Acetophenone sensitization

The reaction products are the same for both direct irradiation and acetophenone sensitization. When the reactant B is used in homochiral form, the product D is nearly racemic (6% e.e.). Relate the formation of the cyclobutanones to the more normal products of type E and E Why does the 5-aryl substituent favor formation of the cyclobutanones Give a complete mechanism for formation of D which is consistent with the stereochemical result. 

The reaction in its basic form is illustrated by the conversion of 274 into 275. This transformation shows the control exercised within the two biradicals A and B on the final outcome of the reaction151. Another typical di-jr-methane process, this time with electron-withdrawing substituents on one of the vinyl moieties, arises on acetophenone-sensitized irradiation of the diene 276. This yields the cyclopropane 277 in 35% yield. The... 

The influence of radical stabilization on the outcome of the rearrangement reactions of a variety of dibenzobarrelenes has been evaluated178. A detailed analysis of the acetophenone-sensitized conversion of the cyano-substituted barrelenes into the corresponding semibullvalenes has been presented179. The outcome of the irradiation of the dibenzobarrelene 331 is dependent upon the excited state involved. Thus direct irradiation affords a cyclooctatetraene and sensitized irradiation converts it into the two... 

The photoreactivity of aldehyde 29a was surprising because of the literature above-mentioned precedents. Therefore, we decided to determine whether this unexpected reactivity could be extended to other related aldehydes, such as 29b and 29c (Structures 29-39). Indeed, acetophenone-sensitized irradiation (5 min) of 29b affords the ODPM product 32b in 90% isolated yield. Irradiation of 29c under the same conditions also gives the aldehyde 32c in 25% yield. In these two instances, the corresponding, 1,3-formyl migration products were not formed [51]. 

Semibullvalenes 374 and 376 are the major products formed from acetophenone sensitized irradiation of 373 and 375, respectively. When 376 is in turn subjected to direct or chlorobenzene-sensitized irradiation, conversion to 377 results... 

The acetophenone-sensitized irradiation of benzobarrelene 5 gives benzosemi-bullvalene 6 in a quantitative yield (Scheme 4.3) [6]. However, the direct irradiation... 

Table 4.1 Photoproducts obtained on direct and acetophenone-sensitized irradiation of 32a,b.

Acetophenone is also a convenient sensitizer for the ODPM rearrangement. Bicyclo[2.2.2]oct-7-ene-2,5-diones 53a-c undergo a highly chemoselective ODPM rearrangement under acetophenone-sensitized irradiation (350 nm) to yield 54a,c and 55b, in excellent yields (Table 4.2) [33]. However, phenols 56a,b, which presum-... 

Bicyclo[2.2.2]oct-5,7-dien-2-ones 57a-c, which are endowed with the structural features required for both the ODPM and DPM rearrangements, undergo acetophenone-sensitized DPM rearrangement to afford the compounds 58a-c in good yields (Scheme 4.24) [34],... 

It is known that simple p,y-unsaturated ketones 69 do not undergo ODPM rearrangement by the acetophenone-sensitization. Instead, under these conditions 69 undergoes alkene E/Z isomerization. However, when thioxanthone is used as a sensitizer, the [ly-unsaturated ketones 69 react to form ODPM rearrangement photoproducts 70 (Scheme 4.28) [41 ]. This result shows clearly that the energy of the triplet sensitizer plays an important role in determining the outcome of the triplet ODPM rearrangement of p,y-unsaturated ketones. 

Acetophenone-sensitized photolysis of l-methyl-3-phenyl-2-cyclohexene-carbaldehyde oxime acetate (56) A solution of 56 (298 mg, 1.16 mmol)... 

The 1-ADPM reaction has been used as the key step in the synthesis of a series of cyclopropanecarboxylic acids present in insecticidally-active pyrethrins and pyrethroids [63]. Thus, acetophenone-sensitized irradiation... 

Scheme 53. Molecular models do not make this mechanism seem very likely and further work on the structures of caulolcatone A and B may be required. A further study of the photochemistry of germacrone (340) has shown that direct irradiation causes isomerization only of the A °-double bond whereas acetophenone-sensitized irradiation induces isomerization of both the and...

With more complex substances such as ethers the nature of free radical attack becomes more difficult to forecast. Thus in the presence of an acetophenone sensitizer, caffeine and diethyl ether gave a 3 1 mixture of the 8-(l-ethoxyethyl) and 8-ethyl derivatives (Scheme 32) (70CJC1716>. The first formed ether radical, presumably (175), is apparently too unstable to produce a reaction product prior to rearrangement (Scheme 33). With... 

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