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 sulfone car- banions which add α -regioselectively as well as anti diastereoselectively to Michael accep- tor 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 methylen- emethane synthon in a (3+2) Michael-initiated ring closure, affording highly functionalized cyclopentane derivatives Such additions proceed with high stereoselectivity and with asym- metric induction leading to nonracemic substituted cyclopentanones. Additions of allyl sul- fone 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 π-system which promotes intramolecular chelation to the Li cation.