Special Topic 6.3 Asymmetric photochemical synthesis

Asymmetric synthesis introduces one or more new features of chirality in a molecule. Several approaches are possible. In general, preferential formation of an enantiomer or diastereoisomer is achieved as a result of the influence of a chiral element present in the substrate, a reagent, catalyst or the environment. Chirality control is also possible in the electronically excited state,584 as demonstrated in the following examples. 

Excitation with circularly polarized light (CPL) is an interesting direct method of asymmetric synthesis, which is based on preferential excitation of one of the enantiomers thanks to a difference in the molar absorption coefficients towards left or right CPL at a given wavelength.585 Unfortunately, only a small enantiomeric excess (ee) can be generated by this method. Some scientists, nevertheless, believe that 

Encmtiodifferentiating photosensitization involves a prochiral or racemic substrate interacting with an optically active sensitizer via quenching (Section 2.2.2) or exciplex formation (Section 2.2.3),585 rather than in the subsequent thermal process. Asymmetric photosensitization requires only a catalytic amount of the chiral sensitizer. An example is provided in Case Study 6.2 (E Z photoisomerization). 

Achiral molecules can crystallize in a chiral space group or form inclusion crystals of achiral guest and chiral host molecules in the absence of any external source of chirality.588-590 Irradiation of such solid-state samples may then generate optically pure compounds in high chemical yields. This type of asymmetric induction is introduced later in Special Topic 6.5 and Case Study 6.21. 

Many other methods of asymmetric induction in photochemistry, such as template-induced enantioselective photoreactions,591 asymmetric photochemistry in zeolites,592 or simple photoreactions of a chiral starting material, parallel their ground-state chemistry counterparts. Diastereoselective photocycloaddition in the Patemo Biichi reaction of chiral reactants (Case Study 6.17) may serve as an additional example. 

---