Hydrogen abstraction, ketones diketones

Upon direct photolysis, the a-(l-hydroxyalkyl)-substituted a, 3-unsaturated ketones 104 gave 1,4-diketones 106 in moderate yields (Scheme 4.47) [71], with the reaction involving an intermolecular 3-hydrogen abstraction. The resultant vinyli-denecydopropanols 105 underwent a formal double tautomerization to afford 106. 

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported. The irradiations involved the crystalline materials using A, > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-(—)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type II hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the cis-cyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trn 5-cyclobutanol (47). A detailed examination of the photochemical behaviour of a series of large ring diketones (48) has been carried out. Irradiation in both the solid phase and solution were compared. Norrish Type II reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive... 

The triplet state of the ecdysone (152) is reactive and aRords the products shown in Scheme 5. The formation of the reduction products (153) and (154) is presumed to follow a path where loss of a hydroxyl radical yields the allyl radical (155) which then gives the products (153) and (154) by hydrogen abstraction. The ketone (156) is formed by a 1,2-bond migration and the cyclobutanol (157) arises by secondary irradiation of the diketone (156). The enone (158) shows solvent dependent photochemistry. Thus in ethyl acetate the deconjugated product (159) is formed while in methanol the reduced ketone (160) is produced. ... 

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