A new biomimetic-like aromatization of the cyclic end groups of terpenoids with stereospecific migration of one of the methyl groups: A convenient route to isorenieratene (φ,φ-carotene)

Article abstract of DOI:10.1002/ejoc.200600794

The synthesis of isorenieratene, a natural carotenoid isolated from the marine sponge Reniera japonica and from some anoxygenic phototrophic bacteria or nonphotosynthetic actinomycetes, was performed from α-, β- and retro-ionones. In this series of cycloh

Full text of DOI:10.1002/ejoc.200600794

FULL PAPER
DOI: 10.1002/ejoc.200600794
A New Biomimetic-Like Aromatization of the Cyclic End Groups of
Terpenoids with Stereospecific Migration of One of the Methyl Groups: A
Convenient Route to Isorenieratene (φ,φ-Carotene)
[a]
[a]
[a]
[a]
[a]
Alain Valla,* Zo Andriamialisoa, Benoist Valla, Roger Labia, Régis Le Guillou,
[b]
[a]
Laurent Dufossé, and Dominique Cartier
Keywords: Isorenieratenes / Carotenoids / Aromaticity
The synthesis of isorenieratene, a natural carotenoid isolated
from the marine sponge Reniera japonica and from some
anoxygenic phototrophic bacteria or nonphotosynthetic acti-
nomycetes, was performed from α-, β- and retro-ionones. In
this series of cyclohexenes, this synthesis is the first to in-
clude a one-pot aromatization of the cyclic end group with
concomitant regioselective migration of one methyl group
from the gem dimethyl functionality.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
We have recently reported a new synthesis of isorenierat- methylcyclohexadiene “tethyatene” (a carotenoid isolated
ene, a natural carotenoid first isolated from the marine from a marine sponge) into φ,χ-carotene, by MnO in a
2
sponge Reniera japonica.[ The biosynthesis of the aromatic mixture of CH Cl /acetone (10% yield), Scheme 2.
1]
[6–8]
2
2
φ,φ-carotene (isorenieratene) is restricted to green or brown
phototrophic bacteria (Chlorobium, Pelochromatium,
[2]
Phaeobium) and a few actinomycetes. It is produced by
Brevibacterium linens, a member of the leading group of co-
[
3]
ryneform bacteria . This compound has also been pre-
viously extracted from Mycobacterium aurum A+.[
4]
It was generally supposed that the aromatic rings of these
compounds resulted from a classical β-ring carotenoid pre-
cursor. Formation of the phenyl end group (1,2,5-trimeth-
ylphenyl) from the original β-ring involved oxidation of the
cyclohexene ring and migration of one methyl of the gem Scheme 2.
dimethyl group. Incubation of Chloropseudomonas ethylica
1
4
with [2- C]-mevalonate showed that the methyl group at
C-1 contained a 14C label; therefore, methyl migration could
proceed as outlined in Scheme 1.[
To the best of our knowledge, in this series of terpenes,
the only related example reported in the literature is the
aromatization of the end group by oxidation of 2,6,6-tri-
This compound was also named renieratene and has
5]
been isolated from a marine sponge, Reniera japonica, along
[9]
with its two isomers, isorenieratene and renierapurpurine.
We report herein that the oxidation of β-ionone 2 and α-
ionone 3, which have the β and the ε end group of carot-
enoids, respectively, leads to C13 aromatic ketone 1. This
reaction proceeds easily with low to moderate yields.
Tables 1 and 2 show the experimental conditions for the
aromatization reaction.
Improved yields, with the use of particularly mild condi-
[
10]
Scheme 1.
tions, could be obtained with retro-ionone 4 (Table 3). It
is known that among the retinoids, retro compounds are
a] FRE 2125 CNRS, Chimie et Biologie des Substances Natu- more unstable and reactive to oxidation.
These aromatization reactions can be rationalized by a
b] Laboratoire de Chimie des Substances Naturelles et des Sci- mechanism involving two hydride transfers from the same
[
[
relles,
6
, rue de l’Université, 29000 Quimper, France
ences des Aliments, Université de La Réunion, Faculté des Sci-
enol. In the case of retro-ionone, its formation may be
easier, which would result in a shorter reaction time and a
better yield. The first hydride transfer from the enol to the
ences et Techniques, E.S.I.D.A.I.,
1
5 avenue René Cassin, B. P. 7151, 97715 Saint-Denis Cedex 9,
La Réunion, France
Eur. J. Org. Chem. 2007, 711–715
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
711

A. Valla et al.
FULL PAPER
Scheme 3.
