Diastereoselective protonation on a radical anion in the photoallylation and photoreduction of 1,1-dicyano-2-methyl-3-phenyl-1-butene by allyltrimethylsilane

Article abstract of DOI:10.1007/s11164-010-0165-y

Diastereoselectivity in the photoallylation and photoreduction of 1,1-dicyano-2-methyl-3-phenyl-1-butene by allyltrimethylsilane in the presence of phenanthrene was dependent on the structures and stoichiometry of the added carboxylic acids. Diastereoselectivity increased up to 72% by the addition of equimolar amount of L-lactic acid based on the alkene. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11164-010-0165-y

Res Chem Intermed (2010) 36:577–585
DOI 10.1007/s11164-010-0165-y
Diastereoselective protonation on a radical anion
in the photoallylation and photoreduction
of 1,1-dicyano-2-methyl-3-phenyl-1-butene
by allyltrimethylsilane
•
Hajime Maeda Nana Nishitsuji Kazuhiko Mizuno
•
Received: 29 December 2009 / Accepted: 9 April 2010 / Published online: 26 August 2010
ꢀ Springer Science+Business Media B.V. 2010
Abstract Diastereoselectivity in the photoallylation and photoreduction of
1,1-dicyano-2-methyl-3-phenyl-1-butene by allyltrimethylsilane in the presence
of phenanthrene was dependent on the structures and stoichiometry of the added
carboxylic acids. Diastereoselectivity increased up to 72% by the addition of equi-
molar amount of L-lactic acid based on the alkene.
Keywords Photoreaction Á Photochemistry Á Electron transfer Á
Diastereoselectivity Á Allylation Á Reduction
Introduction
Photoinduced electron transfer (PET) reaction between organosilicon compounds
and electron-deficient molecules is a powerful tool for organic synthesis [1–11]. In
these photoreactions, radical anions generated from the electron-deficient molecules
and carbon radicals generated from radical cations of the organosilicon compounds
are postulated as reactive intermediates. However, control of stereoselectivity in the
PET reaction has been less discussed so far [12–15]. We have previously reported
that PET reaction of cycloalkylidenepropanedinitriles with allylic silanes afforded
allylated products at a-carbon of cyano groups and reduction products in a highly
diastereoselective manner [16]. Here, we describe the diastereoselectivity in
H. Maeda (&)
Division of Material Sciences, Graduate School of Natural Science and Technology,
Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan
e-mail: maeda-h@t.kanazawa-u.ac.jp
N. Nishitsuji Á K. Mizuno
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
123

578
H. Maeda et al.
CN
CN
NC CN
hν (>280 nm)
SiMe3
+
Ph
Ph
Phen / Additive
CH3CN, rt, 2 h
2
1
anti-3
CN
CN
CN
NC CN
+
+
+
Ph
Ph
CN
Ph
syn-3
syn-4
anti-4
Scheme 1 Photoallylation and photoreduction of 1
photoallylation and photoreduction of an acyclic electron-deficient alkene by use of
allyltrimethylsilane.
Results and discussion
Photoirradiation of an acetonitrile-acetic acid solution [CH3CN:AcOH = 6:1
(wt%)] containing 1,1-dicyano-2-methyl-3-phenyl-1-butene (1) and an excess of
allyltrimethylsilane (2) in the presence of phenanthrene (Phen) as a photosensitizer
by a 300-W high-pressure mercury lamp through a Pyrex filter ([280 nm) for 2 h
afforded allylation product 3 and reduction product 4 in 34% yield (25% de) and
19% yield (30% de), respectively. In the absence of acetic acid, the photoreaction
slowly proceeded to give a complex mixture containing 3 and 4.1 In these
photoreactions, Phen was recovered almost quantitatively. The relative stereo-
chemistry and anti/syn ratio of 3 and 4 were determined by the comparison with
authentic samples2 [17]. Both the major isomers of 3 and 4 were determined to be
anti ones (Scheme 1).
Dependency of diastereoselectivity on structures and stoichiometry of carboxylic
acids added instead of acetic acid was investigated (Table 1). In all entries of
Table 1, anti-3 and anti-4 were the major products, and the diastereoselectivity of 4
was higher than that of 3. In order to obtain higher de, more bulky carboxylic acids
were employed and the stoichimetry of the carboxylic acids was reduced to one
equivalent based on 1 (entries 2–4). As a result, the bulkiness and reduced
stoichiometry of the carboxylic acid made de of products increased, and an
especially good correlation was observed in de of compound 4 (entries 1–4). The de
also increased in the presence of carboxylic acids with additional hydroxy group(s)
(entries 5–8). Protection of hydroxy groups of L-(?)-tartaric acid by benzoyl groups
was not effective, but that of the amino group of L-phenylalanine by the t-Boc group
was effective for the diastereodifferentiation (entries 9–11). The best result was
obtained by the addition of 1 equiv of L-lactic acid based on 1 to give 4 in 72% de
1
In the presence of MeOH or H2O instead of AcOH as a proton source, the reaction proceeded slowly to
give a complex mixture.
2
The hydrogenation of compound 1 by Pd/C has been reported to proceed in a syn selective manner [19].
We prepared syn-rich mixtures of 3 and 4 by the hydrogenation of 1 by Pd/C and allylation of the syn-rich
mixture of 4 by NaH and allyl bromide.
123

