A convenient method for the preparation of cyclohepta[b]indole derivatives

Article abstract of DOI:10.2478/s11532-011-0117-4

Cyclohepta[b]indole derivatives 7 were prepared by subsequent aza-Claisen rearrangement and intramolecular ring-closure of (cycloheptenylmethyl) benzenamine (3). The mechanisms of the reactions are also discussed.

Full text of DOI:10.2478/s11532-011-0117-4

Cent. Eur. J. Chem. • 10(1) • 2012 • 91-95
DOI: 10.2478/s11532-011-0117-4
Central European Journal of Chemistry
A convenient method for the preparation
of cyclohepta[b]indole derivatives
Research Article
Katalin Kupai, Gergely Banoczi, Gabor Hornyanszky,
Pal Kolonits, Lajos Novak*
Department of Organic Chemistry and Technology,
Budapest University of Technology and Economics,
Research Group for Alkaloid Chemistry of the Hungarian Academy of Sciences,
1111 Budapest, Hungary.
Received 7 July 2011; Accepted 2 September 2011
Abstract:
Cyclohepta[b]indole derivatives 7 were prepared by subsequent aza-Claisen rearrangement and intramolecular ring-closure
of (cycloheptenylmethyl)benzenamine (3). The mechanisms of the reactions are also discussed.
Keywords: Cyclohepta[b]indole • Aza-Claisen • Rearrangement • Aza-Alder-ene reaction • Benzenamines
© Versita Sp. z o.o.
Ab initio calculations (DFT-MPW1K) [2] (6-31+G*)
were carried out using Gaussian 2003 [3]. Transition
states were confirmed by IRC method.
1. Introduction
Recently, we have investigated the catalytic aza-Claisen
rearrangement of (cycloalkylmethyl)benzenamine and
the intramolecular ring-closure reaction of the products.
These reactions afforded cycloalkano-indole derivatives,
which were then transformed into the carba analogs of
physostigmine [1]. To explore the scope and limitation of
these new reactions we have examined the mechanism
of the rearrangement reaction of cycloheptylmethyl-
N-Methylbenzenamine (1a) [4], 4-methoxy-N-
methylbenzenamine (1b) [5] and 1-(chloromethyl)
cyclohept-1-ene (2) were prepared by literature
procedures [1].
N - ( ( E ) - C y c l o h e p t e n y l m e t h y l ) - N -
methylbenzenamine (3a). To a stirred suspension
of N-methylbenzenamine (1a; 4.0 g, 37 mmol) and
triethylamine (6 mL, 41 mmol) in water (100 mL)
N-methyl-benzenamines
and
their
applicability
for the preparation of cyclohepta[b]indole derivatives
(7, Scheme 1).
was
added
(E)-1-(chloromethyl)cyclohept-1-ene
(2; 5.35 g, 37 mmol), and the resulting mixture was stirred
at rt for 1 h. The reaction mixture was extracted with ether
(200 mL), the ethereal extract was dried (MgSO4) and
then concentrated in vacuo. The residue was purified by
column chromatography using hexane/EtOAc (7:3) as
eluent to yield 3a as yellow oil (5.54 g, 69.4%). TLC
(hexane/EtOAc 9:1): Rf= 0.56. 1H NMR (CDCl3): 1.48 (m,
4H, 2 CH2), 1.74 (m, 2H, CH2), 2.11 (m, 4H, 2 CH2), 2.9
(s, 3H, CH3), 3.74 (s, 2H, N- CH2), 5.62 (m, 1H, CH=),
6.70 (m, 3H, ArH), 7.21 (d, J= 8.7 Hz, 2H, ArH). 13C NMR
(CDCl3): 26.80, 27.40, 28.34, 30.72, 32.64, 37.94, 59.82,
60.20, 112.29, 116.11, 126.99, 129.23, 137.68, 139.98,
150.19. Anal. Calcd for C15H21N: C, 83.67; H, 9.83; N,
6.50. Found: C, 83.49; H, 10.02; N, 6.43.
2. Experimental procedure
1H NMR and 13C NMR spectra were obtained with
a Bruker DRX-500 spectrometer with internal standard
TMS. Merck precoated silica gel 60 F254 plates
were used for TLC and Kieselgel 60 for column
chromatography. GC chromatographic analysis were
performed with anAgilent 5890D : HP-INNOWAX column
(30
m
×
0.32 mm
×
0.25 μm), hydrogen,
170-210oC/20 min, injector temperature 250oC.
