A stable catalyst of the Pauson-Khand annelation

Article abstract of DOI:10.1016/S0040-4020(02)00420-9

A practical catalytic-in-cobalt version of the PKA is clearly a desirable objective and there has been much research towards this goal in recent years. Systems developed to date, however, require either the use of high temperatures and high carbon monoxid

Full text of DOI:10.1016/S0040-4020(02)00420-9

TETRAHEDRON
Pergamon
Tetrahedron 58 .2002) 4937±4942
A stable catalyst of the Pauson±Khand annelation
Susan E. Gibson,a,p Craig Johnstoneb and Andrea Stevenazzia
aDepartment of Chemistry, King's College London, Strand, London WC2R 2LS, UK
bAstraZeneca UK Ltd, Mereside, Alderley Park, Maccles®eld SK10 4TG, UK
Received 1 February 2002; revised 20 March 2002; accepted 11 April 2002
AbstractÐA practical catalytic-in-cobalt version of the PKA is clearly a desirable objective and there has been much research towards this
goal in recent years. Systems developed to date, however, require either the use of high temperatures and high carbon monoxide pressures
and/or the use of the sensitive cobalt catalyst, octacarbonyldicobablt.0). We report here a detailed account of PKA results obtained using the
more robust cobalt compound, heptacarbonyl.triphenylphosphine)dicobalt.0), 1, which indicate that 1 is an attractive alternative to octa-
carbonyldicobalt.0) as a catalyst of the PKA. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
illustrated by a summary of reports on the reaction in the
last two years. Not surprisingly, in view of current interest in
automation and environmental issues, there have been
several reports concerning the immobilisation of PKA
catalysts .see .1)±.3), Scheme 1). One system employs a
polymer-supported cobalt carbonyl complex .see .1)
Scheme 1)5 whilst the other two .see .2) and .3) Scheme
1) use immobilised cobalt and require relatively high pres-
sures of carbon monoxide to generate cyclopentenones.6,7 In
response to the dif®culties associated with handling and
using Co2.CO)8, Krafft has reported Co4.CO)12 as an alter-
native catalyst precursor using cyclohexylamine to induce
catalyst formation via disproportionation of Co4.CO)12 and
to preserve the active catalyst .see .4) Scheme 1).8,9 Stabi-
lisation of the active catalyst using a phosphane sul®de has
been reported .see .5) Scheme 1)10 and, remarkably, addi-
tion of BINAP leads to excellent enantioselectivity, albeit at
the expense of turnover .see .6) Scheme 1).11,12 Demanding
substrates have been cyclised ef®ciently by the introduction
of sulfur substituents, which presumably also stabilise key
catalytic intermediates .see .7) Scheme 1).13
Some of the most signi®cant contributions to organic
synthesis during the past years have been made by transition
metal-mediated reactions. A prime example is the Pauson±
Khand annelation .PKA), ®rst described in 1971,1 which
involves the synthesis of cyclopentenone derivatives via
the carbonylcobalt.0)-mediated cyclisation of an alkyne,
an alkene and carbon monoxide. During its development
and application, the PKA has mainly been applied using a
stoichiometric amount of the transition metal complex,
usually by prior formation of a stable .alkyne)hexa-
carbonyldicobalt.0) complex, even though the intermolecu-
lar catalytic Pauson±Khand annelation .CPKA) using
Co2.CO)8 under mild conditions was reported as early as
1973.2
Interest in the CPKA was rekindled in the 1990s and signi®-
cant improvements in the scope of this reaction were made
during this decade. A typical example is the report from
Livinghouse in 1996 which describes the application of a
photochemically driven process requiring only mild
temperatures .50±558C) and one atmosphere of carbon
monoxide to a wide range of intramolecular substrates.3 In
1998, the same group reported that careful control of
temperature within a narrow window .60±708C) dispenses
with the need for photolytic promotion.4 Rigorous puri®ca-
tion of the Co2.CO)8 catalyst is essential for the success of
these systems, however, and Livinghouse recommends
recrystallisation or sublimation immediately prior to use,
or opening a fresh commercial sample in a glove-box.
