3-Halopropenyl esters as precursors of a new class of oxygen-substituted
allylic organometallic compounds: Applications in organic synthesis
M. Lombardo, S. Licciulli, and C. Trombini
Dipartimento di Chimica G.Ciamician ,
University of Bologna, Bologna, Italy
Abstract: 3-halopropenyl esters, readily prepared by the addition
of acyl halides to acrolein, react with zinc, indium, and chromium(II),
thus opening a route to a new class of oxygen-substituted allylic organometallic
compounds. Indium and zinc reagents smoothly add to carbonyl compounds,
affording alk-1-en-3,4-diol derivatives in a variety of synthetic procedures
which include typical Grignard stepwise conditions as well as Barbier
one-pot protocols. Using zinc and indium in water or aprotic solvents,
simple diastereoselectivity was found to depend on the nature of the
carbonyl compound; conjugated aldehydes favor formation of syn-adducts
while unconjugated aldehydes favor anti-adducts. Moving to chromium,
a reversal of regioselectivity was observed in favor of (Z)-4-hydroxy-enolacetates,
flexible protected forms of homoaldols. Chromium complexes are generated
in a catalytic cycle based on the combined use of the redox Mn(0)/Cr(III)
couple and of TMSCl. When the Cr-catalyzed reaction is carried out in
the presence of Jacobsen's Salen ligand, the regiochemical outcome of
the reaction is again reversed, and syn-alk-1-en-3,4-diols are
formed in high ee's.
*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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