All solvents were dried by means of standard methods.
All reactions were performed under Ar.
* E-mail: lnovak@mail.bme.hu
91

A convenient method for the preparation
of cyclohepta[b]indole derivatives
Scheme 1. Preparation of cycloheptano-indole derivatives.
N-((E)-Cycloheptenylmethyl)-4-methoxy-N- was added BF3.Et2O (1 mL, 7 mmol) at 170oC and
methylbenzenamine (3b). To a stirred suspension the resultant mixture was stirred at this temperature
of
4-methoxy-N-methylbenzenamine (1b, 1.5 g, for 1 h. After cooling, the reaction mixture was diluted
10.8mmol)inwater(15mL)weresuccessivelyadded(E)- with water (60 mL), extracted three times with CH2Cl2
1-(chloromethyl)cyclohept-1-ene (2; 1.56 g, 10.8 mmol) (100 mL each), and the combined organic extracts were
and triethylamine (3 mL, 22 mmol), and the resulting washed with water. After drying (MgSO4), the solvent
mixture was stirred at rt for 3 h. The reaction mixture was evaporated in vacuo and the residue was purified
was extracted with ether (40 mL), the ethereal extract by column chromatography.
was dried (MgSO4) and then concentrated in vacuo.
Yield 0.45 g (30%), light yellow oil.
The residue was purified by column chromatography
cis-isomer of 7a: Rf= 0.46 (hexane-EtOAc 15:1).
using hexane/EtOAc (9:1) as eluent to yield 3b 1H NMR (DMSO-d6): 1.03 (s, 3H, C5a-CH3), 1.37 (m,
(2.25 g, 85%) as light yellow oil. TLC (hexane/EtOAc 3H, C7-H, C8-H, and C9-H), 1.50 (m, 3H, C7-H, C8-H,
9:1): Rf= 0.65; Rt= 10.25 min. 1H NMR (CDCl3): 1.48 (m, and C9-H), 1.62 (m, 1H, C10-H), 1.71 (m, 2H, C6-H and
2H, 2 CH2), 1.73 (m, 2H, CH2), 2.07 (m, 4H, 2 CH2), 2.83 C10-H), 1.79 (m, 1H, C6-H), 2.57 (s, 3H, N-CH3), 2.88 (m,
(s, 3H, CH3), 3.65 (s, 2H, N- CH2), 3.77 (s, 3H, OCH3), 1H, C10a-H), 6.25 (d, J= 7.5 Hz, 1H, C4-H), 6.49 (t, J=
5.65 (t, J= 6.3 Hz, 1H, CH=), 6.74 (m, 2H, ArH), 6.81 (m, 7.5 Hz, 1H, C2-H), 6.93 (d, J= 7.5 Hz, 1H, C1-H), 6.95 (t,
2H, ArH). 13C NMR (CDCl3): 26.83, 27.42, 28.40, 29.90, J= 7.5 Hz, 1H, C3-H). 13C NMR (DMSO-d6): 23.78 (C-7),
30.81, 38.73, 56.03, 62.04, 114.71, 114.86, 116.45, 24.20 (C5a-CH3), 27.48 (C-9), 27.74 (N-CH3), 31.05
128.00 (2C), 140.44, 144.98 (2C). Anal. Calcd for (C-8), 32.02 (C-10), 35.86 (C-6), 52.81 (C-10a), 68.84
C16H23NO: C, 78.32; H, 9.45; N, 5.71. Found: C, 78.48; (C-5a), 104.97 (C-4), 116.15 (C-2), 123.77 (C-1), 127.56
H, 9.64; N, 5.43.
5 , 5 a - D i m e t h y l - 5 , 5 a , 6 , 7 , 8 , 9 , 1 0 , 1 0 a - C15H21N: C, 83.67; H, 9.83; N, 6.50. Found: C, 83.52 H,
octahydrocyclohepta[b]indole (7a). To stirred 10.01; N, 6.33.
solution of 3a (1.5 g, 7 mmol) in sulfolane (60 mL)
(C-3), 132.29 (C-10b), 150.95 (C-4a). Anal. Calcd for
a
92

K. Kupai et al.
trans-isomer of 7a: Rf= 0.44 (hexane-EtOAc 15:1). (C-10b), 145.83 (C-4a), 152.19 (C-2). Anal. Calcd for
1H NMR (DMSO-d6): 0.83 (s, 3H, C5a-CH3), 1.31 (m, C16H23N: C, 78.32; H, 9.45; N, 5.71. Found: C, 78.28; H,
1H, C7-H), 1.51 (m, 4H, C6-H, C8-H, C9-H, and C10-H), 9.62; N, 5.44.
