Stereoselective chromium- and molybdenum-mediated transformations
of arenes
E. P. K�ndig, R. Cannas, C.-H. Fabritius, G. Grossheimann, M. Kondratenko,
M. Laxmisha, S. Pache, H. Ratni, F. Robvieux, P. Romanens, and S. Tchertchian
Department of Organic Chemistry, University of Geneva,
30 Quai Ernest
Ansermet, 1211 Geneva 4, Switzerland
Abstract: Tricarbonylchromium-mediated dearomatization provides
a rapid access to substituted cyclohexadienes. Efficient asymmetric
routes to planar chiral arene complexes and to substituted cyclohexadienes
have been developed. The article sums up the main features of this chemistry.
Highly enantiomerically enriched ortho-substituted benzaldehyde
complexes are accessible via asymmetric lithiation followed by trapping
with electrophiles. In different solvents, the trimethylsilyl complex
exhibits [alpha] values ranging from 174 to +108 for the same
enantiomer. Details of two asymmetric syntheses of natural products
are given: the alkaloid lasubine I starting from a highly enantiomerically
enriched planar chiral arene complex and the marine furanosesquiterpene
acetoxytubipofuran. The latter is assembled via asymmetric dearomatization
of a benzaldehyde imine complex. Other key steps include an EschenmoserClaisen
rearrangement and a regio- and diastereoselective Pd-catalyzed allylic
substitution. The final section deals with labile arene metal complexes.
For the first time, dearomatization reactions mediated by the Mo(CO)3
group have been realized. The reactions show strong analogies to the
Cr(CO)3-mediated reactions, but exhibit also marked differences:
the areneMo bond is stronger, but more labile, and the sequential
double additions show different selectivities compared to the chromium
analogs.
*Plenary and invited lectures presented at the 12th
International Symposium on Organometallic Chemistry Directed Towards
Organic Synthesis (OMCOS-12), Toronto, Ontario, Canada, 6-10 July 2003.
Other lectures are published in this issue, pp.
453-695.
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