Chemistry Reference
In-Depth Information
M(CO)
6
L,
h
ν
M(CO)
5
LM(CO)
5
HO
HO
•
8.150
CH
3
CHO
L = THF
M(CO)
5
M(CO)
5
O
O
O
8.152
8.151
8.154
L = Et
3
N
PhCHO
Ph
M(CO)
5
O
O
8.153
8.155
Scheme 8.42
(OC)
5
M
Ce(NH
4
)
2
(NO
3
)
6
M(CO)
5
THF
O
HO
OH
OH
M = Cr, W
8.156
8.157
1.
(Ph
3
P)
2
PdCl
2
, CuI,
i
-Pr
2
NH
2. H
2
, Pd/C
I
n
-C
7
H
15
O
O
n
-C
12
H
25
O
O
OH
OH
8
1
8
.
Scheme 8.43
or catalyst for these reactions is generated from the corresponding hexacarbonyl complex (M
Cr, Mo, W),
in the presence of a Lewis base, L, that then becomes a labile ligand. When this ligand is THF, proton transfer
to carbon occurs to give a stable carbene; when this ligand is triethylamine, elimination of the metal occurs to
give an enol ether (compare to Scheme 8.14). In the presence of aldehydes, aldol products,
8.154
and
8.155,
are formed.
This chemistry has been used to synthesize butyrolactone natural products, such as muricatacin
8.159
(Scheme 8.43),
46
taking advantage of the facile oxidation of Fischer carbenes to the carbonyl compounds
(see Scheme 8.13). Treatment of the C2-symmetrical diol
8.156
with the activated metal-carbonyl complex
generated the carbene complex
8.157
. Both the chromium and tungsten complexes were used, with similar
results. Oxidation of the carbene
8.157
yielded the butyrolactone
8.158
. The carbon chain could then be
extended by Sonogashira coupling of the surviving alkyne, and, finally, hydrogenation gave the natural
product
8.159
.
One application of the corresponding enol ether formation is in the synthesis of D-decosamine
8.165
(Scheme 8.44),
47
a deoxysugar that appears in many naturally occurring antibiotics. The required alkynol
8.162
was prepared from a
=
-siloxyaldehyde
8.160
by addition of trimethylsilylacetylene anion. As the
-substituent exerted little stereocontrol over the new stereogenic centre, a chiral ligand was included in this
