Chemistry Reference
In-Depth Information
O
Ph
O
H
O
O
H
O
OH
6.278
X
n
M
MX
n
O
O
6.279
6.280
MX
n
O
O
6.281
6.282
Scheme 6.111
Both syntheses employed a terpenoid starting material, geraniol
6.283
or citronellal
6.288
, and employed
aldol chemistry to install the ketone, leading to slightly different cyclization precursors,
6.286
and
6.291
(Scheme 6.112). For the more complex precursor
6.286
, the choice of catalyst proved to be the key. A gold
NHC complex was found to be most effective. For the simpler alkyne
6.291
, the choice of alcohol protecting
group proved to be critical. When the TBS ether was employed, the ketone did not participate and enyne
6.292
was converted to the diene
6.294
. The free alcohol
6.291
, however, gave the desired bicyclic product
6.293
on treatment with a gold(I) catalyst. Both cyclization products,
6.287
and
6.293
, could then be converted to
the natural product
6.278
through a series of functional group transformations.
6.3
Iron
Complexation of alkenes to iron carbonyl fragments increases the electrophilicity of the alkene, owing to the
electron-withdrawing ability of the carbonyl ligands. The anion of dimethyl malonate will attack the iron
tetracarbonyl complex of methyl acrylate
6.295
(Scheme 6.113).
124
The resulting anion
6.296
is obviously
related to the intermediates involved in the reactions of Collman's reagent (Section 4.5.1). Thus, carbonylation
and addition of an alkyl halide results in acylation.
6.3.1 Fp Complexes of Alkenes
Much better known, and more widely applicable are the complexes developed over many years by Rosenblum:
Fp alkenes.
125
The Fp group is CpFe(CO)
2
. These are complexes of alkenes with the cyclopentadienyldicar-
bonyliron moiety and carry a positive charge. They have been found to be much more stable than the simple