As a result of our interest in the use of carbonylcobalt.0)
complexes immobilised on `polymer-bound triphenyl-
phosphine' .see .1), Scheme 1) in the CPKA,5 we initiated
an `off-polymer' study of phosphorus derivatives of car-
bonyl-cobalt.0) species. Despite .a) the signi®cant use of
phosphine and phosphite substituted alkyne complexes in
the stoichiometric PKA, where co-ordination leads to a
reduction in the rate and overall ef®ciency of the reaction,14
and .b) the observation that the use of triphenylphosphite as
an additive in the Co2.CO)8 mediated CPKA leads to an
improvement in reaction ef®ciency,15 the performance of
preformed complexes of phosphines and phosphites in the
CPKA had not been assessed. We thus conducted a pre-
liminary screening of phosphine and phosphite derivatives
of Co2.CO)8 which revealed that these complexes were
The current state-of-the-art of CPKA is perhaps best
Keywords: catalysis; Pauson±Khand annelation; cobalt.
Corresponding author. Fax: 144-207-848-2810;
e-mail: susan.gibson@kcl.ac.uk
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020.02)00420-9

4938
S. E. Gibson et al. / Tetrahedron 58 /2002) 4937±4942
Scheme 2.
Scheme 3.
Co2.CO)8 at room temperature for 30 min, followed by a
straightforward chromatographic puri®cation .Scheme 2).
In order to compare the activity of catalyst 1 across a range
of PKA substrates, we chose to develop a standard set of
conditions and then apply them to all substrates, rather than
varying the reaction conditions to optimise the yield of each
individual reaction. We chose to identify our standard
conditions using the diethyl malonate derivative 2 .Scheme
3) as the reaction of this substrate is easily followed by GC
analysis. At this stage, we performed our reactions on a
relatively small scale .0.5 mmol) and we chose to use 1,2-
DME as solvent as preliminary experiments using THF and
1,2-DME as solvent had indicated that the latter solvent
gave cleaner reactions. We chose to work under 1 atm of
CO in order to determine the scope and limitations of 1
under relatively mild conditions. A series of experiments
in which the conversion of 2 into 3 was followed over
10 h varying the temperature, and using a range of catalyst
loadings led to the results depicted in Fig. 1. Use of 3 mol%
1 at 70 and 758C .runs A and B) revealed that although the
same conversion .60%) was achieved eventually, the higher
temperature led to a faster turnover of catalyst in the early
stages of the reaction. A temperature of 758C was thus
adopted for the remaining experiments. Reduction of the
catalyst loading to 1 mol% .run C) led to conversion of
20%, whilst addition of an extra 2 mol% of 1 after 4 h
increased conversion to 80% .run D). Adding the catalyst
in portions .3 mol% at tˆ0 and 2 mol% at tˆ4 h) proved to
be less effective than adding it all at the beginning of the
reaction as 5 mol% of 1 added at tˆ0 gave a conversion of
essentially 100% .run E). The use of 6 mol% of catalyst 1
improved the conversion in the early stages of the reaction
.run F). Taking into account conversion, reaction time and
catalyst loading, we decided to adopt the following condi-
tions as our standard conditions: 5 mol% catalyst loading
added at tˆ0, a reaction time of 4 h and a reaction tempera-
ture of 758C. Under our standard conditions, substrate 2 was
converted to cyclopentenone 3 in 94% NMR yield and 80%
isolated yield.
Scheme 1.
indeed catalysts of the PKA.16 We have now examined more
closely one of the catalysts identi®ed by the screen, hepta-
carbonyl.triphenylphosphine)dicobalt.0), 1. We report
herein our study of its scope and limitations which reveals
that this catalyst provides an attractive alternative to the
PKA catalysts currently available.
