Diastereoselectivity photodeconjugation

To minimize this troublesome process, the concentration of the intermediate has to be kept as low as possible by means of catalytic tautomer-ization of the dienol. To make the chiral discrimination as great as possible, a strong interaction between the prochiral intermediate and the chiral environment is needed. When the chiral inductor is introduced into the starting molecule itself, diastereoselective photodeconjugations can be observed. If an external chiral inductor is used, enantioselective protonations are obtained. This approach, which may be catalytic if the chiral auxiliary is not consumed in the process, can take advantage of the acidic property of enols [27] and of the possibility of an acid- or base-catalyzed tautomer-ization of enols [28]. 

For synthetic purposes, 2b only gives satisfactory selectivities, and disappointing results have been obtained with amides [49], a-aminoesters [24s], and a-bromoesters [49]. A more general access to chiral deconjugated esters is needed and diastereoselective photodeconjugations might be the solution to this problem. 

Synthetic applications of the diastereoselective photodeconjugation of unsaturated esters 13 and 17 were explored and compared with the corresponding enantioselective approach. Highly enantioselective syntheses... 

Asymmetric Photodeconjugation Principle Enantioselective Photodeconjugation of a,P-Unsaturated Esters and Lactones Diastereoselective Photodeconjugations. .. 70-7... 

Bargiggia, F. and Piva, O., Diastereoselective photodeconjugation of chiral a,(i-unsaturated esters. Tetrahedron Asymm., 12,1389, 2001. 

With chiral molecules, there is no need for external chiral inductors to induce a diastereoselection. For most esters made from conjugated acids and chiral alcohols, the diastereoisomeric excess (de) of the photodeconjugation process, carried out in apolar solvents containing catalytic quantities of alcohols (and/or amines), remains low. This results from the low steric interactions developed by the alkoxy group during the protonation step of the dienol, as already indicated [32]. However, if protonation of photo-dienols from one enantioface is sterically hindered, as can be observed for isobornyl derivatives 8c and 8d, values of de up to 95% can be attained [50] (Table 4 and Scheme 6). 

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. 

Because the synthesis of chiral fluoro compounds is of great interest, due to their biological properties, the asymmetric photodeconjugation of a-fluoro-a,P-unsaturated esters 46 has been explored. Despite the high acidity of the hydrogen on the C2 atom, the diastereoselective isomerization to the P.y-unsat-urated isomers is effective with levels of induction for 47 similar to those for non-fluorinated substrates (Scheme 25). 

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