Enantioselective reactions photodeconjugation

Accordingly, apolar solvents, in particular n-hexane and dichloromethane, were found to be convenient for these enantioselective photodeconjugation reactions. 

The amide (38a) is photochemically inert on irradiation in ether. The related compound (38b) is, however, photochemically reactive and undergoes fission by a Norrish Type II process to yield a mixture of products.The results of a study of the enantioselective photodeconjugation reactions of the lactones (39) have been published. The behaviour of the ketones (40) and (41) in the isotropic and the two solid phases of heneicosane (CcxH ) has been evaluated. The influence of the various phases on the ratio of elimination to cyclization products of the ketones was discussed. The modification of the photoreactivity of ketones (42) in cyclodextrin has been... 

To be very enantioselective, this reaction has to meet several important requirements. First, photoenols have to be produced as either pure Z or E stereoisomers to allow enantioselective photodeconjugation. Even so, protonation of the Z or stereoisomers from the same, rear side, for example, would produce opposite enantiomers and a low enantiomeric excess (ee) would result (Scheme 3). Fortunately, photoenolization of aliphatic enones is only possible from the Z isomer excited in its singlet state, and the excited molecule has to adopt an s-cis conformation to place the excited carbonyl and the y-H close enough to allow y-H abstraction. Consequently, the enol is formed in a unique configuration. All these observations have led several groups to propose a concerted process involving a 1,5 antarafacial sigmatropic shift for the formation of photodienols [16]. 

Scheme 4. Unsaturated lactones, esters, enones and -aminoalcohols considered in enantioselective photodeconjugation reactions.

While the concept of enantioselective photodeconjugation is of great interest, the process is efficient in relatively few cases and cannot be appHed to the total synthesis of natural products. The diastereoselective reaction has been also examined in parallel to the enantioselective one. High levels of induction (up to 88%) were first achieved using Hehnchen s derivatives as chiral alkoxy moieties (Scheme 20, entry a) " unfortunately, these chiral alcohols are expensive or require numerous steps to be prepared. 

---