2. Results and discussion
Heptacarbonyl.triphenylphosphine)dicobalt.0), 1, was
chosen for further development based on its activity in our
initial screen and the relatively low cost of triphenyl-
phosphine. Complex 1 is a stable solid,17 easily prepared
in good yield by stirring triphenylphosphine with
In order to de®ne the scope and limitations of catalyst 1 for
the PKR, a series of intramolecular cyclisations were
initially screened under our standard conditions. Malonate
and amine derivatives of mono-substituted alkenes and

S. E. Gibson et al. / Tetrahedron 58 /2002) 4937±4942
4939
Figure 1. Effect of temperature and catalyst loading on the conversion of enyne 2 into cyclopentenone 3 over 10 h. All reactions run in 1,2-DME .10 mL)
under 1.05 atm CO using 1.0 mmol of 2. .A) 3 mol% 1, 708C; .B) 3 mol% 1, 758C; .C) 1 mol% 1, 758C; .D) 3 mol% 1 .0 h)12 mol% 1 .4 h), 758C; .E) 5 mol%
1, 758C; .F) 6 mol% 1, 758C.
alkynes, substrates known to undergo PKA relatively
readily, gave very good yields of products using catalyst 1
under our standard conditions .Table 1, entries 1±3).
Enynes disubstituted either at the alkyne or alkene function-
ality, substrates generally accepted as being more
challenging than their monosubstituted counterparts, gave
good to very good yields of annelation products .Table 1,
entries 4±6). Our attention then turned to intermolecular
PKAs and, again we found that catalyst 1 used under our
standard conditions gave good to very good isolated yields
.Table 1, entries 7±9).
4. Experimental
4.1. General
All reactions and manipulations involving organometallic
species were performed under nitrogen or carbon monoxide
using standard vacuum line and Schlenk tube techniques.18
THF and 1,2-dimethoxyethane .1,2-DME) were distilled
over sodium benzophenone ketyl. All other reagents were
dried, distilled and stored according to standard pro-
cedures.19 PK substrates 2,20 4,21 6,22 8,23 and 10,24 were
prepared according to literature procedures. Melting points,
which are uncorrected, were recorded in open capillaries on
a Buchi 510 melting point apparatus. IR spectra were
recorded on a Perkin±Elmer Spectrum One FT-IR spectro-
meter. NMR spectra were recorded at RT on Bruker Avance
360, Bruker Avance 400 and Bruker DRX 500 instruments.
Chemical shifts are reported in ppm relative to residual
undeuterated solvent as the reference and J values are
reported in Hz. Mass spectra were recorded on a JEOL
AX 505W mass spectrometer and elemental analyses were
performed by the North London University microanalytical
services. Flash column chromatography was performed
using Merck silica gel 60 .230±400 mesh). GC analyses
were performed on a Unicam 610 gas chromatograph
equipped with a FID .column: DB-225, 0.25 mm£30 m).
Finally, as catalyst 1 had given reliable and promising
results on a modest-but-typical CPKA scale .0.5 mmol),
we conducted some preliminary experiments to test whether
or not the CPKA could be readily scaled up using catalyst 1.
We were delighted to ®nd that the intramolecular reaction
leading to the production of 7 and the intermolecular reac-
tion leading to the production of 14 could both be performed
without signi®cant loss of isolated yield on a 10 mmol scale
.92 and 90% of 7 and 14, respectively). From a practical
point of view, it is also of note that a sample of 1 stored at
48C in air for 18 months gave identical results in PKA
reactions to a freshly prepared sample of 1.