Table 1. Experimental conditions for the aromatization of 2.
Table 3. Yields and reaction conditions with the use of retro-ionone
4.
Solvent
NMP
Oxidant
equiv.]
Conditions
Temp. [°C], Time [h] [%]
Yield
Yield[a]
[
Solvent
Oxidant
[equiv.]
Conditions
Temp. [°C], Time [h] [%]
30[
a]
a]
b]
a]
a]
a]
CuCl
2
2
100, 15
60, 4
reflux, 48
reflux, 48
C
2
H
4
Cl
2
DDQ 3
DDQ 3
DDQ 4
20[
15[
43[
DMF
DMF
CuCl 4, LiCl 1 100, 3
58
65
50
2
Toluene
Dioxane
DMF
FeCl 4
100, 15
60, 1
3
[b]
C H Cl
2
4
2
DDQ 3
CuCl 4, LiCl 1 120, 5
3
FeCl 4 100, 20
2
45[
24[
[a] Yield determined from the crude product. [b] Yield determined
from the purified product.
DMF
[a] Yield determined from the crude product. [b] Yield determined
from the purified product.
The syntheses of the C15 aldehyde [(2E,4E)-3-methyl-5-
(
2,3,6-trimethylphenyl)-2,4-pentadienal] from the C corre-
Table 2. Experimental conditions for the aromatization of 3.
13
[
12]
sponding ketone, have already been depicted,
as well as
these of the 4-methoxy analogue from the C14 ketone
[
2
(2E,4E)-5-(4-methoxy-2,3,6-trimethylphenyl)-3-methyl-
[
13]
,4-pentadienal].
A new improved synthesis allowed for
the formation of the latter in only two steps, with excellent
yield (82%). Thus, a Knoevenagel condensation of aromatic
ionone 1 with cyanoacetic acid in piperidine/benzene
(Dean–Stark, reflux) led to nitrile 5 [(2E,4E)-3-methyl-5-
Solvent
Oxidant
equiv.]
Conditions
Temp. [°C], Time [h] [%]
Yield
[
30[
20[
a]
b]
C
2
H
4
Cl
Toluene
2
DDQ 3
DDQ 3
60, 4
reflux, 48
(2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4-pentadienenitrile] as
[
a] Yield determined from the crude product. [b] Yield determined a mixture of isomers 9E/9Z, 95:5 in nearly quantitative
from the purified product.
yield (96%).
Piperidine (as base) and benzene (as solvent) were very
oxygen of quinone led to the formation of a cyclohexadiene important in the work up of the reaction (other bases and
via a carbocation or the elimination of the quinone group.
The second hydride transfer occurs from the cyclohexadiene
to the quinone and is followed by concomitant rearrange-
ment of the obtained 1,1-gem-dimethyl carbocation to the
more stable 1,2-dimethyl carbocation and the ultimate for-
[11]
mation of trimethyl-1,2,5-benzene (Scheme 3).
Scheme 4.
Scheme 5.
12
7
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 711–715

A Convenient Route to Isorenieratene (φ,φ-Carotene)
FULL PAPER
Scheme 6.
solvents led to a mixture of cyano acids and nitriles).[14] In Experimental Section
this case, the intermediary formed cyano acid was regiose-
1
All reactions were carried out under an argon atmosphere. H- and
lectively decarboxylated (9E/9Z 95:5) in the reaction mix-
ture. The all-E nitrile was easily purified by recrystallization
in a mixture of pentane/ether, 70:30 (Scheme 4).
Thus, the C15 nitrile was reduced by DIBAL-H in tolu-
ene (0 °C, 30 min.) into the corresponding aldehyde (86%,
Scheme 5).
1
3
C NMR spectra were recorded with a Bruker Avance DPX 400
spectrometer. Chemical shifts are reported in ppm (δ) relative to
TMS. IR spectra were recorded with a Bruker IF 55 spectrometer.
