Ring-closing metathesis dimerizations of enynes and deprotections of propargyl ethers mediated by carbene ruthenium complexes

Article abstract of DOI:10.1002/ejoc.200400630

The ring-forming dimerizations of enynes were catalyzed by the first-generation carbene ruthenium complex, and the effects of the catalysts and ethylene gas were studied. The deprotection of propargyl ethers by the carbene ruthenium complexes is the first to be reported. Wiley-VCH Verlag GmbH & Co. KGaA1 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejoc.200400630

SHORT COMMUNICATION
Ring-Closing Metathesis Dimerizations of Enynes and Deprotections of
Propargyl Ethers Mediated by Carbene Ruthenium Complexes
Dong-Woo Hahn,[a] Dong-Min Byun,[a] and Jinsung Tae*[a]
Keywords: Deprotection / Dimerization / Metathesis / Propargyl ethers / Carbene ruthenium Complexes
The ring-forming dimerizations of enynes were catalyzed by
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
the first-generation carbene ruthenium complex, and the ef- Germany, 2005)
fects of the catalysts and ethylene gas were studied. The de-
protection of propargyl ethers by the carbene ruthenium
complexes is the first to be reported.
Introduction
Metathesis of enyne A tethered by a linker can proceed
by three different ring-forming paths depending on the
length of the tether (Scheme 1). The ring-closing enyne met-
athesis for small-size rings proceeds through path a to give
exo-mode product B. If the linker length is long enough,
endo-mode ring closure occurs to yield 1,3-diene C (path
b).[4] The third option could operate if the linker is too short
and the direct cyclization mode is not possible (path c). In
that case, cross metathesis dimerization can take place to
form D in which the terminal enyne can further cyclize to
give E. The overall tandem enyne-CM/RCM process would
be a new dimerization reaction to the macrocyclic systems
starting from simple enynes. Herein we report the unpre-
cedented ring-forming dimerizations by tandem enyne met-
athesis and other findings.
With the advance of well-defined and functional-group-
tolerant catalysts such as 1 and 2 (Figure 1), olefin metath-
esis[1] has become a powerful tool for synthetic organic and
polymer chemists. Metathesis of dienes, diynes, and enynes
have been utilized in many different types of
carbonϪcarbon bond forming reactions. Especially, enyne
metathesis[2] generates synthetically useful 1,3-dienes that
allow subsequent DielsϪAlder reactions with diverse dieno-
philes to construct complex molecules expeditiously. De-
spite significant recent advances in enyne metathesis, de-
tailed studies on the reaction pathways of cross enyne met-
athesis were limited due to complicated mechanisms in-
volved in enyne metathesis. Ring-forming metathesis
dimerizations by tandem cross and ring-closing metathesis
of dienes have been utilized in the synthesis of symmetric
dimers of natural and unnatural molecules such as [n.n]-
paracyclophanes, sulfones, and lactones.[3] However, to the
best of our knowledge, ring-forming metathesis dimeriza-
tions of enynes mediated by a tandem process of cross and
ring-closing enyne metathesis has not been studied in the
literature until now.
Scheme 1. Metathesis pathways of enynes
Figure 1. Grubbs’ carbene ruthenium complexes
[a]
Center for Bioactive Molecular Hybrids (CBMH) and
Results and Discussion
Department of Chemistry, Yonsei University,
Seoul 120-749, Korea
1,4-Disubstituted benzenes and 1,5-disubstituted naph-
thalenes were selected as the enyne substrates, where the
Fax: (internat.) ϩ82-2-364-7050
E-mail: jstae@yonsei.ac.kr
Eur. J. Org. Chem. 2005, 63Ϫ67
DOI: 10.1002/ejoc.200400630
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
63

D.-W. Hahn, D.-M. Byun, J. Tae
SHORT COMMUNICATION
alkene and alkyne functions could not participate in the fective for enyne-metathesis reactions,[2] and ethylene is
direct ring-forming reactions. Substrates 3Ϫ6 were prepared known to facilitate the enyne metathesis.[8] However, the
readily from commercially available materials (Figure 2).[5]
story was totally different in the case of the present ring-
forming reactions. Treatment of 3a and 3b with the second
generation catalyst 2 gave none of the expected products 7a
and 7b, respectively, but only the starting materials, without
any other metathesis products except some polymeric mate-
rials (Table 1, Entries 4 and 5). The observed reactivity dif-
ference between 1 and 2 is unusual and is an interesting
feature, and may need further studies for a full understand-
ing.
