Synthesis of 4-phenylquinoline-5,8-quinones and their reactivity in hetero Diels-Alder reactions

Article abstract of DOI:10.1055/s-1999-3549

Substituted 4-arylquinolines and 9-aryl-1,2,3,4-tetrahydroacridines can be easily prepared from N-pivaloylanilines by ortho-lithiation/benzoylation and subsequent treatment of the benzophenones thus obtained with ketones in acetic acid. The pivaloyl group is eliminated in situ in these reactions. The reactivity of 3,4-dimethyl-4-phenylquinoline-5,8-dione, obtained by oxidation of the corresponding 5,8-dimethoxyquinoline, as dienophile in hetero Diels- Alder reactions with α,β-unsaturated N,N-dimethylhydrazones to give 1,8- diazaanthraquinones, is compared with that of other quinolinequinones.

Full text of DOI:10.1055/s-1999-3549

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J. Ignacio Úbeda, Mercedes Villacampa, Carmen Avendaño*
Departamento de Química Orgánica y Farmacéutica. Facultad de Farmacia., Universidad Complutense, E-28040 Madrid, Spain
Fax +34(341)3941822; E-mail: avendano@eucmax.sim.ucm.es
Srprv‰rqꢀ ꢀHhꢁpuꢀ (((
sulfuric acid (Scheme 1). These conditions preclude the
$EVWUDFWꢈ Substituted 4-arylquinolines and 9-aryl-1,2,3,4-tetrahy-
use of aldehydes as active methylene compounds.11
droacridines can be easily prepared from I-pivaloylanilines by
‚ꢁ‡u‚-lithiation/benzoylation and subsequent treatment of the ben-
zophenones thus obtained with ketones in acetic acid. The pivaloyl
group is eliminated in situ in these reactions. The reactivity of 3,4-
R5
R3
R2
dimethyl-4-phenylquinoline-5,8-dione, obtained by oxidation of
the corresponding 5,8-dimethoxyquinoline, as dienophile in hetero
Diels–Alder reactions with a,b-unsaturated IꢂI-dimethylhydra-
zones to give 1,8-diazaanthraquinones, is compared with that of
other quinolinequinones.
R5
N
i, iii, iv
ii
R8
R5
R5
Ph
iii, iv
v, vi
.H\ꢃZRUGVꢈꢃFriedländer reaction, pivaloylaniline, 4-arylquinolines,
azaanthraquinones, Diels–alder reaction
NHCO tBu
COPh
R3
R2
R8
1, R5 = OMe, R8 = H
2, R5 = R8 = OMe
NHCOt
N
Bu
R8
R8
The ability of a,b-unsaturated IꢂI-dimethylhydrazones to
react with electrophilic reagents at C-3 has been success-
fully exploited in hetero Diels–Alder cycloadditions with
quinones as electron-deficient dienophiles.1 However,
cinnamyl-IꢂI-dimethylhydrazones are very reluctant to
behave as activated 4-aryl-1-azadienes and they do not re-
act with 1,4-benzoquinones,2 give only moderate yields of
the adducts with 2,5,8-quinolinetriones,3 and have limited
success with naphthoquinones4 and quinolinequinones.5
An alternative approach to 4-aryldiazaanthraquinones is
based on the reaction of 4-arylquinoline-5,8-quinones as
dienophiles with activated 1-aza- and 2-azadienes. This
methodology has been applied as a key step in the synthe-
sis of isoascididemine6 and amphimedine,7 and the start-
ing 4-arylquinolinequinones were prepared through Knorr
cyclizations,7b,7c or Stille palladium-catalyzed cross-cou-
pling reactions between 4-quinolyl triflates and arylstan-
nanes.6,7a,8 Here we report the synthesis of 4-
arylquinolines through a modified Friedländer procedure,
and study the reactivity of 3,4-dimethyl-4-phenylquino-
line-5,8-dione (ꢀꢁ) with different a,b-unsaturated IꢂI-
dimethylhydrazones.
