Chemistry Reference
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Coordination of the copper enolate with the alkynyl moiety onto VIII in-
duced the cyclization, followed by the protonation of the organocopper
intermediate IX. The copper-catalyzed aerobic oxidation of the hemiacetal
X afforded the furanocoumarin product 58.
When the same reaction was performed using CuBr as catalyst and stoi-
chiometric amounts of CuCl
2
, chlorofuro[3,2-c]coumarin was obtained
(Equation 7.12). This product can be further engaged in cross-coupling re-
actions such as Suzuki cross-coupling (Equation 7.13).
CuBr (10 mol %)
CuCl
2
(4.2 equiv)
H
2
O (10 equiv)
O
O
(7.12)
DMF, air, 10 h
75 °C
Cl
O
O
O
59
60
, 77%
Pd
2
(dba)
3
(5 mol %)
P(
t
-Bu)
3
(12 mol %)
60
+
O
(7.13)
CsF (1.5 equiv)
THF, reflux
(HO)
2
B
OMe
OMe
O
O
61
, 70%
Following the work by Cheng and Hu, a similar one-pot cyclization-
oxidation reaction was reported for the synthesis of substituted 2-benzoyl-
furans 63 from b-alkynyl-a,b-unsaturated aldehydes 62 (Scheme 7.15).
39
The proposed mechanism for this copper-catalyzed tandem cyclization-
aerobic oxidation is different, however (Scheme 7.16). The coordination of
the Cu
I
with the alkyne group into 62 leads to an intramolecular attack of the
aldehyde, providing an organocopper intermediate XII. Protonation, fol-
lowed by the conjugate addition of water, gave substituted furfuryl alcohol
XIV. Finally, the copper salts catalyzed the aerobic oxidation of XIV to give
substituted 2-benzoylfurans 63.
Other transition metal complexes, in addition to copper, are known for
their dual activity in different transformations. Recently, He and co-workers
reported the synthesis of carbon-supported Ag nanoparticles.
40
These
were investigated as catalysts for the oxidation of benzyl alcohols under an
air atmosphere and basic DMSO reaction conditions, with the goal of pro-
ducing benzaldehydes. Unexpectedly, they observed instead the formation of
styryl ethers 65 in excellent yields from a variety of benzyl alcohols
(Equation 7.14).
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