Asymmetric Photochemical Transformations Involving Memory of Chirality and Related Strategies

Abstract

The aim of this review is to give an overview of asymmetric photochemical reactions that do not involve the classical strategies of asymmetric synthesis, such as the use of chiral catalysts, reagents or auxiliaries. These reactions can nevertheless start from a chiral compound but in that case, its initial chirality is directly impacted during the transformation—for instance, by trigonalization of a unique stereogenic center—or, on the contrary, the reaction does not contain any chiral reagents. In this latter case, this kind of reaction is called an absolute asymmetric synthesis. In the first part, methods based on memory of chirality (a chiral compound is subjected to a reaction during which its initial chirality is destroyed but retained in a global conformation, allowing a stereoselective transformation and the obtention of an enantioenriched product), on the recombination of solvent-cage radical pairs, on configurationally stable radicals or involving the transfer of an initial axial chirality to a central chirality, are thus described. The second part focuses on absolute asymmetric synthesis reactions. These are all based on the particular property of certain compounds to crystallize as a conglomerate, the chiral crystal being either the reactant, as in frozen chirality, or the product of the reaction.