Synthesis of s-indacene derivatives by double Robinson-type cyclopentene annulation

Article abstract of DOI:10.1055/s-2007-983901

Michael reactions of 2,5-dioxocyclohexane-1,4-dicarboxylates with methyl vinyl ketone yield bis-adducts that can be further cyclized in a Robinson-type annulation to give s-indacene derivatives as single diastereomers. The carboxylate functions of this scaffold can be deprotected and subsequently decarboxylated to yield tricyclic products with a central aromatic ring. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-983901

PAPER
3061
Synthesis of s-Indacene Derivatives by Double Robinson-Type Cyclopentene
Annulation
Synthesis of
s-Ind
a
cene Deriv
w
atives ei Zhang, Jens Christoffers*
Institut für Reine und Angewandte Chemie, Universität Oldenburg, Carl von Ossietzky-Str. 9–11, 26111 Oldenburg, Germany
Fax +49(441)7983873; E-mail: jens.christoffers@uni-oldenburg.de
Received 4 May 2007; revised 28 June 2007
2,5-Dioxocyclohexane-1,4-dicarboxylates (‘succinyl suc-
Abstract: Michael reactions of 2,5-dioxocyclohexane-1,4-dicar-
cinates’) 2 are known compounds that exist almost com-
pletely as the double enol tautomers as shown in
Scheme 2. They are conveniently prepared by twofold
boxylates with methyl vinyl ketone yield bis-adducts that can be
further cyclized in a Robinson-type annulation to give s-indacene
derivatives as single diastereomers. The carboxylate functions of
this scaffold can be deprotected and subsequently decarboxylated to Claisen–Dieckmann condensation of succinates 4. We
yield tricyclic products with a central aromatic ring.
prepared five products 2a–e from succinates 4a–e accord-
ing to a standard protocol reported for methyl and ethyl
esters 2a and 2b; compounds 2c–e and 4c are novel, the
latter was prepared from succinic anhydride and octan-1-
ol in 96% yield.
Key words: annulation, indacene, cyclopentene, conjugate addi-
tion, aldol reaction
The aldol reaction1 and the Michael addition2 are among
the most important methods for C–C bond formation in
carbonyl compounds. When performed as a sequence,
they become a long established3 and extraordinarily pow-
erful tool for six-membered ring formation known as the
Robinson annulation.4 In close relation with our project
on asymmetric metal-catalyzed Michael reactions,5 we
have applied the Robinson annulation to the preparation
of bicyclic cyclohexenone derivatives.6 In extending these
studies to the reaction of bis-b-oxo esters 2 with methyl
vinyl ketone (3) (MVK), we recently observed an unex-
pected and unusual dual cyclopentene annulation under
conditions typical for Robinson annulation (Scheme 1).7
A similar observation has already been reported using ac-
rolein as the Michael acceptor but with low chemoselec-
tivity and unpredictable yield.8 The products of this
process are s-indacene9 derivatives 1. This dicyclopen-
ta[a,d]benzene skeleton is relatively rare in synthetic or-
ganic chemistry,10 however, some natural products,11
ligands,12 and interesting materials13 with this structural
motif have been reported. We report herein full experi-
mental details on the synthesis of the s-indacene deriva-
tives 1 and further transformations of these materials.
In extending our studies of iron-catalyzed Michael
reactions14 of b-oxo esters, we were interested in the reac-
tions of bis(b-oxo esters) 2 with MVK (3). The donors 2
were converted, however, at ambient temperature only
into monoadducts;15 at 60 °C and using ten equivalents of
MVK (3) and 10 mol% of catalyst, the bis-adducts 5 were
obtained. In the case of 5a and 5b, the crude products are
mixtures of cis- and trans-diastereomers, with the rela-
tive cis-configuration predominating. Stereochemically
unique cis-configurated materials (one signal set in the
NMR spectra) are obtained by recrystallization of these
mixtures with yields in the range of 43–79% (Table 1).
The relative configuration of compounds 5 should be elu-
cidable by GLC on a chiral phase, since cis-isomers
should be racemates, whereas trans-isomers would be
meso compounds. However, we were not able to achieve
OH
E
a)
E
E
E
4
OH
2
O
+
O
O
Me
b)
CO2R
Me
E
CO2R
O
3
+
Me
RO2C
O
O
RO2C
Me
O
O
c) or d)
Me
1
2
3
1
E
Me
Scheme 1 Synthesis of s-indacene derivatives 1 from bis-b-oxo
esters 2 and methyl vinyl ketone (3)
O
O
5
Scheme 2 Three-step synthesis of s-indacene derivatives 1 from
succinates 4. For yields and ester substituents E, see Table 1. Reaction
conditions: (a) NaH, DMSO, 50 °C, 1 h; (b) FeCl3·6 H2O (0.1 equiv),
MVK (3) (10 equiv), CH2Cl2, 60 °C, 2 d; (c) concd H2SO4, 0 °C, 1 h,
then 23 °C, 16 h (1a–c); (d) pyrrolidine, AcOH, CH2Cl2, 0 °C, 1 h then
50 °C, 16 h (1d,e).
SYNTHESIS 2007, No. 19, pp 3061–3067
Advanced online publication: 11.09.2007
DOI: 10.1055/s-2007-983901; Art ID: T08007SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
0
7

3062
Y. Zhang, J. Christoffers
PAPER
Table 1 Substituents and Yields of Products 1, 2, and 5
Ester
4a
E
Diester 2
Yield (%)
Tetraketone 5 Yielda (%)
Indacene 1
1ab
Yield (%)
CO2Me
CO2Et
2a
2b
2c
2d
2e
76
84
70
81
76
5a
5b
5c
5d
5e
60
53
43
79
60
57
70
69
46
29
4b
1bb
4c
CO2(CH2)7Me
CO2Bn
CO2allyl
1cb
4d
1dc
4e
1ec
a Yield of pure cis-diastereomer (de >95%) after recrystallization.
b Cyclization with concd H2SO4.
c Cyclization with pyrrolidine–AcOH.