Diastereoselective protonation on a radical anion
579
Yieldsb / % (deb / %)
Table 1 Diastereoselective
photoallylation and
photoreduction of 1
Entry
Additive
3
4
1c
34d (25)
18 (37)
18 (21)
MeCOOH
i-PrCOOH
t-BuCOOH
19 (30)
74 (48)
70 (59)
2
3
4e
5f
25 (36)
55 (65)
25 (63)
50 (43)
COOH
COOH
30 (46)
38 (38)
HO
HOOC
COOH
6g
HOOC OH
OH
Conditions: 1 (0.14 mmol),
2 (0.42 mmol), additive
(0.14 mmol), phenanthrene
(0.07 mmol), CH3CN (8 mL),
high pressure mercury
7
66 (51)
17 (27)
HOOC
HO
COOH
OH
8h
27 (41)
32 (40)
51 (52)
8 (46)
lamp, Pyrex, r.t., 2 h
a
Determined by GC
HOOC
COOH
b
PhCOO
OCOPh
CH3CN:MeCOOH = 6:1
9i
(wt%)
c
HOOC
Ph
COOH
Determined by 1H NMR
COOH
d
10
26 (37)
36 (50)
30 (50)
37 (54)
In MeOH:CH3CN = 1:99
NH2
(vol%)
e
COOH
CH3CN: 50% aqueous solution
of HOCH2CH2COOH = 30:1
Ph
11
12
13
14
NHBoc
OH
(wt%)
f
37d (43)
37d (33)
25d (35)
36d (72)
45d (51)
54d (44)
In MeOH:CH3CN = 11:89
COOH
(vol%)
g
In MeOH:CH3CN = 5:95
OH
Ph
Ph
COOH
(vol%)
h
OH
COOH
CH3CN:Dibenzoyl-L-tartaric
acid = 60:1 (wt%)
(entry 12). The de decreased by increasing the stoichiometry of the carboxylic acid;
for example, when 16 equiv of L-lactic acid based on 1 was added, the de of 3 and 4
decreased to 31 and 40%, respectively.
The photoreaction might proceed via the Phen-sensitized electron transfer
mechanism (redox photosensitization) as shown in Scheme 2 [11]. The primary step
is the generation of radical cation of Phen (Phen?•) and the radical anion of 1 (1-•
)
through a one-electron transfer from the excited singlet state of Phen (1Phen*) to 1.
The secondary electron transfer from 2 to Phen?• produces the radical cation of 2
(2?•), which gives the allyl radical via the nucleophile-assisted cleavage of the C–Si
bond [18–20]. In the presence of a proton source, the protonation at the b-position to
the cyano groups of 1-• predominantly takes place to generate the radical 5. The
radical coupling of 5 with the allyl radical gives 3. The formation of 4 was
explained by hydrogen abstraction of 5, or an electron transfer from 1-• to 5
followed by protonation, or disproportionation of 5 to give 1 and 4. We have
previously reported pKa-dependency of the photoallylation and photoreduction of
123

