Niobium pentachloride activation of enone derivatives: Diels-Alder and conjugate addition products

Article abstract of DOI:10.3390/70500456

Niobium pentachloride has proven to be a powerful activating agent for Diels-Alder or conjugate addition reactions of cycloenones. The Diels-Alder product was obtained only with an unsubstituted enone (cyclohexenone) and the highly reactive diene cyclopentadiene; substituents in the β-position of enones seem to prevent Diels-Alder reaction: oxygenated substituents favor the formation of vinyl chlorides (ethyl ether or dichloromethane as solvents) or enol ethers (ethyl acetate as solvent), while a methyl substituent prevents (any kind of transformation with NbCl₅. Less reactive dienes, furan and 2-methylfuran gave the conjugate addition products of the furan ring to the enone system.

Full text of DOI:10.3390/70500456

Molecules 2002, 7, 456–465
molecules
ISSN 1420-3049
http://www.mdpi.org
Niobium Pentachloride Activation of Enone Derivatives: Diels-
Alder and Conjugate Addition Products
Mauricio Gomes Constantino*, Valdemar Lacerda Júnior and Gil Valdo José da Silva
Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São
Paulo, Av. Bandeirantes 3900, 14040-901 – Ribeirão Preto – SP, Brazil. Phone +55/16 602 3747. Fax
+55/16 633 8151.
* Author to whom correspondence should be addressed. E-mail: mgconsta@usp.br; homepage:
http\\artemis.ffclrp.usp.br
Received: 19 March 2002; in revised form: 15 May 2002 / Accepted: 16 May 2002 / Published: 31 May
2002
Abstract: Niobium pentachloride has proven to be a powerful activating agent for Diels-Alder or
conjugate addition reactions of cycloenones. The Diels-Alder product was obtained only with an
unsubstituted enone (cyclohexenone) and the highly reactive diene cyclopentadiene; substituents in
the b-position of enones seem to prevent Diels-Alder reaction: oxygenated substituents favor the
formation of vinyl chlorides (ethyl ether or dichloromethane as solvents) or enol ethers (ethyl
acetate as solvent), while a methyl substituent prevents any kind of transformation with NbCl .
5
Less reactive dienes, furan and 2-methylfuran gave the conjugate addition products of the furan
ring to the enone system.
Keywords: Niobium pentachloride, Lewis acid, Diels-Alder reaction, vinyl chlorides
Introduction
As part of our studies on the use of the Lewis acid niobium pentachloride in organic synthesis [1], we have
recently started an investigation on Diels-Alder reactions catalysed with this reagent. As many Diels-Alder
.

Molecules 2000, 7
457
reactions proceed quite well without Lewis acids, the use of a catalyst can be justified only with less reactive
substrates, or to obtain a different ratio of product isomers. In our investigations on the synthesis of natural
products [2,3] we found that a considerable simplification would be obtained if we could effect a Diels-Alder
reaction between certain cyclic enones and dienes such as cyclopentadiene and furan derivatives. We have
thus chosen cycloenones 1, 2 (the enolic form of 1,3-cyclohexanedione), 3, 4, 5 as dienophiles and
compounds 6, 7 and 8 as dienes for our first studies. Under normal conditions these compounds do not react
with each other to give Diels-Alder adducts; it is known, however, that some cycloenones do react with
cyclopentadiene in the presence of AlCl [4]. The use of NbCl as Lewis acid for Diels-Alder reactions has
3
5
also been previously reported [5,6].
Reactions were performed under nitrogen atmosphere, at room temperature or at -78ºC, and in anhydrous
solvents: ethyl acetate, dichloromethane or ethyl ether. Excess diene was used in each experiment, while the
molar ratio dienophile/NbCl was maintained constant in 1.0/0.5.
5
Figure 1. Dienophiles and dienes used in experiments with NbCl .
