In 1952, the seminal publication by D. J. Cram and
F. A. Abd Elhafez1b appeared in which a model was
introducedssince then known as the Cram rules
which allowed the analysis of the stereochemical
outcome of such reactions. On the basis of the nature
of the substituents at the chiral center, two different
conformations of the substrate 1 were proposed
together with the favored trajectory of the attacking
nucleophile to explain the preferential formation of
2 or 3, respectively.