We then conducted the reaction under an ethylene atmos-
phere. The effect of ethylene (1 atm) on catalyst 1 was con-
troversial: while the yield of 7c increased from 25% to 36%,
that of 7a slightly decreased from 33% to 31% (Table 1, En-
tries 6 and 7). However, the reaction time and catalyst load-
ing were reduced to 16 h and 10 mol %, respectively, for
completion of the reactions. On the contrary, an atmos-
phere of ethylene gas did not have any beneficial effect on
catalyst 2 in the dimerization reactions. In this case, cross
metathesis between ethylene and the alkyne moiety[9] domi-
nated to give compound 8 instead (Table 1, Entry 8; Fig-
ure 3). The less reactive catalyst 1 therefore was better than
the more reactive catalyst 2 for the enyne-metathesis dimeri-
zations studied herein.
Figure 2. Enyne substrates
By subjecting enyne 3a to 5 mol % 1 under refluxing di-
chloromethane, 7a was produced slowly. Additional cata-
lysts (5 mol % each) were added successively at 8 h intervals
until the reaction was completed.[6] With 20 mol % of total
catalyst loadings, the starting material was consumed com-
pletely within 32 h to give 7a in 33% isolated yield (Table 1,
Entry 1). Conversions of the analogues 3b and 3c were also
slow and required high catalyst loadings for completion of
the reactions (Table 1, Entries 2 and 3). Although the yields
were low, no other products were isolated from the reaction
mixture. The products were proved to arise from the exo-
mode cyclization (see Scheme 1), and both 1,3-diene units
therefore have the same geometries. Saturation of the alkene
isomers by catalytic hydrogenation produced single com-
pounds.[7]
Table 1. Enyne-metathesis dimerizations: the effects of catalyst
and ethylene
Figure 3. Enyne-metathesis products
Entry
3
Catalyst
Ethylene
Products[a]
Additional dimerization examples using substrate 4 and
5aϪc are shown in Table 2. The 1,5-disubstituted naphtha-
lene series 5aϪc were more reactive than the hydroquinone
relatives in terms of total reaction times. The reactions were
completed in 16 h to give 23Ϫ44% of macrocyclic dimers
(Table 2, Entries 2Ϫ4).
At this point, we would like to mention the observed low
yields in the enyne-dimerization reactions. In general, inter-
molecular enyne-metathesis reactions are difficult because
of slow reaction rates and competitive pathways such as
chelation and oligomerization.[10] Therefore, most of practi-
cal intermolecular enyne-RCM reactions employ either ex-
1
2
3
4
5
3a
3b
3c
3a
3b
3a
3c
3a
1
1
1
2
2
1
1
2
no
no
no
no
7a (33)
7b (27)
7c (25)
7a (Ϫ)[b]
7b (Ϫ)[b]
7a (31)
no
[c]
6
1 atm
1 atm
1 atm
7[c]
8[c]
7c (36)
7a (Ϫ)[d] ϩ8 (32)
[a]
[b]
Mixtures of isomers were obtained.
Starting materials were
[c]
[d]
recovered. 10 mol % of catalyst, 16 h. 7a was not detected.
We then examined the more reactive second generation cess alkenes or ethylene gas. However, the enyne dimeriz-
catalyst 2 and the effect of ethylene gas to optimize the reac- ation studied here could utilize neither of these. The varia-
tion conditions (Table 1). In general, catalyst 2 is more ef- tion of alkene concentration is not optional because it is a
64
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2005, 63Ϫ67

Dimerizations of Enynes and Deprotections of Propargyl Ethers
Table 2. Enyne-metathesis dimerizations
SHORT COMMUNICATION
Entry[a]
Substrate
Conditions[b]
Products[c]
1
2
3
4
4
20 mol % 1, 24 h
10 mol % 1, 16 h
10 mol % 1, 16 h
10 mol % 1, 16 h
9 (39)
5a
5b
5c
10a (23)
10b (30)
10c (44)
[a]
[b]
Reaction conditions: 1, CH2Cl2[,c]45 °C, 0.005 m.
Amount of
catalyst 1 used and reaction time. Mixtures of isomers were ob-
tained.
dimerization reaction; the effect of ethylene gas was mini-
mal as mentioned before.
