Diastereofacial selectivity in the reaction of chiral N-trimethylsilyl imines with ester enolates: Preparation of trans -azetidin-2-ones in high stereocontrolled fashion

Highly efficient chiral ester enolate-imine condensation, giving 3,4-disubstituted ?-lactams with excellent trans-syn or trans-anti diastereoselectivity, is reported. The chiral information is included in the electrophilic partner of the condensation, nominally chiral ?-, ?-silyloxy- or alkoxy-N-trimethylsilyl-imines. The high diastereofacial selectivity is determined by the correct choice of the metal cations present in the reaction medium and the very nature of the hydroxy protecting group. By this procedure a number of azetidinones, intermediates in the synthesis of commercially interesting ?-lactam antibiotics have been obtained. Fully assigned 1H and 13C NMR spectra of the so obtained azetidinones are reported.