O
O
O
sufficient resolution with various chiral phases. There-
fore, we envisioned reducing the conformational flexibil-
ity by subsequent Robinson annulation, which has, in the
past, proved to be a useful strategy in our laboratory to
overcome this problem.16 Under standard conditions for
this annulation (concd H2SO4 or pyrrolidine–AcOH) we
did not observe the formation of six-membered annulation
products with an anthracene skeleton, but surprisingly, s-
indacene derivatives 1 were formed. GLC on a chiral
phase now suggests the cis configuration of the two ste-
reogenic centers since these materials turned out to be
racemates, which clearly contradicts an earlier report with
acrolein as the Michael acceptor.8
Me
Me
1
E
H
E
H
H2 (1 bar), Pd/C
i-PrOH, 16 h, 80 °C
8
3a
8a
3
E
E
Me
Me
O
1a (E = CO2Me)
1b (E = CO2Et)
1c (E = CO2Oct)
6a (70%, de >98%)
6b (86%, de >98%)
6c (60%, de >98%)
Scheme 3 Hydrogenation of alkyl esters 1a–c
At first view, this finding implies a formal trans-dihydro-
genation of substrates 1a–c, which is clearly the result of
a cis-dihydrogenation followed by an epimerization at
C8a and C4a. Since the convex face of substrates 1a–c is
blocked by the two ester moieties, the catalytic cis-dihy-
drogenation has obviously occurred from the inner, con-
cave side of the molecule. cis-Dihydrogenation enforces a
double trans annulation of the six-membered and the two
five-membered rings. This extremely strained configura-
tion is readily released by spontaneous epimerization at
the tertiary stereocenters a to the carbonyl groups via the
enol tautomers, resulting in the trans configuration of H1,
H8a and H4a, H5, respectively, and thus simulating for-
mal anti-dihydrogenation.
In order to access the fully saturated s-indacene skeleton
we submitted compounds 1a–c to catalytic hydrogenation
of the C=C bonds yielding tricyclic products 6a–c as sin-
gle diastereomers (one set of signals in the NMR spectra).
The relative configuration was established based on
3
3J(H,H) and J(H,13C) coupling constants: The vicinal
coupling constant 3J(H8a,H1) = 9.1 Hz observed as a dou-
blet for the bridgehead proton H8a (see Scheme 3 for
atom numbering) is compatible only with a close to al-
most antiperiplanar, trans orientation of H8a to H1 in the
five-membered ring. This configuration is in accord with
a detailed analysis of the 13C,1H multiple bond coupling
correlation (HMBC). The HMBC correlation plot dis-
plays prominent cross peaks between H1 and C2, C8a,
and the methyl carbon; these 2J correlations are indepen-
dent of any torsional angle. For the three possible 3J cor-
relations, the 2D plot shows an equally pronounced cross
peak of H1 only to the C8 carbonyl resonance while no
cross peaks appear for the H1/C3 and the H1/C3a correla-
tion. With an all-cis configuration of both Me and both
CO2Me groups, and a boat or tub conformation for the
central six-membered ring, dihedral angles of ca. 90° be-
tween H1 and both C3 and C3a, and of ca. 30° between H1
and C8 can be taken from a molecular model, thus ex-
We have undertaken several attempts to saponify esters
1a–c, but presumably due to the strained neopentyl-type
situation, no nucleophilic attack to the ester carbonyl
groups was observed. Therefore, we prepared the benzyl
(1d–5d) and allyl ester series (1e–5e), since these protec-
tive groups could be cleaved by hydrogenolysis or transi-
tion-metal-catalyzed allylic substitution.
Catalytic hydrogenolysis of the dibenzyl ester 1d gave the
decarboxylated derivative 7 (Scheme 4) as a mixture of
two diastereomeric hydroquinones (rac/meso 2:1 by GLC
on an achiral phase; GLC on a chiral phase gave a 1:1:1
ratio). The product is not stable under ambient conditions,
presumably due to oxidation to the corresponding quinone
derivative 8. Acetylation of partially oxidized hydro-
quinone 7 under reductive conditions (Zn, AcCl,
NaOAc)17 yielded the diester 9, once again in the form of
two diastereomers, which are sufficiently stable under
ambient conditions to allow detailed structural analysis.
3
3
plaining the missing J(H1,13C3) and J(H1,13C3a) cross
peaks in the HMBC spectra. Therefore, we conclude that
all three cycles are cis annulated, with an all-cis configu-
ration for the two methyl and the two ester groups.
Synthesis 2007, No. 19, 3061–3067 © Thieme Stuttgart · New York

PAPER
Synthesis of s-Indacene Derivatives
3063
termined with DEPT experiments. EI-MS and HRMS spectra were
obtained with a Finnigan MAT 95 spectrometer. IR spectra were re-
corded on a Bruker Tensor 27 spectrophotometer equipped with a
‘GoldenGate’ diamond-ATR unit. Elemental analyses were mea-
sured with an EA 1108 from Fisons Instruments. All starting mate-
rials were commercially available, except 2a,19 2b,20 and 4d,21
which were prepared according to literature procedures. The prepa-
ration of and data for 1a, 5a, 5b,15 6a, 7, and 9 have been previously
reported.7 Compounds 2c,22 2d,23 2e,22 and 4c24 have been previous-
ly reported, but without characterization.
O
OH
Me
Me
E
a)
E = CO2Bn
E
Me
Me
O
OH
1d (E = CO2Bn)
1e (E = CO2allyl)
7 (73%)
b)
E = CO2allyl
c)
Diethyl cis-1,5-Dimethyl-4,8-dioxo-2,3,3a,4,6,7,7a,8-octahydro-
s-indacene-3a,7a-dicarboxylate (1b); Typical Procedure
Ice-cold, concd H2SO4 (1.5 mL) was added dropwise at 0 °C to 5b
(200 mg, 504 mmol). The mixture was stirred at 0 °C for 1 h, then at
23 °C for 16 h, and finally sat. NaHCO3 soln (20 mL) was added
dropwise. The aqueous layer was extracted with CH2Cl2 (5 × 10
mL) and the combined organic layers were dried (MgSO4). After
filtration, the solvent was removed and the residue purified by chro-
matography (silica gel, PE–EtOAc, 2:1, Rf = 0.45) to give 1b (127
mg, 70%) as yellowish crystals; mp 89 °C.
O
O
OAc
Me
Me
c)
Me
Me
OAc
8 (80%)
9 (76% from 7, 35% from 8)
Scheme 4 Deprotection of esters 1d and 1e. Reaction conditions:
(a) H2 (1 bar), Pd/C, i-PrOH, 80 °C, 16 h, 73% (rac/meso 2:1); (b)
HCO2H, Et3N, cat. Pd(OAc)2, cat. Ph3P, THF, 23 °C, 4 h, 80%; (c) Zn,
AcCl, NaOAc, 60 °C, 2 h then 23 °C, 24 h, 76% (from 7), 35% (from
8).
