
or
Me
R
CO2Et
CO2Et
Scheme 4
HO
4a (91%)
4b (93%)
4c (95%)
All reactions involving silyl dienol ethers and air or moisture sensi-
tive reagents were carried out under dry N2. Reagents and anhyd
solvents were handled by using standard syringe techniques. Flash
column chromatography was carried out on Merck silica gel 60
(230–400) and TLC analysis using silica gel 60 F-254 thin layer
plates (Merck). Diels–Alder reactions were performed in a thermo-
static bath, in the absence of solvent, using a round-bottomed flask
fitted with a plastic screw cap having an inner teflon seal. CDCl3
was used as solvent for NMR spectra. Melting points are uncorrect-
ed. Isoxazoles 1a–c were prepared according to literature proce-
dures.10,11
Scheme 2
Me
H
OTMS
CO2Me
R
2
+
–Me3SiOH
CO2Me
TMSO
H
Me
3-Cyano-2,4-bis(trimethylsiloxy)penta-1,3-diene (2a)
R
CO2Me
H3O+
To a cooled solution (–78 °C) of LDA (20 mmol) in THF (30 mL)
was slowly added the isoxazole 1a (2.5 g, 20 mmol) under vigorous
stirring. The resulting cloudy solution was stirred for 1 h at this tem-
perature, then TMSCl (60 mmol) was added at –78 °C and the mix-
ture was allowed to warm to r.t. (45 min). When the mixture had
reached the r.t., anhyd ZnCl2 (0.27 g, 2 mmol) was added and the
mixture was stirred overnight. Separation of the lithium salts using
a sintered glass filter funnel under N2 followed by rotoevaporation
of the solvent and vacuum distillation (110–115 °C /2.5 mbar) gave
2a as a yellowish oil (5 g, 93%). 1H NMR of the distillate showed
that 2a was obtained as an equimolar mixture of E/Z-isomers.
3
TMSO
Scheme 3
philes (bromine, methyl iodide or ethyl bromide) and final
workup of the mixture gives the -diketones 5, 6, and 7
(Scheme 4). IR and NMR spectra of 5–7 show that they
are predominantly in enolic form. Since alkylation of pen-
ta-1,3-dienones under basic conditions occurs, in general,
at the C-3 ‘meso’ position, this procedure introduces a
new aspect of the reactivity pattern of penta-1,3-dienones,
thus providing an easy method for the regioselective alky-
lation of the less common C-1 position, via silyl dienol
ethers.
IR (neat): 2220, 1622 cm–1.
1H NMR: = 0.23 (s, 18 H, E or Z), 0.34 (s, 18 H, E or Z), 2.13 (s,
3 H, E or Z), 2.27 (s, 3 H, E or Z), 4.35–4.58 (4 d, J = 1.8, 2.2 Hz, 4
H, E and Z).
13C NMR: = 0.13, 1.02, 21.31, 22.54, 96.07 (br), 98.87 (br),
118.20, 118.93, 148.35, 149.91, 165.41, 166.32. The duplication of
some signals was a consequence of the presence of a mixture of E-
and Z-isomers.
R
MS: m/z (%) = 269 (M+, 3), 180 (60), 165 (45), 137 (100), 109 (53).
R
H3C
Silyl Dienol Ethers 2b,c; General Procedure
O
O
The corresponding isoxazole 1b or 1c (20 mmol) was dissolved in
wet THF (50 mL, 1% H2O) and excess of granulated Li (1 g) was
added at 0 °C. The suspension was stirred at r.t. for 4 h, then the re-
maining Li was removed from the solution. After cooling again to 0
°C, Et3N (40 mmol, 4 g) was added followed by slow addition (15
min) of trimethylsilyl triflate (50 mmol, 11.1 g) dissolved in THF
(20 mL). The mixture was stirred at 0 °C for 15 min, then the bath
was removed and the stirring was continued at r.t. for 1 h. After the
reaction was complete, most of the THF was rotoevaporated
(around 50 mL) and anhyd Et2O was added (50 mL). On standing
two phases appeared. The upper ethereal layer was separated from
the dense phase containing the triflate salts. Rotoevaporation of the
upper layer and vacuum distillation of the residue gave 2b (4.4 g,
91%); bp 85–90 °C/10 mbar (Lit.12 bp 93 °C/11 mbar) and 2c (4.5
g, 87%). NMR spectra of 2b,c show mixtures of E/Z-isomers.
