STEREOCONTROLLED SYNTHESIS OF HIGHLY SUBSTITUTED PROLINE ESTERS VIA [3-METALATED AZOMETHINE YLIDES AND NITROALKENES - ORIGINS OF THE METAL EFFECT ON THE STEREOCHEMICAL OUTCOME(2]CYCLOADDITION BETWEEN N)

The [3 + 2] cycloaddition reaction between several N-metalated azometh ine ylides and nitroalkenes has been studied using AgOAc and LiClO4 as metalating reagents in the presence of triethylamine. The reaction is found to be very versatile and can be extended to homochiral nitroalk enes. In general, lithium and silver salts promote preferential format ion of the endo and exo cycloadducts, respectively. The presence of a phenol moiety induces a shift toward the exo-cycloadduct even when lit hium is used. A model based upon the experimental results obtained and SCF-MO calculations is proposed to explain the variable stereochemica l outcome of these reactions.