3. Conclusion
4.1.1. Heptacarbonyl(triphenylphosphine)dicobalt(0) 1.
Octacarbonyldicobalt.0) .2.00 g, 5.86 mmol) and triphenyl-
phosphine .1.53 g, 5.83 mmol) were dissolved in anhydrous
THF .15 mL) in a dry Schlenk ¯ask under nitrogen. The
mixture was stirred in the dark at room temperature for
30 min and concentrated in vacuo after checking for the
disappearance of the CO absorption at 1845 cm21 by IR
spectroscopy. The resultant brown residue was adsorbed
Heptacarbonyl.triphenylphosphine)dicobalt.0), 1, catalyses
a range of PKAs. The catalyst gives good yields after 4 h
with 5 mol% loading, can be used under mild conditions,
has been used successfully on a 10 mmol scale, and displays
none of the sensitivities and handling problems character-
istic of Co2.CO)8.9

4940
S. E. Gibson et al. / Tetrahedron 58 /2002) 4937±4942
Table 1. Pauson±Khand reaction with various substrates
Entry
1
Substrate
Product
Yield .%)a
80 .94)
2
3
65 .89)
85 .97)
4
5
57 .70)
85 .90)
6
7
45 .62)b
96 .100)
8
9
73
64
All reactions run using 5 mol% of .PPh3)Co2.CO)7 under 1.05 atm CO in 1,2-DME at 758C for 4 h using 0.5 mmol of alkyne substrate.
Isolated .NMR) yield.
de 70%.
a
b
onto neutral alumina .Grade II) under nitrogen and then
charged onto a chromatography column of silica. The
column was eluted under nitrogen ®rst with hexane to
remove any unreacted octacarbonyldicobalt.0) and then
with hexane/diethyl ether .9:1) to collect the title compound
as a brown-red solid .2.17 g, 3.77 mmol, 65%); mp 120±
1258C; IR .CHCl3): n 2077m, 2023w, 1988s, 1958w
and 201.7 .brs, Co.CO)); 31P {1H} NMR .162 MHz;
.CD3)2CO): d 65.3 .CoPPh3); FAB-MS: m/z .%)ˆ548 .2)
[M12CO], 492 .21) [M123CO], 405 .7) [M12Co±4CO],
377 .13) [M12Co±5CO], 349 .17) [M12Co±6CO], 321
.100) .CoPPh31); C25H15Co2O7P .576.21): C, 52.11; H,
2.62; Found: C, 52.1; H, 2.7.
1
.CuO) cm21; H NMR .500 MHz; .CD3)2CO): d 7.50±
4.1.2. Typical 0.5 mmol PKA reaction. Enyne 2 .119 mg,
0.5 mmol) and Ph3PCo2.CO)7, 1, .15 mg, 0.025 mmol) were
dissolved at room temperature in CO saturated 1,2-DME
.5 mL) under a carbon monoxide atmosphere .1.05 atm).
7.70 .m, Ar-H); 13C {1H} NMR .125.8 MHz; .CD3)2CO):
d 129.9 .d, 3JCPˆ10 Hz, m-Ar-C), 132.4 .p-Ar-C), 133.3 .d,
2
1JCPˆ49 Hz, ipso-Ar-C), 133.6 .d, JCPˆ10 Hz, o-Ar-C)

S. E. Gibson et al. / Tetrahedron 58 /2002) 4937±4942
4941
The mixture was heated at 758C with vigorous stirring in the
dark .aluminium foil around ¯ask) for 4 h. The resulting
brown mixture was cooled, ®ltered through a short pad of
celite and concentrated in vacuo. .1H NMR spectroscopy at
this stage revealed a 94% conversion). Puri®cation of the
residue by ¯ash column chromatography .SiO2; hexane/
EtOAc, 6:4) gave the known product 320 .106.5 mg,
0.4 mmol, 80%) as a colourless oil; IR .CHCl3): n 1730s
.1H, m, CHCH2CO), 3.93±3.98 .2H, m, CHHCv and
CHHCHCH2CO), 4.27 .1H, d, Jˆ17 Hz, CHHCHCH2CO),
5.92 .1H, s, CvCH), 7.28 .2H, d, Jˆ8 Hz, m-Ar-H), 7.66
.2H, d, Jˆ8 Hz, o-Ar-H); 13C {1H} NMR .90.6 MHz;
CDCl3): d 21.6 .CH3), 39.8 .CH2CO), 44.0 .CHCH2CO),
47.7 .CH2CHCH2CO), 52.5 .CH2Cv), 126.2 .CvCH),
127.5 .o-Ar-C), 130.1 .m-Ar-C), 133.3 .p-Ar-C), 144.2
.ipso-Ar-C), 178.6 .CvCH), 207.2 .CvO).
