The addition products from the highly stereoselective conjugate additions of lithium (αS)-(α- methylbenzyl) allylamide to α, β-unsaturated tert-butyl esters are efficiently deallylated with tris (triphenylphosphine) rhodium (I) chloride and converted, after transesterification to the methyl esters and cyclisation with methylmagnesium bromide, to the corresponding homochiral N-(α-methylbenzyl)-4-substituted-azetidm-2-ones.