Biomedical Engineering Reference
In-Depth Information
4
GOLDEN OPPORTUNITIES
IN THE SYNTHESIS OF NATURAL
PRODUCTS AND BIOLOGICALLY
ACTIVE COMPOUNDS
CHAPTER
FABIEN GAGOSZ
Laboratoire de Synth`se Organique, UMR 7652 CNRS Ecole Polytechnique,
Palaiseau, France
4.1.
INTRODUCTION
Up to the end of the last century, and despite the fact that the gold element was known
for ages, only little interest had been given to the use of this transition metal for the
design of potential catalysts for organic reactions. The reasons of such a surprising
neglect were undoubtedly due to the preconceived and persistent ideas that gold is a
rare and expensive element, as well as a chemically inert metal. The two first
considerations were obviously unfounded: gold is not only relatively abundant (more
than 2000 tons of gold are currently mined each year), but its price is at least
comparable to those of other transition metals (platinum, rhodium) that are frequently
used in catalysis. The inertness of gold, or more precisely its assumed poor catalytic
potential, was based on the known difficulty of cycling between its oxidation states,
thus limiting its possible utilization as a catalyst in oxidative addition/reductive
elimination processes. As a consequence, only little work concerning the develop-
ment of gold-catalyzed reactions for organic synthesis was done during this period
and the applications of the corresponding methods to the synthesis of natural products
or biologically active compounds were nearly nonexistent. One major exception
concerns the homogeneous gold-catalyzed asymmetric aldol reaction developed by
Ito et al. In 1986, these authors reported that chiral oxazolines
4
could be produced by
the asymmetric addition of an isocyanoacetate
in the presence of a
gold(I) complex formed from the enantiomerically pure ferrocenyl diphosphane
ligand
2
to an aldehyde
1
3
and [Au(
c
-HexNC)
2
]BF
4
(Scheme 4.1) [1]. A cleavage of the oxazoline ring
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