IR (ATR): 2984 (m), 1724 (vs), 1694 (vs), 1613 (s), 1471 (m), 1447
(m), 1422 (s), 1388 (m), 1365 (s), 1332 (m), 1246 (s), 1219 (s), 1200
(s), 1168 (m), 1145 (m), 1125 (s), 1069 (s), 1022 (s), 961 (m), 814
(s) cm–1.
1H NMR (500 MHz, CDCl3): d = 1.25 (t, J = 7.2 Hz, 6 H), 2.31 (s,
6 H), 2.28–2.33 (m, 2 H), 2.43–2.53 (m, 4 H), 2.72–2.76 (m, 2 H),
4.07–4.16 (m, 4 H).
Compound 9 shows a doubled signal set in the NMR spec-
tra (dr 2:1).
13C{1H} NMR (62 MHz, CDCl3): d = 13.9 (CH3), 17.2 (CH3), 31.7
(CH2), 38.9 (CH2), 62.0 (CH2), 70.9 (C), 130.4 (C), 164.8 (C), 171.2
(C), 192.1 (C).
MS (EI, 70 eV): m/z (%) = 360 (50) [M+], 332 (3), 315 (3), 287 (42),
240 (28), 213 (38), 180 (100), 152 (39).
We also cleaved the allyl ester groups in compound 1e by
palladium-catalyzed allylic substitution18 and obtained
quinone derivative 8 as a mixture of tautomers that were
difficult to purify. This mixture showed multiple signal
sets in the NMR spectra, but only one signal in GLC, in-
dicating rapid equilibrium of the tautomers. This mixture
can be converted into diacetate 9 by applying the same
conditions as for 7, but the yield of 9 was low (35%).
Again, compound 9 showed a doubled signal set in the
NMR specta (dr 2:1), and the major diastereomer is the
same as that obtained in the preparation of compound 9
from hydroquinone 7.
Anal. Calcd for C20H24O6 (360.41): C, 66.65; H, 6.71. Found: C,
66.54; H, 6.87.
Dioctyl cis-1,5-Dimethyl-4,8-dioxo-2,3,3a,4,6,7,7a,8-octahydro-
s-indacene-3a,7a-dicarboxylate (1c)
Following the typical procedure for 1b using concd H2SO4 (4 mL)
and 5c (1.00 g, 1.78 mmol) followed by sat. NaHCO3 soln (140 mL)
and extraction with CH2Cl2 (4 × 40 mL). Purification used chroma-
tography (silica gel, PE–EtOAc, 5:1, Rf = 0.43) to give 1c (648 mg,
69%) as a yellowish oil.
In summary, 2,5-dioxocyclohexane-1,4-dicarboxylates 2
(‘succinyl succinates’) are conveniently prepared by con-
densation of two equivalents of succinates 4 under strong-
ly basic reaction conditions. These can be converted with
methyl vinyl ketone 3 in a double Michael reaction into
bis-adducts 5 with high diastereoselectivity. Pure cis-dia-
stereomers of these bis-adducts 5 are obtained by recrys-
tallization. They can be cyclized in a Robinson-type
annulation, a process that yields no six-membered rings,
but s-indacene derivatives 1 as single diastereomers due to
the rather unusual cyclopentene annulation. The C=C
bonds of these tricyclic scaffolds can be hydrogenated to
furnish, again, single diastereomers of diketones 6. Deriv-
atives with benzyl and allyl esters groups can be cleaved
followed by twofold decarboxylation to yield s-indacene
derivatives with a central aromatic ring.
IR (ATR): 2956 (m), 2926 (s), 2856 (s), 1725 (vs), 1694 (vs), 1612
(s), 1458 (m), 1428 (m), 1373 (m), 1328 (m), 1252 (s), 1217 (s),
1164 (s), 1124 (s), 1073 (s), 948 (m), 809 (s) cm–1.
1H NMR (500 MHz, CDCl3): d = 0.84 (t, J = 6.8 Hz, 6 H), 1.17–
1.32 (m, 20 H), 1.57 (quint, J = 6.6 Hz, 4 H), 2.10–2.29 (m, 2 H),
2.27 (s, 6 H), 2.38–2.48 (m, 4 H), 2.68–2.73 (m, 2 H), 3.93–4.05 (m,
4 H).
13C{1H} NMR (125 MHz, CDCl3): d = 14.0 (CH3), 17.1 (CH3), 22.6
(CH2), 25.7 (CH2), 28.2 (CH2), 29.07 (CH2), 29.10 (CH2), 31.66
(CH2), 31.69 (CH2), 38.9 (CH2), 66.0 (OCH2), 70.9 (C), 130.5 (C),
164.5 (C), 171.2 (C), 192.0 (C).
MS (EI, 70 eV): m/z (%) = 528 (31) [M+], 500 (4), 371 (42), 215
(100), 152 (54), 164 (21).
HRMS (EI, 70 eV): m/z [M]+ calcd for C32H48O6: 528.3451; found:
528.3450.
Dibenzyl cis-1,5-Dimethyl-4,8-dioxo-2,3,3a,4,6,7,7a,8-octahy-
dro-s-indacene-3a,7a-dicarboxylate (1d); Typical Procedure
Pyrrolidine (110 mg, 1.5 mmol) and AcOH (93 mg, 1.5 mmol) were
subsequently added at 0 °C to a soln of 5d (400 mg, 768 mmol) in
Preparative column chromatography was carried out using Merck
silica gel 60 with hexanes (PE, bp 40–60 °C) and EtOAc as eluents.
1H and 13C NMR spectra were recorded on a Bruker Avance DRX
500, an Avance DPX 300, and an AC 250. Multiplicities were de-
Synthesis 2007, No. 19, 3061–3067 © Thieme Stuttgart · New York

3064
Y. Zhang, J. Christoffers
PAPER
CH2Cl2 (2 mL). The mixture was stirred at 0 °C for 1 h, then at 50
°C for 16 h. After removal of the solvent, the residue was chromato-
graphed (silica gel, PE–EtOAc, 2:1, Rf = 0.43) to give 1d (170 mg,
46%) as a brown oil.
MS (EI, 70 eV): m/z (%) = 424 (8) [M+], 395 (8), 312 (20), 294 (96),
200 (35), 182 (100), 164 (21), 137 (15).
HRMS (EI, 70 eV): m/z [M]+ calcd for C24H40O6: 424.2825; found:
424.2824.
