Diketones, hydrogen abstraction

The synthesis of a new benzo[6]furan-containing cyclophane has been achieved by irradiation of the starting diketone 113 via a 6-hydrogen abstraction from the o-alkoxybenzophenone. X-ray analysis shows that the cyclophane has a well-defined rectangular cavity <00TL1393>. 

The n,7r triplets of a-diketones are quite reactive in hydrogen abstraction reactions.393-395... 

Of particular interest are the last two examples. Apparently, intramolecular hydrogen abstraction leads quantitatively to cyclobutanols in a-diketones, with no elimination being observed. The phosphorescence yields are, of course, cut drastically by this efficient chemical reaction.396... 

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 detailed study of the photochemical reactivity of the large ring diketones (33) has been carried out. In solution irradiation brings about conventional y-hydrogen abstraction in every case except with the Cjo diketone. The yields are... 

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... 

Type II hydrogen abstraction by the keto group from the adjacent aryl methyl group to give the biradical (60) from which both products (61) and (62) are formed. The reaction is solvent dependent. In cyclohexane solvent only the cyclobutanols (61) are formed, but in methanol a mixture of (61) and the new diketone (62) are obtained in a ratio which is dependent upon steric factors. The photochemically induced proton transfer in 3-methyl-6-hydroxy-m-phthalic acid has been reported. ... 

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. ... 

Scheffer has made use of the intreunolecular hydrogen abstraction reactions in the diketones (3(X)) to synthesize the cage compound (301). This compound was shown to have no anti viral activity.The c e compound (302) is formed easily on irradiation of the dione adduct (303). The synthesis of the cage (302) confirms the e/n/o-arrangement in the adduct. 

The dione (Scheme 22) reacts photochemically with aromatic aldehydes in processes similar to the reactions between quinones and aldehydes. Irradiation of the diketone (343) leads to the formation of the remarkable diol (344). The structure of this product was verified by T-ray analysis. The reaction, brought about by irradiation through Pyrex in various solvents, proceeds via the intermediate keto alcohol (345), a compound isolated at shorter reaction times. There is some doubt in the minds of the authors as to whether the reaction arises by hydrogen abstraction from the y- or the s-positioh. They propose that if the former occurs then the rearrangement by hydrogen migration to yield (I) is kinetically favoured. 

Reactions of 1,3-Diketones - Irradiation of the 1,3-diketone (238) in ethanol or benzene results in the formation of l-hydroxy-2-naphthaldehyde. The site of photochemical hydrogen abstraction reactions within the keto esters (239, X = S) is controlled by SET transfer reactions from the thio substituent to the excited carbonyl group. The usual reaction train following this event yields the biradical (240) which undergoes cyclization to yield (241) in modest to good yields. The involvement of an SET process is proven by the failure of the sulfone derivative (239, X = SO2) to undergo the same reaction. 

Quinones. - Goerner has demonstrated that the yield of semiquinone radicals is low in the absence of hydrogen donating solvents. He notes that the quantum yield for decomposition is substantial in aqueous solution. A detailed account of the photohydroxylation reactions of 1,4-benzoquinone in aqueous solution has been published.The photochemical cyclization of some Diels-Alder adducts of benzo-l,4-quinones has been described. This has provided a path to complex molecules such as 3-bromotetracyclo[5.3.1.0 . 0 ]undec-10(12)-ene-9,l 1-dione. A biradical has been identified as the key intermediate in the photocyclization of phenylbenzoquinone. " A charge-transfer complex is formed initially on irradiation of 2-chloro-5-methoxybenzo-l,4-quinone in the presence of triethylamine and various solvents.2-Chloro-5-methoxyben-zo-l,4-quinone also undergoes addition to arylalkynes to afford oxetenes. The mechanism of this addition reaction was studied.A hydrogen abstraction is involved in the photochemical reactions of chloranil with 3-diketones. ... 

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