Correspondence a-diketone

Both 2-nitro steroids 235 and 239 exist as the enols in ethanol, and are photolysed to give the corresponding a-diketones 237 and 238 (23% in 1 1 ratio)133 (equation 108) and 240 (equation 109), but different monoxime 236 and 241, respectively. On the contrary, 4-nitroketone 242 exists in the keto form, and is photolysed to give the a-oximino ketone 243 and its tautomer 244 without the diketones (equation 110). 

Oxidation of the pyridoins, e.g. with iodine, gives the corresponding a-diketones. The significant electron withdrawal exerted by the 7r-deficient heterocyclic system on carbonyl groups at the a- and y-positions can be demonstrated in the behaviour of the diketones under benzilic acid rearrangement conditions. Whereas 3,3 -pyridil (73) gives a normal... 

Interestingly, as reported in the previous section, two unconnected mechanisms seem to operate in the TBCHD-sensitized photooxidation of several substituted 2,3-diaryl-5,6-dihydro-1,4-dioxins 22a-c, and substituted 2,3,5,6-tetraphenyl-1,4-dioxin, 64a-d, yielding, as major products, 1,2-ethanedioldibenzoates 24a-c, and (Z)-stilbenediol dibenzoates 66a-d respectively, together with variable amounts of the corresponding a-diketones [90, 111] (Eqs. (27)]. 

Cyclic ketones like cyclohexanone and cycloheptanoneyield the corresponding a-diketones in 35% and 90% yields, respectively. 

Hydroxy ketones of the type RCOCHjCHOHCHj are formed in 35-66% yields by partial catalytic hydrogenation of the corresponding /S-diketones over Raney nickel at KX) . Aromatic a-hydroxy ketones (benzoins) are prepared from the corresponding a-diketones (benzils) by catalytic reduction or by reduction with magnesium-magnesium iodide mixture. ... 

Ruthenium- or permanganate-mediated oxidations of internal aikynes are highly dependent on solvent conditions and generally afford the corresponding a-diketones." ... 

With a Ni(cot)2 catalyst in a polar solvent, carbon monoxide is evolved and the stan-nane R3SnR is formed, presumably by oxidative addition to give R3SnNinCOR then elimination of CO followed by reductive elimination, and they react with acyl halides in the presence of a palladium(II) catalyst to give the corresponding a-diketones.65 Reactions of potential synthetic use have been developed, in which oxidative addition is followed by insertion of an electron-rich66 or electron-poor alkyne,67 or of a 1,2-68 or 1,3-diene,69 or of an enone.67 Typical reactions and reaction conditions are shown in Table 6-2. 

The photosensitized singlet oxygenation of 1,4-dioxins in methylene chloride at — 78°C using tetraphenylporphin as sensitizer affords the corresponding bisdioxetanes (74). In all photooxygenations, variable amounts of the corresponding a-diketones (75) and enediol diesters (76) were formed. Thermal decomposition of the bisdioxetanes (74) yielded the corresponding anhydrides... 

The synthesis of 1,4,8,11-tetraphenylpentacene and l,4,8,ll-tetra(2 -thienyl)pentacene has been achieved via photodecarbonylation of the corresponding a-diketone precursors (62) (Strating-Zwanenburg reaction). 

Scheme 8.14. A representation of the acid-catalyzed oxidation of a ketone (cyclohexanone) with selenium dioxide to the corresponding a-diketone (1,2-cyclohexanedione) (e.g., see Singh, K. I Anand, S. N. /. Indian Chem. Soc., 1979,56, 363).

---