CONSTRUCTION OF QUATERNARY STEREOGENIC CENTERS VIA [2-CYCLOADDITION REACTIONS - SYNTHESIS OF HOMOCHIRAL 4,4-DISUBSTITUTED 2-AZETIDINONES AND IMINE SUBSTITUENT EFFECTS ON BETA-LACTAM FORMATION(2])

A study on the asymmetric construction of quaternary stereogenic cente rs via [2 + 2] cycloaddition reaction of ketenes with ketimines is des cribed. Reaction of achiral ketenes and chiral alpha-alkoxy ketone-der ived imines resulted in formation of new beta-lactams as single diaste reomers. The cycloaddition was extended to pyruvate imines, aralkyl ke tone-derived imines, and dialkyl ketimines. In these cases the asymmet ric induction was satisfactorily achieved using beta-silylalkanoyl ket enes and the Evans-Sjogren ketene. C,C-Bis (trimethylsilyl)methylamine ketimines derived from enolizable dialkyl ketones cleanly led to the corresponding C(4) disubstituted beta-lactams without deprotonation. T herefore, a general methodology for a convergent asymmetric synthesis of beta-lactams in which C(4) exists as a quaternary carbon is provide d.