TMSO
OTMS
Figure 1 Siloxybutadienes as synthetic equivalents of -diketone
anions
In conclusion, isoxazoles are efficient precursors of bis(si-
loxy)butadienes by ring cleavage and subsequent silyla-
tion. The latter are versatile intermediates of wide
synthetic scope. They behave as highly reactive dienes in
Diels–Alder reactions that allows the construction of mul-
tifunctionalized aromatic rings. Furthermore, they are
masked -diketones, which upon deprotonation and reac-
tion with electrophiles can be alkylated at C-1, thus en-
larging their synthetic applicability.
Synthesis 2004, No. 3, 401–404 © Thieme Stuttgart · New York

PAPER
Bis(siloxy)butadienes from Isoxazoles
403
3-Methyl-2,4-bis(trimethylsiloxy)penta-1,3-diene (2c)
Colorless oil; yield: 87%; bp 95–100 °C/5 mbar.
13C NMR: = 13.86, 14.01, 17.79, 60.91, 61.35, 104.22, 114.46,
114.68, 125.29, 133.08, 140.61, 160.35, 165.17, 167.70.
IR (neat): 1622 cm–1.
MS: m/z (%) = 277 (M+, 9), 231 (32), 204 (100), 175 (11), 159 (23).
1H NMR: = 0.25 (s, 18 H), 1.62 (br s, 3 H,), 2.07 (br s, 3 H), 4.13
(s, 1 H), 4.27 (br s, 1 H).
Anal. Calcd for C14H15O5N: C, 60.65; H, 5.45. Found: C, 60.9; H,
5.6.
13C NMR: = 0.19, 0.95, 14.38, 15.87, 20.11, 22.16, 92.60, 113.17,
115.01, 145.21, 158.39, 160.03. Duplication of some signals results
from the presence of mixtures of E/Z-isomers.
Diethyl 5-Hydroxy-3-methylphthalate (4b)
Viscous colorless oil; yield: 93%. On standing it solidified after sev-
eral days (low melting solid).
IR (neat): 3450, 3120, 1718, 1603, 1465 cm–1.
MS: m/z (%) = 258 (M+, 13), 243 (100), 229 (33), 147 (90), 73 (85).
Diels–Alder Reactions of Silyl Dienol Ethers 2a–c; General Pro-
cedure
1H NMR: = 1.31 (t, J = 7.4 Hz, 3 H), 1.34 (t, J = 7.4 Hz, 3 H), 2.40
(s, 3 H), 4.31 (q, J = 7.4 Hz, 2 H), 4.42 (q, J = 7.4 Hz, 2 H), 6.81 (br
s, 1 H), 6.94 (br s, 1 H), 7.58 (very br s, 1 H, OH).
13C NMR: = 13.88, 13.99, 21.16, 61.43, 61.82, 114.37, 121.50,
131.19, 136.48, 146.79, 161.32, 166.53, 169.61.
MS: m/z (%) = 252 (M+, 23), 206 (35), 178 (44), 162 (33), 134
(100), 106 (21).
In a sealed flask, a mixture of 2 (5 mmol) and methyl propiolate or
diethyl acetylenedicarboxylate (5 mmol) was heated at 120 °C or
140 °C, respectively for 4 h. The crude mixture was poured into sat.
aq NH4Cl solution (30 mL), slightly acidified with HCl, and stirred
for a 15 min. After extraction with Et2O, drying and removal of the
solvent, the residue was chromatographed (CH2Cl2) to give 3a–c
and 4a–c, respectively.
Diethyl 5-Hydroxy-3,4-dimethylphthalate (4c)
Colorless solid; yield: 95%; mp 40–42 °C (EtOH–hexane).
Methyl 3-Cyano-4-hydroxy-2-methylbenzoate (3a)
Yellowish solid; yield: 75%; mp 208–210 °C (EtOH).