1.68 (m, 1H, C8-H ), 1.88 (m, 1H, C7-H), 1.92 (m, 1H,
1-(4-Methoxy-2-((E)-2-methylcyclohept-1-enyl)
C9-H ), 2.08 (m, 1H, C6-H), 2.21 (m, 1H, C10-H), 2.53 (s, phenyl)-1-methyl-3-phenylurea (6). Light yellow oil.
1
3H, N-CH3), 3.06 (m, 1H, C10a-H), 6.28 (d, J= 7.5 Hz, TLC (hexane/acetone 7:3): Rf= 0.58. H NMR (CDCl3):
1H, C4-H), 6.54 (t, J= 7.5 Hz, 1H, C2-H), 6.88 (d, J= 7.5 1.4-1.8 (m, 9H, 3 CH2 and CH3), 2.2-2.5 (m, 4H, 2 CH2),
Hz, 1H, C1-H), 6.95 (t, J= 7.5 Hz, 1H, C3-H). 13C NMR 3.20 (s, 3H, N-CH3), 3.85 (s, 3H, O-CH3), 6.22 (br s, 1H,
(DMSO-d6): 12.76 (C5a-CH3), 22.79 (C-10), 25.40 (C-8), NH), 6.68 (s, 1H, ArH), 6.85 (d, J= 3.6 Hz, 1H, ArH), 6.98
25.62 (C-9), 26.70 (C-7), 28,50 (N-CH3), 38.94 (C-6), (m, 1H, ArH), 7,22 (m, 5H, ArH). 13C NMR (CDCl3): 23.49
47.78 (C-10a), 72.67 (C-5a), 105.79 (C-4), 116.85 (C-2), (CH3), 26.17, 27.02, 29.48, 29.63, 32.69, 35.92 (N- CH3),
121.63 (C-1), 127.14 (C-3), 132.67 (C-10b), 151.43 55.73 (O-CH3), 113.60, 116.75, 119.22, 122.86, 129.00,
(C-4a). Anal. Calcd for C15H21N: C, 83.67; H, 9.83; N, 130.17, 131.08, 132.49, 135.75, 137.75, 139.30, 154.90,
6.50. Found: C, 83.48 H, 10.12; N, 6.43.
2-Methoxy-5,5a-dimethyl-5,5a,6,7,8,9,10,10a- 7.69. Found: C, 75.51; H, 7.92; N, 7.44,
octahydrocyclohepta[b]indole (7b). To stirred 5 , 5 a - D i m e t h y l - 5 , 5 a , 6 , 7 , 8 , 9 , 1 0 , 1 0 a -
159.37. Anal. Calcd for C23H28N2O2: C, 75.79; H, 7.74; N,
a
solution of 3b (0.54 g, 2.2 mmol) in sulfolane (6 mL) octahydrocyclohepta[b]indol-2-ol (8). To a stirred
was added BF3.Et2O (75 μL, 0.64 mmol) at 170oC and solution of compound 4b (1.1 g, 4.4 mmol) in chloroform
the resultant mixture was stirred at this temperature for (10 mL) was added BBr3 (0.6 mL, 1.56 g, 6.2 mmol)
25 min. After cooling, the reaction mixture was diluted under cooling and the resulting mixture was stirred at rt
with water (20 mL), extracted three times with CH2Cl2 for 1 h. Methanol was then added and the solvents were
(50 mL each). After drying (MgSO4), the solvent was evaporated in vacuo. The residue was basified with
evaporated in vacuo and the residue was purified by saturated NaHCO3 solution and extracted with ether
column chromatography.