580
H. Maeda et al.
hν
2
+
2
Phen
Phen
Nu-
1
1
Phen
1
(Nu- ; nucleophile)
SiMe3
+
NuSiMe3
2
CN
Me
NC
H
NC
Ph
CN
Ph
H+
H+
Me
Me
Me
H
1 (unfavor)
1 (favor)
NC
H
CN
Me
CN
Ph
Me
anti-5 (major)
NC
Ph
Me
syn-5ꢀ(minor)
H
H
Me
H
H • or
H • or
e- / H+ or
dispropor-
tionation
e- / H+ or
dispropor-
tionation
anti-3
anti-4
syn-3
syn-4
Scheme 2 Reaction mechanism for the photoallylation and photoreduction of 1
cyclohexylidenepropanedinitriles with 2 [10]. In the presence of a relatively strong
acid such as carboxylic acids, radical anion is protonated at their b-positions, and
the ratios of pathways of hydrogen abstraction from b-position of the radical anion
followed by protonation at a-position and hydride transfer from the radical anion
increase with decreasing acidity of the additives. Consequently, in the presence of
carboxylic acids used in this study, the mechanism involving protonation of 1-• at
its b-position shown in Scheme 2 should be the predominant pathway.3
The diastereoselectivity can be explained by the Felkin–Anh-type model where
protonation mainly occurs via favored conformation in which the methyl group
locates an anticlinal position against the dicyanomethylene moiety of the radical
anion 1-• [21, 22]. When dependency of heat of formations of 1-• on the dihedral
angles around the Cb–Cc bond of 1-• was calculated by molecular orbital calculation
whose basis set was UHF/3-21G, two local minima were found whose structures
3
The product ratio of allylation and reduction (3/4) listed in Table 1 are lower than the previous report
using cyclohexylidenepropanedinitriles [10, 16]. The difference in the product ratio between previous and
current studies may be due to the increased steric hindrance to protonation, and also due to the decrease of
stoichiometry of carboxylic acids added in order to increase de.
123

Diastereoselective protonation on a radical anion
581
NC
H
CN
C2
Cγ
CN
Cα
Ph
Cβ
C1
CN
Ph
C
2 C1
1
θ
56
55
54
53
52
51
50
49
48
0
50
100
150
200
250
300
350
dihedral angle
θ (°)
Fig. 1 Heat of formations depending on dihedral angles h defined as C1–Cb–Cc–C2 in radical anion of 1
(1•-) calculated with UHF/3-21G
correspond to the favorable and unfavorable conformations predicted by the Felkin–
Anh rule (Fig. 1). Diastereoselectivity should appear in the protonation step of the
radical anion 1-•. Namely, the difference of activation energies between protonation
on Si-face and that of Re-face by carboxylic acids determines the de, and the
difference strongly depends on the bulkiness of the carboxylic acids. In addition, since
the hydroxy group at a- and b-positions of carboxylic acids showed a tendency to
increase de, it is conceivable that interaction of the hydroxy group with a cyano group
of the radical anion 1-• in the protonation step and/or restriction of conformational
change of the carboxylic acid by intramolecular hydrogen bonding [23] are involved.
The de of 4 obtained was higher than that of 3 in all entries, although they were
generated from the same intermediate. This phenomena may suggest the existence of
a disproportionation pathway. If anti-5 abstracts hydrogen from another molecule of 5
faster than does syn-5, or the hydrogen on syn-5 is abstracted faster than hydrogen
does on anti-5, anti-4 accumulates to increase de of 4.
Conclusion
In conclusion, the diastereoselective photoallylation and photoreduction of an
acyclic alkene 1 by 2 in the presence of Phen and carboxylic acids effectively took
place. Diastereoselectivity of the photoreaction was changed depending on the
structure and stoichiometry of the carboxyic acids. Especially, the bulkiness,
hydroxy group, and equimolar addition of carboxylic acids are important to obtain
123

