Asymmetric photodeconjugation

For a complementary approach of asymmetric photodeconjugation to yield secondary stereogenic centers containing fluorine F. Bargiggia, S. Dos Santos, O. Piva, Synthesis 2002, 427-437. 

Henin F, Letinois S, Muzart J. Asymmetric photodeconjugation of ammonium ene-carhoxylates temperature effects and evidence for the a-carhon of the dienohc species as a latent trigonal centre. Tetrahedron Asymm. 2000 11 2037-2044. 

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

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). 

Charlton, J. L., Pham, V. C., and Pete, J. R, Lactate as chiral auxiliary in asymmetric photodeconjugation of unsaturated esters, Tetrahedron Lett., 33, 6073,1992. 

Piva, O. and Pete, J. R, Diacetone D-glucose efficient chiral building block for asymmetric photodeconjugation, Tetrahedron Asymm., 3, 759,1992. 

Bargiggia, R, Dos Santos, S., and Piva, O., Asymmetric photodeconjugation highly stereoselective synthesis of a-fluoro carboxylic derivatives. Synthesis, 427, 2002. 

When photodeconjugation leads to only one asymmetric carbon, photocyclization of a-aminocyclohexanone, involving a y-H abstraction equally in the photochemical step allows the formation of a bicyclic molecule with three new asymmetric centers. Similarly, in the photorearrangement of prochiral cyclohexenones, two new asymmetric carbons are created in the corresponding cyclopropylketone. During the di-II-methane rearrangement of prochiral dibenzobarrelenes, four asymmetric carbons are created, while up to four new asymmetric centers can be obtained in the 2-1-2 photocycloaddition of molecules as simple as 2-cyclopentenone and cyclopentene. 

Considerable effort has been devoted to asymmetric synthesis in the dark. In principle, if photoreactions could produce intermediates with lifetimes long enough to allow them to react in the ground state, it should be possible to apply the well-known principles of asymmetric synthesis to their transformation. As illustrated by Z = isomerization, photodeconjugation and photocyclization have yielded successful results using this approach. 

In conclusion, photodeconjugation of enones or carboxylic acid derivatives is a mild process by which to prepare 3,Y-unsaturated carbonyl and carboxylic derivatives in good yields. These substrates represent interesting frameworks useful for the synthesis of natural products. The asymmetric version of the process also allows the formation of new chiral centers under very mild conditions and with high selectivities. 

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