Hf(OTf)4-doped Me3SiCl-catalyzed aminomethylation of arenes with N,O-acetals: Facile approach to non-natural aromatic amino acid precursors

Article abstract of DOI:10.1055/s-2007-991060

The authors demonstrated a Hf(OTf)₄-Me₃SiCl-system- catalyzed aminomethylation of an aromatic compound, such as a heterocycle or an electron-rich arene, with several new types of N,O-acetals having both a cyano group and a cyclic ami

Full text of DOI:10.1055/s-2007-991060

LETTER
2675
Hf(OTf)4-Doped Me3SiCl-Catalyzed Aminomethylation of Arenes with N,O-
Acetals: Facile Approach to Non-Natural Aromatic Amino Acid Precursors
F
acile Approach to
o
Non-Natura
r
l
Aroma
i
tic Am
o
ino Acid Precursor
S
s
akai,* Junichi Asano, Yuta Shimano, Takeo Konakahara
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science (RIKADAI),
Noda, Chiba 278-8510, Japan
E-mail: sakachem@rs.noda.tus.ac.jp
Received 7 August 2007
dolylglycine derivative with another type of N,O-acetal,
Abstract: The authors demonstrated a Hf(OTf)4–Me3SiCl-system-
such as hydroxyglycine.
catalyzed aminomethylation of an aromatic compound, such as a
heterocycle or an electron-rich arene, with several new types of
N,O-acetals having both a cyano group and a cyclic amino moiety.
This method permits the facile synthesis of artificial aromatic amino
acid precursors.
Initially, to find the best catalytic system, the amino-
methylation of 1-methylindole (1a) with N,O-acetal 2a
using a variety of Lewis acids in the presence of trimeth-
ylchlorosilane (Me3SiCl) as an additive was conducted in
CH2Cl2 as a model reaction.9 The results are listed in
Table 1. For example, when the reaction ran using a
typical Lewis acid, such as BF3·OEt2, AlCl3, ZnCl2, and
Cu(OTf)2, the desired product 3a was obtained in good
yield (entries 1–4). With the exception of using a stoi-
chiometric amount of BF3·OEt2, however, most cases
required a long reaction time (20–24 h) to complete
aminomethylation. Group IV metals also showed a similar
effect for the reaction (entries 5–8). In previous work it
was noted that when the reaction was conducted using 0.2
equivalent of Hf(OTf)4, the desired indole 3a was
obtained in nearly quantitative yield within five minutes
(entry 9). Even when the amount of the hafnium catalyst
was decreased to 0.05 equivalent for the acetal, the
catalyst showed high catalytic activity and the product 3a
was produced in good yield (entry 11). On the other hand,
the reaction without a Lewis acid did not afford the
desired product in practical yield (entry 12), and the
reaction without Me3SiCl did not form the product at all
(entry 13). Consequently, from the viewpoint of cost,
CH2Cl2 solution using 0.1 equivalent of Hf(OTf)4 in the
presence of Me3SiCl showed the best conditions for
aminomethylation.
Key words: N,O-acetal, aminomethylation, amino acid, hafnium
Since the Friedel–Crafts-type reaction is one of the most
efficient methods for carbon–carbon bond formation be-
tween an aromatic compound and a carbon electrophile, a
large number of approaches to establishing synthetic
methods have been developed by organic/medicinal
chemists.1 Among these, aminoalkylation is a powerful
tool used to directly introduce a nitrogen-containing func-
tional group, specifically an aminomethyl group, onto an
aromatic ring. However, most of the electrophilic sources
employed have been limited to an imine or an imino
ester,2,3 and the use of an acyclic N,O-acetal as an ami-
nomethyl group precursor has not been extensively exam-
ined.4 Hafnium-catalyzed aminomethylation of electron-
rich aromatic compounds with an N-silyl-N,O-acetal
leading to the production of aromatic primary amine
derivatives was previously reported.5,6 During ongoing
investigations of aminomethylation of aromatic com-
pounds, the preparation of a new class of an N,O-acetal
having both a cyano group and a cyclic amino moiety was
then attempted. In particular, a cyano group was convert-
ed into a formyl group and a carboxylic group with a sim-
ple operation.7 In addition, the nitrogen-containing rings,
such as piperidine, piperazine, and morpholine, are widely
found in the key structure of a number of natural products
and biologically active substances.8 Consequently, devel-
opment of aminomethylation of an arene with this class of
N,O-acetal would enable a facile approach to producing
an aromatic amino acid precursor, which would be ex-
pected to show potent biological activity. This report doc-
uments the results of Hf(OTf)4-doped Me3SiCl-catalyzed
aminomethylation of an electron-rich arene with an N,O-
acetal having a cyano group leading to the synthesis of an
aromatic amino acid precursor. This catalytic system also
permitted the facile preparation of an underivatized in-
To extend the general process of aminomethylation onto
an indole skeleton, reactions of indoles having several
substituent groups with three types of N,O-acetal 2a–c
were carried out under the optimized conditions. The re-
sults are summarized in Table 2. For example, when reac-
tions of indoles having an electron-donating group with
acetal 2a were examined using the hafnium catalyst, the
reactions reached consumption within 10 minutes to pro-
duce the 3-substituted amino acid precursors 3a–c in good
to excellent yields (entries 1–3). Employment of indoles
1d,e having an electron-withdrawing group, such as an
ester group and a halogen substituent, also underwent the
desired aminomethylation in good yield (entries 4 and 5).