582
H. Maeda et al.
high de. From anti-configuration of products and structure of radical anion of 1,
obtained from molecular orbital calculation, it is concluded that the diastereose-
lectivity appears in the protonation step of the radical anion, whose favorable
structure can be explained by a Felkin–Anh-type model.
Experimental
General
Benzene was distilled from CaH2 and then from Na. Dichloromethane was distilled
from CaCl2 and then CaH2. Acetonitrile was distilled from CaH2 and then from
P2O5. THF was distilled from sodium benzophenone ketyl. Phenanthrene was
recrystallized from hexane. Other chemical sources were used after purification by
distillation or recrystallization.
A 300-W high-pressure mercury lamp (Eikosha, PIH-300) was used as a light
source of the photoreactions. 1H NMR spectra were recorded on a Varian
MERCURY-300 (300 MHz) spectrometer using Me4Si as an internal standard.
Mass spectra (EI) were taken on a SHIMADZU GCMS-QP5050 operating in the
electron impact mode (70 eV) equipped with GC-17A and DB-5MS column (J&W
Scientific; Serial: 8696181). Settings of GC are as follows: INJ = 200 ꢁC,
COL = 80 ꢁC, 0 min, then 2 ꢁC/min to 250 ꢁC, total 120 min. HPLC separations
were performed on a recycling preparative HPLC equipped with Jasco PU-2086
Plus, RI-2031 Plus differential refractometer, Megapak GEL 201C columns (GPC)
using CHCl3 as an eluent. Column chromatography was conducted by using Kanto-
Chemical., silica gel 60 N (spherical, neutral, 0.063–0.200 mm). Geometrical
information of radical anions was obtained from theoretical calculations using
Spartan’04 for Windows with a basis set of UHF/3-21G.
Preparation of 1,1-dicyano-2-methyl-3-phenyl-1-butene (1) [24, 25]
A suspension of AlCl3 (4.17 g, 31.3 mmol) in CH2Cl2 (25 mL) was cooled to
-50 ꢁC. Phenylacetyl chloride (4.84 g, 31.3 mmol) in CH2Cl2 was added dropwise
with stirring over a 15-min period under Ar atmosphere. After stirring for 1 h,
Me3Al (15% solution in hexane, 9.1 mL, 12.5 mmol) in CH2Cl2 (12 mL) was added
dropwise at –30 ꢁC over a 1-h period. The solution was stirred for 2 h at room
temperature. After cooling to 0 ꢁC, H2O was slowly added until evolution of gas
ceased. Organic layer was separated. Aqueous phase was washed with ether
(50 mL 9 2). Combined organic layer was washed with 5% NaHCO3 aqueous
solution (100 mL 9 3). The organic layer was dried over Na2SO4. Filtration and
evaporation of the filtrate gave benzyl methyl ketone (3.022 g, 72% yield). Data for
1
benzyl methyl ketone: H NMR (300 MHz, CDCl3) d 2.15 (s, 3 H), 3.70 (s, 2 H),
7.18–7.34 (m, 5 H) ppm.
A mixture of benzyl methyl ketone (2.01 g, 15 mmol), 50% NaOH aqueous
solution (30 mL), and benzyltrimethylammonium chloride (0.12 g, 0.64 mmol) was
stirred vigorously by a mechanical stirrer. Methyl iodide (3.21 g, 22.5 mmol) was
123