IR (Nujol): 3100–3410, 1715, 1585, 1464 cm& ndash;1
.
IR (Nujol): 3450–3150, 2227, 1698, 1580, 1462 cm–1.
1H NMR: = 2.65 (s, 3 H), 3.4 (very br s, 1 H, OH), 3.92 (s, 3 H),
1H NMR: = 1.41 (br t, J = 7.3 Hz, 6 H), 2.23 (s, 3 H), 2.45 (s, 3
H), 4.26 (q, J = 7.3
6.86 (d, J = 8.8 Hz, 1 H), 7.88 (d, J = 8.8 Hz, 1 H).
Hz, 2 H), 4.40 (q, J = 7.3 Hz, 2 H), 6.81 (br s, 1 H), 6.87 (s, 1 H).
13C NMR: = 18.11, 50.23, 101.31, 112.50, 114.89, 120.17,
135.42, 144.08, 162.73, 165.91.
MS: m/z (%) = 191 (M+, 35), 160 (100), 132 (19), 104 (25).
The OH signal is missing.
13C NMR: = 12.02, 13.90, 14.12, 20.27, 61.34, 61.86, 120.05,
127.82, 128.73, 132.51, 144.33, 159.24, 169.71, 169.84.
MS: m/z (%) = 266 (M+, 25), 220 (52), 192 (100), 174 (31), 148
(80), 120 (21).
Anal. Calcd for C10H9O3N: C, 62.82; H, 4.74. Found: C, 63.1; H,
4.9.
Anal. Calcd for C14H18O5: C, 63.15; H, 6.81. Found: C, 63.3; H, 6.9.
Methyl 4-Hydroxy-2-methylbenzoate (3b)
Colorless solid; yield: 83%; mp 102–104 °C (EtOH).
IR (Nujol): 3505–3420, 1717, 1602, 1587, 1500, 850 cm–1.
1H NMR: = 2.32 (s, 3 H), 3.52 (s, 3 H), 6.42 (s, 1 H), 6.51 (d,
J = 9.0 Hz, 1 H), 7.58 (d, J = 9.0 Hz, 1 H). The OH signal is miss-
ing.
13C NMR: = 21.31, 51.67, 112.88, 116.34, 119.92, 131.12,
141.17, 159.33, 165.91.
Reaction of 2 with Electrophiles; General Procedure
To a stirred solution of the silyl dienol ether 2a,c (5 mmol) in THF
(20 mL), cooled at –78 °C, was added BuLi (5.5 mmol in hexane)
added dropwise. After stirring for 30 min at that temperature, Br2 or
MeIor EtBr (6 mmol) was added and the resulting mixture was left
to warm up from –78 °C to r.t. Then, aq HCl (pH 4, 30 mL) was add-
ed and the mixture was extracted with Et2O (3 × 30 mL). The ex-
tracts were dried and the solvent was removed under reduced
pressure. The products 5a,c, 6a,c and 7a were purified by column
chromatography using CH2Cl2 as eluent.
MS: m/z (%) = 166 (M+, 63), 134 (100), 106 (51), 77 (41).
Methyl 4-Hydroxy-2,3-dimethylbenzoate (3c)
Colorless solid; yield: 87%; mp 140–142 °C (EtOH).
IR (Nujol): 3450–3150, 1720, 1581, 1464, 840 cm–1.
1H NMR: = 2.21 (s, 3 H), 2.65 (s, 3 H), 3.75 (s, 3 H), 6.54 (d,
J = 9.2 Hz, 1 H), 7.58 (d, J = 9.2 Hz, 1 H). The OH signal is miss-
ing.
13C NMR: = 21.48, 23.94, 52.76, 111.93, 120.55, 126.13, 128.29,
140.63, 160.21, 164.16.
1-Bromo-3-cyanopenta-2,4-dione (5a)
Yellow oil; yield: 77%; bp 100–103 °C/0.7 mbar.
IR (neat): 3410 (enol form), 2210, 1735 (weak), 1620, 1600 cm–1.
1H NMR: = 2.57 (s, 3 H), 4.23 (s, 2 H), 5.5 (br s, 1 H, OH enol
form).