Moreover, when introduction of an aminomethyl group
onto an indole ring by acetals 2b,c with N-phenyl-
piperazine or morpholine moiety was conducted, the
desired products were also obtained in excellent yields
SYNLETT 2007, No. 17, pp 2675–2678
1
6
.1
0
.
2
0
0
7
Advanced online publication: 25.09.2007
DOI: 10.1055/s-2007-991060; Art ID: U07407ST
© Georg Thieme Verlag Stuttgart · New York

2676
N. Sakai et al.
LETTER
Table 1 Examinations of Lewis Acids
Table 2 Aminomethylation of Indoles with N,O-Acetals
X
X
R1
Hf(OTf)4 (0.1 equiv)
Me3SiCl (1.2 equiv)
Lewis acid
Me3SiCl (1.2 equiv)
N
N
N
+
CN
CN
N
R2
CH2Cl2, r.t.
R1
+
N
MeO
CN
N
CH2Cl2, r.t.
Me
MeO
CN
1
2a: X = CH2
2b: X = O
2c: X = NPh
N
N
Me
R2
1a
2a
3a
3
Entry
Lewis acid (equiv)
BF3·OEt2 (1.0)
AlCl3 (0.2)
Time (h)
0.1
24
Yield (%)a
Entry R1
R2
Acetal Time (h) Yield (%)a
1
2
98 (84)b
86
1
2
3
4
5
6
7
8
9
H
H
Me
H
(1a)
2a
2a
2a
2a
2a
2b
2b
2c
2c
0.1
0.1
0.1
0.5
5
92 (3a)
94 (3b)
85 (3c)
89 (3d)
78 (3e)
79 (3f)
86 (3g)
85 (3h)
86 (3i)
(1b)
(1c)
(1d)
(1e)
(1a)
(1b)
(1a)
(1b)
3
ZnCl2 (0.2)
24
84
MeO
H
4
Cu(OTf)2 (0.2)
TiCl4 (0.2)
20
84
MeO2C
H
5
24
63
Br
H
H
H
H
H
6
Ti(Oi-Pr)4 (0.2)
ZrCl4 (0.2)
24
75
Me
H
0.5
0.3
0.5
0.1
7
24
73
8
HfCl4 (0.2)
24
82
Me
H
9
Hf(OTf)4 (0.2)
Hf(OTf)4 (0.1)
Hf(OTf)4 (0.05)
–
0.1
1
94
10
11
12
13c
97
a Isolated yield.
24
82
Table 3 Aminomethylation of Heterocycles with N,O-Acetals
24
39
X
X
Hf(OTf)4 (0.2)
24
trace
Hf(OTf)4 (0.1 equiv)
Me3SiCl (1.2 equiv)
a NMR yield.
N
N
b Yield using 0.2 equiv of BF3·OEt2 is in parentheses.
c Reaction was carried out without Me3SiCl.
+
R1
CH2Cl2, r.t.
Y
CN
MeO
CN
4a: R1 = H, Y = NH
4b: R1 = H, Y = NMe
4c: R1 = H, Y = NPh
5: R1 = Me, Y = O
Y
2a: X = CH2
2b: X = O
2c: X = NPh
R1
(entries 6–9). By contrast, when a similar reaction was
examined with the N,O-acetal containing an N-methyl-
piperazinyl group, the desired product did not form, and
the starting indole was recovered. The hafnium catalyst
seems to be coordinated on the tertiary amino group rather
than the activation of the methoxy substituent.