IR (ATR): 2965 (m), 1721 (vs), 1690 (vs), 1608 (vs), 1498 (m),
1454 (m), 1426 (m), 1372 (s), 1328 (m), 1246 (m), 1211 (s), 1158
(s), 1122 (m), 1069 (s), 945 (m), 821 (s), 793 (m), 735 (vs), 696 (vs),
583 (s) cm–1.
1H NMR (500 MHz, CDCl3): d = 2.27–2.29 (m, 2 H), 2.31 (s, 6 H),
2.40–2.52 (m, 4 H), 2.68–2.74 (m, 2 H), 5.04 (A part of an AB sys-
tem, J = 12.6 Hz, 2 H), 5.05 (B part of an AB system, J = 12.6 Hz,
2 H), 7.26–7.35 (m, 10 H).
Dibenzyl 2,5-Dihydroxycyclohexa-1,4-diene-1,4-dicarboxylate
(2d)
A soln of 4d (5.00 g, 16.8 mmol) in abs DMSO (15 mL) was added
dropwise to a cooled (5 °C) suspension of NaH (1.36 g, 34.0 mmol,
60% dispersion in mineral oil) in abs DMSO (20 mL). After stirring
at 0 °C for 1 h, additional abs DMSO (10 mL) was added, and the
mixture stirred at 23 °C for a further 30 min, and then at 50 °C for
1 h. The mixture was cooled to 0 °C, then neutralized by dropwise
addition of 6 M HCl (ca. 30 mL). The precipitate was filtered off,
washed with H2O (4 × 10 mL), dried under high vacuum, and re-
crystallized [PE (50 mL)–CH2Cl2 (100 mL)–toluene (70 mL)] to
yield 2d as colorless crystals (2.60 g, 81%); mp 169 °C.
13C{1H} NMR (125 MHz, CDCl3): d = 17.2 (CH3), 31.5 (CH2), 39.1
(CH2), 67.4 (OCH2), 70.9 (C), 127.8 (2 CH), 128.1 (CH), 128.5 (2
CH), 130.4 (C), 135.5 (C), 165.2 (C), 171.2 (C), 191.8 (C).
MS (EI, 70 eV): m/z (%) = 484 (19) [M+], 456 (2), 349 (7), 91 (100).
HRMS (EI, 70 eV): m/z [M]+ calcd for C30H28O6: 484.1886; found:
484.1886.
IR (ATR): 3300 (w, br), 2896 (m), 1651 (vs), 1620 (vs), 1497 (s),
1456 (s), 1423 (s), 1395 (s), 1336 (vs), 1212 (vs), 1067 (vs), 969
(m), 951 (m), 830 (m), 802 (m), 735 (vs) cm–1.
1H NMR (500 MHz, CDCl3): d = 3.23 (s, 4 H), 5.23 (s, 4 H), 7.33–
7.40 (m, 10 H), 12.12 (s, 2 H).
Diallyl cis-1,5-Dimethyl-4,8-dioxo-2,3,3a,4,6,7,7a,8-octahydro-
s-indacene-3a,7a-dicarboxylate (1e)
Following the typical procedure for 1d using pyrrolidine (1.69 g,
23.8 mmol), AcOH (1.43 g, 23.8 mmol), and 5e (5.00 g, 11.9 mmol)
in CH2Cl2 (6 mL) gave, after chromatography (silica gel, PE–
EtOAc, 3:1, Rf = 0.36), 1e (1.31 mg, 29%) as a brown oil.
13C{1H} NMR (125 MHz, CDCl3): d = 28.6 (CH2), 66.3 (OCH2),
93.1 (C), 128.0 (2 CH), 128.4 (CH), 128.6 (2 CH), 135.5 (C), 168.8
(C), 171.0 (C).
MS (EI, 70 eV): m/z (%) = 380 (8) [M+], 289 (7), 91 (100).
IR (ATR): 3085 (w), 2950 (m), 1725 (vs), 1692 (vs), 1608 (vs),
1426 (m), 1247 (s), 1211 (vs), 1162 (s), 1071 (m), 990 (m), 947 (m)
cm–1.
Anal. Calcd for C22H20O6 (380.40): C, 69.46; H, 5.30. Found: C,
69.56; H, 5.36.
1H NMR (500 MHz, CDCl3): d = 2.31 (s, 6 H), 2.30–2.37 (m, 2 H),
2.40–2.53 (m, 4 H), 2.70–2.79 (m, 2 H), 4.52 (ddt, J = 13.3, 5.7, 1.4
Hz, 2 H), 4.57 (ddt, J = 13.3, 5.7, 1.5 Hz, 2 H), 5.22 (dq, J = 10.5,
1.3 Hz, 2 H), 5.29 (dq, J = 17.2, 1.5 Hz, 2 H), 5.89 (ddt, J = 17.2,
10.5, 5.6 Hz, 2 H).
13C{1H} NMR (125 MHz, CDCl3): d = 17.2 (CH3), 31.8 (CH2), 39.0
(CH2), 66.4 (OCH2), 70.8 (C), 118.5 (CH2), 130.3 (C), 131.6 (CH),
165.1 (C), 171.0 (C), 191.9 (C).
Diallyl 2,5-Dihydroxycyclohexa-1,4-diene-1,4-dicarboxylate
(2e)
Following the typical procedure for 2c using 4e (20.0 g, 101 mmol)
with NaH (8.08 g, 202 mmol, 60% dispersion in mineral oil) in abs
DMSO (50 mL) at 0 °C, and stirring at 0 °C for 1 h; additional abs
DMSO (20 mL) and stirring at 23 °C for 1 h and 50 °C for 1 h.
Workup used 6 M HCl (ca. 30 mL), the precipitate was washed with
H2O (100 mL), and recrystallization [PE (30 mL)–CH2Cl2 (50 mL)]
to yield 2e as colorless crystals (10.7 g, 76%); mp 109 °C.
MS (El, 70 eV): m/z (%) = 384 (70) [M+], 327 (15), 299 (100), 214
(16), 192 (23).
IR (ATR): 3094 (w), 2950 (w), 2832 (m), 1664 (vs), 1646 (vs), 1616
(vs), 1451 (s), 1439 (s), 1424 (s), 1388 (vs), 1372 (s), 1326 (vs),
1201 (vs), 1131 (s), 1099 (s), 1060 (vs), 1002 (s), 960 (s), 928 (vs),
834 (vs) cm–1.