1
.CvOester), 1710s .CvOketone), 1636m .CvC) cm21; H
NMR .360 MHz; CDCl3): d 1.25 .6H, t, Jˆ9 Hz,
2£CH2CH3), 1.72 .1H, dd, Jˆ13, 13 Hz, CHHCO), 2.11
.1H, dd, Jˆ18, 3 Hz, CHHCv), 2.61 .1H, dd, Jˆ18,
6 Hz, CHHCv), 2.77 .1H, dd, Jˆ13, 8 Hz, CHHCO),
3.07±3.10 .1H, m, CHCH2CO), 3.23 .1H, dd, Jˆ19, 1 Hz,
CHHCHCH2CO), 3.33 .1H, dd, Jˆ19, 1 Hz,
CHHCHCH2CO), 4.16±4.25 .4H, m, 2£CH2CH3), 5.91
.1H, d, Jˆ2 Hz, CvCH); 13C {1H} NMR .90.6 MHz;
CDCl3): d 14.0 .2£CH2CH3), 35.1 .CH2CHCH2CO), 38.9
.CH2CO), 42.1 .CH2Cv), 45.0 .CHCH2CO), 60.8
.C.CO2Et)2), 62.0 .CH2CH3), 62.2 .CH2CH3), 125.6
.CvCH), 170.8 .CO2), 171.5 .CO2), 185.7 .CvCH) and
209.6 .CvCHCO).
4.1.4. 10 mmol Intermolecular PKA reaction. Phenyl-
acetylene .1.02 g, 10 mmol), norbornene .4.70 g,
50 mmol) and Ph3PCo2.CO)7, 1, .288 mg, 0.5 mmol) were
dissolved at room temperature in carbon monoxide
saturated 1,2-DME .100 mL) under a carbon monoxide
atmosphere .1.05 atm). The mixture was heated at 758C
with vigorous stirring in the dark .aluminium foil around
the ¯ask) for 4 h. The resulting brown mixture was cooled,
®ltered through a short pad of celite and concentrated in
vacuo. .1H NMR spectroscopy revealed total conversion.)
Puri®cation of the residue by ¯ash column chromatography
.SiO2; hexane/EtOAc, 19:1) gave the known cyclo-
pentenone 1426 .2.02 g, 9.0 mmol, 90%) as a white solid;
mp 92±938C .lit.26 958C); IR .CHCl3):
n
1696s
PK products 3,20 7,25 9,20 11,26 13,26 14,26 1525 and 1627 were
identi®ed by comparison of their spectroscopic data with
literature data.