5
Dienes
Dienophiles
O
O
O
O
OH
OAc
1
2
6
3
7
O
O
O
8
4
5
Results and Discussion
The reaction between 2-cyclohexenone (1) and cyclopentadiene (6) was effected under different
conditions, as summarized in Scheme 1 and Table 1.
Scheme 1
O
H
NbCl5
1
+
6
EtOAc
H
9

Molecules 2000, 7
Table 1. Results obtained in the reaction of 2-cyclohexenone (1) and cyclopentadiene (6).
458
NbCl5 (eq.)
Temperature (ºC)
Time (min)
180
Yield (%)
-
rt or reflux
-
0.5
0.5
rt
180
42
74
-78
90
As shown by these results, no Diels-Alder reaction takes place in absence of catalyst, but addition of
NbCl resulted in formation of Diels-Alder adduct either at room temperature or, in better yield, at
5
-78ºC. The yield improvement by lowering the temperature is a consequence of a decrease in diene
polymerization The stereoselectivity of this reaction (only endo adduct 9 could be isolated) is a remarkable
aspect, since when AlCl is used as catalyst [4] both endo and exo adducts are formed.
3
The other combinations of dienophile and diene from Figure 1 showed that, while a strong activation of the
enone system by NbCl occurred in most cases, Diels-Alder adducts were not the products obtained.
5
Cycloenones 2-5 did not react with cyclopentadiene; only vinyl chlorides (10) or enol ethers (11) were
obtained when these substrates were treated with cyclopentadiene and NbCl . No product containing
5
cyclopentadiene residues was isolated from these reaction mixtures. In fact, products 10 and 11 were
obtained in better yields when the reaction was carried out in the absence of cyclopentadiene (Scheme 2 and
Table 2). From these results it seems that substituents in the b position of enones prevent the formation of
Diels-Alder adducts: oxygenated substituents favor the formation of vinyl chlorides (when ethyl ether or
dichloromethane are used as solvents) or enol ethers (when ethyl acetate is used as solvent), while methyl
substituent prevents any reaction from taking place, starting material being recovered unchanged at the end of
the reaction. A method for the preparation of vinyl chlorides and enol ethers can clearly be elaborated from
these results, and we are already carrying out studies with this purpose.
Scheme 2
O
O
NbCl5
solvent
rt
R
R'
10 R'=Cl
11 R'=OEt
2 R=OH
3 R=OAc
O
NbCl5
R
+
6
NO REACTION
CH3
solvent
rt
R
4 R=CH3
5 R=H

Molecules 2000, 7
459
Table 2. Results of reaction of enones 2 - 5 with NbCl at room temperature.
5
Ratio (%)
10 : 11
100 : 0
100 : 0
5 : 95
100 : 0
100 : 0
20 : 80
-
Enone
Solvent
Time (min.)
Yield (%)
2
2
Et2O
15
30
30
20
20
30
57
68
60
68
80
83
CH2Cl
2
2
EtOAc
Et2O
3
3
CH2Cl
2
3
EtOAc
EtOAc
EtOAc
4a
5a
-
a
These experiments were performed also under reflux and always in the presence of cyclopentadiene; no
product was obtained even after several hours.
Reaction of furan (7) or 2-methylfuran (8) with enones 1 or 5 gave, rather than Diels-Alder adducts,
products 12-16 (Scheme 3 and Table 3); under the same conditions, compound 4 did not react with furan 7.
Scheme 3
O
O
NbCl5
EtOAc
1
+
7
O
12
O
NbCl5
EtOAc
O
O
1
+
8
+
O
O
13
15
O
NbCl5
EtOAc
O
O
5
8
+
+
O
O
14
16

Molecules 2000, 7
Table 3. Results obtained in reactions between enones 1, 5 and furans 7, 8.
