The effect of temperature, solvent, and type of aldehyde on the stereospecific or stereoselective formation of azomethine ylides by the decarboxylative condensation of aldehydes with three types of α-amino acids, (i) cyclic secondary α-amino acids, (ii) cyclic secondary α-amino acids with a benzylic carboxy group, and (iii) acyclic primary α-amino acids, has been studied. Stereospecific anti-dipole formation in (i) and temperature dependent stereoselective anti-dipole formation in (ii) and (iii) is inferred from the stereochemistry of cycloadducts of the azomethine ylides with N-methylmaleimide. These results are used to support a mechanistic scheme involving loss of carbon dioxide from intermediate oxazolidin-5-ones in a stereospecific cycloreversion reaction.