.CvO) cm21; H NMR .500 MHz; CDCl3): d 1.02 .1H,
ddd, Jˆ11, 1, 1 Hz, CHCHHCHCHCO), 1.15 .1H, ddd,
Jˆ11, 2, 2 Hz, CHCHHCHCHCO), 1.32±1.42 .2H, m,
CH2CHHCHCHCO and CH2CHHCHCHCvC), 1.61±1.67
.1H, m, CH2CHHCHCHCO), 1.71±1.77 .1H, m,
CH2CHHCHCHCvC), 2.30 .1H, brd, Jˆ4 Hz,
CH2CHCHCvC), 2.39 .1H, brd, Jˆ5 Hz, CHCO), 2.53
.1H, brd, Jˆ4 Hz, CH2CHCHCO), 2.73 .1H, brdd, Jˆ5,
3 Hz, CHCvC), 7.33±7.42 .3H, m, m-Ar-H and p-Ar-H),
7.67 .1H, d, Jˆ3 Hz, CvCH), 7.71±7.74 .2H, m, o-Ar-H);
1
Data for 5: white solid, mp 145±68C .decomp.); IR
.CHCl3): n 1709s .CvO), 1650m .CvC) and 1601m
1
.CvCAr) cm21; H NMR .360 MHz; CDCl3): d 2.30 .1H,
dd, Jˆ4, 18 Hz, CHHCO), 2.73 .1H, dd, Jˆ6, 18 Hz,
CHHCO), 2.86 .1H, dd, Jˆ8, 10 Hz, CHHCHCH2CO),
3.43±3.52 .1H, m, CHCH2CO), 3.98 .1H, dd, Jˆ8, 8 Hz,
CHHCHCH2CO), 4.06 .1H, d, Jˆ16 Hz, CHHCv), 4.43
.1H, d, Jˆ16 Hz, CHHCv), 6.12 .1H, brs, CvCH), 6.64
.2H, d, Jˆ8 Hz, o-Ar-H), 6.79 .1H, t, Jˆ7 Hz, p-Ar-H),
7.29 .2H, dd, Jˆ7, 8 Hz, m-Ar-H); 13C {1H} NMR
.125.8 MHz; CDCl3): d 40.9 .CH2CO), 44.6 .CHCH2CO),
49.1 .CH2Cv), 52.5 .CH2CHCH2CO), 112.4 .o-Ar-C),
117.7 .p-Ar-C), 125.6 .CvCH), 129.8 .m-Ar-C), 147.6
.ipso-Ar-C), 182.4 .CvCH), 209.3 .CvO); EI-MS: m/z
.%)ˆ199 .99) [M1], 170 .13) [M12CHO], 105 .100)
.C6H5NCH21), 94 .66) [M12C6H5NCH2], 77 .36)
[C6H51]; Acc. Mass. .C13H13NO): calcd 199.0997;
Found:199.0999.
13C {1H} NMR .125.8 MHz; CDCl3):
d
28.8
.CH2CH2CHCHCO), 29.6 .CH2CH2CHCHCvC), 31.7
.CHCH2CHCHCO), 38.8 .CH2CHCHCvC), 39.9
.CH2CHCHCO), 48.1 .CHCvC), 55.4 .CHCO), 127.5
.o-Ar-C), 128.8 .m-Ar-C and p-Ar-C), 131.9 .ipso-Ar-C),
146.5 .PhCvCH), 160.6 .CvCH), 209.5 .CvO).
4.1.5. 2-But-2-enyl-2-prop-2-ynyl-malonic acid dimethyl
ester 12. 2-But-2-enyl malonic acid dimethyl ester .1.6 g,
8.6 mmol) was added over 5 min to a solution of sodium
methoxide .540 mg, 10 mmol) in dry methanol .15 mL).