3-12
Reagents and conditions: i) -BuLi/DMF/THF, 0°C; ii) BuLi/
PhCO2Et/THF, 0°C to r.t.; iii) KHMDS/R2COCH R3, 0°C to r.t.; iv)
2
NH Cl/H2O; v) AcOH/H2SO4/R2COCH2R3, reflux; vi) NH4OH
4
To study the behaviour of 4-arylquinolinequinones as di-
enophiles in hetero Diels–Alder reactions with 1-aza-
dienes $±(,1b we selected the quinoline ꢀꢉ which, after
oxidation with cerium ammonium nitrate (CAN), gave
quantitatively the quinone ꢀꢁ. Quite surprisingly, the re-
actions took place in a few minutes the results being de-
pendent on the 1-azadiene substitution pattern (Scheme
2). Thus, for R3 = R4 = H ($), the fully aromatic adduct
ꢀꢄ, which was produced by elimination of dimethylamine
and spontaneous air oxidation, was obtained together with
the secondary product ꢉꢊ, formed by addition of the elim-
inated dimethylamine to the unreacted starting quinone
ꢀꢁ. However, for substituted azadienes %±(, elimination
of dimethylamine was not followed by in situ oxidation,
and compounds ꢀꢂ±ꢀꢇ were obtained together with vari-
able amounts of ꢉꢊ. In one case, a minor amount of the ox-
idized adduct ꢀꢋ was also obtained.13
We have recently developed the synthesis of quinoline de-
rivatives in a one-pot procedure by ‚ꢁ‡u‚-lithiation of I-
pivaloylanilines, formylation with DMF, and subsequent
condensation with active methylene groups of aldehydes
or ketones in the presence of potassium bis(trimethylsi-
lyl)amide (KHMDS),9 but the method failed when it was
applied to I-pivaloylaminobenzophenones ꢀ and ꢉ,10
probably due to steric interactions in the aldol intermedi-
ates. Fortunately, these benzophenones gave compounds
ꢁ±ꢀꢉ (Table 1) with in situꢀelimination of the I-pivaloyl
group, when the starting materials were treated with ke-
tones in acetic acid as solvent and a catalytic amount of
The first difference between ꢀꢁ and other quinolinequino-
nes so far studied is the effect of the 4-phenyl substituent
on the reaction regioselectivity, since no traces of the 1,5-
diaza-cycloadducts1d,12 were detected in this case. Fur-
thermore, compound ꢀꢁ is more reactive, as shown by the
very short reaction times (longer reaction times did not
improve the yields) and the smooth conditions, which
contrast with those required by other quinolinequinones.
1a,1d,14
Synthesis 1999, No. 8, 1335–1340 ISSN 0039-7881 © Thieme Stuttgart · New York

ꢀꢁꢁꢌ
J. I. Úbeda et al.
3$3(5
Phenylquinolines - Prepared
Carbonyl Reagent
Sub- Prod- R2
strate uct
R3
R8
Yield Picratea
IR (KBr)
(%)
(EtOH)
n (cm–1)
mp (°C)
acetone
butan-2-one
Me
Me
Et
Ph
H
Me
Me
Me
Ph
H
H
H
H
H
H
H
H
53
48
52
54
54
56
80
46
116
171
162
126–127
88–89
175–176
168
1572, 1250
1574, 1254
1576, 1232
1573, 1251
1576, 1231
1565, 1255
1567, 1256
1573, 1251
pentan-3-one
PhCOEt
PhCH2COCH2Ph
cyclohexanone
2-methylcyclohexanone
MeCOCH2COMe
PhCH2
-CH2(CH2)2CH2-
-CH2(CH2)2CH(CH3)-
COMe
Me
Me
112
MeCOCH2CH2COMe
H
38
98
81
1580, 1228
1613, 1262
MeCOCH2CH3
Me
Me
OMe 60
a Satisfactory microanalyses obtained: C 0.39, H 0.14, N 0.44.
OMe Ph
O
Ph
N
Me
i
Me
Me
N
Me
OMe
12
O
13
R4
R3
ii
N
iii, 100% yield
NMe2
A-E
Ph
R4
O
O
Ph
N
R4
N
Ph
N
Ph
R4
O
O
O
O
R3
Me
Me2N
Me
Me
R3
Me
Me
R3
Me
+
+
+
N
H
N
Me
N
N
Me
O
O
NMe2
14, 21-24
15-18
19
20
Comp. R3 R4
% Yield 20 (%)
Comp. R3 R4
%Yield 20 (%)
14
H
H
H
75
99
99
99
12
15
16
17
18
19
CH3
H
H
59
53
40
15
29
32
15
21
CH3
H
CH3
Ph
22
23
24
CH3
Ph
H
H
CH3 CH2-CH3 46
CH3 CH2-CH3 14
CH3 CH2-CH3 99
Reagents and conditions: i) CAN/CHCl3/H2O, 15 min, r.t.; ii) CHCl3, 5–10 min, r.t.; iii) air or MnO2, r.t.
