We describe here the diastereoselective synthesis of cyclic β-aminoketone derivatives which are very powerful synthons for alkaloid synthesis. They were stereoselectively prepared by condensation of a chiral lithium amide on an α,β-unsaturated ester which possesses a leaving group in ω−position. The usefulness of these compounds was demonstrated through the synthesis of indolizidine 209D, an alkaloid isolated from skin extracts of South American frogs and also in the preparation of allosedridine which has been isolated from Sedum acre.