Anal. Calcd for C22H24O6 (384.43): C, 68.74; H, 6.29. Found: C,
68.85; H, 6.32.
1H NMR (500 MHz, CDCl3): d = 3.23 (s, 4 H), 4.70 (dt, J = 5.5, 1.4
Hz, 4 H), 5.27 (dq, J = 10.3, 1.1 Hz, 2 H), 5.35 (dq, J = 17.4, 1.5 Hz,
2 H), 5.95 (ddt, J = 17.2, 10.5, 5.4 Hz, 2 H), 12.12 (s, 2 H).
13C{1H} NMR (125 Hz, CDCl3): d = 28.5 (CH2), 65.2 (OCH2), 93.1
(C), 118.4 (CH2), 131.8 (CH), 168.8 (C), 170.9 (C).
MS (EI, 70 eV): m/z (%) = 280 (33) [M+], 239 (10), 222 (16), 181
(22), 154 (12), 137 (12), 41 (100).
Dioctyl 2,5-Dihydroxycyclohexa-1,4-diene-1,4-dicarboxylate
(2c); Typical Procedure
Compound 4c (20.0 g, 58.4 mmol) was added dropwise to a cooled
(5 °C) suspension of NaH (4.68 g, 117 mmol, 60% dispersion in
mineral oil) in abs DMSO (80 mL). After stirring at 0 °C for 30 min,
additional abs DMSO (20 mL) was added, and the mixture was
stirred at 23 °C for a further 1 h and then at 50 °C for 1 h. The mix-
ture was cooled to 0 °C, then neutralized by dropwise addition of 6
M HCl (ca. 80 mL). The precipitate was filtered off, washed with
H2O (40 mL), dried under high vacuum, and recrystallized [PE (30
mL)–CH2Cl2 (33 mL)] to yield 2c as colorless crystals (8.69 g,
70%); mp 89 °C.
Anal. Calcd for C14H16O6 (280.28): C, 59.99; H, 5.75. Found: C,
59.74; H, 5.83.
Dioctyl Succinate (4c)
IR (ATR): 2956 (s), 2922 (s), 2853 (s), 1737 (s), 1651 (m), 1614 (s),
1469 (m), 1404 (vs), 1329 (s), 1210 (br, vs), 1164 (m), 1066 (s), 944
(s), 816 (s), 779 (vs) cm–1.
1H NMR (500 MHz, CDCl3): d = 0.88 (t, J = 6.9 Hz, 6 H), 1.26–
1.40 (m, 20 H), 1.67 (quint, J = 6.9 Hz, 4 H), 3.17 (s, 4 H), 4.17 (t,
J = 6.6 Hz, 4 H), 12.20 (s, 2 H).
13C{1H} NMR (125 MHz, CDCl3): d = 14.1 (CH3), 22.6 (CH2), 25.9
(CH2), 28.5 (CH2), 28.6 (CH2), 29.15 (CH2), 29.18 (CH2), 31.8
(CH2), 64.9 (OCH2), 93.3 (C), 168.4 (C), 171.4 (C).
A mixture of succinic anhydride (25.0 g, 250 mmol), octan-1-ol
(97.7 g, 118 mL, 750 mmol), and concd H2SO4 (2.45 g, 25.0 mmol)
in toluene (80 mL) was refluxed for 16 h using a Dean–Stark trap.
Subsequently, the mixture was neutralized by dropwise addition of
sat. aq NaHCO3 (200 mL). Then the aqueous layer was extracted
with CH2Cl2 (3 × 50 mL). The combined organic layers were dried
(MgSO4), filtered, and all volatile materials were removed in vacuo.
The residue was submitted to fractional distillation using a 15-cm
Vigreux column to yield 4c (82.6 g, 96%) as colorless liquid; bp 210
°C/0.14 mbar.
Synthesis 2007, No. 19, 3061–3067 © Thieme Stuttgart · New York

PAPER
Synthesis of s-Indacene Derivatives
3065
IR (ATR): 2955 (m), 2925 (vs), 2856 (s), 1736 (vs), 1466 (m), 1352
(m), 1313 (m), 1157 (vs), 877 (m) cm–1.
1H NMR (300 MHz, CDCl3): d = 0.83 (t, J = 6.5 Hz, 6 H), 1.12–
1.33 (m, 20 H), 1.56 (quint, J = 6.8 Hz, 4 H), 2.56 (s, 4 H), 4.02 (t,
J = 6.8 Hz, 4 H).
(silica gel, PE–EtOAc, 2:1, Rf = 0.30) gave the crude product (3.90
g, only one signal set in NMR spectra). Recrystallization (PE–
EtOAc, 90 mL, 2:1) gave a colorless solid (3.24 g, 79%) consisting
exclusively of the cis-diastereomer of 5d (single set of signals in the
NMR spectra); mp 113 °C.
IR (ATR): 2932 (m), 1755 (s), 1707 (vs), 1497 (m), 1454 (m), 1425
(m), 1370 (s), 1300 (m), 1260 (m), 1135 (m), 1097 (s), 1027 (m),
952 (m), 917 (m), 824 (m), 754 (s), 736 (s), 697 (vs) cm–1.
1H NMR (500 MHz, CDCl3): d = 1.98–2.02 (m, 2 H), 2.03 (s, 6 H),
2.23–2.37 (m, 6 H), 2.65 (d, J = 15.8 Hz, 2 H), 3.20 (d, J = 15.8 Hz,
2 H), 5.04 (A part of an AB system, J = 12.3 Hz, 2 H), 5.05 (B part
of an AB system, J = 12.3 Hz, 2 H), 7.27–7.36 (m, 10 H).
13C{1H} NMR (125 MHz, CDCl3): d = 28.6 (CH2), 29.8 (CH3), 38.0
(CH2), 45.0 (CH2), 59.5 (C), 67.9 (OCH2), 128.3 (2 CH), 128.6
(CH), 128.6 (2 CH), 134.9 (C), 169.4 (C), 201.1 (C), 206.6 (C).
MS (EI, 70 eV): m/z (%) = 520 (1) [M+], 502 (3), 450 (3), 393 (2),
321 (4), 181 (3), 91 (100).
HRMS (EI, 70 eV): m/z [M]+ calcd for C30H32O8: 520.2097; found:
520.2098.
13C{1H} NMR (75 MHz, CDCl3): d = 13.9 (CH3), 22.5 (CH2), 25.8
(CH2), 28.5 (CH2), 29.1 (3 CH2), 31.7 (CH2), 64.7 (OCH2), 172.2
(C).