If the above tandem metathesis dimerization operates for
the mixture of diene 11 and diyne 12, we might expect a
Scheme 3. Mechanism of cross metathesis of 11 and 12
dimeric macrocycle 14 through
a
similar process
vinyl ether and deprotection of allylamines are often enco-
untered in the course of metathesis reactions.[11] These non-
metathesis alkene isomerizations mediated by carbene ru-
thenium complexes were proposed to proceed via ru-
thenium hydride species, which might be formed by de-
composition of the carbene ruthenium species under the re-
action conditions.[12] However, isomerization of a triple
bond or deprotection of a propargyl group by carbene ru-
thenium complexes has not been known in the literature
until now.
(Scheme 2). Thus, a 1:1 mixture of 11 and 12 was treated
with 20 mol % of catalyst 1 under the previous metathesis
conditions. However, the expected 14 was not isolated from
the reaction mixture, instead dimer 13 was obtained in 24%
yield with recovered 12. This result was obtained because
of the dimerization of 11 by a tandem diene-CM/RCM re-
action, as shown in our previous report.[3j] This implies that
the overall sequence 11A Ǟ 11B Ǟ 13 is most favorable
under the reversible conditions, although the initially
formed metathesis intermediate could be either 11A or
12A (Scheme 3).
Scheme 4. Deprotection of a propargyl ether
We therefore investigated the scope of the deprotection of
propargyl groups[13] in detail by examining different solvent
systems and reaction temperatures with catalyst 2, as shown
in Table 3. Conditions A (CH2Cl2, 45 °C), B (benzene, 100
°C), and C (toluene, 125 °C) were evaluated with aryl pro-
pargyl ethers 17aϪe.[6] Condition A required longer reac-
tion times, and the yields were low (Table 3, Entries 1 and
2). On the other hand, reactions conducted under condition
Scheme 2. Metathesis of a mixture of diene 11 and diyne 12
An enyne substrate with an electron-deficient alkene was C produced a significant amount of polymeric materials
also considered for the dimerization. When 6 was subjected (Table 3, Entries 3Ϫ6). The best yields were obtained in
to 10 mol % of 1 and 2 under refluxing dichloromethane, benzene solution (condition B) with 8Ϫ20 mol % of 2. Aryl
the dimer 16 was not obtained, but to our surprise com- propargyl ethers 17a and 17c with electron-withdrawing
pound 15 was isolated in 17% and 35% yields, respectively groups at the para-position gave poor yields (34% and 29%,
(Scheme 4). The propargyl group was removed selectively respectively) of the corresponding phenols (Table 3, Entries
without any metathesis reactions. Carbene ruthenium cata- 7 and 9). Compound 17b and 17e with electron-donating
lyzed isomerization of the double bonds of allyl ether to groups (p-OAc and p-OMe) proved to be good substrates,
Eur. J. Org. Chem. 2005, 63Ϫ67
www.eurjoc.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
65

D.-W. Hahn, D.-M. Byun, J. Tae
SHORT COMMUNICATION
and the deprotection yields were 61% and 81%, respectively
(Table 3, Entries 8 and 11). Deprotection of phenyl propar-
gyl ether 17d was completed in quantitative yield to give
phenol (Table 3, Entry 10). The reactions of benzyl propar-
gyl ether 17f and propargylamine 17g were also examined,
but the reactions were slow, and the yields were rather low
relative to those with the aryl propargyl ethers (Table 3, En-
tries 12 and 13).
Conclusion
We have reported a new tandem enyne-metathesis se-
quence from enyne substrates to macrocyclic compounds.
The initial dimeric intermediates formed by cross enyne
metathesis were cyclized by ring-closing enyne metathesis to
give macrocycles with the use of the first generation
Grubbs’ catalyst. Deprotections of aryl propargyl ethers by
carbene ruthenium complexes are the first to be reported in
this communication. The deprotections of propargyl groups
were best catalyzed by the second generation Grubbs’ cata-
lyst in benzene solutions.
Table 3. Deprotection of propargyl groups with 2
Experimental Section
Procedure for the Metathesis Dimerization: A solution of 3a (35 mg,
0.16 mmol) and catalyst 1 (7 mg, 5 mol %) in CH2Cl2 (32 mL) was
refluxed at 45 °C for 7 h under N2. Additional catalysts (5 mol %
ϫ 3) were added at 8 h intervals. A total of 20 mol % of catalyst 1
was added over 32 h before the reaction was completed. The sol-
vent was removed under reduced pressure, and the residue mixture
was column chromatographed on silica gel (hexane/EtOAc, 20:1)
to give 5.2 mg (15%) of 7c (E/Z isomer) and 3.5 mg (10%) of 7c
(E/E isomer) as colorless solids.
