Excited-state reactions acyl migrations, unsaturated

The photochemical 1,3-acyl migration which converts certain jSy-unsaturated ketones into isomeric enones has been investigated for a series of compounds, including cholest-4-en-7-one (651) and its 6,6-dimethyl derivative (652). Photoequilibration affords the isomers (653), considered to arise via n — tt triplet states. The reactions are influenced by conformational and conjugative features which effect the stability of an intermediate radical pair of the type (654). Other workers have studied the phosphorescence spectra and lifetimes of triplet excited states of a number of j y-unsaturated ketones, including some... 

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. 

---