ABSTRACT
By definition, enantioselective reactions are transformations in which a prochiral substrate is converted into a chiral product such that one of the two enantiomers is formed in significant excess. The degree of enantioselectivity is given by the enantiomeric excess (
ee
). Schematically, such a transformation is depicted in Scheme 1, with the triangle (substrate S) representing a prochiral unit and the two tetrahedrons (products P and
ent
-P) representing two enantiomeric chiral products. In photocycloaddition reactions, the degree of enantioselectivity depends on the effectiveness with
which the enantiotopic faces of a prochiral olefin can be distinguished (enantioface-differentiation). The conventional way to achieve this goal is to covalently attach a chiral auxiliary (see Section 61.2) to the prochiral substrate. The consecutive reaction is diastereoselective, and the auxiliary has to be removed afterwards. The degree of diastereoselectivity in the selectivity-determining step is measured by the diastereomeric excess (DE), which is defined by analogy to the enantiomeric excess, and the
ee
corresponds to the DE after removal of the auxiliary. The auxiliary-based process consequently consists of three steps, two of which are, in principle, superfluous as they are not concerned with the reaction under scrutiny.