13C NMR: = 198.18, 184.07, 117.32, 95.85, 41.29, 29.11.
MS: m/z (%) = 203 (M+, 3), 205 (3), 124 (28), 95 (7), 93 (7), 43
(100).
MS: m/z (%) = 180 (M+, 52), 149 (100), 121 (19), 91 (30).
Anal. Calcd for C10H12O3: C, 66.65; H, 6.71. Found: C, 67.0; H, 6.9.
3-Cyanohexa-2,4-dione (6a)
White solid; yield: 75%; mp 53–55 °C (EtOH–hexane).
IR (CCl4): 3450 (enol form), 2213, 1742 (weak), 1650, 1600 cm–1.
1H NMR: = 1.26 (t, J = 8.3 Hz, 3 H), 2.37 (s, 3 H), 2.69 (q, J = 8.3
Hz, 2 H), 4.7 (br s, 1 H, OH enol form).
13C NMR: = 201.34, 187.22, 115.12, 97.09, 32.19, 28.57, 11.48.
MS: m/z (%) = 139 (M+, 20), 124 (11), 110 (13), 82 (27), 43 (100).
Anal. Calcd for C7H9O2N: C, 60.42; H, 6.52. Found: C, 60.7; H, 6.7.
Diethyl 4-Cyano-5-hydroxy-3-methylphthalate (4a)
Colorless solid; yield: 91%; mp 93–95 °C (EtOH).
IR (Nujol): 3650–3100, 2213, 1715, 1580, 1458 cm–1.
1H NMR: = 1.37 (t, J = 7.2 Hz, 3 H), 1.46 (t, J = 7.6 Hz, 3 H), 2.56
(s, 3 H), 4.36 (q, J = 7.2 Hz, 2 H), 4.53 (q, J = 7.6 Hz, 2 H), 7.58 (s,
1 H). The OH signal is missing.
Synthesis 2004, No. 3, 401–404 © Thieme Stuttgart · New York

404
A. Barbero, F. J. Pulido
PAPER
3-Cyanohepta-2,4-dione (7a)
Yellowish solid; yield: 81%; mp 98–100 °C (EtOH–hexane).
(4) Alberola, A.; González, B.; González, A. M.; Laguna, M. A.;
Pulido, F. J. Tetrahedon Lett. 1986, 27, 2027.
(5) Wakefield, B. J.; Wright, D. J. Adv. Heterocycl. Chem. 1979,
25, 147.
IR (Nujol): 3400–3100 (enol form), 2206, 1605 cm–1.
1H NMR: = 0.88 (t, J = 8.1 Hz, 3 H), 1.26 (m, 2 H), 2.16 (s, 3 H),
2.38 (t, J = 8.3 Hz, 2 H), 5.1 (br s, 1 H, OH enol form).
13C NMR: = 200.53, 185.31, 115.66, 99.48, 39.85, 29.14, 21.02,
8.53.
MS: m/z (%) = 153 (M+, 15), 138 (8), 110 (23), 71 (37), 43 (100).
(6) For an overview of the author’s work on silicon chemistry,
see: (a) Pulido, F. J.; Fleming, I. J. Chem. Soc., Chem.
Commun. 1986, 1010. (b) Fleming, I.; Rowley, M.;
Cuadrado, P.; González, A. M.; Pulido, F. J. Tetrahedron
1989, 45, 413. (c) Barbero, A.; Cuadrado, P.; González, A.
M.; Pulido, F. J.; Fleming, I. J. Chem. Soc., Perkin Trans. 1
1991, 2811. (d) Blanco, F. J.; Cuadrado, P.; González, A.
M.; Pulido, F. J.; Fleming, I. Tetrahedron Lett. 1994, 35,
8881. (e) Barbero, A.; Cuadrado, P.; González, A. M.;
Pulido, F. J.; Fleming, I.; Sánchez, A. J. Chem. Soc., Perkin
Trans. 1 1995, 1525. (f) Barbero, A.; García, C.; Pulido, F.
J. Tetrahedron Lett. 1999, 40, 6652. (g) Barbero, A.;
García, C.; Pulido, F. J. Tetrahedron 2000, 56, 2739.