MS (EI, 70 eV): m/z (%) = 343 (1) [M + H+], 231 (21), 213 (6), 119
(26), 101 (100).
HRMS (EI, 70 eV): m/z [M]+ calcd for C20H38O4: 342.2770; found:
342.2769.
Diethyl cis-2,5-Dioxo-1,4-bis(3-oxobutyl)cyclohexane-1,4-di-
carboxylate (5b)
A mixture of FeCl3·6 H2O (158 mg, 0.585 mmol), 2b (1.50 g, 5.85
mmol), and MVK (3) (4.10 g, 58.5 mmol) in CH2Cl2 (5 mL) was
stirred at 60 °C for 2 d in a tightly closed reaction vial. The mixture
was filtered through silica gel, the residue was washed with MTBE
(50 mL), and the solvent was removed to give the crude product
(2.16 g, 93%) as a reddish solid, as a mixture of two diastereomers
(cis/trans 4:1). Recrystallization of the crude product (PE–EtOAc,
60 mL, 1:2) furnished the pure cis-diastereomer (single set of sig-
nals in the NMR spectra) of 5b (1.22 g, 53%) as a colorless solid;
mp 108 °C. Spectroscopic data are in accord with the literature.15
Diallyl cis-2,5-Dioxo-1,4-bis(3-oxobutyl)cyclohexane-1,4-dicar-
boxylate (5e)
Following the typical procedure for 5c using FeCl3·6 H2O (202 mg,
749 mmol), 2e (2.10 g, 7.49 mmol), and MVK (3) (5.25 g, 74.9
mmol) in CH2Cl2 (8 mL), with purification by chromatography (sil-
ica gel, PE–EtOAc, 2:1, Rf = 0.20) gave a crude product (2.91 g,
only one signal set in NMR spectra). Recrystallization (PE–EtOAc,
15 mL, 2:1) gave a colorless solid (1.90 g, 60%) consisting exclu-
sively of the cis-diastereomer of 5e (single signal set in the NMR
spectra); mp 101–103 °C.
Anal. Calcd for C20H28O8 (396.44): C, 60.59; H,7.12. Found: C,
60.23; H, 7.14.
Dioctyl cis-2,5-Dioxo-1,4-bis(3-oxobutyl)cyclohexane-1,4-dicar-
boxylate (5c); Typical Procedure
A mixture of FeCl3·6 H2O (319 mg, 1.18 mmol), 2c (5.00 g, 11.8
mmol), and MVK (3) (8.27 g, 9.82 mL, 118 mmol) in CH2Cl2 (15
mL) was stirred at 60 °C for 2 d in a tightly closed reaction vial. Af-
ter removal of the solvent, the residue was chromatographed (silica
gel, PE–EtOAc, 2:1, Rf = 0.20) to give the crude product (4.82 g,
only one signal set in NMR spectra). Recrystallization (PE–EtOAc,
30 mL, 1:1) furnished a colorless solid (2.88 g, 43%) consisting ex-
clusively of the cis-diastereomer of 5c (single signal set in the NMR
spectra); mp 77 °C.
IR (ATR): 3083 (w), 2975 (m), 2938 (m), 1751 (s), 1708 (vs), 1446
(m), 1425 (s), 1413 (m), 1372 (s), 1356 (m), 1299 (s), 1265 (s), 1203
(vs), 1168 (vs), 1137 (s), 1102 (s), 1060 (m), 1014 (m), 982 (s), 945
(s), 922 (s), 878 (s), 813 (m) cm–1.
1H NMR (500 MHz, CDCl3): d = 2.06–2.10 (m, 2 H), 2.13 (s, 6 H),
2.20–2.29 (m, 2 H), 2.40–2.49 (m, 4 H), 2.68 (d, J = 15.9 Hz, 2 H),
3.23 (d, J = 15.8 Hz, 2 H), 4.60 (dt, J = 5.8, 1.4 Hz, 4 H), 5.27 (dq,
J = 10.4, 1.2 Hz, 2 H), 5.33 (dq, J = 17.1, 1.4 Hz, 2 H), 5.87 (ddt,
J = 17.1, 10.5, 5.8 Hz, 2 H).
13C{1H} NMR (125 MHz, CDCl3): d = 28.6 (CH2), 29.9 (CH3), 38.2
(CH2), 45.1 (CH2), 59.6 (C), 66.8 (OCH2), 119.4 (CH2), 131.0 (CH),
169.2 (C), 201.2 (C), 206.7 (C).
MS (El, 70 eV): m/z (%) = 420 (4) [M+], 362 (16), 321 (19), 251
(22), 231 (25), 207 (17), 169 (9), 125 (20), 41 (100).
IR (ATR): 2954 (m), 2924 (s), 2854 (s), 1706 (vs), 1464 (m), 1370
(s), 1263 (s), 1187 (vs), 1108 (s), 1017 (s), 954 (s), 724 (s) cm–1.
1H NMR (500 MHz, CDCl3): d = 0.88 (t, J = 6.7 Hz, 6 H), 1.11–
1.18 (m, 20 H), 1.62 (quint, J = 6.5 Hz, 4 H), 2.00–2.07 (m, 2 H),
2.13 (s, 6 H), 2.20–2.27 (m, 2 H), 2.39–2.52 (m, 4 H), 2.63 (d,
J = 15.8 Hz, 2 H), 3.23 (d, J = 15.7 Hz, 2 H), 4.10 (t, J = 6.6 Hz, 4
H).
13C{1H} NMR (125 MHz, CDCl3): d = 14.1 (CH3), 22.6 (CH2), 25.7
(CH2), 28.3 (CH2), 28.5 (CH2), 29.1 (2 CH2), 29.9 (CH3), 31.8
(CH2), 38.3 (CH2), 45.1 (CH2), 59.7 (C), 66.6 (OCH2), 169.6 (C),
201.3 (C), 206.6 (C).
Anal. Calcd for C22H28O8 (420.46): C, 62.85; H, 6.71. Found: C,
62.95; H, 6.76.
Diethyl 1,5-Dimethyl-4,8-dioxododecahydro-s-indacene-3a,7a-
dicarboxylate (6b); Typical Procedure
A suspension of 1b (140 mg, 388 mmol) and 5% Pd/C (50 mg) in i-
PrOH (2 mL) was degassed and subsequently stirred at 80 °C for 16
h under 1 atm of H2 (balloon). After filtration, the solvent was re-
moved and the residue was purified by chromatography (silica gel,
PE–EtOAc, 2:1, Rf = 0.39) yielding 6b (121 mg, 86%) as a colorless
solid; mp 40 °C.