6, 7
Entry
Arene
4a
4b
4c
4a
4b
4a
4b
5
Acetal
2a
Time (h)
Yield (%)a
82 (6a)
74 (6b)
66 (6c)
85 (6d)
82 (6e)
82 (6f)
1
2
0.3
0.2
0.5
1
2a
Aminomethylations of other heterocycles 5, such as pyr-
role and furan, with several types of N,O-acetal 2 were
then investigated with the hafnium–Me3SiCl catalytic sys-
tem. The results are summarized in Table 3.10 Although
lowering the nucleophilicity in the pyrrole ring tends to
slightly decrease the product yield (entries 1–3), most
reactions using a pyrrole derivative produced the corre-
sponding amino acid precursors in good yields. When the
reaction using 2-methylfuran (5) was conducted under
similar conditions, all reactions were completed within
three hours to give the corresponding product in satisfac-
tory yields (entries 8–10). Interestingly, when the reaction
was carried out with the substrate involving a pyrrole and
furan moiety, the desired reaction proceeded with high
regioselectivity to yield only the amino acid precursor 8,
where the aminomethyl group was regioselectively intro-
duced onto the pyrrole skeleton. Moreover, amino-
methylation of acetal 2 with an electron-rich arene, such
3
2a
4
2b
2b
2c
5
1
6
1
7
2c
1
94 (6g)
86 (7a)
63 (7b)
83 (7c)
8
2a
0.5
3
9
5
2b
2c
10
5
3
a Isolated yield.
as N,N-dimethylaniline, under optimized conditions also
yielded the aminomethylated product 9 (Figure 1).
Synlett 2007, No. 17, 2675–2678 © Thieme Stuttgart · New York

LETTER
Facile Approach to Non-Natural Aromatic Amino Acid Precursors
2677
In conclusion, it has been demonstrated that the Hf(OTf)4-
doped Me3SiCl system catalyzes the aminomethylation of
an aromatic compound, such as a heterocycle or an elec-
tron-rich arene, with a new class of N,O-acetal having a
cyano group and leads to the production of an amino acid
precursor. This catalytic system was also found to be ef-
fective in the one-step preparation of an indolylglycine
derivative.
X
X
N
N
N
CN
CN
O
Me2N
4 h, 85%
5 h, 77%
18 h, 38%
15 h, 44%
8a: X = CH2
8b: X = NPh
9a: X = CH2
9b: X = NPh
Figure 1
Acknowledgment
This work was partially supported by a fund for the ‘High-Tech Re-
search Center’ Project for Private Universities: a matching fund
subsidy from MEXT, 2000–2004, and 2005–2007. N.S. acknowled-
ges the TORAY Award in Synthetic Organic Chemistry, Japan, for
partial financial support.
Finally, to illustrate the utility of the Hf–Me3SiCl catalytic
system for an N,O-acetal, the aminomethylation of N-
methylindole 3a with another type of the N,O-acetal
(hydroxyglycine)11 10 was examined under optimized
conditions, leading to production of the underivatized
indolylglycine derivative 11 in 75% yield (Scheme 1). In
case using 2-methylfuran (5), desired furylglycine deriva-
tive 12 was also obtained, but the yield (34%) was not
practical, due to low solubility of the product in a typical
solvent.
References and Notes
(1) (a) Olah, G. A. In Friedel–Crafts Chemistry; Wiley-
Interscience: New York, 1973. (b) Heaney, H. In
Comprehensive Organic Synthesis, Vol. 2; Trost, B. M.;
Fleming, I., Eds.; Pergamon: Oxford, 1991, 733–752.
(c) Olah, G. A.; Krishnamurti, A. R.; Prakash, G. K. S. In
Comprehensive Organic Synthesis, Vol. 3; Trost, B. M.;
Fleming, I., Eds.; Pergamon: Oxford, 1991, 293–339. (d)
In Lewis Acids in Organic Synthesis, Vol. 1 and 2;
Yamamoto, H., Ed.; Wiley-VCH: Weinheim, 2000.
(e) Bandini, M.; Emer, E.; Tommasi, S.; Umani-Ronchi, A.
Eur. J. Org. Chem. 2006, 3527.
