Stereoselective and enantioselective synthesis of five-membered
rings via conjugate additions of allylsulfone carbanions*
Alfred Hassner**, Eugene Ghera, Tamar Yechezkel, Victoria
Kleiman, Thiagarajan Balasubramanian, and Daryl Ostercamp
Department of Chemistry, Bar-Ilan University, Ramat-Gan
52900, Israel
Abstract: This lecture describes some of our studies of lithio
derivatives of allyl sulfone carbanions which add a-regioselectively
as well as anti diastereoselectively to Michael acceptor olefins. This
can be ascribed to chelation in the Michael addition step. When the
reaction leads to subsequent ring closure by using a bromoallyl sulfone,
the latter acts as a methylenemethane synthon in a (3+2) Michael-initiated
ring closure, affording highly functionalized cyclopentane derivatives.
Such additions proceed with high stereoselectivity and with asymmetric
induction leading to nonracemic substituted cyclopentanones. Additions
of allyl sulfone carbanions also proceed stereoselectively to C=N systems
containing a chiral auxiliary on N. These can be used in the synthesis
of optically active five- and six-membered ring N-heterocycles. Furthermore,
chiral groups on the allyl sulfone moiety can induce significant remote
asymmetric induction, made possible by the presence of an aromatic p-system
which promotes intramolecular chelation to the Li cation.
*Lecture presented at the 13th International
Conference on Organic Synthesis (ICOS-13), Warsaw, Poland, 1-5 July
2000.
** Corresponding author
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