Oxidation of compounds ꢀꢂ±ꢀꢇ gave quantitatively the In conclusion, we have developed an easy and efficient
aromatic compoundsꢃꢉꢀ±ꢉꢄꢃ(Scheme 2). Compounds with procedure for the synthesis of 4-aryl-2,3-disubstituted
R4 = H or Ph were oxidized when the corresponding chlo- quinolines, and have shown that the hetero-Diels–Alder
roform solution was exposed to air. Those with R4 = Me reaction of their 5,8-dioxo derivatives with activated 1-
or Et, required treatment with MnO2.
Synthesis 1999, No. 8, 1335–1340 ISSN 0039-7881 © Thieme Stuttgart · New York

3$3(5
Synthesis of 4-Phenylquinoline-5,8-quinones and their Reactivity in Hetero Diels–Alder Reactions
ꢀꢁꢁꢍ
1H NMR Data for Compounds
(d, 250 MHz, CDCl3/TMS, in Hz)
a
b
R2, R3
H-8
Prod-
uct
H-6
H-7
C6H5
5-OCH3
3.48
c
7,70 (dd,
8.4, 1.1)
=
=
=
7.58 (dd,
8.4, 7.7)
=
=
=
6.75 (dd,
7.7, 1.1)
=
=
=
2.70 (s)
7.35
7.63 (dd,
7.3, 1.1)
7.48 (dd,
8.3, 7.3)
6.66 (dd,
7.3, 1.1)
2.69 (s), 2.03 (s)
7.09, 7.73,
7.10, 7.32
3.33
7.65 (dd,
8.5, 1.1)
7.48 (dd,
8.5, 7.1)
6.67 (dd,
7.1, 1.1)
3.00 (q, = 7.5),
1.37 (t, = 7.5),
2.06 (s)
3.33
7.71 (dd,
7.3, 1.1)
=
7.48 (m)
6.69 (dd,
7.7, 1.1)
=
7.48 (m), 2.02 (s)
7.42 (m),d 2.71 (s)
7.19, 7.48
3.36
d
d
d
d
–
–
–
–
3.48
3.36
e
e
7.63 (d, = 8.5)
7.63 (d, = 8.4)
7.53 (dd,
8.5, 7.7)
=
=
6.67 (d, = 7.7)
6.63 (d, = 7.7)
1.73 (m); 1.93 (m),
2.42 (t, = 6.5),
2.46 (d, = 6,5)
7.14, 7.36
7.08, 7.37
7.18, 7.39
7,47 (dd,
8.4, 7.7)
1.48 (d, = 7.1),
1.66 (m), 2.21 (m)
2.41 (m)
3.33
3.38
7.65 (dd,
7.3, 1.1)
=
7.38 (m)
7.38 (m)
6.72 (dd,
7.5, 1.1)
=
2.59 (s), 1.90 (s)
7.70 (m)
6.83 (d, = 7.5)
2.23 (s), 2.28 (s)
2.45 (s), 2.03 (s)
7.12, 7.36
7.10, 7.34
3.53
3.26
f
6.60 (d, = 8.5)
6.65 (d, = 8.5)
–
a Multiplet.
b Singlet.
c H-3 : d = 7.05 (s).
d The multiplet at d = 7.42 integrates for 18H.
e Numbers do not correspond to the tricyclic structure.
f 8-OCH3 : d = 4.02 (s).
flash chromatography (silica gel, CH2Cl2) and the products were de-
rivatized to picrates by standard procedures. Spectroscopic data for
compounds ꢁ±ꢀꢉ are summarized in Tables 2 and 3.
azadienes has advantages over the alternative approach
that incorporates the aryl substituent in the heterodiene.
ꢉꢆꢁꢅ'LPHWK\OꢅꢄꢅSKHQ\OꢅꢂꢆꢇꢅTXLQROLQHTXLQRQHꢃꢏꢀꢁꢐ
IR spectra were recorded on a Perkin-Elmer Paragon 1000 spectro-
photometer (KBr pellets). NMR spectra were obtained using a
To a solution of ꢀꢉ (100 mg, 0.35 mmol) in CHCl3 (5 mL) was add-
ed a solution of cerium(IV) ammonium nitrate (384 mg, 0.70 mmol)
in H2O (6 mL). The mixture was stirred for 20 min, extracted with
CHCl3 (2 × 10 mL), and dried (Na2SO4). After removal of the sol-
vent under vacuum, 93 mg (100%) of quinone ꢀꢁ was obtained and
used as such for Diels–Alder reactions.