460
Dienophile
Dien
Temperature ºC
Time (min)
Yield (%)
Product (ratio,%)
e
1
1
1
1
5
5
7
7
8
8
8
8
rt
- 78
rt
15
90
66
74
65
74
68
-
12 only
12 only
5
13 (33) and 15 (77)
13 (95) and 15 (5)
14 (40) and 16 (60)
14a only
- 78
rt
40
300
600
- 78
aAfter 600 minutes only 50% of the starting material had been transformed.
Compounds 12-16 can be regarded either as products of conjugate (nucleophilic) addition of the furan ring
to the enone system, or as products of electrophilic substitution of hydrogen by the enone system in furan ring,
according to the mechanism for 1 and 8 shown in Scheme 4.
Scheme 4
Nb
Nb
O
O
O
H2O
O
O
H
O
13
This type of reaction can be useful in synthesis of natural products; an obvious example is the preparation
of an intermediate for the synthesis of nonactic acid by Beck and Henseleit [7], who used BF3 etherate as
Lewis acid.
Conclusions
In one case, NbCl was shown to be an effective catalyst for Diels-Alder reaction, resulting in higher
5
stereoselectivity as compared to AlCl . When less reactive dienes and/or b-substituted cyclohexenones were
3
used, however, no Diels-Alder adducts were obtained. Furan (7) and methylfuran (8) gave electrophilic
substitution products with cyclohexenone (1) or 3-methylcyclohexenone (5); b-hydroxy-cyclohexenone (2)
and b-acetoxycyclohexenone (3) reacted with NbCl to give a vinyl chloride or enol ether, depending on the
5
solvent used and the addition of cyclopentadiene had no significant effect on these results; b-
methylcyclohexenones 4 and 5 did not react, being recovered unchanged. Further studies can certainly
establish more clear limits to the use of these reactions, each one being a useful transformation for organic
synthesis.

Molecules 2000, 7
461
Acknowledgments
The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), the Conselho
Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and the Coordenadoria de Aperfeiçoamento
de Pessoal do Ensino Superior (CAPES) for financial support and Dr. Adilson Beatriz for helpful comments
and useful suggestions.
Experimental
General
NMR spectra were measured using a Bruker DPX-300 instrument (300 MHz for 1H-NMR and 75 MHz
1
for the 13C-NMR) or a Bruker DRX-400 (400 MHz for H-NMR and 100 MHz for the 13C-NMR);
deuterochloroform was used as solvent and tetramethylsilane as internal standard. GC-MS spectra were
obtained by EI ionization at 70 eV on a HP-5988-A spectrometer. IR spectra were measured with a Perkin-
Elmer 1600 FT spectrometer. TLC was performed on precoated silica gel 60 F254 (0.25 mm thick, Merck),
and for column chromatography silica gel 60, 70-230 mesh (Merck) was used. Analytical gas
chromatography (GLC) separations were performed on a Varian GC 3400 instrument with a fused silica
capillary column (30 m length x 0.25 mm i.d.) coated with DB 1701 (phase thickness 0.25 mm) operating at
temperatures in the range 50-200ºC.
Preparation of substrates
3-Acetoxy-2-cyclohexenone (3) [8]. A mixture of 1,3-cyclohexanedione (1.00 g, 8.93 mmol), acetic
anhydride (2 mL), pyridine (1.5 mL) and 4-dimethylaminopyridine (10 mg) was stirred for 2 hat room
temperature under nitrogen atmosphere. Ethyl ether (20 mL) was added and the organic layer was separated
and washed with 10% aqueous hydrochloric acid (3 x 10 mL), with saturated aqueous sodium bicarbonate (2
x 10 mL), saturated brine (2 x 10 mL), and dried over anhydrous magnesium sulfate. The solvent was
removed under vacuum. Yield of compound 3 as yellow oil: 0.99 g (72%); 1H-NMR (300 MHz): d 5.89 (t,
1H, J1= J2=1.1 Hz), 2.54 (td, 2H, J1 = 6.2 Hz, J2 =1.1 Hz), 2.39 (t, 2H, J1= J2=6.2 Hz), 2.22 (s, 3H), 2.05
(qt, 2H, J = 6.2 Hz); 13C-NMR (75 MHz): d 199.19 (C=O), 169.52 (C), 166.92 (C=O), 116.83 (CH),
36.16 (CH2), 27.78 (CH2), 20.72 (CH3), 20.72 (CH2); IR (film) nmax: 2963, 2887, 1773, 1675, 1641,
-1
1429, 1361, 1196, 1124 cm ; MS m/z (rel. intensity) (%): 112 [(M-42)+] (4), 84 (23), 69 (9), 55 (4), 43
(100), 42 (11), 39 (11), 27 (10).