H2N
CO2H
N
N
Me
3a (2 equiv)
Hf(OTf)4 (0.1 equiv)
Me3SiCl (1.2 equiv)
Me
NH2
CO2H
11: 75%
(2) (a) Speckamp, W. N.; Moolenaar, M. J. Tetrahedron 2000,
56, 3817. (b) Bur, S. K.; Martin, S. F. Tetrahedron 2001, 57,
3221.
+
or
HO
CH2Cl2, r.t., 12 h
or
10
NH2
Me
(3) (a) Xie, W.; Bloomfield, K. M.; Jin, Y.; Dolney, N. Y.; Wan,
P. G. Synlett 1999, 498. (b) Johannsen, M. Chem. Commun.
1999, 2233. (c) Gong, Y.; Kato, K.; Kimoto, H. Synlett
2000, 1058. (d) Janczuk, A.; Zhang, W.; Xie, W.; Lou, S.;
Cheng, J.; Wang, P. G. Tetrahedron Lett. 2002, 43, 4271.
(e) Lei, F.; Chen, Y.-J.; Sui, Y.; Liu, L.; Wang, D. Synlett
2003, 1160. (f) Piper, S.; Risch, N. Synlett 2004, 1489.
(g) Jiang, B.; Huang, Z.-G. Synthesis 2005, 2198.
(4) For selected papers of Friedel–Crafts-type reaction using an
acyclic N,O-acetal, see: (a) O’Donnell, M. J.; Falmagne,
J.-B. Tetrahedron Lett. 1985, 26, 699. (b) Heaney, H.;
Papageorgiou, G.; Wilkins, R. F. J. Chem. Soc., Chem.
Commun. 1988, 1161. (c) Heaney, H.; Papageorgiou, G.;
Wilkins, R. F. Tetrahedron 1997, 53, 2941. (d) DeNinno,
M. P.; Eller, C.; Etienne, J. B. J. Org. Chem. 2001, 66, 6988.
(e) Gong, Y.; Kato, K. J. Fluorine Chem. 2001, 108, 83.
(f) Ge, C.-S.; Chen, Y.-J.; Wang, D. Synlett 2002, 37; and
references cited therein.
O
Me
O
5 (2 equiv)
CO2H
12: 34%
Scheme 1
A plausible mechanism for the aminomethylation of aro-
matic compounds is shown in Scheme 2. First, the coordi-
nation of hafnium triflate to an oxygen atom of acetal 2
forms an iminium complex. Although there is no clear ex-
planation for the role of Me3SiCl at present,12 it may trap
the in situ generated methoxy ion, thus driving the equi-
librium to completion. Finally, the arene would attack the
iminium intermediate to produce the corresponding ami-
nomethylated product.
(5) (a) Sakai, N.; Hamajima, T.; Konakahara, T. Tetrahedron
Lett. 2002, 43, 4821. (b) Sakai, N.; Hirasawa, M.;
Hamajima, T.; Konakahara, T. J. Org. Chem. 2003, 68, 483.
(6) For selected papers of Hf(OTf)4-catalyzed Friedel–Crafts-
type reaction, see: (a) Hachiya, I.; Moriwaki, M.;
Kobayashi, S. Tetrahedron Lett. 1995, 36, 409.
(b) Kobayashi, S.; Iwamoto, S. Tetrahedron Lett. 1998, 39,
4697. (c) Shiina, I.; Suzuki, M. Tetrahedron Lett. 2002, 43,
6391. (d) Song, C. E.; Jun, D.-u.; Choung, S.-Y.; Roh, E. J.;
Lee, S.-g. Angew. Chem. Int. Ed. 2004, 43, 6183.
(7) Larock, R. C. In Comprehensive Organic Transformations,
2nd ed.; Wiley-VCH: New York, 1999.
X
N
X
N
X
N
Me3SiCl
Cl
(TfO)4Hf(OMe)
Me3SiOMe
MeO
Hf(OTf)4
Ar
CN
CN
X
CN
+
Hf(OTf)4
X
N
H
HCl
N
H
Ar
Cl
CN
Ar
CN
Scheme 2 Plausible reaction path for aminomethylation
Synlett 2007, No. 17, 2675–2678 © Thieme Stuttgart · New York

2678
N. Sakai et al.
LETTER
107.1, 107.8, 110.4, 111.2, 114.5, 123.2, 123.9, 142.6,
(8) (a) Carceller, E.; Merlos, M.; Giral, M.; Almansa, C.;
Bartrolí, J.; García-Rafanell, J.; Forn, J. J. Med. Chem. 1993,
36, 2984. (b) Carceller, E.; Merlos, M.; Giral, M.; Balsa, D.;
García-Rafanell, J.; Forn, J. J. Med. Chem. 1996, 39, 487.