1H NMR (250 MHz, CDCl3): d = 7.46 (m, 3 H, H-2’,4’,6’) , 7.00 (d,
1 H, E = 10.0 Hz, H-6), 7.03 (m, 2 H, H-3’,5’), 6.73 (d, 1 H, E = 10.0
Hz, H-7), 2.75 (s, 3 H, 2-CH3), 2.05 (s, 3 H, 3-CH3).
1
Bruker AC-250 (250 MHz for H, 63 MHz for 13C) spectrometer.
Elemental analyses of new compounds were determined by the Ser-
vicio de Microanálisis, Universidad Complutense, on a Perkin-Elm-
er 2400 CHN microanalyser. Melting points were measured on a
Reichert 273 hot stage microscope, and are uncorrected. Reactions
were monitored by TLC, on aluminum plates coated with silica gel
with fluorescent indicator (Scharlau Cf 530). Separations by flash
chromatography were performed on silica gel (SDS 60 ACC, 230–
400 mesh and Scharlau Ge 048). All reagents were of commercial
quality (Aldrich, Fluka, Merck, SDS, Probus) and were purified fol-
lowing standard procedures.
13C NMR (63 MHz, CDCl3): d = 185.1 (C-8), 183.8 (C-5), 163.3 (C-
2), 149.6 (C-4a), 141.0 (C-8a), 139.2 (C-6), 136.8 (C-7), 128.5 (C-
3’,5’), 127.5 (C-4’), 126.8 (C-2’,6’), 24.6 (2-CH3), 16.8 (3-CH3).
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3URFHGXUH
'LHOV±$OGHUꢃ5HDFWLRQVꢎꢃ*HQHUDOꢃ3URFHGXUH
To a solution of quinone ꢀꢁ (10 mmol) in CHCl3 (5 mL) was added
the corresponding hydrazone $±( (1.1 mmol). The mixture was
stirred at r.t. for 5–10 min and the solvent evaporated under vacuum.
The residue was purified by flash chromatography (silica gel,
CH2Cl2/EtOAc).The spectroscopical data for compounds ꢀꢂ±ꢀꢋ are
summarized in Tables 4 and 5.
A solution of ‚-pivaloylaminobenzophenones ꢀ or ꢉ (10 mmol) and
the corresponding ketone (10 mmol) in a mixture of glacial AcOH
(1 mL) and H2SO4 (0,1 mL) was refluxed for 12 h. The cold solution
was neutralized with NH4OH and extracted with CH2Cl2 (3 × 10
mL). The combined organic extracts were dried (Na2SO4) and the
solvent was removed under vacuum. The residue was purified by
Synthesis 1999, No. 8, 1335–1340 ISSN 0039-7881 © Thieme Stuttgart · New York

ꢀꢁꢁꢇ
J. I. Úbeda et al.
3$3(5
13C NMR Data for Compounds
(d, 63 MHz, CDCl3/TMS)
Prod- C-2
uct
C-3
C-4
C-4a
C-5
C-6
C-7
C-8
C-8a
C6H5
R,2 R3
OCH3
55.3
55.7
55.7
55.5
56.4
158.2
124.1
128.2
127.5
128.5
128.5
149.9
147.7
147.9
147.7
149.6
117.1
118.4
118.4
118.5
117.0
156.1
156.2
156.2
155.9
156.5
105.6
105.9
105.9
106.2
105.5
129.5
128.4
128.2
129.0
129.7
121.6
121.5
121.9
122.3
121.1
148.1
145.2
145.6
146.6
148.9
142.8, 127.1, 25.0
128.2, 126.9
158.8
163.0
160.5
158.2
142.4, 127.8, 24.6, 16.8
127.4, 126.2
142.6, 127.4, 30.3, 13.0, 16.2
127.8, 126.1
141.3, 128.2, 142.2, 128.8,
127.5, 126.1
128.1, 126.1, 18.4
29.8, 142.7,
134.8, 133.2
128.0, 127.3,
124.1, 128.4,
126.9, 126.7
a
a
b
156.2
156.0
157.4
128.3
128.1
128.5
145.6
145.4
144.7
121.4
121.4
119.0
158.8
162.5
148.9
105.4
105.3
105.6
128.9
128.4
105.7
118.3
118.0
149.3
147.8
147.8
138.4
141.7, 127.5, 22.8, 23.1, 27.9,
127.4, 126.1 34.1
55.5
55.5
141.9, 127.4, 20.3, 27.1, 28.4,
127.3, 126.0 30.7, 36.8
148.8, 126.9, 24.6, 16.3
127.3, 125.7
55.7,
55.8
a Numbers do not correspond to the tricyclic structure.