Molecules 2000, 7
462
3-Methyl-2-cyclohexenone (5) [9] To a solution of ethyl acetoacetate (12.6 g, 96.9 mmol) and formaldehyde
40% (3.67 g, 48.9 mmol) was added drop by drop a solution of piperidine (0.3 mL) in ethanol (1.2 mL),
maintaining the temperature of the reaction below 30ºC by cooling with a water bath containing some ice.
Stirring was continued for 4 h and the mixture was then allowed to stand overnight. The upper water layer was
separated and the lower oily layer was washed with water (3 x 15 mL). The oily layer was mixed with 50 mL
of 15% aqueous sulfuric acid and refluxed for 11 h. The mixture was cooled and neutralized with ammonium
hydroxide. The crude product was extracted with ethyl ether (3 x 15 mL), dried over anhydrous magnesium
sulfate and the solvent was removed under vacuum. The product was distilled (Kugelrohr) at 74 ºC/1.5
1
mmHg to give 6.82 g (64%) of compound 5 as pale yellow oil: H-NMR (400 MHz): d 5.86 (s, 1H), 2.32
(m, 4H), 2.01 (m, 2H), 1.97 (s, 3H).; 13C-NMR (100 MHz): d 199.96 (C=O), 163.21 (C), 126.87 (CH),
37.89 (CH2), 31.22 (CH2), 24.74 (CH3), 22.85 (CH2); IR (film) nmax: 3039, 2963, 2872, 1677, 1629,
-1
1429, 1383, 1255, 1196, 1022 cm ; MS m/z (rel. intensity) (%): 110 [M+] (5), 82 (89), 67 (8), 54 (32), 41
(10), 39 (40), 32 (25), 28 (100).
General Procedure for the Reactions of Enones with NbCl5.
To a solution of NbCl (0.135 g, 0.5 mmol) in 1.0 mL of anhydrous solvent (ethyl acetate, ethyl ether or
5
dichloromethane), maintained at room temperature or -78ºC under nitrogen atmosphere, was added a solution
of the enone (1.0 mmol) in 1.0 mL of the anhydrous solvent; in cases where a diene (cyclopentadiene or
furans) was added, this solution contained also 2 mmol of diene. The reaction mixture was quenched with a
10% aqueous citric acid solution (2.0 mL, when working at room temperature) or with a 1:1 solution of
water/THF (2.0 mL, when working at -78ºC). The mixture was diluted with water (5 mL) and solvent (10
mL), the organic layer was separated and washed with 5% aqueous sodium bicarbonate (3 x 10 mL),
saturated brine (2 x 10 mL), and dried over anhydrous magnesium sulfate. The solvent was removed under
vacuum and the products were purified by column chromatography through silica gel using a mixture of hexane
and ethyl acetate (8:2) as eluent to give the products as yellow oils.