The mixture was stirred under nitrogen for 15 min. Pro-
pargyl bromide .80% solution in toluene, 1.6 g,
10.8 mmol) was added over 15 min and the resulting
mixture was stirred at room temperature for 16 h. The
mixture was washed with water .20 mL), extracted with
diethyl ether .3£15 mL) and concentrated in vacuo. Puri®-
cation by ¯ash column chromatography .SiO2, hexane/
diethyl ether 9:1) of the residual oil gave the title compound
12 .0.771 g, 3.44 mmol, 40%) as a colourless oil .E/Z,
85:15); IR .CHCl3): n 3308w .CuC±H), 3030w .CvC±
4.1.3. 10 mmol Intramolecular PKA reaction. Enyne 6
.2.5 g, 10 mmol) and .PPh3)Co2.CO)7, 1, .288 mg,
0.5 mmol) were dissolved at room temperature in carbon
monoxide saturated 1,2-DME .100 mL) under a carbon
monoxide atmosphere .1.05 atm). The mixture was heated
at 758C with vigorous stirring in the dark .aluminium foil
around the ¯ask) for 4 h. The resulting brown mixture was
cooled, ®ltered through a short pad of celite and concen-
trated in vacuo. .1H NMR spectroscopy revealed total
conversion.) Puri®cation of the residue by ¯ash column
chromatography .SiO2; hexane/EtOAc, 6:4) gave the
known cyclopentenone 73,25 .2.54 g, 9.2 mmol, 92%) as a
white solid; mp 147±1498C .lit.3 145±1488C); IR .CHCl3):
n 1714s .CvO), 1651m .CvC), 1600w .C±CAr), 1350s
1
H), 1734s .CvO) cm21; H NMR .500 MHz; CDCl3): d
1.67 .3H, brd, Jˆ7 Hz, CvCHCH3.E)), 2.02 .1H, t,
Jˆ3 Hz,
CuCH),
2.75
.2H,
dm,
Jˆ8 Hz,
CH2CHvCHCH3), 2.79 .2H, d, Jˆ3 Hz, CH2CuC), 3.75
.6H, s, 2£OCH3), 5.20±5.26 .1H, m, CvCHCH3), 5.58±
5.65 .1H, m, CHvCCH3); 13C {1H} NMR .125.8 MHz;
CDCl3): d 18.4 .CvCHCH3.E)), 22.9 .CH2Cu), 35.7
.CH2CvCHCH3), 53.1 .2£OCH3), 57.4 .C.CO2Me)2),
71.7 .uCH), 79.3 .CuCH), 124.2 .CHvCHCH3), 131.1
1
and 1162s .NSO2) cm21; H NMR .400 MHz; CDCl3): d
1.99 .1H, dd, Jˆ4, 18 Hz, CHHCO), 2.32 .3H, s, CH3),
2.48±2.58 .2H, m, CHHCO and CHHCv), 3.05±3.15

4942
S. E. Gibson et al. / Tetrahedron 58 /2002) 4937±4942
10. Hayashi, M.; Hashimoto, Y.; Yamamoto, Y.; Usuki, J.; Saigo,
K. Angew. Chem., Int. Ed. Engl. 2000, 39, 631.
11. Hiroi, K.; Watanabe, T.; Kawagishi, R.; Abe, I. Tetrahedron
Lett. 2000, 41, 891.
.CHvCHCH3), 170.7 .2£CO2Me); EI-MS: m/z .%)ˆ224
.8) [M1], 185 .54) [M12CH2CuCH], 165 .42)
[M12CO2Me], 164 .100) [M12HCO2Me], 153 .84), 105
.63) [M12H.CO2Me)2]; Acc. Mass. .C12H16O4): calcd
224.1049; Found: 224.1044.
12. Hiroi, K.; Watanabe, T.; Kawagishi, R.; Abe, I. Tetrahedron:
Asymmetry 2000, 11, 797.
13. Pagenkopf, B. L.; Belanger, D. B.; O'Mahony, D. J. R.;
Livinghouse, T. Synthesis 2000, 1009.
Acknowledgements
14. Billington, D. C.; Helps, I. M.; Pauson, P. L.; Thomson, W.;
Willison, D. J. Organomet. Chem. 1988, 354, 233.
15. Jeong, N.; Hwang, S. H.; Lee, Y.; Chung, Y. K. J. Am. Chem.
Soc. 1994, 116, 3159.
The authors gratefully acknowledge AstraZeneca UK Ltd
for the provision of a studentship .for A. S.).
16. Comely, A. C.; Gibson, S. E.; Stevenazzi, A.; Hales, N. J.
Tetrahedron Lett. 2001, 42, 1183.
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