b 13C NMR spectra of compounds
and were not recorded due to low solubility.
Melting Points and 1H NMR Data of Compounds
Prod- mp (°C)
1H NMR 250 MHz, CDCl3/TMS), d, = (Hz)
ucta
H-1
H-2
H-3
H-4
C6H5
C-CH3
R3c
R4
b
c
183–184 6.66 (s)
5.86 (m)
6.12 (d, = 7.8)
6.36 (m)
–
3.10 (m)
7.01, 7.45
7.02, 7.46
6.80, 7.50
1.93, 2.68
2.99, 2.70
1.95, 2.68
1.66, 2.00
1.56
–
–
–
163
79
6.71 (s)
–
4.86 (m) 3.53 (m)
5.02 (m) 4.72 (m)
–
1.01 (d, = 6.6)
6.80–7.7 (m)
0.74 (t, = 7.5),
1.43 (m)
0.74 (t, = 7.5),
1.36 (m)
d
160
6.77 (s)
6.09 (s)
3.55 (t, = 4.5) 7.01, 7.43
1.66
e
f
–
–
6.13 (s)
–
3.50 (m) 7.01, 7.42
1.97, 2.69
1.66
a Satisfactory microanalyses obtained: C 0.31, H 0.11, N 0.2.
b Multiplet.
c Singlet.
d Not observed.
e N-CH3 : d = 2.66 (s).
f Not determined.
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ꢏꢉꢊꢐ
2[LGDWLRQꢃRIꢃꢀꢆꢄꢅ'LK\GURꢅꢌꢆꢍꢅGLPHWK\OꢅꢄꢅSKHQ\OꢅꢀꢆꢇꢅGLD]DDQꢅ
WKUDTXLQRQHVꢑ
This byproduct was characterized by its IR and NMR spectra and
was not purified.
IR (KBr): n = 1684, 1616, 1559 cm–1.
1H NMR (250 MHz, CDCl3): d = 7.42 (m, 3 H, H-2’,4’,6’) , 7.06 (m,
2 H, H-3’,5’), 5.85 (s, 1 H, H-7), 2.98 [s, 6 H, N(CH3)2], 2.72 (s, 3
H, 2-CH3), 2.04 (s, 3 H, 3-CH3).
13C NMR (63 MHz, CDCl3): d = 163.8 (C-2), 154.5 (C-6), 148.9 (C-
4), 147.3 (C-8a), 138.5 (C-1’), 128.5, 127.7, 126.8 (C6H5), 128.3
(C-3*), 127.5 (C-4a*), 100.1 (C-7), 41.8 [N(CH3)2], 24.5 (2-CH3),
16.3 (3-CH3) (* exchangeable assignments).
Method A, for ꢉꢀꢆꢃꢉꢁ: A solution of the dihydrocompound (0.025
mmol) in CHCl3 was kept in an open flask at r.t. (12 h for ꢀꢂ, 5 d for
ꢀꢍ). After evaporation of the solvent, quinones ꢉꢀ and ꢉꢁ were ob-
tained quantitatively.
Method B, for ꢉꢉꢆꢃꢉꢄ: To a solution of the corresponding dihydro
compound (0.004 mmol) in CH2Cl2 (3 mL) was added MnO2 (20
mg, 0.015 mol) and the mixture was stirred overnight at r.t. The cor-
responding quinones were quantitatively obtained after filtration
and evaporation of the solvent. Data for compounds ꢀꢄꢃ(see text)
and ꢉꢀ±ꢉꢄ are given in Tables 6 and 7.