Spectral Data
1
2,7
Tricyclo[6.2.1.0 ]undec-9-en-3-one (9): H-NMR (300 MHz): d 6.19 (dd, 1H, J =5.7 and J2=2.9 Hz),
1
6.01 (dd, 1H, J1=5.7 and J2=2.9 Hz), 3.26 (br. s, 1H), 2.88 (br. s, 1H), 2.73 (dd, 1H, J1=10.2 and J2=3.6
Hz), 2.67 (m, 1H), 2.32 (dddd, 1H, J1=18.4, J2=6.0, J3=2.5 and J4=1.9 Hz), 2.03-1.92 (m, 1H), 1.93 (ddd,
1H, J1=18.4, J2=12.0 and J3=6.9 Hz), 1.85-1.62 (m, 2H), 1.45 (dt, 1H, J1=8.3 and J2=J3=1.9 Hz), 1.31 (dt,
1H, J1=8.3 and J2=J3=1.5 Hz), 0.77 (ddt, 1H, J1=J2=12.5, J3=10.6 and J4=3.5 Hz); 13C-NMR (75 MHz): d
215.47 (C=O), 137.62 (CH), 134.91 (CH), 51.63 (CH), 48.32 (CH2), 46.52 (CH), 45.19 (CH), 41.38
(CH), 39.36 (CH2), 27.97 (CH2), 21.78 (CH2); IR (film) nmax: 3060, 2932, 2857, 1696, 1607, 1455, 1337,

Molecules 2000, 7
463
-1
1236, 1072 cm ; MS m/z (rel. intensity) (%):162 [M+] (2), 121 (2), 97 (39), 91 (19), 79 (15), 66 (100), 43
(8), 41 (16).
3-Chloro-2-cyclohexenone (10): 1H-NMR (300 MHz): d 6.22 (t, 1H, J=1.5 Hz), 2.69 (td, 2H, J1=J2= 6.5
13
and J =1.5 Hz), 2.40 (t, 2H, J=6.5 Hz), 2.09 (q, 2H, J=6.5 Hz); C-NMR (75 MHz): d 196.85 (C=O),
3
158.56 (C), 128.44 (CH), 36.29 (CH2), 33.86 (CH2), 22.17 (CH2); IR (film) nmax: 3041, 2948, 2887,
-1
1679, 1607, 1455, 1425, 1344, 1289, 1234, 1137, 1026 cm ; MS m/z (rel. intensity) (%):132 [(M+2)+]
(10), 130 (34), 104 (32), 102 (100), 67 (75), 65 (9), 39 (43), 28 (11).
3-Ethoxy-2-cyclohexenone (11): 1H-NMR (300 MHz): d 5.34 (s, 1H), 3.91 (q, 2H, J=7.0 Hz), 2.41 (t,
2H, J=6.4 Hz), 2.33 (t, 2H, J=6.4 Hz), 1.98 (q, 2H, J=6.4 Hz), 1.37 (t, 3H, J=7.0 Hz), 13C-NMR (75
MHz): d 199.69 (C=O), 177.92 (C), 102.67 (CH), 65.18 (CH2), 36.78 (CH2), 29.11 (CH2), 22.29 (CH2),
-1
14.14 (CH3); IR (film) nmax: 3039, 2948, 2887, 1646, 1599, 1465, 1221, 1137, 1026 cm ; MS m/z (rel.
intensity) (%): 112 [(M-28)+] (28), 84 (81), 69 (23), 55 (35), 42 (100), 39 (37), 27 (37), 15 (9).
1
3-[5-(3-Oxo-cyclohexyl)-furan-2-yl]-cyclohexanone (12): H-NMR (300 MHz): d 5.91 (s, 2H), 3.15
(ddt, 2H, J =10.5, J = 8.6 and J =J4= 4.8 Hz), 2.66 (ddt, 2H, J =14.2, J =4.8 and J =J4=0.9 Hz), 2.49
1
2
3
1
2
3
(ddd, 2H; J=14.2, J=10.5 and J =0.9 Hz), 2,35 (m, 4H), 2.10 (m, 4H), 1.82 (m, 4H); 13C-NMR (75
1
2
3
MHz): d 210.11 (C=O), 155.81 (C), 104.80 (CH), 45.51 (CH2), 41.16 (CH2), 37.49 (CH), 29.77 (CH2),
24.26 (CH3); IR (film) nmax: 3041, 3037, 2948, 2872, 1709, 1599, 1557, 1455, 1421, 1221, 1175, 1099,
-1
1018 cm ; MS m/z (rel. intensity) (%): 260 [M+] (2), 203 (3), 55 (3), 42 (4), 41 (3), 32 (20), 28 (100), 27
(2).