Entry
(mol % of 2)[b] time (h)
Substrate Condition[a] Reaction Yield[b]
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
17a
17b
17a
17b
17c
17d
17a
17b
17c
17d
17e
17f
A (20)
A (15)
C (20)
C (20)
C (8)
60
48
24
48
60
60
24
24
24
24
16
48
48
11
19
23
(Ϫ)[c]
25
11
34
61
29
99
81
20
24
Data for 7c (unsymmetric E/Z isomer): Colorless solids; Rf ϭ 0.5
(silica gel, hexane/EtOAc, 5:1); m.p. 123Ϫ125 °C. 1H NMR
(500 MHz, CDCl3): δ ϭ 6.75Ϫ6.50 (m, 8 H), 6.10 (d, J ϭ 16 Hz,
1 H), 6.05Ϫ5.85 (m, 2 H), 5.55Ϫ5.45 (m, 1 H), 5.34 (s, 1 H), 5.17
(s, 1 H), 5.13 (s, 2 H), 4.61 (s, 2 H), 4.48 (s, 2 H), 3.84 (m, 2 H),
3.77 (m, 2 H), 2.50Ϫ2.35 (m, 2 H), 2.35Ϫ2.20 (m, 2 H), 1.95Ϫ1.85
(m, 2 H), 1.85Ϫ1.75 (m, 2 H) ppm. 13C NMR (128.5 MHz,
CDCl3): δ ϭ 153.6, 153.4, 152.6 (2 C), 141.8 (2 C), 132.6, 131.7,
130.4, 128.6, 117.3, 117.1, 116.2, 115.9, 115.6 (2 C), 71.1, 70.0, 67.8,
67.6, 29.5, 29.4, 28.5, 25.4 ppm. IR (neat): ν˜ ϭ 2978, 2913, 2851,
1656, 1514, 1382, 1237, 1202, 1061, 981, 901, 814 cmϪ1. HRMS:
m/z calcd. for C28H32O4 [Mϩ]: 432.2301; found 432.2303.
C (8)
B (8)
B (8)
B (10)
B (8)
B (8)
B (20)
B (15)
17g
[a]
Condition A: 2, CH2Cl2, 45 °C, 0.3 m; Condition B: 2, benzene,
[b]
100 °C, 0.3 m; Condition C: 2, toluene, 125 °C, 0.3 m.
was added in portions (5 mol % each time). Insoluble polymeric
materials were obtained.
Catalyst
Data for 7c (symmetric E/E isomer): Colorless solids; Rf ϭ 0.5 (sil-
ica gel, hexane/EtOAc, 5:1); m.p. 123Ϫ125 °C. 1H NMR
(500 MHz, CDCl3): δ ϭ 6.74 (d, J ϭ 8 Hz, 4 H), 6.68 (d, J ϭ 9 Hz,
4 H), 6.13 (d, J ϭ 16 Hz, 2 H), 6.00Ϫ5.95 (m, 2 H), 5.17 (s, 2 H),
5.13 (s, 2 H), 4.62 (s, 4 H), 3.90 (t, J ϭ 6 Hz, 4 H), 2.35Ϫ2.25 (m,
4 H), 1.95Ϫ1.75 (m, 4 H) ppm. 13C NMR (128.5 MHz, CDCl3):
δ ϭ 153.5, 152.6, 141.6, 131.7, 130.4, 117.3, 116.3, 115.7, 70.0, 67.7,
29.7, 28.6 ppm. IR (neat): ν˜ ϭ 2940, 2870, 1512, 1482, 1282, 1236,
1213, 1110, 1030, 963, 905, 821 cmϪ1. HRMS: m/z calcd. for
C28H32O4 [Mϩ]: 432.2301; found 432.2302.
[c]
The detailed mechanism for the deprotection of propar-
gyl ethers by carbene ruthenium complexes is unclear. But
the ruthenium-hydride mechanism, which is typically used
to explain the deprotection of allyl groups, may also apply
in this case. The allenyl ethers generated by the isomeriz-
ation of the propargyl ethers hydrolyze to the alcohols as
shown in Scheme 5.