4
-(2,3,6-Trimethylphenyl)-3-buten-2-one (1): A mixture of retro-α-
ionone (9.62 g, 50 mmol) and 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone (DDQ) (34 g, 3 equiv.) in 1,2-dichloroethane (100 mL)
were heated at 65 °C for 1 h. The mixture was filtered, and the
solvent was removed under reduce pressure. The crude product was
A Stobbe-like condensation with methyl isopropylidene
[1]
cyanoacetate 7 (reported in ref. ) followed by decarboxyl-
ation of the crude cyanoacid, resulted in nitrile 8 (13E/13Z, purified by column chromatography on silica gel, with the use of
8
0:20). The good stereoselectivity of the reaction can be pentane/1,2-dichloroethane, 1:1, as the eluent. The product (4.72 g,
1
explained by the mechanistic pathway. The opening of the 50%) was obtained as a yellow oil. H NMR (400 MHz, CDCl
obtained intermediary lactone (under kinetic conditions)
led to a cyano acid which possesses a specific configura-
3
):
H), 7.05 and 6.95 (2d, J = 7 Hz,2
H), 6.25 (d, J = 16 Hz, 1 H, C H), 2.40, 235, 2.30, 2.25
) ppm. C NMR (100 MHz, CDCl ): δ = 188.0
CO), 134.6 (Cq), 134.5, 133.5, 143.1 (CH), 133.4, 129.8, 127.5,
δ = 7.72 (d, J = 16 Hz, 1 H, C
H, C H, C
(4s, 12 H, CH
7
3
4
8
1
3
[
15]
3
3
tion (Scheme 6).
(
This latter compound was further decarboxylated into
–
1
2
7.4 (CH
2E,4E)-3-Methyl-5-(2,3,6-trimethylphenyl)-2,4-pentadienenitrile (5):
In a flask equipped with a Dean–Stark trap, cyanoacetic acid
8.21 g, 96 mmol, 3 equiv.) was added to a solution of ketone 1 (6 g,
2 mmol) in benzene (30 mL). The mixture was cooled to 0 °C and
3
), 20.8, 16.9 ppm. IR (film): ν˜ = 1680 cm .
[
16]
nitrile 8 with the use of Corey’s pathway (Scheme 7).
(
(
3
piperidine (25.43 mL, 256 mmol, 8 equiv.) was slowly added. The
mixture was then heated at reflux for 4 h. Benzene was removed
under reduced pressure, and the crude product was extracted with
diethyl ether, the organic layer was washed with brine, water and
dried with MgSO
sure, the crude product was extracted with CH
4
. After removing the ether under reduced pres-
Cl and quickly
2
2
filtered through a mixture of basic alumina and silica (gel) (40:60),
to provide 6.49 g (96%) of yellow ochre crystals of the major iso-
mer 2E,4E-nitrile 6 (ratio determined by H NMR) which was puri-
Scheme 7.
1
Nitrile 8 was reduced into C20 aldehyde 9 (13E/13Z,
0:20) by DIBAL-H. The all-E isomer was easily separated
fied by recrystallization (ether/pentane, 30:70). F: 62 °C (2E,4E).
8
1
7
3
H NMR (400 MHz, CDCl ): δ = 7.02 (d, J = 16.3 Hz, 1 H, H ),
3
4
by column chromatography and the reductive coupling of 6.98 (d, J = 7.5 Hz, 1 H, H ), 6.96 (d, J = 7.5 Hz, 1 H, H ), 6.28
the latter gave isorenieratene 10[ (Scheme 8).
17]
8
10
3
(d, J = 16.3 Hz, 1 H, H ), 5.25 (s, 1 H, H ), 2.32 (s, 3 H, 2-CH ),
Scheme 8.
Eur. J. Org. Chem. 2007, 711–715
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
713

A. Valla et al.
), 1.55 (s, 3 H, 9-CH ) ppm.
FULL PAPER
2
.26 (s, 3 H, 9-CH
3
), 2.25 (s, 3 H, 5-CH
C NMR (100 MHz, CDCl ): δ = 118.0 (CN), 157.1 (Cq), 136.0,
34.9, 134.7, 133.8, 135.9 (CH), 134.5, 129.7, 127.8, 98.3, 21.2
3
), 2.2 (s, 3 H, 1-CH
3
) ppm. (s, 3 H, 5-CH
3
), 2.14 (s, 3 H, 1-CH
3
3
1
3
IR (film): ν˜ = 1660 cm–1.