(±
)-3-(5-Methyl-furan-2-yl)-cyclohexanone (13): 1H-NMR (400 MHz): d 5.88 (d, 1H, J=3.3 Hz), 5.85 (d,
1H, J=3.3 Hz), 3.12 (tt, 1H, J=J2=10.0 and J=J4=4.5 Hz,), 2.66 (ddt, 1H, J = 14.5, J = 4.5 and
1
3
1
2
J3=J4=1.6 Hz), 2.51 (ddd, 1H, J1=14.5, J2=10,0 and J3=1.3 Hz), 2.37 (m, 2H), 2.25 (s, 3H), 2.15 (m, 1H),
2.01 (m, 1H), 1.82 (m, 2H); 13C-NMR (100 MHz): d 210.41 (C=O), 155.38 (C), 150.39 (C), 105.80
(CH), 105.05 (CH), 45.75 (CH2), 41.27 (CH2), 37.65 (CH), 30.04 (CH2), 24.41 (CH2), 13.47 (CH3); IR
-1
(film) nmax: 3115, 2948, 2872, 1713, 1616, 1569, 1446, 1319, 1221, 1022 cm ; MS m/z (rel. intensity) (%):
178 [M+] (6), 121 (15), 108 (10), 77 (4), 55 (6), 42 (13), 32 (28), 28 (100).
(±
)-3-Methyl-3-(5-methyl-furan-2-yl)-cyclohexanone (14): 1H-NMR (400 MHz): d 5.86 (d, 1H, J =3.0
Hz), 5.81 (d, 1H, J=3.0 Hz), 2.71 (dt, 1H, J1=14.0 and J2=J3=1.5 Hz), 2.31 (m, 3H), 2.23 (s, 3H), 2.20 (m,
13
1H), 1.86 (m, 1H), 1.73 (m, 1H), 1.64 (m, 1H), 1.30 (m, 3H); C-NMR (100 MHz): d 210.79 (C=O),
157.89 (C), 150.79 (C), 105.68 (CH), 105.64 (CH), 51.88 (CH2), 40.71 (CH2), 40.44 (C), 35.91 (CH2),
27.18 (CH3), 22.06 (CH2), 13.47 (CH3); IR (film) nmax: 3115, 2948, 2857, 1709, 1607, 1557, 1455, 1349,
-1
1221, 1115, 1022 cm ; MS m/z (rel. intensity) (%): 192 [M+] (4), 149 (4), 135 (10), 122 (8), 98 (3), 77
(4), 55 (35), 28 (100).

Molecules 2000, 7
464
1
(±
)-2-[3,3-Bis-(5-methyl-(2-furan-2-yl)-cyclohexyl]-5-methyl-furan (15): H-NMR (400 MHz): d 6.11
(d, 1H; J=3.0 Hz), 5.90 (d, 1H, J=3.0 Hz), 5.84 (d, 1H, J=3.3 Hz), 5.81 (d, 1H, J=3.3 Hz), 5.76 (d, 1H,
J=3.0 Hz), 5.64 (d, 1H, J=3.0 Hz), 2.81 (tt, 1H, J1=J2=12.0 and J3=J4=3.3 Hz), 2.74 (m, 1H), 2.48 (m, 1H),
2.23 (s, 3H), 2.21 (s, 3H), 2.80 (s, 3H), 1.95 (m, 1H), 1.92 (t, 1H, J=12.0 Hz), 1.84 (dd, 1H, J1=12.0 and
J2=3.5 Hz), 1.73 (m, 1H), 1.55 (ddt, 1H, J=25.0, J =14.0 and J =J4=3.5 Hz), 1.37 (dd, 1H, J =25.0,
1
2
3
1
13
J2=12.5 Hz), C-NMR (100 MHz): d 159.88 (C), 159.02 (C), 155.06 (C), 151.07 (C), 150.70 (C),
150.43 (C), 108.06 (CH), 106.60 (CH), 106.23 (CH), 106.14 (CH), 104.66 (CH), 103.96 (CH), 42.40
(C), 39.12 (CH2), 33.85 (CH2), 33.81 (CH), 31.69 (CH2), 22.70 (CH2), 14.10 (CH3), 14.04 (CH3), 13.97
-1
(CH3); IR (film) nmax: 3115, 2948, 2872, 1612, 1557, 1450, 1378, 1217, 1026 cm ; MS m/z (rel. intensity)
(%): 242 [(M-82)+] (8), 199 (7), 188 (4), 108 (5), 95 (5), 61 (4), 43 (53), 28 (100).