Diastereoselective protonation on a radical anion
583
added slowly with continued vigorous stirring. The temperature of the solution was
kept around room temperature by an ice bath. After stirring for 1 h, H2O (15 mL)
and benzene (20 mL) were added. The organic layer was separated, and was washed
with H2O until its pH became neutral. The organic layer was dried over Na2SO4,
filtrated, and evaporated. The residue was purified by column chromatography on
silica gel (eluent; hexane:ethyl acetate = 2:1) to give a crude mixture containing
1
3-phenyl-2-butanone (2.32 g). Data for 3-phenyl-2-butanone: H NMR (300 MHz,
CDCl3) d 1.39 (d, J = 7.0 Hz, 3 H), 2.05 (s, 3 H), 3.74 (q, J = 7.0 Hz, 1 H),
7.19–7.37 (m, 5 H) ppm.
A part of 3-phenyl-2-butanone with impurity (0.74 g, molar content [90%, ca.
5 mmol) and malononitrile (0.33 g, 5 mmol), and activated Al2O3 (1.50 g) were
placed in a flask. The mixture was stirred for 1 h, and then filtrated by adding ether
(50 mL). The residue after filtration was washed with additional ether (50 mL). The
filtrate was dried over Na2SO4. Filtration and evaporation gave a crude mixture.
Purification by column chromatography on silica gel (eluent; hexane:ethyl
acetate = 3:1) followed by HPLC (GPC, CHCl3) gave of 1,1-dicyano-2-methyl-
3-phenyl-1-butene (1, 0.349 g, 36% yield). Data for 1: 1H NMR (300 MHz, CDCl3)
d 1.58 (d, J = 6.8 Hz, 3 H), 2.06 (s, 3 H), 4.50 (q, J = 7.1 Hz, 1 H), 7.23–7.41 (m,
5 H) ppm.
Preparation of Allyltrimethylsilane (2)
To a stirred mixture of Mg (14.58 g, 0.60 mol) and THF (200 mL), a solution of
Me3SiCl (64 mL, 0.50 mol) and allyl chloride (34 mL, 0.47 mol) in THF (100 mL)
was added slowly at 0 ꢁC under Ar atmosphere. Then, the mixture was stirred
overnight under reflux. Unreacted allyl chloride and THF (60 mL) were removed by
distillation. The mixture was cooled to 0 ꢁC, then saturated NH4Cl aqueous solution
(20 mL) was carefully added below 10 ꢁC to separate two phases. The organic phase
was decanted, and washed with H2O several times to remove THF, and then dried
over Na2SO4. Rectification gave allyltrimethylsilane (2, bp 80–83 ꢁC, 12.0 g, 22%
1
yield). Data for 2: H NMR (300 MHz, CDCl3) d 0.00 (s, 9 H), 1.51 (dt, J = 8.1,
1.1 Hz, 2 H), 4.79–4.87 (m, 2 H), 5.79 (ddt, J = 16.7, 10.4, 8.1 Hz, 1 H) ppm.
General procedure for the photoreaction
A CH3CN (8 mL) solution containing 1,1-dicyano-2-methyl-3-phenyl-1-butene
(1, 27.5 mg, 0.14 mmol), allyltrimethylsilane (2, 66.8 lL, 0.42 mmol), phenan-
threne (Phen, 12.5 mg, 0.07 mmol), and acids listed in Table 1 (0.14 mmol unless
otherwise noted) was placed in a cylindrical Pyrex vessel (/ = 8 mm). Minimum
amount of MeOH was added to dissolve the carboxylic acids in entries 4, 6, and 8.
Then, the solution was degassed by Ar bubbling for 5 min. The solution was
photoirradiated by a 300-W high pressure mercury lamp at room temperature for
2 h. The temperature of the solution was kept around room temperature by
circulated cooling water during irradiation. Yields and diastereomeric excesses
123

