Stereoselective and enantioselective synthesis of five-membered rings via conjugate additions of allylsulfone carbanions

This lecture describes some of our studies of lithio derivatives of allyl s ulfone carbanions which add a-regioselectively as well as anti diastereosel ectively to Michael acceptor olefins. This can be ascribed to chelation in the Michael addition step. When the reaction leads to subsequent ring closu re by using a bromoallyl sulfone, the latter acts as a methylenemethane syn thon in a (3+2) Michael-initiated ring closure, affording highly functional ized cyclopentane derivatives Such additions proceed with high stereoselect ivity and with asymmetric induction leading to nonracemic substituted cyclo pentanones. Additions of allyl sulfone carbanions also proceed stereoselect ively 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-heterocy cles. Furthermore, chiral groups on the allyl sulfone moiety can induce sig nificant remote asymmetric induction, made possible by the presence of an a romatic pi -system which promotes intramolecular chelation to the Li cation .