Synthesis 1999, No. 8, 1335–1340 ISSN 0039-7881 © Thieme Stuttgart · New York

3$3(5
Synthesis of 4-Phenylquinoline-5,8-quinones and their Reactivity in Hetero Diels–Alder Reactions
13C-NMR Data for Compounds
(63 MHz, CDCl3/TMS)
C-3 C-4 C-4a C-5 C-6 C-7 C-8a C-9
C- H3 R3 R4
ꢀꢁꢁꢋ
Prod- C-2
uct
C-9a C-10 C-10a C6H5
117.8 114.2 26.9
122.6 109.0 29.8
126.6 107.4 36.4
119.4 115.4 36.3
118.6 118.2 36.2
110.8 149.2 138.5 161.0 144.5 182.9 139.2a 179.3 130.3 139.1,a 128.3, 20.3, 16.8
127.8, 126.8 24.1
b
b
116.6 149.3 139.0a 161.3
–
179.6
–
182.7 137.4 138.8,a 127.2, 23.9,
127.0, 126.7 25.7
16.8
115.2 149.5 138.0a 161.5 144.5 179.8 128.5 182.7 138.8a 137.5,a 128.3, 16.9,
127.1 24.2
145.9
123.2
112.5 149.3 139.5a 161.1 144.6 182.7 136.7 179.6 128.3 139.2,a 127.2, 18.5, 16.8
127.1, 126.7 24.1
c
135.6 148.5 139.0 161.2 144.6 183.3 117.1 179.8 128.2 127.4, 126.7 18.7, 16.5 25.3
24.1 9.3
a Exchangeable signals.
b Not observed
c N-CH3 : δ = 44.7.
Melting Points and 1H NMR Data for Compounds
,
Prod- mp (°C)
1H NMR (250 MHz, CDCl3/TMS), d, (Hz)
ucta
H-5
H-6
H-7
C6H5
C-CH3
R3c
R4
b
c
108
243
254
156
128
8.37 (dd,
= 8.5, 1.7)
7.64 (dd,
= 8.5, 4.5)
9,08 (dd,
= 4.5, 1.7)
7.09, 7.51
7.06, 7.48
6.99, 7.45
6.80–7.70
7.15, 7.42
2.09, 2.82
2.08, 2.80
2.09, 2.79
2.14, 2.79
2.10, 2.83
–
–
8.14 (d,
= 1.9)
–
8.88 (d,
= 1.9)
2.45
–
–
–
–
–
7.11 (d,
= 8.5)
8.78 (d,
J = 8.5)
2.57 (s)
7,49 (d,
= 4.9)
8.99 (d,
= 4.9)
–
6.80–7.70 (m)
–
8.81 (s)
2.46
1.07 (t, = 7.0),
3.12 (q, = 7.0)
a Satisfactory microanalyses obtained: C 0.06, H 0.03, N 0.25.
b Multiplet.
c Singlet.
13C NMR Data of Compounds
and
C-5
(d, 63 MHz, CDCl3/TMS)a
C-6 C-7 C-8a C-9
Prod- C-2
uct
C-3
C-4
C-4a
C-9a
C-10 C-10a C6H5
-CH3
163.4 138.5 150.6 131.0 135.6 127.8b 155.2 147.0 184.4 147.7 174.0 137.8 142.9, 128.8, 24.8, 17.0
128.2b, 126.9
164.4 138.6b 150.5 130.4 135.2 125.9 155.9 146.0 183.0 147.0 180.7 137.5b 139.2, 128.8, 24.7, 16.9,
127.6, 126.9
19.0
13C NMR spectra of quinones
were not recorded due to low solubility.
a
b Exchangeable assignments.
$FNQRZOHGJHPHQW
5HIHUHQFHV
Financial support from CICYT (project PTR-028-93) is gratefully
acknowledged.
(1) See for instance:
(a) Potts, K. T.; Walsh, E. B.; Bhattacharjee, D. EꢃꢀPꢁtꢃꢀ8ur€ꢃ
ꢀꢋꢇꢍ, $!, 2285.
(b) Waldner, A. Cry‰ꢃꢀ8uv€ꢃꢀ6p‡h ꢀꢋꢇꢇ,ꢀ& , 486 and 493.
(c) Dolle, R. E.; Armstrong, W.; Shaw, A. N.; Novelli, R.
Synthesis 1999, No. 8, 1335–1340 ISSN 0039-7881 © Thieme Stuttgart · New York

ꢀꢁꢄꢊ
J. I. Úbeda et al.
3$3(5
Ur‡ꢁhurqꢁ‚ꢀGr‡‡ꢃꢀꢀꢋꢇꢇ, !(, 6349.