3
1
1
0: The synthesis employed for the reductive coupling of 8 was
–1
(CH
3
), 20.7, 17.4, 16.9 ppm. IR (film): ν˜ = 2206 cm .
[13]
carried out as described by Paust and Manchand. Under an at-
(
2E,4E)-3-Methyl-5-(2,3,6-trimethylphenyl)-2,4-pentadienal (6): At mosphere of argon, powdered LiAlH (190 mg, 5 mmol) was added
4
0
°C, DIBAL-H (1.2  in toluene, 24 mL, 1.2 equiv.) was slowly
added under vigorous stirring to nitrile 6 (5 g) in toluene. The solu-
tion was stirred for 2 h and hydrolyzed by a solution of 2  H SO
After filtration of the aluminium salts, ether (50 mL) and water
50 mL) were added, and the organic layer was then washed with
brine, water and dried with MgSO . The solvent was distilled under
reduced pressure, and the crude product was purified by column
chromatography on silica gel (CH Cl ) to yield the major isomer
E,E) of aldehyde 5, as a yellow oil (85%). H NMR (400 MHz,
to TiCl (1.53 g, 10 mmol) in anhydrous THF (30 mL). After 2 h
at room temperature, 9 (5 mmol) in THF (10 mL) was added. The
3
2
4
.
mixture was stirred overnight and HCl (2 , 50 mL) was slowly
added, at 0 °C. The crude mixture was extracted with diethyl ether
and washed with brine. The solvent was distilled under reduced
pressure, and the crude product was purified by rapid column
chromatography (neutral Al O , pentane/dichloromethane, 50:50)
(
4
2
3
2
2
to yield 85% of 10. A pure sample was obtained by HPLC (Lichro
1
1
(
5 µm CART RP 18 Merck, methanol/hexane, 80:20). H NMR
11
[18]
CDCl
3
): δ = 10.20 (d, J = 8.1 Hz, 1 H, H ), 7.18 (d, J = 16.4 Hz,
H, H ); 7.01 (2d, J = 7.6 Hz, 2 H, H , H ), 6.36 (d, J = 16.4 Hz,
spectrum was identical to the one previously described.
7
3
4
1
1
2
8
10
H, H ), 6.14 (d, J = 8.1 Hz, 1 H, H ), 2.44 (s, 3 H, 9-CH
.26 (s, 6 H, 1-CH , 2-CH ), 2.23 (s, 3 H, 5-CH
): δ = 191.3 (CO), 154.1 (Cq), 135.9, 134.5, 134.4,
33.5, 137.2 (CH), 135.2, 129.7, 129.2, 127.5, 20.8 (CH ), 20.4,
3
),
) ppm. 13C NMR Acknowledgments
3
3
3
(100 MHz, CDCl
3
1
1
3
We are indebted to Pr. B. Corbel and Dr. J.-J. Yaouanc (UBO) for
helpful discussions.
–1
3.0, 9.7 ppm. IR (film): ν˜ = 1659 cm .
Methyl 2-Cyano-3-methyl-2-butenoate (7): In a flask equipped with
Soxhlet apparatus containing CaCl methyl cyanoacetate
177 mL, 1.986 mol), β-alanine (0.88 g, 9.9 mmol, 0.005 equiv.),
acetone (294 mL, 3.972 mol, 2 equiv.) and acetic acid (37.5 mL,
55.4 mmol, 0.33 equiv.) were heated at reflux for 19 h. After cool-
ing to room temperature, the crude mixture was neutralized with a
saturated solution of NaHCO . The solvent was removed under
reduced pressure, and the crude product was rectified to provide 7
a
(
2
,
[
1] A. Valla, D. Cartier, B. Valla, R. Le Guillou, Z. Andriamiali-
soa, R. Labia, D. Breithaupt, S. Savy, L. Binet, A. Dufossé,
Helv. Chim. Acta 2003, 86, 3314–3319.
6
[2] H. Krugel, P. Krubasik, K. Weber, G. Saluz, H. P. Sandmann,
Biochim. Biophys. Acta 1999, 1439, 57–64.