(±
)-2-[1,3-Bis(5-methyl-(2-furan-2-yl))-3-methyl-cyclohexyl]-5-methyl-furan (16): 1H-NMR (400 MHz):
d 5.91 (d, 1H; J=3.0 Hz), 5.87 (d, 1H, J=3.0 Hz), 5.75 (s, 2H), 5.72 (d, 1H, J=3.0 Hz), 5.62 (d, 1H, J=3.0
Hz), 2.47 (d, 1H, J=13.5 Hz), 2.42 (d, 1H, J=13.5), 2.25 (s, 3H), 2.23 (s, 3H), 2.23 (m, 1H), 2.17 (s, 3H),
2.02 (m, 1H), 1.83 (m, 3H), 1.59 (m, 1H), 0.96 (s, 3H); 13C-NMR (100 MHz): d 162.25 (C), 158.26 (C),
156.66 (C), 150.09 (C), 150.00 (C), 149.53 (C), 106.08 (CH), 105.63 (CH), 105.51 (CH), 105.35 (CH),
104.78 (CH), 102.64 (CH), 42.50 (CH2), 40.54 (C), 36.00 (CH2), 35.63 (C), 33.45 (CH2), 25.32 (CH3),
19.07 (CH2), 13.63 (CH3), 13.57 (CH3), 13.55 (CH3); IR (film) nmax: 3100, 2948, 2842, 1612, 1561,
-1
1450, 1319, 1221, 1022 cm ; MS m/z (rel. intensity) (%): 338 [(M)+] (2), 188 (5), 175 (5), 135 (6), 122
(6), 95 (4), 91 (4), 28 (100).
References and Notes
1. Constantino, M. G.; Lacerda Jr., V.; Aragão, V. Molecules 2001, 6, 770-776.
2. Constantino, M. G.; Beatriz, A.; da Silva, G. V. J. Tetrahedron Lett. 2000, 41, 7001-7004
3. Constantino, M. G.; Beatriz, A.; da Silva, G. V. J.; Zukerman-Schpector, J. Synth. Comm. 2001, 31,
3329-3336.
4. Angell, E. C.; Fringuelli, F.; Guo, M.; Minuti, L.; Taticchi, A.; Wenkert, E. J. Org. Chem. 1988, 53,
4325-4328.
5. Howarth, J.; Gillespie, K. Tetrahedron Lett. 1996, 37, 6011-6012.
6. Howarth, J.; Gillespie, K. Molecules 2000, 5, 993-997.
7. Beck, G.; Henseleit, E. Chem. Ber. 1971, 104, 21-29.
8. Hofle, G.; Steglich, W.; Vorbrugzen, H. Angew. Chem. 1978, 17, 569-574.
9. Natelson, S.; Gottfried, S. P. J. Am. Chem. Soc. 1939, 61, 1001-1002.
Sample Availability: Samples of compounds 1-16 are available from MDPI.
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