Photoisomerizations 1.3- sigmatropic shift

Apart from photocyclization reactions, simple trienes undergo many types of other phototransformations, including E Z photoisomerization (Section 6.1.1), sigmatropic shifts (see later) or photoadditions (Section 6.1.4). For example, conformational control and possibly equilibration between the excited conformers result in a... 

Subsequent to the aforementioned photoisomerization, which was later rationalized as a 1,3-acyl sigmatropic shift, photochemical reactions of various types of P,y-enones were examined. It was observed that P,y-unsaturated enones undergo two unique reactions viz. 1,3-acyl sigmatropic shift and oxa-di-7C-methane (ODPM) reaction or 1,2-acyl shift), which are characteristic of their excited states. The direct irradiation of P,y-enones such as 2, in general leads to the formation of another enone 3 as a result of migration of the carbonyl group to the y-carbon and formation of a new 7t-bond between the a- and P-carbons (Scheme 2). Sensitized irradiation, on the other hand, causes a 1,2-acyl shift (or oxa-di-7t-methane reaction), leading to a cyclopropyl ketone of type 4. 

---