Procedure for the Deprotection of Aryl Propargyl Ethers: A benzene
(1.14 mL) solution of 17d (45 mg, 0.34 mmol) and catalyst 2 (9 mg,
3 mol %) was refluxed at 100 °C (bath temperature) in a sealed
tube for 7 h under N2. Additional catalysts (3 mol % and 2 mol %)
were added at 8 h intervals. A total of 8 mol % of catalyst 1 was
added over 24 h before the reaction was completed. The solvent
was removed under reduced pressure, and the residue mixture was
column chromatographed on silica gel (hexane/EtOAc, 10:1) to give
32 mg (99%) of phenol.
Acknowledgments
We thank the Center for Bioactive Molecular Hybrids (CBMH) for
financial support.
Scheme 5. A proposed mechanism for the deprotection of propar-
gyl ethers
66
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2005, 63Ϫ67

Dimerizations of Enynes and Deprotections of Propargyl Ethers
SHORT COMMUNICATION
[8]
[8a] M. Mori, N. Sakakibara, A. Kinoshita, J. Org. Chem. 1998,
[1]
[1a]
For selected recent reviews on olefin metathesis, see:
Grubbs, S. Chang, Tetrahedron 1998, 54, 4413Ϫ4450.
R. H.
[8b]
[1b]
63, 6082Ϫ6083.
A. Kinoshita, N. Sakakibara, M. Mori, J.
A.
Am. Chem. Soc. 1997, 119, 12388Ϫ12389. [8c] A. Kinoshita, N.
Fürstner, Angew. Chem. 2000, 112, 3140Ϫ3172; Angew. Chem.
[8d]
[1c]
Sakakibara, M. Mori, Tetrahedron 1999, 55, 8155Ϫ8167.
Int. Ed. 2000, 39, 3012Ϫ3043.
S. J. Connon, S. Blechert,
H.-Y. Lee, B. G. Kim, M. L. Snapper, Org. Lett. 2003, 5,
Angew. Chem. 2003, 115, 1944Ϫ1968; Angew. Chem. Int. Ed.
[8e]
1855Ϫ1858.
2035Ϫ2038.
E. C Hansen, D. Lee, Org. Lett. 2004, 6,
B. Kang, D.-H. Kim, Y. Do, S. Chang, Org.
2003, 42, 1900Ϫ1923.
For reviews on enyne-metathesis, see:.
[8f]
[2]
[3]
[2a]
S. T. Diver, A. J.
C. S. Poulsen,
M. Mori, Top. Or-
[2b]
Lett. 2003, 5, 3041Ϫ3043.
Giessert, Chem. Rev. 2004, 104, 1317Ϫ1382.
R. Madsen, Synthesis 2003, 1Ϫ18.
ganomet. Chem. 1998, 1, 133Ϫ154.
[9]
[2c]
M. Mori, T. Kitamura, Y. Sato, Synthesis 2001, 654Ϫ664.
See review papers (ref. [2]) for the limitations of intermolecular
enyne-RCM.
[10]
[3a] A. B. Smith, III, C. M. Adams, S. A. Kozmin, D. V. Paone,
[11]
J. Am. Chem. Soc. 2001, 123, 5925Ϫ5937. [3b] A. B. Smith, III,
For non-metathesis behavior of carbene ruthenium complexes,
[11a]
[11b]
see:
B. Schmidt, Eur. J. Org. Chem. 2004, 1865Ϫ1880.
S. A. Kozmin, C. M. Adams, D. V. Paone, J. Am. Chem. Soc.
[3c]
B. Alcaide, P. Almendros, Chem. Eur. J. 2003, 9, 1258Ϫ1262.
2000, 122, 4984Ϫ4985.
A. B. Smith, III, C. M. Adams, S.
[11c]
[3d]
M. Mori, N. Saito, D. Tanaka, M. Takimoto, Y. Sato, J.
A. Kozmin, J. Am. Chem. Soc. 2001, 123, 990Ϫ991.
L. A.
[11d]
[3e]
Am. Chem. Soc. 2003, 125, 5606Ϫ5607.
P. Quayle, D.
´
Paquette, M.-A. Jose, Tetrahedron Lett. 2001, 42, 967Ϫ970.
Fengas, S. Richards, Synlett 2003, 1797Ϫ1800. [11e] B. Schmidt,
Angew. Chem. Int. Ed. 2003, 42, 4996Ϫ4999. [11f] B. Alcaide, P.
