Chemistry Reference
In-Depth Information
On the other hand, the coupling of propargyl alcohol derivatives with cyclopropylcarbene
chromium complexes produces alkylidene cyclopentenones.
67
10.5.2 Rhodium(II)-Catalyzed Strategies
Another interesting route to the synthesis of cyclopentenones is the rhodium-catalyzed
intramolecular cyclization of
-diazo ketones bearing alkyne units. This methodology
gives access to a plethora of substituted cyclopentenones.
The proposed mechanism proceeds via decomposition of the
α
-diazoketone
112
to
generate a rhodium-stabilized carbenoid intermediate
113
as shown in Scheme 10.32. Next,
addition of the rhodium carbenoid onto the alkyne
α
-bond generates a vinyl carbenoid
intermediate
114
, in which the carbene-like character has been transferred to the more
remote carbon of the alkyne. This intermediate may react further in either an intra- or
intermolecular fashion to afford novel products. However, several studies from Padwa
laboratories suggested that themechanism is highly dependent on the solvent employed.
68, 69
O
O
O
Rh(II)
Product
N
2
R
RhL
n
R
RhL
n
R
112
113
114
Scheme 10.32
Rh (II) catalyzed synthesis of cyclopentenones.
-diazido ketone
115
undergoes insertion of the internal alkyne, followed by an external alkyne insertion to
render cyclopentenone
116
, along with several byproducts. Ensuing treatment of
116
with
catalytic amounts of a rhodium(II) complex leads to clean isomerization giving
117
as a
single isomer in good yields (Scheme 10.33).
70-72
In one example of the rhodium-catalyzed intramolecular cyclization, the
α
Me
Pent
Pent
Me
Me
Me
Rh
2
(OAc)
4
H
Rh
2
(OAc)
4
Me
Me
Me
Pent
80 °C
Me
O
N
2
Me
74
O
117
O
116
115
88% yield
Scheme 10.33
Intramolecular Rh(II) catalyzed synthesis of fused cyclopentenones.
Another methodology for the synthesis of fused cyclopentenones is illustrated in
Scheme 10.34. This reaction proceeds via the rhodium(II)-catalyzed diazo decomposi-
tion of a mixture of
-unsaturated carbonyl compounds
118
and
119
.
73
It should be
noted that Rh
2
(acam)
4
has to be used as the source of Rh in this reaction given that when
Rh
2
(OAc)
4
was used, the reaction only afforded complex mixtures.
α
,
β