3] G. Krubasik, P. Sandmann, Mol. Gen. Genet. 2000, 263, 423–
432.
3
[
1
(
(
2
133 g, 49%) as a colourless oil. B.p. (2 mm): 88–90 °C. H NMR
400 MHz, CDCl ): δ = 3.82 (s, 3 H, CO CH ), 2.41 (s, 3 H, CH ),
.32 (s, 3 H, CH ) ppm. IR (film): ν˜ = 2259, 1734 cm .
[
4] M. Viveiros, Ph. Krubasik, G. Sandmann, M. Houssaini-Ira-
qui, FEMS Microbiol. Lett. 2000, 187, 95–101; M. Houssaini-
Iraqui, H. L. David, S. Clavel-Sérès, F. Hilali, N. Rastogi, Curr.
Microbiol. 1993, 27, 317–322; M. Houssaini-Iraqui, S. Clavel-
Sérès, H. L. Rastogi, N. David, Curr. Microbiol. 1993, 27, 317–
3
2
3
3
–1
3
(
2E,4E,6E,8E)-3,7-Dimethyl-9-(2,3,6-trimethylphenyl)-2,4,6,8-nona-
tetraenenitrile (8): At 0 °C, MeOK (2.1 g, 3 mmol) in methanol
10 mL) was added to a mixture of 6 (1 mmol) and 7 (1 mmol).
322.
(
[
[
5] D. J. Moshier, S. E. Chapman, Biochem. J. 1973, 136, 395–404.
6] A. Shimada, Y. Ezaki, J. Inanaga, T. Katsuki, M. Yamaguchi,
Tetrahedron Lett. 1981, 22, 773–774.
After 2 d at r.t., the crude mixture was poured into water (100 mL)
and extracted with diethyl ether. The aqueous layer was acidified
with a solution of 10% HCl and extracted with diethyl ether. The
solvent was removed under reduced pressure, and the crude cya-
noacid (80%) was dissolved in piperidine (20 mL) and heated at
reflux for 4 h. The piperidine was distilled under reduced pressure,
and the crude mixture was extracted with diethyl ether and success-
ively washed with a solution of 5% HCl and water. The solvent
was evaporated, and the major isomer (all-E) was separated from
[
7] Y. Tanaka, T. Katayama, Bull. Jpn. Soc. Sci. Fish. 1977, 43,
1229–1232.
[8] L. I. Vereschchagin, S. R. Gainulina, S. A. Podskrebysheva,
L. A. Gaiboronskii, L. L. Okhapkina, V. G. VoroЈeva, V. P. La-
tyshev, J. Org. Chem. USSR 1972, 8, 1143–1147.
[
9] M. Yamaguchi, Bull. Chem. Soc. Jpn. 1957, 30, 111–114; M.
Yamaguchi, Bull. Chem. Soc. Jpn. 1957, 30, 979–985; M. Yam-
aguchi, Bull. Chem. Soc. Jpn. 1958, 31, 739–742.
crude nitrile 8 (13E/13Z, 80:20, 98%) by column chromatography
[
[
10] P. Ancel, J.-E. Meilland, PCT 0002854 2000 01 20.
1
on silica gel with CH
CDCl
2
Cl
2
(63%). All-E: H NMR (400 MHz,
11] For reviews on 2,3-dichloro-5,6-dicyanobenzoquinone and its
reactions, see: D. Walker, J. D. Hiebert, Chem. Rev. 1967, 67,
153–196; P. P. Fu, R. G. Harvey, Chem. Rev. 1978, 78, 317–361.
3
4
7
3
): δ = 6.99 (m, 3 H, H , H , H ), 6.77 (d, J = 16.4 Hz, 1 H,
H ), 6.32 (m, 2 H, H , H ), 6.19 (d, J = 11.5 Hz, 1 H, H ), 5.21
8
11
12
10
14
(
s, 1 H, H ), 2.27 (s, 6 H, 2-CH
CH , 13-CH ), 2.10 (s, 3 H, 9-CH
CDCl ): δ = 118.1 (CN), 157.8 (Cq), 140.7, 137.0, 134.4, 133.5,
38.3 (CH), 132.2, 130.0, 129.4, 128.5, 127.3, 97.4, 20.9 (CH ),
3
, 5-CH
3
), 2.24, 2.23 (2s, 6 H, 1- [12] R. Le Guillou, Doctorat de lЈUniversité de Bretagne Occiden-
) ppm. 13C NMR (100 MHz,
tale, 21 Avril 2004.