Almendros, J. M. Alonso, Chem. Eur. J. 2003, 9, 5793Ϫ5799.
L. A. Paquette, F. Fabris, J. Tae, J. C. Gallucci, J. E. Hoffer-
[3f]
berth, J. Am. Chem. Soc. 2000, 122, 3391Ϫ3398.
G.-W.
Chen, A. Kirschning, Chem. Eur. J. 2002, 8, 2717Ϫ2729. [3g] C.
W. Lee, R. H. Grubbs, J. Org. Chem. 2001, 66, 7155Ϫ7158.
[11g]
B. Alcaide, P. Almendros, J. M. Alonso, Chemistry 2003,
9, 5793Ϫ5799.
[11h]
[3h]
T. Kitamura, Y. Sato, M. Mori, Chem.
A. K. Chatterjee, J. P. Morgan, M. Scholl, R. H. Grubbs,
[11i]
[3i]
Commun. 2001, 1258Ϫ1259.
B. Alcaide, P. Almendros, J.
J. Am. Chem. Soc. 2000, 122, 3783Ϫ3784.
A. Fürstner, O.
[11j]
[3j]
M. Alonso, M. F. Aly, Org. Lett. 2001, 3, 3781Ϫ3784.
Y. -
R. Thiel, L. Ackermann, Org. Lett. 2001, 3, 449Ϫ451.
Tae, Y.-K. Yang, Org. Lett. 2003, 5, 741Ϫ744.
J.
J. Hu, R. Dominique, S. K. Das, R. Roy, Can. J. Chem. 2000,
[4] [4a]
78, 838Ϫ845.
E. C. Hansen, D. Lee, J. Am. Chem. Soc. 2003, 125,
[12]
[13]
[4b]
S. H. Hong, M. W. Day, R. H. Grubbs, J. Am. Chem. Soc.
9582Ϫ9583.
M. E. Layton, C. A. Morales, M. D. Shair, J.
2004, 126, 7414Ϫ7415.
For references of propargyl ether deprotection, see:
Am. Chem. Soc. 2002, 124, 773Ϫ775.
[13a]
[5]
S.
Substrates 3aϪc and 5aϪc were prepared from hydroquinone
and 1,5-dihydroxynaphthalene, respectively, by double alky-
lations; 1. K2CO3, HCCCH2Br, DMF, 35Ϫ38% of mono alky-
lation products, 2. NaH, Br(CH2)nCHϭCH2, DMF, 85% (3a),
82% (3b), 82% (3c), 99% (5a), 33% (5b), 99% (5c). Substrate 6
was synthesized from hydroquinone; 1. K2CO3, HCCCH2Br,
DMF, 35%, 2. Et3N, ClCOCHϭCH2, CH2Cl2, 95%. Substrate
was prepared from 4-hydroxybenzaldehyde; 1. K2CO3,
HCCCH2Br, DMF, 66%, 2. BrMgCH2CHϭCH2, THF, 83%;
3. Ac2O, pyr, DMAP, CH2Cl2, 99%.
The reactions were performed in sealed tubes, and the tempera-
tures indicated are bath temperatures.
One of the CϪO bonds was cleaved under the hydrogenation
conditions.
Punna, S. Meunier, M. G. Finn, Org. Lett. 2004, 6, 2777Ϫ2779.
[13b]
M. Pal, K. Parasuraman, K. R. Yeleswarapu, Org. Lett.
[13c]
´
H. X. Zhang, F. Guibe, G. Balavoine,
2003, 5, 349Ϫ352.
[13d]
Tetrahedron Lett. 1988, 29, 619Ϫ622.
Kadam, A. Banerji, Synlett 1993, 581Ϫ582.
E. Dun˜ach, Tetrahedron Lett. 1997, 38, 6193Ϫ6196.
Swamy, P. Ilankumaran, S. Chandrasekaran, Synlett 1997,
S. K. Nayak, S. M.
[13e]
S. Oliviero,
[13f]
V. M .
4
[13g]
513Ϫ514.
Synlett 2002, 1752Ϫ1778.
S. K. Mohan, Tetrahedron 1999, 55, 11331Ϫ11342.
Received September 6, 2004
K. R. Prabbu, N. Devan, S. Chandrasekaran,
[13h]
H. B. Mereyala, S. R. Gurrala,
[6]
[7]
Eur. J. Org. Chem. 2005, 63Ϫ67
www.eurjoc.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
67