584
H. Maeda et al.
(de) of 4,4-dicyano-5-methyl-6-phenyl-1-heptene (3) and 1,1-dicyano-2-methyl-3-
phenylbutane (4) were determined by GC and 1H NMR analysis (Table 1). Relative
experimental errors are within 1% (GC) and 5% (1H NMR), respectively. Separation
of syn-4 and anti-4 was performed by recycling preparative HPLC (GPC, CHCl3).
Data for the mixture of anti-3 and syn-3: 1H NMR (300 MHz, CDCl3) d 1.10–1.17
(m, 3 H), 1.40–1.49 (m, 3 H), 2.19–2.30 (m, 1 H), 2.52–2.80 (m, 2 H), 3.27–3.40 (m,
1 H), 5.33–5.46 (m, 2 H), 5.79–5.98 (m, 1 H), 7.10–7.40 (m, 5 H) ppm. Data for
anti-3: MS (EI) m/z (relative intensity, %) = 77 (73), 105 (100), 238 (6, M?);
retention time of GC = 38.6 min. Data for syn-3: MS (EI) m/z (relative intensity,
%) = 77 (79), 105 (100), 238 (9, M?); retention time of GC = 38.3 min. Data for
anti-4: 1H NMR (300 MHz, CDCl3) d 1.13 (d, J = 6.9 Hz, 3 H), 1.38
(d, J = 7.1 Hz, 3 H), 2.35 (m, J = 7.0 Hz, 1 H), 2.94 (quint, J = 7.3 Hz, 1 H),
3.72 (d, J = 5.9 Hz, 1 H), 7.16–7.37 (m, 5 H) ppm; MS (EI) m/z (relative intensity,
%) = 77 (14), 105 (100), 198 (6, M?); retention time of GC = 28.6 min. Data
for syn-4: 1H NMR (300 MHz, CDCl3) d 1.37 (d, J = 6.9 Hz, 3 H), 1.45
(d, J = 6.7 Hz, 3 H), 2.21–2.33 (m, 1 H), 2.65–2.78 (m, 1 H), 3.38 (d, J = 3.6 Hz,
1 H), 7.17–7.37 (m, 5 H) ppm; MS (EI) m/z (relative intensity, %) = 77 (21),
105 (100), 198 (5, M?); retention time of GC = 26.0 min.
Acknowledgments This work was partially supported by Grant-in-Aids for Scientific Research on
Priority Areas ‘‘Reaction Control of Dynamic Complexes (420)’’ (16033252), ‘‘Advanced Molecular
Transformations of Carbon Resources (444)’’ (18037063, 19020060), Scientific Research (B)
(15350026), and Young Scientists (B) (16750039), and the Cooperation for Innovative Technology
and Advanced Research in Evolutional Area (CITY AREA) program from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of Japan.
References
1. U.C. Yoon, P.S. Mariano, Acc. Chem. Res. 25, 233–240 (1992)
2. M.G. Steinmetz, Chem. Rev. 95, 1527–1588 (1995)
3. M. Kako, Y. Nakadaira, Coord. Chem. Rev. 176, 87–112 (1998)
4. K. Mizuno, T. Tamai, A. Sugimoto, H. Maeda, in Advances in Electron Transfer Chemistry, vol. 6,
ed. by P.S. Mariano (JAI Press, New York, 1999), pp. 131–165
5. T. Tamai, T. Hayamizu, K. Mizuno, in Handbook of Photochemistry and Photobiology, ed. by
H.S. Nalwa (American Scientific Publishers, North Lewis Way, 2003), pp. 125–156
6. K. Mizuno, S. Toda, Y. Otsuji, Chem. Lett. 203–204 (1987)
7. K. Mizuno, M. Ikeda, Y. Otsuji, Chem. Lett. 1507–1510 (1988)
8. K. Mizuno, K. Nakanishi, J. Chosa, T. Nguyen, Y. Otsuji, Tetrahedron Lett. 30, 3689–3692 (1989)
9. K. Mizuno, K. Nakanishi, J. Chosa, Y. Otsuji, J. Organomet. Chem. 473, 35–43 (1994)
10. T. Hayamizu, M. Ikeda, H. Maeda, K. Mizuno, Org. Lett. 3, 1277–1280 (2001)
11. K. Mizuno, T. Hayamizu, H. Maeda, Pure Appl. Chem. 75, 1049–1054 (2003)
12. J.-I. Kim, G.B. Schuster, J. Am. Chem. Soc. 114, 9309–9317 (1992)
13. T. Hayamizu, H. Maeda, M. Ikeda, K. Mizuno, Tetrahedron Lett. 42, 2361–2364 (2001)
14. S. Asaoka, T. Wada, Y. Inoue, J. Am. Chem. Soc. 125, 3008–3027 (2003)
¨
15. A. Bauer, F. Westkamper, S. Grimme, T. Bach, Nature 436, 1139–1140 (2005)
16. T. Hayamizu, H. Maeda, K. Mizuno, J. Org. Chem. 69, 4997–5004 (2004)
17. Y. Yamamoto, S. Nishii, T. Ibuka, J. Chem. Soc. Perkin 1. 1703–1705 (1989)
18. J.P. Dinnocenzo, S. Farid, J.L. Goodman, I.R. Gould, S.L. Mattes, W.P. Todd, J. Am. Chem. Soc.
111, 8973–8975 (1989)
19. K.P. Dockery, J.P. Dinnocenzo, S. Farid, J.L. Goodman, I.R. Gould, W.P. Todd, J. Am. Chem. Soc.
119, 1876–1883 (1997)
123

Diastereoselective protonation on a radical anion
585
20. H.J.P. de Lijser, D.W. Snelgrove, J.P. Dinnocenzo, J. Am. Chem. Soc. 123, 9698–9699 (2001)
21. Y. Yamamoto, S. Nishii, T. Ibuka, J. Am. Chem. Soc. 110, 617–618 (1988)
22. M.N. Paddon-Row, N.G. Rondan, K.N. Houk, J. Am. Chem. Soc. 104, 7162–7166 (1982)
´
23. C. Aleman, J. Casanovas, D. Zanuy, H.K. Hall Jr, J. Org. Chem. 70, 2950–2956 (2005)
24. M. Arisawa, Y. Torisawa, M. Nakagawa, Synthesis 1371–1372 (1995)
25. J. Sepiol, Synthesis 504–507 (1983)
123