(h) Barbero, A.; Blanco, Y.; García, C.; Pulido, F. J.
Synthesis 2000, 1223. (i) Barbero, A.; García, C.; Pulido, F.
J. Synlett 2001, 824. (j) Barbero, A.; Castreño, P.; García,
C.; Pulido, F. J. J. Org. Chem. 2001, 66, 7723. (k) Barbero,
A.; Blanco, Y.; Pulido, F. J. Chem. Commun. 2001, 1606.
(l) Barbero, A.; Castreño, P.; Pulido, F. J. Org. Lett. 2003, 5,
4045.
Anal. Calcd for C8H11O2N: C, 62.73; H, 7.24. Found: C, 62.8; H,
7.2.
1-Bromo-3-methylpenta-2,4-dione (5c)
Unstable oil; yield: 70%.
IR (neat): 3250–3600 (enol form), 1650, 1590 cm–1.
1H NMR: = 2.01 (s, 3 H), 2.47 (s, 3 H), 4.39 (s, 2 H), 5.8 (br s, 1
H, OH enol form).
13C NMR: = 197.18, 163.26, 121.43, 42.57, 27.41, 31.20.
MS: m/z (%) = 192 (M+, 1), 194 (1), 150 (10), 152 (10), 121 (5), 123
(5), 93 (7), 95 (7), 43 (100).
3-Methylhexa-2,4-dione (6c)13
Colorless oil; yield: 76%; bp 85–88 °C /11 mbar.
(7) For a review on siloxybutadienes, see: Brownbridge, P.
Synthesis 1983, 1.
IR (neat): 3210–3420 (enol form), 1731 (keto form), 1685, 1600
cm–1.
1H NMR: = 1.11 (t, J = 7.1 Hz, 3 H), 1.34 (d, J = 7.8 Hz, 3 H, keto
form), 2.04 (s, 3 H), 2.26 (s, 3 H), 2.42 (q, J = 7.1 Hz, 2 H), 3.53 (q,
J = 7.8 Hz, 1 H, keto form), 8.6 (br s, 1 H, OH enol form).
MS: m/z (%) = 128 (M+, 5), 114 (18), 99 (24), 86 (33), 71 (43), 57
(30), 43 (100).
(8) (a) Danishefsky, S.; Kerwin, J. F.; Kobayashi, S. J. Am.
Chem. Soc. 1982, 104, 358. (b) Danishefsky, S.; Kerwin, J.
F.; Kobayashi, S. J. Org. Chem. 1982, 47, 1981.
(c) Oppolzer, W.; Weinreb, S. M. In Comprehensive
Organic Synthesis, Vol. 5; Trost, B. M.; Fleming, I.;
Paquette, L. A., Eds.; Pergamon: London, 1991, 315.
(d) Oppolzer, W.; Weinreb, S. M. In Comprehensive
Organic Synthesis, Vol. 5; Trost, B. M.; Fleming, I.;
Paquette, L. A., Eds.; Pergamon: London, 1991, 401.
(9) (a) Oppolzer, W.; Snowden, R. L.; Simmons, D. P. Helv.
Chim. Acta 1981, 64, 2002. (b) Oppolzer, W.; Snowden, R.
L.; Briner, P. H. Helv. Chim. Acta 1981, 64, 2022. (c) See
also: Oppolzer, W.; Snowden, R. L. Tetrahedron Lett. 1976,
4187.
Acknowledgment
We thank the M.E.C. of Spain (BQU2000/0867) and the ‘Junta de
Castilla y León’ (VA023/2001) for financial support.
(10) Alberola, A.; González, A. M.; Guerra, M. D.; Pulido, F. J.;
Rodríguez, J. F.; Torroba, T. An. Quim. 1984, 80, 59; Chem.
Abstr. 1985, 102, 6257.
(11) Alberola, A.; Antolín, L. F.; González, A. M.; Laguna, M.
A.; Pulido, F. J. J. Heterocycl. Chem. 1986, 23, 1035.
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Synthesis 2004, No. 3, 401–404 © Thieme Stuttgart · New York