1,2-Diketones, photocyclization

Herrera, A.J., Rondon, M., and Suarez, E. (2008) Stereocontrolled photocyclization of 1,2-diketones application of a 1,3-acetyl group transfer methodology to carbohydrates. Journal of Organic Chemistry,... 

In contrast, Urry and Trecker had shown in 1962 that a-diketones undergo photocyclization to 2-hydroxycyclobutanones, the intermediate biradical cyclizing but not cleaving (which would form a hydroxyketene) [19]. 

Reactions of 1,2-Diketones and other 1,2-Dicarbonyl Compounds. - Further details of the control of the photocyclizations of enamides such as (142) affording (143) have been described. The results obtained are influenced by the nature of the chiral auxiliaries used. The best results are obtained using ionic and covalent auxiliaries in the crystalline phase or in zeolites. " Photochemically induced a-cleavage occurs on irradiation of the ketoamides (144) in water. This results in the release of the carboxylic acid moiety. The by-product of the reaction is the hemiacetal (145). " The irradiation of propellane in the presence of... 

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. ... 

Herrera et al. [97,98] have reported the usage of Norrish-Yang photocyclization for the synthesis of new spirocyclic monosaccharide derivatives of types 243 and 244 via a hydrogen atom transfer (HAT) reaction promoted by a 1,2-diketone 241, in its excited state, followed by C-C tetrasubstituted bond formation in a diastereo-selective manner (Fig. 8.59). Of special interest is the study of the tendency to inversion at C5 (for examples of epimerization of anomeric and pseudoanomeric radicals, see [99, 100]), probably triggered by conformational changes that the 1,4-diradical intermediate 242 undergoes in its triplet state, within its lifetime (for discussions of the lifetime of diradicals in solution, see [101, 102]) before the intersystem crossing (ISC) occurs. 

Herrera AJ, Rond6n M, Suarez E (2007) Straeocontrolled photocyclization of 1,2-diketones applied to carbohydrate models a new entry to C-ketosides. Synlett 1851-1856... 

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