Chemistry Reference
In-Depth Information
sulfide (DodSMe) is a highly effective promoter of both intra- and intermolecular PK annu-
lations (Scheme 1.20).
42
Additionally, in every case investigated, the reaction efficiencies
and yields obtained when utilising DodSMe were at least comparable with those from the
use of the less practically convenient sulfide,
n
-BuSMe.
MeO
2
C
MeO
2
C
Co
2
(CO)
6
DodSMe, 1,2-DCE,
O
83 °C, 30 min
MeO
2
CC
O
2
Me
75%
O
DodSMe, 1,2-DCE,
Ph
Ph
H
Co
2
(CO)
6
+
83 °C, 30 min
1
66%
Scheme 1.20
In a similar fashion to that described for amine
N
-oxide promoters (
vide supra
), work
from our own laboratory has shown how the sulfur additive can be attached to a solid
support. These polymer-bound and odourless materials display PK promoting abilities in
line with the solution-phase additives, with the further advantage that they can be reused
extremely effectively over a number of cycles.
43
Further practical benefit arises from the
reaction work-up now merely involving filtration and minimal cyclopentenone purification.
Finally in this section, and by way of illustrating how these sulfide-promoted methods can
provide further techniques for use within natural product synthesis (when compared to
other available protocols), Scheme 1.21 illustrates the key PK cyclisations used within our
O
O
O
+
2:1
Co
2
(CO)
6
OO
O
α
-cedrene
2:1;
Z
:
E
O
O
Conditions
Me
3
NO.2H
2
O, acetone, r.t., 16 h: 91%
NMO.H
2
O, DCM, r.t., 16 h: 84%
n
-BuSMe, 1,2-DCE, 83 °C, 30 min: 95%
Polymer-supported sulfide,1,2-DCE,83 °C, 30 min: 80%
O
DodSMe, 1,2-DCE,
83 °C, 40 min
2-epi-
α
-cedren-3-one
O
O
Co
2
(CO)
6
92%
OO
Scheme 1.21
