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<A NAME="RG11202ST-5">5</A>
The palladium-diphenylphosphinoethane
was preformed by heating Pd(OAc)2 (5 mol%) and
dppe (5 mol%) in the presence of 1-heptene (10 mol%,
THF, 50 °C) until a homogeneous dark red solution
was obtained.
The palladium(0) catalyst generated
in situ by reduction of PdCl2(PPh3)2 with n-BuLi has been found particularly effective
in related carbopalladation reactions:
<A NAME="RG11202ST-6A">6a</A>
See ref.
[2d]
<A NAME="RG11202ST-6B">6b</A>
Bottex M.
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<A NAME="RG11202ST-7">7</A>
The demethoxycarbonylation of malonate
ester using Krapcho’s conditions generally needs prolonged
heating in polar solvent. The strain generated by the two contiguous exocyclic
double bonds and the malonate could explain this rather mild conditions.
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Only traces of the corresponding coupling
product were observed which were easily removed by flash chromatography.
<A NAME="RG11202ST-12">12</A>
Experimental Procedure
for the Synthesis of 6a:
Potassium hydride (35% in
mineral oil) was washed with anhyd THF, dried and stored under nitrogen
prior to use.
To a suspension of KH (48 mg, 1.2 mmol) in
3 mL of anhyd THF was added 18-C-6 (53 mg, 0.2 mmol), enyne 2a (191 mg, 1 mmol) in 4 mL of THF, and
iodobenzene (170 µL, 1.5 mmol). This mixture was stirred
at r.t. for 15 min. In a separate flask, n-BuLi
(2 M in hexanes) was added dropwise at r.t. to a suspension of PdCl2(PPh3)2 (35
mg, 0.05 mmol) in 3 mL of THF until the mixture becomes dark green.
After heating at reflux for 5 min and cooling, the mixture turned
to a dark red homogeneous solution and was added at r.t. via a cannula
to the cyanoester anion prepared above. The resulting mixture was
then stirred at 30 °C for 3 h (monitored by TLC
until completion). The solution was then filtered through a short
pad of silica gel (eluting with diethyl ether) and the solvent was
removed under reduced pressure. The residue was purified by flash
chromatography with petroleum ether/Et2O 80/20
to afford 6a as a pale yellow solid (206
mg, 77%). 1H NMR (CDCl3,
300 MHz, ppm): δ = 1.94 (2 H, m), 2.17 (1 H, dt, J = 14.0,
6.5 Hz), 2.43 (2 H, t, J = 6.5
Hz), 2.60 (1 H, dt, J = 14.0,
5.5 Hz), 3.85 (3 H, s), 4.81 (1 H, d, J = 1.1
Hz), 5.02 (1 H, d, J = 1.5
Hz), 6.63 (1 H, s), 7.18-7.30 (3 H, m), 7.34 (2 H, m). 13C
NMR (CDCl3, 50 MHz, ppm): δ = 23.46,
35.36, 35.44, 53.19, 53.66, 117.35, 117.94, 127.44, 128.04, 128.07,
128.95, 135.73, 135.76, 141.05, 167.13. Anal. Calcd for C17H17NO2:
C, 76.38; H, 6.41; N, 5.24. Found: C, 76.43; H, 6.50; N, 5.19.
The E-configuration of this isomer was deduced
from differential NOE experiments in the 1H
NMR.
For related examples of palladium
catalyzed reaction combined with 6-π electrocyclization
processes see:
<A NAME="RG11202ST-13A">13a</A>
See ref.
[2c]
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von Zezschwitz P.
Petry F.
de Meijere A.
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Similar results were recently reported
for palladium catalyzed tandem cyclization/coupling reactions
involving commercial β-bromostyrene:
<A NAME="RG11202ST-14A">14a</A>
See ref.
[6]
<A NAME="RG11202ST-14B">14b</A>
Arcadi A.
Cacchi S.
Cassetta A.
Fabrizi G.
Parisi LM.
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1605
