2-Cyclohexenones, photorearrangements

Keywords cyclohexenone, photorearrangement, intramolecular allylic hydrogen abstraction... 

Zimmerman, H. E., Kutateladze, A. G., Novel Dissection analysis of Spin Orbit Coupling in the Type B Cyclohexenone Photorearrangement What Controls Photoreactivity Mechanistic and Exploratory Organic Photochemistry, J. Org. Chem. 1995, 60, 6008 6009. 

A number of examples of this type of photorearrangement have been observed with cyclic and bicyclic molecules. Some photoreactions involving cyclohexenones are as follows ... 

The photorearrangement of cyclohexenones to bicyclo[3.1.0]hexanones (type A process) has been extensively studied (55 65> ... 

The photorearrangement of 4-methyl-4-phenyl-2-cyclohexenone was shown by Dauben and co-workers to be very solvent dependent/761 In aprotic, nonpolar solvents only two products were reported ... 

Zimmerman and Kutateladze have drawn several qualitative conclusions from a study of spin-orbit coupling in Type B cyclohexenone (17) photorearrangement, evaluated at critical points along the presumed reaction path (full Breit-Pauli... 

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. 

Although many photorearrangements of conjugated molecules have been studied in connection with the Hoffmann-Woodward rules, little attention has been paid to the possibility of asymmetric induction when these reactions are carried out in solution. When the role of stereogenic centers on the selectivity of a rearrangement was indeed examined, as in the photorearrangement of cyclohexenone derivatives, it was more for mechanistic than for preparative purposes. 

The photorearrangement of chiral cyclohexadienones and cyclo-hexenones has been covered in other reviews and is not examined here [173]. However, it is now well established that the formation of lumiketones from rigid enones and dienones (226, 228, and 231), or from monocyclic, chiral cyclohexenones having C-4 as the only asymmetric center, occurs with more than 99% selectivity [174] (Scheme 36). 

---