Cl
O
As previously observed,3,12 the cyclisation of esters with long
unsaturated chains (1a, 2a, 3a) proceeds via an endo pathway.
This is due to the fact that the most stable conformation for the
ester function (s-trans) does not impede the cyclisation process
if the carbon chain is long enough. With the unsaturated ester
4a, only the less favourable conformation (s-cis) can give rise to
the cyclisation product. In this case the formation of a five-
membered ring via an exo pathway is preferred. The but-3-enyl
trichloroacetate lies in an intermediate position. It was im-
possible for us to obtain cyclisation products, we could only
detect cyclic dimers and telomers by mass spectrometry. The
formation of a six-membered ring lactone via the s-cis
conformation is not favourable and the butenyl chain is not long
enough to allow a cyclisation process via the s-trans conforma-
tion. These results are consistent with those reported by O-Yang
i
O
CCl3
Cl
Cl
O
O
1a
1b
9
1
9
4
9% [L1 (0.1 mol%)]
8% [L2 (0.03 mol%)]
0% [L3 (0.03 mol%)]
7% [bpy (0.1 mol%)]
Cl
O
i
O
CCl3
12
Cl
in the case of a-iodo esters.
As a conclusion, the use of new ligands represents a
significant improvement for the catalytic activity of copper(
Cl
O
2a
O
I)
and iron(II) complexes in ATRA reactions by reducing the
required amount of complex by a factor of 10 to 100. Moreover,
they are soluble in the reaction mixture and catalyse reactions
2
b
5
4
3% [L1 (0.1 mol%)]
% [bpy (0.1 mol%)]
I
which are impossible to perform with the Cu Cl–bpy complex.
Scheme 2 Reagents and conditions: i, CuCl–ligand, 1,2-dichloroethane,
0 °C
The metal could also act as a template and allow the notoriously
difficult cyclisation of eight- and nine-membered ring lactone
8
7
precursors as suggested previously. We are currently trying to
relate the redox potentials of these complexes to their catalytic
activity.
We are indebted to Dr B. Maillard for fruitful discussion and
to the Région Aquitaine for financial support.
The cyclisation of hex-5-enyl trichloroacetate 2a gave only a
low yield of lactone 2b in the presence of CuCl–bpy (0.1
mol%), but this yield is subsequently improved by the use of
2
either the CuCl–L1 or the FeCl –L2 systems.¶ In the cyclisation
experiments with 2a, a substantial difference between the yield
of cyclisation products and the total conversion, based on the
disappearance of 2a, was noted. The difference in mass balance
is probably due to the tendency for 2a to give oligomers and
particularly dimers, which have been characterised by mass
spectrometry, because of the increase in ring size.
Notes and References
†
‡
E-mail: j-b.verlhac@lcoo.u-bordeaux.fr
In the presence of an 1:1 mixture of FeCl
2
–ligand (0.03 mol%), the yields
were respectively: L1, 65% (48 h); L2, 75% (18 h); bpy, 4% (18 h).
Replacement of CuCl–L2 by FeCl –L2 allowed us to obtain 1b in 62%
yield.
§
2
We have also observed reactions which were impossible to
¶ A 50% yield of 2b was obtained by using the FeCl –L2 mixture (0.03
mol%)
2
7
perform with CuCl–bpy; for instance the cyclisation of hept-
6
7
-enyl trichloroacetate 3a afforded the cyclic endo lactone 3b in
0% yield in the presence of CuCl–L3 (Scheme 3). It was also
1
B. Giese, Radicals in Organic Synthesis: Formation of Carbon-Carbon
Bonds, Pergamon, New York, 1986.
D. P. Curran, Synthesis, 1988, 417, 489.
F. O. H. Pirrung, H. Hiemstra and W. N. Speckamp, Tetrahedron, 1994,
50, 12 415. N. Baldovini, M.-P. Bertrand, A. Carrière, R. Nougier and
J.-M. Plancher, J. Org. Chem., 1996, 61, 3205
possible to catalyse the exo cyclisation of prop-2-enyl tri-
chloroacetate 4a with 48% yield in the presence of the same
catalyst. In the latter case the FeCl
be effective.
2
3
2
–L2 system also proved to
4
G. M. Lee, M. Parvez and S. M. Weinreb, Tetrahedron, 1988, 44,
4
671
5
6
B. P. Branchaud and G. X. Yu, Organometallics, 1993, 12, 4262
L. Forti, F. Ghelti and U. M. Pagnoni, Tetrahedron Lett., 1996, 37,
2077
F. O. H. Pirrung, H. Hiemstra, W. N. Speckamp, B. Kaptein and H. E.
Schoemaker, Synthesis, 1995, 458; F. O. H. Pirrung, H. Hiemstra, B.
Kaptein, M. E. Martinez Sobrino, D. G. Petra, H. E. Schoemaker and
W. N. Speckamp, Synlett, 1993, 739
O
O
Cl
i
O
7
Cl
O
CCl3
3a
Cl
8
9
H. Nagashima, N. Ozaki, M. Ishii, K. Seki, M. Washiyama and K. Itoh,
J. Org. Chem., 1993, 58, 464
H. Matsumoto, T. Nakano and Y. Nagai, Tetrahedron Lett., 1973,
3
b 70% (CuCl–L3)
3
4% (FeCl2–L2)
5
147
1
1
0 J. C. Phelps and D. E. Bergbreiter, Tetrahedron Lett., 1989, 30, 3915
1 A. Dossing, A. Hazell and H. Toftlund, Acta Chem. Scand., 1996, 50,
O
O
i
O
CCl3
9
5; G. Anderegg and F. Wenk, Helv. Chim. Acta, 1967, 50, 2330.
Cl
Cl
Ligand L2 was obtained by permethylation of 1,4,7-triazaheptane with
a formic acid–formaldehyde mixture
O
Cl
4a
12 F. Barth and C. O-Yang, Tetrahedron Lett., 1990, 38, 1121; J. E.
Baldwin, R. M. Adlington, M. B. Mitchell and J. Robertson, J. Chem.
Soc., Chem. Commun., 1990, 1574.
4
b 48% (CuCl–L3)
5
5% (FeCl2–L2)
Scheme 3 Reagents and conditions: 1,2-dichloroethane, 80 °C, CuCl–L3 or
Received in Cambridge, UK, 14th April 1998; revised manuscript received,
29th June 1998; 8/04915G
FeCl
2
–L2 catalysts (0.1 mol%)
2118
Chem. Commun., 1998