(50 mL). The ethereal extract was dried (MgSO4) and
cis-isomer of 7b: 0.47 g (30%), yellow oil. TLC then concentrated in vacuo. The residue was purified by
1
(hexane/EtOAc 9:1): Rf= 0.66. Rt= 9.22 min. H NMR column chromatography using CH2Cl2/MeOH (20:1), as
(DMSO-d6): 0.99 (s, 3H, C5a-CH3), 1.34 (m, 1H, C7-H), eluent. Yield: 0.82 g (83%), brown solid. TLC (CH2Cl2/
1.38 (m, 1H, C8-H), 1.41 (m, 1H, C9-H), 1.54 (m, 2H, MeOH 20:1): Rf= 0.51. This compound decomposed on
C8-H and C9-H), 1.56 (m, 2H, C7-H and C10-H), 1.67 storage. Therefore it was immediately used up for the
(m, 1H, C6-H), 1.72 (m, 1H, C10-H), 1.81 (m, 1H, C6-H), preparation of carbamate 9, without further purification.
2.51 (s, 3H, N-CH3), 2.81 (m, 1H, C10a-H), 3.63 (s, 3H,
cis-5,5a-Dimethyl-5,5a,6,7,8,9,10,10a-
O-CH3), 6.17 (d, J= 8.3 Hz, 1H, C4-H), 6.55 (dd, J= 8.3 octahydrocyclohepta[b]indol-2-yl phenyl-carbamate
and 2 Hz, 1H, C3-H), 6.64 (d, J= 2 Hz, 1H, C1-H). 13C (9). To a stirred solution of compound 5 (0.41 g,
NMR (DMSO-d6): 23.45 (C5a-CH3), 23.70 (C-7), 27.82 1.7 mmol) in THF (30 mL) was added phenyl isocyanate
(C-9), 28.42 (N-CH3), 31.09 (C-8), 31.98 (C-10), 36.02 (1.85 mL, 2.03 g, 1.7 mmol) and the resulting solution
(C-6), 53.24 (C-10a), 55.59 (O-CH3), 69.00 (C-5a), was refluxed for 36 h. After cooling, the solvent was
105.23 (C-4), 111.60 (C-1), 111.83 (C-3), 134.06 (C-10b), evaporated in vacuo and the residue was purified by
145.47 (C-4a), 151.72 (C-2). Anal. Calcd for C16H23N: C, column chromatography using hexane/EtOAc (7:3) as
78.32; H, 9.45; N, 5.71. Found: C, 78.18; H, 9.40; N, eluent. Yield 0.51 g (84%), light brown crystals, mp
5.49.
121-124 oC; TLC (hexane/EtOAc 7:3): Rf= 0.6. 1H NMR
trans-isomer of 7b: 0.11 g (7%), dark green solid. (DMSO-d6): 1.06 (s, 3H, C5a-CH3), 1.38 (m, 1H, C7-H),
o
Mp 52-54 C. TLC (CH2Cl2/hexane 9:1): Rf= 0.34. Rt= 1.41 m, 1H, C8-H), 1.43 (m, 1H, C9-H), 1.48 (m, 1H,
13.67 min. 1H NMR (DMSO-d6): 0.80 (s, 3H, C5a-CH3), C9-H), 1.50 (m, 2H, C7-H and C8-H), 1.63 (m, 1H, C10-H),
1.29 (m, 1H, C7-H), 1.50 (m, 3H, C8-H, C9-H, and C10-H), 1.70 (m, 1H, C6-H), 1.72 (m, 1H, C10-H), 1.78 (m, 1H,
1.55 (m, 1H, C6-H), 1.67 (m, 1H, C8-H), 1.86 (m, 1H, C6-H), 2.58 (s, 3H, NCH3), 2.91 (m, 1H, C10a-H), 6.23 (d,
C7-H), 1.91 (m, 1H, C9-H), 2.05 (m, 1H, C6-H), 2.19 (m, J= 8 Hz, 1H, C4-H), 6.77 (dd, J= 8 and 2 Hz, 1H, C3-H),
1H, C10-H), 2.48 (s, 3H, N-CH3), 3.04 (m, 1H, C10a-H), 6.81 (s, 1H, C1-H), 7.01 (t, J= 7.5 Hz, 1H, C4’-H), 7.30
3.63 (s, 3H, O-CH3), 6.19 (d, J= 8.2 Hz, 1H, C4-H), 6.53 (t, J= 7.5 Hz, 2H, C3’-H and C5’-H), 7.50 (d,J= 8 Hz, 2H,
(dd, J= 8.2 and 2 Hz, 1H, C3-H), 6.56 (br. s, 1H, C1-H). C2’-H and C6’-H),), 10.00 (s, 1H, NH). 13C NMR (DMSO-
13C NMR (DMSO-d6): 12.45 (C5a-CH3), 22.83 (C-10), d6): 23.71 (C-7), 24.38 (CH3), 27.26, 27.42 (C-9), 27.97
25.47 (C-8), 25.61 (C-9), 26.71 (C-7), 29.07 (N-CH3), (N-CH3), 30.98 (C-8), 31.74 (C-10), 35.89 (C-6), 52.71
39.14 (C-6), 47.96 (C-10a), 55.56 (O-CH3), 72.19 (C-10a), ,69.31 (C-5a), 104.35 (C-4), 118.10 (C-1),