(d) Gesto, C., de la Cuesta, E.; Avendaño, C.ꢀUr‡ꢁhurqꢁ‚
ꢀꢋꢇꢋ, #$, 4477.
(e) Nebois, P.; Fillion, H. Ur‡ꢁhurqꢁ‚ꢀGr‡‡ꢃ ꢀꢋꢋꢀ, "!, 1307.
(f) Bracher, F.ꢀGvrivt†ꢀ6ꢃꢀ8ur€ꢃ ꢀꢋꢋꢉ, 1205.
(g) Chaker, L.; Pautet, F.; Fillion, H. Cr‡rꢁ‚p’pyr† ꢀꢋꢋꢂ, # ,
1169.
(7) (a) Echavarren, A. M.; Stille, J. K. Eꢃꢀ6€ꢃꢀ8ur€ꢃꢀT‚p. ꢀꢋꢇꢇ,
ꢄ, 4051.
(b) Kubo, A.; Nakahara, S. Cr‡rꢁ‚p’pyr† ꢀꢋꢇꢇ, !&, 2095.
(c) Nakahara, S.; Tanaka, Y.; Kubo, A.; Cr‡rꢁ‚p’pyr†ꢀꢀꢋꢋꢌ,
#", 2113.
(8) Echavarren, A. M.; Stille, J. K. Eꢃꢀ6€ꢃꢀ8ur€ꢃꢀT‚pꢃ ꢀꢋꢇꢍ, ꢄ(,
5478
(2) (a)Perumal, P. J.; Bahtt, M. V.; T’‡ur†v† ꢀꢋꢍꢋ, 205.
(b) Brimble, M. A.; Brimble, M. T.; Gibson, J. J.; Eꢃꢀ8ur€ꢃꢀ
T‚pꢃꢂꢀQrꢁxvꢀUꢁh†ꢃꢀ ꢀꢋꢇꢋ, 179.
(c) Echavarren, A. M. EꢃꢀPꢁtꢃꢀ8ur€ꢃ ꢀꢋꢋꢊ, $$, 4255.
(3) (a) Pérez, J. M.; Avendaño, C.; Menéndez, J. C. Ur‡ꢁhurqꢁ‚
ꢀꢋꢋꢂ, $ , 6573.
(9) Úbeda, J. I.; Villacampa, M.; Avendaño, C. T’‡ur†v† ꢀꢋꢋꢇ,
1176.
(10) Walsh, A. T’‡ur†v† ꢀꢋꢇꢊ, 677.
(11) Fehnel, E. A. EꢃꢀPꢁtꢃꢀ8ur€ꢃ ꢀꢋꢌꢌ, " , 2899.
(12) Tapia, R. A.; Quintanar, C.; Valderrama, J. A. Cr‡rꢁ‚p’pyr†
ꢀꢋꢋꢌ, #", 447.
(b) Avendaño, C.; Blanco, M. M.; Menéndez, J. C.; García-
Grávalos, D.; Fernández-Puentes, J. L. U K Patent Appl. 9 708
751.4 (1997).
(13) The isolation of these adducts is not frequent, see for instance:
Bouaziz, Z.; Nebois,P.; Fillion, H. Ur‡ꢁhurqꢁ‚ꢀꢀꢋꢋꢂ, $ ,
6573.
(4) Chigr, M.; Fillion, H.; Rougny, A. Ur‡ꢁhurqꢁ‚ꢀGr‡‡ꢃꢀꢀꢋꢇꢇ, !(,
5913.
(14) Villacampa, M.; Pérez, J. M.; Avendaño, C.; Menéndez, J. C.
Ur‡ꢁhurqꢁ‚ ꢀꢋꢋꢄ,ꢀ$ꢄꢂ 10047.
(5) Kitahara, Y.; Tamura, F.; Kubo, A.; 8ur€ꢃꢀQuhꢁ€ꢃꢀ7ˆyyꢃ ꢀꢋꢋꢄ,
#!, 1363.
(6) Gómez-Bengoa, E.; Echavarren, A. M. EꢃꢀPꢁtꢃꢀ8ur€ꢃ ꢀꢋꢋꢀ,ꢀ
$%, 3497.
Article Identifier:
1437-210X,E;1999,0,08,1335,1340,ftx,en;H01299SS.pdf
Synthesis 1999, No. 8, 1335–1340 ISSN 0039-7881 © Thieme Stuttgart · New York