(c) Yang, R.; Zhao, R.; Chen, D.; Shan, L.; Yun, L.; Wang,
H. Bioorg. Med. Chem. Lett. 2004, 14, 3017.
150.6. MS–FAB: m/z = 269, 243, 185. HRMS–FAB: m/z
calcd for C16H19N3O: 269.1528; found: 269.1529.
[4-(Dimethylamino)phenyl]piperidin-1-yl-acetonitrile (9a):
colorless crystals; mp 148–149 °C. 1H NMR (300 MHz,
CDCl3): d = 1.40–1.50 (m, 2 H), 1.50–1.60 (m, 4 H), 2.40–
2.60 (m, 4 H), 2.96 (s, 6 H), 4.72 (s, 1 H), 6.69 (d, 2 H,
J = 8.5 Hz), 7.33 (d, 2 H, J = 8.5 Hz). 13C NMR (75 MHz,
CDCl3): d = 24.0, 25.8, 40.4, 50.7, 62.5, 112.0, 116.2, 120.7,
128.8, 150.6. MS–FAB: m/z = 244 [M+ + H]. Anal. Calcd for
C15H21N3: C, 74.03; H, 8.70; N, 17.27. Found: C, 74.26; H,
8.79; N, 17.59.
Amino(1-methyl-1H-indol-3-yl)acetic acid (11): colorless
solids; mp 173–174 °C. 1H NMR (300 MHz, DMSO): d =
3.79 (s, 3 H), 5.27 (s, 1 H), 7.11 (t, 1 H, J = 7.8 Hz), 7.21 (t,
1 H, J = 7.8 Hz), 7.46 (d, 1 H, J = 7.8 Hz), 7.53 (s, 1 H), 7.68
(d, 1 H, J = 7.8 Hz), 8.78 (br s, 3 H). 13C NMR (75 MHz,
DMSO): d = 32.6, 48.6, 105.7, 110.1, 119.0, 119.5, 121.9,
125.5, 129.8, 136.5, 170.1. MS–FAB: m/z (%) = 205 (5)
[M+ + H], 188 (100) [M+ – NH2], 159 (52) [M+ – CO2H].
HRMS–FAB: m/z calcd for C11H13N2O2: 205.0977; found:
205.0980 [M+ + H].
(9) General Procedure for the Hf(OTf)4-Catalyzed
Aminomethylation of Aromatics with an N,O-Acetal
To a CH2Cl2 solution (2 mL) was successively added N,O-
acetal 2a (92.5 mg, 0.600 mmol), 1-methylindole (65.5 mg,
0.500 mmol), and freshly distilled trimethylchlorosilane
(65.7 mg, 0.600 mmol) under nitrogen atmosphere. After
1 min, Hf(OTf)4 (38.8 mg, 0.0500 mmol) was added to the
solution, and the thick suspension was vigorously stirred
until the reaction reached completion, as shown by TLC
(hexane–EtOAc, 6:4). The reaction was quenched with a sat.
aq solution of NaHCO3. The combined organic layer was
dried over Na2CO3 and evaporated under reduced pressure.
The crude product was purified by silica gel column
chromatography (hexane–EtOAc) to afford the product 3a in
92% (119.2 mg) yield. All new compounds were fully
characterized by their spectral analyses. Spectral data for (1-
methyl-1H-indol-3-yl)piperidin-1-yl-acetonitrile (3a):
colorless crystals; mp 143–144 °C. 1H NMR (500 MHz,
CDCl3): d = 1.40–1.60 (m, 6 H), 2.50–2.60 (m, 4 H), 3.79 (s,
3 H), 5.06 (s, 1 H), 7.14 (t, 1 H, J = 7.5 Hz), 7.26 (s, 1 H),
7.27 (t, 1 H, J = 7.5 Hz), 7.31 (d, 1 H, J = 7.5 Hz), 7.82 (d, 1
H, J = 7.5 Hz). 13C NMR (125 MHz, CDCl3): d = 24.2, 25.9,
32.9, 50.7, 56.3, 107.9, 109.4, 116.3, 119.6, 120.1, 122.4,
126.3, 128.5, 137.5. MS–FAB: m/z 254 [M+ + H]. Anal.