MS (EI, 70 eV): m/z (%) = 564 (4) [M+], 494 (8), 434 (8), 424 (10),
406 (100), 388 (6), 337 (23), 322 (23), 294 (50), 276 (77), 251 (15),
152 (13), 125 (18).
HRMS (EI, 70 eV): m/z [M]+ calcd for C32H52O8: 564.3662; found:
564.3662.
Anal. Calcd for C32H52O8 (564.76): C, 68.06; H, 9.28. Found: C,
68.46; H, 9.15.
IR (ATR): 2965 (s), 1708 (vs), 1447 (s), 1366 (s), 1303 (m), 1291
(m), 1258 (m), 1231 (s), 1206 (m), 1180 (m), 1100 (s), 937 (m), 892
(s), 838 (m), 721 (m) cm–1.
Dibenzyl cis-2,5-Dioxo-1,5-bis(3-oxobutyl)cyclohexane-1,4-di-
carboxylate (5d)
Following the typical procedure for 5c using FeCl3·6 H2O (316 mg,
1.17 mmol), 2d (3.00 g, 7.89 mmol), and MVK (3) (5.53 g, 78.9
mmol) in CH2Cl2 (10 mL), with purification by chromatography
1H NMR (500 MHz, CDCl3): d = 1.14 (d, J = 6.6 Hz, 6 H), 1.23
(ddt, J = 12.2, 7.1, 11.7 Hz, 2 H, 2-CHaxH), 1.30 (t, J = 7.1 Hz, 6 H,
CH3), 1.75 (ddt, J = 12.5, 2.8, 6.6 Hz, 2 H, 2-CHHeq), 1.87 (ddd,
J = 13.7, 6.9, 2.7 Hz, 2 H, 3-CHHeq), 2.24 (ddquint, J = 10.5, 9.0,
6.5 Hz, 2 H, 1-CHeq), 2.44 (ddd, J = 13.8, 11.6, 6.7 Hz, 2 H, 3-
Synthesis 2007, No. 19, 3061–3067 © Thieme Stuttgart · New York

3066
Y. Zhang, J. Christoffers
PAPER
CHaxH), 3.59 (d, J = 8.9 Hz, 2 H, CH), 4.16–4.22 (m, 2 H, CH2),
4.30–4.36 (m, 2 H, CH2).
Acknowledgment
This work was generously supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie. We are gra-
teful to Dr. Peter Fischer, Universität Stuttgart, for helping us with
interpretation of NMR spectra.
13C{1H} NMR (62 MHz, CDCl3): d = 12.9 (CH3), 18.5 (CH3), 31.5
(CH2), 31.8 (CH2), 35.2 (CH), 60.4 (CH), 61.5 (CH2), 67.3 (C),
171.0 (C), 201.3 (C).
MS (EI, 70 eV): m/z (%) = 364 (29) [M+], 318 (100), 291 (25), 262
(12), 217 (39), 109 (44), 81 (53).
References
Anal. Calcd for C20H28O6 (364.44): C, 65.92; H, 7.88. Found: C,
65.64; H, 7.74.
(1) Modern Aldol Reactions; Mahrwald, R., Ed.; Wiley-VCH:
Weinheim, 2004.
Dioctyl 1,5-Dimethyl-4,8-dioxododecahydro-s-indacene-3a,7a-
dicarboxylate (6c)
Following the typical procedure for 6b using 1c (60 mg, 113 mmol)
and 5% Pd/C (8 mg) in i-PrOH (2 mL) with purification by chroma-
tography (silica gel, PE–EtOAc, 10:1, Rf = 0.41) gave 6c (36 mg,
60%) as a colorless oil.
(2) Christoffers, J. In Encyclopedia of Catalysis, Vol. 5;
Horvath, I., Ed.; Wiley: New York, 2003, 99.
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Reviews: (b) Gawley, R. E. Synthesis 1976, 777. (c) Jung,
M. E. Tetrahedron 1976, 32, 3.
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C. Eur. J. Org. Chem. 2006, 1403. (b) Röttger, S.;
Waldmann, H. Eur. J. Org. Chem. 2006, 2093. (c) Payette,
J. N.; Honda, T.; Yoshizawa, H.; Favaloro, F. G.; Gribble, G.
W. J. Org. Chem. 2006, 71, 416. (d) Gan, Y.; Spencer, T. A.
J. Org. Chem. 2006, 71, 5870.
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2005, 5339. (b) Kreidler, B.; Baro, A.; Frey, W.;
Christoffers, J. Chem. Eur. J. 2005, 11, 2660. (c) Kreidler,
B.; Baro, A.; Christoffers, J. Synlett 2005, 465. Review:
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Scharl, H.; Frey, W.; Baro, A. Eur. J. Org. Chem. 2004,
2701. (c) Christoffers, J.; Scharl, H.; Frey, W.; Baro, A. Org.
Lett. 2004, 6, 1171. (d) Christoffers, J.; Scharl, H. Eur. J.
Org. Chem. 2002, 1505.
IR (ATR): 2955 (s), 2926 (s), 2857 (s), 1712 (vs), 1461 (s), 1376
(m), 1306 (m), 1230 (s), 1176 (m), 1135 (m), 946 (m), 896 (m) cm–1.
1H NMR (500 MHz, CDCl3): d = 0.85 (t, J = 6.7 Hz, 6 H), 1.11 (d,
J = 6.5 Hz, 6 H), 1.12–1.39 (m, 22 H), 1.63 (quint, J = 6.8 Hz, 4 H),
1.74 (ddt, J = 12.4, 2.5, 6.3 Hz, 2 H), 1.87 (ddd, J = 13.7, 6.9, 2.5
Hz, 2 H), 2.24 (ddquint, J = 10.3, 8.8, 6.6 Hz, 2 H), 2.43 (ddd,
J = 13.4, 11.8, 6.7 Hz, 2 H), 3.59 (d, J = 8.8 Hz, 2 H), 4.06–4.15 (m,
2 H), 4.18–4.27 (m, 2 H).
13C{1H} NMR (125 MHz, CDCl3): d = 14.0 (CH3), 19.5 (CH3), 22.6
(CH2), 25.9 (CH2), 28.4 (CH2), 29.1 (2 CH2), 31.7 (CH2), 32.5
(CH2), 32.8 (CH2), 36.2 (CH), 61.4 (CH), 66.7 (OCH2), 68.3 (C),
172.1 (C), 202.3 (C).