3
3
3
[
13] For the synthesis of the 4-OMe analogue, see: A. Valla, Z. And-
riamialisoa, V. Prat, A. Laurent, M. Giraud, P. Labia, R. Pot-
ier, Tetrahedron 2000, 56, 7211–7215; see also: O. A. Shavryg-
ina, S. M. Makin, I. E. Mikerin, E. K. Dobrestova, Zh. Org.
Khim. 1993, 29, 682–686; B. A. Pawson, K. Chan, R. De No-
ble, R. Han, V. Piermattie, A. C. Specian, S. Srisethnil, W. T.
Trown, O. Bohoslawec, L. J. Machlin, E. Gabriel, J. Med.
Chem. 1979, 22, 1059–1067 and references therein.
3
1
2
3
–1
0.5, 17.0, 16.6, 13.0 ppm. IR (film): ν˜ = 2206 cm .
(
2E,4E,6E,8E)-3,7-Dimethyl-9-(2,3,6-trimethylphenyl)-2,4,6,8-nona-
tetraenal (9): As described above for the preparation of 6, aldehyde
was obtained as a brown oil. The major isomer (all-E) was sepa-
rated from the crude product (13E/13Z, 80:20, 98%) by column
9
[
14] In this series, for studies on decarboxylation, see: H. O. Huis-
man, A. N. Smit, J. H. van Leuwen, P. H. Van Rij, Recl. Trav.
Chim. Pays-Bas 1956, 75, 977–1006; A. N. Smit, Recl. Trav.
Chim. Pays-Bas 1961, 80, 891–904.
1
chromatography on silica gel with CH
2
Cl
2
(80%). All-E: H NMR
15
(
(
400 MHz, CDCl ): δ = 10.12 (d, J = 8.1 Hz,1 H, H ), 6.98–7.36
m, 4 H, H , H , H , H ), 6.77 (d, J = 15.2 Hz, 1 H, H ), 6.42 (d,
J = 15.0 Hz, 1 H, H ), 6.28 (m, 1 H, J = 11.8, H ), 5.99 (d, J =
3
3
4
7
11
8
12
10
[15] A. Valla, Z. Andriamialisoa, M. Giraud, V. Prat, A. Laurent,
14
8
.1 Hz, 1 H, H ), 2.33 (s, 3 H, 13-CH
3
), 2.28 (s, 3 H, 2-CH
3
), 2.23
P. Labia, R. Potier, Tetrahedron 2000, 56, 7211–7215; A. Valla,
714
www.eurjoc.org
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Eur. J. Org. Chem. 2007, 711–715

A Convenient Route to Isorenieratene (φ,φ-Carotene)
FULL PAPER
Z. Andriamialisoa, M. Giraud, V. Prat, A. Laurent, P. Potier,
[18] G. Englert in Carotenoids Vol 1B: Spectroscopy (Eds.: G. Brit-
ton, S. Liaaen-Jensen, H. Pfander), Birkhäuser, 1995, p. 179,
ex. 103; N. Okukado, Bull. Chem. Soc. Jpn. 1974, 47, 2345–
2346; S. Akiyama, S. Nakatsuji, S. Eda, M. Kataoka, M. Nak-
agawa, Tetrahedron Lett. 1979, 30, 2813–2816.
Tetrahedron Lett. 1999, 40, 9235–9237.
[
16] G. Corey, E. J. Fraenkel, J. Am. Chem. Soc. 1952, 74, 5897–
5
1
905; G. Corey, E. J. Fraenkel, J. Am. Chem. Soc. 1953, 75,
163–1167.
[
17] J. Paust, P. S. Manchand in Carotenoids Vol. 2: Synthesis (Eds.:
G. Britton, S. Liaaen-Jensen, H. Pfander), Birkhäuser, 1996, p.
Received: September 12, 2006
Published Online: November 27, 2006
329.
Eur. J. Org. Chem. 2007, 711–715
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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715