Calcd for C16H19N3: C, 75.85; H, 7.56; N, 16.71. Found: C,
75.77; H, 7.37; N, 16.71.
Amino(5-methylfuran-2-yl)acetic acid (12): pale yellow
solids; mp 92–93 °C. 1H NMR (300 MHz, DMSO): d = 2.18
(s, 3 H), 5.17 (s, 1 H), 6.12 (d, 1 H, J = 5.0 Hz), 6.49 (d, 1 H,
J = 5.0 Hz), 8.80 (br s, 3 H). 13C NMR (75 MHz, DMSO):
d = 9.1, 47.0, 112.3, 121.4, 174.6, 180.1, 190.9. MS–FAB:
m/z (%) = 156 (80) [M+ + H], 139 (100) [M+ – NH2]. HRMS–
FAB: m/z calcd for C7H10NO3: 156.0661; found: 156.0665
[M+ + H].
(10) When the reaction was examined with an N,O-acetal having
another type of a nitrogen-containing ring, such as pyrrole,
the desired amino acid precursor was not obtained, resulting
in the production of the unknown product.
(11) (a) Hoefnagel, A. J.; van Bekkum, H.; Peters, J. A. J. Org.
Chem. 1992, 57, 3916. (b) Sugiura, M.; Mori, C.; Hirano,
K.; Kobayashi, S. Can. J. Chem. 2005, 83, 937.
(12) For selected papers on reactions enhanced by a coexistence
of Lewis acid and Me3SiCl, see: (a) Corey, E. J.; Boaz, N.
W. Tetrahedron Lett. 1985, 26, 6015. (b) Mukaiyama, T.;
Wariishi, K.; Saito, Y.; Hayashi, M.; Kobayashi, S. Chem.
Lett. 1988, 1101. (c) Yamanaka, M.; Nishida, A.;
(1-Methyl-1H-pyrrol-2-yl)piperidin-1-yl-acetonitrile (6b):
colorless crystals; mp 127–128 °C. 1H NMR (500 MHz,
CDCl3): d = 1.40–1.60 (m, 6 H), 2.40–2.50 (m, 4 H), 3.61 (s,
3 H), 4.71 (s, 1 H), 6.04 (t, 1 H, J = 3.5 Hz), 6.33 (d, 1 H,
J = 3.5 Hz), 6.62 (d, 1 H, J = 3.5 Hz). 13C NMR (125 MHz,
CDCl3): d = 24.1, 25.8, 33.9, 50.3, 56.4, 106.6, 110.8, 114.8,
123.7, 124.4. MS–FAB: m/z = 203 [M+ + H]. Anal. Calcd for
C12H17N3: C, 70.90; H, 8.43; N, 20.67. Found: C, 71.12; H,
8.16; N, 20.91.
(1-Furan-2-yl-methyl-1H-pyrrol-2-yl)piperidin-1-yl-
acetonitrile (8a): colorless oil. 1H NMR (300 MHz, CDCl3):
d = 1.47 (m, 2 H), 1.59 (m, 4 H), 2.52 (m, 4 H), 4.83 (s, 1 H),
4.95 (d, 1 H, J = 10.0 Hz), 5.38 (d, 1 H, J = 10.0 Hz), 6.08 (t,
1 H, J = 5.0 Hz), 6.15 (d, 1 H, J = 5.0 Hz), 6.31 (t, 1 H,
Nakagawa, M. Org. Lett. 2000, 2, 159. (d) Huang, T.; Li,
C.-J. Tetrahedron Lett. 2000, 41, 6715. (e) Lee, P. H.; Lee,
K.; Sung, S.-Y.; Chang, S. J. Org. Chem. 2001, 66, 8646.
(f) Tsuji, R.; Yamanaka, M.; Nishida, A.; Nakagawa, M.
Chem. Lett. 2002, 428. (g) Yang, L.; Xu, L.-W.; Xia, C.-G.
Tetrahedron Lett. 2007, 48, 1599.
J = 5.0 Hz), 6.37 (s, 1 H), 6.73 (s, 1 H), 7.35 (s, 1 H), 13
C
NMR (75 MHz, CDCl3): d = 24.0, 25.7, 43.6, 50.2, 56.3,
Synlett 2007, No. 17, 2675–2678 © Thieme Stuttgart · New York