(C-5a), 105.80 (C-4), 110.07 (C-1), 110.79 (C-3), 134.37 118.45 (C-2’ and C-6’), 120.41 (C-3), 122.80 (C-4’),
93

A convenient method for the preparation
of cyclohepta[b]indole derivatives
128.95 (C-3’ and C-5’), 133.12 (C-10b), 139.12 (C-1’),
For the preparation of the carba analog
141.39 (C-2), 148.60 (C-4a), 152.81 (C=O). MS m/z (%): of physostigmine, compound 7b had been treated with
252 (M+2, 24), 251 (M+1, 100), 231 (M+1 – C7H6NO, 6). BBr3 and the sensitive hydroxyl derivative 8 formed
Anal. Calcd for C22H26N2O2: C, 75.40; H, 7.48; N, 7.99. reacted immediately with phenyl isocyanate. Compound
Found: C, 75.18; H, 7.50; N, 7.76.
9 was then isolated in 84% yield.
The cis-diastereoselectivity can be rationalized
by the inspection of the transition states leading to
the isomers (4b → cis and trans 7b). As a model
compound for the ab initio DFT calculation on
3. Results and discussion
Synthesis of compounds 7 and 9 are depicted in
Scheme 1. Treatment of N-methylaniline (1) with
1-(chloromethyl)cyclohept-1-ene (2) in the presence of
Et3NaffordedthecorrespondingN-(cycloheptenylmethyl)
benzenamine (3), which was subjected to thermal
rearrangement.
TS-4"b
350
300
chair-TS-4'b
250
TS-cis-7b
200
)
l
o
150
m
/
Aza-Claisen rearrangements usually require more
forcing conditions than those required for the classic
Claisen rearrangement [6,7]. Earlier, during the
preparationofpotentialinhibitorsofacetylcholineesterase
we demonstrated that boron trifluoride – diethyl ether
complex (BF3•Et2O) could efficiently catalyze a variety
of aza-Claisen rearrangements [8].
The rearrangement reaction of 3a was performed in
sulfolane at 170oC, in the presence of equal amount of
(BF3•Et2O). The aza-Claisen rearrangement followed by
a ring closure reaction afforded three compounds, but
only two products cis-7a and trans-7a were isolated by
repeated column chromatography.
J
4'b
k
(
100
E
50
3b
0
4"b
- 50
- 100
TS
TS
TS
cis-7b
4"b
cis-7b
4'b
3b
- 150
Figure 1. Energy diagram of the formation of compound 7b.
350
TS-aza-Alder-ene reaction
300
chair-TS-4'b
250
The reaction of 3b was taken approximately under
the same conditions as we used for 3a. The two step
reaction furnished three compounds: cis-7b and trans-
7b (ratio 2:1), and compound 5. The latter obviously
was formed from the product of rearrangement (4b) by
double bond migration.
200
)
l
o
150
m
/
J
4'b
k
(
100
E
50
3b
0
The separation of the above mixture by column
chromatography was difficult and we did not get
a good result. For getting better separation, the mixture
was treated with phenyl isocyanate. The side product 5
reacted with phenyl isocyanate to form adduct 6, which
was easily separated from the diastereomers by column
chromatography.
- 50
- 100
TS
TS
cis-7b
4'b
3b
cis-7b
- 150
Figure 2. Energy
diagram of the aza-Claisen rearrangement
followed by aza-Alder-ene reaction.
Table 1. Calculation on the formation of compound 7b.
