any chemical reaction that affects the structural symmetry in the molecules of a compound, converting the compound into unequal proportions of compounds that differ in the dissymmetry of their structures at the affected centre. Such reactions usually involve organic compounds (organic compound) in which the symmetrical structural feature is a carbon atom bonded to four other atoms or groups of atoms, of which two are alike; in the asymmetric synthesis, one of the identical groups is preferentially modified or replaced, so that the product is a mixture of two dissymmetric compounds, one of which predominates.

Asymmetric reactions result from the influence of some dissymmetry in the reacting system, such as the presence of a dissymmetric centre in the molecule, a dissymmetric solvent or catalyst, or circularly polarized light, in which the plane of vibration of the electromagnetic fields rotates in either a right-handed or a left-handed sense.

Asymmetric syntheses often are called stereoselective; if one of the products forms exclusively, the reaction is called stereospecific.