MS (EI, 70 eV): m/z (%) = 532 (12) [M+], 421 (13), 402 (100), 374
(21), 374 (4), 217 (31).
HRMS (EI, 70 eV): m/z [M]+ calcd for C32H52O6: 532.3764; found:
532.3764.
(7) Christoffers, J.; Zhang, Y.; Frey, W.; Fischer, P. Synlett
2006, 624.
(8) Buchanan, G. L.; Kennedy, W. B.; Lawson, A. M. J. Chem.
Soc., Perkin Trans. 1 1977, 32.
1,5-Dimethyl-1,2,3,5,6,7-hexahydro-s-indacene-4,8-dione (8)
Under an inert atmosphere, a mixture of HCO2H (72 mg, 1.6 mmol)
and Et3N (160 mg, 1.6 mmol) in abs THF (0.5 mL) was added at 23
°C to a stirred soln of Pd(OAc)2 (2.9 mg, 13 mmol) and Ph3P (5.1
mg, 20 mmol) in abs THF (0.5 mL). After vigorously stirring the
mixture for 15 min, a soln of 1e (100 mg, 260 mmol) in abs THF (1.0
mL) was added. The mixture was then stirred at 23 °C for 4 h, sub-
sequently the solvent was removed in vacuo, and the residue chro-
matographed (silica gel, PE–EtOAc, 3:1, Rf = 0.49) to give the
product 8 (45.0 mg, 80%) as a mixture of several tautomers and as
a brown solid. NMR spectra show several signal sets.
(9) (a) García Cuesta, I.; Coriani, S.; Lazzeretti, P.; Sánchez de
Merás, A. M. J. ChemPhysChem 2006, 7, 240. (b) Güell,
M.; Matito, E.; Luis, J. M.; Poater, J.; Sola, M. J. Phys.
Chem. A 2006, 110, 11569.
(10) (a) Dahrouch, M.; Burgos, F.; Castel, A.; Chavez, I.;
Gornitzka, H.; Manriquez, J. M.; Riviere, P.; Riviere-
Baudet, M.; Alvarez, J.; Onyszchuk, M. Appl. Organomet.
Chem. 2007, 21, 31. (b) Lian, J.-J.; Lin, C.-C.; Chang, H.-
K.; Chen, P.-C.; Liu, R.-S. J. Am. Chem. Soc. 2006, 128,
9661. (c) Dahrouch, M. R.; Jara, P.; Mendez, L.; Portilla, Y.;
Abril, D.; Alfonso, G.; Chavez, I.; Manriquez, J. M.
Organometallics 2001, 20, 5591. (d) Hafner, K.; Stowasser,
B.; Krimmer, H.-P.; Fischer, S.; Böhm, M. C.; Lindner, H. J.
Angew. Chem., Int. Ed. Engl. 1986, 25, 630; Angew. Chem.
1986, 98, 646. (e) Birladeanu, L.; Chamot, E.; Fristad, W.
E.; Paquette, L. A.; Winstein, S. J. Org. Chem. 1977, 42,
3260. (f) LeGoff, E.; LaCount, R. B. Tetrahedron Lett.
1964, 5, 1161.
IR (ATR): 3417 (s, br), 2952 (vs), 2863 (m), 1689 (vs), 1476 (vs),
1446 (vs), 1372 (m), 1326 (m), 1262 (vs), 1224 (s), 1144 (s), 1052
(s), 939 (s), 814 (s) cm–1.
MS (EI, 70 eV): m/z (%) = 216 (66) [M+], 201 (100), 186 (5), 174
(12).
HRMS (EI, 70 eV): m/z [M]+ calcd for C14H16O2: 216.1150; found:
(11) (a) Hudlicky, T.; Fleming, A.; Radesca, L. J. Am. Chem. Soc.
1989, 111, 6691. (b) Woodward, R. B.; Hoye, T. R. J. Am.
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216.1150.
4,8-Diacetoxy-1,5-dimethyl-1,2,3,5,6,7-hexahydro-s-indacene
(9)
(12) (a) Esponda, E.; Adams, C.; Burgos, F.; Chavez, I.;
Manriquez, J. M.; Delpech, F.; Castel, A.; Gornitzka, H.;
Riviere-Baudet, M.; Riviere, P. J. Organomet. Chem. 2006,
691, 3011. (b) Roussel, P.; Cary, D. R.; Barlow, S.; Green, J.
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A suspension of quinone 8 (40.0 mg, 185 mmol), abs NaOAc (61
mg, 0.74 mmol), and Zn dust (182 mg, 2.78 mmol) in AcCl (3 mL)
was stirred at 60 °C for 2 h and then at 23 °C for 16 h. The mixture
was poured into ice water (30 mL). The aqueous layer was extracted
with CH2Cl2 (5 × 10 mL). The combined organic layers were dried
(MgSO4), filtered, and concentrated in vacuo. The residue was
chromatographed (silica gel, PE–EtOAc, 3:1, Rf = 0.53) to yield 9
as a mixture of two diastereomers (dr 2:1) and as a yellowish oil (20
mg, 35%). Spectroscopic data are in accord with the literature.7
Synthesis 2007, No. 19, 3061–3067 © Thieme Stuttgart · New York

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Synthesis of s-Indacene Derivatives
3067
(13) (a) Chen, H. Z.; Jin, Y. D.; Xu, R. S.; Peng, B. X.; Desseyn,
H.; Janssens, J.; Heremans, P.; Borghs, G.; Geise, H. J.
Synth. Met. 2003, 139, 529. (b) Halterman, R. L.; Jan, S.-T.;
Abdulwali, A. H.; Standlee, D. J. Tetrahedron 1997, 53,
11277.
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Phys. Chem. Chem. Phys. 2005, 7, 2664. (b) Pelzer, S.;
Kauf, T.; van Wüllen, C.; Christoffers, J. J. Organomet.
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2001, 723.
(17) Christoffers, J.; Mann, A. Eur. J. Org. Chem. 2000, 1977.
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3416.
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35, 4073.
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342, 546.
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Soc. Jpn. 1989, 62, 2353.
(16) Christoffers, J.; Schuster, K. Chirality 2003, 15, 777.
Synthesis 2007, No. 19, 3061–3067 © Thieme Stuttgart · New York