Reagent
∆E (kJ mol-1
)
Reagent
∆E (kJ mol-1
)
Starting compounds 3b
Chair-TS-4’b
0
Starting compounds 3b
Boat-TS-4’b
0
234
122
358
-2.7
211
235
288
251
133
361
-2.7
Intermediate (R,R;S,S) 4’b
TS-4”b
Intermediate (R,S;S,R) 4’b
TS-4”b
Intermediate 4”b
TS-cis-7b
Intermediate 4”b
TS-trans-7b
TS-aza-Alder-ene reaction
94

K. Kupai et al.
the transition states, compound 3b was chosen. was significantly lower than those calculated for the
The formation of 7b can be envisaged in three steps: TS-4”b (ΔΔE = -147 kJ mol-1, Table 1). Therefore, this
aza-Claisen rearrangement (4b’), aromatic stabilization two step reaction pathway with its low activation barrier
(4b”), and nucleophilic attack of the nitrogen on the may be regarded as the mechanism of the formation of
exo-double bond. In the transition state of the first compound 7b.
step, there was a rather large difference between the
energies of the chair and boat geometry (ΔΔE = - 17.3
kJ mol-1), showing improbable boat conformation of
the transition state in the aza-Claisen rearrangement.
4. Conclusions
In summary, cyclohepta[b]indol derivatives (7a,b
and 9) were prepared from aniline derivative (1) and
(chloromethyl)cyclohept-1-ene (2). The convenient
process involved aza-Claisen rearrangement of the
benzeneamine derivative 3, followed by BF3•Et2O
catalyzed intramolecular aza-Alder-ene reaction of
the product 4. This short synthetic scheme would
enable access to a variety of new cyclohepta[b]indole
molecules, which would have interesting biological
properties.
Surprisingly, our calculation for the transition state of
the second step, aromatization, showed high energy
(ΔE = 358 kJ mol-1). For the closing step, the
transition state leading to the cis-product (cis-7b)
had lower energy than the corresponding trans
(ΔΔE = - 23.5 kJ mol-1), suggesting that the “trans”
reaction pathway was unfavorable, in accordance
with experiment (Fig. 1). We got also lower energy for
the cis-product (cis-7b) than for the trans-compound
(ΔΔE = - 7.5 kJ mol-1) (Table 1).
Due to the high calculated activation energy of
intermediate 4”b (358 kJ mol-1), we considered another
possible mechanism for this reaction. We found that
an intramolecular aza-Alder – ene reaction on the
intermediate 4’b might also take place (Fig. 2) [9-13].
The calculated activation energy (4’b→TS-4’b→7b)
Acknowledgements
We thank Attila Csaki for his contribution to this work.
References
[8]
[1] I. Kiraly, G. Hornyanszky, K. Kupai, L. Novak,
Heterocycles 75, 43 (2008)
[2] J. Lynch, P.L. Fast, M. Harris, D.G. Truhlar, J. Phys.
Chem. 104, 4811 (2000)
[3] M.J. Frisch et al., Gaussian 03 Revision C 02
(Gaussian, Inc., Wallingford CT, 2004)
[4] J. Barluenga, A.M, Bayón, G. Asensio, J. Chem. Soc.
Chem. Commun. 1334 (1984)
[5] M.F. Millson, R. Robinson, J. Chem. Soc. 3362
(1955)
(a) W.K. Anderson, G. Lai, Synthesis 1287 (1995);
(b) U. Nubbemeyer, J. Org. Chem. 61, 3677
(1996); (c) M.A. Cooper, M.A. Lucas, J.M. Taylor,
A.D. Williamson, Synthesis 621 (2001) and
references therein
U. Pindur, G.H. Schneider, Chem. Soc. Rev. 23,
409 (1994)
A.G. Leach, K.N. Houk, Chem. Commun. 1243
(2002)
P.S. Baran, C.A. Guerrero, E.J. Corey, J.Am. Chem,
Soc. 125, 5628 (2003)
G.E. Hulson, A.H. Dave, V.H. Rawal, Org. Lett. 9,
3869 (2007)
K. Zeng, J. Shi, X. Liu, X. Feng, J. Am. Chem. Soc.
130, 15770 (2008)
[9]
[10]
[11]
[12]
[13]
[6] (a)A.M.M. Castro, Chem. Rev. 104, 29 (2004);
(b) U. Nubbemeyer, Top Curr. Chem. 244, 149,
(2005)
[7] M.
Hersemann,
U.
Nubbemeyer
(Eds.),
The Claisen Rearrangement (Wiley-VCH Verlag
GmbH, New York, 2007) pp. 512
95